Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction: Design, Properties and Applications 0128207914, 9780128207918

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

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Woodhead Publishing Series in Civil and Structural Engineering

Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction Design, Properties and Applications

Edited by

Fernando Pacheco-Torgal Lech Czarnecki Anna Laura Pisello Luisa F. Cabeza € ran Granqvist Claes-Go

An imprint of Elsevier

Woodhead Publishing is an imprint of Elsevier The Officers’ Mess Business Centre, Royston Road, Duxford, CB22 4QH, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States The Boulevard, Langford Lane, Kidlington, OX5 1GB, United Kingdom © 2021 Elsevier Ltd. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library ISBN: 978-0-12-820791-8 (print) ISBN: 978-0-12-820943-1 (online) For information on all Woodhead publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Matthew Dean Acquisitions Editor: Gwen Jones Editorial Project Manager: Isabella Silva Production Project Manager: Vignesh Tamil Cover Designer: Alan Studholme Typeset by SPi Global, India

Contents

Contributors 1

Introduction to eco-efficient materials for reducing cooling needs in buildings and construction Fernando Pacheco-Torgal 1.1 Climate emergency and global warming runaway patterns 1.2 Heat waves, urban heat island, and cooling materials as a way to save lives in the context of the coronavirus recession 1.3 Outline of the book References

Part One effects 2

3

xi

1 1 3 6 9

Pavements for mitigating urban heat island

High albedo pavement materials Marco Pasetto, Andrea Baliello, Emiliano Pasquini, and Giovanni Giacomello 2.1 Introduction 2.2 Characteristics of pavement albedo 2.3 Pavement albedo variability 2.4 High albedo pavement typologies 2.5 Albedo and pavement aging 2.6 High-albedo benefits and drawbacks 2.7 Conclusions and future trends References Performance of thermochromic asphalt Henglong Zhang, Shuai Zhang, Zihao Chen, and Chongzheng Zhu 3.1 Introduction 3.2 Three-component organic reversible thermochromic microcapsules 3.3 Thermochromic asphalt binders 3.4 Recommendations for future research and applications References

15

15 16 18 19 24 27 28 30 33 33 34 39 57 58

vi

4

Contents

Pavements for mitigating urban heat island effects I.K. Mizwar, Madzlan bin Napiah, and Muslich Hartadi Sutanto 4.1 Introduction 4.2 Thermal performance of pavement 4.3 Impact of pavement temperature on mechanical performance 4.4 Mitigate UHI effect 4.5 Future trends References

61 61 61 66 68 72 73

Part Two Facade materials for reducing cooling needs 5

6

7

Quantitative approximation of shading-induced cooling by climber green wall based on multiple-iterative radiation pathways Louis S.H. Lee and C.Y. Jim 5.1 Introduction 5.2 Factors influencing shading-induced cooling 5.3 Revised radiation apportionment model 5.4 Practical recommendations and future trends 5.5 Conclusion References Experimental study of geometric configuration and evaporative cooling potential of brick elements Philipp Lionel Molter and Ata Chokhachian 6.1 Introduction 6.2 Research methodology 6.3 Experiment: In-situ field measurements 6.4 Results 6.5 Conclusion 6.6 Future work Acknowledgments References Eco-efficient evaporative and ground-coupled system with terra-cotta evaporative walls Nolwenn Le Pierre`s, Guilian Leroux, and Etienne Wurtz 7.1 Introduction and state of the art of evaporative cooling systems 7.2 System description and operating principle 7.3 Terra-cotta characteristics and evaporative tank behavior 7.4 Impact of the evaporative cooling and ground-coupled system on the building performance 7.5 Conclusion and outlooks References

79 80 82 84 96 99 99

101 101 103 104 112 113 113 114 114

117

117 119 121 128 136 137

Contents

8

9

Hemp plaster and passive cooling techniques for retrofit: A case study Haitham Sghiouri, Mouatassim Charai, and Ahmed Mezrhab 8.1 Introduction 8.2 Experimental setup 8.3 Case study 8.4 Results and discussion 8.5 Conclusion Acknowledgment References Performance of multilayer glass and BIPV fac¸ ade structures S. Medved, C. Arkar, S. Domjan, and T. Zˇizˇak 9.1 Introduction 9.2 Object of research 9.3 Overall assessment indicators 9.4 CFD model and boundary conditions 9.5 Validation of CFD model 9.6 Parametric analysis 9.7 Conclusion References

vii

139 139 140 145 156 164 165 165 169 169 173 176 177 182 186 200 200

Part Three Roofing materials for reducing cooling needs 10

11

Green roofs as passive system to moderate building cooling requirements and UHI effects: Assessments by means of experimental data Roberto Bruno, Piero Bevilacqua, and Natale Arcuri 10.1 Introduction 10.2 Design options 10.3 Green roofs to moderate urban heat island (UHI) 10.4 Green roof modeling 10.5 Description of an experimental setup with extensive green roofs Acknowledgments References Thermal evaluation of building roofs with conventional and reflective coatings I. Herna´ndez-P erez and Y. Olazo-Go´mez 11.1 Introduction 11.2 Experimental evaluation of small-scale roof samples 11.3 Numerical model of the building roof 11.4 Thermal comfort evaluation of a building room with conventional and reflective roofs

205 205 207 208 209 215 238 239

247 247 249 257 259

viii

Contents

11.5

Comparison of experimental results with those of the literature 11.6 Conclusions Acknowledgments References

12

Active and passive systems for cool roofs Ming Chian Yew and Ming Kun Yew 12.1 Introduction 12.2 Types of cool roof systems 12.3 Mechanism of active cool roof system 12.4 Mechanism of passive cool roof system 12.5 Future trends 12.6 Sources of further information and advice References

266 271 271 271 275 275 277 282 284 285 286 287

Part Four PCMS and switchable glazing based materials for reducing cooling needs 13

14

Biobased phase change materials for cooling in buildings Luisa F. Cabeza 13.1 Introduction 13.2 Biobased PCM materials 13.3 Enhanced biobased PCM materials 13.4 Biobased PCM composites 13.5 Outlook on cooling applications in buildings and sustainability aspects 13.6 Conclusions Acknowledgments References PCM incorporated bricks: A passive alternative for thermal regulation and energy conservation in buildings for Indian conditions Rajat Saxena, Sana Fatima Ali, and Dibakar Rakshit 14.1 Introduction 14.2 PCM bricks as a sustainable passive alternative 14.3 Temperature and heat transfer assessment for PCM bricks 14.4 Thermal comfort assessment for a room with PCM bricks 14.5 Conclusions and future recommendations Acknowledgments References

291 291 292 294 295 299 299 299 300

303 303 304 311 315 323 324 324

Contents

15

16

17

18

Influence of novel PCM-based strategies on building cooling performance Yuekuan Zhou, Zhengxuan Liu, and Siqian Zheng 15.1 Introduction 15.2 Systematic literature review of novel PCM-based strategies for building cooling performance 15.3 Heat-transfer mechanism and modeling of PCM-integrated building energy systems 15.4 System performance enhancement of novel PCM-based cooling systems 15.5 Discussion of real applications and future prospects Acknowledgment References

ix

329 329 330 339 342 347 349 349

Optically smart thin materials for building cooling Omar Iken, Rachid Agounoun, Imad Kadiri, Miloud Rahmoune, Khalid Sbai, and Rachid Saadani 16.1 Introduction 16.2 Categories of optically smart thin materials 16.3 Fabrication methods of optically smart thin materials 16.4 Applications of optically smart thin materials for building cooling 16.5 Building energy efficiency due to optically smart thin materials applications 16.6 Conclusion and perspectives References

355

Building performance of thermochromic glazing Fabio Favoino, Luigi Giovannini, Anna Pellegrino, and Valentina Serra 17.1 Introduction 17.2 Thermochromic and thermotropic materials and glazing 17.3 Methodology for building performance of TC glazing 17.4 Results 17.5 Discussion 17.6 Conclusions References

401

Passive cooling by means of adaptive cool materials Claudia Fabiani and Anna Laura Pisello 18.1 Introduction 18.2 Generalities

455

356 357 366 376 389 393 394

401 403 413 422 428 432 433

439 440

x

Contents

18.3 18.4

Classification of thermochromic materials Thermochromic materials for passive cooling in the built environment 18.5 Conclusions and future developments Acknowledgments References

Index

440 450 454 455 455 459

Contributors

Rachid Agounoun Advanced Materials and Applications Laboratory, Higher School of Technology, Moulay Ismail University, Meknes, Morocco Sana Fatima Ali Centre for Energy Studies, Indian Institute of Technology Delhi, New Delhi, India Natale Arcuri Mechanical, Energy and Management Engineering Department, University of Calabria, Arcavacata di Rende, CS, Italy C. Arkar Faculty of Mechanical Engineering, Laboratory for Sustainable Technology in Buildings, University of Ljubljana, Ljubljana, Slovenia Andrea Baliello Department of Civil, Environmental and Architectural Engineering (DICEA), University of Padova, Padova, Italy Piero Bevilacqua Mechanical, Energy and Management Engineering Department, University of Calabria, Arcavacata di Rende, CS, Italy Roberto Bruno Mechanical, Energy and Management Engineering Department, University of Calabria, Arcavacata di Rende, CS, Italy Luisa F. Cabeza GREiA Research Group, University of Lleida, Lleida, Spain Mouatassim Charai CERTES, Paris-Est University, Creteil Cedex, France Zihao Chen Key Laboratory for Green & Advanced Civil Engineering Materials and Application Technology of Hunan Province, College of Civil Engineering, Hunan University, Changsha, China Ata Chokhachian TUM Department of Architecture, Technical University of Munich, Munich, Germany S. Domjan Faculty of Mechanical Engineering, Laboratory for Sustainable Technology in Buildings, University of Ljubljana, Ljubljana, Slovenia Claudia Fabiani CIRIAF—Interuniversity Research Center; Department of Engineering, University of Perugia, Perugia, Italy

xii

Contributors

Fabio Favoino Politecnico di Torino, Department of Energy, Turin, Italy Giovanni Giacomello Department of Civil, Environmental and Architectural Engineering (DICEA), University of Padova, Padova, Italy Luigi Giovannini Politecnico di Torino, Department of Energy, Turin, Italy I. Herna´ndez-Perez Academic Division of Engineering and Architecture, Juarez Autonomous University of Tabasco, Cunduacan, Tabasco, Mexico Omar Iken Advanced Materials and Applications Laboratory, Higher School of Technology, Moulay Ismail University, Meknes, Morocco C.Y. Jim Department of Social Sciences, Education University of Hong Kong, Tai Po, Hong Kong Imad Kadiri Advanced Materials and Applications Laboratory, Higher School of Technology, Moulay Ismail University, Meknes, Morocco Nolwenn Le Pierre`s LOCIE, Universite Savoie Mont Blanc, CNRS UMR5271, Le Bourget du Lac, France Louis S.H. Lee Department of Environment, Technological and Higher Education Institute of Hong Kong, Hong Kong, China Guilian Leroux LOCIE, Universite Savoie Mont Blanc, CNRS UMR5271; Department of Solar Technologies, Universite Grenoble Alpes, CEA-LITEN, Le Bourget du Lac, France Zhengxuan Liu College of Civil Engineering, National Center for International Research Collaboration in Building Safety and Environment, Hunan University, Changsha, Hunan, China S. Medved Faculty of Mechanical Engineering, Laboratory for Sustainable Technology in Buildings, University of Ljubljana, Ljubljana, Slovenia Ahmed Mezrhab Mechanics and Energy Laboratory, Mohammed First University, Oujda, Morocco I.K. Mizwar Department of Civil and Environmental Engineering, Universiti Teknologi PETRONAS, Perak, Malaysia Philipp Lionel Molter TUM Department of Architecture, Technical University of Munich, Munich, Germany

Contributors

xiii

Madzlan bin Napiah Department of Civil and Environmental Engineering, Universiti Teknologi PETRONAS, Perak, Malaysia Y. Olazo-Go´mez National Technological Institute of Mexico/CENIDET, Cuernavaca, Morelos, Mexico Fernando Pacheco-Torgal C-TAC Research Centre, University of Minho, Guimara˜es, Portugal Marco Pasetto Department of Civil, Environmental and Architectural Engineering (DICEA), University of Padova, Padova, Italy Emiliano Pasquini Department of Civil, Environmental and Architectural Engineering (DICEA), University of Padova, Padova, Italy Anna Pellegrino Politecnico di Torino, Department of Energy, Turin, Italy Anna Laura Pisello CIRIAF—Interuniversity Research Center; Department of Engineering, University of Perugia, Perugia, Italy Miloud Rahmoune Advanced Materials and Applications Laboratory, Higher School of Technology, Moulay Ismail University, Meknes, Morocco Dibakar Rakshit Centre for Energy Studies, Indian Institute of Technology Delhi, New Delhi, India Rachid Saadani Advanced Materials and Applications Laboratory, Higher School of Technology, Moulay Ismail University, Meknes, Morocco Rajat Saxena Department of Mechanical Engineering, Pandit Deendayal Petroleum University, Gandhinagar, India Khalid Sbai Advanced Materials and Applications Laboratory, Higher School of Technology, Moulay Ismail University, Meknes, Morocco Valentina Serra Politecnico di Torino, Department of Energy, Turin, Italy Haitham Sghiouri Mechanics and Energy Laboratory, Mohammed First University, Oujda, Morocco Muslich Hartadi Sutanto Department of Civil and Environmental Engineering, Universiti Teknologi PETRONAS, Perak, Malaysia Etienne Wurtz Department of Solar Technologies, Universite Grenoble Alpes, CEA-LITEN, Le Bourget du Lac, France

xiv

Contributors

Ming Chian Yew Department of Mechanical and Material Engineering, University of Tunku Abdul Rahman, Kajang, Selangor, Malaysia Ming Kun Yew Department of Civil Engineering, University of Tunku Abdul Rahman, Kajang, Selangor, Malaysia Henglong Zhang Key Laboratory for Green & Advanced Civil Engineering Materials and Application Technology of Hunan Province, College of Civil Engineering, Hunan University, Changsha, China Shuai Zhang Key Laboratory for Green & Advanced Civil Engineering Materials and Application Technology of Hunan Province, College of Civil Engineering, Hunan University, Changsha, China Siqian Zheng Department of Architecture and Civil Engineering, City University of Hong Kong, Hong Kong, China Yuekuan Zhou Department of Building Services Engineering, Faculty of Construction and Environment, The Hong Kong Polytechnic University, Hong Kong, China Chongzheng Zhu Key Laboratory for Green & Advanced Civil Engineering Materials and Application Technology of Hunan Province, College of Civil Engineering, Hunan University, Changsha, China ˇ izˇak Faculty of Mechanical Engineering, Laboratory for Sustainable Technology T. Z in Buildings, University of Ljubljana, Ljubljana, Slovenia

Introduction to eco-efficient materials for reducing cooling needs in buildings and construction

1

Fernando Pacheco-Torgal C-TAC Research Centre, University of Minho, Guimara˜es, Portugal

1.1

Climate emergency and global warming runaway patterns

According to Watts (2018), in February of 2017 the temperatures in the Arctic remained 20°C above average for longer than a week, increasing the melting rate. As a consequence, the replacement of ice by water led to a higher absorption of solar radiation, making the oceans warmer and being responsible for basal ice melting (Tabone et al., 2019) and also for a warmer atmosphere (Ivanov et al., 2016). This constitutes a form of positive feedback that aggravates the aforementioned problem. Wadhams (2017) already stated that an ice-free Arctic will occur in the next few years and that it will likely increase the warming caused by the CO2 produced by human activity by 50%. The latest data on rates of melting combined with new models suggest that an ice-free Arctic summer could occur by 2030 (Screen and Deser, 2019; Bendell, 2019). The warming of the earth will also result in extensive permafrost thaw in the Northern Hemisphere. With this thaw, large amounts of organic carbon will be mobilized, some of which will be converted and released into the atmosphere as greenhouse gases. This, in turn, can facilitate positive permafrost carbon feedback and thus further warming (Schuur et al., 2015; Tanski et al., 2018). Turetsky reported that permafrost thawing could release between 60 and 100 billion tonnes of carbon. This is in addition to the 200 billion tonnes of carbon expected to be released in other regions, which will thaw gradually. Also, recent fires in the Arctic region (Siberia and Alaska) have emitted millions of tons of CO2, constituting another positive feedback situation, and it is expected that in future fires will occur more frequently (Chen et al., 2016; Riley et al., 2019; Schirmeier, 2019). Vicious cycles of drought, leading to fire, leading to more drought have a positive feedback effect that further aggravates carbon dioxide emissions and global warming. Gasser et al. (2018) stated that the world is closer to exceeding the budget (cumulative amount of anthropogenic CO2 emission compatible with a global temperaturechange target) for the long-term target of the Paris Climate Agreement than previously Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction https://doi.org/10.1016/B978-0-12-820791-8.00001-8 © 2021 Elsevier Ltd. All rights reserved.

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

thought. Also, according to Xu et al. (2018), three lines of evidence suggest that the rate of global warming will be faster than projected in the recent IPCC special report. First, greenhouse-gas emissions are still rising. Second, governments are cleaning up air pollution faster than the IPCC and most climate modelers have assumed, but aerosols, including sulfates, nitrates, and organic compounds, reflect sunlight so the aforementioned cleaning could have a warming effect by as much as 0.7°C. Third, there are signs that the planet might be entering a natural warm phase, because the Pacific Ocean seems to be warming up, in accord with a slow climate cycle known as the Interdecadal Pacific Oscillation, which could last for a couple of decades. These three forces reinforce each other. Kareiva and Carranza (2018) stated that positive feedback loops represent the gravest existential risks, and the risks that society is least likely to foresee. To make things worse, the current climate emergency is also impacting microorganisms, not only exacerbating the impact of pathogens and increasing disease incidence, but also having a positive feedback effect on climate change (Cavicchioli et al., 2019). Recently Bamber et al. (2019) found that future sea level rise with the inclusion of thermal expansion and glacier contributions results for 2100 will exceed 2 m, which is more than twice the upper value put forward by the Intergovernmental Panel on Climate Change in the Fifth Assessment Report. This is especially worrisome, because 90% of urban areas are situated on coastlines, making the majority of the world’s population increasingly vulnerable to the current climate emergency (Elmqvist et al., 2019). At the same time, the United Nations estimates that by 2030, 700 million people will be forced to leave their homes because of drought (Padma, 2019). Drought and heat waves associated with this climate emergency are responsible for damaging crop yields, deepening farmers’ debt burdens, and inducing some to commit suicide. A study by Carleton (2017) shows that in India over the last three decades, the rising temperatures have already been responsible for over 59,000 suicides. And this raises the additional issue of determining which countries should take responsibility for climate refugees (Bayes, 2018). It is no wonder that Wallace-Wells (2017) wrote about catastrophic scenarios that include starvation, disease, civil conflict, and war. Even the discreet and circumspect Joachim Schellnhuber, professor of theoretical physics, expert in complex systems and nonlinearity, and founding director of the Potsdam Institute for Climate Impact Research (1992–2018) and former chair of the German Advisory Council on Global Change, spoke out more strongly in his foreword for the paper by Spratt and Dunlop (2018), in which he wrote: “climate change is now reaching the end-game, where very soon humanity must choose between taking unprecedented action, or accepting that it has been left too late and bear the consequences.” Torres (2019), on the other hand, has gone beyond the pessimistic forecasting by mentioning a hypothetical “double catastrophe scenario” in which an ongoing “stratospheric geoengineering project” is interrupted by a destabilizing event—e.g., a terrorist attack, or interstate or civil war—that could have unpredictable consequences for the global climate, for instance bringing about massive agricultural failures. And these are not unrealistic scenarios, because the world economy is so entangled that any random event could have massive consequences, especially for poor people. Coincidentally or not, on September of 2019

Introduction to eco-efficient materials

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several drones attacked the Abqaiq facility in Saudi Arabia, the most important oil processing facility in the world, worsening an already unstable world economy (FT, 2019). This is a clear sign of the consequences of a world economy addicted to nonrenewable resources that are located in one of the most unstable regions in the world. In July of 2018, Professor Bendell authored a dramatic piece warning of the probable social collapse, articulating the perspective that it is now too late to stop a future collapse of our societies because of the current climate emergency, and that we must now explore ways in which to reduce harm. He called for a “deep adaptation agenda” that would encompass: “withdrawing from coastlines, shutting down vulnerable industrial facilities, or giving up expectations for certain types of consumption” (Bendell, 2018). The essence of this deep adaptation lies in the “four Rs”: 1. 2. 3. 4.

Resilience: What do we most value and want to keep? Relinquishment: What must we let go of? Restoration: What skills and practices can we restore? Reconciliation: What can we make peace with to lessen suffering?

And in December of the same year, Read (2018) also shared his views about the dramatic future of our planet and the fate of humanity. Some say Bendell has gone too far in his pessimistic views, but a professor of physics at the University of Oxford wrote the following in a paper published in August of 2019: “Let’s get this on the table right away, without mincing words. With regard to the climate crisis, yes, it’s time to panic” (Pierrehumbert, 2019). In a presentation that Bendell gave in May of 2019 at the European Commission, he mentioned the importance of technologies for deep adaptation (Bendell, 2019), but of course he only named a few. Be that as it may, he was not even considering long-term scenarios defended by Baum et al. (2019). Also on November 5, 2019, Ripple et al. (2019), along with several thousand scientists, issued a warning: “Clearly and unequivocally the planet Earth is facing a climate emergency,” which is none other than tacit support for Bendell’s views.

1.2

Heat waves, urban heat island, and cooling materials as a way to save lives in the context of the coronavirus recession

According to the IPCC, heat waves are the most important and dangerous hazard related to the current climate emergency. Kew et al. (2019) reported that anthropogenic climate change has increased the odds of heat waves at least threefold since 1950, and across the Euro-Mediterranean the likelihood of a heat wave at least as hot as summer 2017 (responsible for temperatures above 40°C in France and the Balkan region and nighttime temperatures above 30°C) is now on the order of 10%. The negative impacts of extreme heat, which are more acute in urban areas, include health risks, higher concentrations of pollutants (Meehl et al., 2018), lower water quality, and decrease in labor productivity. Ironically, Belkin and Kouchaki (2017) even found that heat increases fatigue, which leads to reduction in positive affect, subsequently

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

reducing individual helping. Also, those most affected are the most vulnerable groups among the urban dwellers: the elderly, the individuals with preexisting chronic conditions, communities with weak socioeconomic status, people with mental disorders, and isolated individuals (Smid et al., 2019). In this context it is worth remembering that in 2003 a European heat wave claimed the lives of several thousand people and in 2010 Moscow was hit by the strongest heat wave of the present era, killing more than 10,000 people. Europe, with its growing aging population trend, a population which is more susceptible to heat-wave effects, will in that context be hit in a harder way (Fig. 1.1). If no adaptation measures are undertaken, this could mean an additional several thousand deaths/year from heat waves (and their synergistic effects with air pollution). The consequences associated with heat wave predictions do not even take into account the effect associated with urban heat islands (UHIs). This phenomena is triggered by absorption radiation due to artificial urban materials, transpiration from buildings and infrastructure, release of anthropogenic heat from inhabitants and appliances, and the airflow blocking effect of buildings (Mirzaei and Haghighat, 2010; Pacheco-Torgal et al., 2015). The dark-colored surfaces used (such as dark asphalt pavements) have low reflecting power (or low albedo characteristics); as a consequence they absorb more energy and in summer can reach almost 60°C, thus contributing toward greater UHI effects. UHI is probably the most documented phenomenon of the current climate emergency for various geographic areas of the planet, with a huge increase in the number of publications appearing on this topic since 1990 (Fig. 1.2). This may have something to do with the beginning of the sustainable development movement after the publication of the Brundtland report (Our Common Future) (Brundtland, 1987). As

CDD/year

1981–2100

+8 +6 +4 +2 0 –2 –4 –6

–8 rcp85

Fig. 1.1 Linear trends of cooling degree days (CDDs) per year under the RCP8.5 radiative forcing scenario, i.e., radiative forcing values at the end of the 21st century, relative to preindustrial values of +8.5 W m
2 (Spinoni et al., 2018).

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Fig. 1.2 Characteristics by year of UHI-related publications (Huang and Lu, 2018).

a result of massive urbanization and industrialization of human civilization in the last few decades, UHI has gained a dramatic dimension that jumpstarted the publications in this field. In the future this urbanization trend is expected to become even worse; according to Guerreiro et al. (2018) by 2050 urban systems will be home to 66% of the global population, with the proportion being even higher in the European Union, where currently 75% of the population resides in cities with expected growth to 82% by 2050. At that point, UHI will become more and more important, having more dramatic consequences. Some authors have reported a 10°C temperature increase in the city of Athens due to the UHI effect (Santamouris et al., 2001) and an 8.8°C increase in London (Kolokotroni and Giridharan, 2008), while a recent 3-year investigation in the city of Padua reported an increase up to 6°C (Busato et al., 2014). According to Li et al. (2014), even the waste heat discharged by air conditioners alone was responsible for an increase of almost 2°C in Beijing average air temperature in 2005. Recent projections show that in the northwest area of the United Kingdom, summer mean temperatures could rise by 5°C (50% probability, 7°C top of the range) by the 2080s (Levermore et al., 2018). The expected rise in global temperature is likely to increase the energy needed to cool buildings in the summer. Balaras et al. (2007) mentioned an increase of energy cooling needs more than 2000% between 1990 and 2010. Also, the synergistic effect between heat waves and air pollution causes worse outdoor air quality in the summer and prevents natural ventilation, thus increasing cooling needs. In the heavily polluted city of Beijing, Li et al. (2014) reported that 28.88% of the total air-conditioning energy consumption is due to the UHI effect. Manoli et al. (2019) studied data from some 30,000 cities worldwide, and concluded that cities having a more desert-like surrounding countryside can more easily achieve cooler temperatures by use of careful plantings than cities surrounded by tropical forests,

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

which need far more green spaces to reduce temperatures, thus creating more humidity. In these latter areas, other cooling methods are therefore expected to be more effective, such as increased wind circulation, more use of shade, and new heatdispersing materials. Shandas et al. (2019) used a combination of ground-based measurements and satellite data that accurately identified areas of extreme urban heat hazards. The results showed that the urban microclimate was highly variable, with differences of up to 10°C between the coolest and warmest locations at the same time. Sailor et al. (2019) reported that building occupants in many US cities rely only on air conditioning, to a degree that their health and well-being are compromised in its absence. They found that residential buildings are highly vulnerable to heat disasters and that situation will be exacerbated by intensification of UHIs. A recent review by Santamouris (2019) included a projection that the mortality of the elderly population in Washington State will increase between 4 and 22 times by 2045, and heat-related mortality in three cities in the northeastern United States will increase six to nine times by 2080 under the high-emission scenario, RCP 8.5. Of course, these projections do not account for the economic recession caused by the coronavirus (Michelsen et al., 2020; Fernandes, 2020; Leiva-Leon et al., 2020), which in turn will reduce the number of those who can afford air conditioning. Some estimates are so pessimistic (Sraders, 2020) as to imply that air conditioning could become a luxury expense. On the positive side, Macintyre and Heaviside (2019) concluded that cool roofs could reduce heat-related mortality associated with the UHI effect by 25% during a heat wave. This shows how cooling materials can be important in saving lives, especially in the context of the coronavirus recession. Bai et al. (2018) advocated that research on mitigating urban climate change and adapting to it must be supported at a scale commensurate with the magnitude of the problem, and that funding agencies need to provide grants for cross-disciplinary research and comparative studies, especially in the global south. Sharma et al. (2019) also stated that there exists a huge gap in the literature on such topics. Of particular concern are cities located in developing nations with arid or semiarid climate conditions that are already experiencing very hot and dry summers and, due to low adaptive capacities, are more vulnerable to changing climate. All of these factors constitute a strong justification for this book. In addition, books already on the market do not present a comprehensive review of the full innovative range of eco-efficient materials capable of mitigating UHI effects and meeting building cooling needs. Some publications contain almost no information on cool pavements and others are deficient on subjects such as switchable glazing-based materials (Santamouris, 2019). This book, however, has a balanced coverage of eco-efficient materials for pavements, fac¸ades, and roofs, with a section especially for phase change materials (PCMs) and switchable glazing-based materials. With special contributions from a team of international experts, this book provides an updated state of the art on eco-efficient materials for reducing cooling needs in buildings and other construction.

1.3

Outline of the book

This book provides an updated state-of-the-art review of eco-efficient materials to reduce cooling needs in building and construction.

Introduction to eco-efficient materials

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Part One encompasses an overview of pavements for mitigation of urban heat island effects (Chapters 2–4). Chapter 2 covers particular applications aimed at mitigating the heat-related concerns using high albedo materials. First, albedo significance and relevance and its usefulness for effectively producing thermally optimized pavements are analyzed. After a short contextualization, the physics of albedo is discussed, in order to better understand its theoretical meaning and function in determining the radiative properties of materials that affect the general energetic balance of pavement surfaces. Highalbedo paving solutions are described, giving some details on constituent materials, chromatic characteristics, and surface properties. Some general issues regarding the overall benefits and drawbacks involved with the utilization of high-albedo pavement materials are debated. Chapter 3 introduces three-component organic reversible thermochromic microcapsules, including their classification, merits, components, structure, thermochromic mechanism, and thermal and optical properties. The performance of thermochromic asphalt binders is illustrated, covering physical, optical, thermal, rheological, and antiaging properties, and the adjustment of thermochromic asphalt temperature. Finally, some proposals for future research on thermochromic asphalt are given. Chapter 4 reviews pavements developed to mitigate urban heat islands. This includes information on methods to quantify surface temperature and heat transfer from pavement to urban temperature. Furthermore, the impacts of pavement temperature on mechanical performances are presented. Special attention is given to porous pavement, PCM pavement, and hydronic pavement. Fac¸ade materials for reducing building cooling needs are the subject of Part Two (Chapters 5–9). Chapter 5 presents a revised radiation apportionment model for estimating the benefits of shading against short-wave radiation as cooling-load reduction. Adhering to the principles of an improved radiative transfer model, field data harvested from net radiometers were input into a Microsoft Excel spreadsheet. The Solver function determined the radiative properties of various layers of a windowed building envelope, featuring a climber green wall. Chapter 6 investigates the potentials of different geometrical brick patterns and their behavior on self-shading potential to reduce mean surface temperature of solar exposed brick walls. The study was shaped in two layers, including field measurements for geometric behavior and evaporative cooling potential. The results are discussed and compared for three configurations of solid, extruded, and perforated under varying boundary conditions over a day. Chapter 7 presents an innovative low-energy, low-tech, and low-cost cooling system for buildings. This cooling system simultaneously makes use of three available heat sinks: the ground, evaporation of water, and radiation to the sky. A terra-cotta tank is placed along a northern wall of the building to achieve the two last phenomena. The chapter includes a case study simulation on a 100-m2 house in Bordeaux climatic conditions. Chapter 8 refers to a retrofitting case study of an office building using hemp-based plaster and passive cooling techniques. An economic simulation is also included.

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

Chapter 9 concerns the evaluation of thermal response of advanced glass fac¸ade structures with improved energy performance during the cooling period of the buildings. Two types of glazed fac¸ade structures were analyzed: 6-pane glazing structure upgraded in a BIPV system with and without PCM inserts, and BIPV double glazed fac¸ade with a transparent triple-glazed interior structure. The diurnal transient thermal response of constructions under investigation was evaluated using a CFD technique for extreme daily summer climate conditions for Athens, Ljubljana, and Stockholm. Part Three (Chapters 10–12) deals with roofing materials for reducing building cooling needs. Chapter 10 reviews the importance of green roofs for UHI mitigation. It includes design options and green roof modeling. The chapter also includes a description of an experimental set-up with extensive green roofs, including its cooling energy savings. Chapter 11 discusses the thermal performance of building roofs with conventional and reflective coatings. A numerical model of a building roof validated with experimental data is included. Chapter 12 focuses on utilization of active and passive cool roof systems to enhance the comfort of building occupants with attic temperature reduction. A case study including a thermal reflective coating, MAC-solar powered fan, and a rainwater harvesting system is analyzed. Part Four concerns PCMs and switchable glazing-based materials for reducing cooling needs (Chapters 13–18). Chapter 13 contains a short review of recent developments concerning biobased phase change materials for cooling in buildings. Chapter 14 deals with PCM selection (mapping), based on its thermophysical properties and climatic parameters, which are location specific, followed by technology for PCM incorporation within building components. It also provides a comprehensive review of studies carried out so far in terms of energy savings through PCM incorporation within buildings. Chapter 15 provides a state-of-the-art review of novel PCM-based strategies for building cooling performance enhancement. The investigated strategies include PCM integrated forms (such as distributed and coupled systems) and combined strategies (such as high-reflective coating, radiative cooling wall, and hybrid ventilations). Solutions for system performance enhancement of novel PCM-based cooling systems are comprehensively presented. Chapter 16 reviews optically smart thin materials, such as thermochromic and electrochromic coatings, including all the mechanisms that manage their optical smartness. Classical and new methods used to fabricate these materials are detailed. Applications of these materials, such as smart thermal building insulators, and their impact on cooling and heating energy consumption are discussed. Chapter 17 focuses on the most promising innovative solutions, in particular those whose strong dynamic and adaptable behaviors are able to tailor building energy needs and to optimize their performance and indoor-outdoor functionality. In this chapter, thermochromic materials are presented together with their major potentialities and limitations. Finally, their effect on building energy efficiency is assessed, with particular focus on the existing applications at the single building and urban scales.

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Chapter 18 closes Part Four with an overview of thermochromic glazing products, considering their thermo-optical properties and technological integration. This chapter also includes a quantitative evaluation of their whole building performance. Total energy use and visual comfort aspects are discussed for a typical sun-oriented office building in three different climates, in order to provide an overview of their potential performance improvements as compared to traditional static glazing technologies.

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FT, 2019. Attack on Saudi facility exposes world economy’s ‘Achilles heel’. https://www.ft. com/content/0b89f8e2-d885-11e9-8f9b-77216ebe1f17. Gasser, T., Kechiar, M., Ciais, P., Burke, E.J., Kleinen, T., Zhu, D., Huang, Y., Ekici, A., Obersteiner, M., 2018. Path-dependent reductions in CO2 emission budgets caused by permafrost carbon release. Nat. Geosci. 11 (11), 830. Guerreiro, S.B., Dawson, R.J., Kilsby, C., Lewis, E., Ford, A., 2018. Future heat-waves, droughts and floods in 571 European cities. Environ. Res. Lett. 13 (3), 034009. Huang, Q., Lu, Y., 2018. Urban heat island research from 1991 to 2015: a bibliometric analysis. Theor. Appl. Climatol. 131 (3–4), 1055–1067. Ivanov, V., Alexeev, V., Koldunov, N.V., Repina, I., Sandø, A.B., Smedsrud, L.H., Smirnov, A., 2016. Arctic Ocean heat impact on regional ice decay: a suggested positive feedback. J. Phys. Oceanogr. 46 (5), 1437–1456. Kareiva, P., Carranza, V., 2018. Existential risk due to ecosystem collapse: nature strikes back. Futures 102, 39–50. Kew, S.F., Philip, S.Y., Jan van Oldenborgh, G., van der Schrier, G., Otto, F.E., Vautard, R., 2019. The exceptional summer heat wave in southern Europe 2017. Bull. Am. Meteorol. Soc. 100 (1), S49–S53. Kolokotroni, M., Giridharan, R., 2008. Urban heat island intensity in London: an investigation of the impact of physical characteristics on changes in outdoor air temperature during summer. Sol Energy 82, 986–998. Leiva-Leon, D., Perez-Quiros, G., Rots, E., 2020. Real-time weakness of the global economy: a first assessment of the coronavirus crisis. ECB Working Paper No. 2381, March. Available at SSRN: https://ssrn.com/abstract¼3560170. Levermore, G., Parkinson, J., Lee, K., Laycock, P., Lindley, S., 2018. The increasing trend of the urban heat island intensity. Urban Clim. 24, 360–368. Li, C., Zhou, J., Cao, Y., Zhong, J., Liu, Y., Kang, C., Tana, Y., 2014. Interaction between urban microclimate and electric air-conditioning energy consumption during high temperature season. Appl. Energy 117, 149–156. Macintyre, H.L., Heaviside, C., 2019. Potential benefits of cool roofs in reducing heat-related mortality during heatwaves in a European city. Environ. Int. 127, 430–441. Manoli, G., Fatichi, S., Schl€apfer, M., Yu, K., Crowther, T.W., Meili, N., Burlando, P., Katul, G.G., Bou-Zeid, E., 2019. Magnitude of urban heat islands largely explained by climate and population. Nature 573 (7772), 55–60. Meehl, G.A., Tebaldi, C., Tilmes, S., Lamarque, J.F., Bates, S., Pendergrass, A., Lombardozzi, D., 2018. Future heat waves and surface ozone. Environ. Res. Lett. 13 (6), 064004. Michelsen, C., Clemens, M., Hanisch, M., Junker, S., Kholodilin, K.A., Schlaak, T., 2020. Coronavirus plunges the German economy into recession: DIW economic outlook. DIW Weekly Report 10 (12), 184–190. Mirzaei, P.A., Haghighat, F., 2010. Approaches to study urban heat island – abilities and limitations. Build. Environ. 45 (10), 2192–2201. Pacheco-Torgal, F., Labrincha, J., Cabeza, L., Granqvist, C.G. (Eds.), 2015. Eco-efficient Materials for Mitigating Building Cooling Needs: Design, Properties and Applications (No. 56). Woodhead Publishing. Padma, T., 2019. African nations push UN to improve drought research. https://www.nature. com/articles/d41586-019-02760-9. Pierrehumbert, R., 2019. There is no Plan B for dealing with the climate crisis. Bull. At. Sci. 1–7. Read, R., 2018. This civilisation is finished: so what is to be done? https://ueaeprints.uea.ac.uk/ 69557/1/Read_Rupert_This_Civilisation_is_finished_so_what_is_to_be_done.pdf.

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Riley, K.L., Williams, A.P., Urbanski, S.P., Calkin, D.E., Short, K.C., O’Connor, C.D., 2019. Will landscape fire increase in the future? A systems approach to climate, fire, fuel, and human drivers. Curr. Pollut. Rep. 5 (2), 9–24. Ripple, W.J., Wolf, C., Newsome, T.M., 2019. World scientists’ warning of a climate emergency. Bioscience. https://doi.org/10.1093/biosci/biz088. Sailor, D.J., Baniassadi, A., O’Lenick, C.R., Wilhelmi, O.V., 2019. The growing threat of heat disasters. Environ. Res. Lett. 14 (5), 054006. Santamouris, M. (Ed.), 2019. Cooling Energy Solutions for Buildings and Cities. World Scientific. Santamouris, M., Papanikolaou, N., Livada, I., Koronakis, I., Georgakis, C., Argiriou, A., Assimakopoulos, D., 2001. On the impact of urban climate on the energy consumption of buildings. Sol. Energy 70, 201–216. Schirmeier, Q., 2019. Climate change made Europe’s mega-heatwave five times more likely. https://www.nature.com/articles/d41586-019-02071-z. Schuur, E.A., McGuire, A.D., Sch€adel, C., Grosse, G., Harden, J.W., Hayes, D.J., Hugelius, G., Koven, C.D., Kuhry, P., Lawrence, D.M., Natali, S.M., 2015. Climate change and the permafrost carbon feedback. Nature 520 (7546), 171. Screen, J.A., Deser, C., 2019. Pacific Ocean variability influences the time of emergence of a seasonally ice-free Arctic Ocean. Geophys. Res. Lett. 46, 2222–2231. Shandas, V., Voelkel, J., Williams, J., Hoffman, J., 2019. Integrating satellite and ground measurements for predicting locations of extreme urban heat. Climate 7 (1), 5. Sharma, R., Hooyberghs, H., Lauwaet, D., De Ridder, K., 2019. Urban heat island and future climate change—implications for Delhi’s heat. J. Urban Health 96 (2), 235–251. Smid, M., Russo, S., Costa, A.C., Granell, C., Pebesma, E., 2019. Ranking European capitals by exposure to heat waves and cold waves. Urban Clim. 27, 388–402. Spinoni, J., Vogt, J.V., Barbosa, P., Dosio, A., McCormick, N., Bigano, A., F€ ussel, H.M., 2018. Changes of heating and cooling degree-days in Europe from 1981 to 2100. Int. J. Climatol. 38, e191–e208. Spratt, D., Dunlop, I., 2018. What Lies Beneath: The Understatement of Existential Climate Risk. Breakthrough (Nafional Centre for Climate Restorafion). https://climateextremes. org.au/wp-content/uploads/2018/08/What-Lies-Beneath-V3-LR-Blank5b15d.pdf. Sraders, A., 2020. How much will coronavirus hurt the economy? These new estimates are terrifying. https://fortune.com/2020/03/17/how-much-will-coronavirus-hurt-the-economythese-new-estimates-are-terrifying/. (Accessed 2 April 2020). Tabone, I., Robinson, A., Alvarez-Solas, J., Montoya, M., 2019. Submarine melt as a potential trigger of the North East Greenland Ice Stream margin retreat during Marine Isotope Stage 3. Cryosphere 13 (7), 1911–1923. Tanski, G., Wagner, D., Fritz, M., Sachs, T., Lantuit, H., 2018. Impetuous CO2 release from eroding permafrost coasts. In: 5th European Conference on Permafrost, 23 June–1 July, Chamonix, France. Torres, P., 2019. Facing disaster: the great challenges framework. Foresight 21 (1), 4–34. Wadhams, P., 2017. A Farewell to Ice. Oxford University Press, Oxford. Wallace-Wells, D., 2017. The Uninhabitable Earth: famine, economic collapse, a sun that cooks us: what climate change could wreak—sooner than you think. New York Magazine. . Watts, J., 2018. Arctic warming: scientists alarmed by ‘crazy’ temperature rises. The Guardian. 27. Xu, Y., Ramanathan, V., Victor, D.G., 2018. Global warming will happen faster than we think. Nature 564 (7734), 30–32. https://doi.org/10.1038/d41586-018-07586-5.

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Part One Pavements for mitigating urban heat island effects

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High albedo pavement materials Marco Pasetto, Andrea Baliello, Emiliano Pasquini, and Giovanni Giacomello Department of Civil, Environmental and Architectural Engineering (DICEA), University of Padova, Padova, Italy

2.1

2

Introduction

At present, pavements cover a significant part of urban and suburban surfaces (about 35%–40% of the built areas). Moreover, roads, cycle tracks, footways, parking areas, and squares are mutually integrated with buildings and create a composite space that constitutes a unique high heat-absorbing and transmitting entity. Infrastructures surround buildings and vertical structures and form an interconnected network giving continuity to surfaces; however, they are also an obstacle for natural ground runoff and water disposal and cause warming of the local air. Modification of the ground permeability due to the substitution of natural soil with structures, together with the progressive reduction of vegetation, plants, and green areas, deeply alters the natural thermal equilibrium typical of pristine rural zones. The general lack of green coverage impedes the achievement of adequate levels of evapotranspiration and the dissipation of the environmental heat gathered in the lower strata of the atmosphere. Urbanization impacts the water cycle, since the increasing impermeable surface area affects the quantity of water retained and thus the humidity necessary for keeping the inhabited areas cool. In addition, the widely extended road networks generally have black or dark colors due to the general usage of bituminous mixtures for pavement construction, contributing to concentration of the heat radiation. Consequently, the temperatures in cities increase and promote the formation of so-called surface and atmospheric urban heat islands (Taha et al., 1992). This situation has nonnegligible effects on the urban canopy layer (i.e., the segment of atmosphere contained between the ground level and the medium height of buildings, roofs, and tree tops), influencing perceived comfort and human health. In general, many elements contributing to pavement’s heat-retention/transmission properties can be identified (pavement thickness, layer density, type of materials). One of these factors is the albedo, i.e., the surface radiative property that determines the reflected part of the incident radiation impacting the bodies exposed to the sun. In this regard, the use of high-albedo pavement materials has been recently recognized as one of the most efficient strategies to pursue urban heat island mitigation (Akbari et al., 2012; Dumais and Dor
e, 2016). Since albedo represents one of the main radiative properties of real objects, its understanding is fundamental to any study of the possible use of specific materials able to attenuate the heat-related negative effects developing in the urban environment.

Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction https://doi.org/10.1016/B978-0-12-820791-8.00002-X © 2021 Elsevier Ltd. All rights reserved.

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2.2

Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

Characteristics of pavement albedo

The radiative properties of bodies summarize the responses of objects when subject to electromagnetic and thermal radiation due to the solar effect. More specifically, solar electromagnetic radiation assumes a common wavelength λ ranging between 0.1 and 3 μm and about one-half is composed of visible radiation (λ from 0.4 to 0.76 μm) and the remaining part of ultraviolet (λ from 0.1 to 0.4 μm) and infrared (λ from 0.76 to 3 μm) energy (Dickinson, 1986). Consequently, electrons, atoms, and molecules of solids and fluids subject to solar irradiation (at a temperature higher than 0 K) are characterized by a constant state of microscopic movement and emit/absorb energy in all the directions of space. Considering the typical pavement materials, such a volumetric phenomenon can be well approximated as a surface transfer: this is possible because the incident radiation is absorbed and emitted within a few microns below the external surface, since surfaces are generally opaque (nontransparent) (Irwin, 2007). From a physical point of view, the real radiative phenomena are complex: taking into account a generic body (always hotter than 0°K), it can theoretically emit radiation in all directions and a huge field of wavelengths, depending on the incident energy and its radiative properties (commonly associated with the body’s material and temperature). Therefore, it is evident that diverse objects at the same temperature can emit different radiation spectra. For this reason, the hypothetical concept of a “black body” is commonly introduced, in order to indicate a perfect emitter/absorber of radiation, i.e., an object able to emit the maximum radiation at each temperature and wavelength and to absorb the totality of incident radiation, independently from the wavelength and the direction (Planck and Masius, 2012). With this approach, the sun (i.e., the solar radiation emitter) can be assumed as a black body at the approximate temperature of 5800°K, with a radiation peak located in the region of visible light (Goody and Yung, 1989). Fig. 2.1 shows the relation between the monochromatic emissive power of the black body Enλ and the wavelength λ at different temperatures and depicts the solar radiation shape. Radiative properties are used to summarize the response of objects to external stimuli given by irradiation, starting from the assumption that the entire energy incident on the surfaces is split into three main portions related to the absorbed, reflected, and

Fig. 2.1 Monochromatic emissive power for some selected temperatures.

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transmitted fractions (Foster et al., 2010). Based on this principle, the incident radiation G is given by Eq. (2.1): G ¼ Gabs + Gref + Gtr

(2.1)

where Gabs represents the absorbed radiation, Gref indicates the reflected radiation, and Gtr stands for the transmitted radiation. Then, normalizing the singular components with respect to the total incident radiation G, Eqs. (2.2)–(2.4) define the three main radiative properties of the surface: α ¼ Gabs =G

(2.2)

ρ ¼ Gref =G

(2.3)

τ ¼ Gtr =G

(2.4)

where α represents the absorptivity, ρ is the albedo, and τ indicates the coefficient of transmission. Particularly referring to albedo, ρ is the dimensionless coefficient of reflection that gives information about the reflected part of the radiation: it ranges numerically from 0 to 1, where 1 indicates a total reflection of the incident radiation and 0 a null reflected part (Li, 2016). Normalization allows Eq. (2.1) to be rewritten in order to obtain the relation between the radiative properties (Eq. 2.5): G Gabs Gref Gtr ¼ + + !1¼α+ρ+τ G G G G

(2.5)

Indeed, recalling the assumption made for opaque elements (e.g., pavement materials), for which the energy transmission below the surface is obstructed, the τ term can be neglected and Eq. (2.5) can be transformed into Eq. (2.6). This well-known relationship is very important: in fact, it allows a description of the surface radiative response by means of a single physical parameter (the literature widely prefers albedo rather than absorptivity). 1¼α+ρ

(2.6)

Even if these parameters should assume a bidirectional behavior (i.e., they should depend on the directions of the incident and reflected radiations), such an assumption is generally omitted for the simplicity of calculation. Furthermore, a commonly accepted simplification hypothesizes that such radiative properties are not affected by the temperature of the object surface, but are only dependent on the temperature of the radiation source (e.g., the characteristics of the solar radiation hitting the pavement materials) (Marshall and Plumb, 2008). The tangible effect of the albedo in mitigating the heat must be finally evaluated considering its contribution with respect to the general radiative balance equation that is commonly used in order to describe the heat transfer mechanisms impacting real pavements (Solaimanian and Kennedy, 1993).

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

In general, all thermal models in the literature suppose the existence of three basic components that are relevant to describe heat transfer in the pavement system: conduction, convection, and radiation. Conduction is typically considered as heat transfer along the structure depth, starting from the upper layers toward the lower ones. Convection is ascribed to the phenomena developing in the proximity of the pavement surface (between the wearing course and the surrounding environment above). Radiation is the emission from the sun of shortwaves received by the pavement surface (considering the radiation intercepted by the surrounding elements, e.g., structures, buildings, trees, objects, etc.). In this context, high-albedo materials are typically associated with effective solutions for the heat reduction, since ρ (the ratio between the heat energy reflected and that received by the surface) affects the quantity of radiation effectively absorbed by the pavement and that is reflected onto the environment. The described mechanisms are schematized in Fig. 2.2. Albedo affects the radiative balance in the specific equation used to calculate the ingoing direct radiation coming from the sun that reaches the pavement in the form of (1 – ρ) (Pasetto et al., 2019b). Thus, the increase of albedo is able to reduce the absolute value of the positive terms composing the heat flow balance (sign “+” is conventionally attributed to the ingoing components of the balance equation).

2.3

Pavement albedo variability

From a practical point of view, the albedo is related to many aspects, including the color of pavement, material composition, and surface roughness and texture (Ferrari et al., 2020). Even if the most diffused thermal models fix the albedo value

Fig. 2.2 Conceptual scheme of the heat transfer mechanisms affecting a pavement.

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for any day during simulations, it must be considered that the real situation is very complex and some ρ fluctuations will be recorded day by day. In this context, research efforts were recently made by some authors to enhance the reliability of the predictive thermal models by studying the real albedo fluctuations recorded within in-service pavements. Effectively, several levels of temporal variability have been found for albedo, over short and long time periods, with nonnegligible ρ changes on many pavement surfaces. As an example, the daily variability of albedo was studied by Li et al. (2013) analyzing bituminous and concrete pavements. Both for flexible and rigid structures, significant variations of albedo were observed over a few hours, specifically in the early morning and in the late evening. In this sense, even similar levels of incident radiation and similar material properties offered distinct absolute albedo values and daily variability. When treating bituminous pavements, such an effect was also linked to possible glare problems. However, the identified fluctuations were supposed to be rather insignificant with respect to the overall thermal balance (adsorbed/reflected quantity of radiation) because of the low level of insolation during these periods. For instance, the following figure is extracted from the previously mentioned original research and reports the daily evolution of ρ for 24 h, both for bituminous (Fig. 2.3A) and concrete (Fig. 2.3B) pavements. The same authors (Li et al., 2013) also evaluated the seasonal albedo variability based on extended observation periods. Early and late day variations were again noticed but, in the proximity of midday, the ρ result was substantially different from one season to another. In the case of bituminous pavements, albedo during the spring and winter seasons was noticeably higher than the corresponding values in autumn and summer (increase of about 15% in both cases). This fact was principally ascribed to the different surface conditions of pavement in the different seasons (e.g., presence of dirt), as well as to the variable intensity of solar radiation peaks. In fact, it was reasonably argued that the solar radiation levels incident on the pavements were characterized by more variability during the most intense peak periods (e.g., in the summer season). Otherwise, a potential responsibility was also attributed to the slight different inclination of the radiation due to the different sun-earth positionings. For instance, Fig. 2.4 (from the original research of Li et al.) shows recorded ρ seasonal evolution for a bituminous pavement in winter (Fig. 2.4A) and summer (Fig. 2.4B). Basically, the pavement’s specific seasonal condition can certainly be hypothesized as a key factor in determining the final ρ fluctuation; some studies have also included the possible presence of snow and frost on the surface among the causes of albedo variation, defining conventional mathematical distributions (e.g., seasonal ρ variability approximated with a sinusoidal trend) (Herb et al., 2006; Han et al., 2011).

2.4

High albedo pavement typologies

Concerning surface color, in general, light tonalities present a low absorptivity (thus a high albedo value—see Eq. 2.6), at least in the visual light spectrum of solar radiation (λ from 0.4 to 0.76 μm). This is not the same for the perceived color in the infrared

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

Fig. 2.3 Daily variation of albedo in 24 h: bituminous (A) and concrete (B) pavements. From Li, H., Harvey, J., Kendall, A., 2013. Field measurement of albedo for different land cover materials and effects on thermal performance. Build. Environ. 59, 536–546.

range, where any dependence was lacking (Stuart-Fox et al., 2017). On the contrary, paving materials with darker colors are characterized by low ρ values (black road pavements made of bituminous concrete typically assume the lowest ρ, equal to about 0.005–0.1). Based on this observation, several literature studies have already

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Fig. 2.4 Example of albedo seasonal variability for a bituminous pavement: winter (A) vs summer (B). From Li, H., Harvey, J., Kendall, A., 2013. Field measurement of albedo for different land cover materials and effects on thermal performance. Build. Environ. 59, 536–546.

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

demonstrated strong correlations between the lightness of pavement materials and the ability of structures in reducing heat diffusion in the pavement proximity (Kyriakodis and Santamouris, 2018; Pasetto et al., 2019a). Surface temperature reductions of even 20–25°C have been measured in pavements of different color, even if made of the same product. Therefore, the increase of albedo is often a target point when dealing with thermally optimized pavement: enhanced albedo values can be obtained through different strategies, depending on the pavement type and function. Rigid pavements are made of concrete and can be designed to fulfill particular structural needs, e.g., static or heavy loads (parking lots, piers, airport structures, etc.). Concrete pavements are widely utilized in the urban environment for other purposes, due to their surface characteristics and construction methods (walkways, pedestrian and cycle paths, squares and open spaces, etc.) (Plati, 2019). Commonly, concrete surfaces have a much higher reflectance (albedo value) than other competitive paving materials, since the light color of concrete naturally reflects heat and light. Numerically, albedo values two to three times higher than those of bituminous pavements can be easily found in the literature (Hulley, 2012; Nizˇeti
c and Papadopoulos, 2018). Surface albedo of concrete pavements is developed with the hydration of cement during the first weeks after casting and is typically subject to slight progressive reduction over time (weathering should promote the progressive exposure of the darker aggregates composing the mixture) (Environmental Protection Agency, 2017). Possible concrete modifiers have also been investigated with the aim of increasing the albedo in rigid pavements: as an example, pozzolanic additives were found to increase ρ with respect to conventional cement binders. On the other hand, fly ash added to concrete blends proved to reduce the resulting ρ (Marceau and Vangeem, 2008). Flexible pavements made of bituminous mixtures are commonly employed on road networks because of their mechanical characteristics compatible with the loads produced by vehicles. The composition of a conventional bituminous concrete (a lithic aggregate matrix bound with bitumen) confers the typical gray/dark color to the wearing courses and involves very low albedos (Badin et al., 2019). Therefore, the increase of the spectral reflectance in the visible part of the solar radiation in this case can be achieved by first acting at binder-scale on the black bitumen. Possible color modifications have been recently studied in China, obtaining commercial chemical products able to partially decolor the bitumen or the emulsions, in order to produce gray/light gray mixes. More drastic solutions can even involve the complete replacement of bitumen with clear or transparent synthetic binders, able to produce lighter pavement layers, exalting the color of the aggregate (Pasetto et al., 2019a). Some proprietary products for this purpose have recently started to be experimented with and commercialized, with functional applications in real-scale trial sections and finally on open-trafficked roads. The use of clear binders in flexible pavements opens new possibilities for the selection of suitable paving aggregates to produce clear mixtures for high-albedo layers, which can also assume high-value aesthetic characteristics useful for better infrastructure integration and preservation of landscapes (Pratico` et al., 2012). Namely, light marbles, clear limestones, porphyry aggregates, etc. possess adequate mechanical characteristics to be used for the manufacturing of

High albedo pavement materials

23

clear/colored pavements (in such cases, the economic aspects should be carefully considered). In other situations, the color of flexible surfaces has been modified by pigments, also in order to enhance the operational ease of paving operations since such elements can be directly employed during the mixture manufacturing. They act as colorants of the pavement blends during the mixing phases and confer different chromaticity to the mortar composing the mixtures (Del Carpio et al., 2016). From a construction point of view, hybrid installations midway between flexible and rigid pavements can be adopted using the technique of whitetopping, i.e., placing concrete surface layers over bituminous pavements. Thin overlays can be designed for resurfacing older pavements in order to rehabilitate them, also obtaining clearer surfacing (characterized by higher albedo values), to optimize the thermal efficiency of structures (Santamouris, 2013). With regard to the selected technology, all options must be designed and achieved taking into account the in-service performance of the constructed pavements (above all, if mixture pigments/modifiers are used, sufficient consideration must be given to the possibility that the modification could affect the stiffness of mixes, thus their structural strength and durability). Specifically, such aspects must be contemplated when flexible pavements are used, since the temperature is a factor that can cause possible permanent deformations (rutting, unevenness), especially during hot summers (Alkaissi, 2018). Recent developments in paving technology are also encouraging the use of varnishes and paintings for road surfaces, due to the cheapness and ease of such solutions (Santamouris and Kolokotsa, 2016; Sen et al., 2019). Various commercial products of different types recently started to be successfully applied for heat-related aspects (historically, painting of pavements was promoted in the last few decades only for aesthetic or regulation and safety purposes). However, the actual challenges in using paints are still finding products durable enough and that avoid deterioration (dirtying, color modifications, etc.) in order to also ensure durable efficiency from a thermal point of view. Not least, the use of surface-applied colors must also be evaluated in relation to the pavement’s final uses: possible safety concerns related to the pavement’s skid characteristics and adhesion must be analyzed when trafficked roads are painted and the standard micro- and macrotexture can be altered. Regarding surface characteristics, some recent studies have identified a certain role of roughness/texture in determining the overall pavement albedo value, and therefore its final thermal performance. This derives from the fact that a smoother surface returns a more specular reflected radiation, whereas a more uniform reflection (distributed in all directions) will be given by a rougher surface. Actually, it should also be considered that roughness of pavements, together with macrotexture, can affect comfort and safety of road users (surface friction, hydroplaning, etc.), as well as the degree of deterioration and the real in-service life of the infrastructures. Surface texture objectively exerts an influence on the albedo value of pavement referred to its serviceability (Federal Highway Administration, 2015). Quantitatively analyzing the possible values assumed by albedo, Table 2.1 reports some indicative ρ values for common paving/urban materials and environments extracted from specific literature (Taha et al., 1992; Hansen, 1993; Salleh et al., 2014).

24

Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

Table 2.1 Indicative albedo values for different paving/urban materials and environments.

2.5

Materials

Albedo ranges

Fresh snow Old snow Ice Water surfaces Common soil Green surfaces Pristine lands Crops Forest Sand surfaces Savannah Urban roofs New bituminous Aged bituminous Bituminous—light aggregates Whitetopping—white concrete New concrete Concrete Colored paving areas Artificial stone paving Residential areas—town Residential areas—suburban Asphalt parking lots Freeway/street networks

0.75–0.95 0.40–0.70 0.30–0.75 0.03–0.10 0.05–0.35 0.18–0.22 0.25–0.30 0.10–0.25 0.10–0.20 0.29–0.43 0.15–0.20 0.08–0.16 0.06–0.08 0.08–0.15 0.30–0.40 0.40–0.60 0.35–0.37 0.30–0.32 0.15–0.45 0.15–0.16 0.12–0.23 0.13–0.24 0.08–0.15 0.29–0.30

Albedo and pavement aging

The evolution of pavement performance is another issue to be considered in relation to albedo values; in fact, during their life, clear surfaces are most likely subject to color changes due to aging, from stripping and progressive exposure of aggregates, as well as dirtying caused by vehicle transit (where allowed) and atmospheric agents. In this regard, it is widely recognized that the surface albedo of bituminous pavements increases over time, because of the progressive discoloring of the primitive black aspect (Chen et al., 2019b). Given this background, the influence of aging on albedo has been analyzed in the literature; extensive and prolonged field measurements are reported, performed to prefigure prediction models for different kinds of pavement materials. Pavement type and surface age, as well as surface and coarse aggregate color and texture were taken into account by Alleman and Heitzman (2019) in order to calibrate a regression analysis (Alleman and Heitzman, 2019). A progressive increase of albedo for bituminous pavements and a decrease for concrete ones were effectively demonstrated: in the case of

High albedo pavement materials

25

bituminous mixtures, a logarithmic law furnished highly reliable ρ predictions using the pavement age as input parameter (Eq. 2.7). For cement concrete rigid pavements, power equations were successfully used, including a correlation with surface age and coarse aggregate color (Eq. 2.8). As an example, in absolute terms, the reduction of albedo due to aging in some concrete pavements can be equal to 30% of the initial value, after several years of service (ρ decreases from 0.33 to 0.22) (Taha et al., 1992). ρ ¼ a  ln ðyÞ + b

(2.7)

ρ ¼ c  yd

(2.8)

where y represents (in both the equations) aging (years); a and b are regression coefficients for bituminous pavements (a varies from 0.0338 to 0.0135, b from 0.1275 to 0.0875, depending on the location studied); c and d are regression coefficients for concrete surfaces (c varies from 0.3603 to 0.2029, d from 0.059 to 0.069, depending on the location studied).

Another example of aging effect on flexible pavements has been described by Pomerantz et al. (2005): after 10 years of life, albedo increased from 0.04–0.05 to about 0.16. However, no significant albedo evolution during the lifetime was clearly identified in the study. The continual effect of aging on pavement albedo was alternatively assessed by Li et al. (2014) and Richard et al. (2015). The former provided a qualitative correlation for concrete and bituminous pavements and albedo during an 8-month period of weather exposure; the latter was able to extend the evaluation period up to 32 months after the pavement construction defining the ρ trend, based on a predictive logarithmic model, limited to flexible pavements. Eq. (2.9) describes the regression formula: ρ ¼ 0:0133  ln ðyÞ + 0:1042

(2.9)

where y represents aging (years). Other research generically reports bituminous pavement albedo values varying from 0.04–0.06 (after construction) to 0.09–0.18 (after aging). With regard to concrete pavements, ρ values decrease by about 0.06 to 0.19 due to weather action, soiling, and traffic abrasion (with a strong sensitivity to cement type and aggregate origin). It is noteworthy that in rigid pavements some slight albedo increases in the initial stage of life of the mixture were detected and attributed to the initial carbonation of concrete (Sen, 2015). As a conclusive summary, Alleman and Heitzman (2019) recently published the final version of a technical report in which the previously presented models and other albedo-aging correlations are accurately described. As an overall recap, they reported a list of the main aging effect mechanisms influencing the albedo values of in-service pavements, also giving an estimation of their severity (both in the case of bituminous concrete—BC—and Portland cement concrete—PCC). Table 2.2 shows an extract of the original publication.

Table 2.2 Summary of perceived aging effect mechanisms related to pavement aging. BC Factors

Aging effect mechanisms

Mastic binder (BC) and mortar paste (PCC) changes

Composition and color variations Supplementary material additions Solar insolation-induced heating (in relation to seasonal and geographic variations) Solar insolation-induced photochemical oxidation (in relation to seasonal and geographic variations) Traffic volume-related rolling-wheel erosion of mastic binder or mortar paste leading to coarse aggregate exposure Chemical composition, glassiness, and color variations Traffic volume-related rolling-wheel erosion of mastic or mortar leading to fine aggregate exposure Chemical composition, glassiness, and color variations Traffic-related rolling-wheel mechanical scrubbing against pavement surfaces Physical contaminant deposition (e.g., tire crumb particulates, soil, oil, antifreeze, grease) Chemical reaction (e.g., acid-base, redox, precipitation, chemical binding, complexation) Biological fouling (attachment of microbial bacteria, algae, fungus, mold, etc.) Surface floating Surface brushing Surface tining Rain-induced wetting and washing

Coarse aggregate changes

Fine aggregate changes

Mechanical scrubbing Surface staining and contamination

Construction changes

Nonwinter weather exposure

Low

Med

PCC High

Low

Med

X

High X X

X X

X X

X

X

X

X

X

X

X

X

X X

X X

X

X

X

X

X

X X X

X X X X

X

From Alleman, J., Heitzman, M., 2019. Quantifying pavement albedo. NCAT Report 19-09. U.S. Department of Transportation, National Center for Asphalt Technology, Auburn University, AL, USA.

High albedo pavement materials

2.6

27

High-albedo benefits and drawbacks

In the most general terms, albedo is closely linked to the lightness of the pavement surface. The production of clear pavements as a valuable strategy to obtain cooler structures has been demonstrated; it involves several secondary benefits, not strictly related to the thermal aspects only. Several studies developed cost analyses concerning the energetic repercussions of this choice on urban activities. As an example, it was estimated that the use of concrete or clear pavements (rather than bituminous ones) not only reduces the heat island effect, but also can lead to a savings of between 0.04% and 0.025% of the total air-conditioning energy need during the summer, for each 10% of albedo increase (Santamouris et al., 2018). Obviously, the complexity and variety of factors involved make such estimations quite difficult, while the effects of such a choice can be diversified and not always beneficial: the reduction of the energy supply can affect the pollution levels, greenhouse gases, ozone-related impacts, climate changes, life quality of the citizens, etc. (Chen et al., 2019a; Tukiran et al., 2016). Many of these facets can be difficult to account for and monetize; it involves accounting for the cost of health and human life. Heat can directly threaten human health and can cause headaches, respiratory problems, heat cramps, strokes, and fainting. Vulnerable populations (children, older adults, and people with health critical issues) are subject to higher risks (even mortality) when, during summer heat waves, road pavement reaches a temperature higher than 40°C (Villanueva et al., 2015). An example of the tangible benefits linked to the use of clear pavement surfaces is the artificial lighting needed in populated areas (roads, pedestrian areas, cycle paths, etc.). Some studies have shown that the higher reflectivity of concrete pavements lowers lighting costs: therefore, thanks to the enhanced night visibility due to the clear color of cement concrete, significant monetary savings can be achieved for the lighting equipment and the energy supply. Some researchers have estimated that the clear color of rigid pavements could lower lighting costs up to 30%, in comparison with other types of pavement (Ferguson, 2005). Successful applications of clear flexible pavements (made of synthetic binders and high-performance aggregates) can be found in road tunnels. In these cases, the construction of high-reflective pavements can optimize the tunnel lighting, ensure a cost-saving energy management, and furnish more visible and safer surfaces (Bocci et al., 2012). High-albedo pavement materials associated with clear surfaces can sometimes negatively affect the urban environment and even impact on human comfort and safety. In this respect, visual fatigue and dazzling effects related to these pavements can have a negative contribution to health, being dangerous for driving, as investigated in recent years (Boyce, 2008). Glare is the condition of vision characterized by a discomfort or a reduction in the ability to see details or objects, caused by an unsuitable distribution of luminance or extreme contrast. Disability glare is defined as a type of glare that impairs the vision: it can be caused by scattering of light inside

28

Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

the eye because of the imperfect transparency of the optical components of the eye and by diffuse light passing through the scleral wall or the iris (Yaglou and Minard, 1957). Discomfort glare is the glare that causes discomfort without necessarily impairing the vision. Veiling glare is defined as the reflection-associated contrast reduction that happens when the visual target falls below the required value. In general, it causes a difficult visibility or readability of view targets (Shimazaki et al., 2001). According to the literature, the human eye can encounter a wide range of luminance, from 0.000001 cd/m2 (very dark night) to 10,000 cd/m2 (sunlit beach) (Nitta et al., 2017). The state of adaptation of the human eye dictates the spectral sensitivity, because at different luminance values, different photoreceptors are activated. Within the luminous environment, the human eye acts adaptively and the visual perception changes accordingly. In urban contexts, it is easy to understand the importance of such topics, above all adopting alternative paving solutions that, in general, are obtained using colored and clear materials. Moreover, when innovative clear and traditional (asphalt) dark pavements lay one beside the other, the contrast of their color can influence the visual perception and determine safety issues (Matzarakis et al., 1999). In order to avoid possible problems connected to such questions, an innovative approach is guiding the production of high-albedo pavements without significant lightening of the surface: this, to obtain pavements able to optimize pavement heat transfer and avoid possible glare issues. For example, the use of innovative paint coating technologies has been proposed to increase the albedo of conventional bituminous pavements, without changing their color, by increasing and reducing the reflectivity for the infrared and visible spectra, respectively (Kinouchi et al., 2004).

2.7

Conclusions and future trends

Regarding albedo, specific testing methods have been specified during the last few decades. The American standardization system first proposed some specifications addressed to the evaluation of pavement ability to reflect solar radiation. In this context, the albedo parameter is supported by other concepts such as solar reflectance (SR) (physical meaning equal to albedo) and solar reflectance index (SRI) (calculated starting from SR and thermal emittance). More specifically, ASTM C1549, “Determination of solar reflectance near ambient temperature using a portable solar reflectometer” and ASTM E1980, “Calculating solar reflectance index of horizontal and low-sloped opaque surfaces” can be cited. Typically, by relating the temperature of a test specimen to reference black-and-white control specimens, SRI can be calculated. From an experimental point of view, standard procedures have also been set up to directly determine the albedo value of substantially horizontal (or low-tilted) surfaces. The albedo estimation at laboratory scale is commonly made through the ASTM E903 standard, “Solar absorptance, reflectance, and transmittance of materials using integrating spheres.” Field measurements are performed using special equipment (called albedometers or double pyranometers) consisting of instruments able to capture the total solar radiant energy incident upon a surface per unit of time and

High albedo pavement materials

29

surface area (ASTM E1918, “Measuring solar reflectance of horizontal and low-sloped surfaces in the field”). In the case of external (real-scale) surveys, albedo can be deeply dependent on weather conditions (cloudiness, wind, scattering, etc.): this is mainly due to the intrinsic complexity of the phenomena that regulate the magnitude of the solar irradiance (time and space variations due to the effect of atmosphere surrounding the earth). Alternative procedures to estimate the albedo value of paving materials have been recently employed by some authors, in order to account for ρ through comparative analysis (Pasetto et al., 2019b). Starting from the knowledge of a known-albedo reference material, the photometric characteristics of the test specimens are in this case compared, in relative terms, with those of the control sample (the method supposes a direct proportionality between the ρ value and the luminance of the surfaces in a standard perceptive color model). Analog considerations of the strict relation between albedo and surface color represented the starting point for other research contributions that aimed to establish models for linking ρ with the chromatic characteristics of the employed materials. As an example, Alleman and Heitzman (2019) performed an extensive survey campaign on bituminous concrete pavements, proposing a predictive model for albedo presented in Eq. (2.10): ρ ¼ ða  AggrCLR + bÞ  ln ðyÞ + ðc  AggrCLR + dÞ

(2.10)

where AggrCLR indicates the color of predominant coarse aggregate in the bituminous mixture (in gray-scale value), y represents the age of the pavement surface (years), and a, b, c, and d are calibration coefficients. A similar approach was utilized to propose an equation suitable for concrete pavements; the most advanced computation was obtained by also accounting for the surface texture and is expressed in Eq. (2.11): ρ ¼ ða  AggrCLR=CTL MTD + bÞ  yðcAggrCLR=CTL MTDdÞ

(2.11)

where AggrCLR indicates the color of predominant coarse aggregate in the bituminous mixture (in gray-scale value), CTL MTD represents the mean texture depth of the surface (mm), y is the age of the pavement surface (years) and a, b, c, and d are the new calibration coefficients valid for rigid pavements. Further details on all the coefficient values can be found in the mentioned publication. At present, the common technical specifications concerning pavement construction do not state minimum requirements for the surface albedo of in-service materials. This can be ascribed to the complexity of the phenomenon, which is affected by many factors (environmental boundary conditions, local weather and climate, daily and seasonal variations, etc.) and is also influenced by the pavement condition and aging. Just a few indicative ρ values can be found in scientific production: for instance, the latest AASHTO pavement design guide and its software implementation, published in 2011 (American Association of State Highway and Transportation Officials, 2011) proposed 0.15 and 0.30 as default albedo values for bituminous and concrete pavements, respectively. However, these still constitute only suggested values that, as stated by the previously described specific literature, are not yet currently displayed by

30

Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

commonly utilized paving materials. In conclusion, considering the important repercussions of pavement albedo values on several aspects of everyday life (health in urban areas, energy savings, road safety, etc.), it is well evident that in the near future more attention should be devoted to this parameter and to its role in the design and maintenance of pavements. To this purpose, new standards and regulations as well as specifications will be required, in order to include the thermal properties of pavements among the fundamental parameters to be used in construction and maintenance of environmentally sustainable infrastructures.

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Goody, R.M., Yung, Y.L., 1989. Atmospheric Radiation: Theoretical Basis. Oxford University Press. Han, R., Jin, X., Glover, C., 2011. Modeling pavement temperature for use in binder oxidation models and pavement performance prediction. J. Mater. Civ. Eng. 23 (4), 351–359. Hansen, F.V., 1993. Albedos. Report ARL-TR-57. U.S. Army Research Laboratory, Maryland. Herb, S., Marasteanu, M., Stefan, H.G., 2006. Simulation and Characterization of Asphalt Pavement Temperatures. Minnesota Department of Transportation, St. Paul, MN. Hulley, M.E., 2012. Metropolitan Sustainability—Understanding and Improving the Urban Environment. Woodhead Publishing. Irwin, J.A., 2007. Astrophysics: Decoding the Cosmos. Wiley. Kinouchi, T., Yoshinaka, T., Fukae, N., Kanda, M., 2004. Development of cool pavement with dark colored high albedo coating. In: 5th Conference on Urban Environment. Vancouver, BC, Canada, August 23, 2004. Kyriakodis, G.E., Santamouris, M., 2018. Using reflective pavements to mitigate urban heat island in warm climates – results from a large scale urban mitigation project. Urban Clim. 24, 326–339. Li, H., 2016. Pavement Materials for Heat Island Mitigation: Design and Management Strategies. Butterworth Heinemann. Li, H., Harvey, J., Kendall, A., 2013. Field measurement of albedo for different land cover materials and effects on thermal performance. Build. Environ. 59, 536–546. Li, H., He, Y., Harvey, J., 2014. Cool pavement strategies for enhancing urban sustainability. In: 7th Geotechnical Graduate Student Society at UC Davis Round Table, Davis, CA, April 18, 2014, United States. Marceau, N.L., Vangeem, M.G., 2008. Solar reflectance values for concrete: intrinsic material properties can minimize the heat island effect. Concr. Int. 2008, 52–58. Marshall, J., Plumb, R.A., 2008. Atmosphere, Ocean and Climate Dynamics: An Introductory Text. Academic Press. Matzarakis, A., Mayer, H., Iziomon, M.G., 1999. Applications of a universal thermal index: physiological equivalent temperature. Int. J. Biometeorol. 43, 76–84. Nitta, J., Shimazaki, Y., Aoki, M., Kamidouzono, K., Sakoi, T., Satsumoto, Y., 2017. Experimental evaluation on cooling effects of water retaining pavement on human. In: 8th Japanese-German Meeting on Urban Climatology, Osaka, Japan, March 25–29, 2017. Nizˇeti
c, S., Papadopoulos, A., 2018. The Role of Exergy in Energy and the Environment. Springer. Pasetto, M., Baliello, A., Giacomello, G., Pasquini, E., 2019a. Aesthetic and mechanical suitability of a clear synthetic resin as a unconventional binder for road pavements. Adv. Mater. Sci. Eng. 2019, 1–15. Pasetto, M., Pasquini, E., Giacomello, G., Baliello, A., 2019b. Innovative pavement surfaces as urban heat islands mitigation strategy: chromatic, thermal and mechanical characterization of clear/colored mixtures. Road Mater. Pavement Des. 20 (1), 533–555. Planck, M., Masius, M., 2012. The Theory of Heat Radiation. Project Gutenberg. Plati, C., 2019. Sustainability factors in pavement materials, design, and preservation strategies: a literature review. Constr. Build. Mater. 211, 539–555. Pomerantz, M., Akbari, H., Levinson, R., Pon, B., 2005. Reflective pavements and the urban heat island effect. In: Developing Research and Implementation Strategies Workshop, Washington, DC, United States, June 27, 2005. Pratico`, F., Giunta, M., Marino, C., Attina`, A., 2012. Pavement albedo and sustainability: an experimental investigation. In: 7th International Conference on Maintenance and

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

Rehabilitation of Pavements and Technological Control, Auckland, New Zealand, August 28–30, 2012. Richard, C., Dore, G., Lemieux, C., Bilodeau, J.P., Haure-Touz
e, J., 2015. Albedo of pavement surfacing materials: in situ measurements. In: Proceedings of the 16th International Conference on Cold Regions Engineering, Salt Lake City, Utah, July 19–22. Sponsored by the Cold Regions Engineering Division of ASCE. Salleh, S.A., Latif, Z.A., Pradhan, B., Wan Mohd, W.M.N., Chan, A., 2014. Functional relation of land surface albedo with climatological variables: a review on remote sensing techniques and recent research developments. Geocarto Int. 29 (2), 147–163. Santamouris, M., 2013. Using cool pavements as a mitigation strategy to fight urban heat island: a review of the actual developments. Renew. Sust. Energy Rev. 26, 224–240. Santamouris, M., Kolokotsa, D., 2016. Urban Climate Mitigation Techniques. Routledge. Santamouris, M., Haddad, S., Saliari, M., Vasilakopoulou, K., Synnefa, A., Paolini, R., Ulpiani, G., Garshasbi, S., Fiorito, F., 2018. On the energy impact of urban heat island in Sydney: climate and energy potential of mitigation technologies. Energy Buildings 166, 154–164. Sen, S., 2015. Impact of Pavements on the Urban Heat Island. (MS thesis)University of Illinois at Urbana-Champaign, IL, USA. Sen, S., Roesler, J., Ruddell, B., Middel, A., 2019. Cool pavement strategies for urban heat island mitigation in suburban Phoenix, Arizona. Sustainability 11, 4452. Shimazaki, Y., Yoshida, A., Suzuki, A., Kawabata, T., Imai, T., Kinoshita, S., 2001. Application of human thermal load into unsteady condition for improving outdoor thermal comfort. Build. Environ. 46 (8), 1716–1724. Solaimanian, M., Kennedy, T.W., 1993. Predicting maximum pavement surface temperature using maximum air temperature and hourly solar radiation. Transp. Res. Rec. 1417, 1–11. Stuart-Fox, D., Newton, E., Clusella-Trullas, S., 2017. Thermal consequences of colour and near-infrared reflectance. Philos. Trans. B 372, 1–8. Taha, H., Sailor, D., Akbari, H., 1992. High-albedo materials for reducing building cooling energy use. Technical report LBL-31721 Lawrence Berkeley Lab, CA, USA. Tukiran, J., Ariffin, J., Ghani, A.N.A., 2016. Comparison on colored coating for asphalt and concrete pavement based on thermal performance and cooling effect. J. Teknol. 78 (5), 63–70. Villanueva, K., Badland, H., Hooper, P., Koohsari, M.J., Mavoa, S., Davern, M., Roberts, R., Goldfeld, S., Giles-Corti, B., 2015. Developing indicators of public open space to promote health and wellbeing in communities. Appl. Geogr. 57, 112–119. Yaglou, C.P., Minard, C.D., 1957. Control of casualties at military training centers. A.M.A. Arch. Ind. Health 16, 302–316.

Performance of thermochromic asphalt

3

Henglong Zhang, Shuai Zhang, Zihao Chen, and Chongzheng Zhu Key Laboratory for Green & Advanced Civil Engineering Materials and Application Technology of Hunan Province, College of Civil Engineering, Hunan University, Changsha, China

3.1

Introduction

Asphalt is a viscoelastic material, with rheological and mechanical properties that is broadly applied in pavement construction (Yan et al., 2013). It is known that the rheological properties of asphalt are determined by temperature and time. Due to its temperature susceptibility, asphalt binder is prone to be soft at high temperatures and stiff at low temperatures, resulting in rutting and cracking pavement distresses, respectively. The black color of asphalt binder brings about substantial solar absorption, leading to high surface temperatures of the pavement and harmful environmental issues. A typical result is the so-called urban heat island, in which there is a temperature difference between a city and the ambient environment surrounding the city, which also increases the energy consumption for cooling during the summer (Zhu and Mai, 2019). In addition, high temperatures also make asphalt binder less sticky, consequently degrading the adherence between mineral aggregates and the binders. Conversely, the ductility is degraded at low temperatures. These two phenomena are reasonable explanations for rutting and shoving distress in summer and cracking distress in winter. In addition, being susceptible to thermal and optical conditions, asphalt binder is prone to oxidation. Cool pavement can be achieved using highly reflective material, to reflect the majority of the solar radiation in daylight hours (Zhu and Mai, 2019). Though cool pavement addresses the high-temperature distress issues, the cooling effect on the pavement tends to exacerbate the cracking problem in cold weather. Thermochromic materials, designed to compensate for the deficits of cooling pavement, are materials that can reversibly change their solar reflectance in response to temperature, giving them a wide range of applications (i.e., military, medical, building materials). When the temperature is above a certain value, called the transition temperature, thermochromic materials reflect more solar energy, while they absorb more when the temperature is under the transition temperature. Hu and Yu (2013) carried out pioneering research on thermochromic asphalt binders. They used a differential scanning calorimeter (DSC) and modulated differential scanning calorimeter (MDSC) to characterize the thermal properties of thermochromic materials. According to comparison measurements, they found that the surface temperature of thermochromic asphalt binder was lower than that of conventional asphalt binder on a typical summer day and the temperature dropped lower in thermochromic asphalt than in regular Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction https://doi.org/10.1016/B978-0-12-820791-8.00003-1 © 2021 Elsevier Ltd. All rights reserved.

34

Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

asphalt in winter. They also found thermochromic asphalt binders presented higher specific heat capacity and lower thermal conductivity and thermal diffusivity than conventional asphalt binders (Hu et al., 2014). Innovative thermochromic coatings were designed to modulate the surface temperature of asphalt pavement and the results showed that the surface temperature of the thermochromic coating was lower and higher than that of conventional asphalt coating under hot and cold weather conditions, respectively, which helped the asphalt mixture to improve its resistance to high temperature-related performance degradation and delay ice formation on the surface of the road. Although thermochromic asphalt binders have a positive effect on keeping the surface temperature of pavement within a relatively reasonable range to mitigate the high- and low-temperature pavement distresses, it is also necessary to investigate the aging behaviors of these types of asphalt binders. There are two types of asphalt aging: thermal oxidation and photooxidation. Short-term thermal oxidation aging occurs when asphalt is exposed to heat and oxygen in the process of asphalt mixture production and paving, which is primarily due to the oxidation and loss of volatile components at high temperature. Long-term thermal oxidation aging continues during the pavement service life, as a result of continuous oxidation. In terms of ultraviolet irradiation aging, the structure of the asphalt molecule changes with the absorption of ultraviolet energy, which induces a cleavage bond and produces the oxidation components, and finally causes an increase in the stiffness and brittleness of the asphalt binder. By simulating short-term and long-term thermal aging of asphalt with the methods of thin film oven test (TFOT), pressure aging vessel (PAV), and ultraviolet radiation (UV), Zhang et al. proposed various chemical and rheological aging indexes to evaluate the antiaging properties of thermochromic asphalt binders before and after implementation of the three aging methods. The results showed that the introduction of thermochromic powders could improve thermal stability, low-temperature cracking performance, and antiaging resistance of asphalt binders. This chapter first introduces the properties of three-component organic reversible thermochromic microcapsules, including their classification, merits, components, thermochromic mechanism, and thermal and optical properties. Then, the optical, thermal, physical, rheological, and antiaging properties of thermochromic asphalt binders are discussed. Finally, some recommendations for future research and applications are proposed.

3.2

Three-component organic reversible thermochromic microcapsules

3.2.1 Classification and merits of reversible thermochromic materials A thermochromic material is a material that changes solar reflectance in response to temperature. The temperature at which the solar reflectance changes is called the transition temperature. Thermochromic materials are divided into two categories

Performance of thermochromic asphalt

35

based on reversibility. Reversible thermochromic materials can be classified into liquid crystals, inorganic materials, and organic materials. Although the liquid crystal materials have wide ranges of transition temperature, they are especially sensitive to chemical substances, which can easily damage their sensibility. Their short storage life, poor durability, and high cost all limit their applications to a large extent. As for inorganic reversible thermochromic materials, though they have good optical and thermal resistance and can be manufactured conveniently, their color variation is limited. More importantly, they are highly toxic and corrosive. Compared with inorganic and liquid crystal reversible thermochromic materials, organic reversible thermochromic materials have higher temperature selectivity, brighter colors, and lower cost, which undoubtedly makes them the most attractive materials at present.

3.2.2 Components and thermochromic mechanism Although organic reversible thermochromic materials have the best comprehensive performance among the various thermochromic materials, they still have some shortcomings, such as lack of thermal and chemical resistance. So the organic reversible thermochromic materials are often encapsulated to improve their physical and chemical stability. Encapsulation is a process in which solid particles, liquid droplets, or gases are surrounded with a continuous and thin layer formed on the outside. The inner material is called the core material and the outer material is called the shell material. At present, three-component organic reversible thermochromic microcapsule materials have been extensively applied in many fields, such as the military and medical, construction, and painting industries. Regarding their applications in the asphalt road construction field, three-component organic reversible thermochromic microcapsule materials are under investigation as asphalt binder modifiers. The core material of three-component organic reversible thermochromic microcapsules is composed of color developer, color former, and solvent. The color former is usually a cyclic ester, which determines the color of the final product in its colored state. The color developer is usually a weak acid, which imparts the reversible color change to the thermochromic material and is responsible for the color intensity of the final product. The solvent is usually an alcohol or an ester whose melting point controls the transition temperature at which the color change occurs. The thermochromic mechanism is the electron transfer and acquisition caused by varying temperature between the color former and the color developer. In this case, the structure of the color former transforms from quinone ring to lactone ring when the ambient temperature is above the transition temperature, while it returns to its origin structure when the temperature decreases to below the transition temperature. From a macroperspective, it is shown that the thermochromic microcapsule changes its appearance color in accordance with temperature. Moreover, Fig. 3.1 exemplifies the FTIR spectra of three kinds of thermochromic powders adopted by Zhang et al., who investigated their effects on aging behaviors of asphalt (Zhang et al., 2017). As presented in Fig. 3.1, the existence of absorption bands centered around 3355 cm
1 and 1509 cm
1 correspond, respectively, to hydroxyl (dOH) and benzene skeleton vibration derived from bisphenol A (BPA), a weak acid

36

Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

Fig. 3.1 The FTIR spectra of three types of thermochromic powders (Du et al., 2019).

that functions as the color developer. The characteristic peaks of methyl stearate, an ester used as a solvent here, lie at 2925, 2850, and 1740 cm
1, indicating methyl (dCH3), methylene (dCH2), and carbonyl (C]O) group stretching vibrations, separately. The peaks centered around 1552 and 813 cm
1 are related to thiotriazinone stretching and melamine skeleton bending vibrations, which are characteristics of melamine-formaldehyde resin, a common shell material of microcapsules.

3.2.3 Thermal properties In order to observe the temperatures of thermochromic coatings and compare them with the temperature of corresponding cool and common coatings, Karlessi et al. (2009) calculated the mean daily and mean maximum daily (6:00–20:00) surface temperatures of samples from measured data. The results for each sample are shown in Table 3.1. It can be concluded from a comparison of the groups of thermochromic cool and common coatings with TiO2 that mean daily surface temperature and maximum daily temperature of thermochromic coatings were lower. In addition, when compared with cool and common coatings without TiO2, the thermochromic coatings demonstrated lower mean daily temperature.

Performance of thermochromic asphalt

37

Table 3.1 Mean daily and mean maximum daily surface temperature (°C) for thermochromic, cool, and common coatings in August and September. Thermochromic With TiO2

Without TiO2

Cool With TiO2

Without TiO2

Common Light

Dark

44.6 36.4 42.3

48.5

Mean daily surface temperature (°C) in August Green Yellow Brown Black Blue Gray

33.2 32.2 31 37.6 33.1 34.1

36 32.5 38.4 37.4 35.5

40.9 34.4 40.2 44.6 38.7 40.4

43.8 35.3 45.2 42.4 44.4

39 45.1

47.5 43.9

Mean maximum daily surface temperature (°C) in August Green Yellow Brown Black Blue Gray

44.2 42.5 40.2 50.3 42.7 44.3

49.5 43.8 51.5 49.6 46.7

57 44 54.9 63.8 52.3 56.1

61.1 46.7 64.4 59.2 63

63.6 49.3 59.2 52.8 64.3

69.8

68 62.6

Mean nocturnal surface temperature (°C) in August Green Yellow Brown Black Blue Gray

18 18.5 18.6 21.3 20 21

17.8 18 21.1 20.6 20.5

20.2 17.6 21 20.4 20.7 20.2

21.6 20.5 20.6 20.5 20.8

20.2 20.2 20.6 20.9 20.3

20.7

21.1 20.2

Mean daily surface temperature (°C) in September Green Yellow Brown Black Blue Gray

26.1 24.9 23.8 30.9 26.1 26.6

28.6 26.2 31.6 31.4 29

34 28.1 34.2 38.8 32.2 32.9

37.9 28.6 39.2 35.7 37.4

38.1 29.8 35.9 32.8 37.9

42.3

41 37.7

Mean maximum daily surface temperature (°C) in September Green Yellow Brown Black Blue Gray

38.7 36.4 34.3 45.6 37.2 37.9

44 39.7 46.6 45.9 42.3

52.7 43 51.7 61.3 48.1 51.8

59.3 41.8 61.9 55.5 58.8

60.6 44.2 55.8 49 59.3

67

64.7 59.3

38

Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

Hu et al. (2014) utilized DSC to measure the phase transition temperature and latent heat of thermochromic materials. The phase transition temperatures are determined by the endothermic and exothermic peaks and the latent heat is evaluated from the integrated area of the peak profile. It has been proven that the transition temperature of three-component organic reversible thermochromic microcapsules hinges on the phase transition temperature of the solvent in the thermochromic microcapsules. Commonly, when the temperature increases and the solvent melts, thermochromic microcapsules will fade to be colorless. Conversely, when the temperature decreases and the solvent freezes, the thermochromic microcapsules will change to their original color. The results of melting and freezing temperatures of thermochromic materials are shown in Table 3.2 and Fig. 3.2. It is noticeable that the phase transition Table 3.2 DSC results of thermochromic materials. Melting

Sample Black thermochromic Blue thermochromic Red thermochromic

Freezing

Density (kg/m3)

Phase change temperature (°C)

Latent heat (J/kg)

Phase change temperature (°C)

Latent heat (J/kg)

900

27.28

33,660

20.38

35,680

900

34.41

25,040

23.31

29,370

900

31.76

23,270

22.21

38,510

Fig. 3.2 DSC curves of raw thermochromic powders.

Performance of thermochromic asphalt

39

temperature is not accurate at 31°C, which could be attributed to the fact that the phase transition temperature was dominated by the whole thermochromic microcapsule, rather than simply by the solvent.

3.2.4 Optical properties Karlessi et al. (2009) measured the spectral reflectance of thermochromic coatings for colored and colorless states. Reflectance curves in the visible range of black, gray, and yellow thermochromic coatings with TiO2 in their colored and colorless states indicated that, at high temperature, coatings did not become completely white. All coatings presented very strong absorption in the near-ultraviolet range of the spectrum and all thermochromic coatings were highly reflective in the near-infrared. The comparison between reflectance curves of thermochromic coatings in their colored and colorless states indicated that thermochromic coatings can absorb solar energy at lower temperatures and reduce the absorption at higher temperatures. By measuring the spectral reflectance of three kinds of thermochromic powders at 25°C and 35°C, Hu et al. also found that thermochromic powder showed high solar reflection, especially in the infrared range. In addition, thermochromic powders at 35°C presented higher reflectance in the visible and near-infrared range than those at 25°C，which could be explained by the color changes caused by the rising temperature. Using spectrophotometry analysis, it can be further confirmed that the optical properties of three-component organic reversible thermochromic microcapsules change in accordance with temperature fluctuation.

3.3

Thermochromic asphalt binders

3.3.1 The performance of thermochromic asphalt binders 3.3.1.1 Optical properties In principle, the introduction of thermochromic materials would enable thermochromic asphalt binders to exhibit a different reaction to solar radiation, as compared with conventional asphalt binders. Solar energy consists of a spectrum of wavelengths, including ultraviolet light, visible light, and infrared light. Below the transition temperature of thermochromic powder, where the powder is less infrared reflective, the binder should reflect less solar energy; above the transition temperature, where the powder is more reflective to infrared, the thermochromic asphalt binder should reflect more energy, as is shown in Fig. 3.3 (Hu et al., 2015). Hu et al. (2015) measured the spectral reflectance of various asphalt binders over the wavelength range of 300–1800 nm at 25°C and 35°C. The results are shown in Fig. 3.4. It can be seen that thermochromic asphalt binders are more reflective in the near-infrared range than conventional asphalt binders and they become more reflective with increases in temperature. As a result, the reduction of the spectra reflectance could reduce the surface temperature of thermochromic binders up to 6–8°C, which would mitigate the rutting and heat-island effects. In addition, when aggregates

40

Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

Fig. 3.3 Schematic principle of thermochromic asphalt binder: (A) smaller infrared reflectivity under the transition temperature of thermochromic powders; (B) higher infrared reflectance above the transition temperature.

with a white color are incorporated, the reflectance value of the asphalt mixture could be higher than that of the asphalt binder. Hu and Yu (2016b) further measured the reflectance spectra of asphalt binder mixtures at 25°C and 35°C. As shown in Fig. 3.5, thermochromic asphalt binder mixtures are more reflective than conventional asphalt binder mixtures, particularly in the nearinfrared range. This conclusion corresponds with the former one. However, the influence of thermochromic powders is less obvious in the visible or near-ultraviolet ranges, which could be attributed to the lower depth of penetration of solar rays at that spectral range. Additionally, it could be concluded that the values of the spectral reflectance of asphalt binder mixtures increase as the temperature increases.

3.3.1.2 Thermal properties Thermal properties include specific heat capacity (CP), thermal conductivity (λ), and thermal diffusivity (α). These parameters could be utilized to describe the distribution and variation of asphalt pavement temperature. The diffusivity is the ratio of thermal conductivity to the volumetric heat capacity. Consequently, the third property can be produced by measuring the other two properties using the following formula: α¼

CP λ

Hu et al. (2014) measured the phase transition with differential scanning calorimetry (DSC) and the specific heat capacity (CP), thermal conductivity (λ), and thermal diffusivity of various asphalt binders at different temperatures with modulated differential scanning calorimetry (MDSC). The principle of measuring the thermal conductivity and thermal diffusivity is based on the difference between the heat capacity measured in two different situations, equilibrium and nonequilibrium thermal conditions (Hu et al., 2014). The results are illustrated in Table 3.3 and Fig. 3.6. Hu deemed that the difference between theoretical and experimental values of melting and freezing latent heat might be due to the microstructure and interaction of additives with polymer matrix, which cause irregular heat dissipation patterns

Performance of thermochromic asphalt

41

Fig. 3.4 Spectral reflectance of thermochromic asphalt binders at (A) 25°C and (B) 35°C.

through the samples. Hu also found that the specific heat capacities of thermochromic asphalt binders are higher than those of pure asphalt binders, which could be explained by the effect of the large specific heat capacity of thermochromic powders. Higher specific heat capacity means that the pavement would store more solar energy and would reduce the fluctuation of pavement temperatures. Meanwhile, as is shown in

42

Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

Fig. 3.5 Reflectance spectra of asphalt binder mixtures at (A) 25°C, (B) 35°C.

Figs. 3.7 and 3.8, the specific heat capacity of the samples increased with the temperature, except in the range of 20°C and 30°C, which could be attributed to the phase transition of pure asphalt. Thermal conductivity, in the temperature range from
20°C to 50°C, was much lower in thermochromic asphalt binders than in the pure asphalt binder. This phenomenon could be explained by the low thermal conductivity of thermochromic powders and thermal resistance between thermochromic powders and asphalt binder. Hu also did research on the thermal diffusivity of thermochromic

Performance of thermochromic asphalt

43

Table 3.3 DSC results of the sample. Melting

Sample Black asphalt binder Blue asphalt binder Red asphalt binder

Freezing

Density (kg/m3)

Phase change temperature (°C)

Latent heat (J/kg)

Phase change temperature (°C)

Latent heat (J/kg)

954.2  24.3

27.47

3590

19.83

3120

940.4  11.1

34.80

2980

24.14

2970

953.0  22.3

32.35

1740

23.15

1860

Fig. 3.6 DSC curves of thermochromic asphalt binders.

asphalt binders and found that the thermal diffusivity values of thermochromic asphalt binders are lower than that of pure asphalt binder, implying that thermochromic asphalt binders require a longer time to be heated or cooled than conventional pure asphalt binder. Also, the figure showed that the thermal diffusivity of thermochro1mic asphalt binders remained almost constant (within 5%) from
20°C to 20°C, but that of pure asphalt binder decreased with the temperature. Hu and Yu (2013) further evaluated the effect of thermochromic materials on the thermal responses of pavement. He simulated the surface temperatures of

44

Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

Fig. 3.7 Influence of temperature on specific heat capacity of various asphalt binders.

Fig. 3.8 Influence of temperature on thermal diffusivity of various asphalt binders.

thermochromic asphalt binder samples under typical summer conditions. The difference between surface temperature of thermochromic asphalt binders and pure asphalt binder (Tthermochromic
Tpure) is seen in Fig. 3.9. As shown in Fig. 3.9, the surface temperature of thermochromic asphalt binders is lower than in traditional pure asphalt,

Performance of thermochromic asphalt

45

Fig. 3.9 Difference between surface temperature of thermochromic asphalt binders and pure asphalt binder.

and the maximum reduction in surface temperature is 6.6°C. Hu deemed that the effectiveness of surface temperature reduction was attributed to the increases in the solar reflectance and thermal properties (latent heat) of asphalt binder, due to the introduction of thermochromic materials. Meanwhile, He also compared the performance of thermochromic asphalt binder with regular asphalt. The results are shown in Fig. 3.10. The figure first demonstrates a lower rate of temperature decrease in thermochromic asphalt binders compared with pure asphalt binder, and then shows the higher temperature in thermochromic asphalt binder during a cold winter, indicating that thermochromic asphalt possesses the potential of delaying the formation of ice and mitigating low-temperature cracking.

3.3.1.3 Physical properties The indexes to evaluate the physical properties of asphalt binders include penetration, softening point, and ductility. The softening point can reflect high-temperature stability to some extent, penetration can indicate the consistency, and viscosity can evaluate the flow resistance of asphalt binders. Zhang et al. (2018) measured the physical properties of thermochromic asphalt binders with different black powder contents and a blank sample. The results are shown in Fig. 3.11. It can be seen from the figure that the increment of black thermochromic powder content led to a gradual increasing of softening point and viscosity and a corresponding decreasing of penetration and ductility, indicating that thermochromic powders could improve the thermal stability of asphalt binder. This improvement can be explained by the interference effect between microparticles and asphalt molecules.

46

Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

15 Black asphalt binder Blue asphalt binder Red asphalt binder Pure asphalt binder

10

Surface temperature (°C)

5 0 –5 –10 –15 –20 –25 0

2

4

6

8

10

12

14

16

Time (h)

Fig. 3.10 Surface temperature of asphalt binder samples during winter.

3.3.1.4 Rheological properties It is known that asphalt is an adhesive material and susceptible to temperature. When the temperature is low, the asphalt binder acts like an elastic solid, while when the temperature rises (above the softening point), it acts as a viscous liquid. However, the inherent black color of asphalt causes it to absorb significant amounts of solar energy, which contributes to pavement rutting and shoving distress in hot summers. In addition, the rutting issue can be exacerbated by increasing axle loads, further influencing the driving safety of vehicle occupants. Meanwhile, without sufficient ductility, asphalt pavement is prone to cracking distress in cold winter temperatures. Thus it is necessary to explore the rheological properties of asphalt binders. Zhang et al. (2018) measured complex shear modulus (G∗) and phase angle (δ) of thermochromic asphalt binders and a blank sample with a dynamic shear rheometer (DSR). These two values indicate resistance to deformation and viscoelastic balance of behavior separately. Also, in order to evaluate the deformability and stress relaxation capability of asphalt binders at low temperatures, the stiffness (S) and creep rate (m-value) was acquired using a bending beam rheometer (BBR) on TFOT and PAV residues. Smaller S and larger m-values imply the ability of the binder to resist cracking at low temperature. The results are shown in Figs. 3.12 and 3.13, respectively. It can be seen that G∗ of the thermochromic binders is significantly larger than that of the blank sample and the incrementing of thermochromic powders leads to increasing G∗, though the difference among thermochromic asphalt binders with various content of black thermochromic powders is small. Conversely, the value of phase

Fig. 3.11 Physical properties of different asphalt binders: (A) softening point, (B) penetration at 25°C, (C) ductility at 10°C, (D) viscosity at 135°C.

48

Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

100,000

2500 2000 1500

Complex modulus (Pa)

1000

10,000

500 65.0

1000

67.5

70.0

72.5

75.0

Blank sample 2% BTP binder 4% BTP binder 6% BTP binder 8% BTP binder

100 40

50

(A)

Phase angle (degree)

91 90 89 88 87 86 85 84 83 82 81 80 79 78 77 76

70

80

90

Blank sample 2% BTP binder 4% BTP binder 6% BTP binder 8% BTP binder

88 87 86 85 84 60.0

40

(B)

60

Temperature (°C)

50

60

62.5

70

65.0

67.5

80

70.0

90

Temperature (°C)

Fig. 3.12 Complex modulus and phase angle of different asphalt binders between 40°C and 90°C: (A) complex modulus, (B) phase angle.

angle decreased with growth in powder content. Taking complex shear modulus and phase angle into consideration, it could be concluded that thermochromic materials could intensify the resistance of asphalt binders to deformation at high temperature. As for the stiffness and creep rate of asphalt binders, the opposite trend between the changes in these two values indicates that deformability and the stress relaxation capability of binders deteriorates as temperature decreases. Additionally, the S values of thermochromic asphalt binders are much smaller than that of the blank sample throughout the whole testing temperature range, while m-values are larger than that of the blank sample, implying that thermochromic powder can improve the

400

–6°C –12°C –18°C –24°C

A. Blank sample B. 2% BTP binder C. 4% BTP binder D. 6% BTP binder E. 8% BTP binder

0.50 0.45 0.40 M-value

Stiffness (MPa)

300

200

A. Blank sample B. 2% BTP binder C. 4% BTP binder D. 6% BTP binder E. 8% BTP binder

–6°C –12°C –18°C –24°C

0.35 0.30 0.25

100

0.20 0

(A)

A

B

C

D

E

0.0

(B)

A

B

C

D

E

Fig. 3.13 Creep stiffness (S) and creep rate (m-value) of different asphalt binders at different temperatures: (A) creep stiffness, and (B) creep value.

50

Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

low-temperature and enduring long-term aging properties of asphalt binders. However, it should be noted that the thermochromic material can only improve the lowtemperature properties after long-term aging methods, demonstrating the effect of thermochromic powders on the improved resistance of asphalt binders to long-term aging. Zhang further utilized performance grade to evaluate the performance of thermochromic asphalt binders. The results are shown in Table 3.4, indicating that the introduction of thermochromic powders does not degrade the performance grade but rather enhances it. Though Zhang et al. evaluated the influence of thermochromic materials on the rheological performance of asphalt binders, they utilized simple base asphalt binder as the experimental subject. However, currently modified asphalt has been widely applied on pavement, especially SBS modified asphalt binder, so it is valuable to evaluate the rheological performance of SBS asphalt binder. Du and Zhang did further research on the resistance of SBS asphalt binder influenced by thermochromic materials to rutting, fatigue, and cracking distresses with continuous performance grade (PG) grading, multiple stress creep recovery (MSCR), linear amplitude sweep (LAS), shear stress relaxation (SSR) and temperature sweep (TS) tests, conducted using a dynamic shear rheometer (DSR) and BBR. The results showed that thermochromic microcapsules improved the fatigue resistance of SBS modified asphalt binder. However, the impact on the rutting and cracking performance was not so apparent.

Table 3.4 The continuous PG results of various asphalt binders. Sample Critical temperature (G∗/sin δ  1.0 kPa for original binder, °C) Critical temperature (G∗/sin δ  2.2 kPa for TFOT residue, °C) High continuous grading temperature Intermediate continuous grading temperature (G∗sin δ  5.0 MPa for TFOT + PAV residue, °C) Low continuous grading temperature (S  300 MPa and m-1value  0.3) Continuous grade

Blank sample

2% BTP binder

4% BTP binder

6% BTP binder

8% BTP binder

67.6

69.1

69.6

70.1

70.1

69.6

69.6

70.1

71.1

68.6

67.6

69.1

69.6

70.1

68.6

20.6

19.2

19.5

20.5

20.8


25.0


26.1


27.8


29.2


28.0

67.6–25.0 (20.6)

69.1–26.1 (19.2)

69.6–27.8 (19.5)

70.1–29.2 (20.5)

68.6–28.0 (20.8)

Performance of thermochromic asphalt

51

Fig. 3.14 RV test results for various asphalt binders.

Hu investigated the effect of thermochromic powder type and content on the rotational viscosity of asphalt binders at 135°C. The results are shown in Fig. 3.14. It can be seen that the thermochromic binder containing red thermochromic powders exhibited higher viscosity than the original binder, while the binder with blue and black powders, however, presents lower viscosity than the original asphalt binder. It is also shown that the workability and pumping potentials of the black and blue binders are better than those of the red binder. The pumping potentials are the ability of binders to be pumped out of the equipment during pavement construction. Above all, we can conclude that the type of thermochromic powder plays a significant role in the rotational viscosity of asphalt binders. Furthermore, Hu also evaluated the rutting resistance of thermochromic asphalt binders through dynamic shear rheometer (DSR) tests. The results showed that the addition of thermochromic powder increased G∗, indicating that the introduction of thermochromic powders could improve the rutting resistance of asphalt binders.

3.3.1.5 Antiaging properties Asphalt is an organic mixture composed of hydrocarbons and their derivatives, which makes it susceptible to being aged by heat, oxygen, solar radiation, and water. Aging processes take place throughout the life of the road and influence the physical and mechanical properties of the asphalt binder. After being seriously aged, asphalt becomes extremely stiff and brittle, leading to the low-temperature and fatigue cracking of asphalt pavement and reducing its lifespan (Qin et al., 2014). Asphalt aging may be influenced by several factors, including characteristics and content of the asphalt

52

Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

binder and natural and external conditions (i.e., temperature, light, and time). The aging processes can be divided into short-term aging and long-term aging. Generally, the thin film oven test (TFOT) and rolling thin film oven test (RTFOT) are utilized to simulate short-term hardening conditions that occur in hot-mixing facilities, while the pressure aging vessel (PAV) test was developed to evaluate in-service oxidative aging of asphalt by exposing the asphalt to increasing temperature in a pressurized atmosphere. Meanwhile, it has been proven that ultraviolet radiation causes the upper layer of pavement to become stiffer and more brittle, and this distress is then elevated by subsequent heavy traffic load and varied temperatures. Zhang et al. proposed aging indices that were obtained from the measurement of rheological properties of binders before and after aging, including the complex modulus aging index (CAI) and the phase angle index (PAI). Larger values of CAI and smaller values of PAI mean more serious aging of the asphalt. They can be defined according to the following equations: CAI ¼

Aged complex modulus Unaged complex modulus

PAI ¼

Aged phase angle Unaged phase angle

Meanwhile, oxidation also leads to the creation of highly polar and strongly interacting oxygen functional groups, like the carbonyl group (C]O) (Mouillet et al., 2008; Mill, 1996; Petersen, 1998). Consequently, the aging degree of an asphalt binder can be evaluated by tracking the content of the carbonyl group. So the carbonyl index (CI) and the sulfoxide index (SI) were utilized to quantify the amount of carbonyl group. Their detailed definitions are as follows (Mouillet et al., 2008): AC¼O CI ¼ X A AS¼O SI ¼ X A where AC¼O is the integral area of carbonyl group centered around 1700 cm
1 and AS¼O is the integral area of the 1030 cm
1. The Psulfoxide group
1centered around
1 sum of the area represents: A ¼ A1700 cm + A1600 cm + A1460 cm
1 +
1
1 A1376 cm + A1030 cm + A864 cm
1 + A814 cm
1 + A743 cm
1 + A724 cm
1. Zhang treated various asphalt binders using three aging methods. The rheological aging indices are present in Fig. 3.15 (Zhang et al., 2018). It can be concluded that, after TFOT, thermochromic asphalt binders showed better resistance to short-term oxidation than the blank sample based on smaller CAI and larger PAI within the whole range of temperature. In addition, the sequence in which photooxidation and longterm oxidation resistance of binders are ranked is 4%, 6%, 2%, and 8% BTP binder and blank sample. Zhang further found that the peak area of the carbonyl group

Performance of thermochromic asphalt 3.5

A. Blank sample B. 2% BTP binder C. 4% BTP binder D. 6% BTP binder E. 8% BTP binder

A. Blank sample B. 2% BTP binder C. 4% BTP binder D. 6% BTP binder E. 8% BTP binder

1.02 1.00

PAI after TFOT

3.0

CAI after TFOT

53

2.5

2.0

0.98 0.96 0.94

1.5

0.92 1.0 40

50

70

60

(A)

80

90

40

A. Blank sample B. 2% BTP binder C. 4% BTP binder D. 6% BTP binder E. 8% BTP binder

4.2

70

60

80

90

Temperature (°C)

1.05 A. Blank sample B. 2% BTP binder C. 4% BTP binder D. 6% BTP binder E. 8% BTP binder

1.00 3.5

PAI after UV

CAI after UV

50

(B)

Temperature (°C)

2.8

0.95

0.90

2.1

(C)

0.85 30

40

50

Temperature (°C)

15

(D)

30

40

50

60

70

Temperature (°C)

1.05

A. Blank sample B. 2% BTP binder C. 4% BTP binder D. 6% BTP binder E. 8% BTP binder

A. Blank sample B. 2% BTP binder C. 4% BTP binder D. 6% BTP binder E. 8% BTP binder

1.00

PAI after PAV

12

CAI after PAV

70

60

9

0.95 0.90 0.85

6 0.80 0.75

3 40

(E)

45

50

55

60

Temperature (°C)

65

70

75

40

(F)

45

50

55

60

65

70

75

Temperature (°C)

Fig. 3.15 Rheological aging indices of various asphalt binders after three aging methods: (A) CAI after TFOT, (B) PAI after TFOT, (C) CAI after UV, (D) PAI after UV, (E) CAI after PAV, and (F) PAI after PAV.

gradually increased as aging deteriorated. As illustrated in Table 3.5, ΔCI of the BTP binder is smaller than that of the blank sample after three aging methods, indicating the superior antiaging properties of thermochromic asphalt. Zhang (Du et al., 2019) further investigated the aging resistance of SBS modified asphalt treated with thermochromic microcapsules. Fig. 3.16 shows the CAI values

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

Table 3.5 Carbonyl index (CI) of blank sample and BTP binder after different aging methods. Blank sample

4% BTP binder

Sample

CI

ΔCI

CI

ΔCI

Un-aged TFOT aging UV aging PAV aging

0.0120 0.0167 0.0201 0.0343

– 0.0047 0.0081 0.0223

0.0074 0.0083 0.0136 0.0261

– 0.0009 0.0062 0.0187

4

CAI

3

2

1 70#–TFOT 70#–UV 70#–PAV

70#RT–TFOT 70#RT–UV 70#RT–PAV

0 30

40

50

60

70

80

90

Temperature (°C)

(A) 4 SMA–TFOT SMA–UV SMA–PAV

SMART–TFOT SMART–UV SMART–PAV

CAI

3

2

1

0 30

(B)

40

50

60

70

80

90

Temperature (°C)

Fig. 3.16 Effect of red thermochromic microcapsule on aging susceptibility: (A) base asphalt, (B) SBS modified asphalt.

Performance of thermochromic asphalt

55

after TFOT, UV, and PAV aging. CAI is the ratio of aged complex modulus to unaged complex modulus. The larger CAI indicates more serious aging susceptibility of asphalt binder. Compared with 70#, CAI values of 70#RT were much smaller, indicating that red thermochromic microcapsules were able to improve the resistance of base asphalt binder against both thermal and optical oxidation of asphalt. Furthermore, it can be noted that the improving effect of red thermochromic microcapsules on aging resistance of base asphalt binder was the most obvious after PAV aging, due to the outflowing of methyl stearate from damaged microcapsules. Additionally, the antiaging performance of thermochromic asphalt binders was investigated under a natural environment (Chen et al., 2019). According to the growth rates and amplitudes of the carbonyl and sulfoxide index, in summary, the thermochromic asphalt binder possessed superior weathering aging resistance. As for the aging mechanism, Zhang et al. contended that the asphalt matrix stiffened with the increment of aging severity due to the asphalt colloid structure transforming from sol to gel under the influence of volatilization of light components and the association between resins and asphaltenes. However, the reflectivity of asphalt binders with red thermochromic microcapsules added was higher than that of pure asphalt binder within the near-infrared range at high temperature, which indicates that the thermochromic microcapsules had the potential to reduce asphalt binder solar absorption and consequently inhibit stiffening of the asphalt matrix due to thermal aging. Furthermore, the core material would outflow from damaged shells due to the aging influence. Methyl stearate, which constitutes the largest part of the core material and which has low density and low melting point, acts like a fluid oil and can compensate for light components being volatized and improve the solubility of the micelle. Additionally, methyl stearate is also a phase change material with relatively low phase temperature. When outflowing from microcapsules and melted with asphalt matrix, methyl stearate can absorb a great deal of heat from the asphalt matrix and keep its temperature nearly constant. In this way, the thermal influence on the asphalt matrix would also be lessened.

3.3.2 The temperature adjustment of thermochromic asphalt pavement Conventional asphalt pavement absorbs significant amounts of solar energy due to its black color, which contributes to temperatures as high as 48–67°C and further leads to serious pavement distresses (i.e., rutting, shoving, and aging). Consequently, cool pavement was developed to relieve this issue. The material used in cool pavement has high reflectivity, which reflects more solar energy when the ambient temperature is high. However, there are still some shortcomings of cool pavement. On the one hand, the reduced temperature exacerbates cracking distress in cold weather. On the other hand, Sen et al. (2019) has proven that, while new asphalt surfaces increased the temperature of the surrounding 2 m of air by 0.5°C, replacing aged asphalt with typical concrete with higher albedo did not significantly decrease the temperature. Meanwhile, research has shown that cool pavements may also increase the thermal

56

Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

loading of a pedestrian on the street or sidewalk (Qin, 2015). Lynn et al. found that the maximum net flux of this pedestrian would increase about 80 W/m2 when the pavement albedo rose from 0.15 to 0.50 (Lynn et al., 2009). Thus an innovative material, namely thermochromic material, has been developed and utilized in asphalt binders. As mentioned earlier, thermochromic powders are materials that not only absorb energy when the temperature decreases but also reflect more energy when the temperature rises. This dynamic mechanism enables the temperature of asphalt pavement to change within a certain range. Hu and Yu (2016a) did their research on the characterization of thermochromic asphalt coatings by measuring the surface temperature of asphalt coatings treated with different thermochromic powders, or with none, during a hot summer day and a cold winter day. The results are shown in Figs. 3.17 and 3.18. They found that, under hot conditions, the surface temperature of thermochromic asphalt coatings was lower than that of conventional asphalt coating, with a maximum decrease as high as 6.6°C. Hu argued that the effectiveness of surface temperature reduction was attributed to increases in the solar reflectance of asphalt coatings, due to the introduction of the thermochromic powders. This helps to improve the resistance to high temperaturerelated performance degradation (such as rutting and fatigue). Furthermore, experimental results during cold weather indicated that the surface temperature of asphalt concrete covered with thermochromic asphalt coating is generally 1°C higher than that of conventional asphalt coating. Hu explained that solar reflectance could be neglected, due to the existence of snow. Therefore, the high heat capacity and low

Fig. 3.17 Difference between the surface temperature of thermochromic asphalt coatings and conventional pure asphalt coating on a hot summer day.

Performance of thermochromic asphalt

57

Surface temperature difference (°C)

1.5 Black asphalt binder Blue asphalt binder Red asphalt binder 1.0

0.5

0.0

–0.5

–1.0 8:30

10:00

11:30

13:00

14:30

16:00

17:30

19:00

20:30

22:00

23:30

Time

Fig. 3.18 Difference between the surface temperature of thermochromic asphalt coatings and conventional pure asphalt coating on a cold snowy day.

thermal conductibility of thermochromic asphalt coatings resulted in an increase in surface temperature. Even though this could demonstrate the potential ability of thermochromic asphalt coatings to delay ice formation to some extent, this improvement was not so desirable.

3.4

Recommendations for future research and applications

According to existing investigations, thermochromic microcapsules show good antiaging properties and can adjust asphalt pavement temperature, consequently contributing to alleviating pavement distresses at high and low temperatures, prolonging pavement durability and improving urban heat islands. However, some issues still need to be addressed in future research and applications. First, though the influence of thermochromic microcapsules on the properties of asphalt binders has been investigated, there are few investigations on the process of producing thermochromic asphalt mixtures, such as optimum content, mixing temperature, etc. More importantly, whether compaction in field work would damage thermochromic capsules is still a question. Second, during service life, asphalt pavement suffers from thermal and optical oxidation, and so would thermochromic microcapsules. In this case, the shell material would be less strong and the core material could flow out from it. Consequently, some future work should focus on the aging process of three-component organic reversible

58

Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

thermochromic capsules in asphalt mixtures. Meanwhile, it is also necessary to investigate the functional loss of thermochromic powders, as this would correspondingly influence the service life of asphalt pavement. Above all, future work should emphasize the optimum conditions for producing thermochromic asphalt mix in the field and the aging process of thermochromic powders in asphalt pavement.

References Chen, Z.H., Zhang, H.L., Shi, C.J., Wei, C.W., 2019. Rheological performance investigation and sustainability evaluation of asphalt binder with thermochromic powders under solar radiation. Sol. Energy Mater. Sol. Cells 191, 175–182. Du, P.F., Chen, Z.H., Zhang, H.L., 2019. Rheological and aging behaviors of base and SBS modified asphalt with thermochromic microcapsule. Constr. Build. Mater. 200, 1–9. Hu, J.Y., Yu, X., 2013. Experimental study of sustainable asphalt binder: influence of thermochromic materials. Transp. Res. Rec. J. Transp. Res. Board 2372, 108–115. Hu, J.Y., Yu, X., 2016a. Innovative thermochromic asphalt coating: characterisation and thermal performance. Road Mater. Pavement. Des. 17 (1), 187–202. Hu, J.Y., Yu, X., 2016b. Reflectance spectra of thermochromic asphalt binder: characterization and optical mixing model. J. Mater. Civ. Eng. 28 (2), 04015121-1–04015121-10. Hu, J.Y., Wanasekara, N., Yu, X., 2014. Thermal properties of thermochromic asphalt binders by modulated differential scanning calorimetry. Transp. Res. Rec. J. Transp. Res. Board 2444, 142–150. Hu, J.Y., Gao, Q., Yu, X., 2015. Characterization of the optical and mechanical properties of innovative multifunctional thermochromic asphalt binders. J. Mater. Civ. Eng. 27 (5), 04014171-1–04014171-10. Karlessi, T., Santamouris, M., Apostolakis, K., Synnefa, A., Livada, I., 2009. Development and testing of thermochromic coatings for buildings and urban structures. Sol. Energy 83 (4), 538–551. Lynn, B.H., Carlson, T.N., Rosenzweig, C., Goldberg, R., Druyan, L., Cox, J., et al., 2009. A modification to the NOAH LSM to simulate heat mitigation strategies in the New York City metropolitan area. J. Appl. Meteorol. Climatol. 48, 199–216. Mill, T., 1996. The role of hydroaromatics in oxidative aging in asphalt. In: Preprints of 212th ACS National Meetingvol. 41(4), pp. 1245–1249. Mouillet, V., Lamontagne, J., Durrieu, F., Planche, J.P., Lapalu, L., 2008. Infrared microscope investigation of oxidation and phase evaluation in bitumen modified polymers. Fuel 87 (7), 1270–1280. Petersen, J.C., 1998. A dual, sequential mechanism for the oxidation of petroleum asphalts. Pet. Sci. Technol. 16 (16), 1023–1059. Qin, Y., 2015. A review on the development of pool pavement to mitigate urban heat island effect. Renew. Sust. Energ. Rev. 52, 445–459. Qin, Q., Schabron, J.F., Boysen, R.B., Farrar, M.J., 2014. Field aging effect on chemistry and rheology of asphalt binders and rheological predictions for field aging. Fuel 121 (2), 86–94. Sen, S., Roseler, J., Ruddell, B., Middel, A., 2019. Cool pavement strategies for urban heat island mitigation in suburban Phoenix, Arizona. Sustainability 11, 4452. Yan, K.Z., Xu, H.B., Zhang, H.L., 2013. Effect of mineral filler on properties of warm asphalt mastic containing Sasobit. Constr. Build. Mater. 48, 622–627.

Performance of thermochromic asphalt

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Zhang, H.L., Chen, Z.H., Li, L., Zhu, C.Z., 2017. Evaluation of aging behaviors of asphalt with different thermochromic powders. Constr. Build. Mater. 155, 1198–1205. Zhang, H.L., Chen, Z.H., Xu, G.Q., Shi, C.J., 2018. Physical, rheological and chemical characterization of aging behaviors of thermochromic asphalt binder. Fuel 211, 850–858. Zhu, S., Mai, X., 2019. A review of using reflective pavement materials as mitigation tactics to counter the effects of urban heat island. Adv. Compos. Hybrid Mater. 2, 381–388.

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Pavements for mitigating urban heat island effects

4

I.K. Mizwar, Madzlan bin Napiah, and Muslich Hartadi Sutanto Department of Civil and Environmental Engineering, Universiti Teknologi PETRONAS, Perak, Malaysia

4.1

Introduction

An urban heat island (UHI) is a phenomenon in which the temperature in an urban area is higher than the surrounding area, due largely to the increasing artificial materials used in the urban area. Buildings and pavements that replace green areas in urban centers lead to higher temperatures. Reduced air quality and increased energy use for air conditioning are considered effects of urban heat islands. Studies of pavement materials and also the structure of pavement itself are leading to the development of pavements that can reduce the impacts of urban heat islands. Urban heat island studies utilize many parameters in numerical simulations, including air temperature, surface temperature, sky view factors, wind speed, mean radiant temperature, and physiological equivalent temperature. Although a recent study Farhadi et al. (2019) showed that if lower surface temperatures are the basis for UHI mitigation, a stronger correlation between the UHI effect and thermal comfort can be observed. The surface temperature correlates with the temperature of pavement surfaces.

4.2

Thermal performance of pavement

Thermal conductivity, specific heat capacity, and thermal diffusivity are thermal properties. A study by Luca and Mrawira (2005) found that there were no thermal properties of asphalt concrete that were not related to its physical properties. However, the tests in this study were carried out at room temperature, except for a Marshall test at 60°C. In urban heat studies involving high temperatures, the thermal performance of pavement influenced the mechanical performance of the pavement.

4.2.1 Methods to quantify surface temperature There are two meanings of surface temperature. In pavement studies, it refers to the surface of the pavement. In urban heat island studies, it refers to temperature measurements taken in the air close to the earth’s surface (around 1 m) in an open area. The surface temperature of pavement can be measured by a thermocouple or infrared Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction https://doi.org/10.1016/B978-0-12-820791-8.00004-3 © 2021 Elsevier Ltd. All rights reserved.

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

Table 4.1 Methods to record surface temperature. No.

Method

Advantage

Disadvantage

Application

1

Satellite

Cover extensive spatial area

Limited time intervals, the data are affected by weather and atmosphere

Landsat in city of Split (Leder et al., 2016); MODIS in Europe (Schwarz and Manceur, 2015), Taipei City (Chen et al., 2017)

2

Aircraft scanner

3

Infrared thermometer

Spatial resolution depends on aircraft altitude Spatial resolution depends on sensor distance from the surface, not affected by weather and atmosphere

High cost, irregular coverage Less spatial coverage

Benrazavi et al. (2016), Takebayashi and Moriyama (2012), and Farhadi et al. (2019)

thermometer, while three methods are used to record surface temperatures in UHIs (Table 4.1), which are via satellite, aircraft scanner, and infrared thermometer. In recent studies, Mathew et al. (2016) and Chandrappa and Biligiri (2016) developed models to predict land surface temperature (LST). The Mathew model is based on historical temperatures and parameters representing vegetation (EVI/enhanced vegetation index), road density (RD), and elevation of the area. However, the Chandrappa model is based on climatological factors such as solar incident radiation and air temperature. The advantage of the latter model is that it can be used to estimate heat energy flux from different pavement materials. A study in Malaysia Benrazavi et al. (2016) found that the surface temperature of asphalt was 60.4°C during the time period 12:00–15:00 (the highest temperature during a day). This increased by 35.8°C from the lowest of 24.6°C (at 06:00–09:00). They used field measurement instrumentation of an FLIR E60 infrared thermal imaging camera to compare three different landscape environments: under shade, near water, and open space. The results suggested that shade is more significant than water in influencing surface temperature reduction of asphalt pavement. This study also compared the surface temperature and air temperature. There was thermal variation between them.

4.2.2 Heat transfer from pavement to urban temperature Three types of heat transfer occur in pavement: radiation, convection, and conduction. When solar radiation is incident on pavement, two processes occur: the solar energy is both absorbed and reflected. Both are a function of the wavelength of the solar

10 0 –10 0

6

12

18 0 6 Day time (h)

Aspect ratio: 2 Aspect ratio: 1 Aspect ratio: 0.5 Stand-alone Ambient rural

Wall South: 1.Floor

30 20

Air temp. (°C)

20

63

Temp. difference; Surf.Amb. (K)

Wall North: 1.Floor

30

Air temp. (°C)

Temp. difference; Surf.Amb. (K)

Pavements for mitigating urban heat island effects

10 0 –10 0

6

12

18 0 6 Day time (h)

12

18

0

Fig. 4.1 Differences between the wall surface temperature and the ambient air temperature for a stand-alone building and buildings surrounded by street canyon with different aspect ratios (Allegrini et al., 2012).

radiation, surface color, the average temperature of the pavement, wetness, and age of the pavement surface (Solaimanian and Kennedy, 1993). Pavement can influence air temperature and temperature of the adjacent building wall. The effect of this heat transfer is more obvious in canyon geometry. Allegrini et al. (2012) have proven that surface temperatures of building facades in canyon geometry are higher than on a stand-alone building (see Fig. 4.1). Multiple reflections (reflection from the pavement and building facade) and lower convection contribute to this high temperature. In their following study, Allegrini et al. (2015) showed that different wind flow patterns cause different rates of cooling by the wind. This study answered the question as to why air temperature can vary although the urban morphologies have a similar surface temperature. However, this study was not consistent with Toraldo et al. (2015) who concluded that wind has no significant effect on road pavement temperature. In another study on heat transfer from pavement to urban temperature, Qin (2015) developed a numerical model to evaluate the reflective diffuse radiation from the pavement to adjacent building walls. It was found that the ratio of a building’s height to the road’s width, called the aspect ratio, controls the urban canyon albedo (UCA), while other factors play secondary roles. Reflective pavements in an urban canyon reflect a sizable additional amount of diffuse radiation to the adjacent wall during the summer. It is recommended that reflective pavements be used only if an urban canyon has an aspect ratio no greater than 1.0. Yaghoobian and Kleissl (2012) evaluated local thermal and radiative effects of asphalt (representative of highly absorptive materials) and concrete (representative of more reflective materials) ground surfaces on building thermal loads. The difference in albedo and (less so) other material thermal properties causes a higher ground surface temperature on asphalt than concrete, with a maximum of 9.9°C (see Fig. 4.2). Ground surface materials affect building surfaces directly (through radiation) and indirectly (through convection). Despite the higher ground surface temperature and canopy air temperature over asphalt, building walls are cooler than when over concrete. Yaghoobian et al. (2010) showed that transfer of heat between ground surfaces and walls is affected by net shortwave radiation. Consequently, the higher building wall temperature over concrete is directly related to the higher albedo of concrete than

64

Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction Ground surface

55

36

50

34

45 40 35 30

20

32 30 28 26

22 6

12

18

20

24

(D)

Canopy air

26

6

12

18

24

Transmitted SW

800

25

700

24

600

23 500 22

W/m–2

Temperature (°C)

Asphalt Concrete

24

25

(A)

Inside building surfaces

38

Temperature (°C)

Temperature (°C)

60

21

400

20

300

19

200

18

100

17

6

(B)

12

18

0

24

(E)

Outside building wall

6

12 Qsys

18

24

6

12 Time (LMST)

18

24

0 40

35

Thermal load (kWh)

Temperature (°C)

–20

30

–40 –60 –80

–100

25

–120 20

(C)

6

12 Time (LMST)

18

24

–140

(F)

Fig. 4.2 Comparison of asphalt and concrete ground surface material for summer day in San Diego, California (Yaghoobian and Kleissl, 2012). (A) Ground surface, (B) Canopy air, (C) Outside building wall, (D) Inside building surface temperature, (E) Transmitted shortwave radiation into the building, and (F) Hourly thermal load for asphalt and concrete ground surface material for summer day (July 10th).

Pavements for mitigating urban heat island effects

65

asphalt. Higher reflection from concrete transmitted shortwave radiation into the building. The contribution of transmitted shortwave radiation through windows on indoor temperatures is larger than the effects of conductive heat transfer through the building envelope. Consequently, the daily AC energy use to keep the indoor air temperature at the cooling setpoint increases by 10.2% for a concrete ground surface. Hall et al. (2012) determined the behavior sensitivity to pavement thermophysical properties of pavement surface temperature, using a predictive transient model. The model has confirmed that a pavement surface with high conductivity and low absorptivity will be cooler. Cooler pavement has lower heat emitted to the urban environment. Gui et al. (2007) indicated that both albedo and emissivity have the highest positive effects on pavement maximum and minimum temperatures, respectively, while increasing the thermal conductivity, diffusivity, and volumetric heat capacity helps in mitigating the maximum, but not the minimum, pavement near-surface temperature. Feng and Myint (2016) explored the relationship between the land surface temperature (LST) (from ASTER data) of central building objects and land cover patterns in their neighboring areas. The composition of land cover features has a stronger impact on low-rise building LST than on midrise and high-rise building LST. Moreover, low-rise building LST is highly related to the composition of neighboring vegetation and pavement. This relationship is limited to midrise buildings and high-rise buildings. The high percentage of pavement nearby would normally have a positive effect on the LST of central buildings. However, if these pavements are largely shaded, their ability to absorb energy changes and their contribution to LST decreases. Qaid et al. (2016) investigated UHI in Putra Jaya, Malaysia. This study concluded that the high-rise residential buildings and the boulevard street are thermally comfortable most of the daytime hours, while low-rise buildings suffer from a long period of heat stress. Reflected solar radiation and surface temperature have an influence on increasing outdoor thermal comfort. The high surface temperature was found on the wide, exposed, impervious surfaces in the city, such as pavement (wide roads, wide sidewalks, and parking lots), in line with (Al-ameri et al., 2014). The maximum surface temperature was recorded between 15:00 and 16:00 and reached 57°C. These impervious surfaces elevate the air temperature and the heat within the city during the daytime. Regarding the effect of buildings, although the reflection of sunlight can be increased to some extent, they can provide shade for visitors, to moderate thermal comfort (Yahia and Johansson, 2014). Qaid et al. (2016) reported that the highest intensity of the solar radiation in Putra Jaya occurred between 12:00 and 14:00 and reached 1035 W/m2. From Fig. 4.3, we can see that air temperature is still high at 15:00 and 16:00, although solar radiation has already decreased. This is because of the reradiated heat from the exposed urban surface, including pavement. This study suggests that surface material should be able to balance between reflected and absorbed solar radiation to provide thermal comfort during the day and mitigate the heat island at night.

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

Fig. 4.3 Time series of the solar radiation intensity and the air temperature during mobile survey days (Qaid et al., 2016).

4.3

Impact of pavement temperature on mechanical performance

4.3.1 Main pavement failures Climate damage to the pavement can decrease the serviceability of the road. This damage is related to the development of cracks and ruts. Pavement cracking and rutting are considered to be main pavement failures. To endure climate conditions, some mechanical testing is being done on improving the performance of asphalt mixtures.

4.3.2 Effect of low temperature on pavement Pavement failure in low-temperature areas is thermal cracking. This cracking occurs when tensile stresses (thermal stress) due to low temperatures exceed the tensile strength of the pavement. The effect is similar to fatigue cracking. However, the cause is different, as fatigue cracking is due to repetitive traffic loading. Microcracks initiate at the bottom of the pavement layer (bottom-up) or near the pavement surface (top-down) and grow into macrocracks. This cracking allows water to go to underlying pavement layers, lowering the pavement smoothness and shortening pavement service life. Some materials are mixed with the original pavement mixture to mitigate pavement failure due to low temperatures. A coproduct of refining castor oil can improve low-temperature performance, based on rheological tests using a dynamic shear rheometer (Dong et al., 2018). The oil that can improve low-temperature performance is refined waste oil. The study Lei et al. (2015) showed that the oil, compared with the neat asphalt mixture, increased the fracture temperature by approximately 50% (Fig. 4.4).

Pavements for mitigating urban heat island effects

Fracture temperature (°C)

–35

67

Fig. 4.4 Fracture temperature of asphalt mixture (Lei et al., 2015).

Fracture temperature

–30 –25 –20 –15 –10 –5 0

VA

7% PP-1 6% BO-1 Asphalt type

5% RW

Another material that has better resistance to lower temperatures is a crumb rubber modifier (CRM). A higher CRM percentage resulted in lower stiffness of CRM binders (Lee et al., 2008). The binder source influence on the CRM binder properties at a lower temperature was not as significant as the CRM percentage increase. Considering low-temperature stiffness, the addition of 10% crumb rubber into a control binder was suggested (Wang et al., 2012). Both studies utilized a bending beam rheometer (BBR) to evaluate the CRM. A recent study Kakar et al. (2019) showed that phase change material (PCM) encapsulation is very important for minimizing low-temperature variations. A PCM with change temperature of 6°C was studied with three different hardnesses of bitumen. The rheological characterizations of bitumen and DSC results showed that replacing filler with an equivalent volume of microencapsulation phase change material (MPCM) (3% by mass of bitumen) does not alter the bitumen performance. In terms of a mixture, a study Wu et al. (2007) showed that low temperatures of a stone mastic asphalt (SMA) mixture were improved by using steel slag as a coarse aggregate.

4.3.3 Effect of high temperature on pavement Pavement failures in high temperatures are rutting, deformation/lateral plastic flow, and/or loss of material under the wheel path. This failure occurs when shear stress due to repeated heavy load and high temperature accumulates permanent strain. Rutting can occur in all layers of pavement. A study Li et al. (2015) showed that for pavement structure using three asphalt layers, the middle asphalt layer exhibits the most permanent deformation, followed by the top and bottom asphalt layers. For pavement structures using two asphalt layers, more permanent deformation occurs in the bottom asphalt layer. The study utilized field core samples from highway locations with daily maximum air temperature of 35°C. Modified asphalt mixtures were produced to reduce the high-temperature effects on the pavement. Rutting resistance of modified asphalt mixtures were improved over

68

Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

those of base asphalt. The modified mixture utilized a residue of manufacturing chemical alcohol from corn (Dong et al., 2018). Improving thermal modification of asphalt mixtures can be useful for increasing rutting resistance. Chen et al. (2015) studied the effect of thermally conductive filler. The result showed that the thermal conductivity of asphalt concrete is affected by the content and shape of filler for thermal modification. Modifying the structure of asphalt pavement is an alternative to reducing the effect of high temperature on the pavement. The rutting model developed by Yinfei et al. (2015) based on the heat transfer model showed that bidirectional pavement structure had higher modulus and greater resistance to permanent deformation than the conventional one.

4.4

Mitigate UHI effect

In order to reduce the UHI effect, some methods have been proposed and applied. Urban parks, community green spaces, green roofs, and cool (or permeable) pavements should be prioritized (Chen et al., 2017). Following is a discussion of UHI effect mitigations from the pavement side.

4.4.1 Reflective pavement Reflective pavement is one of the types of cool pavements. This type attempts to reduce heat absorption into the pavement by increasing the reflection of the pavement surface (albedo). Having a lighter color can increase albedo. The lighter color can be produced by changing the aggregate color or by painting the surface of the pavement. Anting et al. (2017) showed that FBP (full body porcelain) and MP (monoporosa) have good potential to be used as cool pavement coating materials, based on their thermal and spectral performance. FBP is able to obtain a surface temperature reduction up to 6.4°C during the peak period. Another study Anak Guntor et al. (2014) applied waste tile as a coating material on asphalt pavement. It could reduce the surface temperature of asphalt pavement up to 4.4°C. The amount of solar radiation emitted back from conventional asphalt pavement contributes to UHI. A study by Chatzidimitriou and Yannas (2015) found that high pavement albedo reduces surface temperatures and increases global temperatures. Comparing the lowest albedo (black marble) and the highest albedo (white marble), the highest mean global temperature (39.4°C) was detected above the surface with the highest albedo (Fig. 4.5). With materials that have similar albedo, such as asphalt (0.15) and dark marble (0.16), the difference in surface temperature must be attributed to other properties, including thermal conductivity and thermal capacity. Asphalt’s volumetric thermal capacity (2100 kJ/m3 K) is lower than that of marble (2800 kJ/m3 K) and its thermal conductivity (0.7 W/m K) is much lower than marble’s (3.5 W/m K). Asphalt also has a lower emissivity than marble (0.90 and 0.95, respectively) and these differences in fundamental properties may explain the differences in surface temperatures.

Pavements for mitigating urban heat island effects

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Site A | 29-07-2007 60

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Fig. 4.5 Global temperature above different pavements (Chatzidimitriou and Yannas, 2015).

Toraldo et al. (2015) reported that both air temperature and solar irradiance greatly affect road pavement temperatures. Air humidity and wind have no significant effect on road pavement temperatures. This showed the potential of lighter colored pavements used in urban areas, such as open-graded asphalt pavements filled with cement mortar, to mitigate the UHI phenomenon. Santamouris et al. (2012) concluded that the use of reflective paving materials is a very efficient mitigation technique to improve thermal conditions in urban areas. Another type of reflective pavement utilizes retroreflective (RR) materials. This is an updated version of reflective material. Rossi et al. (2016) evaluated RR pavement as regards the energy kept inside the urban canyon: the optic interaction between pavement and facades showed a significant decrease in the energy kept inside the canyon. RRMs reflect light back, along with the incident 12 direction (instead of diffuse reflection). This method can solve the problem of reflected sunlight reaching neighboring buildings and roads (Yuan et al., 2015). Rosso et al. (2017) proposed concrete elements with infrared-reflective (IR) pigments. IR concrete is able to maintain surface temperatures up to 10.6°C lower with respect to non-IR samples. IR concrete is not employed as additional paint or a coating layer, but as massive composites. This has numerous advantages: (i) it is homogeneous, not just a top coat, and is consistent all along the prototype thickness, so if cracks or scratches occur on the material, the color and characteristics remain the same, requiring less maintenance; (ii) it does not require additional manufacturing steps such as painting or spraying a topcoat on its surface (the cool additive is poured directly in the concrete mix); (iii) it is suitable for irregular shapes and historical construction, since it can easily replicate almost any color.

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4.4.2 Porous pavement Porous asphalt exhibited higher daytime surface temperatures than the other pavements because of the reduced thermal energy transfer from the surface to subsurface layers (Stempihar et al., 2012). However, porous asphalt showed the lowest nighttime temperatures compared with other materials with similar or higher albedo. This study indicated that pavement impact on UHI is a complex problem and that important interactions between influencing factors such as pavement thickness, structure, material type, and albedo must be considered. Another study by Li et al. (2014) found that large quantities of water available near the surface or large moisture exposure to the atmosphere are critical for the evaporation rate and consequent evaporative cooling effect of pavement materials. A porous asphalt concrete (PAC) can be incorporated with water-retentive slurry (WRS) ( Jiang et al., 2016); the product is called water retentive asphalt concrete (WRAC). The use of WRAC resulted in a temperature drop of about 10°C compared to PAC (Fig. 4.6).

4.4.3 Hydronic asphalt pavement Hydronic asphalt pavement (HAP) is another alternative to mitigate urban heat islands. The thermal behavior of HAP is strongly dependent on the material characteristics, pipe arrangements, and fluid operation parameters (Pan et al., 2015). A study of HAP has also been documented (Basheer Sheeba and Krishnan Rohini, 2014). Using the finite element method, the author obtained optimum pipe spacing, pipe diameter, depth, and pipe arrangement. Another study by Chiarelli et al. (2015) presented a prototype air-powered energy-harvesting pavement. The results showed that the prototype was useful in reducing the UHI effect by lowering the pavement surface temperature by more than 6°C.

Temperature (°C)

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Fig. 4.6 Indoor temperature test results ( Jiang et al., 2016).

5

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4.4.4 PCM pavement PCMs have been utilized in several different applications. Compared to applications of PCM in buildings (roof, floor, and wall), applications of PCM in pavements still need more research, especially regarding the mechanical performance of PCM pavement. The first research paper on this application Ma et al. (2010) showed the results of an experiment on high and low temperatures of asphalt mixtures. However, the result for high temperature was not as expected, due to the leakage of PCM and the choice of phase change temperature. Marshall test stability at 45°C and 60°C was performed on PCM pavement with a phase change temperature of 25°C, instead of 45°C or 60°C. Another study Chen et al. (2011) suggested that 3–5°C less than the softening point of asphalt can be considered as the phase change temperature of PCM. In yet another study, Guan et al. (2011) mixed the PCM into an asphalt mixture. The temperature change rate of the asphalt mixture with PCM decreased, the emergence of extreme temperatures was delayed, and the duration of the extreme temperatures was shortened. However, this study didn’t produce a mechanical performance result. He et al. (2013) prepared a blend of asphalt and shape-stabilized phase change materials (SSPCMs). The results showed that asphalt-SSPCM blends have large phase change enthalpy, good thermal stability, and chemical compatibility. Based on phase change theoretical analysis and numerical calculations, SSPCM applied in asphalt pavement can actively regulate and control the pavement using solar energy conversion and storage, reduce the asphalt pavement disease-related temperatures, enhance the performance of and prolong the service life of the asphalt pavement, reduce need for repairs and maintenance cost, and enhance driving safety. At the same time, it can also save energy resources and protect the environment. Athukorallage and James (2016) conducted heat transfer analysis for a pavement in which the PCM was embedded in the asphalt-concrete layer. The simulation results showed that the pavement system embedded with PCMs yielded a lower surface temperature value than systems without PCM (maximum temperature decrease was 1.5°C for the distributed PCM with a volume fraction of 30%). They observed that an increase in effective thermal conductivity yields lower surface temperature for the PCM embedded pavement system. However, this fraction is too large for asphalt pavement and could reduce the life of the pavement. In their subsequent study Athukorallage et al. (2018) provided the maximum surface temperature value and the corresponding time of occurrence with different PCM volume fractions. The PCM volume fractions influence the temperature and rate of change of temperature in the system (Fig. 4.7). Adding carbon fiber can also increase the thermal conductivity of PCM (Zhang et al., 2018). However, the intended application of this study was for a building envelope system. Qin and Hiller (2014) demonstrated the energy partition at the ground surface, focusing on the sensible heat realized from the pavement surface. Pavements with different surface albedos and different thermal inertia were simulated to demonstrate the influences of surface and heat-storage modifications on the sensible heat released. Most of the solar absorption by the pavement was discharged as sensible heat and

Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

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Fig. 4.7 Maximum surface temperature value and the corresponding time of occurrence with different PCM volume fractions (Athukorallage et al., 2018).

long-wave emissions, whereas the daily cumulative heat conduction was roughly 5% of the absorption. They found that both increasing the surface albedo and enhancing the evaporative flux were effective in suppressing the sensible heat and promoting cool pavements. Raising the thermal inertia of the pavement decreased the sensible heat during the day but increased this factor at night. Therefore, designing cool pavement by varying their thermal inertia should be done with caution. A study by Dehdezi et al. (2013) on incorporating PCM in concrete pavement showed that increasing PCM content in concrete led to lower thermal conductivity and an increase in the heat storage ability of concrete. However, the compressive and flexural strength of the concrete significantly decreased.

4.5

Future trends

This chapter has described some developments in pavements aimed at mitigating urban heat islands. As discussed, several findings are reflective pavements, porous pavements, hydronic pavements, and PCM pavements. PCM pavements and hydronic pavements are still at the study level. One major challenge is to develop an appropriate mixture containing PCM with good mechanical performance in terms of rutting and fatigue. This could include encapsulated PCM, which is micro/macroencapsulated phase change material (MPCM), which can prevent PCM leakage into the asphalt mixture. Another challenge is to find a hydronic pavement with a simple procedure for pipe installation. In addition, there is a clear need for better understanding of the effects of reflective pavement on surrounding areas in order to further improve its function to reduce urban heat islands. Future directions that can be taken to better understand cool pavement include simulations of the effects of cool pavements on surrounding buildings.

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Part Two Facade materials for reducing cooling needs

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Quantitative approximation of shading-induced cooling by climber green wall based on multiple-iterative radiation pathways

5

Louis S.H. Leea and C.Y. Jimb a Department of Environment, Technological and Higher Education Institute of Hong Kong, Hong Kong, China, bDepartment of Social Sciences, Education University of Hong Kong, Tai Po, Hong Kong

Nomenclature English alphabet A E F P W

ambient environment, referring to the outdoor surroundings net energy fenestration, referring to the window panes plant, referring to the climber vegetation canopy wall, referring to the indoor wall surface

Greek alphabet α ρ τ κ

absorptivity reflectivity transmissivity extinction coefficient

Subscript BW GW INDOOR

bare wall green wall indoor space bounded by window and indoor wall

Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction https://doi.org/10.1016/B978-0-12-820791-8.00005-5 © 2021 Elsevier Ltd. All rights reserved.

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PAG SYSTEM

posterior air gap between the climber canopy and the external building wall with windows building envelope system

Acronym LAI PAG RAM RAM*

5.1

leaf area index posterior air gap radiation apportionment model revised radiation apportionment model

Introduction

Ever-increasing cooling needs of buildings have challenged environmental sustainability in cities. Building occupants are more and more reliant on air conditioning to achieve indoor thermal comfort. The energy load peaks in summer due to irritating heat stress induced by intense sunshine. Furthermore, the urban heat island phenomenon contributed to by anthropogenic heat sources engenders extra cooling load in cities, which are relatively warmer than the rural surroundings. Worse still, global climate change has brought more frequent and acute thermal extremities such as heat waves to cities. For centuries, building architects have been devising building materials and dimensional innovations in pursuit of indoor biometeorological comfort. Recently, some cities have steered a paradigm shift to capitalize on the cooling functions of green infrastructures, in which vegetation plays a central role as a nature-based, cost-effective, and sustainable solution. However, urban densification has left little land at ground level to accommodate greenery. To tackle the scarcity of plantable space in compact city areas, green walls as building-integrated vegetation offer an aboveground alternative. Indoor passive cooling by green walls is inaugurated by several mechanisms, of which shading plays a key role. In this chapter, shading is defined as physical blocking of incoming radiation by vegetation foliage on green walls. Research is strongly needed to develop computational tools to enhance the assessment of green-wall cooling potential. Pioneering green wall research, German scientists coined the term € Fassadenbegrunung, which literally means “greening of vertical surface.” Other similar terminologies include green fac¸ade, living fac¸ade, and biofac¸ade, which have different meanings. These technical terms bear no specifications for indoor or outdoor applications. Also, green walls fall into two broad categories, namely climber green walls and herb-shrub green walls. Acknowledging the broad scope, this chapter squarely focuses on outdoor climber green walls, which typically consist of a training system, growing medium, and climber vegetation (Fig. 5.1). Key design features of green walls, with analysis of cooling benefits, are elaborated.

Quantitative approximation of shading-induced cooling

Posterior air gap

81

Indoor space

Outdoor wall Training system Climber vegetation Pane window

Net radiometer

Hanging planter Growing medium Indoor wall

Fig. 5.1 Cross-sectional diagram showing the key components of a climber green wall on a fenestrated building envelope. It also shows the recommended installation position of net radiometers for approximating radiative transfers using the revised radiation apportionment model (RAM*) introduced in this chapter.

When climber seedlings or cuttings are planted, the training system, usually composed of vertically mounted stainless steel wire ropes, supports the twining vines. Attention should be paid to species-specific preference for training-system specifications to optimize the coverage and thickness of the climber canopy. The interrope spacing and depth of the posterior air gap (PAG) can be adjusted to facilitate the establishment of a dense climber canopy quantified by the leaf area index (LAI), which expresses foliage surface area per unit wall area. By intercepting more solar radiation, a higher LAI can better reduce solar penetration into indoor space to alleviate the energy burden for cooling. The provision of a growing medium presents a key determinant of vegetation growth. For climber green walls, the planter is usually provided at the toe. To overcome the limitation of climber-height attainment on tall structures, suspended planters can be installed every or every several floors to maintain plant coverage at high elevation. The thermal conductivity, width and depth of the planter, and its soil composition govern the ambience-and-building heat transfer (Sudimac et al., 2019). The choice of climber species is largely based on color, texture, size, shape, thickness and density of foliage, seasonality, rigor, ornamental traits, growth cycle, and habitat requirements. More importantly, cooling is provided mainly by shading of solar irradiance by foliage, thus reducing the amount reaching the building shell. Leaves and stems reflect and absorb some incoming radiation in the shortwave band (0.3–3.0 μm) to retard radiative heating and deliver cooling. On the other hand, latent

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heat absorption due to evapotranspiration from leaves and soil lowers the air temperature in the canopy boundary layer. In terms of cooling effectiveness, various studies have highlighted the major contributions of shading over evapotranspiration (Hoelscher et al., 2016). Therefore, deeper understanding of critical radiation parameters and behaviors can express the energy pathways subsequent to solar incidence. Green walls provide a wide array of environmental and ecological benefits. They furnish nesting and perching sites for urban wildlife. Fruits and foliage offer foraging resources. Hanging leaves and twigs absorb vibration and insulate noise. Dry deposition of air pollutants on foliage surface and admission into the leaf interior purify polluted air. Integrating vegetation into building designs can soften the rigid building geometry and beautify unpleasant and dull streetscapes. This chapter presents a revised radiation apportionment model (RAM*) for estimating the benefits of shading against shortwave radiation as cooling-load reduction. Based on the theoretical basis of a previous study (Lee and Jim, 2019a), radiative properties and fluxes of the climber canopy of a green wall are mathematically and computationally solved with enhanced accuracy. The crucial factors of shadinginduced cooling are discussed. Practical recommendations are distilled to improve green-wall design and achieve greater cooling-energy savings.

5.2

Factors influencing shading-induced cooling

5.2.1 Role of climate and weather Maximum cooling load in buildings occurs during summer (the hot season) when solar irradiance peaks, and when air conditioning is engaged to lower indoor air temperature. In summer, high-intensity solar radiation and vigorous climber growth allow green walls to bestow effective cooling and reduce indoor cooling requirements. Geographically, the tropical zone has a longer hot season in which shading-induced cooling can be notably realized. With a longer growing season, climbers can achieve a higher LAI for an extended period to enhance shading capacity. Moreover, in lowlatitude regions, relatively constant and long day length throughout the year entails more effective shading benefits. In brief, geoclimatic settings of cities located in the tropics articulates the importance of shading-induced cooling by green-wall vegetation. Apart from climate, local weather conditions set a cap on maximum shading benefits. Shading-induced cooling originates from the interception of incoming solar radiation by climber vegetation through reflection or absorption. The stronger the solar inputs, the greater the magnitude of cooling. Under a clear sunny sky, intensive solar irradiance contains a high flux density of direct beam radiation (Di and Wang, 1999). In addition to blocking direct sunshine, the climber tissues are warmed by absorbing or consuming some of the incident solar radiation, thus contributing to suppressing airconditioning energy use. However, in cloudy and overcast conditions, the predominant mode of solar radiation is diffuse, thus weakening or even nullifying shading benefits. There is relatively

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little radiation available to shade against. The degree of shading attenuation is largely a function of cloud thickness and coverage. However, when solar radiative input is reduced by cloudiness, the corresponding suppression of heat stress diminishes cooling needs. Nevertheless, as long as some shortwave radiation is present, the building envelope benefits from shading-generated cooling. It is therefore necessary to account for shading-induced cooling even in cloudy weather. Therefore, this chapter provides an estimation of shading-induced cooling benefits in both sunny and cloudy scenarios.

5.2.2 Influence of building orientation on shading effect Unlike green infrastructure on a horizontal surface, green walls face a certain aspect. Shading provision is therefore a function of sun position relative to wall orientation. Sun position can be expressed by solar azimuth in conjunction with elevation. Daily evolution of solar azimuth implies self-shading from direct beam radiation by the green wall itself for a certain period of the day during which the sun appears to be “behind the wall.” Also, wall orientation governs the solar elevation or incidence angle at which direct beam radiation is received. For a vertical wall, solar radiation flux density striking the climber surface increases proportionally with decreasing solar elevation angle, thus raising the amount of solar radiation to shade against (Campbell and Norman, 2012). When a green wall is exposed to a sun path with low elevation angle, potential shading increases due to relative increase in solar-radiation flux density. Not only does the sun position vary in a day, but also throughout the year. The annual oscillation of the sun path modulates solar azimuth and elevation angle, causing seasonal variations in potential shading-generated cooling benefits. To summarize, building orientation interacts with the daily sun position and annual sun-path oscillation to dictate the available amount of incoming solar radiation and corresponding shading efficacy. In the Northern Hemisphere, two scenarios with respect to wall orientations deserve in-depth investigation. First, south-oriented walls encounter maximum solar exposure, thus experiencing the greatest shading, especially during summer. Although solar radiation strikes at rather high elevation angles in summer, south-facing walls endure the longest daily sunshine span, covering most solar azimuth angles associated with direct solar beams. Second, walls facing northeast or northwest receive direct beam irradiance in the morning and afternoon, respectively (Lee and Jim, 2019b), during which they are subjected to concentrated flux density arriving at low solar elevation angle. Acquiring climber vegetation radiative properties can help systematically evaluate shading-caused benefits in these scenarios. In the Southern Hemisphere, a literally opposite interpretation can be applied.

5.2.3 Interactions between greenery and window Many microclimate studies monitored building envelopes retrofitted with green walls. To simplify the analysis or concentrate on a limited set of factors, most studies did not take into account the influence of windows on shading-induced cooling. But windows

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

are omnipresent in modern cities, as architects use them to furnish a view, for natural illumination and ventilation, and for general human satisfaction and comfort (Feitosa and Wilkinson, 2018). To maintain these functions, building managers avoid or minimize covering windows with climber vegetation. Therefore, the partial canopy coverage over window panes allows some transmission of solar radiation. A conventional algebraic solution of the climber canopy’s transmissivity usually ignores windowreflected and foliage-arrested fluxes. To understand the holistic real-world radiation regime, it is imperative to determine the main constituent factors of transmissivity more accurately and precisely. Window glass transmissivity determines the indoor-and-outdoor radiation transmission. Fenestration arrangement and material properties such as double-glazing and low-emissivity glass affect the transmission process. Some radiation penetrating indoors is reflected back through the window to the outdoor environment. The indoorto-outdoor return radiation pathway depends similarly on the aforesaid radiative properties. A thick climber canopy with high LAI reduces the proportion of escaping radiation. Some species, e.g., Jasminum sp. and Lonicera sp., showed pronounced shading-induced cooling (Cameron et al., 2014). Radiative exchanges between the multiple layers of building envelopes depend on how the layers transmit, absorb, and reflect radiation fluxes. Microclimate monitoring provides the fundamental data to assist the approximation of such radiative components. This chapter uses field data to accurately solve the radiative properties, taking into account the related complexities using a RAM*.

5.3

Revised radiation apportionment model

5.3.1 Schematic representation of building envelope in the monitoring campaign Hong Kong (22°N, 114°E), located on the southern coast of China, has a monsooninfluenced subtropical climate (Cwa). The hot and humid summer elevates cooling needs to high levels. The compact mode of development has left little room for ground-level plantings. Green walls become a viable, if not necessary, greening option. Field monitoring was conducted on a green wall at a power substation in Tseung Kwan O New Town, Hong Kong (Table 5.1, Fig. 5.2). The measurement campaign covered June 01, 2016 to July 01, 2017. Shortwave radiation readings were recorded with net radiometers (CNR4, Kipp & Zonen, Delft) connected to dataloggers (LOGBOX SD, Kipp & Zonen, Delft) set at a 15-min sampling frequency. Net radiometers were mounted vertically, parallel to the building shell surface. An additional net radiometer was placed in the midpoint of the PAG between the window and the climbers, taking radiation measurements at a 1-min frequency. The green wall building envelope is represented as three infinitely long, parallel layers in a one-dimensional space (Figs. 5.1 and 5.3). In order to expand the applicability of the RAM*, flexible terminologies were adopted, namely plant (P), fenestration (F), and wall (W). It was assumed that P and F were porous and semitransparent, whereas W was opaque. But since this chapter is concerned with the monitored

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85

Table 5.1 Dimension and material details of the building envelope components in the microclimate monitoring. Green wall building envelope parameter

Dimension or material

Orientation of green wall Height of green wall base from ground level Wall dimension Window dimension Window-to-wall area ratio Thickness of reinforced concrete wall Posterior air gap depth Planter dimension Training system Interrope spacing Climber species Leaf-to-wall area ratio

N70°E 19.5 m 4.5 m
4.7 m (width
height) 0.92 m
1.02 m (width
height) 0.177 m2/m2 0.3 m 1.0 m 1.0 m
1.0 m (width
depth) Stainless-steel wire rope 0.3 m Lonicera japonica 0.24

Fig. 5.2 Photograph showing the climber green wall used for field data collection in this study.

building envelope, the terms “climber vegetation canopy,” “window,” and “indoor wall” are employed and represented by the respective abbreviation. On the left, shortwave radiation enters from and exits to the outdoor ambience (A). In the middle, PAG is sandwiched between P and F.

5.3.2 Theoretical basis Before presenting the RAM*, a brief theoretical recap of the previous version is needed (Lee and Jim, 2019a). Readers should familiarize themselves with the development and validation of the initial version. Three dimensionless radiative properties were employed, namely reflectivity (ρ), absorptivity (α), and transmissivity (τ).

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

Plants (P)

Fenestration (F)

Wall (W )

Fig. 5.3 Schematic depiction of radiation fluxes in the original version of radiation apportionment model (RAM) (Lee and Jim, 2019a). The black arrows indicate the considered flux pathways, whereas the red and blue arrows were not included in the original model.

They characterize the respective proportion of per-unit incoming shortwave radiation being reflected away, absorbed by, and transmitted through a layer, indicated by subscripts. By the principle of conservation of energy, these three quantities add up to unity (Cengel, 2007): ρ+α+τ¼1

(5.1)

Radiation fluxes in the original RAM were marked by circled numbers (Fig. 5.3). For every unit of incoming shortwave radiation reaching the climber vegetation canopy (①), immediate foliage-caused reflection (ρP) occurred (②). The exact ρP value depended on foliage color, leaf angle distribution, leaf surface texture, and plant moisture level. The canopy transmissivity (τP) determined the proportion passing through the climber canopy toward the window (③). Back-radiation was reflected by the outdoor window surface, traveling toward the climbers (④), whereas a proportion penetrated indoors (⑤). But the indoor wall would send reflected radiation back toward the window (⑥), the amount of which was jointly controlled by window transmissivity (τF) and wall reflectivity (ρW). The window-reflected plus wall-reflected fluxes, expressed as an overall flux (⑧), escaped through the climbers toward the outdoor ambience (⑨). The iterative radiation processes would persist, starting from reflection at the indoor-facing side of the climbers (⑩), until the flux density dropped to a negligible level. The original RAM somehow overcame the inherent inaccuracies of conventionally determined radiative properties. Traditionally, radiative properties were solved using

Quantitative approximation of shading-induced cooling

87

simple arithmetic division involving pyranometer readings in front of and behind the climber vegetation canopy, causing overestimated ρP values by mistakenly including the escaping fluxes from ②, ⑨, and so on. Similarly, reflection fluxes caused by the indoor-facing side of the climbers, namely ⑩ and so on, led to overestimated τP values. The initial RAM dissected the climber-window-wall radiative interactions and concentrated on ρP, τP, and αP. Unfortunately, the initial RAM suffered from two major drawbacks. First, manual inputs were required for the radiative properties of artificial building elements, namely ρF, τF, and ρW. Inaccurate values could bias the radiative properties of climber vegetation. Second, indoor window-wall reflections were neglected, not to mention the flux escaping toward the outdoor ambience, as indicated by the red and blue arrow shafts in Fig. 5.3. However, in the RAM*, a new climber-window-wall radiative interaction framework has been added to cover these two loopholes and improve accuracy.

5.3.3 Revised radiation apportionment model Updated notations in RAM* express radiative exchanges among layers. The origin and the destination of a flux are connoted by the first and second letters in the notation, respectively, with numerical subscripts conveying the order of iteration (Fig. 5.4). For instance, PF1 indicates the summation of the first stage of iteration summing fluxes from plants (P) to window (F). Curly arrow shafts express the between-layer flux reflections, whose repetitive nature is conveyed by ellipses below every set of arrows

Plants (P)

Fenestration (F)

Wall (W )

AP

Fig. 5.4 Schematic depiction of the revised radiation apportionment model (RAM*), modeling the building envelope of green wall (GW) with three layers, namely plant (P), window (F), and indoor wall (W). The origin and destination of radiation fluxes are indicated by the first and second alphabets, respectively, adjoining the arrows.

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

pointing to the same direction. Then, each curly bracket indicates the summation of direction-specific fluxes as an infinite geometric sequence tending toward zero. Summing up all reflection fluxes stage by stage economizes the calculation by avoiding listing out countless fluxes spawning exponentially. For every unit of incoming shortwave radiation (AP1), some is immediately reflected to the outdoor ambience (PA1). The through-climber transmitted proportion is then distributed by multiple climber-window reflections, splitting into a sum traversing toward the window (PF1) and the sum heading opposite to the climber canopy (FP1), which escapes thereafter (PA2). As PF1 penetrates indoors reaching the wall, repeated window-wall reflections are summarized as FW1 and WF1, with the latter exiting the window and generating the second stage of iterative climber-window reflections. The outgoing flux (FP2) breaks free from the climbers, being lost to the ambience (PA3), whereas the recoiling flux (PF2) reenters the indoor space. Flux densities at subsequent stages tend to attenuate progressively toward the eventuality of zero, or diminish to infinitesimally small values. Finally, RAM* encapsulates the radiative transfers in five summed infinite geometric series, which are subsequently labeled by subscripts connoting the related building envelope layers. PA ¼ PA1 + PA2 + ⋯ + PA"n ¼ ρP +

τ2P  τ2F  ρW

#

ð 1  ρF ρP Þ 2  ð 1  ρW ρF Þ τ2P  ρF  + ð 1  ρF ρ P Þ τ2F  ρW  ρP 1 ð 1  ρW ρF Þ  ð 1  ρF ρP Þ

PF ¼ PF1 + PF2 + ⋯ + PFn ¼ 

τ2F  ρW  ρP  τ2F  ρW  ρP 1 ð 1  ρW ρF Þ  ð 1  ρF ρP Þ

(5.2)

(5.3)

τP  τ2F  ρW FP ¼ FP1 + FP2 + ⋯ + FPn ¼

τ P  ρF ð 1  ρF ρP Þ 2  ð 1  ρW ρF Þ  + ð 1  ρF ρP Þ τ2F  ρW  ρP 1 ð1  ρW ρF Þ  ð1  ρF ρP Þ

(5.4)

τP  τF ð 1  ρF ρP Þ  ð 1  ρW ρF Þ  FW ¼ FW 1 + FW 2 + ⋯ + FW n ¼  τ2F  ρW  ρP 1 ð 1  ρW ρF Þ  ð 1  ρF ρP Þ

(5.5)

τ P  τ F  ρW ð 1  ρF ρP Þ  ð 1  ρW ρF Þ  WF ¼ WF1 + WF2 + ⋯ + WFn ¼  τ2F  ρW  ρP 1 ð 1  ρW ρF Þ  ð 1  ρF ρP Þ

(5.6)

Quantitative approximation of shading-induced cooling

89

All radiative properties can be computed by entering microclimate data. Although only AP, PA, PF, and FP are available, the algebra in Eqs. (5.2)–(5.6) can be determined using Solver in Microsoft Excel. Five empty cells are reserved for solving ρP, τP, ρF, τF, and ρW. A set of four columns tabulates the observed AP, PA, PF, and FP. Then, another set of five columns is filled by Eqs. (5.2)–(5.6) with cell reference to the required radiation properties, modeling PA, PF, FP, FW, and WF by multiplying with observed AP values. Next, the root-mean-square error (RMSE) between modeled and observed PA, PF, and FP is calculated. For optimization, all three RMSEs are summed in a so-called “objective cell” because Solver only solves a single objective at a time by changing the five radiation properties. Solver minimizes the “objective cell” using the GRG Nonlinear Engine under two constraints: (1) Radiative properties of plants add up to unity; and (2) Radiative properties of windows add up to unity.

RAM* can swiftly determine ρP, τP, ρF, τF, and ρW. Moreover, making use of Eq. (5.1), climber canopy absorptivity (αP) and window absorptivity (αF) can be calculated. Then, the climber canopy’s extinction coefficient (κ) is available by inputting the measured LAI: τP ¼ eKLAI

(5.7)

The improvement made by RAM* can be gauged by comparing the radiation properties against those determined using the original RAM.

5.3.4 Model application When the radiation properties become available, FW and WF, which were not monitored, can be subsequently computed by RAM* based on AP observations. For comparison, a building envelope featuring bare wall (BW) as shown in Fig. 5.5 was simulated by setting τP ¼ 1 to bypass the influence of the climber canopy layer, which is only featured on a green wall (GW). But due to the absence of the climber canopy in BW, AFBW indicates the solar radiation from the outdoor ambience (A) reaching the window (F), and vice versa for FABW. For brevity, FABW is calculated in the same manner as Eq. (5.2) and therefore is not shown. Integrating (FWGW  WFGW) and (FWBW  WFBW) with respect to time in a day produces the net energy gain of the indoor space behind GW (EGW, INDOOR) and BW (EBW, INDOOR). In the notation, the letter E denotes the net energy. For example, EGW, INDOOR can be obtained by integrating (FWGW  WFGW) with respect to time in a day. The disparity between net energy (E) between a particular location along the building envelope location of BW and GW is connoted using the subscripted expression BW-GW. For instance, the difference between EBW, INDOOR and EGW, INDOOR is expressed as EBW-GW, INDOOR, conveying the maximum potential reduction in cooling load. A positive figure indicates a higher value of BW than GW. To apply RAM*, the theoretical maximum and minimum shading-created savings throughout the summer of 2017 was quantified using savings in sunny and cloudy

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

Fenestration (F)

Wall (W)

Fig. 5.5 Schematic depiction of the revised radiation apportionment model (RAM*), modeling building envelope of bare wall (BW) with two layers, namely window (F) and indoor wall (W). The origin and destination of radiation fluxes are indicated by the first and second alphabets, respectively, adjoining the arrows. The difference from the green wall building envelope lies in the absence of the plant layer (P) and the posterior air gap (PAG).

scenarios, respectively. Objective selection criteria singled out sunny and cloudy days using meteorological records from a nearby government observatory (Hong Kong Observatory 2017a,b). A sunny day had above-seasonal-mean sunshine hours (6.0 h), above-seasonal-mean daily insolation (16.6 MJ), below-seasonal-mean cloud amount (75.8%), and zero daily total rainfall. A cloudy day had above-seasonal-mean cloud amount, below-seasonal-mean sunshine hours, below-seasonal-mean daily insolation, and below 10 mm daily rainfall. Only daytime data would be considered due to the daytime-exclusive presence of solar irradiance. The avoided cooling energy load in megajoules (MJ) was translated into power usage in kilowatt-hours (kWh) after adjusting by the air conditioner’s coefficient of performance (COP) at COP ¼ 3. Taking the carbon dioxide intensity (0.66 kg CO2/kWh) and the electricity tariff (0.148 USD/kWh) as reported by a local power company (CLP Group, 2019), the savings were translated into commonly understandable monetary and carbon terms, which were extrapolated to monthly and seasonal figures.

Quantitative approximation of shading-induced cooling

91

5.3.5 Radiative properties of climber green wall Schematic revision of shortwave radiative transfers in the RAM* allowed the quick determination of radiative properties, returning each climber canopy radiative property’s value, namely ρP ¼ 0.079, τP ¼ 0.366, and αP ¼ 0.555 (Table 5.2). Comparing to the original model, the sequence of the radiative properties remained unchanged: αP > τP > ρP. However, the RAM* rendered greater αP than any of those determined in other approaches, whereas τP was lower than previously found values. However, ρP identified using the RAM* lay in the range of the values bound by sunny and cloudy scenarios in the original model. The RAM* also advanced the idea of determining radiative properties of the fenestration (F) and indoor wall (W). In the previous model, their reflectivity values were manually decided on, assuming ρF ¼ 0.08 and ρW ¼ 0.5. However, slightly higher ρF but starkly lower ρW were found in the RAM*, with ρF ¼ 0.122 and ρW ¼ 0.117 (Table 5.2). Microsoft Excel Solver minimized possible errors arising from the manual decision of the radiative properties of the building shell layers. As the RAM* revealed τP, the climber canopy’s extinction coefficient (κ) was determined at κ ¼ 4.19 using Eq. (5.7) at LAI ¼ 0.24 (Table 5.1), which is considerably larger than the range values (3.34–4.00) found in the initial model. Such an increase in κ was attributed to the lower τP found by the updated RAM*, implying the climber’s greater ability to attenuate incoming shortwave radiation that would otherwise warm the building envelope. In addition, the RAM* determined a single, universal κ value rather than a weather-dependent range, reducing the bias due to inconsistent weather classification in estimating potential cooling benefits. Of course, as postulated in Eq. (5.7), the κ value still varies with the density of foliage per unit wall area which is expressed by LAI.

5.3.6 Energetic effects of shading generated by green fac¸ade The RAM* modeled the radiative processes befalling building envelopes featuring green wall (GW) and bare wall (BW) using a dataset of incoming shortwave radiation, therefore establishing the mathematical expression APGW ¼ AFBW. Nearly twice as much insolation occurred in sunny weather (11.19 MJ) than in cloudy (6.27 MJ) (Table 5.3). The overall reflectivity, known as albedo, of GW and BW was inquired from the ratios APGW:PAGW and AFBW:FABW, and was computed to be 0.11 and 0.19, respectively. BW had higher albedo because of the higher material reflectivity of glass Table 5.2 Radiative properties of the key layers of the building envelope with green wall (GW) as featured in Fig. 5.1. Building envelope

Reflectivity (ρ)

Transmissivity (τ)

Absorptivity (α)

Plant (P) Fenestration (F) Wall (W)

0.079 0.122 0.117

0.366 0.752

0.555 0.126

Table 5.3 Descriptive statistics of integrated energy in megajoules (MJ) at various positions along the building envelope of green wall (GW) and bare wall (BW) under (a) sunny and (b) cloudy weather conditions. Weather condition (a) Sunny

(b) Cloudy

Green wall

Mean Max Min SD Mean Max Min SD

Bare wall

APGW

PAGW

PFGW

FPGW

FWGW

WFGW

AFBW

FABW

FWBW

WFBW

11.19 12.64 9.80 1.28 6.27 9.64 3.38 2.69

1.24 1.41 1.07 0.16 0.69 1.05 0.38 0.28

4.16 4.70 3.64 0.47 2.33 3.58 1.26 1.00

0.78 0.88 0.68 0.09 0.44 0.67 0.24 0.19

3.17 3.58 2.78 0.36 1.78 2.73 0.96 0.76

0.37 0.42 0.32 0.04 0.21 0.32 0.11 0.09

11.19 12.64 9.80 1.28 6.27 9.64 3.38 2.69

2.12 2.39 1.85 0.24 1.18 1.82 0.64 0.51

8.54 9.64 7.48 0.97 4.78 7.36 2.58 2.06

1.00 1.13 0.87 0.11 0.56 0.86 0.30 0.24

The first and second alphabet letters abbreviate the origin and destination of the radiation flux, i.e., A, P, F, and W for ambience, plant, fenestration, and indoor wall, respectively.

Quantitative approximation of shading-induced cooling

93

(ρF ¼ 0.122) than the climber foliage (ρP ¼ 0.079) (Table 5.2). In other words, GW admitted more energy per unit of incoming shortwave radiation than BW. Deducing from the greater energy admittance of GW, the climber canopy has boosted the energy retention of the building envelope system. It would be relevant to wonder how cooling load reduction could be achieved under such heightened energy intake. Such a phenomenon was pronounced in sunny weather, when average net energy gain of GW (EGW, SYSTEM ¼ 9.96 MJ) well exceeded that of BW (EBW, SYSTEM ¼ 9.08 MJ) (Table 5.4). But when solar inputs were halved on cloudy days, GW still retained 0.50 MJ more shortwave energy than BW. This result brings into question the genuine effectiveness of climber-induced shading in procuring positive energy benefits. By analyzing the composition of GW and BW, shading-induced cooling was demonstrated as the items of the energy budget were elaborated. GW’s net indoor energy gain was obviously lower than that of BW. In sunny scenarios, EBW, INDOOR (7.54 MJ) noticeably exceeded EGW, INDOOR (2.80 MJ), creating a mean daily equivalent cooling of 4.74 MJ (Table 5.4). But the cooling scaled down to 2.65 MJ in cloudy conditions, as the subdued solar inputs partially nullified pro rata the effects of shading. Therefore, despite greater net energy gain by GW as a lumped thermoradiative system, canopy shading still effectuated and lowered indoor cooling demands. In addition, shading was evidenced by the stark disparity between FWGW and FWBW, verifying how climbers intercepted the shortwave radiation. FWGW was approximately 5.37 and 3.00 MJ lower than FWBW in sunny and cloudy weather conditions, respectively (Table 5.3). On the other hand, while FWGW was approximately 28.3% of the insolation, BW windows admitted a higher proportion of solar inputs, raising FWBW to about 76.3% of the insolation. In brief, the attenuation of solar penetration into the indoor space was responsible for the reduced cooling requirements observed with GW. Despite clarifications on indoor cooling benefits, one might still ponder (1) how GW lowered the indoor cooling load, and (2) where the rest of the GW energy surplus went. Given the identical building shell comprising the window and indoor wall, any GW-versus-BW energy budget disparities must be linked to the additional GW climber canopy, whose transmissivity (τP ¼ 0.05) connoted rather efficient shielding against incoming shortwave radiation. Consequently, the climbers as the outermost front of GW trapped a considerable proportion of APGW, allowing penetration of a relatively meager proportion as PFGW and hence effectively suppressing indoor warming (Table 5.3). The shielding effectiveness was quantified by (AFBW  PFGW), which reached 7.03 and 3.94 MJ in sunny and cloudy weather, respectively. This mechanism tackled the first query. Meanwhile, low τP but high αP implied sizeable recapturing of outgoing flux from window panes by climbers (FPGW). Eventually, a considerable proportion of EGW, SYSTEM ended up warming the climbers (Table 5.4). Alternatively speaking, a good proportion of the energy that ventured into the indoor space was returned to warm the climber vegetation instead of the indoor space, sparing building users from thermal discomfort and reducing airconditioning energy consumption.

Table 5.4 Descriptive statistics of net energy gain (E) and cooling in megajoules (MJ) of the whole building envelope system (SYSTEM), posterior air gap (PAG; green wall only), and indoor space (INDOOR) of green wall (GW) and bare wall (BW) under (a) sunny and (b) cloudy weather conditions. Green wall Weather condition (a) Sunny

(b) Cloudy

Mean Max Min SD Mean Max Min SD

Bare wall

Cooling

EGW,

EGW,

EGW,

EBW,

EBW,

EBW-GW,

EBW-GW,

SYSTEM

PAG

INDOOR

SYSTEM

INDOOR

SYSTEM

INDOOR

9.96 11.23 8.72 1.12 5.58 8.59 3.00 2.41

3.38 3.81 2.96 0.39 1.89 2.91 1.02 0.81

2.80 3.16 2.45 0.32 1.57 2.41 0.85 0.67

9.08 10.25 7.95 1.04 5.08 7.82 2.74 2.18

7.54 8.52 6.60 0.86 4.22 6.50 2.28 1.82

0.88 0.77 0.98 0.08 0.50 0.26 0.78 0.23

4.74 5.35 4.15 0.54 2.65 4.08 1.43 1.14

The definition of cooling is defined as the difference in net energy gain between BW and GW. A positive cooling value denotes a lower net energy gain of GW than of BW.

Quantitative approximation of shading-induced cooling

95

The energy savings of shading-induced cooling discovered by RAM* were translated into more understandable terms, namely reduction in power usage, carbon dioxide emission, and electricity tariff, based on a 1 m
1 m areal basis (Table 5.5). Assuming COP ¼ 3 for air conditioners, daily electricity consumption was slashed by 0.44 and 0.25 kWh in sunny and cloudy conditions, respectively, which also marked the maximum and minimum possible savings. The corresponding avoidance of carbon dioxide emission achieved 0.29 and 0.16 kg/day, depending on weather conditions. In monetary terms, the reduced power consumption transcended to daily monetary savings of USD 0.07 and USD 0.04 correspondingly. The savings computed using RAM* discernibly exceeded those from the original version of the model.

5.3.7 Incorporation of the radiation apportionment model in building design The RAM* can be applied as a tool to accurately estimate the shading-generated energy benefits in terms of power consumption, electricity tariff, and carbon dioxide emission. Building designers can employ the RAM* to justify and prioritize green wall installation. For example, on a building featuring vertical walls facing the four cardinal directions, namely north, east, south, and west, the south-facing wall would theoretically enjoy priority in receiving vertical greening due to its highest exposure to solar irradiance. However, shading from nearby structures in the dense and compact urban fabrics would counteract shading against solar radiation, which may be less influenced by nearby buildings at other orientations. Therefore, the designers can briefly monitor the on-site incoming solar radiation with respect to wall orientation. Entering the data into the RAM*, the conceivable savings engendered by shading can be predicted to develop informed selection of the orientation to be equipped with wall greenery. Apart from approximating the gross savings emerging from the shading, the RAM* could assist in the lifecycle cost-and-benefit analysis of green wall systems. In spite of Table 5.5 Estimated savings in electricity use (kWh), carbon dioxide emission (kg CO2), and electricity tariff (USD) generated by shading-induced cooling on a square-meter basis on the green wall in (a) sunny and (b) cloudy scenarios.

Weather condition (a) Sunny

(b) Cloudy

Daily average Monthly total Seasonal total Daily average Monthly total Seasonal total

Electricity use (kWh)

Carbon dioxide emission (kg CO2)

Electricity tariff (USD)

0.44 13.61 40.39 0.25 7.62 22.61

0.29 8.98 26.66 0.16 5.03 14.92

0.07 2.02 5.99 0.04 1.13 3.35

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

the attractive shading-induced cooling benefits, the financial expenditures and carbon dioxide emission arising from the production and transportation of green wall systems could be audited in detail. Accurate shading-generated savings computed by the RAM* boost the precision of comparison against the related costs. Thus, an informed decision can be made with emphasis on the environmental and economic feasibility of vertical greening. In short, the RAM* provides information for evidence-based decision making toward more sustainable urban planning.

5.4

Practical recommendations and future trends

5.4.1 Selection of vegetation species for green wall To optimize the vertical greening prescription, landscape architects can model thermoradiative interactions using the RAM* to match plant light requirements. Decisions can be made on growing heliophytes (sun lovers) or sciophytes (shade lovers). Different climber species feature different foliage dimensions in various arrangements. Not only does a climber canopy with high LAI shield more incoming radiation, but also hinders more outgoing flux. To approximate the sustainability of the building, available radiative properties data of various climber species can be fed into the RAM* aimed at striking a balance between plant performance and energy benefits. However, landscape designers must exercise caution regarding the thermal and mechanical implications of vegetation growth on a training system. First, in the RAM*, the term APGW
[(APGW  PFGW) + (FPGW  PWGW)  ρP] denotes climbers’ absorption of shortwave energy, including the training system. A metallic trellis or wire ropes may reach a high temperature that could thermally damage relatively fresh tissues. In our study location, some buds physically contacting the stainless steel wire ropes were dried and scorched. Second, wire rope distortion happened when climber stems extended from the designated wire rope to adjoining ones, to cause a complex entanglement of metal and plant tissues. Continued growth of lignified stems exerted tremendous pulling force and torsion, putting the wire ropes and the fastening bolts to test. Future research could measure the created tension and ways to reduce the interrope trespassing of climbers. Timely inspection and pruning could be considered before trimming is hindered by thickened, sturdy, and enmeshed branches without compromising plant health.

5.4.2 Recommended green wall design Properly designed green walls promote vegetation growth, procuring attractive ecosystem services. The two key design features of a green wall system are the growing medium and the PAG. The current study presented an exceptional experimentation into the dimensional traits of these two components. The 1 m-deep planter provided ample soil volume with sufficient moisture- and nutrient-holding capacity to promote plant health. The deep planter provided a sizable receptacle at the bottom of the PAG for collecting organic litter, which could release nutrients after natural decomposition.

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Nutrient supply would fuel active growth of soil microorganisms, boosting and maintaining plant vitality. However, the wire ropes of this study’s green wall were provided at the outer edge of the planter, meaning that the nutrients of the litter falling outside the planter were inadvertently lost. A possible improvement would be inserting a setback between the edge of the planter and the wire ropes. The exact distance depends on the expected depth of the climber canopy. The current green wall study featured a much deeper PAG than those in previous studies, which seldom exceeded 60 cm. Three possible thermal benefits apart from shading-induced cooling have been identified. First, a deeper PAG can accommodate a greater volume of air cooled by evapotranspiration, accelerating the convective heat dissipation of the warmed building envelope. Second, the climber canopy located farther from the outdoor window surface could more effectively taper down wind velocity within the boundary layer of the building shell surface. The enhanced insulation would decelerate convective heat transfer of the building shell from ambient warm air. Third, a deeper PAG implied that the wide planter would function as an awning for the floor below, providing extra physical shading. These benefits point toward a deeper PAG in future green wall designs.

5.4.3 Improved planter design To maximize shading-induced cooling, lush and dense climber canopy development can be encouraged by adjusting the training system and adopting an innovative planter design. Field experiments can be conducted to glean objective data to develop a package of precision green wall practice that is well informed by research. The growth performance of various climber species can be investigated using different dimensional specifications of the training system. For instance, the interrope distance can be adjusted to suit the growth of different climber species. Climber performance can be monitored by quantities such as height attainment and LAI with respect to time. The biomass production can be tracked to inform the provision of load-bearing capacity by the training system. The phenological records serve as reference for adapting the training system to suit the inherent traits of target climber species. With a refined training system, a healthy and wholesome climber canopy can be successfully established and sustained. Lee and Jim (2019c) have proposed a rotatable planter that can align the climber canopy perpendicular to the window after sunset. The realignment minimizes the view angle between the building shell and the warmed climber canopy during the daytime but maximizes the exposure to the cool nighttime sky. The resulting steeper radiativethermal gradient thus advances heat dissipation from the indoor space to the ambient environment. Meanwhile, convection replaces the warmed air residing inside PAG with relatively cooler air, recharging the potential of the heat-sink function of the PAG as an insulation layer masking the building shell. Overall, this new planter design optimizes and augments the thermal benefits of climber green wall systems.

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5.4.4 Using green fac¸ade on complex buildings The RAM* models the shading-induced cooling of a flat, vertically established wall with an easily identifiable orientation. It highlights the simplicity of predicting the shading-induced cooling on buildings with Bauhaus architecture, which emphasizes the functional aspects. The clearly delimited surfaces marked by sharp lines allow partial analyses of thermal-radiative behavior of a wall as a localized building feature. The prediction of shading-induced cooling showcases the strength of the RAM*. However, thanks to contemporary advances in building technologies, modernist or even futuristic architecture is manifested in complex building forms, which often feature hard-to-define surfaces to mimic twisting and stretching of extraordinary geometric shapes. Although vertical greening is a commonly used strategy of passive cooling, complex buildings pose a genuine challenge in predicting the shading-induced cooling. Indistinctive wall orientation, undulating surface curvature, and self-shading between adjacent walls complicate the mathematical calculations resolving the shading-induced cooling offered by green walls. A possible remedial measure is to divide a complex surface into smaller fractal units for conducting separate analyses based on the RAM*. The digitized model of a complex surface can be obtained with the aid of advanced technologies, such as three-dimensional laser scanning. The results can be numerically integrated to overall shading-induced cooling. The reliability of this approach rests on the acquisition of accurate information on the orientation and zenith of the divided units. Two technical alerts are provided here. First, climber propagation and establishment on geometrically complex surfaces may not be comparable to those on vertical walls. Surface inclination angle and phototropism jointly control the outcome of the vertical greening efforts. Spots where overabundance of or scanty climber growth is observed should be treated with pruning or fertilization, respectively. Second, the load-bearing capacity of the anchorage on which planters are hung must be checked. On complex buildings, irregularities in load distribution due to nonconstancy in surface dimensions implies that vertical greening should be factored in during the design stage to ensure sufficient reinforcement. Bearing in mind these technical cautions, the sound establishment of green walls can kick off their proper functioning.

5.4.5 Optimizing the multiple functions of green fac¸ade The RAM* testifies to the potential shading-induced cooling benefits. However, green walls concurrently provide many more ecosystem services. Comparable to the shading effect, the potential benefits of some ecosystem services increase with LAI. Increasing climber canopy leaf area density and depth raise the noise attenuation by effectively absorbing and dissipating more sound energy. Increase in foliage surface implies greater air purification function through dry deposition and absorption of airborne pollutants. For landscape beautification, a luxuriant canopy fulfills ornamental and decorative expressions in the designated landscape design. The RAM* can be used in tandem with other tools to gauge the performance of green walls in the context of urban ecological reconciliation.

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However, building managers should not solely pursue high-LAI climber canopy. There are at least three situations in which a high LAI may be unwelcome. First, solar penetration that inaugurates indoor cooling load in summer would turn into a soughtafter natural heating source in winter. Unnecessarily high LAI reduces summer cooling load at the cost of increased winter heating load. To remedy, building managers can operate the RAM* to find the “sweet spot” of LAI to minimize annual total energy consumption for air conditioning. Second, natural illumination decreases with increasing climber canopy closure and thickness. Power savings by shading-induced cooling may be partially offset by the additional power consumption of artificial lighting. To rectify, the shading-induced cooling can be determined by the RAM* at the level of adequate visible light for the purposed activity in the building. Third, excessively high LAI implies vigorous climber growth, which may jade the landscape quality when the vegetation canopy becomes unpleasant and messy. Keeping such a climber canopy neat and visually appealing renders maintenance pruning a laborious and difficult task (Chew et al., 2019). Therefore, clear goals should be formulated in optimizing the benefits of green walls.

5.5

Conclusion

Shading is a very important cooling mechanism of climber green walls. The potential reduction in cooling load generated by shading is computed by a revised and elaborated version of a radiation apportionment model, RAM*. Using pyranometer readings collected in front of and behind the climber canopy, the radiative properties of various layers of a building envelope with green wall retrofit can be reliably and swiftly determined. Using the obtained radiative properties, the shading-induced cooling in sunny and cloudy weather scenarios can be quantified. Extrapolating the estimated daily cooling to a seasonal total permitted the computation of energy savings and thus the equivalent reduction in power usage, electricity tariff, and carbon dioxide emission. The RAM* serves as a powerful tool in understanding shading as a principal cooling mechanism provided by green walls. Practical recommendations have been distilled from the research findings for practitioners handling green wall design and management. Landscape architects can make scientifically justified decisions on climber species selection and green wall specifications. Researchers can innovate planter designs to maximize the effectiveness of climber-generated cooling. Building architects can advance building design by incorporating green walls in their creations, preferably at the incipient state. Building owners and managers can boost building energy and amenity performance by balancing the desired goals of establishing green walls.

References Cameron, R.W., Taylor, J.E., Emmett, M.R., 2014. What’s ‘cool’ in the world of green fac¸ades? How plant choice influences the cooling properties of green walls. Build. Environ. 73, 198–207. Campbell, G.S., Norman, J., 2012. An Introduction to Environmental Biophysics, second ed. Springer Science & Business Media, Berlin.

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Cengel, Y.A., 2007. Heat and Mass Transfer: A Practical Approach, third ed. McGraw-Hill, New York. Chew, M.Y., Conejos, S., Azril, F.H.B., 2019. Design for maintainability of high-rise vertical green facades. Build. Res. Inform. 47 (4), 453–467. CLP Group, 2019. 2018 Sustainable report. Retrieved from: https://sustainability.clpgroup. com/en/2018. Di, H.F., Wang, D.N., 1999. Cooling effect of ivy on a wall. Exp. Heat Transfer 12 (3), 235–245. Feitosa, R.C., Wilkinson, S.J., 2018. Attenuating heat stress through green roof and green wall retrofit. Build. Environ. 140, 11–22. Hoelscher, M.T., Nehls, T., J€anicke, B., Wessolek, G., 2016. Quantifying cooling effects of facade greening: shading, transpiration and insulation. Energy Build. 114, 283–290. Hong Kong Observatory, 2017a. The Weather of June 2017. Retrieved from: https://www.hko. gov.hk/en/wxinfo/pastwx/mws2017/mws201706.htm. Hong Kong Observatory, 2017b. The Weather of July 2017. Retrieved from: https://www.hko. gov.hk/en/wxinfo/pastwx/mws2017/mws201707.htm. Lee, L.S., Jim, C.Y., 2019a. Energy benefits of green-wall shading based on novel-accurate apportionment of short-wave radiation components. Appl. Energy 238, 1506–1518. Lee, L.S., Jim, C.Y., 2019b. Multidimensional analysis of temporal and layered microclimatic behavior of subtropical climber green walls in summer. Urban Ecosyst. 23, 1–14. Lee, L.S., Jim, C.Y., 2019c. Transforming thermal-radiative study of a climber green wall to innovative engineering design to enhance building-energy efficiency. J. Clean. Prod. 224, 892–904. Sudimac, B., Ilic, B., Muncan, V., Anđelkovic, A.S., 2019. Heat flux transmission assessment of a vegetation wall influence on the building envelope thermal conductivity in Belgrade climate. J. Clean. Prod. 223, 907–916.

Experimental study of geometric configuration and evaporative cooling potential of brick elements

6

Philipp Lionel Molter and Ata Chokhachian TUM Department of Architecture, Technical University of Munich, Munich, Germany

6.1

Introduction

Brick structures can be classified as one of the most successful construction techniques for buildings in human history (Coffman et al., 1990). Due to the fact that clay can be found in most areas of human settlements, sun-dried bricks or even burned clay have been used to build shelters and buildings all over the globe during the last millenniums (Love, 2012). Human invention and technological progress have improved the robustness of bricks, leading to extensive use of these modular, weather-resistant, and waterproof building materials in aqueducts, bridges, and cisterns since early Hellenistic time (Ug˘urlu and B€oke, 2009). The brick is still a universal building module for architectural use worldwide, since approximately 30% of the world’s population lives in earthen brick-made structures, based on statistics from 1990 (Coffman et al., 1990). According to the compound annual growth rate (CAGR) for the brick product segment, the estimation is that used of bricks will increase by 3.5% during 2017–27 and it is anticipated to dominate over the forecast period (TMRGL, 2017). Scientific investigations into the structural behavior and performance of bricks and the impact of bricks on energy consumption as well as human comfort may still be questions needing to be addressed by researchers. However, investigations in building physics are an extensive field of research and increasing standards of energy regulations have already pushed brick innovation toward better U-values, especially in northern and central Europe (Wernery et al., 2017). The success of monolithic walls built from a single material as an approach captivates builders and planners by its simplicity and the avoidance of complicated details (Wernery et al., 2017). However, the assembly of brick modules as the smallest entity in a larger constructive reference system offers various possibilities of architectural expression and design alternatives. Diverse geometric configurations in a vast variety of patterns and complex placements allow multifaceted expressive forms of a repetitive brick module (Lynch, 2010). Bonwetsch states that the single brick is a tangible entity that retains its readability in the context of a larger assembly and is thus able to reveal the constructive logic behind it (Bonwetsch, 2015). This design freedom of

Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction https://doi.org/10.1016/B978-0-12-820791-8.00006-7 © 2021 Elsevier Ltd. All rights reserved.

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

bricks allowing multiple patterns resulting in ornamental expression has been explored throughout architectural history worldwide. A number of studies discuss traditional brick properties and patterns. Among the recent studies, Asdrubali et al. (2014) clarified the importance of material thermal flux to evaluate the in situ thermal transmittance of a material to determine actual wall performance. However, only few scientific literature reports can be found on the role of brick patterns and geometrical configurations in the thermal performance of the walls. Previous research has shown a significant energy-saving potential for cooling loads of internal spaces enveloped by monolithic brick walls. Previous studies have investigated the thermal performance and thermal transmittance of solar-exposed brickwork to interior spaces; most of those investigations have observed brick configurations showing flat geometries without three-dimensional patterns providing an extruded surface for self-shading. Rhee-Duverne and Baker (2013) examined the experimentation procedures and modeling software used in testing the thermal performance of various types of traditional solid brick walls. Findings indicate a reasonable relationship between self-shading geometrical configuration of walls and energy performance of the building. According to investigations, less solar absorption and self-shading exterior walls provide extensive potential for indoor as well as outdoor comfort (Tarabieh et al., 2017). The study shows a relationship between the brick bond types, the percentage of projection, and the volume of projection for the tested cases. Besides the indoor comfort and energy consumption effects of brick walls, several studies have proven that the large amount of radiation released through brick facades is partly responsible for increasing the mean radiant temperature, which has a negative influence on outdoor thermal comfort (Chokhachian et al., 2017). Radiative energy exchange accounts for a large share of heat transfer between the human body and the built environment, compared to convection or evaporation (Huang et al., 2014). In dense urban environments, the anthropogenic heat is stored and reradiated by urban structures, decreasing the comfort of the living environment and increasing energy consumption in summer (Rizwan et al., 2008). For this reason, there is a significance in influencing radiative heat transfer in order to improve urban microclimate and energy efficiency. Studies show that brick facades with low reflectivity in comparison with heavily insulated envelopes can decrease extreme heat stress for pedestrians by 26% during the day (Chokhachian et al., 2017). Additionally, there have been several studies on the evaporative cooling potential of building envelopes, with Han et al. (2017) having explored the effect of two passive cooling systems, water-retaining bricks on roof and radiation shield on roof, concluding that the maximum cooling capability can be achieved through on-roof water-retaining bricks. Another study explored the effects of a moist void-brick wall as a passive microclimatic converter and the results show that the wall surface temperatures are on average lower than the ambient air temperature by 5°C over a day (He and Liu, 2012). In previous research, thermal performance and energy analysis of external brick walls have been reported, but mostly those studies investigated traditional and static brick configurations. Only a few groups of researchers have investigated the thermal behavior of solar-exposed brick walls of changing brick patterns showing self-shading

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strategies: one group investigated the impact of brick bonds and the integration of parametric analysis with energy performance simulations conducted on a south facade for a typical residential room in a hot arid climate. A design of a performance-driven brick facade showed a substantial reduction of energy consumption (Abdelmohsen et al., 2019). Further investigation highlighted a significant energy consumption reduction for the base case, indicating a potential impact of nonuniform distribution patterns. The results found that a 60% Flemish bond with quarter brick extrusion was the optimal facade, reducing energy consumption by 27.75% in comparison to a standard Flemish bond facade (Tarabieh et al., 2018). The study showed further that the optimal facade reduced energy loads through the efficient balance between self-shading and thermal mass, only absorbing the required heat. This also reduces the external radiation of heat, improving the microclimate. The approach involved simulating quarter and half brick extrusions at a ratio ranging from 15% to 60% of the facade. This chapter investigates the thermal behavior of solar-exposed brick walls of different geometric configurations as well as the evaporative cooling potential of irrigated brick walls. The research investigates the relationship between the absorption of a solar-exposed wall and different geometrical configurations of brick patterns providing diverse self-shading strategies in comparison with the effect of temperature decrease as a result of evaporation cooling. The investigations have been conducted in field measurements as a comparative analysis of the surface temperature of the diverse patterns and irrigated bricks exposed to solar radiation. The results provide design strategies for brick walls to improve urban outdoor comfort and microclimate due to decreased surface temperatures.

6.2

Research methodology

This study used extensive field measurements of solar-exposed brick mock-ups of different geometric patterns with increased moisture on the brick surface, which were exposed to outdoor conditions on a warm summer day in Munich with a full south orientation. In the first step, the radiation patterns of the brick patterns showing different self-shading strategies were simulated using Grasshopper and Honeybee components. The self-shading of the walls was the result of a three-dimensional brick layering with cantilevers and voids in the walls. As the base case, flat walls without self-shading were simulated in comparison to patterns showing voids and cantilevers. The walls having the biggest decrease of solar radiation due to shading were chosen for one-to-one scale measurements. For the experiment, two different types of bricks in terms of color and materiality were chosen for two different measurement setups. The first measurement setup investigated the geometric configuration with beige bricks. This chosen color absorbs less radiation than darker bricks. Previous research has proven a significant reduction of surface temperature in a comparison of brighter colors of bricks (Molter et al., 2019). The second measurement setup investigated the impact of different self-shading strategies of brick patterns in combination with an evaporative cooling effect.

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The tested bricks of this second experiment were red bricks. Previous studies have proven that the effect of evaporation cooling is stronger with darker brick colors (Molter et al., 2019). The difference of surface temperature between dry bricks and irrigated bricks was higher than with bricks showing a lighter color.

6.3

Experiment: In-situ field measurements

6.3.1 Boundary conditions The objects were monitored on a sunny summer day (July 23, 2019) on a rooftop terrace at 28 m height above the ground floor in the city center of Munich (48.135125–11.581981). The mock-ups were based on concrete ground facing south and were exposed to the sun. The climate conditions showed high direct solar radiation. The climate conditions were measured using an Ahlborn Almemo system and were processed with WinControl V6 software. In order to validate the measured data, a second weather station was used, located at the same height on the rooftop of an adjacent building at 300 m distance (https://www.meteo.physik.uni-muenchen.de/wet ter/index.html). The observations showed that the air temperature reached a maximum of 31°C at 15:30, and stayed constant until 17:30. The radiation levels reached peaks around 910 W/sqm at 13:00 and then slowly decreased. The absorbed radiation heated the bricks, leading to significantly higher surface temperatures during the day (Fig. 6.1). For the experiment, two types of bricks of burned clay from the same manufacturer (GIMA) were chosen. The technical details of the bricks and their material characteristics are given in Table 6.1.

6.3.2 Geometrical configurations The experiment involving geometric configuration was the first investigation to evaluate the impact of different self-shading strategies on brick patterns. First, different self-shading geometric compositions were set up in a 3D modeling software in order to evaluate expected shadow projections and solar exposure potential. Different days representing different solar incident angles were examined: March 20, June 21, December 22, at 09:30, 13:30, and 16:30 h, respectively. The configurations that showed the most promising relationships between solar-exposed exterior surfaces and shaded wall areas were then selected and translated into the in-situ measurements of defined mock-ups (Fig. 6.2). The scale of the mock-up allowed the bricks to be placed in different positions without needing additional structures. The mock-ups showed different patterns and varying potentials to absorb solar radiation during a warm day with high direct solar radiation rates. On average, the mock-ups had 1 m2 of surface exposed to solar radiation. Fig. 6.3 illustrates five different wall patterns that were selected and observed. The following setup (Table 6.2) shows the different wall patterns and layering techniques. The mock-ups provided different shadows on the wall surface during different

Experimental study of geometric configuration

1000 800

850

105

910

900

730

750

850

770

780 700

650

600

650 560

510 400

500 400 230

200

300 100

0 09:30 h 10:30 h 11:30 h 12:30 h 13:30 h 14:30 h 15:30 h 16:30 h 17:30 h

(A)

Global radiation in W/m2

Diffuse radiation in W/m2

Air temperature in °C 35 30 25 20

(B)

09:30 h 10:30 h 11:30 h 12:30 h 13:30 h 14:30 h 15:30 h 16:30 h 17:30 h

Fig. 6.1 (A) Climate conditions during the measurement day: global radiation, diffuse radiation. (B) Climate conditions during the measurement day: air temperature.

Table 6.1 Technical details of the bricks used for the experiment. Product

Type

Luna

Perforated

Luna

Solid

Dimensions (mm)

Weight (kg/piece)

Perforation

290 115 52 290 115 52

2.5

15.5%

3.5

0

sun angles, which was the main objective in the choice of patterns. However, it is important to highlight that the patterns had different architectural expression due to their layering and visual appearance. Since the results of this study will provide design strategies for brick walls to improve urban outdoor comfort and microclimate due to decreased surface temperatures, the architectural expression of the walls is an important aspect of this research. Most of the studies cited in this chapter investigated with regard to physical behavior. However, since brick walls shape our cities and the urban environment, the architectural expression and visual appearance are important.

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Fig. 6.2 Experiment setup and process of measurements.

Fig. 6.3 Preselected patterns showing different self-shading strategies.

6.3.3 Self-shading strategies The brick patterns shown in Table 6.2 have layering geometries that differ from bond types, projections, and ornamentation of traditional brick walls. The tested patterns show extrusions in a layered wall as well as cantilevering brick rows projecting shadows for the layered rows below. The generated wall surfaces vary in their sky view factor and thus their solar exposure, leading to different solar absorption and surface temperatures. The surface temperatures of the mock-ups were monitored as an average of measurement points measured at defined strategic points of the different patterns of the facades. An average of three measurement points was chosen because

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Table 6.2 Geometrical configurations of the tested brick mock-ups. Configuration

Illustration

Description

Full bricks

A mock-up of a traditional flat surface as a running bond in a half brick offset allowing no self-shading

Full overhang

This mock-up consists of a two distinct rows. The first layer is made of perforated bricks. The second layer is made of solid bricks. These bricks are turned 90 degrees on their bed face, providing shadow for the lower (first) layer by a cantilever This type of wall is composed of two distinct layers, both out of perforated bricks. The first layer shows a row of bricks resting in their stretcher face. The next layer combines the bricks by placing both on their stretcher face. The bricks are interspersed in the following way: one that does not stand out, then the next brick lets the header face stand out This mock-up consists of two distinct rows. A first layer of perforated bricks rests in their stretcher face. The other layer of bricks is made out of full bricks. These bricks are supported on their bed face pulling out their stretcher face, shading the lower layer less than the individual overhang mock-up This next type of wall is composed of two distinct layers, with both layers of perforated bricks. One layer is supported on the stretcher face, leaving no gaps between the bricks In the other layer, the bricks are supported in their bed face, leaving a gap equivalent to half a brick between them. These bricks stand out on their header face and shade the layer below This type of wall is composed of two distinct rows, both of them using perforated bricks. A first row of bricks rests on their stretcher face, leaving no gaps. Another row is layered on top; the bricks are resting on their bed, rotated at 45 degrees. This layer shades the layer below

Individual overhang

Half overhang

Perforated wall

Rotated bricks

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

Fig. 6.4 Average of three measurement points.

of the fact that some parts of the walls were exposed to the sun throughout the day, others remained in shadow, and the rest varied depending on the time of day (Fig. 6.4). The extrusion percentage seems to play a significant role in the results of the shaded area and the average surface temperature. The different patterns show significant differences in the shaded areas of the mock-ups. Configuration c. (full overhang) shows a shaded surface of 70% during the time of the measurements (Figs. 6.5 and 6.6, Table 6.3).

6.3.4 Irrigated bricks and evaporative cooling potential Previous research of the authors has proven a significant reduction of surface temperature due to the evaporative cooling effect of irrigated bricks (Molter et al., 2019). The study confirmed that irrigated bricks can have 7°C lower surface temperatures in comparison to dry bricks. Other research in Cairo (Abdelmohsen et al., 2019) investigated the self-shading potential of diverse brick patterns in simulations to examine the thermophysical behavior of traditional masonries using parametric software. The study aimed to identify the optimal brick pattern for external walls using a self-shading strategy for reduced cooling loads. Based on this outcome, a second in-situ experimentation was undertaken to evaluate the impact of different self-shading strategies of brick patterns in combination with evaporative cooling effect. First, a solar exposure analysis was conducted using the Grasshopper and Honeybee tools. As seen in Fig. 6.7, the so-called Flemish bond

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109

100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 09:30 h 10:30 h 11:30 h 12:30 h 13:30 h 14:30 h 15:30 h 16:30 h 17:30 h Regular wall

Rotated bricks

Half overhang

Individual overhang

Full overhang

Perforated wall

Fig. 6.5 Percentage of shaded surface during measurement day.

45 40 35 30 25 20 09:30 h 10:30 h 11:30 h 12:30 h 13:30 h 14:30 h 15:30 h 16:30 h 17:30 h Regular wall

Rotated bricks

Half overhang

Individual overhang

Full overhang

Perforated wall

Fig. 6.6 Average surface temperature during measurement day.

was observed in different variations: solid, extruded, and perforated. The variations show different levels of sun exposure of the wall surfaces. Hereupon, the research design for the in-situ measurements showed a comparison of the evaporative cooling effect on a wet setup to exactly the same setup (a solid brick wall) as a dry wall. The process was repeated for an extruded brick facade and a perforated brick facade. The extruded brick structure chosen was a 60% extruded Flemish brick wall with quarter brick extrusion. This is based on the study conducted in Cairo.

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

Table 6.3 Average surface temperature during measurement day.

09:30 h 10:30 h 11:30 h 12:30 h 13:30 h 14:30 h 15:30 h 16:30 h 17:30 h

Regular wall (°C)

Rotated bricks (°C)

Half overhang (°C)

Individual overhang (°C)

Full overhang (°C)

Perforated wall (°C)

31.25 33.55 35.75 40.15 41.40 42.75 43.05 38.55 33.55

27.00 30.73 32.58 36.00 37.39 39.63 39.89 36.90 33.00

29.21 31.69 33.19 36.48 37.01 36.55 35.62 33.43 31.32

29.72 32.40 33.47 36.00 36.11 35.41 35.44 32.26 30.41

26.61 29.29 30.74 33.72 34.36 34.17 35.06 31.33 30.23

27.00 29.77 31.24 34.57 34.90 35.72 35.98 33.43 32.15

Fig. 6.7 Solar exposure analysis on Grasshopper and Honeybee of so-called Flemish bond in different variations: solid, extruded, and perforated. Total surface facing south: solid 0.687 m2; extruded 1.35 m2; perforated 0.635 m2; surface exposed only to sun during the day: solid 100%; extruded 8%; perforated 80.5%.

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111

The configurations that showed the most promising relationship between solarexposed exterior surfaces and shaded wall areas were selected and translated into the in-situ measurements of the defined mock-ups (Figs. 6.8 and 6.9). The mock-ups were based on concrete soil in a full southern orientation. The surface temperature and correlated surface humidity were observed to evaluate the cooling effect of shadowing and evaporation. The mock-ups were exposed to the sun. The absorbed radiation heated the bricks, leading to significantly higher surface temperatures during the day (Fig. 6.10). An average of three measurement points was chosen because of the fact that some parts of the walls would be exposed to the sun throughout the day, others would remain in shadow, and the rest would vary depending on the time of day (Fig. 6.8).

Fig. 6.8 Thermal image of solar-exposed mock-up: perforated dry at 14:00 h.

Fig. 6.9 Experimental setup and process of measurements.

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

Fig. 6.10 Temperature curves in °C of solar-exposed mockups: (1) extruded dry; (2) extruded wet; (3) perforated dry; (4) perforated wet; (5) solid dry; (6) solid wet.

55 50 45 40 35 30 25 20 13:30 h

14:30 h

15:30 h

Extruded wet Perforated wet

6.4

16:30 h

17:30 h

18:30 h

Extruded dry Perforated dry

Results

The results of the experiments show a significant relationship between surface shadowing and surface temperatures. The standard solid constructed brick wall yields similar results to the research previously conducted by the authors (Molter et al., 2019). The results showed a 7°C difference between the wet and dry brick. The following results show the different effects of different geometric patterns (brick bonds) in combination with the evaporative cooling effect. The comparison of the three different brick bonds in their dry situation throughout the day showed that the extruded brick bond performed the best, due to the large amount of self-shading. The average reduction in surface temperature between the dry solid and the dry extruded brick bond was 4.5°C, and 6.2°C between the dry perforated and the dry extruded brick bond. The solid and the perforated brick bond started in the same way, but by the end of the day the perforated bond had a slightly higher surface temperature. Throughout the day they had an average difference in surface temperature of 1.6°C. This is supported by the fact that the perforated brick bond had more sun exposure per brick and therefore a greater capability to heat up internally. Throughout the day the wet extruded brick bond performed the best, with an average reduction in surface temperature of 3.1°C between the wet solid and the wet extruded brick bond and 2.0°C between the wet perforated and the wet extruded brick bond. Due to the fact that the perforated brick bond had more surface exposed per brick, its ability to utilize the evaporative cooling effect increased. This is why, in the end, the wet perforated brick bond outperformed the wet solid brick bond with an average surface temperature difference of 1.1°C. The comparison between the wet solid brick bond and the dry extruded brick bond showed that the latter had a better performance, with an average difference in surface temperature of 1.2°C. Due to the fact that a lot of water evaporates at the beginning of the day, the dry extruded brick bond and its self-shading effect increased in effectiveness by the end of the day relative to the wet solid brick bond. However, the wet perforated brick bond had a better performance than the dry extruded brick bond throughout the whole day, with an average surface temperature reduction of 4.8°C. The analysis of the six different setups together showed that the

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wet extruded brick bond had the overall best performance. The total reduction in surface temperature by the end of the day between the dry solid brick bond (base case) and the wet extruded brick bond (optimal case) was 13.1°C. The mean difference throughout the day was 9.4°C. Even though the perforated bond had more potential of utilizing evaporative cooling, the combination of self-shading and a small fraction of evaporative cooling performed better. This is due to fact that self-shading is a consistent effect and evaporative cooling lightly wears off through the day. The average reduction in surface temperature from the wet perforated brick bond to the wet extruded brick bond was 2.0°C.

6.5

Conclusion

In this study we experimented with different brick element configurations to quantify the potentials of formal optimization on surface temperatures. The results showed that the geometry of the pattern design has a strong impact on thermal performance of the brick facades, which can potentially have an indirect impact on outdoor spaces, since facade surface temperature can be significantly decreased and have a positive influence on mean radiant temperature on a local scale. In terms of solar exposure, the results show the highest values for the case of perforated and lowest for the extruded configuration. Moreover, the combination of self-shading and evaporative cooling (the extruded wet setup) results in the most desirable surface temperatures. The efficient balance between self-shading and sun exposure is effective in decreasing the surface temperature of the facade, significantly contributing to an improved microclimate. Previous research has shown the success of evaporative cooling and self-shading separately. This research chapter introduced the possibility of combining these aspects to further enhance microclimates in cities. Therefore, the hypothesized “extruded wet” experiment proved most effective in improving microclimates by the means of lowering fac¸ade surface temperatures. The “extruded” experiments showed the least difference between wet and dry readings, averaging only 4.8°C. However, the addition of self-shading resulted in the lowest overall mean temperature. The temperature difference between the “solid dry” experiment (base case) and “extruded wet” is 10.1°C. This further emphasizes that the combination of extrusion and water can decrease surface temperature and improve microclimates substantially. On the basis of the simulations and in-situ measurements, the experiment can be expanded to better understand the relationship between self-shading and evaporative cooling. In order to realize the experiment in building envelopes, the experiment requires the integration of an irrigation system, to test the watering technique.

6.6

Future work

Most research conducted in relation to self-shading strategies and irrigation of bricks has investigated the building physics and thermal comfort of brick walls. The aim of the researchers is to link aspects of physical engineering to architectural and design

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aspects of brick-made structures, since urban outdoor comfort and microclimate is strongly influencing the architectural quality of our cities. The focus area is the impact of performative brick patterns on the architectural expression of the walls. Future work will also provide information and guidelines as well as design strategies for urban brick walls to improve urban outdoor comfort and microclimate due to decreased surface temperatures. However, since brick walls shape our cities and the urban environment, the architectural expression and visual appearance of the walls is an important aspect that needs to be considered.

Acknowledgments The studies discussed in this chapter were part of a larger research project at Technical University of Munich TUM. Within this framework, several junior researchers, research associates, and the authors were involved. The authors would like to thank: Philippe Bareiss, Harsh Bavishi, Keshava Narayan, Himadri Panchal, Mathias Spiessens, Aaron Sa´nchez, Andrea Contreras, Sebastian Palacios, Jaume Bernabeu, Beatriz Sa´nchez, and Eva Gonza´lez.

References Abdelmohsen, S., Tarabieh, K., Elghazi, Y., Hassan, A., El-Dabaa, R., Ibrahim, I., 2019. Coupling parametric design and robotic assembly simulation to generate thermally responsive brick walls. In: Building Simulation Conference, Rome, Italy. Asdrubali, F., D’Alessandro, F., Baldinelli, G., Bianchi, F., 2014. Evaluating in situ thermal transmittance of green buildings masonries—a case study. Case Stud. Construct. Mater. 1, 53–59. Retrieved from: http://www.sciencedirect.com/science/article/pii/S2214509514000102 https://doi.org/10.1016/j.cscm.2014.04.004. Bonwetsch, T., 2015. Robotically Assembled Brickwork: Manipulating Assembly Processes of Discrete Elements. ETH Zurich. Chokhachian, A., Perini, K., Dong, S., Auer, T., 2017. How material performance of building fac¸ade affect urban microclimate. In: Paper presented at the Powerskin 2017, Munich, Germany. Coffman, R., Agnewl, N., Austin, G., Doehnel, E., 1990. ADOBE MINERALOGY: characterization of adobes from around the world. In: Paper presented at the 6th International Conference on the Conservation of Earthen Architecture: Adobe 90. Han, R., Xu, Z., Qing, Y., 2017. Study of passive evaporative cooling technique on waterretaining roof brick. Procedia Eng. 180, 986–992. Retrieved from: http://www. sciencedirect.com/science/article/pii/S1877705817317654. https://doi.org/10.1016/j.pro eng.2017.04.258. He, J., Liu, K.Q., 2012. Numerical analysis of passive microclimatic-modifying effects of a moist void-brick wall. Appl. Mech. Mater. 193–194, 1156–1164. Retrieved from: https://www.scientific.net/AMM.193-194.1156. https://doi.org/10.4028/www.scientific. net/AMM.193-194.1156. Huang, J., Ceden˜o-Laurent, J.G., Spengler, J.D., 2014. CityComfort +: a simulation-based method for predicting mean radiant temperature in dense urban areas. Build. Environ. 80, 84–95. Retrieved from: http://www.sciencedirect.com/science/article/pii/S0360132 314001693. https://doi.org/10.1016/j.buildenv.2014.05.019.

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Love, S., 2012. The geoarchaeology of mudbricks in architecture: a methodological study from C ¸ atalh€oy€uk, Turkey. Geoarchaeology 27 (2), 140–156. Retrieved from: https:// onlinelibrary.wiley.com/doi/abs/10.1002/gea.21401. https://doi.org/10.1002/gea.21401. Lynch, G., 2010. The History of Gauged Brickwork. Routledge. Molter, P.L., Fellner, J., Chokhachian, A., 2019. Adaptive bricks: potentials of evaporative cooling in brick building envelopes to enhance urban microclimate. In: Powerskin Conference Proceedings, Munich. Rhee-Duverne, S., Baker, P., 2013. Research Into the Thermal Performance of Traditional Brick Walls. English Heritage, London. Rizwan, A.M., Dennis, L.Y.C., Liu, C., 2008. A review on the generation, determination and mitigation of Urban Heat Island. J. Environ. Sci. 20 (1), 120–128. Retrieved from: http:// www.sciencedirect.com/science/article/pii/S1001074208600194. https://doi.org/10.1016/ S1001-0742(08)60019-4. Tarabieh, K., Abdelmohsen, S., Elghazi, Y., El-Dabaa, R., Hassan, A., Amer, M., 2017. Parametric investigation of three types of brick bonds for thermal performance in a hot arid climate zone. In: Paper presented at the Proceedings of the 32nd International PLEA Conference DESIGN TO THRIVE. Tarabieh, K., Abdelmohsen, S., Hassan, A., El-Dabaa, R., Elghazi, Y., 2018. Parametric investigation of brick extrusion patterns using thermal simulation. In: Paper presented at the 4th Building Simulation and Optimization Conference, Cambridge, UK. TMRGL, 2017. Concrete Block and Brick Manufacturing Market (Product Type—Concrete Block (Hollow, Cellular, and Fully Solid), Brick (Clay, Sand Lime, and Fly Ash Clay), and ACC Block)—Global Industry Analysis, Size, Share, Growth, Trends, and Forecast 2017–2027. Ug˘urlu, E., B€oke, H., 2009. The use of brick–lime plasters and their relevance to climatic conditions of historic bath buildings. Constr. Build. Mater. 23 (6), 2442–2450. Retrieved from: http://www.sciencedirect.com/science/article/pii/S0950061808003267. https://doi. org/10.1016/j.conbuildmat.2008.10.005. Wernery, J., Ben-Ishai, A., Binder, B., Brunner, S., 2017. Aerobrick—an aerogel-filled insulating brick. Energy Procedia 134, 490–498. Retrieved from: http://www.sciencedirect.com/ science/article/pii/S1876610217347410. https://doi.org/10.1016/j.egypro.2017.09.607.

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Eco-efficient evaporative and ground-coupled system with terra-cotta evaporative walls

7

` sa, Guilian Lerouxa,b, and Etienne Wurtzb Nolwenn Le Pierre a
LOCIE, Universite Savoie Mont Blanc, CNRS UMR5271, Le Bourget du Lac, France, b Department of Solar Technologies, Universit
e Grenoble Alpes, CEA-LITEN, Le Bourget du Lac, France

Nomenclature k m_ T θ Φ φ

permeability (m2) water flux (kg/s/m2) temperature (°C) moisture content (m3/m3) power (W) air relative humidity (%)

Subscripts cap conv dew dif dr eq ev ext int out pr

capillarity convection dew point diffusion drop equilibrium evaporation outside air intrinsic outside surface pressure

7.1

Introduction and state of the art of evaporative cooling systems

To decrease a building’s cooling demand, passive techniques such as solar protection and inertia should be used. But these solutions are difficult to control and may not be effective enough to offer appropriate thermal comfort in a building. Consequently, Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction https://doi.org/10.1016/B978-0-12-820791-8.00007-9 © 2021 Elsevier Ltd. All rights reserved.

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systems to extract and dissipate overheat from the building are often needed. In their report on the state of the art of passive cooling dissipation techniques for buildings, Santamouris and Kolokotsa (2013) list three main axes: –

– –

The first one uses the high inertial potential of the ground. Indeed, the temperature of the ground at a certain depth remains nearly constant throughout the year and thus represents a potential heat sink during the summer. Ground cooling systems have been widely studied. The results, described in the IEA report (International Energy Agency, 1997) for an earth-toair heat exchanger, give a potential of 6–30 W/m of pipe (diameter: 20 cm) depending on the outside temperature, building location, depth of the pipe, etc. The second axis is based on ventilation. The idea is either to increase the speed of the air in the building to decrease the discomfort sensation or to adapt the ventilation flow rate according to the daily outside conditions (Artmann et al., 2007). The third axis concerns the endothermic phenomenon of evaporation of water in contact with dry air, called evaporative cooling or adiabatic cooling.

Over the last few years, evaporative cooling has received considerable attention and different systems have been developed. Most popular applications consist of cooling the building envelope (with green roofs (He et al., 2016), roof ponds (Spanakia et al., 2011), pipes embedded in wall (Shen and Li, 2016)) or cooling the air ventilation flow. Among the latter, direct water evaporation in ventilation air flow is the simplest and best-known technique. The literature also shows the growing interest in and high performance of more complex systems using heat exchangers, called indirect evaporation systems (Duan et al., 2012; Yang et al., 2019). For instance, in an experimental study on indirect evaporative cooling systems, Bruno (2011) reports a coefficient of performance (COP) from 6 to 20, depending on outside conditions with a quasilinear relationship between the COP and the difference between the air temperature and its dew point. Duan (2011) also developed an indirect evaporative cooler with a countercurrent heat and mass exchanger. He obtained COP values from 3 to 12 and showed that performance depends on the incoming air velocity, temperature, and humidity. Porous material can be used as evaporative surfaces with high durability and availability and low cost. Ibrahim et al. (2003), Velasco Gomez et al. (2010), and more recently Boukhanouf et al. (2017) present systems with hollow bricks filled with water set in the ventilation airflow. Depending on the air condition, Boukhanouf reports good performance with a maximum cooling capacity of 225 W/m2 of wet surface. Ibrahim et al. (2003) show similar performance with maximum measured cooling of 224 W/m2 of evaporator surface. Quadratic formulae relating the cooling surface power to the difference between ambient and saturated vapor pressure were derived. More recently, another evaporative cooling system using porous ceramic tubes was evaluated by the same authors (Boukhanouf et al., 2018). For this porous tube design, tests showed a specific cooling capacity of 140 W/m2 of wet ceramic surface and an overall COP of 11. Velasco Gomez et al. (2010) designed a system for both direct and indirect cooling. The performance of these systems improves when outdoor air temperature increases, usually related to low values of relative air humidity. To increase heat and mass transfer in evaporative cooling, Chen et al. (2018) recently proposed a novel configuration of fiber bundle with polymer hollow fibers through which water can permeate, to replace the porous bricks. They could reach a very high cooling capacity of about 800–1000 W/m2 for dry

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air conditions (relative humidity 32%–40%). Further research is to be performed on the packing fraction of the hollow fiber module. Membrane-assisted evaporative cooling is also presented as one of the innovative enhancement techniques for this technology (Yang et al., 2019). However, polymeric membranes are usually fragile, so further research is needed for their development and implementation. Another heat sink is radiative cooling. It uses the low temperature of the sky for radiative exchanges. It was ruled out by Santamouris and Kolokotsa (2013) in their state of the art on passive cooling dissipation techniques for buildings, because of its low power density. However, a review by Lu et al. (2016) reported experimental studies on radiative cooling systems and showed a cooling potential of 40–60 W/m2, highly dependent on the cloud coverage and ambient air humidity. All these studies show the significant potential and the growing interest in lowenergy cooling systems. Nevertheless, these systems are still not widespread because of their cost, installation difficulties (in the case of ground cooling), and varying performance depending on the ambient conditions. A low-consumption and eco-efficient cooling system for buildings was thus imagined and patented (Stephan, 2012). The main idea was to propose a low-tech system, which means low embodied energy, low cost, easy installation, and low maintenance, which could also present high efficiency. This requires the system to use simultaneously different heat sinks (evaporative cooling, radiative cooling, and ground cooling) to dissipate excess heat from the building. It is based on a typical building wall porous material: terra-cotta. This chapter presents a study of this cooling system. In the first section the principle of the system is described. The second section details the terra-cotta material that is optimal for the wall of this system, and in the last section, the impact of this eco-efficient system on the building behavior is analyzed.

7.2

System description and operating principle

The following description of the system focuses on a single-family house application. It could also be used, at a larger scale, for a collective building. The main components of the system—illustrated in Fig. 7.1A—are a storage tank (1) (situated in the basement or crawl space of the house); a heat exchanger (which is used as a heating floor during the winter and can be used as a cooling floor during the summer period) in the concrete slab of the building (2); a porous tank (set outside the building along a shaded fac¸ade), also called an evaporator (3); and a water pump (or circulator) (4). The system follows a daily cycle. During the daytime, when the indoor house temperature exceeds the set comfort temperature, the pump (4) is turned on. Cool water passes from the storage tank (1) to the cooling floor (2), where it removes heat from the building and is then transferred back to the porous tank (3). When this porous evaporator is full, the three-way valve (9) sends water back from the cooling floor directly to the storage tank. During the following night, the water in the evaporator (3) is cooled due to evaporation, radiation, and convection at the porous tank surfaces. In the morning, as soon as the temperature of the water contained in the porous tank

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

Outside

6: Overflow pipe

7: Fill in pipe

Inside

3: Evaporator

9: Three-way valve 5: Automatic valve Ground

2: Cooling floor

Crawl space 8: Draining pipe 4: Pump

1: Water storage tank

(A)

(B) Fig. 7.1 Cooling system integrated into a dwelling: (A) design, (B) possible implementation.

(3) increases, the automatic valve (5) opens and cool water flows from the evaporator into the storage tank (1). This closes the cooling cycle. The evaporator (3) is a vertical flat tank that can be installed at about 30 cm along the northern wall of the dwelling, with porous surfaces (top, bottom, and sides, Fig. 7.1B). This tank should receive as little solar radiation as possible, to avoid

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heating up the tank during the day and to take advantage of the water evaporation to cool down the water inside the tank. The porous material proposed for this tank is terra-cotta because of its very low cost, low embodied energy, and its adequate properties for the evaporation process. The storage tank (1) makes use of ground cooling because it is installed in the basement under the house and is thus in close contact with a large surface of ground. The storage tank can be a plastic flexible container, making its installation in the basement easy and its price low. This component is used both as a storage and as a heat exchanger with the ground. This system simultaneously uses different heat sinks, thus enhancing its overall cooling potential, and in addition takes advantage of the complementarity of the various heat exchanges: the heat dissipation into the ground, which is continuous and slightly dependent on the outside conditions, and the heat dissipation by the evaporator (by convection, radiation, and evaporation), which has a higher potential but is more dependent on the ambient air conditions. The materials and components used for this system are low-tech and low cost and can be installed quickly on existing buildings. The system can also be connected to various existing heating facilities such as heating floors and water-to-air heat exchangers.

7.3

Terra-cotta characteristics and evaporative tank behavior

7.3.1 Functioning principle of the terra-cotta evaporator One of the most important components of the system is the terra-cotta tank on the surface of which the radiative, convective, and evaporative phenomena occur. Terracotta material was selected for this application, thanks to its well-known porosity and permeability that should allow water to cross the wall with the right flow. Moreover, terra-cotta is a natural material that contains low embodied energy and that is widely available. This material has also been used for a long time in arid climates for cooling applications. As it has been used for centuries to make bricks in the building sector, the industrial processes necessary for its production are already existing and widespread, making this material cheap and accessible. Finally, the idea for this system is to be easily integrated from an architectural point of view along a building, and the use of already existing construction materials could help in making this system accepted and adopted from an aesthetic point of view (Fig. 7.1B). The porous structure of the walls and the geometry of the tank have a great impact on its cooling performance. The evaporator wall is a porous material with ambient air on one side and on the other side either liquid water (when the tank is full) or air (when the tank is empty) or both (when the tank is partly filled) as shown in Fig. 7.2. When the tank is filled with water, water flows through the porous wall from inside to outside due to: – –

the capillary forces of the porous structure, creating the corresponding mass flux m_ cap ; and the liquid pressure due to the water column in the tank creating the corresponding mass flux m_ pr .

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Fig. 7.2 Evaporator terra-cotta wall behavior.

In this porous material, water is liquid on the tank side but phase change can occur and the vapor phase is then transported by Fick diffusion, creating the corresponding mass flux m_ dif . On the outside surface of the wall, water: – –

is transported into the ambient air by convective mass transfer, creating the corresponding evaporation mass flux m_ ev or forms drops that run off if the evaporation potential is low, creating the corresponding mass flux m_ dr . This water flux m_ dr is lost to the surroundings of the system, does not contribute to the evaporative cooling effect, and should thus be minimized in the system.

Evaporation thus can happen both inside the porous wall and at the external surface of the evaporator wall. The water mass balance (illustrated in Fig. 7.2) of the evaporator wall can be expressed as: m_ pr + m_ cap + m_ dif ¼ m_ ev + m_ dr

(7.1)

Capillary migration and vapor diffusion flux in the porous material m_ cap and m_ dif are strongly dependent on moisture content gradients in the wall. Indeed, capillary forces are null if the moisture content gradient is null, for example when the whole porous material layer is saturated by liquid water. In these conditions, the water flux through the wall is equal to the pressure driven mass flux m_ pr alone. Consequently, three cases are of interest: –

First case: m_ pr > m_ ev

In this case, the convective evaporation mass transfer flux m_ ev at the outside surface of the evaporator is lower than the pressure-driven mass flux m_ pr inside the material; which means that all the liquid water that reaches the outside surface of the wall cannot be evaporated. The porous material is thus fully saturated with liquid water; the volumetric moisture content in the wall is maximal and equal to the porosity of the

Eco-efficient evaporative and ground-coupled system with terra-cotta evaporative walls

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porous material. The water flux in the material is only due to water pressure as the capillary flux m_ cap is null, since the moisture content gradient is null. The remaining liquid from the pressure-driven mass flux that is not evaporated at the outside surface creates drops that run off on the outside surface of the wall (m_ dr ¼ m_ pr  m_ ev ). In this case, evaporation at the outside surface of the wall is the limiting phenomenon of the evaporator. –

Second case: m_ ev > m_ pr

In this case, evaporation flux at the outside surface of the wall is larger than the pressure-driven mass flux inside the material. This means that the porous material is not saturated. This creates a moisture content gradient across the porous wall (along x, Fig. 7.2) and capillary forces must be taken into account. Phase change occurs within the porous structure and vapor migrates to the external surface by diffusion. In these conditions, the water transfer through the porous wall is the critical phenomenon, the evaporation potential is not fully exploited, and the cooling effect is not maximal. –

Third case: m_ ev ¼ m_ pr

This last case is the ideal functioning condition of the system, when no water is lost at the outside of the wall by droplets running off, and a saturated wall allows full evaporation potential at its outside surface. In this case, the cooling of the water inside the tank is optimal. In a given tank, all the cases can happen, depending on several parameters, which include the filling of the tank with water and the ambient air conditions, which change throughout the day and throughout the year. Moreover, at a given time, as a function of the height in the evaporator, the three cases can happen simultaneously, as the water pressure inside the wall that increases with the depth of water increases the pressure water flux through the wall (Fig. 7.2). The ideal material for this evaporative cooling application is therefore a wall material that allows for maximum evaporation rate and minimum runoff water losses throughout the whole season functioning. Given the impact of the pressure inside the tank and the material-specific transfer properties of the coupled heat and mass transfers through the evaporator wall, it is necessary to define at the same time the suitable material and the geometry of the tank. To find a suitable tank to meet these conditions and calculate the performance of this evaporator, a numerical model can be set up, and long-term simulations should be performed to take into account the different weather conditions that would be encountered by the system during the cooling season. This model was presented in (Leroux et al., 2019a).

7.3.2 Terra-cotta characteristics As weather conditions differ highly depending on localization, the characteristics of the material and the geometry of the tank should be adapted to these local constraints. The hydraulic conductivity of a porous medium determines the amount of liquid that can pass through it under a pressure gradient. This parameter is commonly used for

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

liquid transfers (Darcy’s law). It depends on the viscosity of the fluid, the porous structure, and the volumetric moisture content and gradient of the fluid. A good knowledge of this hydraulic conductivity is thus necessary, in the case of the presented evaporator. The hydraulic conductivity can be expressed as a function, in particular, of the relative permeability and the intrinsic permeability of the porous medium. The relative permeability is a coefficient that makes it possible to account for the dependence of the hydraulic conductivity with respect to the volumetric liquid fraction. The intrinsic permeability kintr of the porous medium represents the ability of a porous medium to be crossed by a fluid. Its value is solely a function of the porous medium geometry and is independent of the fluid. It is this important parameter that can be optimized for choosing the best terra-cotta for the walls of the evaporator. In many cases, terra-cotta bricks are produced using an extrusion process. In that case, the characteristics of the surface of the bricks are very different from the characteristics of the core of the material. A “skin” is indeed formed during the extrusion process, at the surfaces of the bricks that touch the extrusion mold. This clay skin strongly reduces the average intrinsic permeability of the whole wall, and should then be avoided, or removed. In this last case, a simple sandblasting of the tank wall after extrusion and drying is sufficient to remove the clay skin. A model was developed and validated to represent the functioning of this type of terra-cotta wall (Leroux et al., 2019a). In a case study with ambient air temperature of 22°C, relative humidity of 50%, and air speed of 3 m/s, with typical terra-cotta characteristics as presented in Table 7.1, simulations showed that for low permeability (lower than 1.5  1017 m2), the water transfer through the porous layer is only due to capillarity and is equal to the evaporation rate. Water transfer is not sufficient to maintain the outside surface wet and thus the evaporation is low. For an evaporative cooling application, this zone has to be avoided. For an intrinsic permeability larger than 1.5  1016 m2, water transport due to hydraulic pressure m_ pr is larger than the capillary transport m_ cap . A runoff of droplets appears at the outside surface of the wall for an intrinsic permeability higher than 8  1017 m2, and becomes large for values higher than 2  1016 m2. Evaporation flux in this zone is constant and equal to 2.8 g/(m2 min). Finally, for an intrinsic permeability between 1.5  1017 and 8  1017 m2, there is a maximum evaporation rate and a null runoff flux m_ dr . The sum of both capillary forces and pressure forces maintains the outside surface wet and a high evaporation rate. This zone is ideal for an evaporative cooling application.

Table 7.1 Properties of the terra-cotta wall. Porosity

Density

Thermal conductivity

Heat capacity

Wall width

Wall height

0.22 m3/m3

1970 kg/m3

2.0 W/m/K

840 J/kg/K

12 mm

0.6 m

Eco-efficient evaporative and ground-coupled system with terra-cotta evaporative walls

125

A change in the thickness of the porous wall or in the height of the tank would change this ideal range of permeability. Indeed, the high pressure at the bottom of a high tank allows better performance for lower permeabilities of the terra-cotta. However, the higher the tank, the smaller the range of ideal intrinsic permeability. In view of these results, and as environmental conditions change throughout the year, it is advisable to use tanks of small height, as the increase in pressure due to the water column does not significantly improve the performance at low permeability and greatly increases the risk of runoff with high permeabilities. In order to increase the exchange and evaporation surface, these small tanks can be connected in parallel, through an external simple hydraulic network. The same dependence of the ideal range of permeability could also be observed for changing outside conditions. For example, the impact of the wind speed on the ideal permeability range can be investigated. Three values of air speed were simulated: 0.5, 3, and 10 m/s. Simulation results are reported in Fig. 7.3 in the case of an outside temperature of 22°C and relative humidity of 50%. In Fig. 7.3, we can observe that the moisture content in the wall varies between 0 and 0.22 m3/m3, which corresponds to the material porosity (Table 7.1). For high permeabilities, the moisture content in the wall is equal to the material porosity, whereas for intrinsic permeabilities between 4  1018 m2 and 4  1017 m2 a sharp transition brings the moisture content to zero for low permeabilities. This figure also presents the difference between the temperature of the water in the porous reservoir and the temperature of the outside air. This temperature difference is

0.35

Low evaporation zone One ideal zone Two ideal zones Three ideal zones Drop zone

6.0 5.5

Ideal zone 0.5 m/s

5.0 4.5

Moisture content q (m3/m3)

0.3

4.0 Ideal zone 3 m/s

3.5 3.0

0.25 h = 0.22

2.5

0.2

2.0 1.5 1.0

0.15 Ideal zone 10 m/s

0.5

0.1

0.0 –0.5

0.05

DT (Tair – Twater)

qout 0.0 10–18

2

5

10–17

2

5

10–16

–1.0

Air speed = 0.5 m/s Air speed = 3 m/s Air speed = 10 m/s 2

Temprature difference DT (Tair – Twater) (°C)

0.4

–1.5 –2.0 5

kintr (m2)

Fig. 7.3 Evolution of the water temperature and liquid volumetric fraction at the outer surface of the porous medium as a function of the intrinsic permeability of the medium and the air speed.

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small for low intrinsic permeabilities. As the rate of evaporation is low, the temperature of the water is close to that of the ambient air. When the permeability is larger than 2  1018 m2, the temperature difference increases rapidly until reaching a maximum for intrinsic permeabilities larger than 2  1017 m2. The maximum temperature difference (between 4°C and 5°C) is only weakly sensitive to the wind speed. In fact, when the wind speed increases, the rate of evaporation increases. Logically, the higher the air speed, the higher the maximal evaporation rate: these are 1.41, 2.76, and 6.51 g/(m2 min) for air speeds of 0.5, 3, and 10 m/s, respectively. But simultaneously, the convective heat supply between the ambient air and the evaporator wall also increases. Thus the temperature difference for different wind speeds is also representative of the evolution of convective transfer coefficients as a function of this wind speed. This competition will be further discussed at the end of this section. Moreover, when the air speed increases, the ideal intrinsic permeability range moves toward larger values. At the same time, the water drop rate decreases a little. This means that the higher the wind speed, the greater the permeability required to keep the outer surface of the porous wall moist. For low permeability, evaporation is independent from the air speed. In this case, water flowing through the porous layer is the critical phenomenon. In this case, the temperature difference between the water and outside air decreases when the air speed increases, due to the more efficient heat convection coefficient. An intrinsic permeability of 4  1017 m2 is suitable for the three air speeds tested. This permeability offers a maximum evaporation rate without runoff, for the three air speeds tested that are representative of most ambient conditions in moderate climates. A higher permeability would lead to runoff problems and a smaller permeability would reduce the evaporation rate. Focusing on the competition between evaporation and convection, Fig. 7.4 presents, for the porous tank, the evolutions of the convective and evaporative powers according to the surface temperature of the evaporator for four different outside air temperatures and humidity conditions and two wind speeds, for a fixed intrinsic permeability of 7.5  1017 m2 for the terra-cotta. The evaporative power Φev has the same evolution on these four figures, as the dew point temperature is equal (Tdew point ¼ 15°C). This is not the case for the convective power Φconv, which decreases with the decrease of the outside temperature. A point of equilibrium (Teq) is observed when the sum of the evaporative and convective powers is equal to 0. This point of equilibrium corresponds to the temperature that the water would reach if the steady state is reached in the evaporator. For a steady ambiance (null wind speed), Text ¼ 28°C and φext ¼ 45%, the temperature of the water in the evaporator would reach 22°C at steady state with evaporative and convective powers of approximately 500 W. If the wind speed reaches 3 m/s, the evaporative and convective powers increase, the tank heats up, and reaches a new equilibrium temperature at 22.6°C with evaporative and convective powers of approximately 1800 W. Indeed, the convective power increases faster than the evaporative one when the air speed increases. In addition, at high air speeds, the powers involved are large and the evaporator tends to reach its equilibrium point quickly. This can be an advantage in the cooling phases of the system at the beginning of the night but also a disadvantage when the evaporator is colder than its equilibrium temperature, typically at the end

Eco-efficient evaporative and ground-coupled system with terra-cotta evaporative walls

Fconv Fev Fev+Fconv

2500

2000

1000

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1500

1000 500 0

–1500 –2000 –2500 –3000 14

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22

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3000

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500 0 –500 –1000 –1500 –2000 –2500

24

–3000 14

26

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2000 1500

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–500

–2000 –2500 –3000 14

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–1000

Fconv Fev Fev+Fconv

2500

Teq (v=3 m/s)=17.7°C

Power (W)

2000

22

Text = 16°C, Tdew point = 15°C (jext = 95%) 3000

Fconv Fev Fev+Fconv

2500

20

Tsurf (°C)

Text = 20°C, Tdew point = 15°C (jext = 75%) 3000

Air speed = 3 m/s Air speed = 0 m/s

Teq (v=3 m/s)=20.2°C

Text = 28°C, Tdew point = 15°C (jext = 45%)

Teq (v=0 m/s)=19.8°C

3000

127

16

18

20

22

24

26

Tsurf (°C)

Fig. 7.4 Evaporative and convective powers and corresponding equilibrium temperatures for different outdoor conditions.

of the night. Consequently, an increase in the evaporation rate thanks to a better exposure of the tank to outside winds is not necessarily synonymous to a colder evaporator water temperature, but it can make it possible to converge more quickly toward the equilibrium temperature during the night. When the outside temperature decreases and gets closer to the dew point temperature, the difference between the evaporator equilibrium temperature Teq and the ambient one decreases, as well as the differences in equilibrium temperatures at different wind speeds. The dew point temperature is the minimum possible value for the evaporator equilibrium temperature, and the closer the system gets to this value, the lower the evaporated power. Finally, from these different discussions and others discussed in (Leroux, 2016), the cooling of the water in the evaporator during the night is due to evaporation and radiation to the sky. Conversely, convection with outside air and absorption of solar radiation penalize the cooling of the tank and should be avoided. However, it

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is impossible to avoid convection without impairing the evaporation phenomenon. Moreover, there exists an optimum intrinsic permeability of the terra-cotta, which depends on the geometry of the tank and the outside climatic conditions, that can be found and should be looked for.

7.4

Impact of the evaporative cooling and groundcoupled system on the building performance

7.4.1 Evaluation of the building summer behavior The impact of this eco-efficient system on a single-family house behavior was evaluated using a model presented (Leroux et al., 2019a), calibrated, and validated before using a real-scale experiment (Leroux et al., 2019b). The house used for this study is a one-floor 100-m2 house with a basement. The dwelling is divided into three 10-m2 bedrooms, a 50-m2 sitting room/kitchen, a bathroom, and a technical room (Fig. 7.5). The height under the ceiling is 2.5 m, which gives an internal volume of 250 m3. The roof has four West 4.00

10.56

1.20 1.35

2.81

1.20 1.35

1.35

1.14 1.20 1.35

Room 3

Room 2

Room 1

1.61 90 2.15 1.41

10.56

WC

2.40 2.15

Sitting room

Bathroom

Car park

Kitchen

1.90

Technical room

1.20

1.60 2.15

Fig. 7.5 Case study house.

3.26

East

90 2.15 13.56

3.60

Eco-efficient evaporative and ground-coupled system with terra-cotta evaporative walls

129

40 38 36 34 32 30 28 26 24 22 20 18 16 14 12 10 8 6

Outside air temperature Water storage temperature Tground,lim theoretical ground temperature (depth:–1.15 m)

Temperature (°C) / COP

20/06 00:00

10/07 00:00

30/07 00:00

54 52 50 48 46 44 42 40 38 36 34 32 30 28 26 24 22 20 18 16 14 12 10 8 6

20/08 00:00

10/09 00:00

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20/06 00:00

10/07 00:00

30/07 00:00

20/08 00:00

10,000 9500 9000 8500 8000 7500 7000 6500 6000 5500 5000 4500 4000 3500 3000 2500 2000 1500 1000 500 0

Power (W)

Temperature (°C)

sides with a 20-degree slope. Windows account for, respectively, 27%, 21%, 12%, and 2% of the south, east, west, and northern building fac¸ades. This house is assumed to be situated in Bordeaux, south of France. The properties of the terra-cotta simulated are those presented in Table 7.1, and its intrinsic permeability is 7.5  1017 m2. The model allowed the simulation of the thermal behavior of the house in Bordeaux during the hot season. This makes it possible to compare with the simulated thermal behavior of the house if there were no cooling system. For this, two simulations were performed, one with the cooling system, the other without it. Fig. 7.6 (top) presents the seasonal evolution of the temperature of the water in the storage tank, the temperature of the outside air, and of the ground at 1.15 m depth. The storage tank temperature varies during the summer between 16°C and 22°C. The increase of the storage temperature occurs after several days of intense heat, for example at the beginning of July (Fig. 7.6). A moderate outside temperature allows the storage temperature to decrease progressively over a few days, thanks to the operation of the system, for example at the end of July. The temperature of the ground increases

10/09 00:00

Fig. 7.6 Evolution of the simulated water temperature in storage tank, of the ambient temperature and of the ground temperature during the summer (top) and performance of the system during the summer (bottom).

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

progressively during the summer between 14°C and 17°C, its cooling potential being minimal at the end of August. The bottom graph of Fig. 7.6 shows the evolution of the coefficient of performance (COP) of the system during the season. This COP is defined as the ratio of the heat extracted from the house by the cooling floor to the electricity consumed by the system’s pump ((4) in Fig. 7.1). The cooling power of the system and the temperature of the water in the storage tank are also presented. There is a clear correlation between the temperature of the water in the storage tank and the COP. When the storage tank temperature is low, the COP is very high (the COP can be above 40 in the early summer). Changes in the storage tank temperature have a direct impact on the COP value, which follows the same variations in reverse. For example, during hot periods, with a water tank temperature above 20°C, the COP is less than 20. At the end of the summer, because of the warmer ground, the system performance is lower than at the beginning of the summer season, but remains higher than 14. The cooling capacity of the system is also linked to the temperature of the water storage tank and varies between 4500 and 1000 W, depending on the period. The air temperatures in the living room of the house, simulated with and without the cooling system, are calculated in both cases as shown in Fig. 7.7 for a fortnight. In this figure, the impact of the system is strongly visible. On the one hand, the air temperature in the living room with cooling (blue) remains generally below 26°C (the set comfort temperature). On the other hand, very high air temperatures are observed without cooling, which can exceed 30°C during daytime and not fall below 28°C during the night during some periods. The average air temperature in the case with the cooling system is 25°C, which means 3°C below the average air temperature that Tair,inside without cooling system (simulated) Tair,inside with cooling system (simulated)

Tair,inside

30 28

Temperature (°C)

26 24 22 20 18 16 14 11/08 00:00

13/08 00:00

15/08 00:00

17/08 00:00

19/08 00:00

21/08 00:00

23/08 00:00

25/08 00:00

Fig. 7.7 Temperatures of the house during 2 weeks of the summer, with and without the cooling system.

Eco-efficient evaporative and ground-coupled system with terra-cotta evaporative walls

131

would have happened without cooling. The cooling system thus largely contributes to maintaining a satisfactory level of comfort inside the house. For both cases, however, the temperature of the air inside the house is almost always higher than the temperature of the outside air. This means that there are significant solar gains that overheat the house. In addition, at night, the temperature in the house remains high and takes advantage of little external coolness. This is indicative of a very low rate of air renewal and poor management of the windows of the house. Thus intelligent management of the openings of the house (shutters to decrease solar gains and windows to ventilate at night) could have limited the rise in the building temperature. Building management in this case has not been optimized and should be a first step toward summer comfort in a real building. The main performance indicators for the system were integrated and calculated over the 4-month duration of the simulation. During the 4 summer months simulated, the system ran for 310 h and evacuated 576 kWh of excess heat from the dwelling. The average COP of the system over this period was 23.2. The use of the system maintained a good level of comfort in the dwelling, since only 123°C h were recorded with a temperature inside the house above 26°C, and 11°C h above 27°C. This result is well below that found for the case simulated without use of the cooling system, which gave 1489°C h higher than 26°C and 771°C h higher than 27°C. The system consumed 3718 L of water over the 4 months, of which 741 L were lost by runoff, the rest being evaporated. This gives a specific water consumption of 6.4 L/kWh cooling. Focusing on the dissipation of the 576 kWh of heat extracted from the housing, simulations allowed the distribution of this energy to be determined in the various heat sources or sinks in interaction with the system. The heat extracted from the housing is dissipated half into the ground by the storage tank and half by the evaporator. The exchanges with the internal air of the basement are nearly null. This means that there is thus no benefit or disadvantage in insulating the upper surface of the storage in the basement. On the other hand, if the system works at lower temperature regimes, these exchanges with the basement air could become penalizing. For the heat dissipated at the evaporator, the same conclusions as those given in Section 7.3 are found: the amount of heat dissipated by evaporation is large (more than 900 kWh) but strongly offset by the energy absorbed by convection. Only 280 kWh of cooling are obtained, overall. Finally, the heat dissipated by the system shows a good balance between the ground cooling and the evaporator.

7.4.2 Optimization of the system dimensioning Following the system behavior study, the next step is to look for the minimum dimensions of the system that can guarantee good summer comfort and a high coefficient of performance. The four dimensioning criteria chosen are: l

l

l

l

maximum seasonal COP minimum degreeshours of discomfort minimum storage tank volume minimum evaporator surface

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

In this situation, with several criteria for the selection of a solution, two main approaches are possible: either to build a single function from the different criteria, or to sort the solutions and to keep a set of so-called “optimal” solutions; the decision maker then chooses from among those solutions the ones considered the best. The first possibility requires the creation of a single function integrating all the criteria. This imposes the choice of weighting values between the different criteria. This weighting choice is subjective and generally difficult to establish. It strongly guides the final dimensioning result. In the second case, the subjective character favoring certain criteria appears later in the decision process and the choice is performed among a small number of optimal solutions that can be more easily represented graphically. In this work this second approach is used. The selection of the different optimal solutions is based on the notion of Pareto dominance. In other words, a solution is said to be dominated if it does not improve any criterion. This sorting makes it possible to discard all the dominated solutions since better solutions on all the criteria exist and are preferable. It is common to couple this method with evolutionary algorithms (e.g., genetics). In the cooling system, given the small number of degrees of freedom for the dimensioning of the system parameters (evaporator surface, evaporator thickness, storage tank surface, storage tank volume), a grid with discrete values for each parameter and all the combinations is used. The sizing is here realized for the house in Bordeaux presented in Section 7.4.1 with a normal management of the shutters. The set of nondominated solutions is shown in Fig. 7.8. For an easy understanding, the solutions are presented with respect to two criteria or the product of two criteria (surface  volume). In Fig. 7.8A, the panel of solutions forms an arc. This boundary is usually called a “Pareto front” with reference to the solution sorting method. It can be seen from the proposed solutions that the larger the storage volume, the higher the COP. Solutions offering a COP less than 10 are also part of so-called “optimal” solutions. Indeed, these solutions have been kept, since they minimize the size of the storage and could be useful in the case of limited installation space. In this figure, dots come out of the front and do not optimize the COP or the volume. These solutions have still been retained since they are interesting with respect to the other criteria. Indeed, the front shown in two dimensions does not allow visualizing the four dimensions corresponding to the four different criteria. The COP increases rapidly with the first cubic meters of storage (up to 5 m3), and then this increase is much weaker when the volume continues increasing. In spite of very large volumes proposed by some solutions, the maximum COP achieved is about 30. Fig. 7.8B shows an overall trend to improve the COP with the evaporator surface. This trend is not as clear as for the other criteria, as the front is quite wide. Fig. 7.8C shows that when the discomfort level decreases, the COP increases. Finally, Fig. 7.8D aggregates the two geometric surface and volume criteria into one to facilitate representation. There is a characteristic front of the solutions. The presentation of all these nondominated, and therefore potentially interesting, solutions makes it possible to pick the ones that will be chosen according to the weight given to each criterion. Among the solutions proposed, five were selected. They are illustrated in Fig. 7.8 by a color code and reported in Table 7.2. These solutions are

Eco-efficient evaporative and ground-coupled system with terra-cotta evaporative walls 50

133

80

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70

40 60

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10 10

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COP Solution 3 Solution 4 Solution 5

Fig. 7.8 Results of optimal solutions in the sense of Pareto. (A) Storage tank volume as a function of the COP; (B) evaporator surface as a function of the COP; (C) evaluation of the comfort as a function of the COP; (D) storage volume times evaporator surface as a function of the COP.

examples of the variety of possible solutions and should allow adaptation to the different implementation constraints. A first solution with maximum dimensions that gives a COP close to 30 was selected (solution 5). This solution is preferred if the goal is to have a maximum yield regardless of the size and price of the system. A solution with small dimensions is proposed (solution 1), which provides a COP of 11 and can be installed in a case where installation spaces are limited. Finally, the other three solutions are compromises between performance and dimensions. This range of solutions makes it possible to

Table 7.2 Details of the five selected solutions over a summer season (June 1 to September 30) in Bordeaux.

Solution 1 Solution 2 Solution 3 Solution 4 Solution 5

Storage tank surface (m2)

Storage tank volume (m3)

Evaporator surface (m2)

13.2 44 44 26.4 44

0.66 2.2 4 7.96 35.2

7.2 7.2 15.84 15 36

Evaporator volume (m3)

Discomfort level (deg. hr > 26°C) (°C h)

Seasonal COP

Heat extracted from the building (kWh)

0.215 0.215 0.95 0.75 1.08

419 157 115 103 63

11.4 19 22.2 23.7 29.5

380 547 569 582 621

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135

adapt the dimensions as accurately as possible according to the constraints of the installation. For example, solution 2 seems to be a good compromise and can be chosen as the most interesting. It allows a high COP of 19, while having reasonable size (storage volume of 2.2 m3 and evaporator of 0.215 m3).

7.4.3 System performance It is also interesting to compare the performance of this low-tech system with a conventional air-conditioning system operating under the same conditions. It is not easy to compare air-conditioning systems. Indeed, the coefficient of performance varies a lot depending on the conditions of use. Thus, to compare systems with each other, performance must be measured in an environment and operating regime specified by a standard (e.g., Eurovent certification). For a heat pump with air-cooled condenser and water-heated evaporator (cooling floor), the system must be tested with an outdoor temperature of 35°C and must be able to reduce the return water temperature from the floor from 23°C to 18°C. For comparison, the energy classes give a rating of A for cooling floor and air evaporator connected heat pumps with COPs greater than or equal to 4.05, and G for COPs below 3.3. The use of these machines in cooler outdoor conditions and requiring a higher cooling temperature gives a higher operating COP. Also, the COPs announced by the manufacturers generally take into account only the electrical consumption of the compressor without accounting for the consumption of auxiliaries, like for example the pumps of the auxiliary networks (cooling floor) or the fans. So to compare a typical air conditioner with the evaporative cooling system, they are both put under the same operating conditions and the same electricity consumption is accounted for. The evaporative cooling system is compared here with the air conditioner Aqualis 2 from Ciat (Ciat, 2016). It is an air conditioner with an air-cooled condenser and a water-heated evaporator. The performance given by the manufacturer is available in Table 7.3. It gives the performance (COP) of the system according to the exit temperature of the water in the exchanger and the outside temperature. By interpolating this table on the same operation points as that of the evaporative system, the Table 7.3 Reference air-conditioner characteristics: COP as a function of the outside air and cooling temperatures. Outside air temperature Texit,water

24°C

26°C

28°C

32°C

36°C

40°C

16°C 18°C 20°C 22°C 23°C

4.4 4.7 4.9 5.2 5.3

4.2 4.5 4.7 5.0 5.1

4.0 4.2 4.5 4.7 4.9

3.5 3.8 4.0 4.3 4.4

3.1 3.4 3.6 3.9 4.0

2.7 2.9 3.1 3.3 3.4

Adapted from Ciat, 2016. groupe d’eau glac
ee et pompes a` chaleur r
eversibles air/eau aqualis CIAT N 04.528 C, technical sheet.

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

performance that the air conditioner would have had if it had rendered the same service is obtained—that is, if it had produced the same cooling power under the same external conditions. In Bordeaux climatic conditions, the COP of the air conditioner remains around 5 and varies little throughout the 4-month summer season. The COP of the evaporative system meanwhile is much higher (between 50 and 15) and presents strong variations during the season. The air conditioner performance is thus quite low but presents also little variation as a function of outdoor conditions. It can produce cold continuously, which guarantees maintaining a set building temperature. The evaporative system studied here is not, strictly speaking, an air-conditioning system, but is a cooling one. It works with a very high COP but it cannot strictly guarantee a set temperature. The use of this system, sized with care, can, however, offer a very good level of comfort with performance far superior to that of a conventional air conditioner. The impact of the evaporative cooling system on the thermal behavior of a building was studied. The installation of this system allows significant improvements in the internal comfort. In the case of very high solar gains, it cannot insure a set temperature, but very significantly decreases the indoor temperature. This system is therefore recommended in addition to intelligent building management, for an adaptive comfort. The exchanges with the ground play a very important role in the acceptable functioning of the system and make it possible to resist short heat waves efficiently. The evaporator, meanwhile, helps to reduce the storage tank temperature and operate at lower temperature levels, which guarantees high COPs. This system makes it possible to operate with an average coefficient of performance of 24, which is approximately five times greater than a conventional commercial air conditioner.

7.5

Conclusion and outlooks

This chapter presents the impact of a low-tech cooling system on the thermal behavior of a building. It is possible to choose the best material characteristics for the evaporator walls to fit the ambient conditions outside of the building with high efficiency of the evaporation and low water losses to the surroundings. The installation of this system allows significant improvement of the building thermal comfort. In the case of very high solar gain, the cooling system cannot follow a set temperature, as airconditioning devices do, but it does very significantly decrease the indoor temperature. This system is therefore recommended in addition to smart building management for an adaptive comfort. It is possible to propose an optimized sizing of the system through the notions of Pareto dominance. This system allows operation with an average coefficient of performance of 24 in given climatic conditions, which is approximately five times greater than a conventional commercial air conditioner in the same condition. The heat exchanges of the storage tank with the ground in the basement play a very important part in the acceptable functioning of the system and allow the system to adapt to heat waves efficiently. The terra-cotta evaporator, meanwhile, helps to cool down the storage tank and to operate at lower temperature levels, which guarantees

Eco-efficient evaporative and ground-coupled system with terra-cotta evaporative walls

137

high COPs. The complementarity between the different heat sinks allows good performance to be maintained throughout the summer. The system is very cheap and simple, as expected, and could be an efficient alternative to electrical compression chiller technology in terms of performance. However, this system cannot be an option in regions where water scarcity is an issue, its evaporative performances would be strongly reduced in humid climates, and it requires a basement or a crawl space, not available in all buildings. Other studies, for example on the aging of the system, should be undertaken to prove long-term reliability. Indeed, possible limitation of the heat transfer to the ground could appear after long heating periods, as the ground temperature would be affected by this heat load. Moreover, the material characteristics of the evaporator wall and the coupled convection and evaporation phenomena could be affected by limestone deposits or algae growth over the evaporator external and internal surfaces, respectively. Finally, the technical problems that could happen during winter, for example when negative temperatures occur, are still to be experimentally evaluated. From a modeling point of view, this system should also be studied to optimize its overall performance as a function of climatic and surroundings conditions as well as energy and power demand, and compared with other low-energy consumption systems.

References Artmann, N., Manz, H., Heiselberg, P., 2007. Climatic potential for passive cooling of buildings by night-time ventilation in Europe. Appl. Energy 84 (2), 187–201. Boukhanouf, R., Alharbi, A., Ibrahim, H.G., Amer, O., Worall, M., 2017. Computer modelling and experimental investigation of building integrated sub-wet bulb temperature evaporative cooling system. Appl. Therm. Eng. 115, 201–211. Boukhanouf, R., Amera, O., Ibrahim, H., Calautit, J., 2018. Design and performance analysis of a regenerative evaporative cooler for cooling of buildings in arid climates. Build. Environ. 142, 1–10. Bruno, F., 2011. On-site experimental testing of a novel dew point evaporative cooler. Energy Build 43, 3475–3483. Chen, X., Su, Y., Aydin, D., Ding, Y., Zhang, S., Reay, D., Riffat, S., 2018. A novel evaporative cooling system with a polymer hollow fibre spindle. Appl. Therm. Eng. 132, 665–675. Ciat, 2016. groupe d’eau glac
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International Energy Agency, 1997. Energy Conservation in Buildings, and Community Systems Programs. Selection guidance for low energy cooling technologies, Annex 28 Low Energy Cooling. Leroux, G., 2016. Etude d’un syste`me innovant de rafraıˆchissement a` basse consommation pour le b^atiment. Thesis from Universit
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Hemp plaster and passive cooling techniques for retrofit: A case study

8

Haitham Sghiouria, Mouatassim Charaib, and Ahmed Mezrhaba a Mechanics and Energy Laboratory, Mohammed First University, Oujda, Morocco, bCERTES, Paris-Est University, Cr
eteil Cedex, France

8.1

Introduction

Building rehabilitation is a major energy-economy opportunity. However, it may require a huge investment and heavy reconstruction work. The use of passive techniques helps with these two hindrances and can provide significant improvements to the building’s energy consumption without a major investment or a heavy reconstruction effort. These techniques include the use of performant plasters; in the literature, several researchers used local eco-materials to produce plaster composites that perform better than regular plaster (Carbonaro et al., 2015, 2016; Pedren˜o-Rojas et al., 2017; Sair et al., 2019). Passive cooling is also a major method to improve buildings’ performance. It is basically an approach that aims to control heat gains and heat removal through passive techniques with low or no energy consumption. Among the techniques used for passive cooling of buildings are natural ventilation (Sanchez et al., 2016; Weerasuriya et al., 2019); the use of materials with a very high thermal mass, such as “rocks,” (Mastouri et al., 2017) that help modulate the temperatures in the building; and shifting the cooling load by several hours to allow natural ventilation to be more effective when the outdoor temperature is colder. Another technique is shading, which can be architectural shading such as overhangs (Sghiouri et al., 2018) or the use of movable shading devices (Sghiouri et al., 2019) that can be automated. These passive techniques can prevent overheating of insulated buildings, since their insulation against heat loss and airtightness can reduce heating needs significantly, especially if the building is occupied, whereas its cooling needs might increase. This observation was obtained from our on-site study, which is briefly summarized in the following paragraph. In our study we monitored an office building whose envelope was well insulated with 10 cm of EPS and XPS insulation. The results obtained from the monitoring were used to create a numerical model to validate the insulated building and to simulate the same building, but lacking insulation and double glazing, to provide a reference case that could reasonably be considered as a typical office building in Morocco, i.e., a building without insulation, since building insulation in Morocco is not widespread Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction https://doi.org/10.1016/B978-0-12-820791-8.00008-0 © 2021 Elsevier Ltd. All rights reserved.

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

and almost all buildings, whether residential or office buildings, are not insulated. Then, for both buildings, several passive solutions were incorporated into them to solve the overheating problem that occurs during the summer. With the aim of not having to significantly alter the reference building in the retrofit process, we chose the following passive solutions to be integrated into the monitored and the reference buildings: l

l

l

l

Adding a hemp plaster layer to the roof’s interior side; hemp plaster was developed using local eco-materials in our laboratory; Increasing the thickness of the hemp plaster layer of the reference building; Using a cool painting technique on external walls, because the roof cannot be modified due to being covered with solar PV panels; Integrating automated external shading devices to the building.

In this chapter, the methodology applied to this case study and the most interesting results are highlighted. The second section presents the experimental setup for the thermophysical characterization of the hemp plaster eco-material, the meteorological station, and the monitoring equipment from which the weather data and the indoor measurements needed for the numerical validation of this case study were obtained. The third section introduces the case study, i.e., the description of the building and the climate prevailing in that location, the validation of its numerical model, and the hypothesis and description of the simulation adopted as a tool to extrapolate the monitoring results to a more general case representative of the current situation of office buildings in Morocco, in view of introducing passive energy efficiency measures to mitigate the demand for cooling and improve thermal comfort. Then, the fourth section shows the results obtained and discusses them. Finally, a conclusion is drawn, including the most important results, observations, shortcomings, and outlook.

8.2

Experimental setup

8.2.1 Thermophysical characterization of hemp plaster As part of our work, we designed a new eco-material based on plaster and hemp produced locally in the eastern region of Morocco, with the contents of hemp ranging from 0 wt% (regular plaster) to 6 wt%, although for this chapter we used only regular plaster and plaster with 6wt% of hemp, which presented the best thermal performance (Fig. 8.1). Experimental measurements were carried out by the hot disk method (Gustafsson, 1991; Gustavsson et al., 1994, 1997; Gustavsson and Gustafsson, 2005), using the TPS 1500 instrument (Fig. 8.2), which meets the ISO 22007-2 standard (ISO 220072, 2008). This method allows the thermal conductivity and the thermal diffusivity of the tested materials to be determined in a transient regime by supplying constant power for a limited time to the probe, consisting of two Kapton-wrapped nickel spirals; the Kapton coating improves the mechanical resistance and the electrical insulation of the

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Fig. 8.1 Samples of regular plaster and hemp plaster.

Fig. 8.2 TPS 1500 Hot Disk instrument.

sensor. The power supplied is intended to raise the temperature of the material being tested by a few degrees and then the temperature change is measured with the same probe by recording the variation in its electrical resistance using a Wheatstone bridge. The characteristics of the temperature increase, directly linked to the evolution of the electrical resistance of the probe, are precisely recorded in 200 data points, and the analysis of this variation in a transient regime allows both the conductivity and the thermal diffusivity to be determined. Moreover, this measurement is absolute and therefore requires no prior calibration or correction factors. In addition, the instrument systematically detects and suppresses contact resistance that may be caused by a nonoptimal contact between the probe and the tested material. To conduct the measurement, the probe was installed between two samples of the same material with a minimum size of 3 mm
13 mm diameter, each as shown in Fig. 8.3, and the measurement was carried out at room temperature. According to the manufacturer, the uncertainty of the thermal conductivity probing is better than 5% for the interval 0.01–400 W/m K, and the repeatability is better than 1%.

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Fig. 8.3 Sample thermal conductivity measurement illustration.

Fig. 8.4 Aerial view of the building and the meteorological station.

8.2.2 Meteorological station Weather data were measured using a meteorological station less than 500 m away from the monitored office building (Fig. 8.4). It is installed on the rooftop of one of the university’s buildings and mainly includes the following sensors: Tracking Cleanliness Sensor (TraCS) developed by scientists from DLR, Germany’s national aeronautics and space research center; pyranometers to measure the global and diffuse irradiation; two pyrheliometers that monitor the direct normal irradiance (DNI) coming directly from the sun and the reflected DNI from a mirror mounted on a SOLYS2 solar tracker (Fig. 8.5); a CS215 sensor from Campbell to measure outdoor temperature and humidity; and an NRG 200 wind direction sensor and NRG 40H anemometer to measure wind direction and wind speed, respectively. All the measurements of the meteorological station were carried out using a 1 min time step.

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Fig. 8.5 Meteorological High Precision (MHP) station with TraCS accessories installed on the rooftop of the University of Oujda, Morocco.

Table 8.1 Location and some relevant weather data. Location

Oujda, Morocco

Weather file data format WMO station Latitude Longitude Elevation (m) Highest average monthly temperature (°C) Lowest average monthly temperature (°C) Average annual global horizontal solar irradiance (GHI) (kWh/m2/day) K€oppen climate classification

TMY2 601,150 34.8°N 1.9°E 470 26.5 (July) 9.4 (January) 5.4 BSk (cold semiarid)

Some relevant data for the building’s location are presented in Table 8.1, including latitude and longitude, K€ oppen climate classification, and World Meteorological Organization (WMO) station data used for the typical meteorological year (TMY) calculation. Note that the TMY weather file is not used for validation. Instead, the

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validation data used are from the weather station presented previously. However, only 8 years of data was available from the meteorological station, which is not enough to create a TMY file; thus the 2003–17 (15 years) data from the WMO station was used to generate a TMY weather file suitable for the energy analysis of the building, which was performed after the validation with the Meteorological High Precision station (MHP) near the building.

8.2.3 Building monitoring instruments The monitoring of the building was carried out using a Testo 480 instrument equipped with the appropriate probes for the evaluation of visual and thermal comfort inside the building (Fig. 8.6). The instrument measures major parameters related to the indoor conditions, such as air temperature by the means of a type K thermocouple, operative temperature using a TC type K globe thermometer, relative humidity, air velocity in the room using a hot wire probe, illuminance by means of a class C lux probe according to DIN 5032-7, and finally ambient CO2. Another instrument used to monitor the office was the Testo 174H (Fig. 8.7), which measured the indoor temperature and relative humidity and had a long-lasting battery and a data logger, in case a power failure occurred and the Testo 480 stopped working,

Fig. 8.6 TESTO 480 for indoor comfort measurements.

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145

Fig. 8.7 TESTO 174H data logger for temperature and relative humidity measurements.

Fig. 8.8 Professor’s Office Building south fac¸ade view.

a contingency that did not occur. The measurements made by the Testo 480 were completed successfully throughout the duration of the study covering the month of July.

8.3

Case study

The building selected as the case study was the Professors’ Office Building of the Faculty of Sciences at Universit
e Mohammed Premier d’Oujda (Morocco) (Fig. 8.8). The reason behind this choice was the fact that it has 10 cm of polystyrene thermal insulation and double glazing, which are quite unique in Morocco, since most office buildings are not insulated and have huge single-glazed fac¸ades. In summer, the building has some serious overheating issues, with very high and uncomfortable temperatures, even in the north-facing office monitored, which makes it a candidate for retrofit and testing passive cooling techniques. The second major reason for choosing this building is that it has the same envelope composition as typical Moroccan office buildings except for its insulation and double glazing, which makes the extrapolation of its validated numerical model to noninsulated typical office buildings fairly accurate.

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8.3.1 Building description The office building is south-facing and consists of two levels, as shown in (Fig. 8.8). It is built on a floor area of 485 m2 distributed over 35 offices, two bathrooms, a meeting room, and corridors, as shown in Figs. 8.9 and 8.10. The external roof is composed of a 16-cm hollow-core slab (HCS), above which there are 10 cm of reinforced concrete and 10 cm of extruded polystyrene (XPS) insulation, covered from the top by a 5-cm layer of screed. The HCS in turn is covered from the bottom by 2 cm of regular gypsum plaster, as shown in (Fig. 8.11). The U-value of the roof is 0.25 W/m2 K, and its external solar absorptance is 0.6, which

Fig. 8.9 Ground floor architectural plans.

Fig. 8.10 First-level architectural plans.

Fig. 8.11 External roof’s composition.

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147

is the solar absorptance of a rough grey surface such as concrete (ASHRAE, 2017; Parker et al., 1993). Note that the roof is covered with PV panels that decrease the albedo of the roof and contribute to increasing the roof’s surface temperature through absorption of radiation from the sun and emitting it to the roof. The same reasoning goes the other way: the PV panel array decreases the roof’s temperature at night by emitting radiation to the night sky. Note that this phenomenon and the production of the solar PV panels were not accounted for in the modeling. The external wall thickness is 54 cm, consisting of two 20-cm layers of fired hollow bricks covered on both sides by 2 cm of mortar and between which there is 10 cm of expanded polystyrene (EPS) insulation (Fig. 8.12). The U-value of these walls is 0.19 W/m2 K and their external solar absorptance is 0.4, which is the solar absorptance of a rough white coating. The ground floor is composed of 30 cm of a concrete slab covered from the top with 6 cm of XPS insulation and 5 cm of screed, above which tiles of a thickness of 2 cm are superimposed; the U-value is 0.4 W/m2 K (Fig. 8.13). All glazed areas of the monitored building consist of double glazing with a U-value of 2.82 W/m2 K and a g-value of 0.64. The most glazed fac¸ade is the north-facing one, since it contains a huge glass door that leads to a window-to-wall ratio (WWR) of 25.2%. The second-most glazed fac¸ade is the one that faces the south direction, as it has a WWR of 14.4%, followed by the eastern and western fac¸ades, both with WWRs of 6% (Table 8.2).

Fig. 8.12 External wall’s composition.

Fig. 8.13 Ground floor’s composition.

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

Table 8.2 WWR and glazing area for each fac¸ade. Orientation

South

North

East

West

Total

Glazed area (m2) Fac¸ade area (m2) WWR (%)

41.4 309 14.4

77.9 309 25.2

6.8 112.5 6

6.8 112.5 6

132.9 843 15.8

8.3.2 Building model The building’s thermal model is based on the architectural plans obtained from the architect of the building. However, considering each office as a thermal zone on its own is time consuming and not efficient when simulating; thus we grouped together offices with the same solar and occupancy gains and similar outdoor conditions, resulting in 33 thermal zones, as described in Fig. 8.9, describing the ground level’s thermal zoning, and Fig. 8.10, describing the first level’s thermal zoning. Based on the architectural plans, a height of 3 m was considered for each level. In this study, TRNSYS 18 (Klein, 1976) was used to simulate the building’s thermal performance using the TRNSYS3D SketchUp plugin to draw the 3D geometry to use in TYPE 56, the component that models buildings. The time step chosen was 1 h, so as not to drag out the simulation time unnecessarily. TRNSYS software models buildings and systems based on a transient approach; it can model a wide range of systems, including pipes, controllers, and heating and cooling systems, using components called TYPES that can be interconnected through their outputs and inputs based on a black-box methodology. Fig. 8.14 shows the 3D model created using the TRNSYS3D plugin in SketchUp; note that the glazing areas of a thermal zone were grouped together, as in zone F (refer to Fig. 8.10), containing three offices with a window each. These windows were considered as a single window with the same area of the three windows combined. Also, the PV panels on the roof were considered as shading elements. However, a better modeling would be to consider the interaction between them and the roof. Fortunately,

Fig. 8.14 3D model of the building with thermal zoning in SketchUp.

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149

in this study this approximation did not impact the validation of the building, which was validated successfully as described later in this chapter.

8.3.3 Hypotheses To perform the simulations, several hypotheses were considered: 1. The building was divided into 33 thermal zones, as described in Figs. 8.9 and 8.10. 2. The initial air temperature was set to 20°C and humidity to 50% in each thermal zone. These initial values were chosen assuming the building’s heating system was already operating at 20°C before the simulation began in January. This hypothesis does not apply to the freefloating mode (without HVAC systems) and validation. 3. Inside surfaces’ convective heat transfer coefficients were calculated internally by TYPE 56 using the following equation: hinside ¼ C (Tsurf – Tair)n (W/m2 K) (SEL et al., 2004). 4. Outside surfaces’ connective heat transfer coefficients were calculated using the following equation: houtside ¼ 3*W + 2.8 (W/m2 K) (Mastouri et al., 2017), where W is the wind speed in m/s. 5. Kusuda’s correlation was used to model the thermal behavior of the soil underneath the building, to set the boundary condition on the ground floor at 1 m of depth (Kusuda and Achenbach, 1965) (Eq. 8.1).


π 0:5  T ¼ Tmean  Tamp
exp depth
365α ( "   #) 2π depth 365 0:5
tnow  tshift 

cos 365 2 πα

(8.1)

First, the simulation was done in free-floating mode to get the air temperature on an hourly basis to compare it to the monitoring data for validation. After the validation of the building, the HVAC systems were activated to evaluate the energy demand for cooling and heating using 20°C as a setpoint for heating and 26°C as a setpoint for cooling, as in ISO 7730 (ISO, 2005). Then the building roof’s U-value was improved using a locally developed eco-material (hemp plaster) and its impact on the energy performance was assessed. Afterwards, passive cooling techniques, namely cool painting and shading, were integrated into the building to help deal with the overheating issue of the monitored building. Eventually, the HVAC systems were switched back to the off mode to study the thermal comfort using the ASHRAE 55 adaptive comfort model (ASHRAE, 2013, p. 55). After having improved the thermal performance and the thermal comfort of the monitored building, results were generalized to a typical office building in Morocco (i.e., an office building without insulation). First, the HVAC systems were turned on to assess the energy demand for a reference case that represents a typical office building in Morocco. Then, the impact of adding a layer of hemp plaster and increasing its thickness on the cooling and heating demand was evaluated. Afterwards, passive cooling techniques were implemented into the typical office building to help improve

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summer comfort. Eventually, the HVAC systems were switched off to evaluate the thermal comfort using the ASHRAE 55 adaptive comfort model. The following indicators were used to assess the impact of hemp plaster and passive cooling techniques on the energy and comfort performance of the office building: 1. Heating and cooling demand calculated using setpoints for cooling and heating established at 20°C and 26°C, respectively, according to international standard ISO 7730 (ISO, 2005). 2. Maximum and minimum air temperatures. 3. Operative temperature calculated using Eqs. (8.2)–(8.4).

Top ¼ aTa + ð1  aÞTmr

(8.2)

where Top is operative temperature, Ta is ambient temperature, Tmr is mean radiant temperature, and a is a coefficient calculated using Eq. (8.3). a¼

hc hc + hr

(8.3)

where hc and hr are the heat exchange coefficient by convection and radiation, respectively. To get the mean radiant temperature Tmr we used a simple model (Eq. 8.4) assuming the mean radiant temperature is the area-weighted mean surface temperature. Tmr ¼

T1 S1 + T2 S2 + … + Tn Sn S1 + S2 + … + Sn

(8.4)

where Tn and Sn are the temperature and the area of the surface n, respectively. 4. The number of discomfort hours, which is the difference between the operative temperature in the office and the acceptable operative temperature from the adaptive comfort model described later in this chapter. 5. The area weighted discomfort hours, based on the calculation of the number of discomfort hours in each occupied zone of the building and the area of that zone, following Eq. (8.5):

DH w ¼

DH 1 A1 + DH 2 A2 + ⋯ + DH m Am A1 + A2 + ⋯ + A m

(8.5)

where DHi is discomfort hours in zone i and Ai is the floor area of zone i. Note that the discomfort hours were calculated only when the building was occupied. In summary, the cases that were studied are presented in Table 8.3.

8.3.4 Thermal comfort model In this study, the thermal comfort of the occupants is assessed and compared for the simulated cases presented earlier; it is based on an adaptive comfort model from standard ASHRAE 55, which is widely used in the literature and in industry. Both

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Table 8.3 Summary of the studied cases.

Insulation

Cool painting (c_paint)

Automated shading (autoshade)

Adding a 2-cm hemp plaster layer (h_plaster)

Increase of the roof’s hemp plaster thickness to 5 cm (h_plaster +)

Monitored Mon. + c_paint Mon. + h_plaster Mon. + autoshade Mon. + all

✘ ✘ ✘ ✘

✘ ✘

✘ ✘

✘ ✘

Reference Ref. + c_paint Ref. + h_plaster Ref. + h_plaster + Ref. + autoshade Ref. + all Ref. + all +

✘

✘ ✘

✘ ✘ ✘

✘ ✘

✘

✘ ✘

✘

acceptability limits of the ASHRAE 55 adaptive comfort model (i.e., 80% and 90%) were considered. The adaptive thermal comfort model defines acceptable operative temperature Tac based on comfort temperature Tc calculated using monthly mean outdoor temperature Tmo in addition to the acceptability limit. Following, the adaptive comfort is described through equations, where Tc is the comfort temperature and Tmo is the monthly mean outdoor dry-bulb air temperature: 8
33:5°C Not applicable

(8.6)

For the 90% acceptability limit, the acceptable operative temperature is the following: Tac ¼ Tc  2:5

(8.7)

For the 80% acceptability limit, the allowed operative temperature is the following: Tac ¼ Tc  3:5

(8.8)

Note that in our simulations, when Tmo is larger than 33.5°C, the Tc is considered equal to 28.185°C, and when Tmo is below 10°C, Tc is considered equal to 20.9°C (Samani et al., 2016).

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Table 8.4 Plaster thermophysical characteristics. Plaster type

Thermal conductivity (W/m K)

Specific heat capacity (J/kg K)

Density (kg/m3)

Regular plaster Hemp plaster

0.570 0.364

1034 942

1295 1051

8.3.5 Hemp plaster In the beginning of this chapter, we talked about a new local eco-material for construction we developed in our laboratory and called hemp plaster. Its thermal characteristics are far better than regular plaster (Table 8.4); thus we studied its effect on the heating and cooling demand of the building. Table 8.4 compares the thermophysical characteristics of hemp plaster to regular plaster in Oujda City. Note that these values are mean values of three measurements conducted on each material. These characteristics were used in the simulation. The regular plaster is the plaster used in the monitored building and is also the plaster typically used in Oujda City. In this study, the hemp plaster was applied to the internal side of the roof as a substitute for regular plaster to help improve the U-value of the roof. It is well known that roofs have a large contribution to the heat losses of a building, so having a better thermally performing roof can help decrease the energy demand, especially for heating.

8.3.6 Passive cooling techniques 8.3.6.1 Cool painting Heat gains through the roof and the south-facing walls (for our case study) contribute to a large part of the cooling energy demand. To decrease this cooling energy demand caused by solar gains through the external walls, highly reflective materials and coatings referred to as cool painting can be used (Parker and Barkaszi, 1997). Cool painting on walls reduces cooling energy demand and improves indoor comfort by lowering the indoor air temperature in summer. However, it also increases the heating energy demand and may cause glaring issues because of its high reflectivity (Krarti, 2017). To model the cool painting, the solar absorptance of the external walls was set to 0.3, which is the solar absorptance of a smooth white surface (SEL et al., 2004). The base case scenarios for both monitored and reference buildings had a solar

Table 8.5 Solar absorptance, reference case vs cool case.

External walls

Monitored and reference cases

Cool painting case

0.4

0.25

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absorptance of external walls set to 0.4, which is the solar absorptance of a rough white surface (Table 8.5).

8.3.6.2 Automated shading devices Shading helps deal with the excess of solar irradiance that passes through the glazed areas of the building and causes overheating issues and increased cooling demand for air-conditioned buildings. In this study, shading was achieved by the use of automated external movable shading devices. External shadings were chosen as a solution to overheating by blocking direct solar radiation before it enters the building. In addition, the shading devices were automated to improve their efficiency in blocking harmful radiation causing overheating and letting natural light pass through when there is not an overheating issue. Automated external movable shading devices were set up to block a maximum fraction of 70% of solar radiation in summer only. The control strategies were as follows: the devices were switched ON when the solar radiation on the window exceeded 140 W/m2 and the indoor air temperature exceeded 24°C, and were then switched back to OFF mode when the irradiance on the window fell below 120 W/m2 or when the indoor air temperature fell below 22°C. The condition on solar radiation was set using the integrated mode in TYPE 56 in TRNSYS.

8.3.7 Model validation The numerical model can be validated using two methods: by comparing the simulated HVAC demand to energy bills, as in the studies from (Tavares et al., 2015; Ascione et al., 2017b); and by comparing simulated indoor air temperature to measured indoor air temperature, as in the studies from (Şahin et al., 2015; Carlon et al., 2016; Cacabelos et al., 2017; Ascione et al., 2017a). Using metrics such as mean bias error (MBE) and coefficient of variation (CV) (RMSE) and meeting guidelines such as ASHRAE Guideline 14 (ASHRAE, 2002), the process of calibrating the model to fit the measured data can be automated as in (Cacabelos et al., 2015). In this study, we validated our model of the building manually (i.e., trial and error) by comparing the simulated air temperatures in office 19 to data from a monitoring campaign that lasted from July 5 to July 25, 2019. We chose this method because the building is naturally ventilated and does not have HVAC facilities such as air conditioners, except for the meeting room and offices 29 and 30, which do have a split air-conditioning system. The validation of the model was performed by changing the thermal characteristics of the glazing and the solar absorptance of the roof slightly, to account for the presence of the PV panels. The other components of the envelope were provided to us by the architect (thicknesses of building materials, composition of the walls, and so forth). All the data used for the validation of the building are shown in Table 8.6; we note that the thermophysical characteristics of the building materials are from the database of software developed by the Agence Marocaine pour l’Efficacit
e Energ
etique (AMEE, n.d.) called BINAYATE.

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Table 8.6 Thermophysical characteristics of the building’s envelope. Construction element External walls

Ground floor

Roof

Partitioning walls Intermediate floor

Glazing

a b c

Materials Mortar Hollow Brick EPS Hollow Brick Mortar Tiles Screed XPS Reinforced concrete Soil layera Regular plasterb Hollow-core slab Reinforced concrete XPS Screed Mortar Hollow Brick Mortar Regular plasterb Hollow-core slab Reinforced concrete Screed Tiles Double glazing Single glazingc

Thickness (cm)

Thermal conductivity (W/m K)

Thermal capacity (kJ/kg K)

Density (kg/m3)

2.0 20.0 10.0 20.0 2.0 2.0 5.0 6.0

0.7 0.274 0.03 0.274 0.7 1.3 1 0.03

1.00 0.741 1 0.741 1.00 0.84 1.0 1

1350.0 664.0 30.0 664.0 1350.0 2300.0 1700.0 37.5

30.0 25.0

2.5 0.995

1.0 0.96

2500.0 2100.0

2.0

0.57

1.0

1295.0

16.0

1.25

1.0

2000.0

10.0 10.0 5.0 2.0 20.0 2.0

2.5 0.03 1 0.7 0.274 0.7

1.0 1.0 1.0 1.00 0.741 1.00

2500.0 37.5 1700.0 1350.0 664.0 1350.0

2.0

0.57

1.0

1295.0

16.0

1.25

1.0

2000.0

2500.0 1.0 2.5 10 1700.0 1.0 1 5.0 2300.0 0.84 1.3 2.0 U-value ¼ 2.82 W/m2 K and g-value ¼ 0.64 U-value ¼ 5.69 W/m2 K and g-value ¼ 0.82

Mean value of five on-site measurements using TLS-100 instrument. Measured using hot disk method. The single glazing is used for the reference/typical office building.

To assess the accuracy of the model’s calibration, the following metrics were used as suggested in the literature (Ruiz and Bandera, 2017) for hourly calibration: R2 no less than 0.75 (ASHRAE, 2014; E. V. Organization, 2012), CV (RMSE) not exceeding 30 according to ASHRAE and FEMP criteria (ASHRAE, 2014; Webster et al., 2015)

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or 20 according to IPMVP (E. V. Organization, 2012), and NMBE no more than 5% for IPMVP and 10% for both ASHRAE and FEMP. –

R2 or coefficient of determination shows how close the simulation results are to the linear regression of the measured ones. R2 is defined between 1 and 0; the closer it is to 1, the more it indicates that the modeled values match the measured ones. R2 is calculated using Eq. (8.9):

0

12

Xn Xn Xn B C n mi  si  mi  s B C i¼1 i¼1 i¼1 i C ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi s R ¼B    B  X
Xn 2
Xn 2 C Xn @ A n m2  m s2  s n  n i¼1 i i¼1 i i¼1 i i¼1 i 2

(8.9) where n is the number of measurement points, mi is the measured temperature, and si is the simulated temperature. For this study an R2 value of 0.9589 was obtained, as shown in Fig. 8.15. –

CV (RMSE), or coefficient of variation of the root mean square error, reflects the variability of the errors between measured and simulated values without having the problem of cancellation of errors as in NBME, as described later. CV (RMSE) is calculated using Eq. (8.10):

CVðRMSEÞ ¼

sX ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi n ðm i  si Þ2 1 i¼1 m

n


100 ð%Þ

(8.10)

After the calibration of the model, a CV (RMSE) value of 0.97% was obtained. –

NMBE, or normalized mean bias error, is the average of the individual errors calculated by subtracting the simulated value from the measured one, divided by the mean value of the

Measured temperature (°C)

33 32 31 R2 = 0.9589

30 29 28 29

30 31 32 Simulated temperature (°C)

Fig. 8.15 Simulated vs measured temperature.

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

Fig. 8.16 Measured and simulated temperatures in Office 19 in July. measured values, as in Eq. (8.11). Note that, as opposed to CV (RMSE), this metric suffers from cancellation of errors and is not sufficient to judge the accuracy of our model.

Xn 1 NMBE ¼  m

i¼1

ðmi  si Þ n


100 ð%Þ

(8.11)

In this study, the NMBE equals 0.36%. Fig. 8.16 shows the prediction of our validated model for the period lasting from July 5, 2019 at 10 a.m. to July 25, 2019 at 4 p.m. It is clear that the simulated temperature only exceeds the uncertainty of 0.5°C of the measuring instrument occasionally, for less than 24 h in total, out of a total of 487 h of monitoring time, which represents less than 5%, thus explaining the low values of NMBE and CV (RMSE) and the high value of R2.

8.4

Results and discussion

8.4.1 Energy demand 8.4.1.1 Monthly heating demand Fig. 8.17 shows the heating energy demand of the monitored building, simulated using the TMY weather file, for the heating season lasting from December to March. January was the coldest month with 0.53 kWh/m2 of heating energy demand that changed very slightly with the added passive techniques. The same reasoning goes for the rest of the months. Note that the hemp plaster was not increased for the monitored building, since the change in heating demand was insignificant. “Mon. + all” notation means that the simulated case is the monitored building containing all the techniques combined. For the reference/typical office building this time, the heating demand was significantly larger and highly affected by the passive measures installed in the building.

Monitored cases

Hemp plaster and passive cooling techniques for retrofit

Fig. 8.17 Heating demand of the monitored building in the heating season.

Mon. + all Mon. + autoshade Mon. + h_plaster Mon. + c_paint Monitored 0 Dec.

Reference cases

157

0 0 1 1 1 1 Heating energy demand (kWh/m2) Jan.

Feb.

Mar.

Fig. 8.18 Heating demand of the reference/typical building in the heating season.

Ref. + all+ Ref. + all Ref. + autoshade Ref. + h_plaster+ Ref. + h_plaster Ref. + c_paint Reference 0

5

10

15

20

25

Heating energy demand (kWh/m2)

Monitored cases

Nov.

Dec.

Jan.

Feb.

Mar.

Apr.

Fig. 8.19 Cooling demand of the monitored building in the cooling season.

Mon. + all Mon. + autoshade Mon. + h_plaster Mon. + c_paint Monitored 0

5

10

15

20

Cooling energy demand (kWh/m2) May

June

July

Sep.

Oct.

Nov.

We recall that the reference building is the same as the monitored one, except for the fact that it is lacking thermal insulation and only has single glazing instead of the double glazing installed in the monitored building. The reference building had 6.59 kWh/m2 of energy demand in January, which was still the coldest month (Fig. 8.18). Note that “Ref. + all+” means that the reference building contains all the passive cooling techniques with an increased hemp plaster thickness.

8.4.1.2 Monthly cooling demand Fig. 8.19 shows the monthly cooling demand of the monitored building in the cooling season lasting from May to November: the monitored building + cool painting, the monitored building + hemp plaster, the monitored building + automated shading devices, and the monitored building with all the techniques combined. The hottest

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

month was July with a cooling demand peaking at 5.66 kWh/m2 and the building was highly affected by the passive measures used. Note that the month of August was not included, since the building was not occupied and did not contain any temperaturesensitive process or product. Fig. 8.20 shows the cooling energy demand for the reference/typical building in the cooling season lasting from June to October, much shorter than the cooling season for the monitored/insulated building, which suffers from serious overheating issues. With no surprises, July was the hottest month and the cooling demand peaked at 5.15 kWh/m2 for the reference building without any passive technique.

8.4.1.3 Yearly energy demand

Fig. 8.20 Cooling demand of the reference/typical building in the cooling season.

Reference cases

As shown in Fig. 8.21, the energy demand for heating of the monitored base case was 1.05 kWh/m2; it increased to 1.12 kWh/m2 for the monitored building + cool painting on the external walls. Adding a 2-cm hemp plaster on the roof’s bottom side facing the inside of the office led to a decrease to 1.02 kWh/m2, while the energy demand did not change with automated shading devices due to its control system; finally, the energy demand also increased when all the techniques were combined to reach 1.09 kWh/m2, which remained insignificant. On the other hand, the change in the cooling demand Ref. + all+ Ref. + all Ref. + autoshade Ref. + h_plaster+ Ref. + h_plaster Ref. + c_paint Reference 0

Fig. 8.21 Total energy demand of the monitored building with passive techniques.

Total energy demand (kWh/m²)

June

2 4 6 8 10 Cooling energy demand (kWh/m2) July

Sep.

Oct.

25 20

17.4

16.63

17.23 14.59

15

14.09

10 5

1.05

1.12

1.02

1.05

1.09

0 Monitored Mon. + Mon. + Mon. + Mon. + all c_paint h_plaster autoshade Monitored cases Heating

Cooling

Total energy demand (kWh/m2)

Hemp plaster and passive cooling techniques for retrofit

159

30 25

21.3

22.09

20.33

20 15 10

9.74

9.28

9.74

18.47

21.3

9.7

7.86

21.1

7.4

19.24

7.29

5 0 Reference Ref. + Ref. + Ref. + Ref. + Ref. + all c_paint h_plaster h_plaster+ autoshade Reference cases Heating

Ref. + all+

Cooling

Fig. 8.22 Total energy demand of the reference/typical building with passive techniques.

was significantly dependent on the passive technique used: the cool painting reduced the cooling demand of the monitored building by 4.4%; adding a 2-cm layer of hemp plaster decreased the cooling energy demand by 1%; installing automated shading devices helped to decrease the cooling demand by 16.1%; and combining all the techniques together reduced the cooling demand by 19%. For the reference building (Fig. 8.22), the total energy demand for heating was 21.3 kWh/m2; adding the cool painting increased the total energy demand for heating by 3.7% to reach 22.09 kWh/m2, while reducing the cooling energy demand by 4.7% from 9.74 to 9.28 kWh/m2. Adding 2 cm of newly developed hemp plaster did not affect the cooling demand but it reduced the heating demand by 4.6%, from 21.3 to 20.33 kWh/m2. Increasing the thickness of the hemp plaster from 2 to 5 cm led to a reduction in both the cooling and the heating demands by 0.4% and 13.3%, respectively, which is very significant. Installing automated shading devices did not affect the heating demand but reduced the cooling demand by 19.3% from 9.74 to 7.86 kWh/m2. Including all the passive cooling techniques in the building with the use of hemp plaster did not affect the heating demand much. However, the cooling demand was significantly decreased from 9.74 to 7.4 kWh/m2, which represents a 24% reduction. Increasing the thickness of the hemp plaster in the typical office building with all the passive cooling techniques decreased the cooling and heating demand even more, by 25.2% and 9.7%, respectively.

8.4.2 Indoor air temperature Using the free-floating mode in the simulation of the building, the temperatures were obtained in Office D, which is on the ground floor and faces north so is not subject to direct solar radiation. Fig. 8.23 shows that, in the case of the monitored building, the passive techniques reduced the indoor air temperature in Office D by about 0.9°C, while in the case of the reference building, the change was not as significant, since the passive cooling techniques reduced the indoor air temperature by less than 0.4°C. However, in Office G, which is on the first level of the building and facing south, the

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

37 Monitored 32

Mon. + all Reference

27

8 PM

10 PM

4 PM

6 PM

2 PM

12 AM

8 AM

10 AM

6 AM

4 AM

Ref. + all+ 2 AM

22

Ref. + all

12 PM

Temperature (°C)

42

Tamb

Time of day

Fig. 8.23 Indoor air temperature in Office D.

Temperature (°C)

42 37 Monitored 32

Mon. + all Reference

27

Ref. + all Ref. + all+ 12 PM 2 AM 4 AM 6 AM 8 AM 10 AM 12 AM 2 PM 4 PM 6 PM 8 PM 10 PM

22

Tamb

Time of day

Fig. 8.24 Indoor air temperature in Office G.

reduction of indoor air temperature was significant, as shown in Fig. 8.24. Thanks to the passive cooling techniques, the indoor air temperature in Office G was reduced by about 2°C in the monitored building and by about 1.2°C in the reference/typical office building.

8.4.3 Thermal comfort Fig. 8.25 shows the area weighted discomfort hours in all the buildings, according to the ASHRAE 55 adaptive comfort model in summer. The results show that in May, when all the passive techniques were included in the monitored building, the area weighted discomfort hours were reduced by 4.4% when considering the 90% acceptability limit, while when we considered the 80% acceptability limit, the reduction hit 8.5%. However, in June and July the passive cooling techniques did not improve the thermal comfort when we consider the number of discomfort hours. In September

Area wighted discomfort hours

Hemp plaster and passive cooling techniques for retrofit

161

Fig. 8.25 Area weighted discomfort hours of the monitored building according to ASHRAE 55.

300

250 200 150 100 50 0 Mai

June

July

Sep.

Oct.

Area weighted discomfort hours

Months Monitored (Ash-90)

Mon. + all (Ash-90)

Monitored (Ash-80)

Mon. + all (Ash-80)

250

Fig. 8.26 Area weighted discomfort hours of the reference building according to ASHRAE 55.

200 150 100 50 0

June

July Months

Sep.

Reference (Ash-90)

Ref + all (Ash-90)

Ref + all+ (Ash-90)

Reference (Ash-80)

Ref + all (Ash-80)

Ref +all+ (Ash-80)

there was a 3% reduction in discomfort hours when considering the 90% acceptability limit, and a 9% decrease when considering the 80% acceptability limit. The change was insignificant for the month of October when considering the 90% acceptability limit, while for the 80% acceptability limit, the reduction in discomfort hours was about 10%. Note that the discomfort hours were only calculated when the building was occupied, as stated in the hypotheses. Compared to the monitored case, the change in the reference building was quite significant when we added passive cooling techniques and hemp plaster. Fig. 8.26 shows that the discomfort hours were reduced in the building with all the techniques by about 22% and 24%, respectively, in June for an 80% and 90% acceptability limit, compared to the base case scenario. The increase in hemp plaster thickness did not affect the result significantly. In July, which is the hottest month, the discomfort hours were reduced by 19.2% and 29.5%, respectively, for 80% and 90% acceptability limit

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

Table 8.7 COP, EER, and electricity price. COP

EER

3.2 2.8 Electricity price: 0.1316 €/kWh (tax exclusive)

in the building with passive techniques, compared to the building lacking them. Finally, in September the discomfort hours were decreased by 72% for the 90% acceptability limit, and 83% for the 80% acceptability limit. We recall that August was not included because the building was not occupied during the summer vacation.

8.4.4 Economic aspects To evaluate the economic aspects of implementing hemp plaster and passive cooling techniques into the building, we considered a COP of 3.2 and an EER of 2.8, according to (AMEE Guide, n.d.) recommendations. We also considered an electricity price of 0.1316 €/kWh for public administration and office buildings (ONE, n.d.) as shown in Table 8.7. Table 8.8 shows the yearly cost of electricity used to heat and cool the building if we assume the building actually has an HVAC system. Also, the extra costs resulting from the implementation of the passive techniques are summarized in Table 8.9. The data provided in Tables 8.7–8.9 were used to evaluate the payback period for implementing each passive cooling technique; from a retrofit point of view, adding a 2-cm hemp plaster layer costs 5.5 €/m2 in total, and increasing its thickness to 5 cm costs 11.5 €/m2, including the materials needed and the implementation costs. However, for new buildings, using hemp plaster instead of regular plaster is cost efficient. We estimated the cost of hemp added to the regular plaster to be 1 €/m2 for a 2-cm layer and increasing its thickness costs an additional 5.5 €/m2, totaling 6.5 €/m2, since implementing costs and regular plaster raw materials are not accounted for and only the hemp added is included in the final cost. Finally, the cost of each automated movable shade was estimated to be 600 € and its implementation costs 25 € for each window on the south fac¸ade (CYPE Ingenieros, S.A., n.d.). The payback period was calculated without accounting for inflation and risk, and was done from a retrofit point of view. The payback periods are summarized in Table 8.10, and show that only the use of cool painting in the monitored building, and the reference building with a 5-cm layer of hemp plaster, have a reasonable payback period, with 37 and 53 years, respectively. Note that NA stands for not applicable, since the use of cool painting with the reference/typical building increased heating demand to a point that led to the total energy demand of the building being higher than the reference building. Therefore, it could not be considered as an improvement to the building.

Table 8.8 Economies in electricity and HVAC operation costs. Yearly price in €

Electrical

Monitored + c_paint + h_plaster + autoshade + all Reference + c_paint + h_plaster + h_plaster + + autoshade + all + all +

Heating

Cooling

Heating

Cooling

Heating

Cooling

Total

768.6 819.8 746.6 768.6 797.9 15,591.6 16,169.9 14,881.6 13,520.0 15,591.6 15,445.2 14,083.7

12,736.8 12,173.2 12,612.4 10,679.9 10,313.9 7129.7 6793.0 7129.7 7100.4 5753.5 5416.8 5336.3

240.2 256.2 233.3 240.2 249.3 4872.4 5053.1 4650.5 4225.0 4872.4 4826.6 4401.2

4548.9 4347.6 4504.4 3814.2 3683.5 2546.3 2426.1 2546.3 2535.9 2054.8 1934.6 1905.8

31.6 33.7 30.7 31.6 32.8 641.2 665.0 612.0 556.0 641.2 635.2 579.2

598.6 572.1 592.8 502.0 484.8 335.1 319.3 335.1 333.7 270.4 254.6 250.8

630.2 605.9 623.5 533.6 517.6 976.3 984.3 947.1 889.7 911.6 889.8 830.0

Economy

24.4 6.8 96.7 112.7 8.0 29.2 86.6 64.7 86.5 146.3

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

Table 8.9 Investment needed to implement passive techniques.

Acrylic cool painting Hemp plaster (2 cm) Hemp plaster (5 cm) Automated aluminum Venetian blinds

Quantity

Unit

Unit price

Implementation cost

Investment (€)

300 400 400 18

m2 m2 m2 –

1.5 4 10 600

1.5 1.5 1.5 25

900 2200 4600 11,250

Table 8.10 Energy economy and payback period.

Mon. + c_paint Mon. + h_plaster Mon. + autoshade Mon. + all Ref. + c_paint Ref. + h_plaster Ref. + h_plaster + Ref. + autoshade Ref. + all Ref. + all+

8.5

Energy economy (€)

Payback period (years)

24 7 97 113 8 29 87 65 87 146

37 326 116 88 NA 75 53 174 166 114

Conclusion

In this chapter we investigated the use of an eco-material we recently developed, called hemp plaster, as a material for the retrofitting of typical office buildings in Morocco in order to improve their thermal performance. It was found that adding a hemp plaster layer can reduce significantly the heating energy demand of a typical Moroccan office building in the Oujda region, reaching about 13% of reduction in total energy for heating over a typical meteorological year simulation. Increasing the thickness of the hemp plaster can also help reduce the cooling demand by improving the thermal resistance of the roof. We also included passive cooling techniques to mitigate the cooling demand of a monitored building that has good insulation and airtightness but suffers from severe overheating issues. The passive cooling techniques reduced the indoor air temperature by about 2°C in a south-facing room. When including all the passive cooling techniques to the monitored building, the energy demand for cooling dropped by 19%, especially with the help of automated movable shading devices. The passive cooling techniques were also included in the typical office building and contributed to a reduction of about 25% of the cooling energy demand.

Hemp plaster and passive cooling techniques for retrofit

165

In addition to the energy demand and indoor air temperatures, we also conducted an analysis of the thermal comfort of the occupants during summer, based on a metric we called “area weighted discomfort hours” that is based on the ASHRAE 55 adaptive comfort model, since the building is naturally ventilated. The results show quite insignificant improvement in the monitored/insulated building, while in the reference/typical building the reduction of discomfort hours was significant due to the passive cooling techniques implemented in the building. Also, an economic study was performed, without including inflation and risk, to evaluate the energy economies in € and the payback period, taking into account a split air conditioner with a COP of 3.2 and an EER of 2.8. The results show that only two variants achieved reasonable payback periods, which are 37 years for cool painting used in the monitored/insulated building and 53 years for adding a 5-cm layer of hemp plaster to the roof’s composition of the reference/typical office building. Finally, in a future work we will account for the PV panels on the rooftop of the building and their electrical production, and study the interaction between the building and the PV panels.

Acknowledgment The authors would like to thank the Moroccan National Center for Scientific and Technical Research (996183890) for funding this work through the PPR project, “Promotion of solar energy and energy efficiency in the oriental region of Morocco.” EnerMENA funds provided by the German Federal Foreign Office assisted in the procurement of the weather station. The collaboration between DLR and Universit
e Mohammed Premier of Oujda was made possible by the Morocco Meteonet Solar Project cofinanced by the German Federal Ministry of Education and Research and the Universit
e Mohammed Premier of Oujda, Morocco.

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Performance of multilayer glass and BIPV fac¸ ade structures

9

S. Medved, C. Arkar, S. Domjan, and T. Zˇizˇak Faculty of Mechanical Engineering, Laboratory for Sustainable Technology in Buildings, University of Ljubljana, Ljubljana, Slovenia

9.1

Introduction

The latest developments in energy-environmental policy in the European Union (EU) show the commitment of EU countries to a faster transition to a carbon-free society. Because buildings in the EU are responsible for almost 40% of energy consumption and contribute 36% of CO2 emissions, ambitious goals have been set for the residential sector as well as nonresidential, which are put into force through several directives on energy efficiency of buildings. The latest, the “EPBD recast” published in 2018 (EPBD, 2018), introduces targets expressed as net-zero energy building (NZEB) requirements. These requirements will lead to increased energy efficiency and sharing of onsite or nearby produced-energy carriers, needed to maintain optimal living conditions in buildings. The goals will be particularly challenging to meet in nonresidential buildings, which are often built with large window-to-wall ratios or even as all-glass buildings. Such buildings have low demand for heat but high demand for electricity for cooling, ventilation, and lighting (Medved et al., 2019). Nevertheless, advanced glazed building envelope structures could have an important role to play in the development of new and refurbished NZEBs, since they can be built as low thermal transmittance fac¸ade structures with integrated technologies for electricity production. Medved et al. (2019) presented research on semitransparent glazed fac¸ade structures (semitransparency was achieved by checkerboard m-silicon PV cells), showing that energy needs for cooling and heating of 1 m2 of advanced glazed fac¸ade structure can be covered by energy produced by PV cells with an area of between 45% in the Central European climate to 60% of total glazed area for the Stockholm climate. In general terms, advanced glazed fac¸ade structures can be related to static or dynamic properties of the glazing, such as light transmittance adjustable to light flux density or temperature, color rendering adjustable according to the physiological needs regarding daytime or age of the inhabitancies, or solar gain control adjustable to the incident angle of solar rays. Among the static properties, low thermal transmittance is the most important property of glazing. A thermal transmittance, or U-value, of the glazing of around 0.3 W/m2 K can be achieved for single glass and 0.26 W/m2 K for triple glass, double evacuated glazing (Ghoshal and Neogi, 2014), if the gap between two glass panes with low emission coating is evacuated Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction https://doi.org/10.1016/B978-0-12-820791-8.00009-2 © 2021 Elsevier Ltd. All rights reserved.

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

to a pressure below 0.1 Pa (Fang et al., 2010). After solving durability problems with edge sealing, marketed products have emerged (LANDVAC, 2020) with thermal transmittance of 0.42 W/m2 K. Inserting transparent insulation material (TIM) in the gap between the glass panes in the form of aerogel, a silica-based material with low thermal conductivity (λ
0.021 W/mK), a thermal transmittance of double glazing of 0.5 W/m2 K (Buratti and Moreti, 2012) was achieved. Recently, Kralj et al. (2019) reported on thermal properties of six-pane multilayer glazing that can be defined as the best-available technology (BAT) on the market. Such multilayer glazing can be produced in sizes up to 4 m high and 1.25 m wide and can have a center glazing thermal transmittance as low as 0.248 W/m2 K. Photovoltaics are a widely used technology for on-site production of electricity in buildings. Regardless of the type of PV cell technology installed in building integrated photovoltaic (BIPV) structures, the efficiency of electricity generation depends on the cell’s temperature. As open-circuit voltage decreases more significantly compared to increasing the short-circuit current, the power of the PV cell decreases with increased temperature. The impact of PV cell temperature on the efficiency is determined by the temperature coefficient, and values of 0.45 (%/K) for poly-c Si cells, to 0.36 (%/K) for thin-film CIGS, 0.25 (%/K) for CdTe, and 0.20 (%/K) for a-Si PV cells (Makrides et al., 2012) are typical. In the case of PV systems integrated into the (vertical) fac¸ade of the building, PV cells are sunlight by direct solar radiation shorter time compared to free-standing PV modules. When the sun is not shining, the density of (diffuse) radiation is significantly lower. The decrease of efficiency is commonly modeled by the solar radiation coefficient γ () and the log of the ratio of global solar irradiation on the surface of the BIPV Gglob,b (W/m2) to reference solar irradiation (γ log10 (Gglob,β/1000). A value for γ of 0.12 is commonly used (Skoplaki and Payvos, 2009). On the other hand, the vertical fac¸ade BIPV has a noticeable advantage with regard to dust and soiling (Lee et al., 2018). The decrease in efficiency of a PV system in the range between 14.3% (30-degree slope) and 23.1% (horizontal) was reported if PV modules have not been cleaned for a long time, while only a 2%–4% decrease was observed for south and west fac¸ade BIPV structures. The most common ways to mitigate the negative effect of the PV cell temperature are cooling by wind, buoyancy, or forced ventilation and accumulating the heat during the day using a phase change material (PCM) structure in contact with the PV cells, which then releases the heat during the night. Goverde et al. (2015) analyzed steady temperatures of a PV module (with a 3-mm polyethylene cover) exposed to a sun simulator in a wind tunnel. At a density of solar irradiation of 400 W/m2 the PV cell temperatures were 11°C lower at a wind speed of 1 m/s, 16°C at a wind speed of 2 m/s, and 21°C at a wind speed of 5 m/s, compared to nonwind conditions. Kaldellis et al. (2014) showed long-term experimental data on the impact of wind speed on the thermal loss factor, approximated by the sum of a constant and a wind speed-related component Uw ((W/m2 K), equal to the product of the empirical coefficient λw (W/(m2Km/s)) and the wind speed v (m/s). An average value for λw of 8.5 (W/(m2Km/s)) was determined for a windy location (average wind speed 1.6 m/s) and free-standing, slightly tilted PV modules; meanwhile, almost no wind-speed impact was noticed in the case of a building

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integrated PV system at a calm-wind site. Agathokleous and Kalogirou (2016) presented a nondimensional analysis of bounce and a forced ventilated gap between BIPV and the wall structure and reported that convective surface heat transfer coefficients are in the range between 1 and 15 W/m2 K in the case of natural convection and 10–50 W/m2 K in the case of forced ventilation. A comprehensive worldwide study on photovoltaic energy output was carried out by Smith et al. (2014). The authors analyzed the impact of PV cell cooling by adding aluminum plate and PCM to the bottom of the PV module for increased electricity production, and pointed out the regions where this technique can have the largest impact—the increased electricity production was 16 kWh/m2 per year. In a document published by the European Photovoltaic Platform (Solar Power Europe, 2019), it is claimed that solar photovoltaic electricity has become the lowest-cost source of electricity in most parts of the world and BIPVs are presented as a new business opportunity for successful energy transition in European cities. Such systems can be either building attached (BAPV) or building integrated (BIPV). BAPV systems are added onto the building using separate construction and have no direct effect on the building structure’s function (Biyik et al., 2017), while BIPV systems are integrated into the building envelope (roof or fac¸ade) and can replace conventional building materials. Besides energy supply, BIPV structures in general improve durability and reduce the need for maintenance of the building envelope, although such systems must be carefully assessed, because they are more sensitive to the shading from surrounding objects (Saretta et al., 2019; Walker et al., 2019). BIPV structures can be installed on the opaque building envelope or as part of transparent envelope structures. In the first case, besides electricity generation, BIPV structures can be functionally upgraded into a BIPV/T structure to provide heat to the building as well. Such structures are designed to operate as an open circulation system to preheat ventilation air while decreasing the temperature of the PV cells (Quesada et al., 2012), or as a closed-loop system for space heating (Aelenei et al., 2014). The decrease of cooling load and cooling needs through opaque envelope structure due to shading and ventilation cooling is commonly presented as one of the advantages of BIPVs. Peng et al. (2013) studied heat transfer in opaque walls with BIPV structures and reported that summer heat gains can be decreased by 51% due to ventilation cooling and shading, although the wall has a high thermal transmittance of 3.3 W/m2 K. Peng et al. (2015) also investigated air velocities in ventilated BIPV fac¸ade structures through in-situ experiments and reported velocities up to 0.15 m/s in the case of buoyancy-driven flow; they also reported that wind velocity below 1 m/s has no significant impact on the air-flow velocities in a ventilated cavity. Taking into account that contemporary opaque envelope structures have low thermal transmittance, the decreasing of cooling loads is moderate to low. Several studies were carried out to increase the impact of the shading and ventilation of the gap formed between BIPVs and the building envelope by storing latent heat in PCMs. An extended overview of potential PCM materials for low-to-high temperature applications in buildings was published by Cabeza (2013); meanwhile, Karthick et al. (2020) focused on the specification of PCM custom optimization for BIPVs. Curpek et al. (2019) investigated the thermal response of ventilated nontransparent BIPV and BIPV/PCM in the front of an

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opaque fac¸ade and reported that peak heat flux through the fac¸ade structure decreased from 4.2 to
1 W/m2 in the case of a BIPV/PCM fac¸ade. Kant et al. (2019) developed a multiparametric model of heat flux transferred through a concrete wall with BIPV in contact with a PCM layer and ventilated gap. They used computational fluid dynamics (CFD) techniques; nevertheless, they optimized the thermal response under steady-state conditions. Pereira and Aelenei (2019) optimized air velocity, ventilated gap, and PCM layer thickness as
10 m/s, 0.06 m, and 0.05 m (QL 35.152 J/kg), respectively. The evaluation of BIPV glazed fac¸ade structures is even more complex due to the multifunctionality of such structures. Besides the impact on heating and cooling loads and energy needs, the effect on lighting, as well as the possibility of hybrid ventilation and the free cooling from night-time ventilation, should all be taken into consideration. Domjan et al. (2019a) showed that the final energy demand of an office building with an advanced all-glazed fac¸ade for cooling and ventilation can be decreased in the Athens climate from 58.3 to 44.2 kWh/m2an in the case of a south BIPV glazed fac¸ade, and down to 27.0 kWh/m2an in the case of BIPV glazed fac¸ade structures on all facades and if hybrid ventilation and free cooling is implemented. Under the same conditions, final energy demand for cooling and ventilation decreases from 34.0 to 24.6 kWh/m2an (south BIPV) and to 14.6 (all-fac¸ade BIPV) for the Stockholm climate. This can be achieved by only 2.5 kWh/m2an (Athens) and 2.75 kWh/m2an increased use of electricity for lighting, while the final energy demand for heating differs by less than 1 kWh/m2an. To further improve the energy performance of glazed BIPV structures, similar measurements comparing opaque BIPV can be implemented. Peng et al. (2015) presented the study of a double a-Si BIPV ventilated fac¸ade in front of a single glazed window. They reported that air velocity of bouncy driven flow u was 0.12 m/s and that the inner surface of the window pane had a 4.8°C lower average diurnal maximum temperature in the case of force ventilated BIPV. To provide daylighting, BIPVs consisting of thin film PV cells were selected. Transparency of BIPV glazed fac¸ade structures can also be achieved with m-Si or p-Si PV cells installed in a checkerboard pattern. Xu et al. (2014) defined the PV cell coverage ratio based on the window-to-wall ratio and the office depth. They proposed 60% of PV cell coverage for the all-glazed fac¸ade structure in the case of a deed office (with depth-to-height ratio of
4). Domjan et al. (2019b) presented a multiparametric model for optimizing thermal and visual properties of advanced multipane glazing, including PV coverage ratio. Research on thermal performance of semitransparent CdTe BIPV windows at temperature climate was presented by Alrashidi et al. (2020). The temperatures of window glass inner surface and thermal transitivity were given, although single glazing was studied. This chapter looks at studies on thermal response of advanced BIPV glazed fac¸ade structures in the form of multipane glazing and triple glazing with a ventilated glass fac¸ade. Market-available six-pane glazing with thermal transmittance as low as is required for opaque structures in most of the national regulations on energy-efficient buildings was analyzed in the first study case, and a prefabricated double-glazed fac¸ade was the second example examined. Both products were upgraded to BIPV in the form of a glass-laminated phase change (PC) module.

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As the main research was focused on the evaluation of cooling loads, the impact of PCM inserts in glazed structures was analyzed in extreme diurnal summer weather conditions in different locations.

9.2

Object of research

Recently, Kralj et al. (2019) and Arkar et al. (2019) reported on thermal properties of innovative six-pane glazing that can be defined as the best-available technology (BAT) on the market. A multilayer glazing structure can be built with up to six glass panes with size up to 4 m high and 1.25 m wide. In the case of six-pane glazing, central glass thermal transmittance can be as low as 0.248 W/m2, which fulfills the criteria on energy efficiency of buildings with opaque fac¸ade structures in most EU countries. Thermal and radiative properties of such structures were tested in-situ and the results confirmed the modeled values (Kralj et al., 2019). In the context of the EU-supported project TIGR4smart (2019), such multilayer fac¸ade structures were upgraded to BIPV fac¸ade structures (Fig. 9.1). According to previous optimized distributions of PV cells in a checkerboard arrangement, an area corresponding to 60% of the structure area was found to be optimal regarding the overall final energy demand for heating, cooling, and lighting (Medved et al., 2019). Because solar irradiation is absorbed in each of the glass panes, the structure acts as a low thermal capacity structure. To decrease the cooling load, the dynamic thermal response of a BIPV glazed multilayer fac¸ade structure was determined for a structure with PCM inserts the size of the PV cells, which are installed on one of the glass layers inside the cavities. In this way, the architecture design, visibility of outdoor surroundings, and daylighting are not influenced negatively. For the second BIPV case study, a fac¸ade structure having triple glazed windows, with ventilated glass facade panes acting as physical/safety barriers in front of the windows, was selected. This market-available product is shown in Fig. 9.2. In the research this structure was upgraded to a BIPV fac¸ade with a force ventilated air gap. Besides

Fig. 9.1 Multilayer glass fac¸ade structure with thermal transmittance or U-value of 0.248 W/m2, available in sizes up to 4  1.25 m, with up to six glass panes (Trimo, 2019) (left); advanced multilayer glass structure with integrated PV cells (middle, right) (TIGR4smart, 2019).

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Fig. 9.2 Glass pane with an air gap in front of the triple glazed window; in the study the glass pane was replaced by a BIPV glass structure and PCM inserts were installed in the form and on the spot of PV cells.

electricity generation, this type of structure enables decentralized ventilation with preheated air during the heating season and decreases the cooling load during the cooling season by heat accumulation and night-time ventilation. A sketch of the research objects is shown in Fig. 9.3. The six-pane glass fac¸ade structure and the triple window glazing structure with glazed fac¸ade and ventilation cavity were taken as reference objects. The thermal transmittances of the reference multilayer structure and the triple glazing structure were 0.268 and 0.9 W/m2 K, respectively. The goal of the research was to investigate the cooling loads defined by the maximum heat flux and daily cooling needs, defined by heat transferred from the surface of the interior glass pane to the building interior. Thus the diurnal thermal response of the structures was studied for a clear summer day under extreme climate conditions. Three sites were analyzed: Athens, Ljubljana, and Stockholm. For the multipane glass fac¸ade structures (Figs. 9.1 and 9.3, left), transient numerical simulations were performed to determine the temperature field within the structure and heat fluxes on the inner surface of the structure. Based on those results, the following properties were determined: (a) average PV cell temperature during sunlight hours, (b) maximum glass pane temperature, (c) maximum heat flux at inner surface of the structure, which defines cooling load, and (d) heat transferred into the indoor space as a consequence of the absorbed solar radiation in the fac¸ade structure. With known heat flux at the inner surface of the structure and transitivity of solar radiation, the total solar transmittance g can be determined and used in the computer models of the thermal response of the building. Besides the multipane glass fac¸ade structure as a reference structure, the transient thermal response of the advanced multipane BIPV glass fac¸ade structure with and without PCM inserts in the shape of the PV cells was also determined. Because the glass structure was narrow (110 mm), it was assumed that inserts could be built on any of the inner glass surfaces without disturbing the view toward the outdoors. As the multipane glass structure itself limited

Fig. 9.3 Objects of the research: reference six-pane glass fac¸ade structure (left) upgraded with BIPV (left top) and with integrated PCM inserts (left bottom); reference triple glazed structure with glazed fac¸ade and ventilated gap (right) upgraded with BIPV (right middle) and with integrated PCM inserts (right left).

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Table 9.1 Approximated climate data of reference summer days as boundary conditions used in transient CFD simulations.

Geographical location Maximum solar irradiation Gglob,90,max Average daily outdoor air temperature Te,avg and amplitude Ae Solar irradiation

Athens

Ljubljana

Stockholm

λ 23°430 , Φ 37°580 400 W/m2 30°C, 5 K

λ 14°300 , Φ 46°030 500 W/m2 22.5°C, 7.5 K

λ 18°040 , Φ 59°200 800 W/m2 20°C, 5 K

t < 18,000 s, Gglob,90,(t) ¼ 0 W/m2 t ¼ 18,000–39,579 s, Gglob,90,(t) ¼ (Gglob,90,max)/ 21,600)(t  18,000) t ¼ 39,580–43,209 s, Gglob,90,(t) ¼ Gglob,90,max t ¼ 43,210–61,200 s, Gglob,90,(t) ¼ ((Gglob,90,max)/21600)(t  43,210) t > 61,200 s, Gglob,90,(t) ¼ 0 W/m2

Outdoor air temperature Te,(t)

Te,(t) ¼ Aesin ((2πt/86,400) + 2π0.46) + Te,avg (°C)

the thickness of the inserted PCM elements, the inserts were 10.4 mm thick (corresponds to two layers of encapsulated PCM). The optimum melting temperature and most effective position of the PCM inserts inside the glass structure were points of interest. Second sets of numerical simulations were performed for the case of the ventilated BIPV glazed structure installed in front of the triple glazing in such a way that it formed a 100-mm wide ventilated cavity (Figs. 9.2 and 9.3, right). As in the case of the multilayer glazed fac¸ade structure, the BIPV consisted of monocrystalline silicon cells with total area corresponding to 60% of the glass fac¸ade area. The shape and properties of the PCM inserts as well as reference climate conditions were equal to those in the study case of the multipane glass facade structure (Table 9.1). The impact of the BIPV structure, PCM inserts, and ventilation air flow rate on cooling load were studied for extreme summer climate conditions.

9.3

Overall assessment indicators

In the presented study, it was assumed that the advanced glass and BIPV fac¸ade structures were built into a nonresidential building that is occupied daily between 08:00 and 17:00. Because of this, energy performance indicators were developed taking into account the thermal response of the advanced BIPV fac¸ade structure during the occupied period. Energy performance indicators were developed as relative figures that compare the performance of the advanced BIPV fac¸ade to the reference glass fac¸ade structures (Fig. 9.3). The heat flux damping factor fd was defined based on the maximum heat flux q_ +i,max at the inner surface of the fac¸ade structure directed toward the

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177

building interior. The heat transfer ratio fh was defined based on the diurnal heat transferred into the building per unit area of glazed fac¸ade structure during the period when the building was occupied. fd ¼ 1  Z fh ¼

q_ i,BIPV, max ð Þ q_ i, ref

17:00

+ q_ i,BIPV

Z8:0017:00 8:00

¼

q_ i,+ref

qi,BIPV ðÞ qi, ref

It must be emphasized that both fd and fh take into account only the convective and radiation heat fluxes at the internal surface of the structure as transient heat fluxes. The total cooling load and heat gains can be determined by the principle of superposition, adding solar heat flux, which can be treated as nontransient heat flux. By developing the heat transfer ratio fh it was assumed that heat flux that enters the building after working time is extracted from the building by natural cooling techniques, for example by nighttime ventilation. For the BIPV fac¸ade structures, overheating hours OHHBIPV (h/day) were determined to be additional indicators, indicating the diurnal difference between actual PV cell temperature and the reference temperature of 25°C, and as such they indicate a decrease of the PV cells’ efficiency. Z OHHBIPV ¼

0 sunset

1 25°C zfflffl}|fflffl{ @TPV  TPV, ref Adt ðKh=dayÞ

sunrise

Schematic procedure for determination of fd, fh, and OHHBIPV is shown in Fig. 9.4.

9.4

CFD model and boundary conditions

The research was focused on the dynamic modeling of cooling loads that result from heat transfer in advanced glazed and BIPV glass fac¸ade structures. Thus numerical simulations were performed for clear summer day and extreme climate conditions. Three sites were analyzed: Athens, Ljubljana, and Stockholm. A dynamic model was used due to the relatively high thermal mass of the multipane glass structure, high sensible heat accumulation of absorbed solar irradiation in the outer glass pane in the case of the BIPV fac¸ade structure, and also to enable modeling of the latent heat accumulation in the PCM inserts. A CFD technique was used for solving temperature, pressure, and velocity fields in the 2D fac¸ade structures. The PHOENICS 2019 (2019) computer software was used to solve the set of Navier-Stokes equations. Static thermal and radiative properties of the multiglass fac¸ade structures were determined by WINDOW 7.6 (2018) software. Transient CFD simulations were performed in 300-s time steps for a time period of 30 h at constant air indoor temperature of 26°C. Numerical results for the period between 06:00 to 06:00 of the next day were

Fig. 9.4 Values taken into account in the determination of the proposed energy performance indicators of the advanced BIPV glazed fac¸ade structures (left) and performance of the on-site renewable energy utilization (right).

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Fig. 9.5 Average daily outdoor air temperature Te,avg and amplitude Ae for selected locations for four summer months; red dots indicate values taken into account in reference days shown in Table 9.1.

used for the evaluation of diurnal thermal properties of the advanced glazed BIPV fac¸ade structures to avoid the influence of initial conditions. Boundary conditions were defined by the indoor Ti and outdoor Te air temperature, constant combined surface heat transfer coefficients hc+r,i and hc+r,e, and timedependent source terms defined for both the glass pane and PC cell. Meanwhile, the indoor air temperature was set as a constant according to thermal comfort requirements, and the outdoor air temperature was approximated by the sin function, taking into account site climate conditions. Since extreme temperature conditions were assumed, the outdoor air temperature was defined in such a way that only
2% of summer days had higher average outdoor air temperature Te,avg and, according to that, the outdoor air temperature amplitude Ae was also chosen. In Fig. 9.5, climate data for four summer months in selected cities are shown. The data were taken from the Meteonorm data library (Meteonorm, 2005). Climate boundary conditions are presented in Table 9.1. Source terms were defined according to the solar irradiation Gglob,90 on the vertical south-oriented fac¸ade. Trapeze-shaped pattern distribution was assumed. The following maximum daily values of Gglob,90,max were assumed: 800 W/m2 for Stockholm, 500 W/m2 for Ljubljana, and 400 W/m2 for Athens. Transmissivity of solar irradiation was compared to modeled values in the range of incident angles 15 degrees by in-situ experiments. It was found that the data modeled by WINDOW 7.6 were adequate for dynamic modeling [x]. For dynamic modeling, the absorptivity of solar irradiation of each of six glass panes αs,n were taken from WINDOW 7.6 generated data. For the diffuse solar irradiation, αs,n was determined at a constant incident angle of 55 degrees.

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Fig. 9.6 Temperature dependence of specific heat capacity cp,PCM on the encapsulated PCM used in numerical modeling.

Thermal properties of PCM inserts were taken from manufacturer’s data (DuPont, 2012). Inserts, consisting of aluminum-sheet encapsulated paraffin, were built using two PCM panes with a total width of 10.4 mm. A latent heat storage capacity of 104.5 kJ/kg and sensible heat storage capacity of 71 kJ/kg (in the range of 5–30°C) and thermal conductivity of 0.18 and 0.22 W/mK were assumed for solid and liquid state, respectively (Kuznik and Virgone, 2009). The melting point of PCM was defined by the temperature Tm,PCM,max at which the specific heat capacity cp,PCM has a peak value. The temperature at which PCM starts to melt was taken as Tm,PCM,max  12°C and the temperature at which PCM is liquid was set as Tm,PCM,max + 4°C. The shape of the specific heat capacity cp,PCM curve (Fig. 9.6) was established based on the study from Eddhahak-Ouni et al. (2013) and was approximated by the function presented in the article from Arkar et al. (2018). For optimum PCM melting temperature purposes, Tm,PCM,max was changed, but the shape of the specific heat capacity curve remained unchanged. The thermal response of the PCM inserts was modeled using a CFD modeling procedure based on discrete tabulated values in the form of an enthalpy gradient measured from absolute zero temperature, which were read by computer code during each time step. The share of liquid PCM was established according to the shape of the specific heat capacity curve and melting point Tm,PCM,max. Only conductive heat transfer was assumed in PCM, since paraffin in the panel is encapsulated. As an example of numerical modeling, the temperatures in the cross-section of the advanced six-pane glazed, BIPV, and BIPV with PCM inserts glass fac¸ade structures are shown in Fig. 9.7 (left). Fac¸ade structures are 3 m high, with PCM inserts installed on the third glass pane. Temperatures are shown for the Stockholm reference day

Temperature (°C) 102 97 92 87 82 77 72 67 62 57 52 47 42 37 32 27 22

Temperature (°C) 54 51 48 46 43 40 38 35 33 30 27 25 22 19 17 14 12

Temperature (°C) 23 21 20 19 18 17 16 15 14 12 11 10 9 8 7 6 5

Fig. 9.7 Temperatures in cross section of the multipane glazed, BIPV, and BIPV glazed fac¸ade structure with PCM inserts at 50,100 s for Stockholm reference day (left); temperatures in the cross section of the ventilated BIPV fac¸ade in front of triple glazing simulated with PCM insert on the second to bottom PV cell row using in-situ measured climate data and at volume flow rate 36.0 m3/h at 48,000 and 59,400 s of simulation time (right); note that scale of structure thicknesses is 10.

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(Table 9.1) at the simulation time 50,100 s (13:55 solar time) at which maximum temperatures in the structure occur. Edge heat losses were taken into account by surface heat transfer coefficient defined on the basis of steady numerical calculation by PHYSIBEL TRISCO code (2010). The second case (Fig. 9.7, right) shows temperatures in the BIPV ventilated fac¸ade structure determined by actual climate data (Fig. 9.8) measured on January 31, 2020 on the experimental fac¸ade structure shown in Fig. 9.1 at time 48,000 s (13:20) and 59,400 s (16:30 local time).

9.5

Validation of CFD model

An in-situ experiment was carried out to validate the CFD model of transient thermal response of the multipane BIPV glass fac¸ade structure and ventilated BIPV structure with PCM inserts. The test glass BIPV was installed in front of the south-oriented opaque thermal-insulated wall of the conditioned building. A PV module with 24 156 x 156 mm monocrystalline cells with reference efficiency of 18.8% was installed as the BIPV structure (Union Glass, 2019). The test fac¸ade was divided into two similar vertical sections, in one of which two layers of encapsulated PCM (DuPont, 2012) were installed on the opaque wall, as shown in Fig. 9.8 (left). Temperature, heat flux, and air velocity sensors, as well as solar and IR irradiance and wind velocity sensors, were installed. K-type thermocouples were used for temperature measurements; a hot-wire anemometer for continuous measurements of the air velocity, and heat flux meters sized at 120  120 mm were used for measurement of heat flux on the internal surface of the fac¸ade structure. Velocity sensors were installed in the pipe duct with 100-mm diameter at 10 D distance from the bend. At air velocity in the pipe of
1 m/s, the flow was turbulent, as Re > 6000 in the case of internal flow in the duct. The Blasius formula was used to obtain mean velocity in the pipe. Global solar irradiation was measured by a Kipp&Zonnen CM11-P pyranometer and longwave IR irradiation from the sky was measured by a Kipp&Zonnen CG1 pyrgeometer. A meteorology station was installed for measuring other meteorological parameters. The accuracy of the thermocouples was validated in a thermostatic bath and the hot-wire anemometers were validated by vane anemometer. Each gap was force ventilated by two highly efficient DC fans and counter draft flaps were built to prevent bouncy driven flows in the opposite direction. A computer data acquisition system recorded data in 60-s periods. Validation was carried out by adapting PHOENICS simulation code to read measured values of the outdoor air temperature and solar irradiation on the surface of the BIPV structure from a *.TXT file through InForm command SOURCE of TEM1 at PATCH is coval(fix,PWLF(temp.txt,TIM)) and SOURCE of TEM1 at OBJECT is coval(fixflu,PWLT(sol.txt,TIM)) with WHOL. Data from the clear and calm day of December 31, 2019 were used for validation of the transient thermal response of the BIPV ventilated fac¸ade with a PCM layer with a thickness of 10.4 mm, installed on an opaque wall, as shown in Fig. 9.8. Constant values of the combined external he,r+c and internal hi,r+c surface heat transfer coefficient of 12 and 8 W/m2 were assumed, respectively. The value of he,r+c was determined according to the balance of IR heat fluxes (incoming was measured,

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183

Fig. 9.8 BIPV ventilated fac¸ade structure built for validation of CFD transient simulations; the test fac¸ade was divided into two vertical sections, in one of which two layers of PCM were built. To enable further all-year optimization, the PCM panel was painted with a high solar absorptance coating. The lightweight wood fac¸ade wall with inserted thermal insulation layer has a thermal transmittance of 1.120 W/m2 K.

outgoing flux calculated by the Stefan-Boltzmann law) and McAdams correlation. The solar irradiation absorptivity of the PCM panel and IR emissivity equal to 0.90 were used. The volume flow rate of ventilation air was set as a constant and was equal to 20.6 m3/h. In Fig. 9.9, a comparison of the PV cell temperature of the middle PV cell in the fourth and sixth rows is shown. Experimental data correspond to inner glass BIPV surface temperature; nevertheless, observation of CFD results showed that the difference in the temperature of the observation points was negligible.

Fig. 9.9 Validation of the CFD model of BIPV ventilated fac¸ade with PCM inserts on PV cell temperature in fourth and sixth rows in BIPV fac¸ade structure (left); temperature of inner glass pane behind the PV cells in BIPV (middle); specific heat flux at inner surface of the structure (right).

186

9.6

Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

Parametric analysis

Parametric analysis was carried out for the advanced glazed fac¸ade structures presented in Section 9.2. The aim of the parametric analysis was to determine to what extent the BIPV structure and the BIPV structure with PCM inserts improve overall assessment indicators regarding the reference structures. The melting point temperature of the PCM inserts was optimized and in the case of the multipane glazed fac¸ade structure, the optimal position of the PCM inserts was found. The optimization criteria were: (a) the lowest maximum heat flux at the inner surface of the structure q_ i,max during the building occupied hours, which define cooling load; (b) the lowest amount of heat that is transferred into the buildings during the building occupied hours, which defines cooling needs; and (c) minimum q_ i at the end of the observed period (this will be at 06:00 the next morning) which indicates overheating of the structure. However, it can be assumed that consecutive extreme days will rarely occur, and that free cooling of the building during the night will eliminate overheating of the glazed fac¸ade structures.

9.6.1 Optimization of PCM inserts position in multipane glazed fac¸ade structure To define the optimum PCM insert position, the inserts were installed in the numerical model on the inner surface of the first to fifth glass panes. As was mentioned, the thickness of the inserts was designed with consideration to the market-available encapsulated PCM panels and the distance between glass panes in the advanced multipane glass structure (18 mm). The climate data for the Athens reference day was used in the numerical simulations. In all cases the PCM melting temperature Tm,PCM,max was 43°C, so that latent heat storage starts at a temperature 2 K above the daily average outdoor air temperature (Fig. 9.6). Heat flux q_ i at the inner surface of the fac¸ade structure for the reference summer day in Athens for glazed, BIPV, and the BIPV fac¸ade structure with PCM inserts is shown in Fig. 9.10. Positive values indicate heat flux direction toward the building interior. The BIPV fac¸ade structure significantly improved the energy performance of the multipane glass fac¸ade structure, as maximum heat flux q_ i,max decreased from 52.2 to 26.4 W/m2. The amount of heat transferred at the inner surface of the structure during the occupied hours (08:00 to 17:00) decreased from 325 to 164 Wh/m2. It can be seen that the PCM inserts further improved energy performance of the multipane BIPV glass fac¸ade structure and that the optimum position of the PCM inserts is on the third glass pane. In this case, the cooling load decreased to almost three-quarters compared to the glazed structure and to almost half compared to the BIPV glazed structure. The decrease in cooling needs is approximately equal to the decrease of the cooling loads in all cases. Complete results are shown in Table 9.2. The same position of the PCM inserts was found for the Ljubljana and Stockholm reference days.

Six pane glazing BIPV

45

35

Occupate hours

BIPV, PCM,1 BIPV, PCM,2 BIPV, PCM,3 BIPV, PCM,4 BIPV, PCM,5

qi (W/m2)

qi (W/m2)

55

25

BIPV BIPV, PCM,1 BIPV, PCM,2 BIPV, PCM,3 BIPV, PCM,4 BIPV, PCM,5

15

25

15

5

Occupate hours

5

–5

Sun noon

21.6 28.8 36.0 43.2 50.4 57.6 64.8 72.0 79.2 86.4 93.6 100.8 108.0

Time x 103 (s)

–5

Sun noon

21.6 28.8 36.0 43.2 50.4 57.6 64.8 72.0 79.2 86.4 93.6 100.8 108.0

Time x 103 (s)

Fig. 9.10 Heat flux q_ i at inner surface of the six-pane glazed fac¸ade structure and BIPV glazed fac¸ade structure determined for Athens reference day (left); precise presentation of heat flux q_ i at inner surface of the BIPV glass fac¸ade structure with PCM inserts installed on first to fifth glass pane (right).

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Table 9.2 Assessment indicators determined for multipane glass fac¸ade structures for the reference extreme summer day in Athens.

9.6.2 Optimization of multipane glazed fac¸ade structure PCM insert melting temperature After optimization of the PCM insert positions, the PCM melting temperature Tm,PCM, max was optimized. Optimization criteria remained as in the previous case. Numerical simulations were performed for the range of Tm,PCM,max between 28°C and 44°C. This was carried out by maintaining the shape of specific heat capacity cp,PCM of the encapsulated PCM equal in all cases (Fig. 9.6). Fig. 9.11 shows heat flux q_ i at the inner surface of the structure over the reference day, for the Ljubljana and Stockholm reference days. It was discovered that Tm,PCM,max has no significant impact on cooling loads; nevertheless, an optimal value can be noted. At least for the selected places, it can be claimed that a Tm,PCM,max of 12°C above the average outdoor air temperature will lead to the lowest cooling load. Of course, there are significant decreases of cooling load and cooling needs compared to the six-pane glass and BIPV multipane glazed fac¸ade structures: by 65%–75% compared to the six-pane glazed and 44%–52% compared to the BIPV fac¸ade structure (Table 9.3).

9.6.3 Maximum temperatures of glass panes in the multipane glazed fac¸ade structure Due to the fact that absorption of solar irradiation occurs in each glass pane and because of low thermal transmittance of the multipane glazing, glass pane temperatures in extreme climate conditions are high (Kralj et al., 2019). This causes high thermal stress in the glass panes as well. Despite the fact that PV cells shade the interior glass panes, the high temperature of the BIPV structure prevents the inner glass pane from being cooled toward the outdoors. PCM inserts installed on the glass pane can decrease extreme glass pane temperatures, but how effective this is depends on the PCM melting temperature and position of the PCM inserts. Taking into account the claim that the optimal PCM melting temperature is
12°C above daily average outdoor temperature, transient numerical simulations were performed to evaluate maximum glass pane temperatures. Fig. 9.12 shows the maximum diurnal temperatures of glass panes at the cross section at a height of 2.8 m, where the influence of

Fig. 9.11 Heat flux q_ i at inner surface of six-pane glazed fac¸ade structure and BIPV glazed fac¸ade structure determined for Ljubljana (top) and Stockholm (bottom) reference day.

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

Table 9.3 Maximum heat flux q_ i,max at the inner surface of the advanced glass fac¸ade structures during occupancy of the building, heat flux q_ i at the end of the calculation period (at 06:00 in the morning following the reference day) and heat transferred into the building interior by convection and radiation from the inner surface of the fac¸ade structures.

the spacer and edge heat bridges are no longer detected. This corresponds to the middistance between the 15th and 16th (last) row of the PV cells and it was assumed that the inner glass panes are not shaded by the PV cells at that point. Regarding the position of the PCM inserts, the highest temperatures were noticed between 48,600 and 52,800 s of simulation time. In both cases, the highest temperature in the six-pane glass structure occurred in the third glass pane (BIPV, PCM 3), and meanwhile in the BIPV glazed structure, the second glass pane (BIPV, PCM 2) was the hottest, while the maximum temperatures of the central internal glass panes were lower by 10–15°C. The temperature of those glass panes can be lowered further by approximately 20°C if PCM inserts are installed on the second glass pane. Excluding the second glass pane, the decreasing of temperature in all other internal glass panes, as well as the cooling loads, was greatest when the PCM inserts were installed on the third glass pane.

9.6.4 PV cell temperature in multipane glazed fac¸ade structure Several techniques can be implemented to decrease the temperature of the PV cell (module) and, as a consequence, to increase electricity generation. In this research, we investigated the extent to which the PC cell overheating can be reduced by latent heat storage in PCM inserts. Transient numerical simulation was performed to evaluate the diurnal maximum of the average PV cell temperature. Individual PV cell temperatures were replaced by the average value, since the temperature gradient of PV cell temperature at extreme conditions was only 3–5 K (1st to 16th PV cell). Fig. 9.13 shows the maximum average PV cell temperature in the south-oriented BIPV glazed fac¸ade with and without installed PCM inserts for Athens and Stockholm reference days.

Fig. 9.12 Temperature profile across the six-pane glazing, BIPV structure, and BIPV structure with PCM inserts at height 2.8 m (in the middle of the 15th and 16th (last) row of the PV cells for Athens (left) and Stockholm (right)); for the Athens case Tm.PCM,max, equal to 43°C, is taken into account, as the Stockholm Tm,PCM,max was 32°C. Note that temperatures of the outer glass (at width 0.00 mm) do not defer significantly between six-pane glass and BIPV glazed fac¸ade structure due to the observation plane—it crosses the glass structure between the last two rows of PV cells and not the PV cells.

Fig. 9.13 Average PV cell temperature in BIPV determined for reference day in Athens (left) and Stockholm (right).

Performance of multilayer glass and BIPV fac¸ade structures

193

Table 9.4 Overheating hours OHHBIPV of PV cells in BIPV six-pane glazed fac¸ade structure.

Overheating hours OHHBIPV (h/day) were determined by taking into account the period of the day when the fac¸ade is sunlit: between 23,400 and 59,400 s in Stockholm and between 32,400 and 57,600 s were expressed in the simulation running time. The results are shown in Table 9.4. Compared to the BIPV fac¸ade structure, the maximum PV cell temperature can be decreased by 2°C (in Stockholm) to 4°C (in Athens), if PCM inserts are installed on the first glass pane. Obviously, the PCM inserts do not affect the PV cell overheating, since only in the case of Athens could some influence be noticed if the PCM inserts were installed on the first glass pane. This is due to the low thermal conductivity of glass and high temperature of the second glass pane.

9.6.5 Cooling load and cooling needs of ventilated BIPV fac¸ade in front of triple glazing As the second case study of transient thermal response of advanced glazed fac¸ade structures, cooling loads and cooling needs were investigated for the ventilated BIPV glazed fac¸ade installed in front of triple glazing, as is shown in Figs. 9.2 and 9.3, for different reference summer diurnal climate conditions. Details on the reference days determined for Athens, Ljubljana, and Stockholm climate conditions are presented in Table 9.1. As in the case of multilayer glazed fac¸ade structures, the BIPV consisted of monocrystalline silicon cells with total area corresponding to 60% of the glass fac¸ade area. The impact of the PCM inserts in the shape of the PV cell installed on the inner surface of the BIPV structure was investigated as well. The thickness of the PCM inserts was 10.4 mm (Fig. 9.2, right) and the thermodynamic properties of the PCM material are presented in Section 9.4 (DuPont, 2012). Since it was found that the average PV cell temperatures were similar to those in the multipane glazed fac¸ade structure, PCM inserts with the same melting temperature for the particular location were used (for Stockholm 30°C, for Ljubljana 32°C, and for Athens 43°C). It was assumed that the gap force ventilated with average inlet air velocity of 0.2 and 0.5 m/s (range corresponds to the buoyancy driven ventilation) and at 1 m/s. This corresponds to a volume flow rate of 360 m3/h per 1-m width of the fac¸ade. The temperature of ventilation air is equal to the outdoor air temperature (Table 9.1).

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

The specific heat flux q_ i at the inner surface of the glazed structures toward the interior of the building, evaluated for different climate conditions, is shown in Fig. 9.14. Unless otherwise stated, an average air velocity at the inlet opening of the ventilated gap of 1 m/s was considered. The BIPV structure had a decreased cooling load, which refers to the maximum diurnal q_ i, by more than 22–42 W/m2 compared to the glazed structures. The PCM inserts further decreased cooling loads: by 10% in Athens, 16% in Ljubljana, and 13% in Stockholm reference climate (Table 9.5). A sensitivity analysis was carried out, focused on the PCM melting temperature and average inlet air velocity of the inlet opening of the gap. From Fig. 9.14 (middle) it can be seen that the lower PCM melting point doesn’t change the thermal response of the observed structure, at least for the case of cooling load evaluation. The average inlet air velocity has a greater impact (Fig. 9.14, right), since cooling loads can be decreased up to 7% of the inlet velocity increase, from 0.2 to 1 m/s. A similar impact can be seen for the BIPV and BIPV glazed fac¸ade structures with PCM inserts.

9.6.6 Temperature of PV cells in ventilated BIPV fac¸ade Based on transient numerical simulations, the average temperatures of the PV cells in BIPV glazed structures were evaluated. Although air flow in the ventilated gap caused a temperature gradient, the gradient was relatively low (up to 5 K at gap air velocities >0.5 m/s) and the average PV cell temperature can be used for the assessment of the PV cell efficiency. Fig. 9.15 (left) shows the average temperature of different glazed fac¸ade types over the reference summer day in Ljubljana and Athens. In the case of the PCM inserts, the PCMs with Tm,PCM,max of 34°C (Ljubljana) and 43°C (Athens) were assumed based on minimum cooling load criteria. It can be seen that the maximum average BIPV fac¸ade temperature will increase 7–9°C above the temperature of the glazed fac¸ade structure in both climate conditions. It can also be concluded that, for particular boundary conditions, PCM inserts will cause an additional increase in maximum daily temperature of the BIPV glazed fac¸ade (and PV cell temperature) of 2.2–2.5°C. At lower inlet air velocities, the average PV cell temperature increases, but the influence of cooling by ventilation is obviously limited (Fig. 9.15, middle). In Fig. 9.15 (left), the temperature distributions in cross section at the height of 2.7 m on the structures are presented at simulation time 48,600 s (daily maximum). It can be seen that the surface temperature of the PCM inserts is lower than the surface temperature of the BIPV fac¸ade structure, but low thermal conductivity kPCM (W/mK) of the PCM material is the reason for the PV cell overheating (Table 9.6). For the examples presented, it was found that the air velocity in the ventilated gap did not have a high impact, which means that natural ventilation is an option for cooling the BIPV fac¸ade structures. The parametric study was carried out to evaluate under which conditions PCM inserts in the ventilated cavity decrease peak diurnal PC cell temperature and overheating during sunlight hours OHHBIPV. The thermal diffusivity a (m2/s), the thermal effusivity b (Ws0.5/mK) of the PCM, and the ratio between the conductance resistance

Fig. 9.14 Heat flux q_ i at inner surface of ventilated BIPV glazed fac¸ade structure, evaluated for reference climate conditions for Athens (right), Ljubljana (middle), and Stockholm (left); ventilated glazed, ventilated BIPV, and ventilated BIPV with PCM insets were evaluated; unless otherwise stated, PCM melting temperature for Athens was 43°C, for Ljubljana 32°C, and for Stockholm 30°C, and average air velocity at the inlet of the ventilated gap was 1 m/s.

Table 9.5 Maximum diurnal heat flux q_ i,max at the inner surface of the ventilated glass fac¸ade structures and heat transferred into the building interior during the period of occupancy of the building determined for different climate conditions and fac¸ade technological solutions. Athens

q_ i,max (W/m2) Qi,(8–17) (Wh/day) fd () fh ()

Ljubljana

Glazed

BIPV

BIPV + PCM

43.2 275

21.8 141 0.49 0.52

19.6 128 0.54 0.47

Stockholm

Glazed

BIPV

BIPV + PCM

47.2 282

20.2 113 0.57 0.40

17.6 98 0.62 0.34

Glazed

BIPV

BIPV + PCM

81.7 495

37.0 214 0.54 0.43

33.7 199 0.59 0.40

Fig. 9.15 Average glass fac¸ade structure temperatures by structure type: clear glass, BIPV, or BIPV with PCM inserts, evaluated for the reference day for Athens and Ljubljana at inlet air velocity in the ventilated gap of 1.0 m/s (left); the impact of inlet air velocity in the gap on diurnal maximum heat flux q_ i for Stockholm case (middle); cross-section profile of temperatures at the height of the structure of 2.7 m determined for Stockholm for different glass fac¸ade types at the simulation time 48,600 s (left).

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

Table 9.6 Overheating hours OHHBIPV of PV cells and in ventilated BIPV glazed structure installed in front of triple glazing.

in PCM inserts and surface heat transfer resistance expressed as Biot number, equal to hc+r. dPCM/kPCM were defined as influencing parameters (Fig. 9.16). Results of the parametric study of south-oriented BIPV and BIPV glazed fac¸ades for the Stockholm reference summer day are shown as PV cell overheating hours OHHBIPV (defined in Fig. 9.4) in Fig. 9.17. In the case of the BIPV fac¸ade with PCM inserts, the results are shown for different thicknesses of PCM inserts. It can be concluded that, in the case of low thermal conductance PCM material, OHHBIPV is lower compared to the BIPV glazed fac¸ade only at hc+r below 7.5 W/m2 K for the reference thickness of the PCM insert (dPCM 0.0054 m) and 10 W/m2 K for the PCM with four times the reference thickness. In the case of high thermal conductivity of PCM (k 1 W/mK), OHHBIPV could be decreased to half of that

Fig. 9.16 Properties that influence the transient heat transfer in BIPV and BIPV glazed fac¸ades with PCM inserts with ventilated cavity (Fig. 9.3, right).

Fig. 9.17 Overheating hours OHHBIPV of PV cells in BIPV and BIPV with PCM insert glazed fac¸ades with ventilated cavity (Fig. 9.3, right), determined using Stockholm reference day data and theoretical thermal conductivity k of the PCM, and combined convection and radiation surface heat transfer coefficient hc+r on the surface of the BIPV or PCM inserts toward the ventilated air gap.

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

characteristic for the BIPV glazed fac¸ade; nevertheless, the advantage of the PCM inserts can be seen only at an hc+r below 25 W/m2 K. For the assumed boundary conditions, the PCM inserts only cause a decrease in the maximum daily PV cell temperature TPCM,max if the thermal conductivity of the PCM material is above 0.5 W/mK.

9.7

Conclusion

In global warming mitigation planning, very ambitious targets are set regarding increasing the energy efficiency of buildings. They include not only energy conservation but on-site energy production as well. Two products in the form of advanced glazed building envelope structures that fulfill both targets are evaluated in this chapter. Taking into account the topics covered in this book, we have focused our research on the analysis of building cooling loads and cooling needs and how they can be reduced by the use of advanced technologies, such as multilayer glass structures and PCM materials. Both products are developed in such a way that they decrease energy demand for cooling, enable adequate daylighting, and provide for the buildings to become energy independent by electricity generation through BIPV solutions.

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Part Three Roofing materials for reducing cooling needs

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Green roofs as passive system to moderate building cooling requirements and UHI effects: Assessments by means of experimental data

10

Roberto Bruno, Piero Bevilacqua, and Natale Arcuri Mechanical, Energy and Management Engineering Department, University of Calabria, Arcavacata di Rende, CS, Italy

10.1

Introduction

Improvement of city sustainability, due to its inherent complexity, requires a deep transformation and reorganization of urban spaces, and represents a challenging target to attain in the near future (Cajot et al., 2017). Moreover, this target must be achieved by addressing the difficult combination of growth and social equity, which presents open questions affecting cities at different levels of development (Duvier et al., 2018). In this context, the scientific community should consider the spread of natural ecosystems as a contributor to the ongoing battle against climate change and depletion of fossil energy reserves (Nicoletti et al., 2016). The majority of the world’s resources, in fact, find their final end-use in cities, with negative environmental impacts at the local level. An eco-friendly city makes urban spaces more sustainable, especially with interventions that reduce energy consumption and pollutant emissions. The employment of eco-systems such as green covers, installed on flat or slightly pitched roofs, can improve urban livability and increase the efficiency of building-plant systems, with consequent reduction of the energy demand from fossil sources. The green roof, also called an eco-roof, living roof, or roof garden (Saadatian et al., 2013), is a passive solution to building covers that can reduce thermal energy demands with limited operating costs. Actually, these systems are not a novelty for the building sector, because they have been used since ancient times, with the most famous example being the Babylonian gardens in Mesopotamia (Ayata et al., 2011). Applications that are more recent can also be found, especially in Northern European countries; however, these systems are predominantly used in cold climates to improve the building envelope insulation and to facilitate storm-water management. Nevertheless, green roofs can also limit cooling demands (Sailor, 2008; Theodosiou, 2003; Zinzi and Agnoli, 2012; Jaffal et al., 2012; Parizotto and Lamberts, 2011; Coma et al., 2016) and have other benefits, such as the reduction of rainwater runoff (Mentens et al., 2006; Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction https://doi.org/10.1016/B978-0-12-820791-8.00010-9 © 2021 Elsevier Ltd. All rights reserved.

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Lee et al., 2013; Carbone et al., 2015), the mitigation of the urban heat island effect (UHI) (Susca et al., 2011; Bowler et al., 2010), the improvement of membrane durability and the extension of roof life (Teemusk and Mander, 2009; Liu and Minor, 2005). Furthermore, other advantages can be listed, such as carbon dioxide sequestering, reduction of localized air pollution (Saadatian et al., 2013; Currie and Bass, 2008; Rowe, 2011; Baik et al., 2012), improvement of water runoff quality (Vijayaraghavan et al., 2012), all with positive effects on human health. Beyond the aspects mentioned, the creation of new ecosystems providing habitat for wildlife and recreational opportunities (Dvorak and Volder, 2010; Schrader and B€ oning, 2006; Williams et al., 2010; Ignatieva et al., 2011), the reduction of noise pollution (Van Renterghem and Botteldooren, 2011; Van Renterghem et al., 2013; Korol et al., 2018a), and the spread of social gardens in urban environments for the production of “zero kilometer” food (Wilkinson and Dixon, 2016) must also be contemplated. Regarding energy savings evaluations for air-conditioning applications, several results can be found in the literature that discuss both experimental and theoretical data. However, the latter studies often bring up the lack of ways to link green roof models with the simulation tools employed in building simulations. Usually accurate numerical approaches are used (Sailor and Hagos, 2011; Lazzarin et al., 2005; Tabares-Velasco and Srebric, 2012; Feng et al., 2010; Del Barrio, 1998; Ouldboukhitine et al., 2011); however, interacting with building models has been addressed by many authors in simplified ways. For instance, in Niachou et al. (2001) the vegetated roof was simulated by modifying the thermal conductance of the building cover. In Spala et al. (2008), the vegetated roof was considered as a further thermal resistance in the cover layering by implementing the whole building in the TRNSYS environment, which currently is not equipped with a dedicated mathematical model of vegetated walls. Conversely, a detailed green roof model is available in EnergyPlus (Crawley et al., 2000), by also considering latent exchanges, and it was employed in Ascione et al. (2013) to determine the energy savings attainable in different European climatic conditions. A more exhaustive approach for the evaluation of green roof energy performance could be represented by using experimental data as input in building simulation tools. Indeed, these data permit consideration of the actual interaction between the building cover and the passive system. For instance, the monitored values of the temperature at the structural roof interface can be set as boundary conditions in TRNSYS for the external surface of the ceiling. The interface temperature, in fact, is the resultant of all the thermal exchanges that affect the green roof by taking into account every term involved in the energy balance in the actual operational conditions (Bevilacqua et al., 2020). Experimental data have demonstrated that green roofs reduce cooling demands not only in warm locations, since summer heat flows were reduced by 70%–90% also in the humid continental climate of Toronto (Liu and Minor, 2005). Moreover, the maximum interface temperature between the green roof and the structural roof decreased more than 20°C, whereas daily temperature fluctuations reduced of about 30°C. In Estonia, with similar climatic conditions, the same reduction was detected in green roof solutions during the summer; moreover, the roof membrane cooling in the

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intermediate seasons was decelerated with a positive impact on heating requirements (Teemusk and Mander, 2009). Even in the US Midwest climate, green roofs reduced heat flows by 167% during the summer (Getter et al., 2011). Comparison with a bare roof solution located in Singapore showed that a green roof reduced the internal air temperature, in free-floating conditions, by an average value of 0.5°C (Qin, 2013). In an experimental setup in Ancona (Italy), a green roof was proven to mitigate summer heat fluxes and indoor temperatures; however, the cooling potential was hindered by high roof insulation layers (D’Orazio et al., 2012). In the same location, other investigations have proven that green roofs can reduce the entering heat flux 60% more than solutions without vegetation, by proving the noticeable role of plants in the energy balance (Olivieri et al., 2013). However, these data highlight the fact that green roof performance cannot be generalized, because it depends on the adopted layering and especially on the climatic conditions. Indeed, many publications have considered the impact of green roofs on building energy consumption, but these studies have shown a wide variability of results, referring both to heating and cooling demands (Vera et al., 2017). These sources of uncertainty increase when numerical models evaluate green roof thermal performance. For instance, in Ascione et al. (2013) cooling energy savings of a reference one-story office located across different warm European climates ranged between 1% and 11%, whereas they were reduced up to 7% in continental climates. Cooling and heating load reductions up to 19% and 11%, respectively, were determined for a supermarket located in Greece that was equipped with a vegetated roof over a concrete slab (Foustalieraki et al., 2017). Conversely, a study carried out for a small-scale building in Pennsylvania (United States) showed negligible annual energy savings connected with the employment of a vegetated roof on the building cover (DeNardo et al., 2003), because the reduction in the cooling demand was counterbalanced by a similar increase in heating demand. Globally, green roof energy performance is still a matter of debate among scientists, who are still evaluating the potentially achievable annual energy savings. For instance, regarding the climatic zone, the lack of an insulation layer in the building cover facilitates a reduction in cooling requirements; however, as already mentioned, this advantage can be counterbalanced by a correspondent increase in heating demands.

10.2

Design options

Vegetated roofs are classified into different categories as a function of the weight and the maintenance levels, as follows: intensive (heavyweight systems), semiintensive, and extensive (lightweight configuration) (Cascone, 2019). Usually, extensive and intensive green roofs are the most widespread solutions, with the first being more suitable for building refurbishment (Korol et al., 2018b). Intensive green roofs, in fact, are deep and heavy, require a high level of maintenance and are often designed in order to provide complete accessibility. Therefore, the extra weight on the structure has to be considered during the design phase, and consequently they are more suitable for new buildings. The extensive green roofs, instead, are developed with a limited thickness

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1 2

1

Growing media

3

2

Drainage and storage

3

Protection layer

4

Antiroot

5

Lightweight slab (slope purpose)

6

Waterproof layer

7

Lightweight slab

8

Insulation and structural roof

4 5 6 7 8 7

GSPublisherVersion 0.50.100.100

Fig. 10.1 Typical layering of an extensive green roof.

of porous growing media, so are lightweight and less invasive from a structural point of view, with lower maintenance and operating costs. Indeed, limited water consumption for irrigation can be obtained by planting self-sustaining native vegetation species adapted to the local climatic conditions, especially in dry climates. Usually, the presence of several layers is mandatory to achieve good performance, including the vegetation, the substrate, and the drainage layers. Actually, green roof layering is often more complex, as depicted in Fig. 10.1, requiring the installation of other materials, such as: l

l

l

l

an antiroot membrane to preserve the roof structure; a slightly tilted lightweight concrete slab to drive the discharge of rainfall water; a waterproof layer to avoid water infiltration; the structural roof as the support of the whole system.

10.3

Green roofs to moderate urban heat island (UHI)

Green roofs have also been regarded as suitable systems to mitigate urban heat islands (UHIs), by quantifying the overall effects with the introduction of proper indexes (Bevilacqua et al., 2017). The validity of these systems is confirmed by legislative plans adopted by several countries to stimulate the spread of vegetated roofs on buildings as a strategy to combat UHIs (Carter and Fowler, 2008). The largest cities worldwide, in fact, are characterized by internal elevated summer temperatures when compared with their outlying surroundings, due to the low surface albedo in buildings that favors the absorption of the incoming solar radiation. This issue is further amplified by city geometry, in which the presence of tall buildings and narrow streets leads to the reduction of self-shading effects and the solar radiation remains trapped inside the urban canyons, due to the multiple reflections, contributing to the air-temperature

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increase. Furthermore, in summer this situation is further aggravated by the anthropogenic heat released by human activities, such as the heat released from heat pumps for air-conditioning purposes. Finally, the heat stored in the building structures cannot be dissipated into the air due to the limited view factors; consequently, this effect impedes the radiative infrared exchange with the sky. The UHI phenomenon has been widely investigated in several studies (Rizwan et al., 2008; Ward et al., 2016; Alexandri and Jones, 2008; Hart and Sailor, 2009; Ramamurthy and Sangobanwo, 2016) highlighting the negative effects in terms of human health and environmental and energy consumption implications (Santamouris et al., 2015; Wang et al., 2016; Sun and Augenbroe, 2014). UHI effects can be attenuated using different techniques, such as the adoption of highly reflective surfaces, urban geometry planning and urban vegetation growth, but cool and green roofs are more promising solutions. The first is less expensive, due to the employment of reflective paints to limit solar radiation absorption and the consequent reduction of surrounding temperatures; however, in winter this could promote a worsening of heating demands due to lack of exploitation of solar gains (Ascione et al., 2013; Akbari and Matthews, 2012; Santamouris, 2014; Marino et al., 2015; Coutts et al., 2013; Costanzo et al., 2016). Green roofs, on the other hand, appear more appropriate to reduce air-conditioning demands without compromising the winter thermal performance, when the latter is relevant, because proper layering can add further thermal resistance on the building cover to limit thermal losses, especially in dry climates (Gagliano et al., 2015). It is worth mentioning that the presence of vegetation on the building roof also affects photovoltaic (PV) performance positively, because it improves the panel back surface cooling (Chemisana and Lamnatou, 2014). The overall effect on a large scale in urban spaces is due to the transpiration processes of the vegetation planted on green roofs. This, in fact, allows for the reduction of the air temperature around the passive system, helping to moderate the heat island. Furthermore, the presence of vegetated surfaces allows for the limitation of solar radiation absorbed from building surfaces by means of shading effects. However, a possible critique could be the variability of the vegetation properties during the year, which could influence thermal performance considerably (Bevilacqua et al., 2015), with an amplified effect when evaluated on a city scale.

10.4

Green roof modeling

Green roof solutions involve different physical phenomena that often include simultaneous heat and mass transfer processes, with another source of uncertainty being represented by the thermal property variation in the function of water content. Furthermore, the latter represents one of the more crucial parameters affecting the thermal performance of green roofs due to the latent exchange that removes the heat from the roof structure. The presence of vegetation species instead allows for a shading effect on the growing media surface, reducing the absorption of solar radiation. However, other heat fluxes influence the thermal performance, such as long-wave radiative exchange with the sky, the convective exchange with the outdoor air, and the

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precipitation rate, all appearing in the green roof energy balance. All these features make green roof modeling very difficult and, consequently, many computer models have been implemented in simplified way. Moreover, the validation of these models was carried out by referring to specific layering. Such models cannot be employed in a generalized way, because they are tuned to precise climatic conditions and for specific green roof configurations. Consequently, the reliability of the results could be questionable when projected for other situations. Accurate heat and mass transfer models are essential to consider the interaction between the vegetation cover and the underlying building. Moreover, as mentioned before, the correct modality to couple green roof models with more detailed building simulation tools is still being debated. Nevertheless, accurate computer modeling of green roofs requires a preliminary mathematical simulation of the main layers constituting the building cover, in particular the vegetation and the growing media. Regarding vegetation modeling, the main parameters required in computer programs are the leaf area index (LAI), the stomatal resistance, the plant height, and the leaf reflection and emissivity coefficients. Regarding the substrate, the thermal properties considered are the soil thermal conductivity, heat capacity, density, and thickness. Nevertheless, the latter parameters often are set as constants without considering the influence of water content, especially if the transfer function model is used to implement opaque surfaces. In Wong et al. (2003), a sensitivity analysis carried out by considering the influence of the vegetation type, water content, and the growing media thickness caused an energy savings variability between 1% and 15%, with reference to a nonresidential building in Singapore. Again, other investigations carried out with other weather conditions have shown that LAI (Theodosiou, 2003; Vera et al., 2017, 2015; Sailor et al., 2012) and growing media thickness (Reyes et al., 2016) influence thermal performance consistently. In addition, the green roof layering plays a decisive role in the energy savings evaluation: for instance, the presence of a thermal insulation layer caused lower cooling energy savings than with noninsulated green roofs, because in summer the insulation hinders the extraction of heat from the roof structure ( Jaffal et al., 2012; Niachou et al., 2001; Bevilacqua et al., 2020; Silva et al., 2016; Zhao et al., 2014; Jim, 2014). Globally, few specific studies can be found in the literature involving numerical heat and mass transfer models. In addition, only a small number of numerical heat and mass transfer models have been included in building simulation tools to consider the interaction between the envelope and the green roof surface. Furthermore, validated software specifically developed to investigate on the green roofs role to moderate the UHI effect, are not available. The majority of green roof model development was implemented in the last decade (Vera et al., 2018), and among these computer models many similar issues can be easily identified. For instance, common aspects of concern are the hypotheses of considering only homogeneous plants, neglecting the growing media properties and water content along the horizontal direction, employing monodimensional heat and mass transfer processes, the presence of saturated air in proximity of stomata, the absence of plant photosynthesis in the energy balance equations, and no heat conduction

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transmitted by plants. At least 10 models are very simplified because they do not consider the mass balance, with results that refer exclusively to the energy balance by setting the water content constant during the simulations. In general, from a thermal point of view, two energy balances can be written with reference to the plants (pl) and the growing media (med), respectively, as: S_ abs,pl ¼ Q_ conv,pl + Q_ rad,plsky + Q_ rad,pl + Q_ trasp

(10.1)

S_ abs,med ¼ Q_ conv,med + Q_ rad, medsky  Q_ rad,med + Q_ cond, med + Q_ evap

(10.2)

where the first member represents the actual absorbed solar radiation (S), whereas on the other side the convective (Qconv) and radiative (Qrad) heat fluxes appear, as well as the latent rates (transpiration Qtrasp for the plant and evaporation Qevap for the growing media). Despite its contribution being negligible, one model also includes the role of photosynthesis in the plant energy balance (Heidarinejad and Esmaili, 2015). The mass balance takes into account the transport of water by means of the variation of the volumetric water content in the growing media (wg) with density ρ and depth Δz, by including the rates of precipitation (P), evaporation (E), and drainage (D): ρ  △z 

dwg ¼PDE dt

(10.3)

The majority of the mathematical models differ in their complexity levels in determining the variation of the water content in the substrate. In detailed computer model calculations, often the 1D Richards equation is used to determine the variation of the volumetric water content: 
   dwmed δ dΛp δ δwmed Ψ med  Dmed  ¼ +1 ¼ + Ψ med δz δz dt δz δz

(10.4)

The time variability is determined as a function of the media hydraulic conductivity (Ψ , namely the aptitude to transfer water inside the substrate), the moisture potential of the substrate tension (Λp), and the drainage. In turn, the first can be calculated as a function of the correspondent properties in a saturated condition as:


wg Ψ med ¼ Ψ med,sat  wg,sat

2b + 3 (10.5)

where b is a calibration parameter. The water diffusion coefficient Dmed through the substrate can be evaluated as: Dmed ¼ 


 b  Ψ med,sat  Λp, sat wmed b + 2  wmed,sat wmed,sat

(10.6)

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More simplified evaluations of the water content usually produce errors in the surface substrate temperature with a magnitude of 2°C (Ouldboukhitine et al., 2011), and these deviances obviously increase in rainy climates (Chan and Chow, 2013). Moreover, the most famous Sailor model (Sailor, 2008), widely employed because it is implemented in EnergyPlus, also did not include the water balance in its older version. Later, the same model was updated adequately by including other terms in the balances, such as the precipitation rate intercepted by leaves and successively vaporized (Heusinger et al., 2018). Nevertheless, an evident simplification in EnergyPlus concerns the growing media properties that are set as constant with the volumetric water content. More articulated models instead incorporate the water mass balance inside Eqs. (10.1), (10.2) directly, providing more robust results (Lazzarin et al., 2005; Del Barrio, 1998; Ouldboukhitine et al., 2011; Alexandri and Jones, 2008; Heidarinejad and Esmaili, 2015; Takebayashi and Moriyama, 2007; Djedjig et al., 2012; Wang et al., 2013; Rakotondramiarana et al., 2015; De Munck et al., 2013). The growing media, or substrate, constitutes a critical aspect of green roofs, affecting the thermal performance by influencing the system inertia, the thermal resistance, the transmitted heat flux, and attenuating in summer the temperature oscillations inside the layering. The approaches usually employed to state the thermal characteristics of the growing media use a transfer function or finite difference methods. The first is widely adopted in building modeling, for instance by TRNSYS, because it is reliable but difficult to generalize. The weighting coefficients, depending on the wall layer properties, are calculated at the initial time-step and they are set constant for the whole simulation duration. Consequently, the transfer function method does not consider the variability of the thermal properties with the volumetric water content. With the finite difference method, the green roof is discretized in a finite number of nodes and, for each of them, the balance equations are written by considering also the capacitive effects and consequently the transient behavior. For internal nodes, the energy equations involve heat conduction and mass transfer; conversely the upper node is affected also by convective, radiative, and latent heat exchanges. These features have suggested the latter method as being mainly employed in the mathematical models to attain reliable results in transient conditions and with variable inputs. Some approaches use the finite difference method also in steady-state conditions; however, the effects due to the thermal mass growth, as well as the timeshift and the attenuation of the thermal flux transmitted through the structure, cannot be determined. With reference to transient models, steady-state finite difference methods have showed deviances on the interface substrate temperature, with overestimates of over 2°C, with a transmitted heat flux greater than 10 W/m2 and peak power shifts of 3 h (Djedjig et al., 2012). For the internal nodes, balance equations involve the substrate thermal conductivity, which often is determined by applying the Johansen and the De Vries methods (Farouki, 1981). Soil and substrate have to be well distinguished, because of the very different properties: the latter, in fact, is modified to attain a lower density, a major organic content, and granular media with large size. For instance, as a function of the water content, the growing media conductivity can be over 1 W/m K in saturated conditions (Sailor and Hagos, 2011; Sailor et al., 2008), whereas the soil can also

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reach values greater than 2 W/m K (Simo˜es and Serra, 2012). Other investigations report that thermal conductivity can vary by a factor equal to 10 as a function of  the water content (Medved and Cerne, 2002). Similar problems also affect the specific thermal capacity, which in wet conditions can assume values 35%–45% greater than the correspondent values measured in dry substrates (Sailor and Hagos, 2011; Zhao et al., 2014; Sailor et al., 2008). In the simplified models, usually a linear function is employed to consider the thermal property variability as a function of the volumetric water content. In addition, the optical properties, especially the albedo and the emission coefficient, are affected by the water content. The latter, in fact, makes the surface darker, facilitating the solar radiation absorption (Marasco et al., 2014). Emissivity, instead, is widely influenced by the media granular size (Mira et al., 2007). However, the role of the last two parameters is less significant in a green roof well covered by plants. By considering that the water content is a decisive parameter in determining the green roof performance, two different approaches are usually suggested in the literature: models in which the volumetric water content is set as an input (TabaresVelasco and Srebric, 2012; Takebayashi and Moriyama, 2007) and more accurate models that perform the water mass balance in order to evaluate the actual water content as a function of the external conditions (Lazzarin et al., 2005; Del Barrio, 1998; Ouldboukhitine et al., 2011; Alexandri and Jones, 2008; Heidarinejad and Esmaili, 2015; Djedjig et al., 2012; Wang et al., 2013; Rakotondramiarana et al., 2015; De Munck et al., 2013). As mentioned, the absorption of solar radiation is strongly affected by LAI and plant coverage, since they block part of the shortwave radiation absorbed by the substrate. However, some models do not consider the shading effects due to the plant coverage (Takebayashi and Moriyama, 2007; Quezada-Garcı´a et al., 2017). Conversely, two approaches can be implemented to take this phenomenon into account: a simplified method where a fractional factor is introduced to reduce the incoming solar radiation on the horizontal surface (Sailor, 2008; Djedjig et al., 2012; Rakotondramiarana et al., 2015; Alexandri and Jones, 2007; He and Jim, 2010), and a more detailed method implementing Beer’s law in order to consider the short-wave extinction coefficient in the energy balance equations (Lazzarin et al., 2005; Tabares-Velasco and Srebric, 2012; Del Barrio, 1998; Heidarinejad and Esmaili, 2015; De Munck et al., 2013; Chen et al., 2015; Gaffin et al., 2005; Tabares-Velasco et al., 2012). The more debated aspect, concerning its analytical evaluation, is surely the evapotranspiration contribution (Cascone et al., 2019a), which combines the effects connected to the plant transpiration and the water evaporation in the growing media, because it could be weighted as up to 90% of the net absorbed solar radiation in the energy balance (Tabares-Velasco and Srebric, 2011; Maa et al., 2018). Surely, evapotranspiration is driven by the vapor pressure deficit, in turn depending on the outdoor weather conditions, vegetation properties, and available moisture inside the substrate. The calculation of the evapotranspiration can be carried out by the simplest Bowen ratio, namely the ratio between latent and sensible heat fluxes, which ranges between 0.12 and 0.45 (Martens et al., 2008), by determining the first with the sole convective heat exchange rate. In Takebayashi and Moriyama (2007), an efficiency factor was introduced in order to evaluate evapotranspiration in the same

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manner but as a function of the water latent vaporization heat. Other detailed models report a formulation with the evapotranspiration contribution evaluated as a function of the vapor pressure deficit, assuming the plants as a single vegetation entity with average properties, realizing the so-called Penman-Monteith model (Lazzarin et al., 2005; Ouldboukhitine et al., 2011; Chen et al., 2015; Ray and Glicksman, 2010; Arkar et al., 2018). However, this formula relates to the potential evapotranspiration, namely the maximum rate obtainable involving exclusively the weather conditions and referring to well-watered grass. The vapor pressure deficit model, instead, is more accurate because evapotranspiration is related to both plant (stomatal resistance) and substrate (water content) properties, in order to determine the actual evapotranspiration, as described in Sailor (2008), Tabares-Velasco and Srebric (2012), Ouldboukhitine et al. (2011), Djedjig et al. (2012), and Alexandri and Jones (2007). These models also include the vapor internal resistance of plants through the leaf surface that, usually, is set as constant, despite the fact that it depends on the vegetation species, weather conditions, and water content. The stomatal resistance, in fact, is inversely proportional to the volumetric water content, because the plants tend to minimize the water losses when the substrate is dry. The literature reports other sophisticated models, such as that developed by Maa et al. (2018), which includes the effects due to wind speed, moisture, and solar radiation. In order to make the modeling phase more complex, the connection between evapotranspiration processes and convection exchange, the latter occurring above the plant canopy and the substrate and quantified by applying an overall heat transfer coefficient, determined as a function of the Nusselt number (Nu), has to be considered. Many models impose laminar flows on the building coverage and this simplification leads to underestimating the convective heat exchange up to three times less than actual operational conditions, which seem to show the instauration of mixed convection. The latter is usually modeled as a function of the Grashof (Gr) and Reynolds (Re) numbers and the characteristic length (L) of the building cover using the following correlation (Maa et al., 2018):
  Gr 0:33  Nu ¼ 2:53   3 + 1:25  0:0253  Re0:8  L0:067 2:2 Re (10.7)  5 if 0:068Re2:2 < Gr < 55:3 Re 3 A more accurate model (Allen et al., 1998) also takes into account the aerodynamic resistance due to the vegetation presence to modify the wind profile and, consequently, the convection heat transfer coefficient. Conversely, in a simplified way, by assuming an average plant height of 10 cm, the convective coefficient as function of the wind velocity (v) is instead determined in a simplified manner as (Allen et al., 1998): h ¼ 5:6  v

(10.8)

Moreover, many correlations concerning mostly extensive green roof configurations can be found in the literature (Tabares-Velasco and Srebric, 2011; Onmura et al., 2001), both for laminar and turbulent air flows. Nevertheless, the employment of a

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215

predominant turbulent flow in the mathematical models leads to detection in summer of deviances of up to 2.4°C on the surface temperature compared to experimental data (Alexandri and Jones, 2007). Regarding the radiative heat fluxes, beyond the evaluation of plant and substrate emission coefficients, a further error source is connected with the evaluation of the sky temperature, which can be calculated by specific correlation as a function of the main weather data but developed for specific climatic conditions (Oliveti et al., 2012; Bruno et al., 2016). Globally, many models have been validated referring to specific contexts and mostly by means of statistical indexes, such as RMSE and MBE, calculated from the interface surface temperature (Feng et al., 2010; Vera et al., 2018; TabaresVelasco et al., 2012; Pianella et al., 2017; Fioretti et al., 2010). Nevertheless, no models have been validated with reference to the whole building-green roof system for the evaluation of thermal energy requirements. By considering the difficulties in adopting the aforementioned models in a generalized way and the potential low reliability grade of the results for different climatic contexts and layering solutions, the analyses of experimental results to determine thermal performance and energy savings currently seems to be the most appropriate approach.

10.5

Description of an experimental setup with extensive green roofs

Different extensive green roofs, characterized by different layering, typology of drainage systems and vegetation, have been installed on the building cover at the University of Calabria (South Italy) and have been monitored since 2012. The structure hosting the passive systems is an office building with a square plan, 21.30  21.30 m, and a global height over the ground of 18 m, consisting of five floors and one underground. Three different types of extensive green roofs, with a net surface area of 50 m2 each, were installed on the 380 m2 of the available plane surface (see Fig. 10.2). A fourth sector (bare roof) was left unchanged and used as reference in order to quantify the difference in thermal performance between the green and traditional roofing solutions. The structural roof consists of reinforced concrete joists with a total thickness of 31 cm. The main difference between the first and second solution (Plot 1 and Plot 2, respectively) is the hydraulic properties of the installed materials; however, both are noninsulated and they have been planted with the same native Mediterranean species (Dianthus grantianopolitanus, Carpobrotus edulis, and Cerastium tomentosum). The third green roof (Plot 3) consists of the same materials with the addition of a 3-cm insulation layer under the water storage layer and it was initially not planted. In the successive years, instead, spontaneous sparse vegetation was observed on the growing media. The thermal properties of the several materials employed for the green roofs are listed in Table 10.1. In particular, the substrate was made with reduced content of pumice and mainly lapillus with different sizes and shapes, and organic matter at a percentage lower than 6%. As mentioned, the reference roof was represented by

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Fig. 10.2 Experimental setup for green roof monitoring.

Green roofs as passive system to moderate building cooling requirements

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Table 10.1 Thermal properties of layers employed in the experimental extensive green roofs. Material Substrate Drainage Insulation Protection Waterproof Lightweight slab a

Thickness (mm)

Thermal conductivity (W/m K)

Specific heat (J/kg K)

Density (kg/m3)

80 55 30 3 4 75

0.270 0.138a 0.034 0.042a 0.200 0.098

1307 800 800 1200 1925 1000

1210 400 55 2000 900 400

Equivalent value.

Table 10.2 Sensor characteristics for the experimental setup monitoring. Sensor

Range

Accuracy/resolution

Ambient temperature sensor Relative humidity Wind speed Rain gauge RTD Pt 100

40°C to +80°C 0%–100% 1.5–79 m/s – 50°C to 250°C

0.4°C 3% 5% 0.2 mm 0.1°C at 0°C

Sensor

Sensitivity

Spectral range

Pyranometer Pyrgeometer

5–20 μV/Wm2 4 μV/Wm2

295–2800 nm 4.5–42 μm

the original configuration of the building cover, with 50 m2 of surface equipped with a black waterproof membrane with an albedo of 0.05. In Fig. 10.2, a representative picture of the four sectors taken in 2016 is shown, with the particular detail of the spontaneous vegetation that grew on the third plot. The sensor network was assembled in order to measure the main meteorological parameters as well as the temperatures inside the substrate and, in particular, at the interface with the structural roof. Weather data were acquired by a meteorological station placed in a corner of the roof to monitor dry bulb air temperature, relative humidity, precipitation rate, atmospheric pressure, wind direction, and wind speed. In addition, horizontal solar radiation was measured by a secondary standard pyranometer, whereas infrared radiation toward the sky was measured by means of a precision infrared radiometer. In Table 10.2 the operative ranges and correspondent measure accuracy/resolution are listed for the sensors used. For each plot, vertical direction single probes to detect temperature and water content simultaneously were installed in the substrate, with a proper distance from the edges to avoid boundary effects. Furthermore, two RTD (resistance temperature detector) sensors were installed to measure the

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1 2 3 4

1

Growing media

2

Drainage and storage

3

Storage and protection

4

Antiroot concrete

5

Lightweight concrete

5

Temperature and water content sensor Temperature sensor Temperature and relative humidity sensor

1 2 3 4 5 6

1

Growing media

2

Drainage and storage

3

Insulation

4

Storage and protection

5

Antiroot concrete

6

Lightweight concrete Temperature and water content sensor Temperature sensor Temperature and relative humidity sensor

GSPublisherVersion 0.53.100.100

Fig. 10.3 (A) Particulars of the sensor locations inside Plots 1 and 2. (B) Particulars of the sensor locations inside Plot 3.

temperature at the interface between the lightweight slab and waterproof layer and at the interface with the structural roof (see Fig. 10.3A and B). Exclusively for Plot 3, an additional RTD was installed on the top of the insulation layer. For the reference plot, instead, other RTDs, opportunely shielded, were installed to monitor the surface temperature. The automatic irrigation system, also employing the same water stored from rainfall, is activated as a function of limited water content in the substrate exclusively in summer, to avoid stress on the vegetation, excluding Plot 3. The influence of the green roofs was considered mainly for the top floor of the hosting building by installing probes to detect the surface temperatures on the ceilings,

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indoor air temperature, and relative humidity in the rooms located beneath the passive systems and the reference roof. The data logger employed to collect the measurements stored the values at intervals of 5 min. Since the monitored rooms have different exposures, the green roof performance was quantified by referring mainly to the transmitted heat flux through the structure rather than the energy consumption, considering that the latter is also strongly affected by the occupant behavior. In order to limit these effects, measured indoor variables refer to holidays and periods with suspended work activities.

10.5.1 Summer behavior of the extensive green roofs To evaluate the summer behavior of the experimental extensive green roofs, monitored data of a typical summer week in 2017 were considered (Fig. 10.4). The selected period was representative of extreme hot weather conditions with no precipitation, intense solar radiation, and air temperature that exceeded 40°C for several days. It is worth mentioning that occupants are not present due to the summer holiday period, so the indoor environmental conditions can be regarded as free-floating conditions. The overall beneficial effect of the presence of green roofs on the building cover can be appreciated when looking at the temperature trends recorded at the interface between the structural roof and the green roof bottom layer (Fig. 10.5). These temperatures are compared with the surface temperature of Plot 4 (reference roof), which is the value monitored at the same position of the other green plots. Furthermore, Fig. 10.5 also reports the trends of the ceiling temperature of the indoor rooms directly placed under the correspondent plots in order to appreciate the mitigation of the temperature field inside the structure.

Fig. 10.4 Trends of horizontal solar radiation and air temperature for a hot summer period in 2017.

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Fig. 10.5 Trends of surface temperature of the reference plot (Ref. Roof), temperature at the interface with the structural roof for Plot 1, Plot 2, and Plot 3, and indoor ceiling temperature of the room correspondent to the green plots (Int 1, Int 2, Int 3) and reference roof (Int Ref).

The bituminous reference roof, due to the higher solar absorption, reached daily maximum temperatures above 70°C during the middle hours of the day, whereas the daily thermal excursions in the period under consideration were of an average value of 50°C. Obviously, these features also contribute to the UHI effects negatively. At night, instead, the surface temperature of the traditional bituminous roof drops to lower values when compared with the green plots, because of the more intense radiative heat exchanges with the sky due to the higher emissivity. Nevertheless, the average value over the considered time period is 43.8°C, significantly higher than the average temperatures for the green plots. Considering that the temperature of the indoor ceiling of the room below this plot showed an average value of 32.6°C, it is already evident that the traditional roof generated a conspicuous average component of the thermal flux directed toward the indoor environments. Plot 1 and Plot 2 showed very similar trends with considerably small differences resulting in an average value at the interface, respectively, of 28.2°C and 28.8°C. Here the cooling effect of the green roof appears more clearly, since the indoor ceiling temperature of the rooms associated with these plots showed an average value of 30.7°C and 31.8°C, respectively, indicating a thermal flux directed toward the outdoor environment, which dissipates thermal energy of the indoor rooms. The effect of the additional thermal insulation layer on the green roof stratigraphy is clearly visible in the temperature trend of Plot 3, which is on average 6.9°C higher

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than Plot 1 and 6.2°C higher than Plot 2. This situation is favorable in winter, but it could reflect a worsening of the cooling performance of the green roof. Nevertheless, despite the presence of the insulation layer, still Plot 3 is able to reduce the ingoing average component of the thermal flux, compared to the reference roof.

10.5.2 Effects on the UHI phenomena Performance indexes, calculated as a function of external surface temperatures of the green roofs, have been used in the literature (Bevilacqua et al., 2017; Cascone et al., 2019b; Teemusk and Mander, 2010) to characterize the behaviors of green roofs in relation to the urban heat island phenomenon, energy savings, and lifespan of the roof cover. Such indexes are easy to calculate and allow immediate comparison of different extensive green roof packages. In particular, the following indicators have been used to analyze the behavior of the experimental green plots: l

Surface temperature reduction (STR), evaluated as the ratio of the daily average surface temperatures of green roofs compared to the reference roof. STR ¼

l

(10.9)

This index is representative of the sensible heat flow exchanged between the extensive green roof and the indoor environment and, consequently, is proportional to the energy consumption for heating and cooling. Temperature excursion reduction (TER), evaluated as the ratio of the ith plot surface temperature excursion to the reference plot surface temperature excursion. TER ¼

l

Tav, i Tav, Ref

Tmax, i  Tmin, i Tmax, ref  Tmin, ref

(10.10)

This index is representative of the fluctuation in the daily external surface temperature. Since the fluctuation of surface temperatures is a cause of thermal stress, influencing the durability of the roof membrane, this index provides useful information about the mitigation of this phenomenon. External temperature ratio (ETR), evaluated as the ratio of the ith plot surface temperature to the average temperature of external air. ETR ¼

Tav, i Tav, air

(10.11)

The latter represents the contribution to mitigation of the urban heat island effect due to the installation of the green roof. Reduced ETR implies a greater reduction in the effect of the urban heat island, because of the smaller sensible heat exchanges with the external air. Considering the similar behavior demonstrated by Plot 1 and Plot 2 in the temperature attenuation, the results for the first one are not reported in the following graphs. Fig. 10.6 reports the average monthly value of STR and TER indexes calculated for

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Fig. 10.6 Average monthly values of STR and TER indexes for Plot 2 and Plot 3, calculated in the summer months.

Plot 2 and Plot 3 in the summer months from June to September. The red dotted line represents the threshold at which the green plot behaves like the reference roof with an STR ratio value equal to one. It clearly appears from Fig. 10.6 that both plots were able to generate a lower average surface temperature compared to the reference roof. The STR index for Plot 3 in the months of June was slightly greater than 1, producing a temperature comparable with that for the bituminous roof. Also, the values obtained in July and August were close to 1 (0.969 and 0.954, respectively). This behavior can be attributed to two causes. On the one hand, the sparse spontaneous vegetation and the absence of irrigation led to low water content and higher temperature in the growing medium because of the absence of shading effects on the surface and the evapotranspiration phenomenon. On the other hand, the presence of the additional insulation layer just below the drainage layer, causing an increase in thermal resistance, resulted in a more limited capacity of this plot to dissipate the accumulated solar radiation towards the underlying layers. The STR index for Plot 2 was considerably lower, being on average lower than 0.8 for the entire summer period. The minimum of 0.717 was reached in July, denoting a conspicuous capability of limiting the surface temperature and therefore the sensible heat exchanges with the indoor environment. Considering the TER index (Fig. 10.6), it is possible to appreciate how Plot 2 was able to significantly reduce the daily surface temperature fluctuations. Indeed, the TER ranged from 0.346 in July to 0.486 in June, denoting a halving of the fluctuation compared to the reference roof solution. Plot 3, instead, showed a minimum value of 0.797 in September, but overcame the unity in all the remaining summer months, with a maximum of 1.163 in June. The temperature reached by the substrate of this plot on sunny summer days was elevated because of the aforementioned phenomena that caused overheating; still the night cooling of the growing media allowed low minimum temperatures to be obtained, resulting therefore in daily fluctuations comparable, or even greater, than the black bituminous roof. Nevertheless, when analyzing the surface temperature in relation to the external air temperature, thanks to the ETR index it can be seen that both green plots can significantly contribute to the reduction of the urban heat island effect (Fig. 10.7). On average, the reference roof registered temperatures markedly higher than the external air, with an ETR ranging from 1.265 to 1.414, indicating therefore greater sensible heat exchanges with the external environment. Plot 2 generated instead values of ETR

Green roofs as passive system to moderate building cooling requirements

223

Fig. 10.7 Average monthly values of ETR index for Plot 2 and Plot 3 calculated in the summer months.

considerably lower, in the range 0.958–1.061, indicating therefore a surface temperature very close to that of the external air. Plot 3 behaved worse than Plot 2, mainly because of the greater temperatures reached by this plot in summer, but was still able to perform better than the reference roof, producing an index that ranged from 1.024 to 1.423.

10.5.3 Achievable energy savings in cooling applications As mentioned previously, numerical simulation programs lead to a certain degree of inaccuracy in energy performance evaluations, since there is still uncertainty regarding how to set the parameters describing the vegetation and the substrate properties (Peri et al., 2016). In particular, suitable models that consider the interaction between buildings and green roofs are often lacking in available building energy simulation tools. Furthermore, evaluations of the trends of coverage ratio as a function of the planted species are not available in the literature and consequently it is not involved in green roof modeling (DeNardo et al., 2003; Cascone et al., 2019b). In order to overcome these limitations, a possible solution that takes these effects into account is the used of experimental data as input in simulation tools. The thermal performance of a real installation, in fact, can be determined by TRNSYS 18 dynamic software (TRNSYS 18, 2016; TRANSSOLAR and Energietechnik GmbH, 2016), despite the fact that the current version does not contain a specific green roof model that accounts for the physical behavior. Nevertheless, the effects on the building energy consumption can be evaluated by the use of available temperatures registered at the interface between the green roof bottom layer and the structural roof, used as a boundary condition for the external ceiling surface. In this position, in fact, the value of the experimental temperature condenses the overall effect of the green roof thermal exchanges. The simulated building is the same as that hosting the three experimental extensive green roofs. It was modeled with consideration of proper division into thermal zones, attributing internal gains and work schedules in order to represent the real

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configurations as accurately as possible. Moreover, for a precise evaluation of the actual energy performance, the values acquired by the weather station were also used in the simulation as external climatic conditions to determine the green roofs behavior as a function of real climatic data. In order to evaluate the effects produced by the design parameters on the energy results, the simulation considered the insulated and the noninsulated green roof configurations, by hypothesizing two different operating regimes of the air conditioning plant: the first continuous and the second one intermittent, in accordance with the schedule described in Table 10.3. The use of experimental data renders the study realistic and reliable, thanks to the long acquisition period, allowing the limitations arising from the use of the computer program to be overcome. The accuracy of the results depends on an accurate description of the real building from both geometrical and physical points of view, because other terms appear in the energy balance, such as thermal losses, solar gains, and the ventilation rate. In Fig. 10.8 the south and west exposures are shown with the precise locations of the transparent surfaces. The opaque envelope is constituted of two different types of external dispersing wall, as described in Tables 10.4 and 10.5, related to the functional subdivision of the internal spaces. The internal partitions, participating in the building thermal mass, are described in Table 10.6. In addition, two different types of floor decks can be found in the structure, as described in Tables 10.7a and 10.7b with the correspondent layering section. The investigated building is equipped with six different types of windowed system, as shown in Table 10.8; however, these have in common a metallic frame 45 mm thick without thermal break and a 4/12/4 glazing with double clear glass filled with air. Table 10.3 Operating schedule of the cooling system. From Daily (h) Weekly (days) Seasonal (months)

08:30 Monday 15 April 01 September

To 18:30 Friday 31 July 15 October

GSPublisherVersion 0.46.100.100

Fig. 10.8 South and west fronts of the investigated university building.

Green roofs as passive system to moderate building cooling requirements

225

Table 10.4 External “light” wall layers and thermal properties.

Layer Albanit Plasterboard Glass fiber Air gap Concrete Quartz painting a

Thickness (mm)

Density (kg/m3)

Thermal conductivity (W/m K)

Specific heat (J/kg K)

14 10 50 500 300 5

600 900 16 – 2200 1300

0.440 0.210 0.046 2.780a 1.480 0.300

1600 1090 670 – 880 840

Equivalent value.

Table 10.5 Massive external wall layers and thermal properties.

Layer Quartz painting Concrete Quartz painting

Density (kg/m3)

Thermal conductivity (W/m K)

Specific heat (J/kg K)

5

1300

0.300

840

300 5

2200 1300

1.480 0.300

880 840

Thickness (mm)

Table 10.6 Internal partition layers and thermal properties.

Layer

Thickness (mm)

Density (kg/m3)

Thermal conductivity (W/m K)

Specific heat (J/kg K)

Plasterboard Plywood Glass fiber Plywood Plasterboard

5 10 45 10 5

1400 1300 16 1300 1400

0.700 0.440 0.046 0.440 0.700

1010 840 670 840 1010

For the energy evaluations, internal gains were set by considering two persons in each air-conditioned room, seated doing light writing, in order to associate a degree of activity in accordance with the standard ISO 7730 (ISO, 2005). Moreover, 280 W were included for personal computers and other electric appliances, whereas the artificial lighting system was set at a thermal power of 5 W/m2. These endogenous sources were scheduled in order to consider an operation from the 8:30 a.m to the

Table 10.7a External roof slab layers and thermal properties. Layer

Thickness (mm)

Density (kg/m3)

Thermal conductivity (W/m K)

Specific heat (J/kg K)

Waterproof membrane Light cellular concrete Waterproof membrane Light cellular cement Precast compressed concrete Lighting elements and PCC Precast compressed concrete Internal plasterboard

5 10 5 20 60 130 60 20

1200 1800 1200 1800 2400 500 2400 1400

0.170 0.930 0.170 0.930 1.800 0.110 1.800 0.700

1470 1000 1470 1000 1000 1240 1000 1010

Table 10.7b Floor deck layers and thermal properties. Layer

Thickness (mm)

Density (kg/m3)

Thermal conductivity (W/m K)

Specific heat (J/kg K)

Waterproof membrane Light cellular concrete Waterproof membrane Light cellular cement Reinforced concrete Hollow brick Internal plasterboard

5 10 5 20 60 120 20

1200 1800 1200 1800 2400 2200 1400

0.17 0.93 0.17 0.93 1.80 0.50 0.7

1470 1000 1470 1000 1000 840 1010

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Table 10.8 Windowed systems in the investigated building. Type

Shape

Position

Dimensions

Opaque surface

W1

North South

205  180 cm

½

W2

North South West

130  130 cm

0

W3

South West

205  435 cm

½

W4

East

205  270 cm

½

W5

East

205  270 cm

5/6

D1

East

205  270 cm

0

6:30 p.m. from Monday to Friday every month, with the exception of August (summer vacation). These gains were differentiated also as a function of the specific use of the thermal zone (office spaces, hallways, and passage areas), as shown in Fig. 10.9. Regarding the ventilation losses, a natural airflow of 0.5 h1 was set for every conditioned room, in accordance with the Italian regulation UNI 11300-1 (Italian Unification Institution, 2008). In order to prevent indoor overheating, the airstream was scheduled exclusively at night to produce free-cooling. Globally, the thermal analyses involved the top three floors of the building to evaluate whether the beneficial effects of the green roofs could be detected beyond the upper floor. The building was geometrically modeled with the TRNSYS3D plug-in, which allows for a better calculation of the distribution of the solar radiation transmitted in the indoor environment through the glazed surfaces, by dividing the upper and the second to the upper floors into 16 and 18 thermal zones, respectively (see Fig. 10.9). The indoor environment

Green roofs as passive system to moderate building cooling requirements

Room 1 (10.13 m2) Double height room (56.97 m2)

Room A (40.05 m2)

Room B (31.00 m2)

Room C (32.16 m2) Atrium (42.62 m2)

Room 3 (16.07 m2) Double height room (36.73 m2)

Service 1 (11.30 m2)

229

Service 4 (40.05 m2) Room 2 (21.12 m2)

Room 1 (27.64 m2)

Service 3 (9.20 m2)

Service 2 (9.20 m2) Room 4 (16.38 m2) Stair (10.88 m2)

Room D (33.21 m2) Corridor (17.80 m2)

Atrium (85.27 m2)

Stair (40.05 m2)

Room 5 (15.13 m2)

Corridor (60.17 m2)

Room 6 (14.96 m2) Double height room (56.51 m2)

Room E (30.37 m2)

Room F (29.22 m2)

Room 2 (20.77 m2) Room G (13.17 m2)

Double height room (48.32 m2)

Elevator (10.88 m2) Room 7 (26.12 m2)

Room 8 (23.21 m2)

Service 1 Serv. 2 (9.90 m2) (3.24 m2)

LEGEND People, Computers, Artificial lighting Artificial lighting No internal gains

Fig. 10.9 Considered internal gains for the upper (on the left) and the second to upper (on the right) floors.

under the second to the upper floor was considered as a unique thermal zone, since it hosts a university laboratory. Cooling requirements were determined with reference to the data monitored and acquired during 2017, because in the last decade it was one of the hotter registered years, producing significant cooling demands. The indoor setpoint temperature was 26°C. The comparison of the results allowed the determination of the differences in energy needs between the buildings equipped and not-equipped with a green roof. Furthermore, long-wave radiative exchange with the sky and the soil were considered by means of the measured experimental sky temperature, as well as the solar radiation incident on the external surfaces, determined after a projection of the acquired global horizontal radiation using the Reindl correlation for the diffuse component (TRNSYS 18, 2016). The climatic data employed for simulation are summarized on an annual level in Fig. 10.10. It is clear that for the considered location 2017 was characterized by a hot and dry summer with an annual average temperature of 17.2°C and an amount of precipitation lower than 600 mm. The maximum temperature of the outdoor air reached 43.6°C in August with a mean daily excursion of 30°C and absence of precipitation. Table 10.9 lists the outdoor temperature and humidity arranged in precise intervals, highlighting the dry conditions due to the number of hours with low relative humidity. In the TRNSYS environment, the effects of the extensive green roofs on the building energy consumption were computed by imposing the temperature measured in each plot at the interface with the structural roof as an input vector. Indeed, these temperatures are representative of the overall effect of the several thermal exchanges (evapotranspiration, long-wave radiation, upper convection, conduction in the soil,

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Fig. 10.10 Monthly average values of external air temperature, maximum and minimum outdoor air temperature, cumulated rainfall, and average solar radiation measured at the experimental site during 2017.

Table 10.9 Summer temperatures and relative humidity distribution. Summer Temperature >15 36.97%

Relative humidity >20 26.55%

>24 17.18%

>26 12.86%

>30 6.43%

85 3.85%

and so on). In this regard, it should be noted that the interface temperatures are also influenced by the operation of the air-conditioning systems in internal spaces. Nevertheless, the presence of a false ceiling in the upper floor of the monitored building somehow operated in the direction of decoupling the internal air node temperature (directly influenced by the mechanical systems) from the roof internal surface temperature, which is mostly influenced by the thermal balance of the upper layers. The green roof external surface temperature, in fact, is obtainable from an energy balance in which different important contributions participate, determining also the lower layer temperatures, up to the interface with the structural roof. Conversely, the indoor conditions generate much less significant forcing. This approach allowed simulations to be carried out in which building energy consumptions were quantified without the need to simulate the whole green roof package with proper models and the correspondent limitations (Ferrante et al., 2016). Finally, the building thermal behavior was determined by considering three different scenarios (differentiated as a function of

Green roofs as passive system to moderate building cooling requirements

231

the considered building cover) supposing a continuous and an intermittent operation of an ideal cooling system (unlimited power) in every considered thermal zone: -

Building with traditional reference roof (RR); Building with the extensive noninsulated green roof (GR); Building with the extensive insulated green roof (I-GR);

In the simulations, only Plot 1 was considered because it differs from Plot 2 only for hydraulic properties, producing almost the same thermal behavior. The simulation results were compared in order to quantify the differences among the different simulated scenarios. Fig. 10.11 justifies the choice to set April 15 as the initial date for cooling: it emerges that during 2017 positive values of the thermal load (denoting cooling, conversely negative values meaning heating loads) are necessary to maintain the indoor set-point temperature. Heating is required from the end of October to March; consequently for the locality the tendency is to have 7 months of cooling and 5 of heating, showing the great potential of energy savings achievable by a green roof on the cooling demand front. By supposing a continuous operation regime and summarizing the results for all the thermal zones of the upper floor, the traditional roofing solution (reference roof) needed a cooling peak power of 12 kW (Fig. 10.11A), whereas the same peak was reduced to 9 kW for the insulated green roof configuration (Fig. 10.11B). Passing to the noninsulated configuration (Fig. 10.11C), a further decrement to 8 kW was observed, by confirming the counterproductive effect of the insulation layer in warm climates. Moreover, in winter the peak powers were almost coincident between the two configurations and equal to 4 kW, so Plot 1 does not seem to be affected negatively by the insulation absence. Nevertheless, a slight worsening of the thermal performance of the noninsulated green roof was observed exclusively in the intermediate seasons (spring and autumn), producing a little advance/delay of the heating season that, potentially, could lead to a worsening of the heating requirements than for the insulated green roof (Plot 3). Fig. 10.12 shows the time percentages in hours in which the cooling demands fell in precise power ranges, differentiated with a step of 2 kW. It is evident how green roofs shift the required cooling loads toward the lower classes; in particular, the percentage dropped considerably for Plot 1 in the last interval when cooling loads were compared with the reference roof, and the same sector for the 50% of the hours needing cooling power lower than 2 kW. These results prove the actual green roof aptitude to limit cooling powers, allowing for the sizing of a smaller air-conditioning plant with all the consequent advantages in terms of primary energy consumption. For instance, a dedicated heat pump with a smaller size has less chance to operate with a capacity ratio lower than unity, avoiding affecting the EER (Energy Efficiency Ratio) negatively for nonmodulating devices. For the reference roof, the percentage of hours in which the cooling powers exceeded 4 kW was 12%, approximately 7% higher than the insulated green roof configuration and over 10% higher than the noninsulated one. In order to find the optimal solution for green roofs, Table 10.10 lists the annual energy consumption for the three investigated plots. It is clear that the noninsulated green roof offered the best performance, reducing the cooling demand by 43.8% when

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction 12

Sensible thermal power [kw]

10 8 6 4 2 0 –2 –4 –6

(A) –8 12

Sensible thermal power [kw]

10 8 6 4 2 0 –2 –4 –6

(B) –8 12

Sensible thermal power [kw]

10 8 6 4 2 0 –2 –4 –6

(C) –8 Fig. 10.11 Trend of the thermal powers required for three analyzed green roof configurations supposing a continuous operation of the air-conditioning plant: (A) reference roof, (B) insulated green roof, and (C) noninsulated green roof.

compared with the reference roof. Although Plot 1 was able to also reduce the heating demand by 6.3%, instead of the 27.9% detected for the insulated configuration (Plot 3), on an annual level it provided the best results, because its summer performance prevailed over the slight winter improvement. In particular, when compared to the reference roof, Plot 1 produced an annual thermal energy savings of 34.9%, whereas Plot 3 “only” showed savings of 24.1%. A more realistic scenario was carried out by hypothesizing an intermittent functioning of the air-conditioning plant. Clearly, the observed cooling peak powers were greater than those detected in the continuous operation regime, with the effects connected with the building fabric thermal inertia being more influential, especially when

Green roofs as passive system to moderate building cooling requirements

233

Fig. 10.12 Time percentage of the hours in which the cooling power falls in a precise interval assuming a continuous operation of the air-conditioning plant.

Table 10.10 Heating and cooling energy demand for the air-conditioning of the building top floor for the three considered roof configurations in continuous operation.

Reference roof Insulated GR Noninsulated GR

Heating (kWh)

Cooling (kWh)

Total (kWh)

4996.8 3602.9 4680.6

15,943.5 12,291.7 8956.0

20,940.4 15,894.5 13,636.6

the plant activates after an unconditioned period. The reference roof during 2017 showed a cooling power peak of about 22 kW in July and August (Fig. 10.13A), whereas the installation of the insulated green roof produced a maximum cooling peak shifted to September (after the interruption of the air-conditioning plant during August) that assumed a value of 17.5 kW (Fig. 10.13B). The noninsulated solution instead offered the maximum cooling loads in July and August with strongly reduced powers, up to 13 kW (Fig. 10.13C), further emphasizing the positive effects due to the absence of an insulation layer in summer. Again, by arranging the required cooling powers in intervals in order to determine the correspondent frequency (Fig. 10.14), the reference roof showed cooling powers greater than 8 kW for 41.9% of the conditioned hours, whereas the same percentage dropped to 33.5% for the insulated green roof and to 28.9% for the noninsulated one. The monthly energy demands for all the thermal zones of the upper floor are presented in Fig. 10.15. Again, it emerges that the noninsulated green roof in the considered climatic context represents the most effective solution in summer, whereas in winter the insulated green roof produced major energy savings. Nevertheless, the beneficial effect of the green roof is quite evident in summer months (from June to September), where the energy demand reductions ranged from 30.3% to 35.2%.

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction 25

Sensible thermal power [kw]

20 15 10 5 0 –5 –10 –15 –20 –25

(A)

25 20

Sensible thermal power [kw]

15 10 5 0 –5 –10 –15 –20 –25

(B)

25

Sensible thermal power [kw]

20 15 10 5 0 –5 –10 –15 –20 –25

(C)

Fig. 10.13 Trend of the thermal powers required for three analyzed green roof configurations, supposing an intermittent operation of the air-conditioning plant: (A) reference roof, (B) insulated green roof, and (C) noninsulated green roof.

The insulated green roof, instead, was able to generate energy savings from November to February ranging between 29.0% and 52.4%. At the seasonal level, the greater energy savings were found in summer for the noninsulated green roof and the intermittent plant operation determined more conspicuous cooling demand savings, with a percentage of 37.9%, but with much more limited savings in winter when compared with the insulated configuration.

Green roofs as passive system to moderate building cooling requirements

235

Fig. 10.14 Time percentage of the hours in which the cooling power falls in a precise interval assuming an intermittent operation of the air-conditioning plant.

Fig. 10.15 Monthly energy requirements of the upper floor with intermittent operation of the air-conditioning plant.

Still, on an annual level (Table 10.11), the noninsulated configuration outperformed the insulated one with a maximum energy reduction of 34.7% in 2017. Despite the major energy savings produced by the green roofs being appreciated most prominently in the rooms underlying the green cover, it is possible to detect positive effects also on the second to upper floor. Fig. 10.16 depicts the trend of the obtained monthly energy demands that show a similar trend to that observed for the upper floor. Table 10.12, instead, lists the monthly energy demands for intermittent operation. Nevertheless, the obtained energy reductions are very limited when compared with those detected for the upper floor. In particular, for the insulated green roof, the savings ranged between 1.7% and 3.5% during the hotter months, whereas higher percentages, between 3.5% and 5.4%, were obtained for the noninsulated

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Table 10.11 Heating and cooling energy demand for the air-conditioning of the building top floor for the three considered roof configurations in intermittent operation.

Reference roof Insulated GR Noninsulated GR

Heating (kWh)

Cooling (kWh)

Total (kWh)

1768.0 1144.5 1460.7

9530.5 7557.6 5914.2

11,298.5 8702.1 7375.0

Fig. 10.16 Monthly energy requirements of the second to upper floor with intermittent operation of the air-conditioning plant.

Table 10.12 Heating and cooling energy demand for the air-conditioning of the building second to upper floor for the three considered roof configurations in intermittent operation.

Reference roof Insulated GR Noninsulated GR

Heating (kWh)

Cooling (kWh)

Total (kWh)

1132.2 1028.8 1091.1

10,349.9 10,051.5 9735.4

11,481.1 11,080.3 10,826.5

configuration. Again, the insulated green roof was able to produce better results in winter, with the noninsulated one performing better in summer. On an annual level, for the second to upper floor, the noninsulated green roof also represents the best choice.

10.5.4 Assessment of green roof effects on thermal comfort conditions Beyond the reduction of the building energy needs, green roofs also positively affect the quality of life in the internal and external environment (Gagliano et al., 2016). Regarding the air-conditioned spaces, the standard EN ISO 7730 (ISO, 2005)

Green roofs as passive system to moderate building cooling requirements

237

introduces physical quantities to take into account the thermal sensations. In particular, two parameters to use as indexes for thermal comfort analysis are proposed: the operative (Top) and the mean radiant (Tmr) temperatures, which in a simplified way can be determined as an average temperature of the walls constituting the airconditioned room weighted as a function of the correspondent surfaces. The operative temperature, instead, is defined as that uniform temperature of the air and of the walls of the environment that would cause, for the human body, the same thermal exchange by convection and radiation of the real environment. Therefore, it depends on the air temperature, the mean radiant temperature, and the convective and the radiative heat transfer coefficients of a clothed human body. These parameters are indicated especially for short-term applications, whereas for the long-term evaluation of the general comfort conditions of indoor environments, both for mechanically heated and cooled buildings and for buildings without mechanical cooling, the EN 15251 standard (Nicol and Wilson, 2010) is more recommended. Indeed, it indicates some methods where four levels of categories (I, II, III, and IV) concerning the thermal comfort conditions are defined. For instance, in category II, to which a predictive mean vote (PMV) in the range 0.5 < PMV < 0.5 can be associated, for single offices with sedimentary activity (1.2 met), the suggested operative temperature should vary in the interval 20.0– 24.0°C in winter and 23.0–26.0°C in summer. The lowest values are indicated as the lower operative temperature limits (Top,limit,lower) and the higher ones as the upper operative temperature limits (Top,limit,upper). In addition, the degree hours criteria can also be used to define the duration in which the operative temperature exceeds the aforementioned intervals during the occupied hours, weighting the detected deviances by a factor (wf) depending on by how many degrees the range has been exceeded. In particular, the weighting factor is 0 when: Top,limit,lower  Top  Top,limit, upper

(10.12)

and conversely wf is determined as: wf ¼ Top  Top,limit

(10.13)

when Top < Top,limit,upper or Top > Top,limit,lower Finally, for a precise season, the products of the weighting factor and the time have to be summed, using the following rule: Warm period: X

wf  time if Top > Top, limit,lower

(10.14)

Cold period: X

wf  time if Top < Top, limit,upper

(10.15)

Regarding the same investigated building, assuming an intermittent operation of the air-conditioning plant to better represent the real operation of the plant, the results for

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

Table 10.13 Discomfort degree hours for the occupied room of the building top floor. Cold

Room E Room F Room G Double height room Room D Room C Double height room Room B Room A

Warm

RR (h)

I-GR (h)

GR (h)

RR (h)

I-GR (h)

GR (h)

1105.78 870.41 1356.42 4304.10

600.91 489.01 1017.05 2960.46

1021.22 843.75 1456.41 4029.16

7453.04 7056.34 7241.25 10,213.45

2141.42 2006.74 2490.68 4137.80

1506.84 1478.84 2004.47 2628.38

1034.08 1080.02 4829.99

554.50 599.50 3554.64

956.53 1009.63 4598.07

8518.39 8463.14 9554.31

2629.46 2619.41 3808.45

1955.12 1950.40 2408.58

1619.85 2477.97

1065.18 2015.85

1558.60 2502.14

6887.64 6794.31

1924.11 2304.26

1340.85 1782.82

the nine occupied rooms of the upper floor (Fig. 10.9 on the left) are reported in Table 10.13. Clearly, the positive effects of the installed extensive green roofs were also found in terms of thermal comfort conditions. The main beneficial effect of the green roof was to lower the ceiling surface temperature of the indoor spaces connected with the green surfaces, determining as a consequence a reduction of the mean radiant temperature of the rooms, improving the comfort conditions. In cold periods, the results were qualitatively similar to those obtained for the energy savings, where the insulated configuration allowed a conspicuous reduction of the degree hours in each room, with percentages in the range 18.6%–46.4%. The noninsulated green roof solution showed instead in Room 3 and Room 9 a worsening of the degree hours in winter, with an increment, in absolute value, of the index, while maintaining a reduction lower than 10% in the remaining rooms. Regarding the warm period (summer), the results were more unbalanced in favor of the noninsulated green roof, reaching a maximum reduction of 80.5% for Room 8, whereas the insulated roof was able to reduce the discomfort index by 72.1%. Therefore, the results demonstrated the relevant ability of the green roof to improve the thermal comfort of the indoor space directly connected to the building cover.

Acknowledgments This research was partially supported by the Italian National Operational Programme (PON)— Research and Competitiveness, Action II: Interventions supporting industrial research through the project PON01_02543 “Integrated and sustainable management service for the waterenergy cycle in urban drainage systems.”

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Thermal evaluation of building roofs with conventional and reflective coatings

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I. Herna´ndez-P
ereza and Y. Olazo-Go´mezb a Academic Division of Engineering and Architecture, Juarez Autonomous University of Tabasco, Cunduacan, Tabasco, Mexico, bNational Technological Institute of Mexico/CENIDET, Cuernavaca, Morelos, Mexico

11.1

Introduction

Many factors influence the temperature of the indoor air of non-air-conditioned buildings and the cooling and heating loads of air-conditioned buildings: the type of weather, the construction materials, the area of opaque and semitransparent components, the orientation, among others. The building envelope is a barrier against the effects of the environment, and its purpose is to provide comfort to the inhabitants; given this definition, for those buildings located in warm weather the energy gains should be reduced as much as possible, while in cold weather energy losses should be avoided. Different technologies are available to reduce energy gains or losses in buildings. The semitransparent areas play a vital role in thermal comfort and in providing lighting levels in a building (Gorgolis and Karamanis, 2016). Significant advances have been made in technologies for glazing: for instance, solar control films, glazing with a high level of insulation, low-emissivity coatings, double glazing (with vacuum space between glass or inert gas-filled), smart glazing, as well as improvements in frames and shading systems, to name a few. In residential buildings, the opaque components generally cover most of the area of the building envelope; therefore, their efficient thermal behavior is also essential. Regarding building materials, the walls are usually made of wood, metal, concrete, or bricks, but other types of walls are used to improve the energy efficiency and comfort levels of buildings, such as Trombe walls, thermal storage walls with phase change materials, ventilated walls or double skin walls, and green walls, among others (Omrany et al., 2016). The roof is the component most exposed to the weather effects. The impact of solar radiation on summer days, the heat loss from infrared radiation at night, and, in winter, rain or snow all affect the roof more than any other component (Givoni, 1994). In hot climates, it can contribute up to 50% of the thermal energy load (Nahar et al., 2003), and in cold climates it serves as a potential route for heat loss. The exterior surface of the roof is subject to the largest temperature fluctuations, depending on its type and external color. Many technologies such as thermal insulation, phase change materials, reflective coatings, or even vegetation are available today for the energy retrofitting of Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction https://doi.org/10.1016/B978-0-12-820791-8.00011-0 © 2021 Elsevier Ltd. All rights reserved.

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building roofs. In particular, reflective or cool materials applied to building roofs (cool roofs) have become a well-known measure to reduce electricity consumption from air conditioners and to improve thermal comfort in unconditioned buildings (Herna´ndezP
erez et al., 2014; Gagliano et al., 2015; Pisello, 2017). These materials have a high solar reflectance and high thermal emittance and keep the roof at lower temperatures than ordinary materials when exposed to the sun (Levinson et al., 2007; Santamouris et al., 2011). Cool or reflective roofs are not a new concept; one of the earliest strategies used to improve comfort conditions inside buildings, especially in the Mediterranean and Middle Eastern countries, was to whitewash the opaque building components. Painting a building roof with a conventional white material keeps its surface cooler than a nonpainted roof, because of its ability to reflect the solar energy in the visible spectrum. Because color is the morphological characteristic that most impacts the thermal behavior of roof coatings (Alchapar and Correa, 2016), in the last two decades reflective roofs or cool roofs have gained popularity, and many researchers worldwide have developed reflective white materials (Xue et al., 2015; Ullah et al., 2019). These materials can have values of solar reflectance of up to 0.9. Not only white materials are available, but also reflective colored elements can be used in sloped roofs to avoid glare problems (Rosado et al., 2014), or to conserve the visual conditions of traditional architecture (Pisello, 2017). Buildings around the world generally use different roofing materials depending on their availability, the regional weather, and the nature of the supporting structure. In the United States, most of the residential buildings have steep-sloped roofs with asphalt shingles (Berdahl et al., 2012). Concrete and clay tiles are used in the roofs of residential and school buildings in Italy (Pisello and Cotana, 2014; Fantucci and Serra, 2019). Buildings with a flat concrete roof are popular in Greece (Kolokotsa et al., 2012, 2018). Residential buildings use roof tiles or metal roof materials in Australia (Suehrcke et al., 2008; Seifhashemi et al., 2018). Multistory and rural buildings with concrete or cement roofs are common in India (Dhaka et al., 2012; Garg et al., 2016). Many public housing apartment blocks are fitted with doubleskin roofs in Singapore (Tong et al., 2014; Zingre et al., 2017). Despite the variety of roofing materials, reflective coatings are available for nearly every type of roof. Currently, there are reflective coatings in the form of prefabricated sheets or single-ply products of EPDM (ethylene propylene diene monomer), TPO (thermoplastic polyolefin), CPE (chlorinated polyethylene), and PVC (polyvinyl chloride) (Gartland, 2008; Gaffin et al., 2012). However, technology developments in reflective materials have focused on improving the optical properties of liquid coatings due to their easy installation (they are applied in the same way as painting). The liquid application products can be elastomeric, polyurethane-based, or acrylic coatings (Santamouris, 2014). This chapter presents the thermal evaluation of building roofs with conventional and reflective coatings. The chapter is divided into three main sections. The first section is focused on the experimental monitoring of two outdoor test cells, the roofs of which were covered with different coatings. This section also shows the procedure followed to measure the optical properties of three coatings used in the test cells. The second section presents a numerical model that considers the roof as a single

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component that was validated with experimental data obtained as described in the first section. The third section provides the thermal evaluation of a building room model with three different roofs with the coatings already mentioned. This section shows the coupling of the roof model from the second section and a computational fluid dynamics model of the room, from which the benefits of installing a reflective coating, in terms of thermal comfort, were determined.

11.2

Experimental evaluation of small-scale roof samples

This section presents the experimental testing of three small-scale roof samples. A gray roof was used as the reference, while the other two roof samples were covered with a white reflective and a black coating, respectively. Two outdoor test cells were used to measure the surface temperatures of the roofs and to determine the heat transferred by them. In addition, because two main physical properties of the roof coatings affect their thermal behavior, the solar reflectance and thermal emittance of the traditional and reflective coatings were also measured.

11.2.1 Optical properties of conventional and reflective coatings The properties that determine the temperature of an opaque surface when exposed to the sun are solar reflectance and thermal emittance. Solar reflectance is an overall value that indicates how much energy a material reflects, regarding the solar energy received. Thus, an opaque material can have a solar reflectance between 0 and 1. On the other hand, thermal emittance is a measure of how well an opaque material radiates energy as compared with an ideal surface, known as a black body, operating at the same temperature. The optical properties, solar reflectance, and thermal emittance of a black material, gray-concrete sample, and white reflective material were determined as shown shortly. To measure the solar reflectance of the samples, a spectrophotometer with an integrating sphere (Fig. 11.1) was used. As reported by the reference standard ASTM E 903-12 (2012), the integrating sphere allows one to obtain the spectral reflectance from 0.3 to 2.5 μm. Fig. 11.2 shows the spectral reflectance of the three types of surface studied in this work. The solar reflectance (SR) was obtained by integrating the spectral reflectance over the standard spectral irradiance distribution (ASTM G173-03-2012). To measure the thermal emittance of the different coatings (ε), a portable emissometer (Fig. 11.3) and the standard ASTM C1371-15 (2015) were used. Table 11.1 presents the optical properties of the materials studied in this chapter.

11.2.2 Outdoor test cells to evaluate small-scale roof samples Two outdoor test cells were used to evaluate the thermal performance of small-scale roof samples. Each test cell was 1 m long, 1 m wide, and 1 m high. A gray roof was used as the reference, while two additional roof samples were covered with a white reflective and a black coating, as mentioned in the previous section. The next section

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Fig. 11.1 UV-VIS-NIR spectrophotometer.

presents the results of two comparative tests. Fig. 11.4 shows a representation of the test cell facility used for the experimental thermal evaluation of the building roofs. The roofs of the test cells were removable to allow evaluation of different roof samples. During sunlight hours, the roof and the walls of the test cells received solar irradiance and exchanged heat with the outdoor environment, and because it was only desired to quantify the energy transferred by the roof, the test cells had a guarded zone between the external walls and the heat flux transducer. In each test cell, the guarded zone was maintained at constant temperature by an air conditioner. The air conditioners were set on an adjustable base and connected to the test cells by outlet and inlet air ducts. Fig. 11.5 shows a schematic of the cross-sectional view of one of the test cells; the heat flows through the roof and the heat flow transducer. Because the experimental tests are performed outdoors, the entire exterior surface of the roofs is exposed to solar radiation (G), but the test cells were designed so that only the area of the roof above the heat flow transducer had an effect on the measurements (AR). Part of the radiation received by the roof was reflected back to the atmosphere (SR*GAR), and the remainder was absorbed by the roof (αGAR). A portion of this absorbed energy was transferred to the heat transducer and the remainder was transferred to the outdoors by convection (qconv-out) and by thermal radiation (qrad-out). Moreover, because of the temperature difference between outdoor air (To) and the air in the heat transducer (Ti), the roof has another heat flow from the outdoor environment. Therefore, the total heat flow passing into the transducer because of the roof (QR) is the sum of the fraction of the absorbed radiation transferred to the indoor air and the heat flow due to the temperature difference between outdoor and indoor air. Then, the heat flow is transferred by convection (qconv-int) and by thermal radiation (qrad-int) to the indoor air. As shown in the same figure, the test cells have a guarded zone maintained at constant temperature by an air conditioner (Tgz). This guarded zone surrounds the outer

1.0

uv

vis

nir 1.0

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0.9

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SR = 0.05, u = 0.05, v = 0.05, n = 0.06

0.8

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0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

(C)

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Fig. 11.2 Spectral reflectance of the samples studied (A) gray sample, (B) black sample, (C) white sample. n, near-infrared reflectance; SR, solar reflectance; u, ultraviolet reflectance; v, visible reflectance. The tinted square indicates the aspect of the coatings when new.

Fig. 11.3 Portable emissometer. Table 11.1 Properties of the coatings studied in this work. Coating

Solar reflectance (SR)

Thermal emittance (ε)

Gray White Black

0.33 0.80 0.05

0.87 0.90 0.88

Fig. 11.4 Configuration of the test cell facility used to evaluate building roofs (Herna´ndezP
erez et al., 2018).

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253

. To qconv-out

G

qrad-out Roof sample

aGA R

QR

qconv-int . Ti Thermal insulation

Qw

qrad-int

Heat flux transducer Qw Guarded zone

. Tgz

Qw

Fig. 11.5 Schematic of the cross-sectional view of one of the test cells (Herna´ndez-P
erez et al., 2018).

surface of the heat transducer and because Tgz is lower than Ti, the heat flow from the roof was transferred from the transducer to the guarded zone. The heat transferred from the other walls of the cells was removed by the cold air circulating in the guarded zone. Further, as the exterior surface of the walls and the floor of the test cells were also in contact with the outdoor environment, these surfaces also transferred heat to the test cells; however, this energy was removed by the air conditioner, which maintains the constant temperature of the guarded zone. The test cells were equipped with instrumentation to perform measurements of the surface temperatures of the roofs, indoor air temperatures, air temperature in the guarded zones, and voltage of the heat flux transducers. The measurements of voltage were used to determine the heat flux through the roof. The three concrete roof slabs had a thermocouple and a thermopile to measure the temperatures of the exterior and interior surfaces. The three analyzed roof samples had a differential thermopile and one thermocouple on the inner surface used as a temperature reference. The heat transducer had a differential thermopile between the interior surface and the exterior surface in the guarded zone. Another thermopile was installed in each test cell to measure the temperature difference between Ti and To. This array had nine points uniformly distributed inside the heat flow transducer and nine points located outside in a shading device. All the thermopiles and thermocouples used in the test cells were type T thermocouples with an uncertainty of
0.5°C. The climatic variables such as solar irradiance, outdoor air temperature, and wind speed were monitored using a weather station with a data acquisition system, which was located 5 m away from the test cell facility. A high precision multimeter was used as a data acquisition system to monitor and record the voltages and temperatures from all connections. The thermopiles and thermocouples of the components were connected to an acquisition card, which was inserted into the multimeter, and was connected to a PC where the data was stored for processing.

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11.2.3 Influence of conventional and reflective coatings on the surface temperatures and heat flux of roof samples In the first comparative test, gray roof and white reflective roof samples were examined. It is worth mentioning that the experiments shown in this section were performed in a city with temperate weather (Cuernavaca, Mexico). Fig. 11.6 shows a photograph of the gray and white roof samples installed on the outdoor test cells. The location of the test cells was carefully selected to avoid any shadows and to ensure that the roofs did not receive energy reflected or emitted by neighboring buildings. This test began on October 26 and ended on October 30, 2014. Fig. 11.7A presents the wind speed and the solar irradiance obtained from the weather station for the 5 days of the first comparative test. All days were clear without clouds, the peak solar irradiance for this period was about 900 W m2, and the wind speed oscillated between 0.3 and 2.1 m s1. Fig. 11.7B shows the behavior of the exterior surface temperature of the two roofs and the temperature of the outdoor environment. This figure indicates that during the hours with sunlight, the white roof had a lower temperature than the gray roof. The average peak temperature of the gray roof was 41.3°C, while that of the white roof was only 30.2°C. These values demonstrate that the white roof remained up to 11.1°C cooler than the gray roof. Moreover, because the average peak outdoor air temperature was 26.9°C, the white roof had a peak surface temperature only 3.2°C higher than the outdoor air, while the gray roof had a peak surface temperature up to 14.4°C higher. Fig. 11.7C shows the behavior of the heat flux passing through the gray and white roofs. Because the heat flux is proportional to the temperature that the roofs reached, the heat flux of the white roof was smaller than that corresponding to the gray roof. The average peak heat flux of the gray roof was 47.6 W m2, and that of the white roof was 18.6 W m2. The percentage difference between the previous values demonstrates that the white roof had a peak heat flux 61% smaller than the gray roof. In the second comparative test, gray roof and black roof samples were analyzed. Fig. 11.8 shows the gray and black roof samples installed on the outdoor test cells when performing the second experimental test. This test started on April 4 and ended on April 13, 2015.

(A)

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Fig. 11.6 Photograph of two roof samples tested using the test cells. (A) Gray roof sample. (B) White roof sample.

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Fig. 11.7 Variables monitored in the first experimental test. (A) Solar irradiance and wind speed. (B) Exterior surface temperature of the roofs. (C) Heat flux that passes through the roofs.

(A)

(B)

Fig. 11.8 Photographs of two roof samples tested using the test cells. (A) Gray roof sample. (B) Black roof sample.

Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

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Fig. 11.9 Variables monitored in the second experimental test. (A) Solar irradiance and wind speed. (B) Exterior surface temperature of the roofs. (C) Heat flux that passes through the roofs.

Fig. 11.9A shows two environmental variables monitored by the weather station for the second test: the solar irradiance and the wind speed. Because of the season of the year, in this test, the peak solar irradiance was 1000 W m2. It can be seen that the third day (April 11) was cloudy because of the behavior of the solar irradiance. The wind speed had higher values during the hours with sunlight than during the night, and there were periods when the maximum wind speed ranged between 2.0 and 2.7 m s1. Fig. 11.9B shows the behavior of the exterior surface temperature of the gray and black roofs. The average peak temperature of the gray roof was 50.2°C, while the peak temperature of the black roof was 64.8°C. When comparing the previous

Thermal evaluation of building roofs

257

temperatures, it can be noticed that the black roof was up to 18.2°C hotter than the gray roof. Further, when comparing the peak temperatures of the gray and black roofs with the average peak outdoor air temperature (32.1°C), it can be observed that the roofs were up to 18.2 and 32.7°C hotter than the outdoor air. This comparison indicates that the surface temperature of the black roof was two times the outdoor air temperature. Fig. 11.9C shows the heat flux that passes through the gray and black roofs. As mentioned earlier, the heat flux is proportional to the temperatures that the roofs reached, with the heat flux of the gray roof being smaller than that of the black roof. The average peak heat flux of the gray roof was 54.2 W m2, and that of the black roof was 75.9 W m2. The percentage difference between the previous values demonstrates that the black roof had a peak heat flux 29% greater than the gray roof.

11.3

Numerical model of the building roof

It is well known that experimental data provide more reliable results than theoretical models. However, it is not always possible to perform an experiment for each research study. For instance, in order to test the roof samples in a different location with distinct weather, the test cell facility would have to be transported or a new one built in that location. Instead, a theoretical model was developed to evaluate the conventional and reflective coatings in a city with warm weather, as shown in the next sections. This roof model was validated using experimental data, and therefore it can reproduce the thermal behavior of a real building roof. The numerical model uses as input data the thermophysical properties of the roof, the optical properties of the coatings from Section 11.2.1, and data from the weather station, and it provides the temperature field of the building roof.

11.3.1 Physical model of the slab type building roof Fig. 11.10 shows the physical model of the building roof. It is a two-dimensional horizontal roof slab with constant thermophysical properties; the exterior surface is exposed to the outdoor environment and the interior surface is in contact with the indoor air. The building roof has a thickness (y1) equal to 6 cm and it has a width (W) equal to 1 m, which are the same thickness and width of the roof samples used

Tout .

qrad-out

qconv-out y

y1

Concrete x

Coating

G

Tin .

qconv-int W

Fig. 11.10 Physical model of the building roof.

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in the test cells presented in the previous section. In the physical model, it is considered that the conventional and reflective coatings have a negligible thickness.

11.3.2 Mathematical model of the roof The usual two-dimensional unsteady-state expression for the heat conduction in a solid medium was used to determine the temperature field of the building roof model:
     ∂ ρcp T ∂ ∂T ∂ ∂T ¼ λ λ + ∂t ∂x ∂x ∂y ∂y

(11.1)

where ρ is the density, cp is the specific heat, and λ is the thermal conductivity. The boundary conditions of the roof model can be described as follows. The exterior surface of the roof receives solar irradiance (G) and it exchanges heat by convection (qconv-out) and radiation (qrad-out) with the outdoor environment. λ

  ∂T 4 ¼ αG + hout ðT  Tout Þ + σε T 4  Tsky for y ¼ y1 , ∂y

0xW

(11.2)

The interior surface of the roof exchanges heat by convection with the indoor air inside the building. λ

∂T ¼ hin ðT  Tin Þ for y ¼ 0, ∂y

0xW

(11.3)

The two vertical surfaces of the roof are considered adiabatic. ∂T ¼ 0 for x ¼ 0, ∂x ∂T ¼ 0 for x ¼ W, ∂x

0 < y < y1 0 < y < y1

(11.4) (11.5)

The outdoor convective heat transfer coefficient hout of Eq. (11.2) was computed by using the well-known correlation of (Clear et al., 2012) for horizontal roofs. The temperature of the sky in the same equation was calculated with the experimental correlation Tsky ¼ 0.0552T1.5 ext (Duffie and Beckman, 2013). The indoor convective heat transfer coefficient hin of Eq. (11.3) was equal to 9.26 W m2 K1 when the heat flow went from the indoor air to the roof, and it was equal to 6.13 W m2 K1 when the heat flow went from the roof to the indoor air (ASHRAE, 2001). A numerical code based on the finite volume method was developed (Xama´n and Gijo´n-Rivera, 2015) and written in FORTRAN to solve Eq. (11.1) with its boundary conditions. The convergence of the numerical code is achieved when the energy balance of each of the control volumes of the grid used for the roof model is within a value of 1010 and the residual for the temperature is smaller than 1010. These convergence

Thermal evaluation of building roofs 45

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35 T°(C)

Texp, White Texp, Gray Tnum, Gray

30 25 20 15 10 00:00

12:00

00:00

12:00

00:00

12:00

00:00

12:00

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Fig. 11.11 Experimental and calculated temperatures of the exterior surface of the roofs.

criteria provide a reliable solution. The numerical solution can be summarized as: (I) the initial temperature value for each of the control volumes is guessed, (II) the heat conduction equation is solved for the roof with its corresponding boundary conditions, and (III) the convergence criteria are applied for all the temperatures for each control volume and for every time step, if the convergence criteria is not fulfilled, one must return to step (II).

11.3.3 Validation of the roof model To validate the numerical code that solves the heat conduction equation, experimental data from the previous section was used. The numerical code was validated for two cases: a conventional gray roof and a white reflective roof. The temperature of the roofs measured every 10 min for 5 days was used. Fig. 11.11 shows the temperatures obtained by solving the model and the experimental temperatures. This figure shows that the model satisfactorily predicts the behavior of the gray roof and the white reflective roof. The maximum deviations of the temperatures obtained for the gray and white roofs were 5.5% and 4.6%, respectively. Therefore, this model can be used to study the thermal performance of concrete roof slabs in different weather conditions or in a season of the year different than that in which the experiments were performed.

11.4

Thermal comfort evaluation of a building room with conventional and reflective roofs

As shown in the previous section, the type of coating on a building roof has a direct effect on the surface temperature that this component can reach. Therefore, when a roof with a reflective coating reduces the surface temperature, it benefits the thermal comfort conditions of non-air-conditioned buildings. In this section, a computational model was used to determine the influence that a roof coating can have on the thermal performance of non-air-conditioned buildings. This section presents the thermal evaluation of a building room with the three roof coatings shown in previous sections, under the weather conditions of a city with warm weather (M
erida, Yucata´n).

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11.4.1 Physical and mathematical model of the building room The thermal behavior of a room during the warmest day of 2014 in M
erida, Mexico is presented. This city has humid, warm-weather conditions. To analyze the thermal performance of the building room, three configurations were compared: a room with a roof coating (gray), a room with a white roof, and a room with a black roof (Fig. 11.12). The room is represented as a two-dimensional closed cavity; the left surface is considered isothermal at T ¼ 24°C (297 K); on the right vertical surface there are two adiabatic walls, including a clear glass window; and the height of the window is considered to be 0.8 m with a glass thickness of 6 mm. The height from the floor to the base of the clear glass window is 1.20 m. The floor is considered adiabatic, and the roof is considered a conductive heat roof with a thickness of 0.1 m. Those four walls form a cavity with a size of 3 m (H ¼ W). The emissivity values of the opaque surfaces of the walls inside the room are considered to be 0.90. It is also considered that the clear glass window surface has an emissivity equal to 0.85. Convective and radiative heat gains or losses on the outside of the glass and the roof are also considered. In the same way, the interior surfaces exchange heat by convection with the indoor air and the interior surfaces exchange heat by radiation between them.

11.4.1.1 Mathematical model of natural convection in the room The conservation equations of mass, momentum in the x and y directions, and the energy equation for natural convection can be expressed as follows: ∂ðρui Þ ¼0 ∂xi

(11.6) Black

Gray

White

H4

Air

glass

H

T = 24°C

H3

H2

H1 y x

W

x

1

Fig. 11.12 Physical model of the building room with different roofs and a single pane window.

Thermal evaluation of building roofs


    ∂ ρui uj ∂P ∂ ∂ui ∂uj ¼ + μ +  ρu0i u0j  ρgi βðT  T∞ Þ ∂xi ∂xj ∂xj ∂xj ∂xi !
 ∂ ρuj T 1 ∂ ∂T 0 0 ¼ λ  ρuj T Cp ∂xj ∂xj ∂xj

261

(11.7)

(11.8)

The terms u0i u0j and u0j T from these equations are the Reynolds stress tensor and the turbulent heat flux vector, respectively. These two terms can be written as: "

# ∂u ∂u 2 j i ρu0i u0j ¼ μt + ρκδij + 3 ∂xj ∂xi ρu0j T 0 ¼ 

μt ∂T σ T ∂xi

(11.9)

(11.10)

The turbulent kinetic energy equations and the turbulent kinetic energy dissipation are obtained from their transport equations. The resulting equations k-ε, from the HH turbulence model proposed by Henkes et al. (1991) can be written as follows: ∂ðρui κÞ ∂ ¼ ∂xi ∂xi ∂ðρui εÞ ∂ ¼ ∂xi ∂xi

  μt ∂k μ+ + Pκ + Gκ  ρε σ k ∂xi  μ+

 μt ∂ε ε ρε2 + Cε1 ½PK + Cε3 Gκ   Cε2 κ σ ε ∂xi κ

(11.11)

(11.12)

The turbulent viscosity (μt), the production of shear stress (PK), and the buoyancy production-destruction of the turbulent kinetic energy (GK) are defined, respectively, as follows: μt ¼ Cu

ρk2 ε

PK ¼ ρu0i u0j

(11.13) ∂ui ∂ui ¼ τij ∂xi ∂xj

GK ¼ βρu0j Ti0 gi ¼

μi ∂T gi β ∂xj σt

(11.14)

(11.15)

To complete the mathematical model for the room system with a window and different types of roof, where the right wall of the room is composed of an upper and lower wall considered insulated, and between them the window, together with a roof considered as a heat conductive solid, it is necessary to apply an energy balance for each of these elements.

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11.4.1.2 Mathematical model for heat conduction in the window and the roof The mathematical model that represents the heat transfer by conduction in the glass is: ! ! ∂ λg ∂Tg ∂ λg ∂Tg 1 ∂ΘðxÞ ¼0 + + ∂x Cpg ∂x ∂y Cpg ∂y Cpg ∂x

(11.16)

where the attenuation function is Θ(x) ¼ G exp [ sg(D  x)]; sg is the extinction coefficient of the glass; and D is the thickness of the glass. The conductive model for the roof was presented in the previous section. The validated roof model in the building room model presented in this section was used. The nonslip condition was imposed for all internal surfaces of the cavity to obtain the boundary conditions for the momentum equations. The boundary conditions for the energy equation in the different internal surfaces of the cavity can be written as: q1conv-int + q1rad-int ¼ 0 for y ¼ 0 0 < x < W

(11.17)

T ¼ 24°C for x ¼ 0 0  y  H

(11.18)

q3cond ¼ q3conv-int + q3rad-int for y ¼ H 0 < x < W

(11.19)

q4conv-int + q4rad-int ¼ 0 for x ¼ W 0  y < H1

(11.20)

q4cond ¼ q4conv-int + q4rad-int for x ¼ W H1  y  ðH1 + H2 Þ

(11.21)

q4conv-int + q4rad-int ¼ 0 for x ¼ W ðH1 + H2 Þ < y  H3

(11.22)

where qcond is the conductive heat flux in the surface of the wall or glass, while qrad is the resulting radiative heat flux in the corresponding wall.

11.4.2 Methodology for the numerical solution and verification of the room model The governing equations were solved using the finite volume method. For the coupling of the mass and momentum equations, the SIMPLEC algorithm was used (Patankar, 1980), in which a hybrid scheme was adopted for the convective terms. Grid independence was carried out for the building room system with the climatic conditions at 15:00, as this time corresponds to a solar radiation value of 619 W m2 and an ambient temperature of 41.5°C at M
erida, Yucata´n, Mexico. The mesh was varied from 71  111 to 121  111 with increments in the x-direction of 10 nodes. The final mesh for the window was 11  51 and for the room was 91  121, of which 10 nodes in the y-direction were for the roof, with a total of 101  121 nodes for which to

Thermal evaluation of building roofs

263

perform the computational runs, with a percentage error of less than 0.1% for maximum air temperatures. To verify the developed numerical code, the results obtained from it were compared with the results from studies available in the literature. Therefore, the differentially heated cavity with turbulent flow and surface radiative exchange with the Radiosity-Irradiation Method (RIM) was modeled as reported by Velusamy et al., 2001. They reported two cases in their work: Case A, a cavity with left and right wall surface temperatures of THOT ¼ 328 K, TCOLD ¼ 318 K, and Case B, the same surface temperatures of THOT ¼ 348 K and TCOLD ¼ 298 K, both cases with an emissivity ε ¼ 0.9 on all walls, for a number Ra ¼ 1011. From the comparison of the results, it can be concluded that the results provided by the program developed are acceptable, because maximum deviations of 3.1% and 3.2% were obtained for Cases A and B, respectively. Thus it was demonstrated that the developed code successfully predicts the solution reported in the literature.

11.4.3 Indoor air temperature and average total heat flux inside the building room on a warm day This section presents the thermal evaluation of the building room with three different types of roofs. The influence of the roof on the heat transfer and the thermal comfort of the room were analyzed for the warmest day of 2014. The weather data used for the hourly simulations for these 2 days were obtained from a meteorological weather station situated in M
erida. Variables including outdoor air temperature (Text), wind speed (v), solar irradiance for a horizontal surface (Groof), and solar irradiance for a vertical surface oriented to the south (Gglass) were used as external boundary conditions for the room model. Table 11.2 presents the hourly weather data used in the simulations for the warmest day. Fig. 11.13 shows the interior surface temperature of the roofs. As shown in the previous section, during the hours with solar irradiance, the black roof had higher surface temperatures than the gray and the white roof. At 13:00 the three roofs reached the peak temperatures, 69.3, 59.7, and 39.3°C, with the values corresponding to the black, gray, and white roof, respectively. Therefore, the white roof reduced the interior surface by 20.4°C compared to the gray roof, while the black roof increased it by 9.6°C. Because the interior surface interacts with the indoor air, the temperature of this surface affects the behavior of indoor air temperature. One of the advantages of using the finite volume method to numerically solve the mathematical model presented in Section 11.4.1 is that this technique provides the temperature values in each of the nodes of the computational domain. Because one part of the computational domain is the indoor air, the temperature distribution of the air in the room can be obtained. Fig. 11.14 shows the positions inside the room where the indoor air has a constant temperature (isothermals) for the hour with the peak heat flow from the roof (qroof avg-int ) to the indoor air (13:00). The figure demonstrates that the temperature of the indoor air around the center of the room with the gray roof is approximately 35°C, while the corresponding values for the rooms with

264

Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

Table 11.2 Weather conditions for the warmest day in Yucata´n, M
exico in 2014. Warmest day 04/07/2014 Time (h)

Groof (W m22)

Gglass (W m22)

T (°C)

v (m s21)

01:00 02:00 03:00 04:00 05:00 06:00 07:00 08:00 09:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 24:00

0 0 0 0 0 17 192 513 715 885 986 999 959 845 619 389 192 9 0 0 0 0 0 0

0 0 0 0 0 34 273 870 807 695 521 302 73 0 0 0 0 0 0 0 0 0 0 0

25.8 25.4 25.3 25.1 24.8 24.5 26.2 29.3 31.5 34.6 36.3 37.9 39.2 39.9 41.5 40.3 37.9 32.2 29.8 28.6 28.2 28.0 28.2 28.1

3.8 4.0 3.9 3.4 3.5 3.5 4.7 4.8 5.4 4.5 3.6 2.9 1.5 1.5 1.4 2.2 4.2 5.0 3.2 3.5 3.0 2.0 2.4 1.5

75

Gray White Black Tout

Tavg (°C)

60

45

30

00:00 04:00 08:00 12:00 16:00 20:00 24:00

T (h)

Fig. 11.13 Average temperature of interior surface of the roof.

Thermal evaluation of building roofs

40

37

265

65

75

40

31

40

38

30 35

33

35

28

(A)

(B)

(C)

Fig. 11.14 Isothermals for the indoor air at 13:00 for the building room with (A) gray roof, (B) white roof, and (C) black roof.

a white roof and a black roof are 30 and 38°C, respectively. These values indicate that the white roof improved the thermal comfort around the center of the room by up to 5 and 8°C compared to rooms with the gray and black roof, respectively. It is also observed that the isothermals in the upper part of the cavity, or air near to the roof interior surface, in the room with the gray roof have a temperature of 65°C. In the same zone, the air temperature in the room with the white roof is 40°C and the black roof is 75°C. When comparing these temperatures, it is evident that the effect of the white coating is more significant in the upper zone of the room. By averaging the temperature of all points in the domain that correspond to the indoor air, the average indoor air temperature can be obtained. Fig. 11.15 shows the behavior of the average indoor air temperature throughout the day. It is observed that, in periods without solar radiation, between 01:00–06:00 and 18:00–24:00, the average indoor air temperature of the three rooms is the same, reaching approximate values of 24°C. On the contrary, during the hours with solar radiation, the average indoor air temperature of the room with the white roof is less than that of the rooms with gray and 45

Gray White Black Tout.

Tavg.(°C)

40 35 30 25

20 00:00 04:00 08:00 12:00 16:00 20:00 24:00

T (h) Fig. 11.15 Average temperature of interior surface of the roof.

266

Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

black roofs. The maximum average indoor air temperature of the room with the gray roof is 35.6°C, with the white 30.0°C, and with the black 38.4°C. These temperatures demonstrate that the white roof can improve the indoor comfort conditions by up to 5.6 and 8.4°C. The previous values are essentially the same as those described in the previous paragraph, when comparing the isothermals in the center of the room. Other important variables when analyzing the thermal performance of buildings are the heat flux and the daily heat gain. Table 11.2 shows the average heat flux that the glass transfers to the indoor air (qglass avg-int ¼ qconv + qrad + qtrans ), and the heat flux that the roof transfers to the indoor air (qroof avg-int ¼ qconv + qrad ). The previous fluxes (qroom avg-int ¼ |qglass avg-int + qroof avg-int |) were added to determine the total amount of heat transferred to the indoor air, as shown in the table. During the hours without solar incidence (01:00–06:00, 18:00–24:00) the room loses heat to the outdoor environment. On the other hand, in the hours with solar incidence, the room receives energy from the outdoor environment. At 13:00, the peak heat flux (qroof avg-int,max ) of the gray, white, and black roofs is 167.0, 58.93, and 225.94 W m2, respectively. These values indicate that at this hour the white roof had a heat flux 65% smaller than that of the conventional roof, while the black roof had a heat flux 35% higher. However, because of the optical properties of clear glass, the energy that passes through the window is almost the same amount of radiation that this component receives. The last row of Table 11.3 contains the daily heat gain of the three roofs. This variable provides the net amount of thermal energy gained by the air from the roof during the whole day. The daily heat gain was obtained by integrating the heat flux over the 24-h period. The room with the gray roof had a daily heat gain of 4475.68 W h m2, while the rooms with white and black roofs had a daily heat gain of 4082.75 and 4694.02 W h m2, respectively. By comparing these values, it was found that the room with the white roof had a daily heat gain 8.8% smaller than the room with the gray roof. On the contrary, the room with the black roof had a daily heat gain 4.9% higher than the room with the gray roof.

11.5

Comparison of experimental results with those of the literature

Similar experimental tests from other countries to those performed in Mexico in the test cells with and without reflective coating are available in the literature. One of these studies was reported by Kachkouch et al. (2018), in which outdoor test cells with passive techniques in the semiarid weather of Morocco were evaluated. The results for the climate of Morocco indicated that the use of a reflective coating reduced the temperature of the roof surface up to 13°C with respect to the test cell with a concrete slab without reflective coating. As previously shown, similar results were found for Mexico, with a roof surface temperature reduction of 11.1°C. The heat flux reduction due to the white reflective coating in Morocco was 66%, while for the tests performed in the climate of Mexico, it was 61%. Further, Stavrakakis et al. (2016) found for locations in Greece roof surface temperature reductions up to 14.2°C for a school building due to the use of a white reflective coating (Fig. 11.16). In addition, during the summer

Table 11.3 Average heat flux from the roof and the window to the indoor air for the warm day. qglass avg-int (W h m22)

qroof avg-int (W h m22)

Gray

White

Black

Gray

White

Black

11.59 10.60 9.99 9.30 8.31 34.79 235.12 740.51 688.88 600.04 452.25 269.23 71.25 17.83 36.01 48.47 54.81 37.20 26.03 21.46 19.77 18.63

11.71 10.76 10.05 9.40 8.42 35.70 241.59 756.99 710.93 629.40 487.33 306.71 111.09 52.94 61.95 63.92 61.36 37.54 26.12 21.53 19.84 18.73

11.65 10.68 10.02 9.35 8.37 34.42 231.83 731.53 676.74 583.86 432.96 248.74 49.87 1.00 22.16 40.19 51.30 37.08 26.08 21.49 19.81 18.68

5.98 6.89 7.32 8.61 9.31 8.59 10.71 42.84 65.29 98.14 124.84 142.51 167.00 152.81 125.59 82.52 47.36 14.01 4.29 1.52 0.45 3.16

6.17 7.08 7.51 8.82 9.52 10.31 2.21 7.41 18.22 32.61 42.81 51.14 58.93 57.79 55.07 43.96 33.04 13.31 4.15 1.36 0.63 3.35

6.08 6.98 7.41 8.72 9.42 7.88 17.66 62.50 91.63 134.87 170.69 193.27 225.94 204.68 164.06 103.65 55.13 14.27 4.22 1.44 0.54 3.26

Total heat flux |qglass avg-int + qroof avg-int | (Wh m22)

Time (h) 01:00 02:00 03:00 04:00 05:00 06:00 07:00 08:00 09:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00

5.61 3.72 2.67 0.68 1.00 26.20 245.84 783.36 754.17 698.18 577.10 411.74 238.24 170.64 161.60 130.99 102.17 51.21 30.32 22.98 19.32 15.48

5.54 3.68 2.54 0.58 1.10 25.39 239.37 764.40 729.15 662.01 530.14 357.85 170.02 110.74 117.02 107.88 94.40 50.84 30.27 22.89 19.21 15.38

5.57 3.70 2.61 0.63 1.05 26.54 249.50 794.03 768.37 718.73 603.65 442.01 275.80 203.68 186.21 143.84 106.43 51.35 30.30 22.94 19.26 15.43 Continued

Table 11.3 Continued qglass avg-int (W h m22)

qroof avg-int (W h m22)

Gray

White

Black

Gray

White

Black

19.56 19.05

19.63 19.17

19.59 19.11

1.67 4.30

1.85 4.51

1.76 4.41

Total heat flux |qglass avg-int + qroof avg-int | (Wh m22)

Time (h) 23:00 24:00 Numerical Integration by Trapezoid rule 24:00 R qðtÞdt 1:00

17.89 14.75 4475.68 (Wh m2)

17.77 14.66 4082.75 (Wh m2) (8.8%)#

17.83 14.71 4694.02 (Wh m2) (4.9%)"

Thermal evaluation of building roofs

269 54.6°C 50 45 40 35 30 27.6°C

(A) 35.7°C 34 32 30 28 26 24 23.5°C

(B) Fig. 11.16 Pilot-building roof’s thermographs: (A) before and (B) after the application of the cool roof (Stavrakakis et al., 2016).

in US weather, Akbari (2003) found from an experimental study a reduction of 19°C of the roof surface temperature due to the use of white reflective coatings in nonresidential buildings. The results reported for different cities with different types of weather show the potential use of reflective coating technology. It should be noted that the differences between the results of the studies available in the literature are mainly due to the different environmental conditions, the roofing materials and roof configurations, and the values of solar reflectance of the reflective coatings. The results also show that this passive cooling technique can have a significant impact on the temperature of the roof surface and the heat flow through the roof, as discussed in this chapter. Additionally, Costanzo et al. (2014) and Romeo and Zinzi (2013) reported improvements in thermal comfort and a reduction in the energy consumption destined for cooling. The effects of using a white reflective coating as a passive technique have demonstrated reductions in the external surface temperature of roofs. As a consequence, this temperature reduction causes the heat flow of this component to decrease, and therefore it contributes significantly to the comfort improvements or the reduction of cooling loads of buildings in different locations with different weather conditions, such as is the case with studies conducted in the United States, Kuwait, Italy, Morocco, and Greece. Details of the studies mentioned are presented in Table 11.4.

Table 11.4 Studies that have measured the passive cooling effect of white reflective roofs. Reference

Location

Akbari (2003)

Image

Cases of study

Results

Nevada, USA

(1) Conventional roof (SR ¼ 0.26) (2) White roof (SR ¼ 0.72)

The surface temperature of the conventional roof was up to 8°C hotter than the outdoor air temperature, while the white roof was up to 2°C cooler than the outdoor air.

Costanzo et al. (2014)

Catania, Italy

(1) White roof (SR ¼ 0.42) (2) Roof with clay tiles (SR ¼ 0.30)

The white reflective coating reduced the exterior surface temperature of the roof between 3 and 4°C.

Kachkouch et al. (2018)

Marrakech, Morocco

The white roof had the highest thermal efficiency, with a surface temperature up to 13.0°C colder than reference roof. The white roof reduced heat flow through the roof slab by up to 66%.

Romeo and Zinzi (2013)

Traponi, Italy

Stavrakakis et al., 2016

Athens, Greece

(1) Gray concrete roof (2) Shaded roof (3) Insulated roof (4) White roof (SR ¼ 0.77) (1) Concrete tiles, SR ¼ 0.25, ε ¼ 0.9 (2) White roof, SR ¼ 0.82, ε ¼ 0.88 Gray roof (SR ¼ 0.172); White roof (SR ¼ 0.89)

The white roof reduced by 2.3°C the indoor air temperature. It also reduced the annual energy demand 4.6% and 13%.

The external surface temperatures were 40.4 and 54.6°C for the gray and the white roof, respectively. The hours of thermal comfort increased in summer 24% when using the white roof.

Thermal evaluation of building roofs

11.6

271

Conclusions

In this chapter, the thermal performance of slab-type building roofs was evaluated with conventional and reflective coatings in three stages. In the first stage, small-scale roof samples were experimentally assessed using two outdoor test cells situated in a city with temperate weather. It was found that the white roof remained up to 11.1°C cooler than the gray roof, and the black roof remained up to 18.2°C hotter than the gray roof. The heat flux through the three roof samples was also determined; the white roof had a peak heat flux 61% smaller than the gray roof, while the black roof had a peak heat flux 29% greater. In the second stage, a numerical model was developed of a building roof, validated with the experimental data obtained in the test cell experiments. The results provided by the numerical model had a maximum deviation of 5.5% and 4.6% from experimental data for the gray and white roof, respectively. Then, this model was coupled to a computational fluid dynamics model of a room located in a city with a warm climate, to study the effect of the coatings on thermal comfort. It was found that the white reflective roof improved the indoor thermal comfort of the room by around 6 and 8°C compared with the gray and the black roof, respectively. It was also found that the room with the white roof had a daily heat gain around 9% smaller than the room with the gray roof. On the other hand, the room with the black roof had a daily heat gain around 5% higher than the room with the gray roof. It was concluded that the white roof is the best option in non-air conditioned buildings located in a warm zone, because it improved the thermal comfort inside the building room, compared to the gray and black roofs.

Acknowledgments Part of the information contained in this chapter was published in previous works of the authors. The authors acknowledge the National Council of Science and Technology (Conacyt-Mexico) for the support given through the National System of Researchers Program (Sistema Nacional de Investigadores, SNI).

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Active and passive systems for cool roofs

12

Ming Chian Yewa and Ming Kun Yewb a Department of Mechanical and Material Engineering, University of Tunku Abdul Rahman, Kajang, Selangor, Malaysia, bDepartment of Civil Engineering, University of Tunku Abdul Rahman, Kajang, Selangor, Malaysia

12.1

Introduction

Cool roofing technologies play significant roles in moving us towards a low-carbon, cleaner world, as energy conservation through building energy efficiency has acquired prime importance for Mother Earth. The four main aspects for improving energy efficiency in building cooling are: (i) near zero energy passive building roof design, (ii) usage of low thermal conductivity roofing materials, (iii) utilization of smart technologies and energy-efficient equipment for cool roof systems, and (iv) integration of active and passive cooling roof technology systems for sustainable development. Hence, the sustainability assessment of buildings is becoming necessary for sustainable development worldwide, especially in the building sector. The main goals of developing building cooling technologies are to reduce depletion of critical resources, such as electricity and raw materials, resulting in a lower carbon footprint; to prevent environmental degradation caused by facilities, infrastructure, and greenhouse gas emissions; and to create built environments that are safe and productive, and that effectively utilize renewable energy (Kolokotroni et al., 2013; Stavrakakis et al., 2016; Boixo et al., 2012). Many factors influence the energy demand of a building, including its purpose, intended use, and location. The thermal properties of the materials used for the external walls and roof can have a major influence on the surface temperature and in turn the amount of heat conducted through the surface of the building. A cool building surface (roof and/or walls) uses a coating with high thermal emissivity and solar reflectance properties to decrease the solar thermal load of a building, thus reducing its energy requirements for cooling (Pisello, 2017; Akbari and Kolokotsa, 2016). Many experimental and modeling studies have been published that compare building energy efficiency benefits of cool roofing techniques with computational and experimental studies, reporting positive results for residential buildings (Santamouris et al., 2011; Santamouris and Kolokotsa, 2015; Synnefa et al., 2007). The tropical region is an uncomfortable climate zone that receives a large amount of solar radiation, high temperatures, high level of relative humidity, and long periods of sunny days throughout the year. Recently, this region has undergone increased urbanization, particularly developed countries such as Malaysia, Singapore, and Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction https://doi.org/10.1016/B978-0-12-820791-8.00012-2 © 2021 Elsevier Ltd. All rights reserved.

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction 1970–1980 6 1980–1990 1990–2000

Percent growth in urban population

6

5

5 4

4

3

3

2

2

1

1

0

SWAsia SCAsia

India

SEAsia

China EastAsia Africa

Europe

NAm

CSAm Oceania

0

Fig. 12.1 Annual rates of the change in urban population measured by region and by decade (Seto et al., 2011).

Indonesia, because of the rapid growth of the urban population, as shown in Fig. 12.1. This development has further increased the intensity of the urban heat island (UHI) phenomenon (Wong et al., 2011). This effect is associated with an increase in energy consumption, specifically the cooling issue, which is the primary concern of this region. As far as we know, energy is an essential quantitative property that empowers everything in our daily lives. As a developing country, the rate of energy consumption in Malaysia has been rising significantly due to the expansion of construction and population increase. Based on the energy consumption report, building is one of the major energy consumers, consuming up to 40% of the total global energy. By the year 2030, about 50% of the global energy is expected to be consumed by the building sector. A statistic produced by the National Energy Balance (NEB) shows that the building sector in Malaysia consumed up to 48% of the generated electricity in the country. Residential buildings consume 24,709 GWh while commercial buildings consume up to 38,645 GWh (Hassana et al., 2014). Since Malaysia is a tropical country, most of its locations experience a hot and humid climate with constant mean air throughout the year. Thus an electricity consumption of about 1167 kWh was contributed by air conditioners, which are considered to be the biggest electricity consumer in Malaysia (Kubota et al., 2010). Comparing the roof and the vertical wall of the building, most of the heat is gained by the roof due to the fact that its exposure rate from the sun is higher during the daytime. Most of the rooftop designs in Malaysia have an uppermost rooftop and a gypsum ceiling board below it. The roofing materials commonly used in Malaysia are concrete roof tiles (85%) followed by clay tiles (10%) and also metal deck (5%) for low-cost houses and factories (Akbari and Kolokotsa, 2016). When these materials are used without an insulation coating, heat is easily transmitted through the roof, which generates heat in the attic region (Yew et al., 2013). Furthermore, most of the buildings have poor ventilation and are fairly airtight, mainly due to the lack of air circulation. As a result, the heat transmitted through the roof circulates in the attic

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region and is transferred to the ambient air inside the building. To counteract this, several effective cool roof systems have been developed. A cool roof system has the capability of reflecting the radiated heat from the sun and reducing the rate of heat transfer through the roof in order to reduce the heat transmitted, as compared with the standard roofing system designs (Yew et al., 2018). Using this type of roofing system in tropical countries improves the comfort level inside the buildings, and also represents a lowcost method of energy savings. In addition, attic ventilation is improved by the cool roof system due to the reduction in the heat transference mechanism through the roof to the attic region. Consequently, the heat gained by the ceiling of the building is greatly decreased, which reduces the need for mechanical cooling systems such as air conditioners.

12.2

Types of cool roof systems

12.2.1 Active cool roof system Active cooling roof technology consists of several innovative practices that cool buildings using smart techniques, including: (1) rainwater harvesting systems integrated with sensors, (2) water-to-air heat exchangers with evaporative cooling systems, and (3) evaporative cooling for water tanks.

12.2.1.1 Rainwater harvesting system integrated with sensors A rainwater harvesting system is mainly implemented to cool down the rooftop temperature to reduce the rate of heat transfer from the rooftop to the attic region. As the name of the system indicates, rainwater is utilized as a cooling agent that directly absorbs the heat from the rooftop. This system applies sustainable environmental concepts by using a renewable source as the main cooling agent to perform the cooling without any environmentally damaging effects. The rainwater harvesting system usually comprises three major components: the catchment area, conveyance system, and water storage system. The cooling flow cycle is illustrated in the schematic drawing in Fig. 12.2. The rainwater harvesting system incorporated in this model is controlled and triggered by the thermostat, which applies the cooling effect at a specific temperature. In this model, the area of the roof tile is the catchment area for the harvesting system. Water flowing from the roof tile is collected at the gutter, which is a half-cut cylindrical pipe. Conveyance devices including gutter, funnel, cylindrical plastic container, and L-shape fixed support are set up to convey the water from the rooftop to the cooling tower. After the water flows through a small-scale self-fabricated cooling tower, it then enters the plastic container used to store the collected water. The cooling system of this rainwater harvesting system was investigated as a means to reduce the temperature of the attic region. The water collected in the storage tank is used as a cooling agent for the rooftop, which then indirectly reduces the attic temperature. When the rooftop temperature reaches 40°C, a water pump is triggered by the thermostat to pump cool water to the rooftop through the rubber pipe. The heated water is then collected by the gutter and flows to the cooling tower in order

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

Roof top Gutter Funnel Attic Water pipe Cooling tower Water tank Water

Fig. 12.2 Cooling flow cycle of rainwater harvesting system.

to release the heat absorbed during the flow path on the rooftop. As result, the water is cooled and stored in the water tank, completing the flow cycle of the system.

12.2.1.2 Water to air heat exchanger (WAHE) with evaporative cooling system This system is mainly implemented to improve the performance of green roof systems, through the heat of the building being transferred out or cooled down by the heat exchange of a water tank installed to perform a phase change and evaporative cooling (Mishra, 2016). This study was carried out by Umberto Berardi and his coworkers to examine the improvements in thermal comfortability inside a building obtained through the conceptual heat transfer mechanism from the phase change of water to an air heat exchanger with an evaporative cooling system (Berardi et al., 2017; La Roche et al., 2016).

12.2.1.3 Evaporative cooling for water tank This evaporative cooling system was introduced to improve the cooling performance of the WAHE system by releasing the heat absorbed from the building through a water tank. The indirect evaporative cooling implemented in this system due to the cooling process is not directly applied to the ambient air inside the building. The evaporative cooling system is implemented for the purpose of cooling the water that absorbs the heat from the building. An indirect cooling process is incorporated in this system, as it does not have direct interchange of heat with the air in the building. During the night the heat that is absorbed by the water is released through evaporative cooling due to the temperature differential between the ambient air and

Active and passive systems for cool roofs

279

warmed water inside the tank ( Jos
e Manuel and Pablo, 2019). The heat transfer mechanism for the cooling process in this system can be categorized into two stages. First, the hot air of the building passes through the heat exchanger so that the heat in the air is cooled by the cool water in the tank. Second, the water inside the water tank is cooled by the evaporative cooling system through the spray above the insulation board. Therefore, the cooled water inside the water tank acts as an intermediate sink while the ambient air outside the building is the final sink. Based on previous research, the thickness of the thermal insulated board on the water tank was designed to be 3 cm. In addition, the white color paint was specially designed as a solar radiation reflector, reducing the heat transfer from the sun to the water tank. For the evaporative cooling system, the spray was suggested to be placed 0.5 m above the center of the water tank, as this height was known as the minimum suggested height for this system (Aurelio Diaz and Osmond, 2017). The performance of the cooling effect through this evaporative cooling system was evaluated using the sensors placed on the floating insulation board during the nighttime, when the spray above the water tank operated. Two sensors were utilized in this system to evaluate the cooling effect. The first sensor was placed at the center of the insulation board in order to measure the temperature of the water that directly emerges from the spray. Then the second sensor was utilized to evaluate the temperature of the water after it passed through the insulation board, in order to study the evaporative cooling effect. The results showed that the water temperature in the tank was successfully reduced by the evaporative cooling effect, which cooled the building indirectly (Zhang et al., 2018).

12.2.1.4 Postrainfall evaporation from porous roof tile for building cooling Rainfall occurs frequently in tropical countries such as Malaysia and greatly impacts the thermal performance of the buildings due to the evaporative cooling effect generated by the rainwater. The energy performance of buildings affected by evaporative rainfall was investigated by Rao in Singapore (Kamal, 2012). An experiment was carried out in order to simulate rainfall over the buildings, with a plastered brick wall spayed by water during a hot sunny day. As a result, the energy consumption of the test room was reduced up to 25% by the effect of evaporative cooling. In addition, the effect of an evaporative cooling wall of porous tile on energy savings in the Chinese city of Guangzhou was studied by Zhang et al. Based on the research done by Zhang, a 35.29%–68.27% reduction in the air-conditioning energy consumption was seen with the rainfall simulated room as compared with another room with the same construction orientation and dimension (Givoni, 1994).

12.2.2 Passive cool roof system The passive cooling technique plays an important role in providing buildings with comfortable conditions through natural means. Reflective and radiative processes are the methods used to decrease heat gain by facilitating the elimination of excess

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

heat in a building’s interior to maintain a comfortable environment using a sustainable approach (Al-Obaidi et al., 2014). This strategy involves a controller that limits the total effect of the heat gain to provide an interior temperature lower than that of the natural surroundings (Mumovic and Santamouris, 2009). In general, the flow of energy in a passive design is based on natural means, such as radiation, convection, or conduction. Passive cooling systems do not eliminate the use of a fan or a pump, when their application can boost performance. In fact, several studies have been carried out by Al-Obaidi and his coworkers in the tropics to enhance passive cooling using hybrid systems (Al-Obaidi et al., 2014). Passive cooling strategies generally consist of all the preventive measures against overheating in the interior of buildings. Such cooling strategies should cover three levels: (1) Passive cooling strategies should prevent heat gains inside the building. The parameters that should be considered include the envelope’s insulation, the solar shading of the facade, and surface properties, such as the color of the external surfaces. (2) Heat gains should be modulated by effective solar control to achieve a balance between controlling solar gain and admitting enough daylight, while ensuring the architectural and structural requirements of the building envelope. Moreover, a comfortable level of heat load should be permitted by modulating the required temperatures for the different uses of internal spaces during the design phase. (3) The heat in the building’s interior should be reduced by heat sinks (natural or hybrid cooling) through air infiltration and surface properties, such as the color of the internal surfaces, and energy-efficient equipment that can considerably reduce internal heat gains.

12.2.2.1 Thermal reflective coating (TRC) and thermal insulating coating (TIC) Roofs coated with thermally reflective material reduce heat transfer through the coating and 90% of solar infrared radiation and 85% of ultraviolet radiation is radiated back from the coated surface. The first generation of TRC used in cool roofs consisted of natural materials, while the second generation was based on the development of artificial white materials designed to present very high albedo; the third phase is the development of colored highly reflective materials (Santamouris, 2014). Pisello et al. (2015) performed an experimental analysis of a combined albedo measurement campaign for the characterization of an innovative waterproof polyurethane-based membrane for cool roof applications. Cozza et al. (2015) did research to formulate exterior building paints as smart coatings with high IR reflectance to decrease the use of energy for cooling buildings Raj et al. (2015) developed pigments based on terbium-doped yttrium created with high NIR reflectance for cool roof and surface coating applications. Yew et al. (2013) focused on a TIC integrated with a series of aluminum tubes installed on the underside of a metal roof. Gagliano et al. (2015) compared the energy and environmental behavior of cool, green, and traditional roofs and identified that green and cool roofs provide higher energy savings and environmental benefits than highly insulated standard roofs.

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TIC plays an important role in promoting measures against the heat island phenomenon and limiting carbon dioxide emissions. The heat island phenomenon is air pollution caused by changes in the surface heat balance, in which the temperature in an urban region rises above that of the surrounding area. It is caused by reductions in greenery and water surfaces and by an increase in artificial heat resulting from energy consumption. Several different thermal insulation materials are used in the building industry today. Conventional materials, such as glass wool, rock wool, expanded polystyrene (EPS), and extruded polystyrene (XPS), require a thick building envelope to reach a sufficiently low thermal transmittance. Using TIC on the roof surface with white solar-reflective paint is a very efficient way to reduce heat discomfort conditions for single-story buildings located in cities with hot and humid climates. TRC has the potential to contribute to the reduction of the heat island phenomenon, as well as carbon dioxide emissions, and creates new opportunities for the design of energyefficient buildings. The demand for thermal insulation products has significantly increased because of high energy prices. With the focus on cost efficiency, there is growing interest in the use of TRC. In this study, the TRC is designed to reduce the surface temperature of the roof by reflecting heat. The thermal insulation paint was formulated using titanium dioxide pigment and chicken eggshell waste as biofiller, bound together by a polyurethane resin binder with low thermal conductivity (0.65 W/m K). The addition of high purity reflective titanium dioxide pigment increases solar reflection, which results in an enhanced thermal insulating function.

12.2.2.2 Moving-air-cavity (MAC) ventilation Natural ventilation, as the name implies, is a system using natural forces to supply fresh air for comfort and heat dissipation. As an alternative to mechanical (fan-forced) ventilation, this approach relies on the natural forces of wind and buoyancy to deliver fresh air to indoor spaces (Schulze and Eicker, 2013). Natural ventilation can be divided into two categories: (1) Controlled natural ventilation is intentional displacement of air through specified openings such as windows, doors, and ventilators. It is usually controlled to some extent by the occupant. (2) Uncontrolled ventilation (infiltration) is the random flow of air through unintentional infiltration through cracks, gaps, or crevices in the building structure. It is less desirable and can be controlled only by plugging the gaps.

Natural ventilation has several benefits, such as low operating cost, zero energy consumption, low maintenance, and probably lower initial cost. It is regarded as healthier, and the way it connects with the outside is seen as a psychological benefit. The effectiveness of natural ventilation is determined by the prevailing outdoor conditions, depending on wind speed, temperature, humidity, and surrounding topography, and the building itself (orientation, number of opened attic inlets, number of windows, size, and location). The alternative to natural ventilation is mechanical ventilation, which uses one or more electrical fans or blowers to move air in and out. The primary advantage of this approach is the consistency and controllability of the rate of ventilation. Other advantages include the opportunities for air filtration and possible heat

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

recovery. The disadvantages are the capital costs, the running costs, the noise, and continuous maintenance. The design of natural ventilation systems necessitates knowledge of the mechanism of air flow through buildings and also of factors that have a bearing on air-flow patterns indoors. In this section we define and discuss three essential aspects of natural ventilation: (i) Natural driving forces. The first aspect is the natural forces utilized to drive the ventilation. The driving forces can be wind, buoyancy, or a combination of both. (ii) Natural ventilation principles. The second aspect is the ventilation principle used to exploit the natural driving forces to ventilate a space. This can be done by single-sided ventilation, cross ventilation, or stack ventilation. (iii) Architectural elements. The third aspect is the characteristic architectural elements used to enhance natural ventilation. The most important characteristic elements are wind towers, wind scoops, chimneys, double fac¸ades, atria, and embedded ducts.

Natural ventilation in a building is provided from two sources: thermal buoyancy and wind, as follows: (1) Wind—The air moves from higher (positive) pressure regions to lower (negative) pressure regions. This phenomenon is based on Bernoulli’s principle, which uses pressure differences to move air. The pressures generated by natural wind are typically
0.004 to 0.14 inches of water column (in-wc). (2) Buoyancy—The warm air is less dense than cool air, so it rises and creates a difference in pressure, which in turn induces air movement. This phenomenon is called thermal buoyancy and is sometimes referred to as the stack effect. The pressures generated by buoyancy are quite low, ranging from
0.001 to 0.01 inches of water column (in-wc).

The magnitude and pattern of natural air movement through a building depend on the strength and direction of these natural driving forces and the resistance of the flow path. Stack ventilation can operate when no wind pressure is available. It can also operate in deep plan buildings where the distance from openings in the perimeter, and the presence of partitions, make wind-driven cross ventilation impractical. This roof model system is designed to integrate aluminum tubes that provide the cavity ventilation of the roof. The moving air gap in the aluminum tube forces the hot air to flow out through the cavity, which is located on the underside of the metal roof. Hot air rises due to the buoyancy effect. Hence, the plumes of hot air promptly enter the air gap. The moving air from the exterior guides the air in the space all the way to the ridge before being released to the exterior of the building. In this study, the dispersion of hot air from the aluminum tubes and opened attic inlet keeps the interior temperature low for human comfort. A comfortable condition reduces the usage of air-conditioning systems in a building, thus saving electricity costs.

12.3

Mechanism of active cool roof system

12.3.1 Role of solar-powered fans and opened attic inlet Solar-powered fans were used to increase the airflow rate inside the cavity by pulling hot air out before transferring to the attic. During a sunny day, solar-powered fans automatically switch on to improve the efficiency of ventilation and the cooling

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system. Conversely, the solar-powered fans automatically switch off at night when there is no incoming solar radiation. Accordingly, this solar-powered fan act as an automation system for this cool roof design. However, the opened attic inlet leads to the transmission of hot air in the attic, which keeps the attic temperature low in the building, meaning the combination of solar-powered fans and attic inlet features can effectively reduce the air-conditioning power consumption. As with any relatively new concept, there is often a fair amount of confusion as to how to best operate attic inlets. Though it is impossible to come up with a single best way to operate them, the following are a few simple rules to keep in mind when trying to get the most out of an attic inlet system (Yew et al., 2013). (1) Attic inlets should open at most 3 inches. During cold weather, the smaller the air jet coming from an air inlet, the quicker the incoming air heats up, and the lower the probability that the incoming air will decrease house temperature or cause a draft. Larger inlet openings can lead to drafty side walls and uneven house temperatures. (2) Attic inlets should generally be operated at a lower static pressure than traditional side wall inlets. There are several reasons for this. First, attic inlets are better designed to throw the cool incoming air farther than most side wall inlets at a lower pressure. Second, an attic inlet is not fighting gravity. Side wall inlets must throw the heavy cold incoming air “uphill” whereas with an attic inlet, gravity works with the air jet to get the air to the side wall. Finally, the air entering through an attic inlet is already where it is wanted, namely near the peak of the ceiling where hot air generally collects. (3) Because attic inlets do not generally require a high static pressure or a large opening, fewer timer fans can be used, operating for a longer period, and still obtain proper air mixing and distribution. Operating fewer minimum ventilation fans over a longer period leads to more stable house temperatures and air quality. For instance, it is generally better to operate two 36-inch fans on a timer for 1 min out of 5 than four fans 30 s of 5 min. Although in both cases the same amount of air is being brought in, operating four 36-inch fans in a typical house results in exchanging 20% of the warm inside air with much colder outside air in just 30 s, which can cause a fairly drastic change in house temperature and air quality. Operating only two 36-inch fans for 1 min results in a much slower exchange of air, which tends to lead to a more stable and consistent house environment.

12.3.2 Role of water cooling An evaporative roof cooling system is designed to reduce the temperature of a roof from 165°F to about 90°F. This cooling reverses the heat flow through the roof. Heat is now transferred out of the building through the roof. This effect is equivalent to that of R-7 roof insulation during the day. Rather than absorbing heat, as does insulation, the evaporation of water carries heat away from the building. This evaporation is achieved by periodically misting a small amount of water onto the surface of the roof. Sensors on the surface of the roof continuously monitor the roof temperature. Once water is misted onto the roof’s surface, the system pauses, allowing the water to evaporate, reducing the temperature of the roof. The system will not mist again until the temperature increases to a point at which more water is required for cooling. This precise control eliminates excessive water runoff while achieving maximum cooling. The no-clog/no-drip system is composed

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

of few moving parts and requires little maintenance. UVRPVC water pipe rests in specially designed pipe supports that do not penetrate the roof membrane. Water is dispersed evenly across the roof using spray nozzles.

12.4

Mechanism of passive cool roof system

12.4.1 Roles of thermal reflective coating (TRC) and moving-air-cavity (MAC) The role of TRC is aimed at reflecting sunlight to the ambient surroundings, while MAC is aimed at providing ventilation for hot air before transferring it into the attic. Both applications are main features in eco-friendly roof systems, as they use environmentally friendly methods to attain the cool roof system performance.

12.4.2 Mechanism of heat transfer of the integrated cool roof system Generally, there are two main heat transfers in a cool roof system, which are heat transfer from the ambient environment to TRC and metal roof (first control volume), and heat transfer from metal roof to MAC and attic (second control volume). The second law of thermodynamics states that heat always flows from high temperature to low temperature; there is no exception such that heat would flow in the opposite direction. Therefore, heat flows in a cool roof system only in one direction, from ambient surroundings to the metal roof and finally penetrating the attic. Eqs. (12.1), (12.2), (12.3) represent the first control volume (CV1) and second control volume (CV2), respectively. Qs ¼ QRad,out + QConv,out + QCond

(12.1)

where Qs ¼ heat energy from two spotlights, W QRad, out ¼ radiation heat transfer that reflected on metal roof, W QConv, out ¼ convection heat transfer on metal roof, W QCond ¼ conduction heat transfer through metal roof, W

QCond ¼ QRad,in + QConv,in + QVe where QCond ¼ conduction heat transfer through metal roof, W QRad, in ¼ radiation heat transfer into the attic, W QConv, in ¼ convection heat transfer into the attic, W QVe ¼ ventilation heat transfer out of the attic, W

The amount of removed heat in MAC can be calculated using Eq. (12.3).

(12.2)

Active and passive systems for cool roofs

285 Thermal insulation coating

Heat reflection QConv,out

Outdoor

CV1

Qs

Metal roof

QRad,out

QCond Qve

Solar powered fan QConv,in

QConv,out

CV2

Aluminium tube QRad,in QRad,out

QConv,in

Moving air cavity

QRad,in

Fig. 12.3 Mechanism of heat transfer in cool roof system.

_ p ðTout
Tin Þ Qve ¼ mC

(12.3)

where Qve ¼ ventilation heat transfer out of the attic, W m_ ¼mass flow rate, kg/s Cp ¼ specific heat at atmospheric pressure, J/kg K Tout ¼ temperature at the outlet of MAC Tin ¼ Temperature at the inlet of MAC

The overall mechanism of heat transfer in a cool roof system is shown in Fig. 12.3. As shown, CV1 and CV2 reduce the heat transfer, mainly due to TRC and MAC, and heat from the source is blocked and reflected. At CV1, TIC provides good thermal reflection and low thermal conductivity, while at CV2, MAC provides good ventilation to the attic. To further reduce heat transmission from CV1 to CV2, choosing high TRC and low thermal conductivity materials could be a more effective method.

12.5

Future trends

The Mediterranean is a complex region, with differences in terms of environmental hazards, population growth, urban sprawl, and economic development. Climate change affects all the area and common risks are already being detected. An urban heat island was monitored in a large and medium urban area. The energy end uses in buildings are another crucial aspect, even if with different situations and perspectives between European countries and North African and Middle East countries. The common trend is the dramatic increase in electricity consumption for cooling buildings, a trend bound to increase in the next few years because of the energy demand of transition economies. Passive building technologies may be fruitfully applied to mitigate the cooling demand increase, reducing the energy consumption in cooled

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

buildings and improving the thermal comfort in noncooled buildings. Cool materials stay cool under the sun because of high solar reflectance and thermal emittance. A review of the products and technologies available on the market or in an advanced research state has been performed in this chapter. Studies have demonstrated the positive impact of the technology in terms of cooling and total energy savings as well as on the indoor thermal conditions in Mediterranean buildings. Cooling is one of the major concerns in building tropical houses. This problem is exacerbated by the heat gain of the roof, which constitutes 70% of the total heat gain. The passive cooling technique is one of the innovative practices and technologies that provide buildings with comfortable conditions through natural means. Reflective and radiative processes are the methods used to decrease heat gain by facilitating the elimination of excess heat in a building’s interior to maintain a comfortable environment. Given that the potential of these techniques varies from region to region, their application in the tropics should be examined. Exploring these approaches in detail allows us to rethink how to effectively adapt these techniques to overcome the buildup of heat in modern tropical houses in Southeast Asia. This study reviews the physical characteristics of these approaches to guide architects and building designers. Results indicate a great reduction in operational cost. However, the significant differences in the performance of color and material properties should be considered, given that the selected approach strongly affects the required thermal conditions of a building.

12.6

Sources of further information and advice

Implementing active and passive approaches in cool roofing systems will enhance occupants’ comfortability by minimizing the effect of heat penetrating, as well as contributing to the reduction of internal gains through day lighting and appliances. However, their efficiency depends on the building type, the occupancy patterns, and climatic boundaries (e.g., air temperature, relevant humidity, velocity and direction of winds), which differ from day to day and from one region to another. Therefore, to effectively improve heat rejection from buildings by natural means, the physical characteristics of the building should be sufficiently understood by the designer. Moreover, selecting an inappropriate technique may result in an unpleasant indoor environment. Hence, the limitations in evaluation tools and insufficient information for both designers and building users are found to be the major reasons for not adopting passive cooling strategies in tropical houses. In Malaysia, about 68.8% of the respondents in tropical regions had no objections to modifying their roof to reduce the need for air conditioning. Such a result shows that the respondents are willing to adopt active and passive cooling strategies in the buildings for economic reasons, as such strategies constitute the most cost-effective methods for creating an optimally cool environment.

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Pisello, A.L., Castaldo, V.L., Pignatta, G., Cotana, F., Santamouris, M., 2015. Experimental inlab and in-field analysis of waterproof membranes for cool roof application and urban heat island mitigation. Energ. Buildings 114, 180–190. Raj, A.K., Rao, P.P., Sameera, S., Divya, S., 2015. Pigments based on terbium-doped yttrium cerate with high NIR reflectance for cool roof and surface coating applications. Dyes Pigments 122, 116–125. Santamouris, M., 2014. Cooling the cities—a review of reflective and green roof mitigation technologies to fight heat island and improve comfort in urban environments. Sol. Energy 103, 682–703. Santamouris, M., Kolokotsa, D., 2015. On the impact of urban overheating and extreme climatic conditions on housing, energy, comfort and environmental quality of vulnerable population in Europe. Energ. Buildings 98, 125–133. Santamouris, M., Synnefa, A., Karlessi, T., 2011. Using advanced cool materials in the urban built environment to mitigate heat islands and improve thermal comfort conditions. Sol. Energy 85, 3085–3102. Schulze, T., Eicker, U., 2013. Controlled natural ventilation for energy efficient buildings. Energ. Buildings 56, 221–232. https://doi.org/10.1016/j.enbuild.2012.07.044. Seto, K.C., Fragkias, M., G€uneralp, B., Reilly, M.K., 2011. A meta-analysis of global urban land expansion. PLoS One. 6(8), e23777. Stavrakakis, G.M., Androutsopoulos, A.V., Vy€orykk€a, J., 2016. Experimental and numerical assessment of cool-roof impact on thermal and energy performance of a school building in Greece. Energ. Buildings 130, 64–84. https://doi.org/10.1016/j.enbuild.2016.08.047. Synnefa, A., Santamouris, M., Akbari, H., 2007. Estimating the effect of using cool coatings on energy loads and thermal comfort in residential buildings in various climatic conditions. Energ. Buildings 39, 1167–1174. Wong, N.H., Jusuf, S.K., Syafii, N.I., Chen, Y., Hajadi, N., Sathyanarayanan, H., Manickavasagam, Y.V., 2011. Evaluation of the impact of the surrounding urban morphology on building energy consumption. Sol. Energy 85 (1), 57–71. Yew, M.C., Ramli Sulong, N.H., Chong, W.T., Poh, S.C., Ang, B.C., Tan, K.H., 2013. Integration of thermal insulation coating and moving-air-cavity in a cool roof system for attic temperature reduction. Energ. Convers. Manage. 75, 241–248. https://doi.org/10.1016/j. enconman.2013.06.024. Yew, M.C., Yew, M.K., Saw, L.H., Ng, T.C., Chen, K.P., Rajkumar, D., Beh, J.H., 2018. Experimental analysis on the active and passive cool roof systems for industrial buildings in Malaysia. J. Build. Eng. 19, 134–141. https://doi.org/10.1016/j.jobe.2018.05.001. Zhang, L., Zhang, R., Hong, T., Zhang, Y., Meng, Q., 2018. Impact of post-rainfall evaporation from porous roof tiles on building cooling load in subtropical China. Appl. Therm. Eng. 142, 391–400. https://doi.org/10.1016/j.applthermaleng.2018.07.033.

Part Four PCMS and switchable glazing based materials for reducing cooling needs

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Biobased phase change materials for cooling in buildings

13

Luisa F. Cabeza GREiA Research Group, University of Lleida, Lleida, Spain

13.1

Introduction

Considering the recent climate-related international compromises, such as the Paris Agreement 2015, and with recommendations such as those from the Intergovernmental Panel on Climate Change (IPCC) (Lucon, 2014), the need to reduce the cooling load in buildings and the built environment has increased interest within the research community. Cabeza et al. (2014a) showed the importance of materials in the life cycle assessment (LCA) of buildings, since they impact two of the three phases of LCA (manufacturing and demolition). With this in mind, using biobased materials would help in decreasing the environmental impact of buildings (Sandak et al., 2019). The main advantages of biobased materials are that they can be found locally, they need no or very little transportation, and they usually have low processing needs or are processed in an ecological manner. The fact that biobased materials are usually transported shorter distances and usually suffer less transformation means that they have lower embodied energy. Phase change materials (PCMs) have been reported as good candidates to help in decreasing the cooling demand of buildings or improving the thermal performance of cooling systems in buildings (Cabeza et al., 2011, 2014b; Zalba et al., 2003; Navarro et al., 2016a,b). Kylili and Fokaides (2016) reviewed the LCA studies carried out in buildings and building applications with PCM, and concluded that, considering the manufacturing, operational, and disposal phases, PCM-incorporated building construction is more environmentally friendly than their reference cases. Therefore, again looking at biobased PCM materials is one of the best ways to decrease the embodied energy of building materials and contribute to climate-change mitigation (Cabeza et al., 2013). The embodied energy contribution of the LCA of a building increases in lowenergy buildings compared to conventional ones (Chastas et al., 2016); embodied energy ranges between 5% and 83% of the total life-cycle energy in low-energy buildings, while in conventional buildings it accounts for 5%–36%. This decrease of embodied energy is due to the selection of building materials and also in the construction method. Biobased materials tend to use more traditional building methods, which is another reason for their impact in the decrease of the embodied energy in buildings.

Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction https://doi.org/10.1016/B978-0-12-820791-8.00013-4 © 2021 Elsevier Ltd. All rights reserved.

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The aim of this chapter is to review the literature to identify which materials have been developed, studied, or characterized to be used as biobased PCMs for cooling applications in buildings. A bibliometric analysis of the topic “biobased phase change materials for building applications” carried out in November 2019 showed that this is quite a new topic, with the first documents appearing in Scopus in 2010. Until 2017, there were between 3 and 5 documents published per year, but in 2018 there were 10, and in 2019 15, showing important growth in the field. In the search, 50 documents were found. The authors with the most publications were Sumim Kim from Korea and Shohel Mahmud from Canada, with five documents each, and Pierre Blanchet, Soroush Ebadi, and Damien Mathis from Canada and Su-Gwang Jeong from Korea with four documents each (these authors belong to three different research groups). It should be highlighted that not all these studies are directly related to biobased PCM with an adequate melting temperature to be used in cooling applications of buildings, so not all of them are included in this chapter.

13.2

Biobased PCM materials

Kenar (2010) developed new biobased oleochemical carbonates as potential PCMs complementing fatty acids, fatty alcohols, and fatty acid esters, and as an alternative to paraffin and salt hydrates. The preparation was through a carbonate interchange reaction between fatty alcohols and dimethyl or diethyl carbonate in the presence of a catalyst. The PCMs had a melting temperature between 2.2°C and 51.6°C and a melting enthalpy between 144 and 227 J/g. Later on, Kenar (2012) developed binary mixtures of such oleochemical carbonates to improve the thermophysical properties of the new PCM. The new binary PCMs had a melting temperature between 12°C and 37°C and a melting enthalpy between 135 and 175 J/g. Moreno Balderrama et al. (2018) synthesized a new biobased PCM based on butyl stearate and encapsulated it with sodium trimetaphosphate under alkaline aqueous conditions, leading to an interconnected porous network, therefore an encapsulated material. The melting temperature of the new PCM was 27°C. Ravotti et al. (2018) investigated new highly pure carboxylic fatty esters to be used as PCMs. The fatty esters examined were derived from saturated fatty carboxylic acids (myristic, palmitic, stearic, and behenic). The fatty acids were coupled with primary linear alcohols of different length (methanol and 1-decanol) and were synthesized through Fischer esterification (Fig. 13.1). The new biobased PCMs had a purity higher than 89% and had enthalpies above 190 J/g, with melting temperatures between 20°C and 50°C. Their stability was also demonstrated. In a follow-up study, Ravotti et al. (2019a) examined the use of lactones as an innovative biobased PCM. Lactones are biobased, biodegradable cyclic esters; the ones considered in this study were the commercial ε-caprolactone and γ-valerolactone and the lab-synthesized via Baeyer-Villiger oxidation 1,2-campholide, oxaadamantanone, and dibenzochromen-6-one. The synthesized lactones had different melting temperatures (from 40°C to 290°C), with the commercial ones being those

Biobased phase change materials for cooling in buildings

293

H+ (H2SO4, catalyst)

O

R

R

O

+

H

O

: :

R

:O: R

O

R

O +

O

H

H

Proton transfer

Nucleophilic attack by alcohol

H

:

Elimination H H2O

: :

1

1

O

:

H

H

:O:

– H+

R

: :

+

:

:

R

: :

OH

: :

:

: :

R

H

O

:

H

:O:

O

R

1

Deprotonation

1

Fig. 13.1 Reaction mechanism of Fischer esterification to produce novel biobased PCM (Ravotti et al., 2018).

of interest for cooling applications. Their melting enthalpy was 121.7 J/g and 109.8 J/g, respectively, but some materials also showed high subcooling and high degradation at low temperatures, and therefore are not suitable as potential commercial PCMs. Finally, in Ravotti et al. (2019b) the relation between the chemical structure of the biobased fatty esters and their thermal properties was analyzed, giving correlations between the melting temperature and the chemical structure. Fabiani et al. (2019) evaluated the potentiality of a new animal fat biobased PCM for the cooling of buildings. The PCM was an animal fat coming from slaughtering residues, mostly composed of nonedible fatty pig and chicken parts. The components were gathered at a slaughterhouse and mixed to a uniform blend by a mechanical treatment at low temperatures. The final product presented around 52% of unsaturated and 42% of saturated fatty acids. Results showed an interesting double transition range globally associated with a melting enthalpy of about 29 J/g with promising thermophysical properties. The melting temperatures of the two peaks were 2°C and 25°C. Ghadim et al. (2019) developed new biobased PCMs composed of binary mixtures of fatty acid esters and fatty alcohols at their eutectic compositions. The eutectic mixtures found to be used for cooling in buildings were 1-dodecanol/methyl stearate (22.46°C melting temperature and 201.91 J/g melting enthalpy), 1-dodecanol/methyl palmitate (20.34°C melting temperature and 224.45 J/g melting enthalpy), 1-tetradecanol/methyl stearate (32.05°C melting temperature and 209.38 J/g melting enthalpy), and 1-tetradecanol/methyl palmitate (26.72°C melting temperature and 210.15 J/g melting enthalpy). All these components showed low subcooling (below 2°C) and no significant changes in thermophysical properties after 1000 thermal cycles.

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

Enhanced biobased PCM materials

Jeong et al. (2013) evaluated the improvement of the thermal properties of a biobased PCM using exfoliated graphite nanoplatelets. The PCM considered was an organic fatty acid ester made from soybeans and palm oils, with a melting temperature of 29.38°C and a melting enthalpy of 149.2 J/g. The enhanced biobased PCM composite was prepared using vacuum impregnation reaching 75% content of PCM, and it had a melting temperature of 27.45°C and a melting enthalpy of 110.6 J/g. Results showed that the new composite had lower subcooling than the pure biobased PCM and that the thermal conductivity was increased 375% (from 0.154 to 0.577 W/m K). This study also enhanced the fact that biobased PCMs have significantly more flammable properties than conventional organic PCMs, and these new compounds have better thermal resistance than the original ones. A follow-up study by Yu et al. (2014) increased the amount of exfoliated graphite nanoplatelet content in this biobased PCM with the aim of increasing the thermal conductivity of the PCM while maintaining the melting enthalpy. Results showed that the addition of 5 wt% nanomaterials increased the thermal conductivity of the biobased material by 336%, with almost no change in the melting enthalpy. This study also showed that the addition of more than 10 wt% nanomaterials increased the subcooling of the PCM; therefore, a low weight of nanomaterials should be used to avoid thermal disadvantages. Alomair et al. (2018) studied, numerically and experimentally, the use of CuO nanoparticles in coconut oil as PCM inside a concentric cylindrical thermal energy storage (TES) system. The authors investigated numerically the effect of the nanoparticle fraction and the size of the cylinder tank on the melting and the heat transfer performance. This model could be used to calculate the melting process, to track the interface between the solid and liquid phase of the system, to calculate the transient heat transfer rate, and to determine the melt fraction. Then, experiments were carried out to compare the melting process of the PCM without and with 0.25% nanoparticles. The authors claim an improvement of the melting process, being faster with nanoparticles. Ebadi et al. (2018a) studied experimentally a similar configuration of TES system also with coconut oil as a biobased PCM and also with CuO nanoparticles. The objective of this study was to investigate the effects of the height of the PCM, the temperature of the hot wall, and the weight fractions of nanoparticles on the melting of a nano-PCM. The concentration of nanoparticles used was 0.1 wt% and 1 wt%, obtaining a decrease in the melting enthalpy of the new PCM (1.94% and 8.25%, respectively) and an increase in the thermal conductivity (4.8% and 7.5%, respectively). Results show that the increase in thermal conductivity is not at the beginning of the melting process, but later on, when the PCM is already partially melted. The melting process, heat transfer, and energy storage characteristics of this system were numerically examined in a subsequent study (Ebadi et al., 2018b). This study demonstrated that adding nanoparticles does not change the patterns of the melt fraction, Nusselt number, and energy storage capacity when compared to conventional PCM.

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Al-Jethelah et al. (2019) studied experimentally the use of nanoparticles in coconut oil as a PCM (food grade coconut oil, with melting temperature 24°C) to improve the thermal conductivity of this PCM and it was compared to the inclusion of the PCM in an open-cell metal foam to ensure constant heat flux. Several nanoparticles (Al2O3, CuO, Fe3O4, and TiO) were compared. With the use of nanoparticles, the melting process was enhanced by 1.2% and with the metal foam by 41.2%, compared to the pure PCM. This study also developed a mathematical model to evaluate the effect of the nanoparticles (volume fraction and size) on the melting process and the heat transfer performance of the PCM. The experimental and numerical results showed that the melting rate is faster when nanoparticles are added to the PCM.

13.4

Biobased PCM composites

13.4.1 Encapsulated biobased PCM Hu and Yu (2012) investigated the encapsulation of biobased PCM with coaxial electrospun ultrafine fibers. Natural soy wax was used as a biobased PCM, and the shell material was polyurethane. The encapsulated material, a uniform fiber with homogeneous wax distribution, showed good thermal performance and thermal stability after 100 heating-cooling test cycles. Oktay et al. (2019) studied two different ways of encapsulating coconut oil. First, the coconut oil was microencapsulated via suspension polymerization with stearyl methacrylate and hydroxyethyl methacrylate. The second method was the UV curation of the PCM to produce a form-stable composite, trapping the PCM within the polymeric network without covalent bonding, avoiding leakage. The melting enthalpy of the microencapsulated PCM was 119 J/g, higher than the pure PCM (with 106 J/g) and the UV-cured form-stable one (with 47 J/g).

13.4.2 Composites with concrete Cellat et al. (2015) developed concrete composites incorporating a biobased fatty acid PCM for building applications. The PCMs developed were two binary mixtures (70:30 wt% capric acid/lauric acid and 75:25 wt% capric acid/myristic acid) with a melting enthalpy of 109.0 and 155.4 J/g, respectively, and a melting temperature of 22.57°C in the case of capric acid/lauric acid and of 27.40°C in the case of capric acid/myristic acid. Then the PCMs were added to concrete mixtures in a 1 wt% and 2 wt% composition, having good mechanical performance in the first case, but not in the second one. Moreover, no leakage was detected when up to 10 wt% PCM was added into the concrete. These new concrete mixtures are recommended by the authors to be used in prefabricated building elements, but not in structural elements. In a follow-up step, Cellat et al. (2019) incorporated the developed capric acid/ myristic acid in concrete, but this time microencapsulated and tested it during 2 years

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in test cabins. The energy savings that were measured, around 13%, were similar to those obtained in similar studies with conventional microencapsulated PCMs (Cabeza et al., 2007). The microencapsulation method was developed by Beyhan et al. (2017).

13.4.3 Composites in the form of sheets Boussaba et al. (2018) developed a novel PCM composite for buildings. These authors recovered coconut fat from underused feedstock (a commercial mixture of fatty acids in Tunisia) and they incorporated it in a composite matrix prepared with natural clay and cellulose fibers (Fig. 13.1). The coconut fat (PCM) had a melting temperature of 22.63°C and a freezing temperature of 17.44°C, and a melting enthalpy around 107 J/g. This biobased PCM was incorporated in its matrix following the direct immersion process with low-cost and eco-friendly components. Then, panels in a sandwich structure were prepared, protecting the composite PCM between two stainless-steel sheets. These panels incorporated 56 wt% PCM and showed no leakage. In a similar study, Boussaba et al. (2019) developed a new PCM-composite using a similar concept, but with a hydrogenated palm kernel vegetable fat as PCM. The biobased PCM had a melting temperature of 26.53°C and a melting enthalpy of 74.35 J/g. The composite had 53 wt% PCM content and its final properties were 27.33°C melting temperature and 40.27 J/g melting enthalpy, and with good thermal and chemical stability. Moreover, results confirmed that there is no leakage in the new composite. Jeon et al. (2019) developed a novel functional board with biobased PCM (Fig. 13.2). The selected PCM was coconut oil impregnated in biochar; the biochar was produced from waste pine cone, pine sawdust, and paper mill sludge. The pores of the biochar served as support material for the PCM. The new composite had a melting temperature of 24°C and a melting enthalpy of 74.6 J/g. The authors claimed this composite to be a good candidate as insulation material, since its thermal conductivity was 0.034 W/m K (Figs. 13.3 and 13.4).

Melting

Stirring

Thermometer

Thermometer

Sonication

Injection

Thermometer

xGnP or CNT Bio-based PCM

Composite PCM

Composite PCM

Hot plate at 60°C

Hot plate at 60°C

Hot plate at 60°C

Composite PCM

Fig. 13.2 Scheme for the preparation of nanoenhanced biobased PCM (Yu et al., 2014).

Biobased phase change materials for cooling in buildings

Natural clay Cellulose power Graphite fibers

Distilled water (1:1)

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The resulted past

Uni-axial compression (2kN)

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Stainless steel sheet The compsite-PCM

Stainless steel sheet

(B) Fig. 13.3 Novel PCM-composite based on coconut fat (Boussaba et al., 2018). (A) Manufacturing procedure, (B) final PCM composite.

13.4.4 Composites with wood Mathis et al. (2018a) impregnated two species of hardwood (red oak and sugar maple) with a microencapsulated biobased commercial PCM with a melting point of 23°C. The impregnation was carried out via vacuum impregnation and with various concentrations of PCM microcapsules (between 10 wt% and 50 wt%). Results showed an enhancement of heat storage of 77% in comparison with untreated wood. Sugar maple was more difficult to impregnate than red oak. Moreover, varnished samples were

Materials Paper mill sludge

Pine cone

Analysis Morphology and Physical properties

Pine sawdust

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Microstructure of biochar and LHSBC Bulk density of LHSBC Pine cone biochar

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Bio based latent heat storage insulation

PCM : coconut oil

Biochar

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PCM impregnation into biochar

Fig. 13.4 Preparation of PCM-based functional board ( Jeon et al., 2019).

Filtering

Latent heat storage bio composite

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tested for adhesion performance. This material was studied in experimental huts (Mathis et al., 2018b), showing good performance in avoiding overheating and, therefore, decreasing the cooling demand by a maximum of 41%. Later on, Mathis et al. (2019) developed decorative wood panels with biobased PCM comparing the use of a blend of capric and lauric acids with the previously mentioned commercial PCMs, achieving similar results with all the tested biobased PCMs.

13.5

Outlook on cooling applications in buildings and sustainability aspects

Only a brief synopsis of applications of biobased materials for cooling in buildings is given in this section. Vik et al. (2017) studied the impact of a biobased PCM on the operating temperature and cooling power demand of an office, testing it in a full-scale test room located inside a climate laboratory. No details on the biobased PCM were given. Bianco et al. (2018) compared experimentally the use of a biobased PCM melting at 35°C (no more specifications of the PCM were given) to other organic and inorganic PCMs in a shading system to decrease the cooling load of buildings. Heidari et al. (2019) studied the sustainability of a biobased PCM-wood panel (fatty acid PCM incorporated in a high-density fiberboard panel). The melting point of the PCM was 20.6°C and the melting enthalpy 47.5 J/g. The study corroborated previous research (De Gracia et al., 2010; Menoufi et al., 2013), stating that the PCM has a major impact on the LCA.

13.6

Conclusions

Different biobased PCMs have been developed with potential use in cooling applications in buildings. The main materials considered have been fatty acids, fatty esters, coconut oil, and coconut fat. Researchers have modified biobased PCMs, mainly coconut oil, to improve the thermal conductivity, having identified that this is one of the main drawbacks of such materials. Such enhancement is carried out with the addition of nanoparticles, mainly CuO. The achieved enhancement varies from around 3% up to 300%. Biobased PCMs have also been modified to produce composites. Such modifications range from microencapsulation to the inclusion of the material in concrete, in polymers to form plate-sheets, or in wood structures.

Acknowledgments This work was partially funded by the Ministerio de Ciencia, Innovacio´n y Universidades de Espan˜a (RTI2018-093849-B-C31—MCIU/AEI/FEDER, UE). Dr. Cabeza would like to thank the Catalan Government for the quality accreditation given to her research group, GREiA (2017 SGR 1537). GREiA is a certified agent TECNIO in the category of technology developers from the Government of Catalonia. This work is partially supported by ICREA under the ICREA Academia program.

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Ebadi, S., Humaira Tasnim, S., Abbas Aliabadi, A., Mahmud, S., 2018a. Geometry and nanoparticle loading effects on the bio-based nano-PCM filled cylindrical thermal energy storage system. Appl. Therm. Eng. 141, 724–740. https://doi.org/10.1016/j. applthermaleng.2018.05.091. Ebadi, S., Tasnim, S.H., Aliabadi, A.A., Mahmud, S., 2018b. Melting of nano-PCM inside a cylindrical thermal energy storage system: numerical study with experimental verification. Energy Convers. Manag. 166, 241–259. https://doi.org/10.1016/j.enconman.2018.04.016. Fabiani, C., Pisello, A.L., Barbanera, M., Cabeza, L.F., Cotana, F., 2019. Assessing the potentiality of animal fat based-bio phase change materials (PCM) for building applications: an innovative multipurpose thermal investigation. Energies 12, 1111. (18 pages). https://doi. org/10.3390/en12061111 Ghadim, H.B., Shahbaz, K., Al-Shannaq, R., Farid, M.M., 2019. Binary mixtures of fatty alcohols and fatty acid esters as novel solid-liquid phase change materials. Int. J. Energy Res. 43, 8536–8547. https://doi.org/10.1002/er.4852. Heidari, M.D., Mathis, D., Blanchet, P., Amor, B., 2019. Streamlined life cycle assessment of an innovative bio-based material in construction: a case study of a phase change material panel. Forests 10, 160. (16 pages). https://doi.org/10.3390/f10020160. Hu, W., Yu, X., 2012. Encapsulation of bio-based PCM with coaxial electrospun ultrafine fibers. RSC Adv. 2, 5580–5584. https://doi.org/10.1039/c2ra20532g. Jeon, J., Park, J.H., Wi, S., Yang, S., Ok, Y.S., Kim, S., 2019. Characterization of biocomposite using coconut oil impregnated biochar as latent heat storage insulation. Chemosphere 236, 124269. (9 pages). https://doi.org/10.1016/j.chemosphere.2019.06.239. Jeong, S.G., Chung, O., Yu, S., Kim, S., Kim, S., 2013. Improvement of the thermal properties of bio-based PCM using exfoliated graphite nanoplatelets. Sol. Energy Mater. Sol. Cells 117, 87–92. https://doi.org/10.1016/j.solmat.2013.05.038. Kenar, J.A., 2010. Latent heat characteristics of biobased oleochemical carbonates as potential phase change materials. Sol. Energy Mater. Sol. Cells 94, 1697–1703. https://doi.org/ 10.1016/j.solmat.2010.05.031. Kenar, J.A., 2012. Thermal characteristics of oleochemical carbonate binary mixtures for potential latent heat storage. Eur. J. Lipid Sci. Technol. 114, 63–70. https://doi.org/10.1002/ ejlt.201100043. Kylili, A., Fokaides, P.A., 2016. Life cycle assessment (LCA) of phase change materials (PCMs) for building applications: a review. J. Build. Eng. 6, 133–143. https://doi.org/ 10.1016/j.jobe.2016.02.008. € Lucon, O., Urge-Vorsatz, D., Ahmed, A.Z., Akbari, H., Bertoldi, P., Cabeza, L.F., Eyre, N., Gadgil, A., Harvey, L.D.D., Jiang, Y., Liphoto, E., Mirasgedis, S., Murakami, S., Parikh, J., Pyke, C., Vilarin˜o, M.V., 2014. IPCC Fifth Assessment Report: Chapter 9— Buildings. In: Clim. Chang. 2014 Mitig. Clim. Chang. Contrib. Work. Gr. III to Fifth Assess. Rep. Intergov. Panel Clim. Chang. https://doi.org/10.2753/JES1097-203X330403. Mathis, D., Blanchet, P., Landry, V., Lagie`re, P., 2018a. Impregnation of wood with microencapsulated bio-based phase change materials for high thermal mass engineered wood flooring. Appl. Sci. 8, 2696. (14 pages). https://doi.org/10.3390/app8122696. Mathis, D., Blanchet, P., Lagie`re, P., Landry, V., 2018b. Performance of wood-based panels integrated with a bio-based phase change material: a full-scale experiment in a cold climate with timber-frame huts. Energies 11, 3093. (15 pages). https://doi.org/10.3390/ en11113093.

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Mathis, D., Blanchet, P., Landry, V., Lagie`re, P., 2019. Thermal characterization of bio-based phase changing materials in decorative wood-based panels for thermal energy storage. Green Energy Environ. 4, 56–65. https://doi.org/10.1016/j.gee.2018.05.004. Menoufi, K., Castell, A., Farid, M.M., Boer, D., Cabeza, L.F., 2013. Life cycle assessment of experimental cubicles including PCM manufactured from natural resources (esters): a theoretical study. Renew. Energy 51, 398–403. https://doi.org/10.1016/j.renene.2012.10.010. Moreno Balderrama, J.A., Dourges, M.A., Magueresse, A., Maheo, L., Deleuze, H., 2018. Emulsion-templated pullulan monoliths as phase change materials encapsulating matrices. Mater. Today Commun. 17, 466–473. https://doi.org/10.1016/j.mtcomm.2018.10.012. Navarro, L., de Gracia, A., Colclough, S., Browne, M., McCormack, S.J., Griffiths, P., Cabeza, L.F., 2016a. Thermal energy storage in building integrated thermal systems: a review. Part 1. active storage systems. Renew. Energy 88, 526–547. https://doi.org/ 10.1016/j.renene.2015.11.040. Navarro, L., de Gracia, A., Niall, D., Castell, A., Browne, M., McCormack, S.J., Griffiths, P., Cabeza, L.F., 2016b. Thermal energy storage in building integrated thermal systems: a review. Part 2. Integration as passive system. Renew. Energy 85, 1334–1356. https:// doi.org/10.1016/j.renene.2015.06.064. Oktay, B., Bas¸ t€urk, E., Kahraman, M.V., Apohan, N.K., 2019. Designing coconut oil encapsulated poly(stearyl methacrylate-co-hydroxylethyl metacrylate) based microcapsule for phase change materials. ChemistrySelect 4, 5110–5115. https://doi.org/10.1002/ slct.201900340. Ravotti, R., Fellmann, O., Lardon, N., Fischer, L.J., Stamatiou, A., Worlitschek, J., 2018. Synthesis and investigation of thermal properties of highly pure carboxylic fatty esters to be used as PCM. Appl. Sci. 8, 1069. (18 pages). https://doi.org/10.3390/app8071069. Ravotti, R., Fellmann, O., Lardon, N., Fischer, L.J., Stamatiou, A., Worlitschek, J., 2019a. Investigation of lactones as innovative bio-sourced phase change materials for latent heat storage. Molecules 24, 1300. (15 pages). https://doi.org/10.3390/molecules24071300. Ravotti, R., Fellmann, O., Lardon, N., Fischer, L.J., Stamatiou, A., Worlitschek, J., 2019b. Analysis of bio-based fatty esters PCM’s thermal properties and investigation of trends in relation to chemical structures. Appl. Sci. 9, 225. (15 pages). https://doi.org/10. 3390/app9020225. Sandak, A., Sandak, J., Brzezicki, M., Kutnar, A., 2019. Biomaterials for building skins. In: BioBased Building Skin. Springer, Singapore, pp. 27–64. https://doi.org/10.1007/978-981-133747-5_2. Vik, T.A., Madessa, H.B., Aslaksrud, P., Folkedal, E., Øvrevik, O.S., 2017. Thermal performance of an office cubicle integrated with a bio-based PCM: experimental analyses. Energy Procedia 111, 609–618. https://doi.org/10.1016/j.egypro.2017.03.223. Yu, S., Jeong, S.G., Chung, O., Kim, S., 2014. Bio-based PCM/carbon nanomaterials composites with enhanced thermal conductivity. Sol. Energy Mater. Sol. Cells 120, 549–554. https://doi.org/10.1016/j.solmat.2013.09.037. Zalba, B., Marı´n, J.M., Cabeza, L.F., Mehling, H., 2003. Review on thermal energy storage with phase change: materials, heat transfer analysis and applications. Appl. Therm. Eng. 23, 251–283. https://doi.org/10.1016/S1359-4311(02)00192-8.

PCM incorporated bricks: A passive alternative for thermal regulation and energy conservation in buildings for Indian conditions

14

Rajat Saxenaa, Sana Fatima Alib, and Dibakar Rakshitb a Department of Mechanical Engineering, Pandit Deendayal Petroleum University, Gandhinagar, India, bCentre for Energy Studies, Indian Institute of Technology Delhi, New Delhi, India

14.1

Introduction

With increased emphasis on energy conservation in buildings, researchers have come up with several solutions for accomplishing it. From a building perspective, there are efforts to utilize natural lighting (Sharma and Rakshit, 2016; Sharma et al., 2018a,b) and sensor control for more effective operations, among other approaches. However, a major concern is still controlling the indoor temperatures with minimal power consumption. Locations with extreme high temperatures (Middle East, Afghanistan, parts of India, Dubai, parts of Australia, Egypt, etc.) during the summer often require cooling. Similarly, places with cold winters (Canada, Sweden, European nations, Russia, China, etc.) have high heating requirements. The cooling/heating of buildings in these locations constitutes the major share of power consumption. Thus there is a need to utilize and implement methods that provide temperature stability of living spaces with minimal power intervention. In addition, owing to the constraints of building space and optimum use of construction materials, there is also a requirement for techniques that can be incorporated with effective space and material utilization. This can be achieved by implementing passive methods for cooling and heating. One such passive alternative is phase change material (PCM) incorporation within buildings. PCM incorporation increases both the heat storage capacity and the thermal resistance of the wall, thus serving the purpose of heat storage as well as insulation (Saikia et al., 2018). For cooling applications, the stored heat may not be of much use and is rejected to the outside, for effective charging (heat gain) in the subsequent cycle. In the case of cold locations where the outside temperature is lower, this stored heat can be a heat reservoir during the off-sunshine hours, thus helping to maintain the inside comfort temperature with less input energy. Some studies have examined PCMs within buildings for thermal regulation applications. There are studies that discuss PCM properties Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction https://doi.org/10.1016/B978-0-12-820791-8.00014-6 © 2021 Elsevier Ltd. All rights reserved.

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(Zalba et al., 2003; Abhat, 1983; Sharma et al., 2009; Kant et al., 2017a; Nkwetta and Haghighat, 2014), their selection (Nghana and Tariku, 2016; Navarro et al., 2019; Saxena et al., 2020a), and their implementation (Navarro et al., 2016a; Ling and Poon, 2013; Hawes et al., 1989; Sa´ et al., 2012; Mavrigiannaki and Ampatzi, 2016). A number of studies on incorporating PCMs within building elements, such as bricks, have been carried out, followed by their assessment in terms of temperature reduction and other aspects. This chapter provides a comprehensive review of studies carried out so far on PCM bricks, discussing the various brick designs, their manufacturing, PCM selection, methods of PCM encapsulation and their thermal assessment. It is important to state here that some studies have examined PCM incorporation using different methods, which are discussed in this chapter. There are various ways in which PCM can be incorporated within a building envelope, mainly by either microencapsulation or macroencapsulation. A study carried out by Rathore and Shukla (2019a) reviewed different techniques to integrate macroencapsulated PCMs within the building envelope and suggests macroencapsulation as an effective way to improve the indoor thermal environment with minimal effect on the mechanical strength of the building. They even carried out an experimental investigation for the thermal behavior using OM 37 macroencapsulated within tubular-shaped aluminum (Rathore and Shukla, 2019b). They observed about 7.19%–9.18% reduction in the peak temperatures, with a time lag of 60–120 min. So far, only PCM incorporation through the macroencapsulation method has been found suitable for practical application; other methods for PCM incorporation are still in the research stages or have been rejected due to leakage issues. This fact, along with research being carried out on direct incorporation of PCM, has been discussed and highlighted in this chapter. In addition, the effect of macroencapsulated PCM bricks on the indoor space temperature and the difference in comfort level temperature with and without PCM have been assessed for the composite climate of New Delhi.

14.2

PCM bricks as a sustainable passive alternative

14.2.1 Review of PCM bricks for building applications The major advantage of using a PCM within bricks is that it acts as heat storage and increases the thermal resistance due to the low thermal conductivity (Saxena et al., 2018a). Several researchers have studied the impact of PCM within bricks, both numerically and experimentally. Research on PCM within bricks is especially gaining attention for thermal regulation in buildings located in Indian climatic conditions. Kant et al. (2017b) numerically analyzed the effectiveness of three different PCMs by placing them in cylindrical cavities within bricks and also varied the quantity of the PCMs. They suggested that it is an effective strategy for space conditioning within buildings and concluded that, out of the three PCMs studied, capric acid had the maximum effect. Another numerical study carried out by Saxena et al. (2018b) studied the behavior of three PCMs within a building; however, the PCMs were rejected based on their characterization results from differential scanning calorimetry (DSC), which

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showed significant subcooling. Another study with more PCMs was undertaken, and it suggested OM 35 and eicosane as suitable for application in New Delhi, based on simulation and DSC analysis (Saxena et al., 2020b). An experimental study was carried out by Mukram and Daniel (2018) wherein HS 26 PCM was encapsulated in an aluminum container and placed within the central cavity of cement bricks. Their results suggested that the indoor temperature was reduced by 5°C for the case when PCM was incorporated within cement bricks. Madhumathi and Sundarraja (2012) also carried out an experimental analysis for bricks integrated with polyethylene glycol (PEG E600); the PCM was placed in glass test tubes for application in buildings for hot and humid climatic conditions. The study tested the solution in both E-W and N-S directions, and suggested that there was a substantial reduction in the heat gain and the indoor temperature and a reduction in the difference of the comfort temperature. An important finding of these studies was that PCM needs to be carefully selected based on thermal characteristics and location, which is further elaborated in Section 14.2.2. Moreover, PCM in bricks as a technique for temperature regulation within an indoor space is being studied worldwide. A study carried out using TRNSYS for summers in China (Meng et al., 2017) marked a temperature reduction of 4.3–7.7°C and a drop in indoor temperature fluctuation by 28%–67%. Alawadhi (2008) also performed a numerical analysis for bricks containing one, two, and three cylinders and compared them with a solid brick without the incorporation of PCMs. Three different types of PCMs were used in this analysis, viz., n-octadecane, n-eicosane, and P116. The results suggested n-eicosane as the best-performing PCM out of the three studied for hot climatic conditions. It was also reported that a reduction of 17.55% in the heat flux was observed when the brick was filled with three PCM cylinders. Experimental studies have been carried out in which modified bricks with PCM were prepared and tested (Saxena et al., 2019). The PCMs were selected based on DSC characterization and showed a temperature reduction up to 6°C during peak hours. An overall heat transfer reduction up to 15% was observed. Apart from incorporating PCM within traditional bricks, the thermal behavior of PCM-concrete bricks has also been studied. Cheng et al. (2013) examined the assessment of the thermophysical properties of a PCM-concrete brick mixture. The traditional methods of measuring the thermophysical properties of such materials were reviewed. They then proposed a new methodology to predict the properties of the composite material. Their current work, however, deals with assessing the thermophysical properties of pure PCM, to be able to select it for brick incorporation. Another such study was carried out by Li et al. (2017) wherein shape-stabilized PCMs (SSPCMs) mixed into cement mortar were used for making PCM bricks. These PCM bricks were used as building walls in an actual scaled test cubicle in China to assess and evaluate their effects in comparison to a regular energy-saving wall for summer as well as midseason days. The SSPCM used in this study was GH-20, which is composed of 15% highdensity polyethylene, 15% expanded graphite, and 70% paraffin wax by weight, mixed with cement mortar to form bricks. The results showed that the PCM-brick wall temperature was in the phase change temperature range for a major duration in midseason days. The temperature fluctuation and temperature of the inner wall surface

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were also found to be reduced. During the summer, it was observed that the PCM selected for the study remained in a liquid state all the time and hence performed poorly with respect to the regular energy-saving wall. It was thus concluded that the composite-PCM wall would work effectively in a single season only, owing to the restricted phase change temperature range. Castell et al. (2013) performed a life cycle assessment analysis on alveolar bricks consisting of macroencapsulated PCMs (salt hydrates, SP-25 A8) within a compact storage module panel. The study was carried out on a test setup (cubicles) made of alveolar bricks, for which temperature and energy consumption were monitored. The study also aimed at determining the most suitable conditions for utilizing the current proposed strategy by analyzing different weather conditions and space conditioning methods. The analysis concluded that embedding PCMs within alveolar bricks did not considerably decrease the overall environmental impact with respect to the experimental conditions considered. It was also concluded that this technique is most effective when summer weather conditions throughout the year are theoretically assumed. Similar observations were made in the case of PCM implementation for winter conditions in the Netherlands (Entrop et al., 2016) and Montreal (Guarino et al., 2017), with a corresponding reduction in heating load by 30%. PCM melting temperatures between 16°C and 24°C were selected in these studies. All the studies presented indicate that PCM selection is an important parameter for implementation of PCM within buildings; thus there needs to be a clear and standard method for rendering a particular PCM suitable for application within a building.

14.2.2 Parameters for PCM selection for brick incorporation PCM incorporation increases the thermal mass of building elements such as bricks. This is the result of PCM melting, which takes place during the day, as it is heated above its melting temperature. PCM absorbs heat and undergoes a phase change, thus inhibiting temperature elevations. It continues to store the incoming heat until it is completely melted. This phenomenon is easy to comprehend; however, in order for it to take place on a regular basis, it is necessary for the PCMs to release the heat during the night. The temperature difference between the PCM and the ambient air is responsible for heat gain and loss during the day and night. Thus PCM melting temperature must lie within the mean maximum and mean minimum temperatures for a particular season. To explain this with an example, mean minimum and maximum temperatures for Jaipur (28.9°N latitude, 75.8°E longitude and altitude: 431 m), India are shown in Fig. 14.1. This ambient temperature in conjunction with other parameters, such as incident solar radiation, surface orientation, wind velocity, and time of day and year, are responsible for temperature over the surface, referred to as the sol air temperature (Duffie and Beckman, 2013). This temperature is responsible for energy storage in PCMs during the day. The thickness of PCMs, if optimally designed, is sufficient to withstand the solar radiation and checks the temperature rise inside the building. During the off-sunshine hours, the temperature of the surroundings is lower, compared to the PCM temperature, which is close to its melting point. Thus the cycle is now

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Fig. 14.1 Mean temperatures in Jaipur.

reversed, and the heat stored during the day is now discharged. The difference in PCM melting temperature and the minimum temperature during the night must be sufficient for the PCM to discharge and become available for absorbing and storing heat on the subsequent day. An experimental study (Saxena et al., 2019) has shown that PCM melting temperature must be kept just below the average maximum temperature of the location during the day so that the PCM may undergo charging and discharging on a daily basis. Thus PCM melting temperature must lie within the mean minimum and mean maximum temperature of that location. Other important PCM properties are high latent heat capacity with lower volume change ratio. It must have low subcooling (difference in melting and solidification temperature) and no phase segregation. To ensure these properties, PCMs are often characterized using differential scanning calorimeter (DSC). This technique provides PCM properties, such as specific heat, latent heat, range of melting, and solidification temperatures. The degree of subcooling can also be easily identified using the DSC characterization curve. The stability of PCMs selected must also be high (>10,000 cycles of melting-solidification). They should be nonflammable and nontoxic. Another important aspect is their cost (should be low), to make this solution economically viable (Gil et al., 2013).

14.2.3 PCM encapsulation and methods for incorporation Different researchers worldwide have tried PCM implementation using different methodologies. PCMs can be incorporated either directly, or through immersion, vacuum impregnation, encapsulation, or through shape and form stabilization. The first three methods may not be suitable, as has been reported by a few researchers (Soares

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et al., 2013), owing to the leakage that is prone to occur once the PCM undergoes a phase change from solid to liquid. Thus, from a building perspective, encapsulated PCMs as well as form/shape-stabilized PCMs are considered, as discussed in Section 14.2.1. Encapsulations are of two types, micro- and macroencapsulations (based on size), and this has been dealt with in detail by many researchers. Research is being carried out to find suitable materials to be used as a sheath for encapsulating PCMs. A study on a water-dispersible PCM sheath has been carried out by Khan et al. (2019). These sheaths provide a good solution to overcoming the water-repellent nature of most sheath materials. These will ensure proper mixing of microencapsulated PCMs with cement and mortar and avoid coagulation to ensure a homogeneous PCM concentration within the mixture. Other research on enhancing the heat transfer rate with proper shape and size selection has also been carried out (Konuklu et al., 2015; Khadiran et al., 2015; Su et al., 2015). Navarro et al. (2016b) has provided examples of macro- and microencapsulated PCM constructions for testing in Paris, Spain, and other parts of the world. In the case of shape- and formstabilized composites, the PCMs are mixed in definite proportions with supporting materials, such as high density polyethylene (HDPE), etc., by melting them along with the PCMs, thereby forming composites at higher temperatures. On cooling, these can be used with construction materials directly to avoid leakages. However, a detailed analysis of cost, water dispersability, compatibility, and long-term behavior are still in the testing phase for building implementation of these form/shape-stabilized PCMs.

14.2.4 Modified brick preparation for PCM incorporation 14.2.4.1 Different brick geometries studied There are different geometries of bricks, as shown in Fig. 14.2. A few studies on different types of bricks that can be used for PCM incorporation have been carried out. Silva et al. (2012) tested bricks with a rectangular cavity in combination with air spaces, as shown in Fig. 14.2A. These bricks were prepared to compare two bricks, one with a macroencapsulated PCM and the other without PCM. The test conditions were simulated within a test chamber. The results showed a 5°C reduction in temperature fluctuation and a time delay/lag of 3 h was observed. Wang et al. (2016) used a solid PCM-mortar brick with an SSPCM containing high-density polyethylene, graphite, and 70% by mass of paraffin, as shown in Fig. 14.2B. This PCM-mortar brick wall showcased an approximately 24% reduction in cooling load during summers and 10%–30% heating load during winters. Principi and Fioretti (2012) tested hollow thermal bricks with the incorporation of sodium sulfate decahydrate PCM stabilized by different substances, as shown in Fig. 14.2C. Utilizing these bricks resulted in a reduction of approximately 25% in the thermal peak loads and temperature fluctuations. Vicente and Silva (2014) analyzed horizontally hollowed fire clay bricks and integrated macroencapsulated organic paraffin PCMs to study their thermal behavior. It was observed that the thermal amplitude decreased by approximately 50%. The brick used in this study is shown in Fig. 14.2D. Zhang et al. (2019) investigated

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(A)

(C)

(E)

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Fig. 14.2 Different brick geometries studied. (A) Brick having rectangular cavity filled with air spaces (Silva et al., 2012); (B) Solid PCM-mortar brick (Wang et al., 2016); (C) Hollow thermal brick (Principi and Fioretti, 2012); (D) Horizontally hollowed fire clay brick with macroencapsulated PCM (Vicente and Silva, 2014); (E) Aluminium hollowed bricks (Zhang et al., 2019); and (F) Hollow clay bricks with squared PCM pockets (Hichem et al., 2013).

the thermal characteristics of aluminum hollowed bricks as bearing structures filled with paraffin as PCM, as shown in Fig. 14.2E. A 2.5 times better temperature control was reported in the case of application of PCM than the case without PCM bricks. Hichem et al. (2013) studied hollow clay bricks with square holes filled by PCMs, as shown in Fig. 14.2F, to analyze their thermal behavior in hot arid areas. They

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reported that incorporating PCM within the square holes significantly enhanced the thermal inertia of the bricks. A similar study with rectangular macroencapsulated PCMs, which were incorporated within modified bricks, was also carried out (Saxena et al., 2020c). Along with assessing the PCM impact within a brick under different PCM configurations, this study also discussed the method for the modified brick making.

14.2.4.2 Brick making Brick making is a simple process and has been used through the ages. It basically comprises of four major steps: clay preparation, molding, drying, and burning, as illustrated in Fig. 14.3. Clay Preparation: The constituents of the clay, the particle size, and moisture are some of the key parameters that need to be kept in mind during clay preparation. Moisture content must be optimum, as increased moisture content results in developing cracks during drying, which may further propagate and result in reduced brick strength or breaking during burning. Molding: Molding is the next step, in which bricks of the requisite shape and size are prepared. Bricks can be either hand molded or machine molded. In order to prepare the slotted/ modified bricks, the mold design needs to be appropriately modified, as shown in Fig. 14.3, which shows a modified mold for preparing a slotted brick with proper allowances. Once the slotted bricks are prepared, the hollow spaces/slots are reinforced with fuel wood. This is done to maintain the dimensional accuracy of the slot, which may otherwise shrink or result in warping of the bricks. Drying: There are two ways to handle the drying. One is solar drying (often used in villages) for small- or medium-scale production. It takes around 5–14 days. The modern-day method is to use ovens for brick drying. The typical time lies between 12 and 48 h. The moisture content after drying is between 5% and 7% and they are referred to as green bricks. Most of the moisture is removed during this process, thus saving fuel and time during the burning stage. Drying increases the strength of the molded bricks and they can be stacked within the conventional flame kiln (Bhatta) or modern kilns for burning. Burning: The temperature is approximately 900–1200°C. A flame kiln, as shown in Fig. 14.3, often uses conventional fuels such as coal, wood charcoal, etc. These are now

Wire cutting Bricks

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Fig. 14.3 Brick-making process.

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decreasing with environmental regulations becoming more stringent, as it is an inefficient method of burning fuel with a high amount of particulate, smoke, and GHG emissions. Thus, this method is becoming obsolete, and is being implemented only at the village level for small- or medium-scale production. Modern electric kilns are generally used these days, as they are more efficient and heat losses are minimized. These are often used for mass-scale production. These bricks, once baked, are cooled slowly to the ambient temperature within 24–48 h.

When prepared, these bricks need to comply with the building codes in terms of compressive strength, moisture absorptivity, etc.; thus, the bricks are tested for their compliance before being implemented within any construction.

14.3

Temperature and heat transfer assessment for PCM bricks

Thermal assessment here in this chapter is referred to as the temperature and heat transfer assessment across the PCM bricks. The comfort assessment, which is generally carried out for an entire building space, is dealt with separately in detail in Section 14.4.

14.3.1 Temperature reduction across PCM bricks To assess the impact on the temperature reduction under different PCM configurations, a study was carried out by Saxena et al. (2020c). This study compared the temperature across the bricks with and without PCMs. The study also assessed the impact of having two PCM layers within the bricks. Fig. 14.4A shows the brick configuration with a schematic view and Fig. 14.4B shows the actual profile of temperatures across the bricks for peak summer (last week of May 2018) for New Delhi, India. The thickness of the PCM considered was 1 cm. The results clearly indicate that peak temperature in the case of single PCM layer brick was reduced by 3.5–6°C and temperature fluctuation was reduced by around 9°C. In the case of double layer PCM bricks, it was observed that a temperature reduction up to 9°C occurred and the reduction in temperature fluctuation was found to be between 12°C and 14°C. PCM selection was based on temperature variation from March to September; however, the temperature across the bricks was also assessed during the winter, i.e., from 24 December to 31 December, 2018, as shown in Fig. 14.5. It was seen that temperature fluctuation was still reduced in the case of PCM bricks. The minimum inside temperature of the PCM brick was around 0.5°C higher as compared to conventional brick. The reason is the thermal resistance. In the case of PCM bricks, the thermal resistance is higher due to lower thermal conductivity of the PCMs. However, the PCMs only act as a sensible heat storage, as phase change is not initiated and latent heat does not come into play. Thus, it is observed that even if PCMs are selected based on cooling load reduction in summer, they reduce heating load during winter, as they act as a sensible heat material and provide an insulating effect due to their low thermal conductivity.

1

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Fig. 14.4 (A) Brick configuration with a schematic view; (B) temperature profile for bottom and top surface of different bricks (Saxena et al., 2020c).

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Temperature profile (Winter)

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Fig. 14.5 Temperature profile for bottom and top surface of different bricks for winter.

14.3.2 Assessment of reduction in heat transfer Heat transfer for a PCM-incorporated building element is a complex phenomenon. When solar radiation is falling on a surface, its temperature rises above the ambient temperature, often referred to as the sol air effect, in which the temperature is given by the equation: Tsol ¼ Ta +

  αI  εΔR ho

(14.1)

Tsol is the solar air temperature, Ta is the ambient temperature, α is the surface absorptivity, I is incident solar radiation falling on the surface, ho is the heat transfer coefficient, ε is emissivity, and ΔR is the long wavelength radiation exchange between the surface and sky (due to the temperature difference); ho can be calculated using McAdam’s equation (Tiwari, 2012), which is a linear correlation, based on the wind velocity: ho ¼ 5:7 + 3:8 vo for 0 < vo < 5 m=s

(14.2)

The difference in sol air temperature and indoor temperature is responsible for heat transfer taking place. The heat flow rate before and after the PCM layer is different. If the temperature of the PCM is below its melting point, then the PCM will undergo sensible heating and the heat transfer to the inside will follow a gradual increase with time. However, once the phase change temperature is reached (schematic diagram is shown in Fig. 14.6), the PCM starts melting. Even with increased solar radiation during the day, the heat flow to the inside only shows a slight variation, which may occur due to the congruent melting, a phenomenon often shown by most of the PCMs used for building applications. The temperature rise is also very gradual. During this period,

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PCM

Indoor space

Brick

Fig. 14.6 Schematic diagram for heat transfer through a PCM layer.

most of the incoming heat (Q1) is absorbed by the PCM (Qs) and only a part of it flows to the indoor space (Q2), which is dependent on the difference between the PCM temperature and indoor air temperature. This continues until complete melting of the PCM takes place, beyond which sensible heat of the PCM starts and the temperature rises relatively faster. As the sun sets, the ambient temperature goes down. The PCM is in a partially or completely melted state. The temperature of the PCM is above both the ambient and indoor air temperatures. Thus, heat flow takes place from the PCM both towards the indoor and ambient air. As the temperature gradient is greater between the PCM temperature and the ambient air temperature, the major amount of heat flow takes place in that direction. Thus, only a part of the heat stored during the day flows to the inside, reducing the overall heat flow during the day. ∂tbrick ∂2 tbrick ¼α ∂τ ∂x2

(14.3)

The heat transfer is determined using the heat conduction equation (Eq. 14.3), which is simplified to Eq. (14.4) as the boundary conditions are applied. This is the generalized equation used for calculating the heat transfer: Q ¼ Ueff Aðts  ti Þ

(14.4)

  1 1 δ1 δ2 2  δ3 δ4 δn 1 ¼ + + + + …+ + Ueff ho k1 k2 k3 k4 k n hi

(14.5)

The amount of heat stored within different elements is calculated using an electrical analogy, where heat stored (Qs) for any element △ x is given by: Qs ¼ ρbrick Abrick Cs △x

(14.6)

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315

where Cs is the heat storage capacity with respect to temperature, which is constant for most materials except for PCMs. The heat storage capacity for PCM C∗s is given as: C∗s ¼ Cps T < Tonset

(14.7a)

   hji  hj1 i L  Tonset < T < Tendset C∗s ðT Þ ¼ Cps + ¼ ðTendset  Tonset Þ Tij  Tij1 

C∗s ¼ Cpl T > Tendset

(14.7b)

(14.7c)

where Tonset is the temperature when solid-liquid transformation starts and Tendset is the temperature at which PCM is completely liquid; h is the enthalpy. Cps is specific heat of the solid phase and Cpl is specific heat of PCM in the liquid phase (Pasupathy et al., 2008). The studies show that a heat transfer reduction of up to 30% during the day can be achieved for summer conditions. For a typical study carried out in New Delhi, India, a heat transfer reduction up to 20% was observed as the result, as shown in Fig. 14.7 (Saxena et al., 2020c).

14.4

Thermal comfort assessment for a room with PCM bricks

14.4.1 Model for thermal comfort assessment With India’s increasing population and rapid urbanization, there has been a tremendous growth in energy consumption in buildings, owing to two main factors: maintaining thermal comfort and lighting (Vaishnani et al., 2020). The study

Heat transfer 80

Heat flux (W/m2)

60 40 20 0 -20 -40 -60 1

145

289

433

577

721

Time step (10 min) HT Brick 1

HT Brick2

HT Brick3

Fig. 14.7 Heat transfer measurement across bricks with and without PCM (Saxena et al., 2020c).

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

presented in this chapter deals with passive means of space conditioning, which can in turn lead to providing thermal comfort to building occupants. Thermal comfort is essentially that state of mind of the occupants that expresses satisfaction with their thermal environment within a building (ASHRAE, 2010). For an occupant to have a healthy mind and body and higher productivity, it is a necessary condition for their bodies to remain at thermal ease. The thermal comfort sensation may vary from person to person; however, it depends on several factors. The main environmental factors influencing thermal comfort are the air temperature, mean radiant temperature, relative humidity, and air velocity, while metabolic rate and clothing insulation are the physiological factors affecting the thermal comfort (ASHRAE, 2010). Numerous indices have been reported in the literature that help in assessing the thermal comfort condition of building occupants; these indices have been formulated based on the factors affecting thermal comfort. These indices include effective temperature (ET), corrected effective temperature (CET), operative temperature (OT), and tropical summer index (TSI), among others (Auliciems et al., 1997). ASHRAE Standard 55 suggests the two most widely recognized indices developed by Fanger as the standard indices for assessing thermal comfort, which are the predicted mean vote (PMV) and the percentage people dissatisfied (PPD) indices (ASHRAE, 2010). The PMV gives the mean vote of the thermal sensation of the occupants in a building on a 7-point thermal sensation scale ranging from 3.0 for a cold environment to +3.0 for a hot environment (Ali et al., 2020; Ali and Rakshit, 2020). It is established on the basis of the heat transfer balance equations for the thermal load on a human body (Ali et al., 2020; Ali and Rakshit, 2020; Albatayneh et al., 2019) and depends on the four environmental factors and the two personal factors mentioned earlier. The ideal thermal environment according to ASHRAE Standard 55 is the one in which the assessed PMV value lies in the range of 0.5 to +0.5 (ASHRAE, 2010). However, whether the occupants are actually feeling satisfied by their thermal environment or not is given by the other index developed by Fanger (Ali et al., 2020; Ali and Rakshit, 2020). The PPD gives the percentage of people dissatisfied by their thermal environment, and can be assessed based on a relationship between the occupant’s satisfaction and the PMV value obtained. ASHRAE Standard 55 recommends the value of PPD to not exceed 20%, indicating that for an environment to be identified as thermally acceptable, at least 80% of the occupants should feel thermally satisfied (ASHRAE, 2010). Besides these indices, the thermal comfort is also shown by the behavioral actions of the occupants that are initiated, such as changing clothing, activity, posture, location, etc. The indices described, however, are based on static models that analyze thermal comfort as the measure of a deterministic sequence of cause-and-effect and ignore the adaptive role of the occupants (Nicol and Humphreys, 2002). They do not take into consideration the psychological factors of the occupants, which essentially play a vital part in regulating and adjusting the thermal comfort condition (Albatayneh et al., 2019). Hence, researchers have now shifted to adaptive comfort models based on the principle that if there is a change in the thermal environment of the occupants that produces thermal discomfort to them, they react in manners that tend to reestablish their comfort (Nicol and Humphreys, 2002).

PCM incorporated bricks

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The adaptive thermal comfort theory suggests that there is a connection between the occupants and their outdoor environment and the control over their immediate indoor environment that allows the occupants to adapt to a wider range of comfort conditions than what is generally considered comfortable (Manu et al., 2014). Many researchers who proposed a linear relationship between the outdoor temperature and the comfort temperature for the occupants in naturally ventilated buildings have studied this adaptive approach for comfort assessment. On the basis of the results for adaptive comfort obtained by researchers, the model for adaptive comfort was integrated into the ASHRAE Standard 55 for naturally ventilated buildings (Santamouris, 2006). Since the proposed model in ASHRAE Standard 55 does not recognize the climatic and workplace context of Indian buildings, application of this model for assessing comfort conditions leads to an elevation in the Indian thermal comfort expectations to amounts that involve providing unsustainable energy without significant improvement in the comfort levels (Centre for Advanced Research in Building Science and Energy, 2012). An Indian-specific model, called the Indian Model for Adaptive Comfort (IMAC), was hence introduced in order to give design and operation assistance for air-conditioned and naturally ventilated buildings. It was developed by conducting survey studies in office buildings for 1 year for each of the climatic zones of India. The studies conducted in naturally ventilated, mixed-mode, and air-conditioned buildings in India led to the development of relationships for finding the temperature at which the occupants would feel neutral towards their thermal environment. It also specifies the range of temperature within which 90% of the occupants feel satisfied with their thermal environment. The IMAC model provides a broader comfortable temperature band for occupants of the building, thus aiding in reduced energy use for space cooling (Manu et al., 2014). The linear correlations developed for estimating the indoor operative temperature are as follows (ECBC, 2017): For naturally ventilated buildings:
Ti,op ¼ 0:54  To + 12:83 (14.8) 90%acceptability range ¼ Ti,op  2:38 For mixed-mode buildings:
Ti,op ¼ 0:28  To + 17:87 90%acceptability range ¼ Ti,op  3:46 For air-conditioned buildings:
Ti,op ¼ 0:078  To + 23:25 90%acceptability range ¼ Ti,op  1:5

(14.9)

(14.10) (14.11)

(14.12) (14.13)

where Ti, op: Indoor operative temperature at which the occupants feel neutral to their thermal environment (°C). To : Thirty-day outdoor running mean air temperature (°C).

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

90 % acceptability range: Range of temperature within which 90% of the occupants feel satisfied with their thermal environment (°C).

14.4.2 Assessing influence of PCM-incorporated bricks on thermal comfort The composite climatic condition of Delhi has been chosen to study the effect of PCM-incorporated bricks on thermal comfort. The simulation was carried out for the last week of May, signifying the peak summer conditions for Delhi. A test room of dimensions 3 m  3 m  3 m was simulated. The simulation study for the test room was carried out using the ISHRAE weather file for Delhi. Five different PCMs—PCM 1–5 viz., eicosane, OM 35, HS34, n-nonadecane, and (48% CaCl2 + 4.3% NaCl + 0.4% KCl + 47.3% H2O), respectively—were selected for carrying out the thermal comfort study for a room made of PCM-incorporated bricks with PCM thickness of 2 cm sandwiched between the brick thickness of 3 and 6 cm. The brick was assumed to be plastered on both sides with a thickness of 1.5 cm. The IMAC model was selected for assessing the thermal comfort conditions due to the PCM-incorporated bricks. Using the weather data for Delhi, the 30-day running mean temperatures for the last week of May were estimated. The case of an airconditioned building has been taken into consideration to analyze the effects of PCM incorporation on the thermal comfort levels within such buildings since the test room was assumed to be under controlled conditions. Eqs. (14.12), (14.13) have been used to assess the indoor operative temperatures at which the occupants would feel thermally satisfied. These indoor operative temperatures were compared with the indoor temperatures achieved as a result of the application of PCM-incorporated bricks, to evaluate the performance of the different PCMs selected. The corresponding plots and results are discussed in Section 14.4.3.

14.4.3 Observations and findings The simulation results obtained for the modeled test room show that, for the base case, i.e., the case without the application of PCM, the inside surface temperature ranged from 28°C to 45°C. After simulation of the test room with different PCMs, it was observed that the inside temperature variation ranged from 32°C to 42°C. It was noticed that for the cases of application of PCM 1 and PCM 2, the temperature fluctuations were minimum, varying between 32°C and 37°C. The cases of PCM 3 and PCM 4 showed marginally greater temperature variations ranging between 32°C and 38°C. On the other hand, it was observed that for the case of PCM 5, the temperature fluctuations ranged between 33°C and 42°C, indicating that the phase change process was not adequately taking place. Since the test room was assumed to be under controlled conditions at 25°C, this indicates an air-conditioned building, so Eqs. (14.12), (14.13) were utilized for assessing the indoor operative temperatures for the last week of May, along with the comfort band satisfying the criteria of 90% of occupants being thermally satisfied with their environment. Fig. 14.8 shows the plots of all the indoor temperatures

PCM incorporated bricks

319

50.00 48.00 46.00 44.00

Temperature (°C)

42.00 40.00 38.00 36.00 34.00 32.00 30.00 28.00 26.00 24.00 20.00

1 6 11 16 21 26 31 36 41 46 51 56 61 66 71 76 81 86 91 96 101 106 111 116 121 126 131 136 141 146 151 156 161 166

22.00

Max. indoor operative temp.

Min. indoor operative temp.

Indoor surface temp. PCM 1

Indoor surface temp. PCM 2

Indoor surface temp. PCM 3

Indoor surface temp. PCM 4

Indoor surface temp. PCM 5

Indoor surface temp. (No PCM)

Indoor Operative temp.

Fig. 14.8 Comparison on the basis of thermal comfort analysis for May (last week).

resulting from the application of different PCMs selected, indoor operative temperature, and the resulting comfort band for the occupants. The indoor operative temperature of the building was found to be between 25.84°C and 25.82°C, with the comfort band ranging between 1.5°C. This could be because in an air-conditioned building, the indoor conditions are controlled, and hence the occupants adapt themselves to that controlled environment. The upper and lower extents of the comfort band are demarcated by the yellow colored area in Fig. 14.8, while the brown line indicates the indoor operative temperature for the air-conditioned building. From the figure, it can be observed that for none of the cases do the temperatures fall within the comfort zone and they are above the comfort zone demarcated, thus representing a sense of a hotter environment than is considered comfortable. However, the comparison could be made on the basis of the degree of fluctuation a particular PCM shows, hence exhibiting whether the variation within the maximum and minimum range is lying closer to the comfort zone or not. Owing to the large degrees of fluctuations in the indoor temperatures for the base case, the peak maximum temperatures obtained are far away from the comfort zone, thus necessitating the need for a passive strategy, such as PCM incorporation within bricks in this case, bringing the temperature range closer to a comfortable environment for the occupants. The indoor temperatures for the cases of PCM 1 and PCM 2 show that no point lies in the comfort band; nonetheless, the temperature variation from maximum to minimum for both the cases is closer to the comfort zone. On the other hand, the indoor temperature profiles of PCM 3, PCM 4, and PCM 5 show poor performance based on thermal comfort, as the lines indicating the indoor temperatures are away from the comfort band demarcated in

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

the graph based on Eq. (14.2). Since these PCMs show higher fluctuations, they are not suitable for providing a thermally comfortable environment for the occupants. A graph of the difference in indoor temperatures for different PCMs and the indoor operative temperature for the last week of May has also been plotted, to look at the deviations of the temperatures resulting from the application of PCMs from the comfort temperature of the occupants, as shown in Fig. 14.9. It shows that the base case, i.e., the case without the application of PCM, shows a large amount of deviation and, hence, suggests the need for some measure to be adopted in order to condition the indoor space in accordance with the thermal comfort requirement of the occupants. The graph also indicates that PCM 1 and PCM 2 are showing good results, with minimum deviations in the indoor temperature, with the average difference in temperatures for PCM 1 being 9.46°C and that for PCM 2 also being 9.46°C. This indicates that, although PCM 1 performed slightly better than PCM 2, due to the marginal differences between the temperature differences for both these PCMs, it can be stated that these two PCMs perform the same based on the average difference in the indoor temperatures and the indoor operative temperature. It can be observed that, out of the remaining three PCM cases, PCM 4 performs better with the average temperature difference of 10.02°C since the differences in temperatures are higher than those for PCMs 1 and 2, while lower than for the cases of PCMs 3 and 5. The plots for PCM 3 and PCM 5 show that the degree of fluctuation for PCM 3 is lower compared to PCM 5, since the difference in temperatures for the peak minimum cases for PCM 5 is less than that for PCM 3. Thus it can be inferred that PCM 5 is performing slightly better than PCM 3. The average difference in temperatures for PCM 5 is 11.03°C while that 20.00 18.00

Difference in temperature (°C)

16.00 14.00 12.00 10.00 8.00 6.00 4.00

0.00

1 6 11 16 21 26 31 36 41 46 51 56 61 66 71 76 81 86 91 96 101 106 111 116 121 126 131 136 141 146 151 156 161 166

2.00

PCM 1

PCM 2

PCM 3

PCM 4

PCM 5

No PCM

Fig. 14.9 Difference in temperatures for different PCMs from indoor operative temperature for May (last week).

PCM incorporated bricks

321

for PCM 3 is 11.63°C. It can hence be concluded from the two graphs (Figs. 14.8 and 14.9), that out of the five PCMs studied, PCM 1 and 2 perform the best, while PCM 4 is mediocre in performance. PCMs 5 and 3 perform almost alike, exhibiting the worst performance with respect to the provision of a thermally comfortable environment for the occupants. A similar procedure was carried out for performing a thermal comfort analysis of the different PCM-incorporated bricks for the winter season as well. For the airconditioned building considered for study, the analysis was carried out for the last week of December, since that represents the worst case for the winter season. The results shown in Fig. 14.10 were obtained, with the yellow demarcated area signifying the comfort band for such buildings based on the climatic conditions of Delhi for the last week of December and the brown colored line representing the indoor operative temperature at which the occupants feel thermally neutral. The neutral temperature in this case was reduced to the range of 23.92–24.30°C with a comfort band between 1.5°C. The plot for the base case represents that the temperatures are far from thermally satisfying the occupants, due to large temperature variations, while it can be seen that the plots for all the cases are lying below the comfort zone, indicating a cooler environment than is comfortable. The cases with incorporation of PCM 1 and 2 indicate that for an air-conditioned building, the temperatures are not reaching the comfort zones of the occupants; however, the fluctuations between the maximum and minimum temperatures demonstrate that they are lying closer to the comfortable zone, with both the PCMs functioning comparably. For the case of the winter season, it was observed that, although PCMs 1 and 2 still performed better than the other PCMs, PCM 3 also performed at par with PCMs 1 and 2, unlike during the summer season. However, in terms of year-round applicability, out of the three PCMs selected, PCM 3 30.00 28.00 26.00 24.00 22.00 20.00 18.00 16.00 14.00 12.00

8.00

1 6 11 16 21 26 31 36 41 46 51 56 61 66 71 76 81 86 91 96 101 106 111 116 121 126 131 136 141 146 151 156 161 166

10.00

Max. indoor operative temp.

Min. indoor operative temp.

Indoor surface temp. PCM 1

Indoor surface temp. PCM 2

Indoor surface temp. PCM 3

Indoor surface temp. PCM 4

Indoor surface temp. PCM 5

Indoor surface temp. (No PCM)

Indoor operative temp.

Fig. 14.10 Comparison on the basis of thermal comfort analysis for December.

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

could be rendered unsuitable owing to large temperature fluctuations in the summer season. Comparison of the comfort band with temperature profiles resulting from PCMs 4 and 5 indicates that neither of the temperatures resulting from the application of these two PCMs fall in the comfort zone, with PCM 5 performing the worst in terms of providing a thermally comfortable environment out of the PCMs selected for the study. This suggests that the greater the fluctuations between the peak maximum and minimum indoor temperatures, the farther are the temperatures from the comfort zone, and the poorer is the performance of those PCMs in providing thermal comfort to the occupants. Fig. 14.11 represents the plots of the difference in the indoor temperatures resulting from different PCMs from the neutral temperature estimated for the last week of December, representing the worst case for the winter season. The graph depicts that, for the case where the PCM has not been incorporated within the building elements, large amounts of fluctuation can be observed, indicating a need for intervention to reduce the difference between the indoor operative temperature and the temperature resulting due to the incorporation of that measure, hence providing a thermally comfortable environment. The plots for PCM 1, PCM 2, and PCM 3 depict that the fluctuations in the temperatures for all the three cases are similar and for the case of December, these three PCMs are performing the same. The average deviation in temperatures for PCM 3 was observed to be the least, with a value of 6.33°C, making it the best-case scenario for the winter season, while there were very negligible differences in these values for the cases of PCMs 1 and 2, with the values being 6.44°C and 6.46° C, respectively. However, as already stated, owing to large fluctuations and a large 20.00 18.00 16.00 14.00 12.00 10.00 8.00 6.00 4.00

0.00

1 6 11 16 21 26 31 36 41 46 51 56 61 66 71 76 81 86 91 96 101 106 111 116 121 126 131 136 141 146 151 156 161 166

2.00

PCM 1

PCM 2

PCM 3

PCM 4

PCM 5

No PCM

Fig. 14.11 Difference in temperatures for different PCMs from indoor operative temperature for December.

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323

temperature difference between the neutral temperature and the indoor temperature due to the application of PCM in the summer season, resulting in poor performance of PCM 3, it can be rendered unsuitable for year-round applicability. In addition, just as with the case for the summer season where PCM 5 was not performing satisfactorily, in this case as well, due to larger differences in temperatures, PCM 5 is less suitable than PCM 1 and PCM 2. The average value of the differences in temperatures for the case of PCM 5 was 6.80°C. PCM 4, on the other hand, performed the worst for the case of the winter season, owing to maximum deviations from the comfort temperature, with the average difference in temperatures being 11.79°C. It can hence be concluded from the graph for the winter season that PCMs 1, 2, and 3 perform the best; however, due to poor performance in the summer season, PCM 3 is found unsuitable, whereas PCMs 4 and 5 performed poorly in providing a thermally comfortable environment for the occupants. Hence, overall, it can be observed that for a year-round applicability of a particular PCM, the preceding plots based on providing a thermally comfortable environment to the occupants proved to be quite helpful. For the worst cases of the summer and winter seasons, both PCMs 1 and 2 performed alike, showing minimum fluctuations, and were closer to the comfort band of the occupants; hence, these two PCMs can be identified as suitable PCMs out of the five PCMs studied. However, even if PCM 1 and PCM 2 are selected, other means of space conditioning would still be required in order to bring the indoor temperature of the building within the comfortable zone. Additionally, for selecting one PCM out of the two PCMs identified, other factors such as cost effectiveness and availability should be considered.

14.5

Conclusions and future recommendations

This study provides a review of different studies carried out on PCM bricks worldwide. Thermal assessment of PCM bricks in terms of temperature and heat transfer reduction across them, compared to conventional bricks, has been discussed. In addition, methods of modified brick preparation along with precautions to be taken during preparation, so that the bricks can be used for PCM incorporation and comply with the mandatory standards, have been examined. PCM bricks with different PCM cavities have been shown and their impact has been discussed. A few studies and their findings for a full-scale model using PCM bricks have also been presented. It is observed that, overall, PCM incorporation within bricks can reduce the temperature across them, up to 6°C. This has been found to hold true for both experimental and simulation studies carried out for summer, with a heat transfer reduction of up to 30%. In the case of cold locations where PCMs are used for space heating, heating load reductions up to 50% have been reported in the literature. Based on the design condition and application, PCMs must be appropriately selected or else the actual advantage of utilizing the latent heat to increase the temperature and time lag may not occur, thus reducing the utility of applying PCMs within bricks.

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

Another important aspect of PCM-incorporated bricks is their effect on the thermal comfort condition, for which a simulation study was carried out for five PCMs for both summer and winter. It was found that the PCM may not be completely capable of maintaining the comfort temperature; however, comfort conditions can be achieved with much less input energy. This is because the inside temperature difference from the comfort temperature range is lower for the PCM-incorporated room as compared to the conventional room, without the PCM. PCM is a passive solution that can have a significant impact in terms of temperature reduction and may result in energy savings. However, this is only possible if cheaper PCMs are found, for which significant research in the field is required. For example, paraffin possesses good thermal properties but is relatively costly, whereas salt hydrates are cheap but they show subcooling. Thus research is required to find nucleating agents that can be used to avoid this problem. Similarly, for other properties, such as stability, research needs to be carried out appropriately. Another important aspect is the use of water-dispersible PCM coatings, so that their direct incorporation within clay and concrete becomes possible, thus reducing the overall cost and complexity. Cheaper PCMs, having reasonable stability and good thermal properties, could result in a significant amount of energy savings and could make their application in buildings possible.

Acknowledgments The authors acknowledge Department of Science & Technology, India [project no. TMD/CERI/ BEE/2016/084(G)] and Yardi Software Pvt. Ltd. for providing fundings for this study.

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composite climatic conditions. J. Braz. Soc. Mech. Sci. Eng. 42, 1–16. https://doi.org/ 10.1007/s40430-020-2219-4. Vicente, R., Silva, T., 2014. Brick masonry walls with PCM macrocapsules: an experimental approach. Appl. Therm. Eng. 67, 24–34. https://doi.org/10.1016/j.applthermaleng. 2014.02.069. Wang, X., Yu, H., Li, L., Zhao, M., 2016. Experimental assessment on the use of phase change materials (PCMs)-bricks in the exterior wall of a full-scale room. Energy Convers. Manag. 120, 81–89. https://doi.org/10.1016/j.enconman.2016.04.065. Zalba, B., Marı´n, J.M., Cabeza, L.F., Mehling, H., 2003. Review on thermal energy storage with phase change: materials, heat transfer analysis and applications. Appl. Therm. Eng. 23, 251–283. https://doi.org/10.1016/S1359-4311(02)00192-8. Zhang, J.J., Yang, C.H., Zhang, J.S., 2019. Thermal characteristics of aluminium hollowed bricks filled with phase change materials: experimental and numerical analyses. Appl. Therm. Eng. 155, 70–81. https://doi.org/10.1016/j.applthermaleng.2019.03.103.

Influence of novel PCM-based strategies on building cooling performance

15

Yuekuan Zhoua, Zhengxuan Liub, and Siqian Zhengc a Department of Building Services Engineering, Faculty of Construction and Environment, The Hong Kong Polytechnic University, Hong Kong, China, bCollege of Civil Engineering, National Center for International Research Collaboration in Building Safety and Environment, Hunan University, Changsha, Hunan, China, cDepartment of Architecture and Civil Engineering, City University of Hong Kong, Hong Kong, China

15.1

Introduction

Latent thermal energy storage using phase change materials (PCMs) is promising for reducing cooling energy consumption and carbon emissions in subtropical regions. In academia, researchers have been focusing on the contribution of novel PCM-based strategies for cooling performance enhancement, from perspectives of multidiversified system designs and novel control strategies. A systematic overview of PCM-integrated cooling systems is strongly needed to present the current situation along with its challenges. Depending on the application forms, PCMs can either be integrated with building components (such as building envelopes and roofs, forming a distributed cooling system) or with centralized thermal storage. Strategies for building cooling performance enhancements include high-reflective coatings, radiative cooling walls, hybrid ventilation with ventilated roof, and intelligent control of coupled systems. In the academic world, researchers are mainly focused on deterministic parameterbased analysis, with respect to temperature response (such as supply air temperature (Ling et al., 2019) and indoor air temperature (Ling et al., 2019)); energy efficiency (such as heat transfer (Zhou et al., 2019a, 2020a), heat storage and release efficiency (Ling et al., 2019)); and energy performance (such as accumulated heat (Yan et al., 2020), net cooling energy (Erell and Etzion, 2000), cooling power (Hanif et al., 2014), cooling energy consumption savings (Liu et al., 2018a), heating/cooling load (Zhou and Yu, 2018), and heat storage density (Ling et al., 2019)). The heat transfer mechanism has been mainly investigated through an enthalpy-porosity approach. However, studies offering a systematic review of cooling performance enhancements through integrated strategies are rare, from the perspectives of thermophysical properties of PCMs (Li et al., 2019a), novel systems design (such as PCM-integrated side-wall cooling (Zhou et al., 2016a) or PCM ventilated cooling systems (Zhou et al., 2019b)), scenario uncertainty-based performance analysis (Zhou et al., 2020b), and system optimization (optimal designs and robust Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction https://doi.org/10.1016/B978-0-12-820791-8.00015-8 © 2021 Elsevier Ltd. All rights reserved.

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operations (Zhou et al., 2019a,c, 2020a; Liu et al., 2019a)). Cooling performance analysis under multilevel uncertainty is necessary for improving the reliability and robustness of PCM-integrated cooling systems. Furthermore, compared to deterministic parameter-based studies, multilevel uncertainty-based optimization can avoid the overestimation or underestimation of system performance (Zhou and Zheng, 2020; Zhou et al., 2020d,e). In this study, a holistic and systematic literature review was conducted on system design and operation of PCMintegrated cooling systems, together with a comprehensive literature review of novel PCM-based strategies. The heat-transfer mechanism and modeling were characterized to demonstrate the underlying mechanism. Solutions for cooling performance enhancement are presented, with respect to distributed PCM integration (such as building envelopes and roofs) and coupled systems. Scientific gaps based on the current literature are listed, which can serve as avenues for upcoming research, including exergy-based optimization, multiobjective optimizations for technoeconomic performance improvement, and accurate descriptions of multilevel uncertainties of scenario parameters through combinations of various distributions of density function.

15.2

Systematic literature review of novel PCM-based strategies for building cooling performance

15.2.1 PCM-integrated cooling system design 15.2.1.1 Building envelopes and roofs In academia, both passive and active systems have been designed, with respect to different PCM-integrated systems. PCMs can be integrated with building envelopes, roofs/ceilings, and combined building components. Research parameters include PCM location, melting temperature, PCM thickness, pipe diameter, and so on. Research objectives include indoor air temperature, indoor thermal comfort, cooling energy consumption, temperature fluctuation rate, and energy savings, among others. Table 15.1 gives a holistic overview of parametric analysis on PCM-integrated cooling systems. With respect to PCM-integrated passive systems, Singh and Bhat (2018) designed a PCM-integrated ceiling to reduce the building cooling load. The results indicated that the melting temperature of the single PCM gypsum board at 40°C would contribute to a low cooling load. With respect to PCM-integrated active systems, Yan et al. (2020) designed a combined system with PCM wall and nocturnal sky radiation roof. Research results indicated that 54.7%–81% of the heat could be removed by the nocturnal radiation cooler. Researchers have also focused on integrated passive and active systems with flexible control for natural energy utilization and indoor thermal comfort improvement. Zhou et al. (2019b) proposed a combined system, with a ventilated roof. Through the parametric analysis, the solar cell temperature was reduced from 70.9°C to 32.8°C, and the photovoltaic efficiency was increased from 11.5% to 14.99%. Belmonte et al. (2015) studied the energy

Table 5.1 A holistic overview of parametric analysis on PCM-integrated cooling systems. Components Building envelope

Studies

System forms

Variables

Objectives

Exterior fac¸ade

Arıcı et al. (2020)

Passive systems

Optimization of PCM location, melting temperature, and thickness

Energy savings, decrement factor, and time lag

Interior fac¸ade

Plytaria et al. (2019) Lizana et al. (2019)

–

Indoor temperature

System configurations and energy-related occupants’ behaviors

Indoor thermal comfort

Singh and Bhat (2018) Belmonte et al. (2015)

PCM melting temperature

Cooling load

–

Cooling demand

Thermal conductivity, PCM thickness

Energy consumption

Roof/ceiling

Combined components Building envelope

Exterior fac¸ade Interior fac¸ade

Roof/ceiling

Combined components Combined components

Zhou et al. (2016a) Zhou et al. (2019a) Hanif et al. (2014)

Active system

PCM thickness and pipe diameter –

Zhou et al. (2017) Weinl€ader et al. (2014) Yan et al. (2020) Zhou et al. (2019b, 2020c)

–

Photovoltaic efficiency Indoor air temperature Removed accumulated heat

PCM melting temperature, PCM thickness, and mass flow rate of cooling water

Removed accumulated heat

Inlet water temperature, mass flow rate –

Integrated passive/ active systems

Equivalent overall energy generation Cooling energy storage

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performance of a passive PCM floor system, coupled with a chilled ceiling system. The PCM floor system reduced the cooling demand by 50%. Plytaria et al. (2019) studied the impact of PCM on the indoor air temperature, with PCM integrated in a radiant wall. Compared to the system without PCM, the PCM-integrated building fac¸ade reduced the indoor temperature by 0.6°C. Integrated systems with both passive and active solutions have attracted researchers’ attention, to maximize the cooling energy storage and release efficiency of PCMs. Zhou et al. (2019b) proposed a new PCM-integrated Trombe wall, with passive and active cooling and hybrid ventilations. Flexible operations between passive, active, and hybrid modes are available, depending on meteorological parameters and the indoor built environment. Weinl€ader et al. (2014) studied the cooling performance of a PCM ceiling with mechanical ventilation. Zhou et al. (2017) proposed a PCMintegrated PV/T system, with active air-based and water-based cooling, nocturnal radiation, and passive natural ventilation. The proposed system and control strategy show promising potential for energy performance enhancement.

15.2.1.2 Combined strategies and systems Geocooling + PCM thermal energy storage Geothermal energy, one of the promising renewable sources, has been extensively investigated in recent studies. Below a certain depth (commonly, 3–4 m), the soil temperature generally remains fairly stable. Geothermal energy can be a promising cooling source for space cooling in summer. As one of the most commonly utilized forms of geothermal energy, the earth to air heat exchanger (EAHE) has been investigated to reduce energy consumption of buildings and increase indoor thermal comfort (Hollmuller and Lachal, 2014), due to its simple structure and low operating costs (Li et al., 2019b; Cuny et al., 2020). The underground soil acts as a heat sink in the EAHE system, which consists of a blower and buried pipes. Ambient air is precooled by the buried pipes to regulate the indoor and built environment. A traditional EAHE for building cooling in summer is shown in Fig. 15.1. Belatrache et al. (2017) adopted the EAHE as air-conditioning equipment for building cooling in Algeria. Tittelein et al. (2009) indicated that the EAHE can reduce energy consumption by 7 kWh/(m2 year). Fazlikhani et al. (2017) analyzed the thermal performance of an EAHE system in two different climates (i.e., a hot-arid climate for Yazd and a cold climate for Hamadan). The EAHE system can lead to energy savings of 50.1%–63.6% and 24.5%–47.8% for these two areas, respectively. EAHE systems are promising for passive cooling of buildings. However, several issues need to be resolved. First, the conventional EAHE systems are mostly designed with horizontal buried pipes. For precooling purposes, a large land area is required to bury long pipes, which is infeasible for densely populated regions, such as the subtropical region of Hong Kong. Although the pipe can be organized in some special forms to reduce the land use, e.g., multiple pipes and horizontal loop pipes (Mustafa Omer, 2008) (as shown in Fig. 15.2), a relatively large occupied area is still necessary. Second, with the consideration of construction costs of buried pipe, the pipes of EAHE systems are commonly designed to be buried at a depth of about 2–5 m. The cooling

Influence of novel PCM-based strategies on building cooling performance

333

Fig. 15.1 Operation schematic diagram of an EAHE system for cooling applications in summer.

Fig. 15.2 The horizontal loop and multiple pipe types for an EAHE system (Mustafa Omer, 2008). Partially reprinted and partially redrawn from Mustafa Omer, A., 2008. Ground-source heat pumps systems and applications. Renew. Sust. Energ. Rev. 12, 344–371, Copyright (2008), with permission from Elsevier.

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performance is dependent on the local rainfall due to the shallow depth of the buried pipes. In academia, the deviation of the required soil temperature in summer was investigated. For instance, Soni et al. (2016) reported that, at the buried depth of 2.7 m, the soil temperature was about 30°C in Bhopal. Benhammou et al. (2017) reported that the surrounding soil temperature of buried pipe at 3 m depth was 27.43°C. It is clear that soil with such high temperatures cannot be considered as an appropriate heat sink for building cooling in summer. Third, with the conventional EAHE, it is not easy to collect and discharge condensate water on tube walls in a timely manner in summer, due to the low slope of the buried tubes. Although some researchers have set the slope of the buried tube at 1–10 degrees to collect the concentration of condensate water in a more timely manner, the slope is still too small to effectively solve this technical issue (Uddin et al., 2016). Furthermore, the increased slope of the buried pipes increases the initial investment and construction difficulty of the EAHE systems. In addition, for a given EAHE system, the outlet air temperature is dependent on the ambient air temperature (Li et al., 2019c), and a 3°C temperature oscillation can be noticed in the literature (Yıldız et al., 2012; Menhoudj et al., 2018; Bansal et al., 2012; Ahmed et al., 2016). Further studies are needed to focus on the system design and operation of EAHE systems with stable outlet air temperature. To solve the previously mentioned issues, a vertical earth to air heat exchanger (VEAHE) coupling with an annular PCM component was proposed in (Liu et al., 2019b), with the schematic diagram as shown in Fig. 15.3. In the first stage, the U-tube of the VEAHE system was buried in a foundation pit. The diameter and depth of this foundation pit were 1000 mm and 16.5 m, respectively. A bypass section was designed at the bottom of the U-tube to collect the condensate water and drainage. In the second stage, an annular PCM component was placed in the 3.6-m depth tube. The PCM used was paraffin due to its low environmental impact and long-term chemical stability (Liu et al., 2018b, 2019b). The specific dimensions and construction implementation of the proposed system are presented in Fig. 15.4. This proposed system has four main advantages compared to traditional EAHE systems. First, it has a small land usage (around 1 m2 for each U-tube). Such a small land requirement allows this system to be used in more situations, including highly densified built-up areas. Second, compared to conventional EAHE systems with their horizontal buried tubes, the VEAHE system is typically designed with a depth of below 10 m. In this case, the soil temperature is more stable and closer to the appropriate heat sink temperature (e.g., 18–20°C) (Xi et al., 2017; Zhou et al., 2016b). Third, as the slope of the buried pipe is 90 degrees, the VEAHE system can centrally discharge the condensate water from the pipe wall in a timely manner and avoid the long-term adhesion of the condensate water on the pipe wall (Liu et al., 2019c). In addition, the annular PCM component in the VEAHE system can absorb and release a large quantity of latent heat during its phase transition process and thus provide a stable outlet air temperature. It should be noted that, compared to traditional EAHE systems, the PCM used in the VEAHE system will lead to a higher initial investment (Liu et al., 2019d). System investors need to conduct a technoeconomic analysis to comprehensively evaluate the feasibility.

Influence of novel PCM-based strategies on building cooling performance

Fan

335

Air valve

Filter

Indoor air

15.5 m

Insulation

A 16.5 m

7.5 m

A Annular PCM

3.6 m

Outdoor air

Bypass structure

Æ 19

0

m

m

Æ319 mm

Inlet 681 mm Æ219 m m

Outlet Insulation Annular PCM

Æ 10

00

A-A 1:2

mm

Fig. 15.3 Schematic diagram and cross-section of a PCM vertical earth to air heat exchanger (VEAHE) system (Liu et al., 2019b). Partially reprinted and partially redrawn from Liu, Z., Yu, Z., Yang, T., El Mankibi, M., Roccamena, L., Sun, Y., Sun, P., Li, S., Zhang, G., 2019. Experimental and numerical study of a vertical earth-to-air heat exchanger system integrated with annular phase change material. Energy Convers. Manag. 186, 433–449, Copyright (2019), with permission from Elsevier.

High-reflective coating, radiative cooling wall, and hybrid ventilations with ventilated roof In academia, the PCM-based strategies for building cooling performance include high-reflective coatings, intelligent ventilations for heat dissipation, and radiative cooling, together with on-site PV generation for self-consumption. A state-of-theart review of each technology is presented here to show the research progress over the past years. The high-reflective coating layer In academia, a high-reflective coating layer in building envelopes and roofs is an effective solution. Herna´ndez-Perez et al. (2018) and Karlessi et al. (2011) experimentally studied the impact of different coatings on the daily heat gain. The research results indicated that roof daily heat gain of the white reflective coating was 80% lower than the gray roof. Guo et al. (2012)

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Fig. 15.4 (A) On-site experimental test rig of hole excavation and U-tube burial; (B) on-site experimental test rig of annular PCM and geometrical dimensions (Liu et al., 2019b). Partially reprinted and partially redrawn from Liu, Z., Yu, Z., Yang, T., El Mankibi, M., Roccamena, L., Sun, Y., Sun, P., Li, S., Zhang, G., 2019. Experimental and numerical study of a vertical earth-to-air heat exchanger system integrated with annular phase change material. Energy Convers. Manag. 186, 433–449, Copyright (2019), with permission from Elsevier.

indicated that a high-reflective coating on an exterior wall surface could decrease the peak temperature about 8–10°C, together with electricity consumption savings at 5.8 kWh/(m2 month). In addition to the high-reflective coating only, the integration of a high-reflective coating with PCMs showed promise for cooling applications. Meng et al. (2019) experimentally studied the impact of a high-reflective phase change material (PCM) roof on the indoor thermal environment. Results indicated that

Influence of novel PCM-based strategies on building cooling performance

337

the high reflectivity film could significantly reduce the heat gain from solar radiation. Karlessi et al. (2011) indicated that PCMs coated with high-reflective coatings could improve thermal inertia and achieve considerable energy savings. Intelligent ventilations Arkar and Medved (2007) adopted a PCM in ventilation systems for free cooling applications. The optimal fusion temperature was between 20°C and 22°C for a continental climate. In a hot summer and cold winter region, Zhou et al. (2018a) optimized the melting temperatures of exterior and interior PCM at 26°C and 22°C, through the heat transfer mechanism for the maximization of energy storage and release efficiency. Liu et al. (2017) studied the geometric dimensions of a PCM-integrated ventilation system. The average thermal energy was 1.02 kJ, when height and length were 0.03 and 1.5 m. The nighttime ventilation was promising for building energy savings, especially when being integrated with latent storage. Barzin et al. (2015) indicated that, compared to the PCM system without the ventilation strategy, a weekly electricity saving of 73% could be realized when adopting night ventilation to charge the PCMs. Alizadeh and Sadrameli (2019) experimentally studied the indoor thermal comfort of a PCM-integrated ventilation system. Research results indicated that the overheating could be reduced by 13.83% with a PCM efficiency of 56% for the cooling scenario. Panchabikesan et al. (2019) conducted a technical feasibility assessment on PCM cooling systems. The results indicated that, contrarily to the temperate and cold climates, the applicability of PCM-based free cooling systems in hot-dry, warm-humid climates was questionable. Effective solutions need to be well investigated to improve the operational feasibility, such as the evaporative cooling. It should be noted that the power consumption of active devices, such as fans and pumps, might affect the technical effectiveness of the ventilation solutions. Researchers have focused on the optimization of ventilation solutions to enhance the technical competitiveness. Ousegui et al. (2019) adopted the inverse method to optimize the air flow rate of fans to achieve complete melting/solidification of PCM for free cooling. The proposed model was quite accurate, with the relative error lower than 5
103. Tang et al. (2020b) conducted an exergy-based optimization on a PCM system using a supervised machine-learning method. Results indicated that the overall exergy was improved by 2.6%, from 849.9 to 872.06 kWh. Radiative cooling—Active water-based and radiative sky cooling In a semiarid climate, Katramiz et al. (2020) comparatively studied the diurnal and daytime radiative cooling, in terms of the system storage capacity and the operational cost. Research results indicated that, compared to the night only radiative cooling panel, the diurnal radiative cooling could reduce the storage system size by 12.3% with a cost savings of 10%. He et al. (2019) studied the thermal and energy performances of a novel radiative PCM cooling wall. Compared to the average indoor temperature of a brick wall at 27.8°C and a PCM wall at 27.3°C, the average indoor temperature was 27.0°C for the radiative PCM cooling wall. Furthermore, the cooling load of the room with the radiative PCM cooling wall was reduced by 47.9% and 23.8%, compared to rooms with a brick wall and PCM wall, respectively. In addition to active water-based radiative cooling systems, radiative sky cooling has attracted researchers’

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

interest. Zhao et al. (2020) studied the impact of radiative sky cooling on the performance of a thermoelectric cooling system. The radiative sky cooling can contribute 45.0%–55.0% on annual cooling energy generation, with improvement of annual COP at 1.87. The state-of-the-art review indicated that each solution is promising for improving the cooling performance of PCM-integrated systems. A hybrid system was proposed by Zhou et al. (2017), as shown in Fig. 15.5. An integrated solution, coordinating a

Fig. 15.5 An integrated hybrid renewable system (Zhou et al., 2017, 2020a): (A) structural configuration; (B) system operation during the daytime; (C) system operation during the night. Partially reprinted and partially redrawn from Zhou, Y., Zheng, S., Zhang, G., 2020. Machine learning-based optimal design of a phase change material integrated renewable system with onsite PV, radiative cooling and hybrid ventilations—study of modelling and application in five climatic regions. Energy. https://doi.org/10.1016/j.energy.2019.116608, Copyright (2018), with permission from Elsevier.

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high-reflective coating, intelligent ventilations for heat dissipation, radiative cooling, together with on-site PV generation for self-consumption, was adopted to fully utilize the advantages of each technology. A temperature-dependent control strategy was implemented in the system to intelligently control the operating mode of each ventilation vent, the power of active fans and pumps, and the selective coating layers on the surface of exterior PCM wallboard. The transmitted solar radiation was reflected by the exterior high-reflective coating, to mitigate the heat accumulation on the exterior PCM wallboard. As shown in Fig. 15.5B, during the summer daytime, air vents 1, 2, 4, and 5 were open, to dissipate the accumulated heat through natural and mechanical ventilations. The active cooling water system in the interior PCM wallboard was turned on to provide radiative cooling for the regulation of the indoor thermal environment. The active cooling water in the PV/T—PCM system was turned on to facilitate the PV cooling. The extracted heat could be utilized for domestic hot water heating purposes. During the nighttime, as shown in Fig. 15.5C, the nocturnal sky radiation was designed to charge the PCMs for cooling storage. The stored cooling energy could then be used for heat removal during the daytime. Furthermore, air vents 2, 4, and 6 and the ventilation fan were open for two purposes: (1) heat transfer enhancement between air and the PCM wallboard; (2) the fresh air supply to the indoor environment.

15.2.2 System assessment criteria of the novel PCM-based cooling system In academia, as listed in Table 15.2, the multicriteria of the novel PCM-based cooling performance can be generally summarized as: (1) the cooling load reduction (Liu et al., 2018a; Zhou and Yu, 2018); (2) thermal storage density (Ling et al., 2019) and cooling energy storage and release efficiency (Ling et al., 2019); (3) thermal energy storage (Nagano et al., 2006) and operative temperature of indoor air (Weinl€ader et al., 2014). Table 15.1 lists the multicriteria of PCM-integrated cooling systems. The analysis of the PCM-integrated Trombe wall (Zhou and Yu, 2018) indicated that the cooling load can be reduced by 14.8%. Ling et al. (2019) proposed a fitting curve to guide the selection of thermal storage density of PCMs. The heat transfer mechanism of the sophisticated hybrid renewable system was characterized in (Zhou et al., 2018a). Weinl€ader et al. (2014) indicated that a PCM ceiling can reduce the room temperature by 2°C.

15.3

Heat-transfer mechanism and modeling of PCM-integrated building energy systems

15.3.1 Building envelopes and roofs For PCM-integrated building envelopes and roofs, several researchers are focused on the characterization of the underlying heat transfer mechanism, regarding various operational modes. A holistic overview of cooling scenarios was demonstrated in

Table 15.2 Multicriteria of PCM-integrated cooling systems. System PCMbuilding envelopes and roofs

Combined strategies and systems

Variables

System assessment criteria

PCM-ventilated wall (Ling et al., 2019; Liu et al., 2018a; Zhou and Yu, 2018)

Melting temperature (Ling et al., 2019; Liu et al., 2018a; Zhou and Yu, 2018) and latent heat (Ling et al., 2019)

Ceiling (Weinl€ader et al., 2014)/ floor (Nagano et al., 2006)

Inlet air temperature and air velocity (Nagano et al., 2006); volume flow rate (Weinl€ader et al., 2014); inlet and outlet air temperature difference (Nagano et al., 2006) Mass flow rate (Erell and Etzion, 2000)

Cooling energy consumption savings (Liu et al., 2018a), heating/cooling load (Zhou and Yu, 2018), heat storage density (Ling et al., 2019), heat storage and release efficiency (Ling et al., 2019), and minimum indoor air temperature (Ling et al., 2019) Operative room temperature (Weinl€ader et al., 2014); cooling power (Weinl€ader et al., 2014) and heat storage (Nagano et al., 2006)

PCM-integrated radiative system (Yan et al., 2020; Erell and Etzion, 2000; Hanif et al., 2014; Cuny et al., 2018) Geocooling + PCM thermal energy storage (Liu et al., 2019b) High-reflective coating, radiative cooling and hybrid ventilations

PCM thermal conductivities, PCM thicknesses, PCM length (Liu et al., 2019b) PCM thickness, thermal conductivity, fusion temperature, pipe diameter, mass flow rate, chiller capacity, supply water temperature, and power of active devices (Zhou et al., 2019a, c, 2020a,b)

Heat flow (Weinl€ader et al., 2014); accumulated heat (Yan et al., 2020); net cooling energy (Erell and Etzion, 2000) and cooling power (Hanif et al., 2014) Outlet air temperature, static payback period (Liu et al., 2019b) Heat transfer (Zhou et al., 2019a, 2020a), equivalent overall output energy (Zhou et al., 2019a,c, 2020a,b), and overall exergy (Tang et al., 2020b)

Influence of novel PCM-based strategies on building cooling performance

341

(Zhou et al., 2018a). In order to evaluate the energy storage and release efficiency of PCMs, the moving boundary method was adopted to characterize the solid-liquid interface. In the phase change period, both conductive and convective heat transfer processes occur in PCMs, simultaneously. The convective heat transfer is due to the density difference between the solid and liquid PCMs, resulting in the mass flow inside the PCM. The most widely adopted approach for modeling the latent heat during the dynamic charging/discharging process is the enthalpy-porosity approach (Brent et al., 1988). The convective heat transfer in the melted PCM was ignored in most numerical models, and only a few studies were focused on the Boussinesq approximations to quantify the density difference. Most researchers are focused on a simplified one-dimensional model to characterize the sophisticated heat transfer mechanism, due to the considerable computational efficiency and accuracy.

15.3.2 Coupled systems The thermal performance of EAHE systems is mainly dependent on the air mass flow rates, tube materials, tube diameters, tube depths, soil types, and other design parameters. Numerical models are necessary for the technoeconomic feasibility assessment, such as a hybrid EAHE system model (Kaushal et al., 2015) and a three-dimensional model (Zhou et al., 2018b). However, there are few numerical models of VEAHE systems. In addition, for the existing models of EAHE systems, the soil temperature around the tube is commonly considered as uniform, without considering the temperature variation at different depths. The main reason is that the heat transfer between flowing air and surrounding soil mainly occurs at a depth of about 2–5 m due to the horizontally buried tubes (Singh et al., 2018). However, for the VEAHE systems, heat exchange between air and soil occurs at different depths, due to the vertically buried tubes. Contrary to the assumption of the constant soil thermophysical properties for existing models of EAEH systems, the soil thermophysical properties in VEAHE systems vary with the increase of depth. The variation of soil thermophysical properties along the vertical direction should be considered in modeling VEAHE systems (Bansal et al., 2013; Cuny et al., 2018). Liu et al. (2019b) proposed a numerical model of VEAHE systems coupled with annular PCM components. The numerical model can be divided into four portions based on the system’s structure, as shown in Fig. 15.6. For the development of a system’s model, the output of each divided portion model can be considered as the input of another divided portion. Therefore, the model of each divided portion can be developed independently, and then they are linked together through MATLAB/ Simulink to investigate the entire system’s performance. The nodal discretization of different media is shown in Fig. 15.7. The temperatures of different media at time t and t  dt are denoted as Tt
and Ttdt. The average temperature between the time t and t  dt is T ¼ T t + T tdt =2. The heat exchange governing equations can be built based on the law of energy conservation (Bahrar et al., 2018). The prediction accuracy is shown in Fig. 15.8, with a maximum relative error of 1.34%.

342

Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

Fig. 15.6 The division of the vertical earth to air heat exchanger (VEAHE) system integrated with the annular PCM (Liu et al., 2019b). Partially reprinted and partially redrawn from Liu, Z., Yu, Z., Yang, T., El Mankibi, M., Roccamena, L., Sun, Y., Sun, P., Li, S., Zhang, G., 2019. Experimental and numerical study of a vertical earth-to-air heat exchanger system integrated with annular phase change material. Energy Convers. Manag. 186, 433–449, Copyright (2019), with permission from Elsevier.

15.4

System performance enhancement of novel PCM-based cooling systems

15.4.1 Building envelopes and roofs To improve the cooling performance of the PCM-integrated systems, researchers are mainly focused on the thermal conductivity enhancement (Li et al., 2019a), PCM-integrated side-wall cooling (Zhou et al., 2016a), scenario uncertainty-based performance analysis (Zhou et al., 2020b), multivariable optimization (Zhou et al., 2019a,c; Liu et al., 2019a), and optimal designs for widespread applications (Zhou et al., 2020a). Zhou et al. (2016a) numerically studied the cooling performance of a PCMintegrated side-wall cooling system, with energy consumption savings at 4.53%. In order to maximize the energy performance of the PCM-integrated cooling system, Zhou et al. (2019a) developed a surrogate model to be flexibly integrated with advanced optimization algorithms to optimize the design and operation of a new hybrid system for cooling applications. For widespread application purposes in different climatic regions, Zhou et al. (2020a) optimized a novel PCM-based cooling system for climate-adaptive designs. Compared to the Taguchi standard

Fig. 15.7 Demonstration of nodes for different media (Liu et al., 2019b). Partially reprinted and partially redrawn from Liu, Z., Yu, Z., Yang, T., El Mankibi, M., Roccamena, L., Sun, Y., Sun, P., Li, S., Zhang, G., 2019. Experimental and numerical study of a vertical earth-to-air heat exchanger system integrated with annular phase change material. Energy Convers. Manag. 186, 433–449, Copyright (2019), with permission from Elsevier.

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

Fig. 15.8 Modeling validation on outlet air temperature and PCM temperature.

orthogonal array, the machine-learning optimization can improve the equivalent overall output energy by 4.2% in Shanghai, by 7.4% in KunMing, by 4.3% in GuangZhou, by 1.1% in Hong Kong, and by 5.4% in HaiKou. General approaches for machine learning-based surrogate models development of PCM-integrated cooling systems are shown in Fig. 15.9; it can be seen that multiple input variables of the renewable system act as an input matrix. An artificial neural network (ANN) was thereafter configured and structured. The output layer includes the multiobjectives (such as charging/discharging rate, energy storage, equivalent overall output energy, energy storage and release efficiency, and so on). Furthermore, the proposed strategy can also be used for multilevel uncertainty optimizations (Zhou and Zheng, 2020). As parameters have uncertainties, Zhou et al. (2020b) investigated multidimensional energy predictions with high-level uncertainties. The research results indicated that the multilevel uncertainties can improve the peak power from 11.5 to 20 kW, and the on-site renewable generation from 1776.9 to 2635.6 kWh by 48.3%. The main principles for the multilevel parameters’ uncertainty-based performance study, demonstrated in Fig. 15.10, are: (1) The first stage is the selection of multidimensional scenario parameters, including both geometrical and operating parameters. (2) Stage 2 is the uncertainty quantification, to quantify both aleatory and epistemic uncertainties. (3) In Stage 3, the cooling performance is predicted using the trained surrogate model, with high computational efficiency. (4) Afterwards, the robust analysis is studied in Stage 4 to show the cooling performance response for multidimensional scenario uncertainties.

Influence of novel PCM-based strategies on building cooling performance

System design-geometrical and operating parameters

345

Supervised machine learning: feature extraction and classification

Output layer: multiobjective

Subsystem 1 (I) Inlet cooling water temperature (°C) (II) Thickness of PCM of exterior PCM (mm) (III) Thickness of PCM of interior PCM (mm) ……

Subsystem 2 (I) Diameter (m) (II) Mass flow rate in (kg/s) (III) Inlet cooling water temperature (°C) ……

wi,j wi,j (I) PCM Charging/discharging rate; (II) Energy storage; (III) Equivalent overall output energy; (IV) Energy storage and

Subsystem 3

release efficiency

(I) Diameter (m) (II) Mass flow rate in (kg/s) (III) Inlet cooling water temperature (°C) ……

Fig. 15.9 General steps for machine learning-based surrogate models on PCM-integrated cooling systems.

15.4.2 Coupled systems In the last several decades, the usages of different PCM components for energy-saving systems have been considered as one of the most promising technologies (Liu et al., 2018b; Tang et al., 2020a). The influence of the annular PCM on the VEAHE system performance was investigated in the study (Liu et al., 2019b), as shown in Fig. 15.11. As shown in Fig. 15.11, the system with PCM has a smaller outlet temperature fluctuation. Specifically, as the air velocity is 1 m/s, the outlet temperature of the VEAHE system with annular PCM ranges from 21.67°C to 23.48°C, and the temperature oscillates within a range of 21.39°C to 24.02°C for the VEAHE system without annular PCM. When the air velocity becomes 2 m/s, the outlet temperatures of the PCMintegrated VEAHE system fluctuate between 22.11°C and 24.84°C, and between 21.91°C and 25.75°C for the VEAHE system without annular PCM. Thus the annular PCM can effectively decrease the magnitude of the outlet temperature fluctuation of 0.82°C and 1.11°C for the air velocity of 1 and 2 m/s, respectively. This indicates that PCMs can effectively decrease the outlet temperature fluctuation and the peak temperature through the melting and solidification processes, especially for a larger air velocity.

346

Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

X

Fig. 15.10 Demonstration of the multilevel uncertainty-based analysis of PCM-integrated cooling systems with deterministic parameter-based optimal design.

In addition, the influences of PCM thicknesses and PCM length on the VEAHE system performance were also investigated in (Liu et al., 2019b), as shown in Figs. 15.12 and 15.13. As shown in Fig. 15.12, the outlet temperature fluctuation decreases from 3.34°C to 2.83°C when the PCM thickness increases from 1 to 5 mm. A slight decrease from 2.83°C to 2.69°C can be noticed when the PCM

Influence of novel PCM-based strategies on building cooling performance

347

Fig. 15.11 Parametrical analysis of different air velocities. (Note: the PCM length is 3.6 m and PCM thickness is 10 mm.)

thickness increases from 5 to 20 mm. It can be noted that the PCM thickness at 5 mm can be recommended as a cost-effective solution, due to the almost saturated outlet temperature and the initial PCM cost. As shown in Fig. 15.13, the outlet temperature variation decreases with the increase of PCM lengths. When the PCM length increases from 1 to 7 m, the temperature fluctuation range of the outlet temperature varies from 3.20°C to 2.17°C with a temperature change of 1.03°C. The outlet temperature fluctuation ranges from 2.17°C to 1.99°C with a temperature change of 0.28°C when the PCM length increases from 7 to 13 m. It is obvious that a downward trend for the outlet temperature fluctuation can be noticed with the increase of PCM length. Through the parametrical analysis, the PCM length at 7 m is recommended as an effective solution.

15.5

Discussion of real applications and future prospects

15.5.1 Building envelopes and roofs The state-of-the-art review presents a systematic methodology for the multilevel uncertainty-based performance, optimal design, and robust operation of PCMintegrated cooling systems. The proposed technique can provide concrete guidance to system designers and operators. Regarding the active system, the energy contradictions between the input electricity for pumps and fans and the output energy can be noticed. These contradictions can be effectively and efficiently solved by the exergy-

348

Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

Fig. 15.12 Parametrical analysis of PCM thicknesses. (Note: the PCM length is 3.6 m, and air velocity is 2 m/s.)

Fig. 15.13 Parametrical analysis of PCM lengths. (Note: the PCM thickness is 10 mm, and the air velocity is 2 m/s.)

based optimization. Furthermore, regarding the uncertainty analysis, difficulties can be found in the mathematical description of multilevel scenario uncertainties. Future studies can focus on combinations of various distributions of density function to accurately characterize the percentage of uncertainty.

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15.5.2 Coupled systems Compared to traditional EAHE system, the VEAHE system coupled with the annular PCM component has several advantages, e.g., less occupied area, higher efficiency of energy utilization, more conducive to centralize discharge of condensate water, and smaller outlet temperature fluctuation (Liu et al., 2019b,c). However, this system may require a larger initial investment due to the construction cost of the buried tube and annular PCM component (Liu et al., 2019b,d). Future studies could focus on system optimization for practical applications, comprehensively considering the initial investment, coefficient of performance (COP), and economic efficiency. In addition, for continuous long-term operation, the increase of soil temperature due to the released heat from the buried tube will affect the system’s operating efficiency. The combinations of the VEAHE system and night ventilation or PCM wall technologies can be promising solutions to improve the stability and robustness for long-term operations.

Acknowledgment This research is supported by the Hong Kong Polytechnic University, the City University of Hong Kong, and Hunan University. The authors also thank the editors for their useful comments and suggestions.

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Zhou, T., Xiao, Y., Liu, Y., Lin, J., Huang, H., 2018b. Research on cooling performance of phase change material-filled earth-air heat exchanger. Energy Convers. Manag. 177, 210–223. Zhou, Y., Zheng, S., 2020. Multi-level uncertainty optimisation on phase change materials integrated renewable systems with hybrid ventilations and active cooling. Energy. https://doi. org/10.1016/j.energy.2020.117747. Zhou, Y., Zheng, S., Zhang, G., 2019a. Artificial neural network based multivariable optimization of a hybrid system integrated with phase change materials, active cooling and hybrid ventilations. Energy Convers. Manag. https://doi.org/10.1016/j.enconman.2019.111859. Zhou, Y., Zheng, S., Zhang, G., 2019b. Study on the energy performance enhancement of a new PCMs integrated hybrid system with the active cooling and hybrid ventilations. Energy 179, 111–128. Zhou, Y., Zheng, S., Zhang, G., 2019c. Multivariable optimisation of a new PCMs integrated hybrid renewable system with active cooling and hybrid ventilations. J. Build. Eng. 26, 100845. Zhou, Y., Zheng, S., Zhang, G., 2020a. Machine learning-based optimal design of a phase change material integrated renewable system with on-site PV, radiative cooling and hybrid ventilations—study of modelling and application in five climatic regions. Energy. https:// doi.org/10.1016/j.energy.2019.116608. Zhou, Y., Zheng, S., Zhang, G., 2020b. Machine-learning based study on the on-site renewable electrical performance of an optimal hybrid PCMs integrated renewable system with high-level parameters’ uncertainties. Renew. Energy. https://doi.org/10.1016/j.renene. 2019.11.037. Zhou, Y., Zheng, S., Zhang, G., 2020c. A review on cooling performance enhancement for phase change materials integrated systems—flexible design and smart control with machine learning applications. Build. Environ. https://doi.org/10.1016/j.buildenv. 2020.106786. Zhou, Y., Zheng, S., Liu, Z., Wen, T., Ding, Z., Yan, J., Zhang, G., 2020d. Passive and active phase change materials integrated building energy systems with advanced machinelearning based climate-adaptive designs, intelligent operations, uncertainty-based analysis and optimisations: a state-of-the-art review. Renew. Sustain. Energy Rev. https://doi.org/ 10.1016/j.rser.2020.109889. Zhou, Y., Zheng, S., Zhang, G., 2020e. A state-of-the-art-review on phase change materials integrated cooling systems for deterministic parametrical analysis, stochastic uncertainty-based design, single and multi-objective optimisations with machine learning applications. Energy Build. https://doi.org/10.1016/j.enbuild.2020.110013.

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Optically smart thin materials for building cooling

16

Omar Iken, Rachid Agounoun, Imad Kadiri, Miloud Rahmoune, Khalid Sbai, and Rachid Saadani Advanced Materials and Applications Laboratory, Higher School of Technology, Moulay Ismail University, Meknes, Morocco

Nomenclature Ab a as b c ΔT EF g he Ielec ING LiNbO3 Na2WO
H2O NH4F NiOH:Li Rs Sr, b SnCl4
5H2O Ta Tc Tsa Tsurr TL U VO(acac)2

The floor space (m2) ˚) Length of the crystallographic unit cell (A Surface absorptivity ˚) Width of the crystallographic unit cell (A ˚) Height of the crystallographic unit cell (A Transmittance modulation Fermi energy (eV) Solar factor External convective-radiative heat transfer coefficient (W/K m2) Carbon intensity due to electricity generation Carbon intensity due to natural gas Lithium niobate Sodium tungstate dehydrate Ammonium fluoride Alloy of nickel hydroxide and lithium Solar radiation (W/m2) Total building stock surface (m2) Tin chloride pentahydrate Air temperature (°C) Transition temperature (°C) Solar-air temperature (°C) Average surrounding surface and sky temperature (°C) Light transmittance factor Thermal transmittance (W/m2 K) Vanadyl acetylacetonate

Abbreviations APCVD APPJ CFD CG

Atmospheric pressure chemical vapor deposition Atmospheric pressure plasma jet Computational fluid dynamics Clear glass

Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction https://doi.org/10.1016/B978-0-12-820791-8.00016-X © 2021 Elsevier Ltd. All rights reserved.

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CVD DBEC DC FTO IR ITO LPCVD M1 M2 MC MIT MOCVD PAD PCM PEC PECVD PET PLD PVD PVP R RF SC SDSF SG SPT TCC TCO USPD VIS WWR

Chemical vapor deposition Dual band electrochromic glazing Direct current Fluorine-doped tin oxide Infrared Indium tin oxide Low pressure chemical vapor deposition Monoclinic 1 Monoclinic 2 Monte Carlo Metal insulator transition Metal organic chemical vapor deposition Polymer-assisted deposition Phase change material Primary energy consumption Plasma-enhanced chemical vapor deposition Polyethylene terephthalate Pulsed laser deposition Physical vapor deposition Poly(vinylpyrrolidone) Rutile Radio frequency Solar control glazing Smart double skin facade Selective glass Structural phase transition Total construction cost Transparent conducting oxide Ultrasonic spray pyrolysis deposition Visible spectrum Window-to-wall ratio

Greek symbols β e λ σ

Angle formed in the unit cell between the length a and the height c Surface emissivity Wavelength (nm) Stefan-Boltzmann constant (W/m2 K4)

16.1

Introduction

The buildings sector is considered to be the highest energy consumer with 20%–40% of the global energy consumption (US Department of Energy, 2017). This energy is mainly used for ventilation, lighting, and air conditioning, especially in hot and humid regions (Al-Rabghi and Hittle, 2001; Kwak et al., 2010). In order to reduce this level of consumption, massive traditional thermal insulation materials based on thermal

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inertia have been used. In the last 20 years, architectural standards have moved to more visual contact with the external environment (Zhu and Wang, 2019) in order to create a more natural living atmosphere. Thus a big demand in the glass market for building glazing has been seen in the last few years, estimated at 9.2 billion m2/year (The Freedonia Group, 2013). This trend to glazed buildings has generated an increase in air-conditioning energy consumption due to the low thermal insulation properties of the glass (Grynning et al., 2013). Consequently, the need to thermally insulate the glazed surfaces has oriented researchers toward the development of smart optically thin films known as “chromogenics” in order to regulate the radiative heat transfer through the coated glazing. The optical smartness of these materials can be defined as a reversible variability of their optical properties depending on an external stimulus, such as an electrical signal for electrochromic materials (Somani and Radhakrishnan, 2003), heat for thermochromic materials (Ke et al., 2018), and light for photochromic materials (Yao et al., 1992). In fact, as a response to the external stimulus, these materials switch reversibly between a transparent and a reflective state in the case of thermochromics, and between transparent and optically absorbing states in the case of electrochromics and photochromics for visible and near-infrared radiations, which are responsible for radiative heat transfer. The objective of this chapter is to shed light on two types of smart optically thin materials with a high potential in building energy-saving applications. The mechanisms that manage thermochromic and electrochromic materials are detailed in addition to their classical and new fabrication methods, which are necessary for construction market products. The integration of optically smart thin materials in the building envelope such as smart windows, smart roofs, and smart walls is presented based on numerical and experimental studies of these smart building components. At the end of this chapter, energy consumption and economic analysis of smart building envelope applications are discussed, using results reported in the recent literature.

16.2

Categories of optically smart thin materials

Optically smart thin materials represent a new and intelligent solution for thermal insulation for glazed buildings. These materials are considered optically smart because their optical properties are highly dependent on external stimuli. In this section, two categories of optically smart thin materials are presented.

16.2.1 Thermochromic materials Materials that can change their coloration depending on their temperature value are known as thermochromic materials. Several types of materials, such as liquid crystals and transition oxides, present a thermochromic effect. This phenomenon is related to the crystalline and electronic band structure dynamics in transition metals associated

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Table 16.1 Thermochromic materials reported in literature. Materials

Transition type

Transition temperature

VO2

Monoclinic/tetragonal

68°C

V2O3

Monoclinic/rhombohedric

124°C

Fe3O4 Ag2S FeS

Orthorhombic/cubic Monoclinic/cubic Tetragonal/hexagonal

154°C 180°C 157°C

References Morin (1959) and Barker Jr et al. (1966) Griffiths and Eastwood (1974) and MacChesney et al. (1968) Verwey (1939) Hebb (1952) Murakami (1961)

with modifications in their absorption spectrum. Those dynamics are thermally induced according to a transition temperature, which depends on the material. Table 16.1 shows several thermochromic materials and their transition temperature. Among these materials, vanadium dioxide (VO2) presents the nearest transition temperature to a building comfort temperature, which makes it the best candidate for smart building cooling applications. In fact, VO2 is a material that changes its optical behavior from a transparent state for temperatures under the transition value Tc ¼ 68°C, to a reflective state for temperatures above the transition. This change in the optical behavior is related to a thermally induced metal/insulator transition (MIT) reported by Morin (1959). This MIT is intrinsically coupled with a dynamic structural phase transition (SPT), which features changes in the electronic band structure and the stoichiometric crystal structure of VO2.

16.2.1.1 Stoichiometric crystal structure of VO2 Indeed, above Tc, VO2 presents a tetragonal rutile (R) crystal structure with the following lattice constants (unit cell dimensions and angles between the sides): a ¼ b ’ ˚ and c ’ 2.85 A ˚ and a space group P42/mnm (#136) (Fig. 16.1A). Below Tc, VO2 4.55 A switches to a monoclinic crystal structure (M1) with low strain according to its lattice ˚ , b ’ 4.53 A ˚ , c ’ 5.38 A ˚ , β ¼ 122.6 degrees, and a space group constants: a ’ 5.75 A P21/c (#14) (Fig. 16.1B). The structural transformation from R to M1 is accompanied with by a doubling of the unit cell volume, which explains the optical transparency of VO2 at the M1 structure and the high optical reflectivity of VO2 at its metal-like R structure. In addition to M1, a second monoclinic structure M2 is also found in the insulator phase of VO2. The difference between M1 and M2 can be noticed at the level of V atom chains. In fact, along the c-axis of the monoclinic unit cell, V chains present a dimerization pattern in the M2 and a zig-zag pattern in the M1 (Fig. 16.1B). On the edges, the two structures M1 and M2 present a similar zig-zag pattern. Furthermore, V atoms form an octahedral coordination with oxygen atoms.

Optically smart thin materials for building cooling

R

x

359

V

x′

O

z′ y′ y

z dx2−y2

(A) V O1 O2

dxz

dyz V1 V2 O1 O2 O3

(B)

M1

M2

Fig. 16.1 VO2 crystal structures. (A) Rutile structure (above Tc), (B) monoclinic structure (under Tc) (Liu et al., 2018).

16.2.1.2 Electronic band structure of VO2 According to Goodenough’s model of the band structure of VO2 at its rutile phase (Goodenough, 1971), the octahedral coordination formed between V and O atoms is responsible for the split of the V 3d orbital into two symmetry states eg and t2g. The second type of symmetry state is also split into two dπ orbitals and one dk orbital (Fig. 16.2). The zig-zag and dimerization patterns formed by V atoms across the c-axis of the unit cell are responsible for the increase of the bandgap while switching from the metallic-like rutile structure R to the insulating monoclinic structure M1 of VO2 (Liu et al., 2018). In fact, the zig-zag distortions induce an increase in the energy level π* overcoming the Fermi energy EF. The association of dimerization and zig-zag patterns along the c-axis generates bonding and antibonding states (dk and dk∗ ) of the dk orbital (Fig. 16.2B).

16.2.1.3 Extrinsic doping effect on the MIT temperature and thermochromic properties The MIT temperature of VO2 is around 68°C according to Morin (1959). In order to make the VO2 applicable as a thermal building insulator, its MIT temperature must be decreased to around 25°C, which is the building comfort temperature. Several studies have reported since 1986 ( Jorgenson and Lee, 1986; Burkhardt et al., 1999; Mlyuka et al., 2009; Jin et al., 1994) that doping the VO2 with materials such as W, Mo, F, Mg, and Sr can tune its MIT temperature and its optical transmittance depending on the

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

s*

s* d*II

p* V 3d

d II

p*

V 3d d II O 2p

O 2p

s

(A)

p

s

(B)

p

Fig. 16.2 VO2 band structures. (A) Metallic structure (above Tc), (B) semiconductor structure (under Tc) (Liu et al., 2018).

doping rate (x at.% material). Jin and Tanemura (1995) and Hu et al. (2016) have found that W doping with a rate of 1.9 at.%W can decrease the MIT temperature to values around the comfort temperature. In fact, the first impact of the doping element is to increase the number of free charges (electrons or holes), which means a decrease in the value of the energy barrier necessary for the MIT and as a consequence decreasing Tc. The inconvenience of doping is that it reduces the optical modulation interval ΔT noted on transmittance spectra between the two states of the VO2 MIT. Indeed, in the case of a doping rate of 1.9 at.%W, the optical modulation ΔT is reduced by around 20% (Fig. 16.3).

16.2.2 Electrochromic materials Electrochromism is a phenomenon analogous to the electrochemical battery principle (Scrosati, 1992). It occurs when an electrical field is applied between two oxide layers forming an EC device (Lampert, 1984). The first oxide layer is a chromic cathode because it changes its color under ion insertion. Several inorganic oxide films based on tungsten (W), molybdenum (Mo), titanium (Ti), and niobium (Nb) can be used as cathodic EC films (Granqvist, 1994a). The second oxide layer changes its color under ion extraction, so it is considered a chromic anode. Materials such as iridium (Ir) and nickel (Ni) are used as anodic EC films (Granqvist, 1994b). In fact, the color change under ion insertion/extraction is governed by the following electrochemical equation (16.1). MeOn + xI + + xe , Ix MeOn

(16.1)

with Me for a specific metal, I for a specific ion, x the number of electrons e, and n the number of oxygen atoms. Some organic materials also show a polymeric EC behavior, such as polyaniline (Gospodinova and Terlemezyan, 1998) and viologens (Cinnsealach et al., 1998). Various combinations of cathodic and anodic oxide films were tested by

Optically smart thin materials for building cooling

361

Fig. 16.3 Transmittance spectra of (A) undoped, (B) doped VO2 films (Iken et al., 2019a).

Granqvist (2014) and Lampert (2004). The combination with the highest optical modulation was based on WO3 as the cathode and NiO as the anode (Seike and Nagai, 1991). In addition to cathodic and anodic oxides, the vanadium pentoxide (V2O5) structure is considered to be a hybrid or intermediate oxide because it can play the role of both cathode and anode (Talledo et al., 1990). The electrochromism of all these materials is related to their electronic band structures and crystallographic structures.

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16.2.2.1 Crystallographic structures of electrochromic materials Electrochromism has been noticed in several materials, as discussed earlier. The common point between these materials is the nature of their microstructure, which can be divided into three groups: Perovskite-like (MeO3), rutile-like (MeO2), and layer and block structures (Me2O5) (Granqvist, 1994a). Generally, the microstructures are built of octahedral blocks the MeO6 formed from edges and corner sharing between unit cells of the three groups’ structures (Granqvist, 1994b). This octahedral global structure directly impacts the electronic band structure of the metal by splitting the d atomic level into two levels eg and t2g (Fig. 16.4). The electronic band structure is considered to be the relevant criterion that decides on the nature of the oxide electrochromism to be anodic or cathodic.

16.2.2.2 Electronic band structure of cathodic oxides Cathodic oxides are in need of electrons to fill their empty energy levels. The insertion of electrons and intercalation of ions into the oxide structure increases the optical absorption of the oxide, which produces a change in its coloration. In fact, the split d atomic level is almost empty in cathodic oxides, and the gap energy is considered to be between the O 2p and the t2g (Fig. 16.4A). Thus electron insertion in the t2g level decreases the gap energy and increases the mobility of free electrons, producing a polaron absorption with a peak in the near-infrared spectrum. This polaron absorption is responsible for the color change of the oxide. Indeed, as a result of electron insertion, WO3 as a well-ordered crystal presents metallic-like properties such as high reflection in large wavelengths (Granqvist, 1994b). The MoO3 switches to an α layered phase with very thin layers (0.7 nm) of link distorted MoO6 octahedral (Granqvist, 1994b), due to covalent forces applied in the (100) and (001) directions Fig. 16.4 The impact of the octahedral symmetry on the atomic levels (Granqvist, 1994b). (A) Polaron abs.; (B) intraband abs.; (C) interband abs..

MxWO3 β-MxMoO3 MxTiO2 MxNb2O5?

HxlrO2 HxRhO2 Li1–xCoO2 HxNiO2?

E

V2O5

E

E

eg

eg

t2g

t2g

O2p

O2p

O2p

(A)

(B)

(C)

V3d

Optically smart thin materials for building cooling

363

and van der Waals forces applied in the (010) direction (Diaz et al., 2010). This change in the stoichiometric structure of MoO3 induces a color change of the oxide to dark blue. In the case of TiO2, the intercalation of lithium ions Li+ reduces the length of links between Ti and O (Zhang et al., 2019). Hence, the optical bandgap decreases till becoming a blue light reflector, which explains the change from bleached to blue colored TiO2. The niobium pentoxide Nb2O5 also shows a polaron absorption in the case of disordered structure. In fact, the inserted electron moves between Nb sites by absorbing a phonon (Granqvist, 1994b). The absorption peak is related to the brown color of the oxide.

16.2.2.3 Electronic band structure of anodic oxides Anodic oxides such as NiO2, RhO2, and IrO2 generally have an almost full t2g level. The insertion of electrons (and ion intercalation for charge-balancing) into the band structure of this type of oxides induces an optical change from an absorbing to a transparent state (Granqvist, 1994b). Indeed, the Fermi level of anodic oxides is localized in the gap between the t2g and eg (Fig. 16.4B). Thus the transparency is related to that bandgap, knowing that transitions between t2g and eg levels are parity-forbidden according to the Laporte rule for octahedral complexes.

16.2.2.4 Electronic band structure of V2O5 as a hybrid oxide Vanadium pentoxide (V2O5) is considered to be a hybrid electrochromic oxide because it shows both cathodic and anodic behavior due to its particular band structure. In fact, V2O5 belongs to the layer structure family of oxides that have an electronic 3d band with a split-off lower portion (Granqvist, 1994b) (Fig. 16.4C). The filling of this split-off low portion with electron insertion causes an optical bandgap widening, which produces a short-wavelength electrochromism controlled by the transitions between the O 2p band and this split-off low portion, since transitions are parity-forbidden between the two portions of the V3d band.

16.2.2.5 Electrochromic devices for solar radiation control An electrochromic device (ECD) is a combination of anodic and cathodic electrochromic materials in addition to some transparent ion conductors and glass or transparent flexible polymer (Granqvist et al., 2009; Fig. 16.5). All these components are arranged in successive layers in order to ensure the ion movement between the anode and the cathode when an electric field is applied. In fact, the ECD may be composed of five layers, two transparent conducting oxide (TCO) layers and between them, respectively, an ion storage layer (cathodic EC material), an ion conducting layer (electrolyte), and a counter electrode (anodic EC material). The reversible ion intercalation and deintercalation between the ion storage and the counter electrode materials are responsible for the coloration or transparency of both cathode and anode in the ECD. This reversible change in coloration directly affects the solar radiation

El ec Tr troc an h sp rom ar ic en la Po t c ye lye on r, st du WO er ct or ,I TO

O te IT iO ly r, ro to er, N ct c e u y l e nd la ng co ic tio nt rom c e r h r u e c a st nd sp tro co lye an ec nPo Tr El o I – + –– ++ – + – + – + + + +

Ions +/–

Cross-section of device 0.4 mm

Fig. 16.5 Prototype of an electrochromic device (Granqvist et al., 2009).

– – –

Optically smart thin materials for building cooling

365

transmittance level of the ECD (Fig. 16.6). Thus this device can control the transmitted amount of solar radiation through the building envelope components (windows, roofs, and walls) by varying an electrical field. The applications of electrochromic materials for building cooling are further discussed in this chapter.

90

Transmittance (%)

Bleached Colored

Bleached Colored

80 70 60 50 40 30 20 10

Reflectance (%)

35 30 25 20 15

PET TCO WOx PE NiOx TCO PET

PET TCO NiOx PE WOx TCO PET

10 5

Absorptance (%)

90 80 70 60 50 40 30 20 10 550

1100

1650

Wavelength (nm)

2200

550

1100

1650

Wavelength (nm)

2200

Fig. 16.6 Transmittance, reflectance, and absorption spectra of an electrochromic device (Granqvist et al., 2009).

366

16.3

Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

Fabrication methods of optically smart thin materials

The fabrication of optically smart thin materials is based on the same concepts as in microelectronics and thin films preparation. Physical vapor deposition (PVD) and chemical vapor deposition (CVD) with all their derived techniques are generally used to fabricate EC and TC thin films deposited on transparent substrates. These two techniques are considered as classical deposition methods because of the complexity of the process and the need for atmosphere regulation with vacuum pumps and substrate heaters. Despite their complexity, these classical techniques provide high-quality thin films with a high production rate. In the last 20 years, some novel processes based on chemical solutions have been proposed in several works. They ensure a film deposition under room temperature and atmospheric pressure. In this section, classical and novel fabrication methods for optically smart thin films are presented with a literature review on recent findings related to deposition techniques used for their fabrication. At the end of this section, durability issues and fabrication solutions are also discussed.

16.3.1 Classical methods of fabrication The PVD technique refers to both sputtering and evaporation methods. It is based on plasma generation between the substrate and a polarized material target to attract energetic ions. This plasma is composed of bombing ions (high energy ions) and reactive gases in a controlled vacuum chamber (Wen, 2015; Mazumder et al., 2019; Voznesenskaya et al., 2019). In fact, the atoms of the target material are dislodged by the energetic ions, so they can react freely with other atoms, such as oxygen, to form oxide films and travel toward the substrate. PVD includes several techniques, such as DC magnetron sputtering (Wang et al., 2015), cathodic arc deposition (Khakzadian et al., 2018), and pulsed laser deposition (PLD) (McGee et al., 2017). CVD is a deposition technique based on a chemical reaction of several precursors able to react at a specific pressure and temperature to produce the desired composition and reach the growth surface (Guo et al., 2017). Generally, precursors are synthesized in a liquid or a solid phase. Liquid precursors can be injected into the reaction chamber using several methods, such as flash vaporization with controlled temperature, gas showerhead nozzles, and bubblers. In the case of solid precursors, sublimation is required. Thus heating until reaching high temperatures is needed and a careful control of the temperature of all components (valves and mass flow controllers) of the chamber is required to avoid potential clogging. CVD includes several subtechniques depending on the parameters used for the chemical reaction. Among those subtechniques, low pressure CVD (LPCVD) (Uny et al., 2019) and plasma-enhanced CVD (PECVD) (Ge et al., 2020) are the most commonly used for oxide films deposition.

16.3.1.1 The impact of fabrication methods on the quality and enhancement of VO2 thin films Several studies have shown that the choice of an adequate thin film preparation method can influence the thermochromic properties of VO2. Gagaoudakis et al. (2018) have prepared a VO2 thin film using a low-temperature radio-frequency

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367

(RF) magnetron sputtering method. In fact, tin dioxide (SnO2) coated glass was used as a substrate and the deposition temperature was fixed at 300°C, which is relatively low. The impact of two deposition parameters was verified. The first one is the oxygen percentage in the ArO2 plasma, which was varied between 2% and 4%. It was found that films deposited with the highest O2 percentage have a wider transmittance hysteresis loop and a sharper transition. In addition, the IR transmittance modulation at λ ¼ 2000 nm has dropped from 19.5% to 12.4% while increasing the O2 amount from 3% to 4%. However, a positive impact was noted on the solar modulation with a small increase from 5% to 6% and an increase in the luminous transmittance from 36.4% to 40.1% with oxygen content increasing. The second parameter is the thickness variation between 35 and 260 nm. It was noticed that thickness increasing positively affected the solar modulation with an increase from 3% to almost 9%. Ho et al. (2019) prepared high quality VO2-based thermochromic films by the magnetron sputtering method with a V2O5 target and an in situ annealing. Indeed, the films were deposited on Si and quartz substrates and the reported results showed interesting findings. First, large optical transmittance modulations of 74% and 77% were noticed in the near-infrared region for wavelengths of 2.5 and 3 μm, respectively. Second, stable luminous transmittance values were found with a small variation of 2% between room temperature and high temperature. Third, a relatively high solar modulation efficiency of about 6% was noticed compared with other VO2 films deposited by sputtering. Kumar et al. (2019) also fabricated VO2 films using an RF magnetron sputtering method at room temperature. In fact, the impact of an ex situ annealing duration on the films’ thermochromic properties was investigated. The annealing temperature was 600°C for durations between 2 and 60 min. It was found that the transition temperature increases and the hysteresis loop width decreases for longer duration annealed films. However, the optical modulation at λ ¼ 2500 nm showed an increase from 40% for 2 min annealing to 70% for 60 min annealing time. Hajlaoui et al. (2020) prepared boron (B) doped and undoped VO2 thin films using the PLD method. Indeed, vanadium and boron targets were ablated using a KrF excimer laser (λ ¼ 248 nm and pulse duration ¼ 25 ns). B doping reduced the transition temperature of VO2 at a rate of 31.5°C at.%B. This rate is the highest among all doping materials including tungsten (W). Nevertheless, the IR modulation of doped films decreased, as in most VO2 doping studies. McGee et al. (2017) also processed VO2 thin films on SiO2 substrates using the PLD technique (Fig. 16.7). Three different VO2 phases (M1, T, and A) were successfully deposited. It was found that the MIT hysteresis loop width highly depends on the phase type and the deposition conditions. Gaskell et al. (2016) have processed monoclinic phase VO2 films using the atmospheric pressure CVD (APCVD) method. Vanadium tetrachloride (VCl4) was used as a vanadium liquid precursor and H2O was considered as the oxygen source. The chosen substrate was a borosilicate glass with 1.1 mm thickness. Characterization results showed that the formed VO2 films are polycrystalline and the thermochromic properties highly depend on the precursor ratio. A picosecond MIT was noticed with a large change in the dielectric function of the resulting films. In a recent study carried out by Rajeswaran and Umarji (2019), a low pressure metal-organic CVD (MOCVD) method was used to deposit VO2 films on Si substrates at different temperatures

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Eco-efficient Materials for Reducing Cooling Needs in Buildings and Construction

er pp r e St oto m

Va ch cuu am m be r

g tin ta et l o R arg se t ou rr ca

KrF2 laser beam

Gas inlet (O2)

et rg e Ta um Pl b

e at str

ew Vi r t po

Su

Heating stage holder ing at e He tag s

Fig. 16.7 Schematic description of the pulsed laser deposition equipment (McGee et al., 2017).

between 520°C and 550°C. Vanadyl acetylacetonate (VO(acac)2) was used as the metal-organic precursor. Resulting films showed morphological variations depending on the growth temperature. In fact, it was noticed that films deposited at 535°C presented grain formations with sizes around 200 nm. At 550°C the films showed an increase of the transition strength, width, and hysteresis. Those differences were related to the presence of several vanadium oxidation states (V3+, V4+, V5+) in the films. Thus those defects fractions can offer an additional control possibility for the transition quality of VO2. All the studies presented here show a solid dependence between the thermochromic properties of VO2 and thin films fabrication methods. It was seen that all the process parameters such as temperature, pressure, and gas flows impact the MIT characteristics of VO2 thin films, such as IR transmittance and solar modulation and the transition hysteresis loop, which are the mean parameters used in smart building insulation.

16.3.1.2 The impact of fabrication methods on the quality and enhancement of ECDs ECDs are composed of several thin layers as shown in Fig. 16.8. Thus the fabrication process is more complex than that of a monolayer deposition. In this section, we will present and discuss several recent works that used classical fabrication methods to process ECDs and to enhance their properties and durability.

Optically smart thin materials for building cooling

369

Fig. 16.8 Schematic description and images of the EC device, (A) and (C) while bleached; (B) and (D) while colored (Lee et al., 2020).

Lee et al. (2020) recently used a reactive DC magnetron sputtering technique to deposit a W doped Ni oxide layer as the counter electrode of the ECD with different W concentrations on an indium tin oxide (ITO) glass substrate under an oxygen and argon atoms mix pressure of 0.47 Pa at room temperature and with a power density of 1218 W/m2. The ion storage layer of WO3 was also deposited on an ITO glass substrate under a 30% ratio between oxygen and argon at room temperature. Results have shown that the W doping minimized the NiO film degradation caused by repetitive electrochemical cycling. In fact, with a doping rate of 0.024 at.%W, reduction in the optical modulation at λ ¼ 550 nm was stable around 2% between 100 and 1000 cycles. Lee et al. (2020) have also succeeded in the fabrication of a flexible ECD by depositing the electrodes (WO3 and Ni1xWx) on a c-ITO/graphene/PET configuration with an electrolyte of Li ions. This flexible ECD showed a stable optical modulation of around 40% at λ ¼ 550 nm with a color change from yellow to dark blue between (1.5 V) and (1.5 V), respectively. The effect of deposition parameters on nanocrystalline WO3 thin films was studied by Pandurangarao and Kumar (2019). In fact, tungsten oxide films were deposited using DC magnetron sputtering. Substrate temperature (300, 400, 500, and 600 K) and sputtering pressure (5 and 10 Pa) were tested to see their impact on the films. Results showed that the pressure does not affect

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the optical transmittance. However, the increase of the substrate temperature decreased the transmittance due to the enhancement of the crystallinity at higher temperatures. Coskun and Atak published two studies (Cos¸ kun and Atak, 2018; Atak and Cos¸ kun, 2019) on all solid-state ECDs. In both studies, an RF magnetron sputtering deposition technique was used. The objective of the first work (Cos¸ kun and Atak, 2018) was to investigate the impact of dry and wet lithium atom insertion in the ECD layers. In fact, the cathodic WO3 layer was bombarded with Li atoms by LiNbO3 RF magnetron sputtering at room temperature. The ion-conducting Ta2O5 layer was additionally wet lithiated (Li ions insertion) by electrochemically cycling it in a Li+ solution. It was found that the additional wet lithiation increased the optical transmittance modulation from 34.8% to 71.7% at 550 nm. In the second study, Atak and Cos¸ kun (2019) inspected the effect of the anodic layer thickness on the optical modulation of the ECD. For this purpose, several NiO films were deposited using RF magnetron sputtering with different thicknesses (140, 230, 310, and 480 nm), and then two ECDs were fabricated with the thinner and the thicker NiO films. Results showed that the optical modulation was highly dependent on the anodic layer thickness. Indeed, the optical transmittance modulation at λ ¼ 550 nm has drastically decreased from 36.4% for the thinner anodic film (140 nm) to 1.5% for the thicker NiO film (480 nm). It should be noted that the magnetron sputtering technique is the most commonly used for ECD fabrication. In fact, ion transport through all the thin films composing the ECD requires a sufficient degree of porosity to make the ion mobility easier (Granqvist et al., 2018). Generally, evaporation and sputtering are known as the most controllable growth techniques (Thornton, 1977) with several control parameters. The plasma pressure and substrate temperature are the ones that determine the deposited film porosity. Indeed, a process with the combination of a low substrate temperature and a high sputter plasma pressure can result in porous films belonging to “zone 1” according to Thornton (1977) sputtered films classification.

16.3.2 Novel solutions of fabrication Novel fabrication processes are generally based on chemical solutions mixing and annealing under atmospheric pressure. Sol-gel, polymer-assisted deposition (PAD), and hydrothermal processing techniques are the most used to fabricate optically smart thin materials. Since 2009, Kang et al. (2009), Gao et al. (2011a), and Du et al. (2011, 2013) have published several papers and deposited a patent, Gao et al. (2011b) presenting a novel solution process based on the PAD method for the synthesis of VO2 thin films with different doping materials. This technique uses polymers as filmforming promoters due to their capacity to stabilize the precursor’s solution. In fact, a sol-gel preparation based on several precursors, such as VOCl2, an aqueous suspension based on V2O5 and sodium tungstate dehydrate (Na2WO
H2O) as a doping material was mixed with a gravimetrically determined proportion of poly (vinylpyrrolidone) (PVP). The resulting solution was then spin coated on a fused silica substrate. After 3 h of heat annealing at 600°C in a nitrogen atmosphere, a VO2 thermochromic film was formed on the substrate. Characterization results have shown that the PAD can make VO2 films with high optical modulation of around 41.5% at

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2000 nm. W doping has decreased the transition temperature from 69°C to 54°C. It was found that the PVP influences the film formation in two ways. First, it improves the film gelation using the organic links between carbonyl groups and amine groups. Second, it enhances the uniformity of the mixed-gel based film after an annealing step due to the interaction between negatively charged carbonyl groups and VO2+. ECD has also been prepared using new manufacturing methods based on chemical solutions of mixed organic and inorganic materials, especially for the fabrication of flexible ECD using polymers. Recently, Lin et al. (2019) have published a study describing the synthesis of flexible ECD based on an organo-tungsten-iron oxide (WFexOyCz). An atmospheric pressure plasma jet (APPJ) method was used at low temperature to deposit the hybrid film on a polyethylene terephthalate/indium tin oxide (PET/ITO) substrate (Fig. 16.9). During the process, oxygen gas flow was intentionally reduced to create an oxygen deficiency in the processed films in order to make the Li+ intercalation/deintercalation cycling easier through the porous ECD. The resulting films have shown high optical modulation of about 70.3% for a wavelength of 800 nm. Ultrasonic spray pyrolysis deposition (USPD) is one of the techniques that have great potential of use for the fabrication of low-cost, flexible, and large surface films (Dominguez et al., 2017). Kim et al. (2018) used the USPD technique to deposit fluorine-doped tin oxide (FTO) films in order to investigate the impact of the sheet resistance (10, 6, and 3 Ω/cm2) on their electrochromic performances. Indeed, the USPD is a technique based on mixing the precursor solutions (tin chloride pentahydrate [SnCl4
5H2O]

Fig. 16.9 Schematic description of the atmospheric pressure plasma jet method (Lin et al., 2019).

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and ammonium fluoride [NH4F]) to be sprayed on a heated (420°C) and rotated (5 rpm) glass substrate using an ultrasonic atomizer with a specific frequency (1.6 MHz). Sheet resistance was controlled by deposition duration. In fact, sheet resistance decreases while increasing deposition time. Thus durations of 18, 28, and 33 min have given, respectively, sheet resistances of 10, 6, and 3 Ω/cm2. FTO films were used as the transparent conducting electrode and WO3 spin coated films were used as the cathode. The results of this study showed that the ECD switching speed increased while sheet resistance decreased. The ECD with 6 Ω/cm2 had the higher coloration efficiency value (50.9 cm2/C); this result could be related to the high transmittance modulation of this sample due to the uniformity of the FTO surface morphology, which decreases light scattering in the interface with the WO3 film.

16.3.3 Durability issues of optically smart thin materials and fabrication solutions Thermochromic and electrochromic materials are known to have some durability issues related to their smart properties, which are required in building cooling applications. A fast deterioration of the switching capacities can cause these materials to be unable to regulate the radiative heat transfer through the building envelope. In fact, the durability issues are generally related to the environment interactions (dry and humid conditions) in the case of VO2 ( Ji et al., 2014), and related to the degree of crystallinity, parasitic electrochemical reactions, and mechanical failure during repeated ion insertion/extraction in the case of ECD (Granqvist et al., 2019). VO2 is a thermodynamically nonstable oxide. Indeed, under aggressive weather conditions, such as high humidity and high temperature, a natural oxidation phenomenon transforms it to a more stable oxide, which is V2O5. This resulting oxide is a nonthermochromic material. The time that it takes this natural transformation to occur is short compared with the required lifetime in building cooling applications. Thus this oxidation phenomenon should be stopped or delayed in order to maintain the thermochromic properties as long as possible. In fact, several studies have proposed solutions to this issue. Long et al. (2018) suggested the use of a vanadium sesquioxide (V2O3)-based buffer layer between the quartz substrate and the VO2 layer (Fig. 16.10). This configuration (Quartz/V2O3/VO2) has shown higher durability than the monolayer of VO2. Indeed, the two samples with and without the V2O3 buffer layer have been placed in a sealed chamber with high temperature (around 60°C) and high humidity (90% relative humidity) for 9 days in order to accelerate the oxidation phenomenon. Results of this aging process showed that the degradation in the monolayer of VO2 was faster than in the one with the buffer layer (60 nm V2O3/60 nm VO2). In fact, as shown in Fig. 16.11A, the optical transmittance modulation of the monolayer VO2 at 2000 nm decreases with time from 60% (just deposited) to almost 0% (after 6 days). The solar transmittance modulation (ΔTsol) presented in Fig. 16.11C shows a drastic decrease after the 4th day under aggressive climatic conditions. It drops from around 8% to around 3% in 2 days. The integration of the buffer layer (60 nm V2O3) has largely ameliorated the TC properties durability of the VO2 thin film. As shown in

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Fig. 16.10 Scanning electron microscopy image of the V2O3 buffer layer (Long et al., 2018).

Fig. 16.11 Time-dependent transmittance spectra of (A) 60 nm VO2, (B) 60 nm V2O3/60 nm VO2, and corresponding optical and solar modulation of (C) 60 nm VO2 and (D) 60 nm V2O3/60 nm VO2 (Long et al., 2018).

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Fig. 16.11B, the optical transmittance modulation decreases from 50% (just deposited) to around 10% (after 9 days). In Fig. 16.11D, ΔTsol showed a slow decrease with a value of 10% after 6 days aging. In fact, this amelioration in TC properties durability can be explained by the compensation ensured by the buffer layer of V2O3 (which gradually switches to VO2) when the surficial VO2 layer starts switching to non-TC V2O5. In the same context, Ji et al. (2014) proposed to deposit (10-, 30-, and 150-nm thick) Al oxide top coatings on an 80-nm thick VO2 in order to protect it and delay the oxidation phenomenon. The prepared samples were first tested under high temperature conditions (dry air at 300°C) for 30 h. After this thermal treatment, films with 10and 30-nm thick Al oxide have shown good TC properties with optical transmittance modulations at 2000 nm of 42% and 46%, respectively. The unprotected VO2 film has shown a total conversion to the non-TC oxide V2O5 after 1 h of the thermal treatment. A second test was carried out under high relative humidity (80% and 95%) and high temperatures (60°C and 80°C) for 1 week. The results of this second test also showed high durability enhancement. In fact, the optical transmittance modulation results of the test under (60°C and 95% of relative humidity) are, respectively, 33% and 41% for 10- and 30-nm thick Al oxide top coating at 2000 nm and after 5 days under test conditions. By changing the test conditions to 80°C and 80% of relative humidity, the optical transmittance modulation showed a small decrease after 1 week under test, with values around 15%, 25%, and 23% for 10-, 30-, and 150-nm thick Al oxide top coating, respectively, at 2000 nm. Those two discussed studies present encouraging solutions for the natural oxidation phenomenon that converts the TC VO2 to nonTC V2O5. More research and solutions development are required to enhance the durability of VO2 and make it more suitable for building cooling applications. ECDs have also shown some critical durability issues related to the thin oxide films degradation after a few number of switching cycles. With the aim of preventing or reversing this degradation phenomenon, Granqvist et al. (2019) have discussed three methods that can be used to improve ECD durability and rejuvenate degraded thin films. The first method was the electrochemical pretreatment (potentiostatic pretreatment) (Arvizu et al., 2018). It consists of immersing the cathode film of WO3 with a Li/Li+ counter electrode in an electrolytic solution based on a mixture of lithium perchlorate (LiClO4) and propylene carbonate (PC). In fact, by applying a voltage of 6 V between the cathode film and the counter electrode, Li+ ions are expulsed from the EC film (WO3) for several hours. Fig. 16.12 shows the impact of the potentiostatic pretreatment on the EC cycling durability of a 300-nm thick WO3 film. Indeed, Fig. 16.12A and B gives, respectively, the cyclic voltammograms (CV) of the nonpretreated and the pretreated EC films. After 40 CV cycles, the nonpretreated film shows a radical decrease in the charge exchange from 53 to 6 mC/cm2 contrary to the pretreated film, which shows a relatively stable charge exchange between 33 and 26 mC/cm2. These results mean that the electrochemical properties of the pretreated film remain more robust and stable for several cycles than ones of the nonpretreated film. In terms of optical transmittance switching durability, Fig. 16.12C and D shows, respectively, the transmittance at 550 nm of the nonpretreated and the

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Fig. 16.12 Cyclic voltammograms for different cycle number of (A) an as-deposited 300-nm thick WO3 film, (B) after potentiostatic pretreatment, and corresponding optical transmittance at 550 nm for (C) as-deposited 300-nm thick WO3 film and (D) after potentiostatic pretreatment (Granqvist et al., 2019).

potentiostatically pretreated films. In the case of the nonpretreated film, the optical transmittance modulation decreases from 80% in the first 10 cycles to 6% at the end of the 40 cycles. The pretreated film has shown a stable transmittance during all the 40 cycles with a small decrease from 80% in the beginning of the cycles to 77% after the end of the voltage cycling process. Based on those results, it could be concluded that the potentiostatic pretreatment of WO3 films can considerably enhance the durability of their electrochromic and electrochemical properties. The second method was the rejuvenation of degraded films by electrochemical posttreatment. This method consists of detrapping the ions intercalated in the EC film due to long voltage cycling duration. Amorphous oxides such as WO3 with highly disordered crystal structure present a complex ions intercalation process. Wen et al. (2016) have found that the potentiostatic technique (previously described) can extract ions from two types of traps in the amorphous EC WO3. Those two types of ion traps are hollow traps and deep traps. Hollow traps are responsible for the increase in the optical absorption due to the trapped ions that fill the reversible intercalation sites. This optical absorption occurs due to electronic transitions from occupied to empty localized conduction band states. In fact, those electronic transitions are triggered

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by ion intercalation. Thus, when the hollow traps are filled, the optical absorption increases drastically and causes the degradation of the colored state of the EC film. In the case of deep traps, the degradation also reaches the bleached state of the EC film and more energy is required to extract deep trapped ions using the potentiostatic technique. There is a third trap type known as irreversible traps. This type produces a weak residual absorption and it is due to ions that cannot be retrieved by the potentiostatic technique. NiO anodic coloring-based films can also be rejuvenated by immersion in the same electrolyte solution (LiClO4-PC) (Qu et al., 2017). The third method that can enhance the durability of EC devices is oxidic admixtures in WO3 and NiO films. Several studies (Faughnan and Crandall, 1977; Wen et al., 2015; Granqvist, 2014) have been conducted on W and Ni oxides mixed with Mo, Nb, Ir, and V and it was found that oxide mixing improved their optical performances and durability. In fact, the oxide mixing shifts the absorption peak of the film toward higher energy band levels and increases the contrast ratio due to the enlargement of the optical density difference between the oxidized and the reduced states of the mixed oxide film (Sato and Seino, 1982).

16.4

Applications of optically smart thin materials for building cooling

The integration of optically smart thin materials such as thermochromic and electrochromic materials in the building envelope for efficient cooling can take the form of smart envelope components such as smart windows, smart roofs, and smart walls. At this point, the nanomaterials engineering domain meets construction and civil engineering. Several challenges related to the application of optically smart materials in building envelopes are facing researchers (Lee and DiBartolomeo, 2002; Czanderna et al., 1999). Thus various experimental and numerical studies have been conducted in order to assess the applicability of smart building envelopes. In this section, a detailed literature review of recent works concerning the technical issues experienced while integrating smart thin materials in building envelopes is presented.

16.4.1 Smart windows The concept of smart windows is based on a new window technology offering dynamic control of solar heat and daylighting in buildings. This dynamic control is a result of optically smart variable properties depending on an external stimulus such as electrical or thermal signals for electrochromic and thermochromic windows, respectively. Smart windows are considered promising solutions for energy-efficient buildings. In fact, by smart control of the solar heat gains between outdoors and indoors, smart windows impact on the air conditioning energy consumption by reducing the heating and cooling peak and transmission loads. In order to assess this impact, several numerical and experimental studies have been carried out on different types of buildings (commercial, residential, and offices) and have shown encouraging results.

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Dussault and Gosselin (2017) realized a numerical study on the integration of electrochromic windows in office building envelops with the aim of analyzing the impact of design parameters on energy consumption and thermal and visual comfort. Design parameters considered in this study were the location (10 cities from United States and Canada), the facade orientation (north, east, south, or west), window-to-wall ratio (33%, 50%, or 75%), internal gains (low and high), air infiltration rate (tight and leaky envelope), thermal mass (low and high), and the smart window control (three rulebased control strategies). A total of 7680 scenarios were built based on all these design parameters. Results of this study include several analyzing outputs: peak load demand, percentage of persons dissatisfied, and useful daylight index. It was shown that the use of smart windows has a considerable impact on the building energy consumption. In fact, smart windows decrease cooling loads and slightly increase heating and lighting loads. Their impact on cooling loads is similar to that of location, orientation, and window-to-wall ratio. Rule-based control has no considerable influence on heating and cooling loads, while it does affect lighting loads. It was concluded that smart windows are a more appropriate for warmer climates due to their negative impact on the heating loads demanded in cold climates (northern cities). The same results were reported earlier by Ajaji and Andr
e (2016) in a numerical study using TRNSYS software. Indeed, an office building in Brussels (Belgium) with a glazing area of 90% was simulated with passive glazing (reference) and with a smart electrochromic glazing system in order to evaluate the influence of the EC glazing on heating, cooling, and lighting energy loads. A dynamic control strategy for the smart glazing was set based on outdoor illuminance and outdoor temperature data. The findings showed that the EC glazing system only reduces cooling energy loads. Heating and lighting loads presented relatively high penalties, which means that EC glazing did not equalize the building energy balance. Another recent study using experimental and numerical investigations to assess the energy performance of a residential building integrating an electrochromic smart window was conducted by Piccolo et al. (2018). The experimental part of this study was carried out using a test-cell with a small area of EC double glazing (Fig. 16.13). In fact, the ECD used in this study was an all-solid-state device with WO3 as the active layer and NiOH:Li as the ion storage material with a polymeric ion conductor electrode in between. The optical transmittance visible modulation of the ECD was around 55% (between 70% for the bleached state and 15% in the colored state) (Fig. 16.14). The simulation was realized using EnergyPlus software as an energy consumption calculation tool. In order to compare the effect of the ECD on heating and cooling energy loads, two locations in Italy with cold and warm weather were chosen. The first location was Messina, with a typical Mediterranean climate in which cooling periods dominate. The second location was Bolzano in the Alps with a cold continental climate where heating periods dominate. Results showed the same influence of EC windows reported in the previously discussed works. It was noted that EC windows were more appropriate for cooling-dominated climates in terms of overheating reduction rather than cold winter climates where a building integrating EC windows showed an increase in energy demand for heating and lighting due to the lower transmittance value in the clear state (70%) compared with a passive clear glazing (85%–94%).

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Fig. 16.13 Image and schematic description of the test cell used by Piccolo et al. (2018) where P and T are labels of photosensors and temperature sensors, respectively.

Fig. 16.14 Transmittance spectra of the home-made solid-state ECD at clear and colored states (Piccolo et al., 2018).

Recently, Frattolillo et al. (2019) conducted an experimental study with the aim of characterizing the impact of EC glazing on building energy consumption for air conditioning and lighting. Two wooden rooms with and without EC glazing (Fig. 16.15) were used to evaluate the indoor comfort and heating and cooling loads. Those two

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Fig. 16.15 (A) Image of the EC panels in the transparent and colored states, (B) images of the test-rooms from indoors and outdoors (Frattolillo et al., 2019).

rooms were located in Cagliari (Italy) in the middle of the Mediterranean Sea with a warm winter and a high amount of solar radiation. Before the EC panels’ integration in the test room, their thermal conductivity was measured using a climatic chamber. The measurements were conducted for the clear and fully tinted states of the EC panels and showed, respectively, a solar factor g ¼ 48% and 10%, a light transmittance factor TL ¼ 62% and 2%, and a thermal transmittance of U ¼ 1.1 W/m2K. In fact, the solar factor g refers to the amount of the incident solar energy on the glazing that is transmitted to the indoor. The energetic performance of the EC glazing showed a positive impact in a Mediterranean summer with over 80% cooling load reduction (Fig. 16.16). Nevertheless, as found in the previously discussed works, smart EC windows show heating load penalties of around 35% during winter, which reduces the global energy gain. Concerning TC-based smart windows, several recent studies have been conducted in order to investigate the impact of this type of smart coating on the building energy consumption, indoor comfort, and lighting. In fact, Liang et al. recently published two studies (Liang et al., 2018, 2019) about VO2 coatings applied to building windows. The first study (Liang et al., 2018) was carried out with the aim of investigating the influence of several parameters related to TC coatings, building architecture, and the location climate. The TC coating varied parameters were the transition temperature (varying between 20°C and 41.3°C using W doping and nanoparticle forming techniques) and solar transmittance (varying between 0.412 and 0.690). The building architectural parameter was the window-to-wall ratio (WWR, varying between 0.1 and 1). The simulations were conducted using EnergyPlus software for a typical office room (Fig. 16.17) located in five Chinese cities with different climatic conditions. Results showed that the energy performance of the smart window does not increase by lowering the transition temperature. Nevertheless, it was found that increasing the solar transmittance modulation was required for all the climates studied. The solar

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Fig. 16.16 Comparison between low emission glass and EC glazing in terms of monthly heating (QH, nd) and cooling (QC, nd) loads. Qh, nd, on refers to the heating energy loads reference when the EC switching program is permanently on (Frattolillo et al., 2019).

Fig. 16.17 The office room model used in EnergyPlus, with a large EC glazing (over 60% of the wall) (Liang et al., 2018).

absorbance of the coating was found to have a negative impact on the energy consumption, especially during the cooling season. The calculated building energy savings were around 20% for all the TC windows studied. As for the EC windows, it was found that TC glazing was also more appropriate to use in warm climates than in cold ones. In the second study conducted by Liang et al. (2019), the optimization of the solar transmittance modulation of the TC window was the main objective. In fact,

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a combination of VO2 as NIR radiation controller and an innovative ionic-liquidbased TC material (IL-NiII) known as a good visible (VIS) radiation controller was numerically tested as the glazing system of an office room located in three Chinese cities with different climatic conditions. The results of this study showed that the combination of an NIR radiation TC controller and a VIS radiation TC controller could reduce the overheating and overlighting during summer days. Based on all these recent studies, it was noted that the smart switching properties of EC and TC-based smart windows do not yet allow an equalized building energy balance between cooling and heating loads. In fact, during the hot season, smart windows are in the reflective state and exhibit a high energy performance by decreasing the radiative overheating. Thus the cooling energy loads are reduced up to 80% depending on the building location climate conditions. However, during winter, the smart window coatings are in the transparent state, with a transparency around 70% to 80%, which is lower than the classic clear glass transparency (up to 90%). This relatively low transparency during the cold season decreases the natural heating due to incident solar radiation on the window, which causes an increase in the heating energy consumption by up to 35%. These heating penalties affect considerably the annual energy savings balance, especially in cold climates where the heating period exceeds the cooling period over the year.

16.4.2 Smart roofs Roofs are known to be responsible for around 25% of the heat losses exchanged through the building envelope, as shown in Fig. 16.18. Thus the integration of optically smart thin materials in this part of the building envelope would help considerably in decreasing the air-conditioning energy consumption and maintaining the indoor comfort. To this point, instead of the smart roofs concept, the nonsmart cool roof concept has seen strong interest, with the aim of only reducing the overheating occurring during cooling periods. Since 1990, in the context of climate change action plans for heat island mitigation, several studies were conducted by the Energy and Environment Floors 15% Roof 25%

Walls 35% Windows & doors 25%

Fig. 16.18 Energy losses that occur through building envelope (Gonc¸alves et al., 2016).

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Division of the Lawrence Berkeley Laboratory (Akbari et al., 1990, 1992, 1997; Rosenfeld et al., 1995) on cool roof applications based on high albedo materials with a high static capacity to reflect solar radiation. Berdahl and Bretz (1997) have categorized high albedo materials into different types: white paint, white roof coatings, white roof membranes, and aluminum pigmented roof coatings. The application of these reflective coatings on the external surface of roofs leads to significant seasonal energy savings of up to 80% of cooling energy consumption (Akbari et al., 1997). Recently, Testa and Krarti (2017) published a comparison-based review between static and switchable (smart) cool roof materials. Based on a large bibliographic study, it was concluded that static cool roof materials can unfortunately lead to a considerable increase in the heating energy loads during the cold season. Consequently, those heating penalties negatively affect the yearly energy savings. As a promising solution to this penalties problem, the authors suggested the use of smart roofing materials. In fact, it was reported that very few studies have investigated the impact of TC coating applications to building roofs and no research has been conducted up to now on the integration of EC coatings to building roofs. VO2 as a TC coating was applied to roofs in a unique study conducted by Gonc¸alves et al. (2016). In this work, a smart optically active VO2 layer was coated on roof-type ceramic tiles in order to enhance the building energy efficiency. The first part of this study was dedicated to the preparation of W doped VO2 nanoparticles-based powder using hydrothermal synthesis assisted by microwave irradiation, which is one of the newest fabrication solutions for smart thin films presented in the previous section. In fact, the nanoparticle powder was obtained from a redox reaction of V2O5 using oxalic acid. The transition temperature was decreased to around 49°C by adding 3% WO3 concentration to the redox solution. The resulting solution was irradiated with microwaves in order to accelerate the reaction and enhance the uniformity of the nanoparticles (Li et al., 2013). After centrifugation, washing, and drying, the collected powders were deposited using spray coating method on ceramic glassy tiles as can be seen in Fig. 16.19. The spray coating process was followed by a nitrogen atmosphere annealing at several temperatures to promote the film adhesion on tiles. The second part of this study was dedicated to the characterization of optical and thermal performance of the smart

Fig. 16.19 Photographs of the (A) pristine regular ceramic tile; (B) VO2 coated tile after thermal treatment (Gonc¸alves et al., 2016).

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Fig. 16.20 Reflectance spectra of (A) undoped VO2, (B) 3% W doped VO2 (Gonc¸alves et al., 2016).

Fig. 16.21 The apparent temperature of regular and coated tile detected by IR camera while heated. The inset presents the photograph and IR camera shot of both tiles, respectively (Gonc¸alves et al., 2016).

tiles. In fact, the optical reflectance of the tiles with and without the W doped VO2 coating was measured and the results showed that the smart coated tiles could reflect 2.43 times more than the uncoated ones (Fig. 16.20). In addition, the two tiles were heated on a hotplate and filmed by an IR camera in order to see the impact of the hotplate temperature on the IR reflectance and the tiles’ surface temperature. Fig. 16.21 shows the impact of the transition on the apparent temperature of the smart coated tile compared with the regular tile. The IR images showed

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that the smart coated tile had a lower temperature than the regular tile due to the lower IR emission provided by the W doped VO2 coating at high temperatures. This study demonstrated the high application potential of VO2 as an optically smart thin material on building roofs. However, more numerical and experimental studies are required to investigate the thermal and energy performance of smart roofs.

16.4.3 Smart walls As shown in Fig. 16.18, walls are the source of around 35% of heat losses through the building envelope. Thereby, wall thermal insulation has seen considerable interest from researchers for decades. The majority of the studies conducted on wall insulation were focused on passive thermal insulation based on the application of high thermal inertia materials (Verbeke and Audenaert, 2018). In the last 10 years, new active insulation materials known as phase change materials (PCMs) have been investigated for their capacity of heat storage using latent heat (Baetens et al., 2010; Zhou et al., 2012; Lai et al., 2010; Zalba et al., 2003). However, regarding the application of optically smart thin materials (TC and EC) to building walls, there are no reported studies on the subject before 2019. In fact, two studies were published in 2019 by Iken et al. (2019a, b) concerning the integration of W doped VO2 as an optically smart wall component with the aim of investigating the thermal and energy performance of two smart wall configurations. In the first study (Iken et al., 2019a), the objective was to calculate temperature distribution and heating/cooling energy loads for a wall configuration with a thin film of W doped VO2 coated on a glass substrate and fixed on the outdoor surface of the wall (Fig. 16.22). Indeed, the numerical calculations were carried out using a Matlab program based on the finite difference method, considering the wall absorptivity as dynamic variations of the smart thin film given by Eqs. (16.2), (16.3), depending on variable outdoor limit conditions (solar radiation and air temperature) in a hot-summer Mediterranean location (city of Meknes, 33.9°N; 5.5°W and 549 m of altitude, Morocco). In fact, Eq. (16.2) gives the absorptivity values of the smart wall during the increase of the daily solar-air temperature (Tsa) (see Eq. 16.4) (morning and afternoon). During the decrease of the daily solar-air temperature (evening and night), the absorptivity values of the smart wall are given by Eq. (16.3). Solar-air temperature given by

Indoor Tin hin

q(t) as = f(Tout) Outdoor Tout he S doped VO2 200 nm Glass 5 mm

20 cm 2 cm

Brick

Cement plaster

Brick

2 cm Cement plaster

20 cm 2 cm Cement plaster

q(t) as = 0.55 Outdoor Tout he

Cement plaster

2 cm

Indoor Tin hin

Fig. 16.22 The studied wall configurations with and without W doped VO2 coating (Iken et al., 2019a).

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Eq. (16.4) is a combination of the air temperature and solar radiation. It describes the impact of solar radiation on temperature near building external surfaces. 8