Personal Comfort Systems for Improving Indoor Thermal Comfort and Air Quality 9819907179, 9789819907175

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Indoor Environment and Sustainable Building Series Editors: Angui Li · Risto Kosonen

Faming Wang Bin Yang Qihong Deng Maohui Luo Editors

Personal Comfort Systems for Improving Indoor Thermal Comfort and Air Quality

Indoor Environment and Sustainable Building Series Editors Angui Li , Xi’an University of Architecture and Technology, Xi’an, Shaanxi, China Risto Kosonen, Mechanical engineering, Aalto University, VANTAA, Finland

The book series “Indoor Environment and Sustainable Buildings” publishes insights and latest research results on indoor environment and sustainable building, and aims to provide a more energy-efficient, safer and healthier space engineered for human to live and work. The intent is to cover all the technical contents, applications, and multidisciplinary aspects of the engineering techniques for improving indoor environment quality and the energy efficiency of heating, ventilation, and air conditioning (HVAC) systems, as well as the smart home devices, sustainable architectures and buildings. Topics in the book series include: ● ● ● ● ● ● ● ●

indoor environment thermal comfort heating, ventilation, and air conditioning (HVAC) systems smart home devices energy efficiency building physics building services sustainable architectures and buildings, etc.

The objective of the book series is to publish monographs, reference works, selected contributions from specialized conferences, and textbooks with high quality in the field of indoor environment and sustainable building. The series provides valuable references to a wide audience in the community of HVAC researchers, indoor space designers, policy makers and architects.

Faming Wang · Bin Yang · Qihong Deng · Maohui Luo Editors

Personal Comfort Systems for Improving Indoor Thermal Comfort and Air Quality

Editors Faming Wang Department of Biosystems KU Leuven Leuven, Belgium

Bin Yang School of Energy and Safety Engineering Tianjin Chengjian University Tianjin, China

Qihong Deng School of Public Health Zhengzhou University Zhengzhou, Henan, China

Maohui Luo School of Mechanical Engineering Tongji University Shanghai, China

ISSN 2730-7042 ISSN 2730-7050 (electronic) Indoor Environment and Sustainable Building ISBN 978-981-99-0717-5 ISBN 978-981-99-0718-2 (eBook) https://doi.org/10.1007/978-981-99-0718-2 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Preface

This book begins with editors explaining the publication purpose of this book as well as why this book is so unique and important. Personal comfort devices are not a new concept, and electric fans, one of the most well-known but outmoded personal comfort devices, were invented in 1886. Personal comfort devices such as electric fans were widely used indoors to mitigate heat stress prior to the invention of air conditioning. Similarly, in the past, electrical heating blankets were commonly used under the bed to help people stay warm while sleeping. Despite the aforementioned personal comfort systems were the most important way to assist individuals in achieving comfort in the past, they were gradually phased out due to the invention of air-conditioning. In 1902, an American engineer named Willis Carrier invented the first modern air-conditioning system. Later, H. H. Schultz and J. Q. Sherman invented the first room air conditioner in 1931; it sat on a window ledge, similar to today’s portable AC units. Barreca et al. [1] published a study on heat-related deaths in the United States, as well as the impact of air conditioning on the country’s mortality rates. They discovered that the mortality effect of an extremely hot day decreased by roughly 80% from 1960 to 2004, when compared to 1900–1959. The adoption of residential air conditioning contributed significantly to the reduction in heat-related mortality. Since 1960, when ordinary families began to instal air conditioning in their homes, air conditioning has played a critical role in both residential and commercial buildings. Today, conventional built environments are normally maintained as homogeneous to help occupants attain comfortable thermal working environments. Unfortunately, maintaining homogeneous indoor environments could lead to unnecessary energy consumption. Moreover, a homogeneous indoor environment failed to fulfill individual needs on thermal comfort. Over the past few decades, various novel personal comfort systems have been developed to fulfil the growing needs of high-performance HVAC systems with better performance of energy efficiency, thermal comfort, and occupancy health and wellbeing. Intensified conditioning of occupants’ occupied areas and less intensified conditioning of surrounding built environment will be able to effectively improve the overall thermal environment satisfaction and also, achieve maximum energy efficiency. The four novel personal thermal comfort enhancement v

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systems include the task ambient conditioning (TAC) system, personal environmental control system (PECS), personal comfort system (PCS), and wearable personal thermal management system (PTMS) [4]. Presently, there is a lack of a reference book to in-depth overview various personal comfort systems and their recent development and applications. More recently, due to the EU energy crisis and increased attention paid to indoor energy savings by governments in many EU countries, it is expected that the use of personal comfort systems will re-gain ever-increasing popularity in the coming years with the help of new policy and regulations to achieve energy-efficient thermal comfort. Further, deep learning and computer vision are hastening the pace of change in indoor built environments toward the highest occupant comfort, health and wellbeing standards. The application of computer vision and deep learning based on long-term unobtrusive and continuous tracking of vital human body signs could assist our society in moving toward precision health, which encourages disease prevention and earlier detection by monitoring health and disease based on an individual’s risk [2] and [5]. In the light of the foregoing, this book also includes a chapter on the application of computer vision technology in indoor built environments. This book first describes fundamental knowledge on human thermal comfort and assessment of indoor thermal comfort, adaptive thermal comfort, thermal comfort in sleeping environments, modelling of human thermal comfort, and thermal comfort assessment using human trials. For the first time, systematic guidelines and a checklist to guide field researchers conducting thermal comfort research with indoor occupants are proposed. The authors hope that the information presented in Lei et al.’s chapter [3] will result in better experimental design when conducting the experiment on thermal comfort using indoor occupants in both occupational and home-based environments. Next, the book presents an in-depth review of recent concept progress and evaluation of various personal comfort systems, summarises important findings and feasible applications, current gaps as well as future research needs. The eights chapters included in this section are task/ambient conditioning systems, personalized ventilation systems, electric fans, personal comfort systems, thermoelectric systems, personal thermal management systems, wearable personal thermal comfort systems and contactless sensing of indoor thermal comfort and air quality using computer vision technology. We hope that this reference book will provide up-to-date information for building and architecture scientists, engineers, and designers in universities, research institutes, and commercial firms. We also hope that the book will be useful to HVAC students, architects, and public health and wellness practitioners who work on indoor thermal comfort assessment, built environment management and indoor environment design. Lastly, many authors became infected with the SARS-CoV-2 omicron variant while preparing their book chapters due to the post-lockdown COVID surge in Mainland China (December 2022). Despite the challenging time constraints and unfavourable health conditions, all authors submitted their chapters on schedule. We hereby would like to express our appreciation to all of the authors who contributed

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to this book, especially Prof. S. Hu and Dr. S. Liang (Qingdao University of Technology), Prof. S. I. Tanabe (Waseda University), Dr. T. H. Lei (Hubei Normal University), Prof. L. Lan (Shanghai Jiaotong University), Prof. Y. Zhai (Xi’an University of Architecture and Technology), Prof. D. Zhao (Southeast University), Dr. W. Song (Guangdong University of Technology) and Dr. X. Cheng (Nanjing University of Posts and Telecommunications). Our thanks also go to the entire Springer production team. Thank you for your perseverance and hard work in bringing this book to fruition. Leuven, Belgium Tianjin, China Zhengzhou, China Shanghai, China January 2023

Faming Wang Bin Yang Qihong Deng Maohui Luo

References 1. Barreca A, Clay K, Deschenes O, Greenstone M, Shapiro JS (2013) Adapting to climate change: the remarkable decline in the U.S. temperature-mortality relationship over the 20th century. NBER Working Paper 18692, National Bureau of Economic Research, Cambridge, MA 2. Jazizadeh F, Pradeep S (2016) Can computer visually quantify human thermal comfort? In: BuildSys ’16: Proceedings of the 3rd ACM international conference on systems for energyefficient built environments, pp 95–98. https://doi.org/10.1145/2993422.2993571 3. Lei TH, Lan L, Wang F (2023) Indoor thermal comfort assessment using human trials. In: Wang F, Yang B, Deng Q, Luo M (eds) Personal comfort systems for improving indoor thermal comfort and air quality. Springer, https://doi.org/10.1007/978-981-99-0718-2_5 4. Yang B, Ding X, Wang F, Li A (2021) A review of intensified conditioning of personal microenvironments: moving closer to the human body. Energy Built Environ 2(3):260–270 5. Yang B, Li X, Hou Y, Meier A, Cheng X, Choi JH, Wang F, Wang H, Wagner A, Yan D, Li A, Olofsson T, Li H (2020) Non-invasive (non-contact) measurements of human thermal physiology signals and thermal comfort/discomfort poses—a review. Energy Build 224:110261

Contents

Thermal Comfort . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Guangtao Fan, Yu Chen, and Qihong Deng

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Adaptive Thermal Comfort . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Maohui Luo

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Thermal Comfort in Sleeping Environments . . . . . . . . . . . . . . . . . . . . . . . . . Songtao Hu and Shimin Liang

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Human Thermal Comfort Modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Shin-ichi Tanabe, Akihisa Nomoto, Yoshito Takahashi, and Yutaro Ogawa

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Indoor Thermal Comfort Assessment Using Human Trials . . . . . . . . . . . . Tze-Huan Lei, Li Lan, and Faming Wang

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Task/Ambient Conditioning Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bin Yang, Yuyao Guo, Xiaojing Li, and Zhiyu Song

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Personalized Ventilation Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 Bin Yang, Yihang Liu, Xin Zhu, and Xiaojing Li Electric Fans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 Bin Yang, Shuang Yang, Xiaojing Li, and Dacheng Jin Personal Comfort Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151 Wenfang Song, Yongchao Zhai, and Faming Wang Thermoelectric System for Personal Cooling and Heating . . . . . . . . . . . . . 185 Haodan Pan, Xueying Li, and Dongliang Zhao

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Personal Thermal Management Materials (PTMMs) . . . . . . . . . . . . . . . . . . 213 Wenfang Song and Wenyue Lu Wearable Personal Thermal Management Systems (PTMS) . . . . . . . . . . . 245 Faming Wang Contactless Sensing of Indoor Thermal Comfort and Air Quality Using Computer Vision Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265 Xiaogang Cheng

About the Editors

Faming Wang is a professor in human health engineering at the Department of Biosystems, KU Leuven, Belgium. He is the head of Bio-environmental Control group under M3-BIORES. Faming earned his Lic. Phil. and Ph.D. degrees in Working Environment (arbetsmiljö) from Lund University (Sweden, Ph.D. degree conferred on December 16, 2011). He was a Marie Curie Postdoc Fellow (Switzerland) and a Killam Fellow (Canada) prior to becoming a tenured faculty member in October 2013. His research interests are mainly focused on built environment, human thermoregulatory system modelling, precision health and occupational safety. Professor Wang has supervised three Ph.D. students as the main supervisor and five postdocs in the area of ergonomics, clothing physiology, smart wearables, human thermoregulatory system modelling and mechanical/HVAC engineering; He has secured over 20 grants as Principal Investigator from different funding bodies in China, Hong Kong SAR, Canada, and EU (total granted amount: 2.7 million Euro). Professor Wang has been ad hoc reviewers for over 100 journals and has reviewed over 600 journal manuscripts, conference submissions, book proposals and Ph.D. theses. He serves as Associate Editor for Indoor Air, Journal of Thermal Biology and Frontiers in Built Environment, and is also an editorial board member for several field journals including Ergonomics, Frontiers in Physiology, International Journal of Occupational Safety and Ergonomics, Energies and Frontiers in Energy Research; He has been ranked among the top 2% of world scientists in building and construction and human xi

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About the Editors

factors (single year and career long lists, published by Stanford University) since 2020. To date, Prof. Wang has published extensively with 269 publications including 140 peer-reviewed journal articles, 80 conference papers, 24 books/chapters, 5 technical reports, 20 computer copyrights and patents. Dr. Bin Yang is a chair professor and school dean in School of Energy and Safety Engineering, Tianjin Chengjian University, China. He got his joint Ph.D. from Technical University of Denmark (DTU) and National University of Singapore (NUS). He got his bachelor and master degree from Tianjin University (China). He is a researcher with more than 20 years research experience in thermal comfort, indoor air quality, building energy efficiency and HVAC. He has worked in different countries with world famous research groups such as DTU and University of California Berkeley (UCB). He was a tenured professor in Umeå University, Sweden. He has obtained several awards such as REHVA Young Scientist Award, ASHRAE Ralph Nevins Physiology and Human Environment Award, ASHRAE Graduate Grant-in-Aid Award, Chinese Government Award for Outstanding Self-financed Students Abroad, and so forth. He has given presentations as one invited speaker several times in globally famous Universities and research institutes such as University of Southern California, Alberta University, Tsinghua University, REHVA, etc. He has more than 100 journal publications. Dr. Qihong Deng is a chair professor in Zhengzhou University School of Public Health and vice president for the International Society of Indoor Air Quality and Climate (ISIAQ). He has mainly focused on the health risk of air pollution and global warming, with the objectives to design comfortable and healthy buildings/cities. He is now the associate editor for the International Journal of Environmental Health Research (Taylor and Francis), and editorial board members for several international journals, such as Environment International (Elsevier), BMC Medicine (BioMed Central), International Journal of Hygiene and Environmental Health (Elsevier), Annals of Epidemiology (Elsevier), Building and Environment (Elsevier), Indoor Air (Wiley), Energy and Buildings (Elsevier), Urban Climate (Elsevier) and

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Journal of Thermal Biology (Elsevier). He is now the Fellows of the International Society of Indoor Air Quality and Climate (ISIAQ), New York Academy of Medicine (NYAM), Royal Society for Public Health (RSPH), Royal Meteorological Society (RMetS), and International Society of the Built Environment (ISBE). He has published more than 150 papers in international journals (more than 80 during the past five years) and has been selected as the Most Cited Chinese Researchers since 2015 (Elsevier). Maohui Luo is an assistant professor at Tongji University in Shanghai, China. His research interests mainly focused on human thermal comfort and human factors engineering for the built environment. Currently, Dr. Luo hosts more than 10 comfort-related projects, including micro-scale thermal sensitivity, human body thermoregulation modelling, intelligent thermal preference detecting, and thermal comfort applications in the kitchen, bathroom, automobile, and medical equipment. To date, he has published more than 40 papers and received over 2800 citations (according to Google Scholar). In 2022, Dr. Luo was honored as the leading talent in Shanghai.

Thermal Comfort Guangtao Fan, Yu Chen, and Qihong Deng

Nomenclature ADP ANSI ASHRAE ATP CEN EPBD ET ET* ISO MV PMV PPD RH SET

Adenosine diphosphate American National Standards Institute American Society of Heating, Refrigerating and Air-Conditioning Engineers Adenosine triphosphate European Committee for Standardization Energy Performance of Buildings Directive Effective Temperature New Effective Temperature International Organization for Standardization Mean vote Predicted mean vote Predicted percentage of dissatisfied Relative humidity Standard Effective Temperature

G. Fan · Y. Chen School of Water Conservancy and Civil Engineering, Zhengzhou University, Zhengzhou, China Q. Deng (B) School of Public Health, Zhengzhou University, Zhengzhou, China e-mail: [email protected]

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 F. Wang et al. (eds.), Personal Comfort Systems for Improving Indoor Thermal Comfort and Air Quality, Indoor Environment and Sustainable Building, https://doi.org/10.1007/978-981-99-0718-2_1

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Variables E req E max C c E H K L M m Pv R RES S T ta T comf T lim t mrt To t op tr T rm W α

Required evaporative cooling for heat balance Maximal evaporative cooling from the environment Convective heat transfer Specific heat Evaporative heat transfer Heat transfer Conductive heat transfer Thermal load of the body Metabolic heat production Mass of the object or tissue Partial water vapor pressure in ambient air Radiative heat transfer Respiratory heat loss Heat storage Temperature Air temperature Optimum temperature for comfort Limits of the acceptable temperature Mean radiant temperature Prevailing mean outdoor temperature Operative temperature Radiative temperature Exponentially weighted operating average of outdoor temperature External work Sensitivity coefficient

1 Definition of Thermal Comfort and Influential Factors According to ASHRAE standard 55 [1] and ISO 7730 [2], thermal comfort is defined as “a condition of mind that expresses satisfaction with the thermal environment”. In a nutshell, it means that a person in thermal comfort feels neither too warm nor too cold. This definition is easily comprehended, but it is difficult to translate into physical parameters because thermal comfort is affected by a variety of factors, including thermal environmental parameters and personal factors. According to heat balance equation of human body, there are six basic factors that must be considered when defining conditions for thermal comfort. These six basic factors are air temperature, mean radiant temperature, air velocity and vapor pressure of the environment to which a person is exposed, as well as human metabolic rate and clothing insulation [1], as shown in Table 1. All of these six factors may vary with time and are needed to maintain thermal homeostasis in order to sustain the

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Table 1 Six basic factors influencing thermal comfort Factors

Description

Air temperature

A common component of thermal comfort; it can be easily influenced with passive and mechanical heating and cooling

Mean radiant temperature The weighted average temperature of all exposed surfaces in a room. Combined with the air temperature, it allows defining the operative temperature which is the most essential component of thermal comfort Air velocity

Quantifies the speed and direction of the air movements in the room. Rapid air velocity fluctuations might result in draught complaints

Vapor pressure

The moisture content of air. Too high or too low humidity levels may induce discomfort

Clothing level

The amount of insulation added to the human body. Higher clothing levels will reduce the heat lost through the skin and lower the environment’s temperature perceived as comfortable

Metabolic rate

(also called physical activity level) has an influence on the amount of heat produced by the human body and therefore also in the perception of a hot or cold environment

occupants’ satisfaction with their surrounding thermal environment. In addition to these six basic factors, the adaptive opportunity afforded by the environment people occupy can also affect human thermal comfort. For example, in naturally ventilated environments, when a change occurs to produce discomfort, people react in ways which tend to restore their comfort [3]. Since the emergence of air-conditioning in the built environment, the research and practice on thermal comfort has been expanding all the time. In particular, the interaction of the six basic factors above is well researched. However, designing for adaptive opportunities is at the emerging stage in terms of environmental design and assessment for thermal comfort and it provides major opportunities for innovative ideas to achieve thermal comfort. Thermal comfort is an interdisciplinary field of study involving various scientific fields, such as building science, thermal physiology and psychology, etc. Maintaining thermal comfort for occupants in buildings or other enclosures is one of the primary goals of heating, ventilating, and air conditioning design engineers.

2 Human Thermoregulation and Thermal Comfort Maintaining thermal homeostasis of the body is a necessary condition for thermal comfort. Thermal homeostasis is affected by the heat generated by human metabolism and the heat exchange between the human body and its environment. When expose to either cold or heat stress environment, the thermoregulatory function of the human body will be actively engaged to ensure thermal homeostasis. Thermoregulation

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has always been an integral component of thermal physiology and it is becoming increasingly important due to global warming [4].

2.1 Key Concepts of Thermoregulation 2.1.1

Human Heat Production

Heat in the body is mainly produced by our cellular metabolic activity with more than 80% of it being converted into heat. The energy produced in the metabolic process is mainly stored in adenosine triphosphate (ATP) [5]. When energy is needed to sustain the basal metabolic rate or to perform physical work, one of the phosphate bonds in ATP is split to result in a unit of phosphate and an adenosine diphosphate (ADP) (see Fig. 1) [5]. The splitting of the phosphate bond produces heat, which is harnessed as energy to drive muscle contraction to produce mechanical work. In general, only about 25–30% of metabolic heat is used by muscles to perform mechanical work [6, 7]. The rest of the metabolic heat (70%) is stored in the body with no physiological functions [5]. Because the metabolic system functions continuously to meet energy demands in the body and heat is a byproduct of metabolism, metabolic heat production is always positive. The regulation of thermogenic activity is influenced by neural and humoral factors. Sympathetic excitation, as well as adrenaline and thyroid hormones, can increase the body’s metabolic level, thus increasing heat production.

2.1.2

Biophysics of Heat Transfer

Heat loss and metabolic heat production generally occur simultaneously. The net amount of heat stored in the body is a function of the balance between heat gain and loss. The major organ of the body that dissipates heat is the skin, as shown in Fig. 2. Heat loss between the skin and the environment is mainly through radiation, conduction and convection between the skin and the environment, as well as evaporative heat loss. The amount of heat loss between the skin and the environment depends mainly on the temperature difference between the skin and the surrounding environment. The magnitude of evaporative heat loss is primarily influenced by the water vapor pressure in the environment. When the skin temperature is higher than environmental temperature, such as during exposure to winter conditions or when sitting in a cold room, the skin can lose heat to the environment through radiation, conduction and convection. When the skin temperature is equal to or lower than environment temperature, physical means for heat loss, including heat radiation, conduction and convection, will not work. Evaporative heat loss becomes the only way of heat loss of the body. Evaporation heat loss has two forms, including insensible perspiration and sweating. Insensible perspiration is the loss of water through the skin which does not occur as perceivable sweat. Sweating is the secretion of sweat through the eccrine and apocrine sweat glands.

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Fig. 1 The ATP-ADP cycle

In addition to heat loss between the skin and the environment, respiratory heat loss is also a non-ignorable part. A person breaths in air that contains water vapor. The respiratory tract and lungs raise the temperature of the air to internal body temperature and saturate the air at that temperature. Therefore, expired air can transfer heat from the body by evaporation and convection. The evaporative heat loss is driven by the difference of the amount of water vapor between inspired air and expired air. The convection heat loss is affected by the difference of the dry-bulb temperature between inspired air and expired air.

2.1.3

Core Temperature

Core temperature is the deep body temperature in the internal body, such as the abdominal, thoracic, and cranial cavities [8]. From a measurement perspective, core temperature refers to the temperature of venous blood returning to the heart, which stores excess metabolic heat produced in the organs. Because the measurement of venous blood temperature is invasive, the most common sites for indirect measurement of core temperature are the rectum, esophagus, and gastrointestinal tract for research [9, 10]. Core temperature has been used as a main indicator for defining hypothermia and hyperthermia, and for indicating the associated risk of heat and cold injuries.

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Fig. 2 The main avenues of heat loss from the human body during cold and heat stress

2.1.4

Extent of Heat Stress

The extent of heat stress can be classified into compensable and uncompensable heat stress. This is mainly determined by the ration between the required evaporative cooling for heat balance (Ereq ) and maximal evaporative cooling from the environment (Emax ). Compensable heat stress is defined as when Ereq /Emax ≤ 1, through which a thermoregulatory steady state can be obtained. This means that heat dissipation is at least equal to heat production and therefore, there is no further increase in core temperature. On the contrary, uncompensable heat stress is defined as Ereq /Emax ≥ 1 where heat production exceeds maximal heat dissipation power. As a result, the core temperature increases continuously and thus it increases the likelihood of heatrelated illnesses. Furthermore, it is worth mentioning that compensable heat stress can easily switch to uncompensable heat stress, if the rate of evaporation from the skin’s surface is limited [11].

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Heat stress is the net heat load to which a person may be exposed from the combined contributions of metabolic heat, environmental factors and clothing requirements. Heat strain is a series of physiological response resulting from heat stress. The physiological responses are dedicated to dissipating excess heat from the body, such as elevated body temperature, elevated heart rate, and dehydration due to acute sweating.

2.1.5

Heat Tolerance

Heat tolerance is defined as cellular adaptation caused by a sub lethal heat exposure, which allows organisms to survive in subsequent lethal heat stress [12]. This definition is based on the protective effect of heat shock protein on cell structure, that is, the protective effect of cellular structure against lethal heat stress after a single dose of exposure to sub-lethal heat stress [5]. In terms of human thermal comfort, heat tolerance is generally defined as the ability to tolerate heat stress without physiological and work failures.

2.2 Human Thermoregulation Process Human thermoregulation is a very complicated process. When the body is stimulated by the environment, skeletal muscle, respiratory system, nervous system, endocrine system and skin play an important role in the process of thermoregulation. Human thermoregulation is achieved via autonomic (i.e., involuntary) and behavioral (i.e., voluntary) responses [13]. These two responses are entirely different branches of the thermoregulatory system, sharing the same sensors for detecting changes in thermal homeostasis but having discrete pathways for afferent and efferent information relay, as well as for central (i.e., brain) information processing [14]. Figure 3 illustrates the autonomic and behavioral branches of human thermoregulation together with warmand cold-specific thermoeffector responses, respectively. The asterisks denote that the provided responses are some examples of the multitude of behavioral response options.

2.2.1

Autonomic Thermoregulatory

Autonomic thermoregulation is based on sensing changes in temperature by thermosensors which are spread throughout the body and transmit afferent information to the central nervous system. These thermal signals are integrated and, subsequently, transmitted downstream via the autonomic nervous system to the thermoeffectors (e.g., cutaneous vasculature, sweat glands, skeletal muscle, white or brown adipose tissue) to initiate heat dissipating or heat conserving/generating responses, thereby

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Fig. 3 The autonomic and behavioral branches of human thermoregulation together with warm-and cold-specific thermoeffector responses [14]

maintaining the dynamic balance of heat production and heat dissipation [15]. Specifically, if the body is exposed to a warmer environment or strenuous exercise to increase metabolic heat production, heat dissipation mechanisms, such as eccrine sweating, skin vasodilation and tachypnea, will be stimulated, if the body is exposed to a colder environment or a person removes clothing insulation, heat conservation or generation mechanisms, such as peripheral/skin vasoconstriction, shivering and non-shivering thermogenesis, will be stimulated [15]. Shivering thermogenesis is performed in the muscle, while non-shivering thermogenesis is performed in the muscle and the brown adipose tissue [16, 17]. These autonomic processes are key players in thermoregulation, and can alter both heat production and dissipation from the body within a short period, maintaining body core temperature at a relatively stable level, thereby enabling humans to survive even in extreme conditions, and to perform strenuous exercise for longer periods. However, these processes place large demands on energy expenditure, circulatory blood redistribution, and the body’s water supply [18].

2.2.2

Behavioral Thermoregulatory

Behavioral thermoregulation refers to that the body controls thermal balance by activating conscious activities to maintain a constant core temperature. These conscious activities can include a wide array of simple or complex activities, such as changing environment and posture, or altering physical activity. Well-observed examples of behavioral thermoregulation in humans include taking off or putting on clothing and using air conditioners [19]. Behavioral thermoregulation is based on autonomic thermoregulation, which is preceded by the change of cutaneous vascular tone [20]. It even reduces the requirement for autonomic thermoregulation, and is proposed as the first line of thermal

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defense [21]. This is based on the fact that autonomic thermoregulation has limited capacity to regulate our core temperature, whereas behavioral thermoregulation has a near-infinite capacity to regulate our core temperature [18, 22]. Behavioral thermoregulation is initiated at a given level of thermal discomfort, which is regulated by the change of mean skin temperature if core temperature remains unchanged. Sometimes behavioral thermoregulation occurs in anticipation of a thermal challenge. In the winter, for example, we put on a down jacket and change our clothing based on the weather forecast [23]. However, common signals activating and determining behavioral thermoregulation would be thermal information about the environment. Behavioral thermoregulation is different between male and female with females utilizing their behavioural response more than their male counterpart during acute heat exposure [24].

2.3 Active and Passive Systems of Heat Transfer The human body is a system that detects fluctuations in the internal environment and subsequently initiates an appropriate feedback control mechanism, in order to restore the internal environment [25–28]. This feedback control mechanism can be quantified using mathematical models. In addition, a good mathematical model can work cohesively with the physiological system, to explain the physiological response. To date, most models are centered on passive and active systems [29]. The passive system (open loop) is defined as a material that is responsible only for heat transfer to or from the surrounding area. Heat transfer (H) for the passive system is illustrated by the equation below: ΔH = mcdT /dt where m is the mass of the object or tissue; c: is the specific heat of the object; and dT /dt represents changes in the object or core temperature with respect to time. This equation from above gives an indication of heat storage rates at rest and during exercise (via the passive system). At rest, positive heat gain is achieved when ambient temperatures exceed the skin temperature. On the contrary, heat loss occurs when the skin temperature is greater than the environmental temperature. However, when calculating positive heat gain during exercise, metabolic heat production must be considered, in addition to the net heat flux from the ambient environment. The structure of the passive system includes all anatomical structures, such as the body surface area and muscle tissues. Furthermore, this system behaves like an open loop control, with the absence of an effector mechanism [30]. The passive system indicates the direction of heat transfer between the core to the skin and from the skin to the environment. This provides information about maximal evaporative cooling, which includes the effect of clothing, on heat dissipation. Maximal evaporative cooling is defined as the maximal heat loss achieved by the combination of the environment and clothing.

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The active system (closed loop) includes sensors that detect changes in the initial value, a central controller and an effector mechanism. This system behaves like a closed-control loop and it is composed of thermal receptors, ascending and descending pathways and the effector mechanisms. The active system initiates a feedback control and a physiological control mechanism, in order to maintain thermal homeostasis. Collectively, both active and passive systems behave like a closedcontrol loop, in order to maintain the core body temperature between approximately 37.0 and 37.5 °C. Collectively, by understanding the active and passive systems of heat transfer would result in the better understanding of the biophysics model and the Fanger’s model of thermal comfort as the main theory of those two models are based on both active and passive systems of heat transfer.

2.4 Human Thermoregulation Model Representation of the human in a thermoregulatory model is most often done by sectioning the human into nodes, segments, and elements, typically using one of four different designs: (1) one-node, (2) two-node, (3) multi-node, or (4) multi-element. The multi-element approach is more realistic human shape [31]. A classical multisegment multi-node human thermoregulation model divided the human body into 6 segments (head, trunk, arms, hands, legs and feet), each segment consisting of 4 concentric layers (core, muscle, fat and skin), and a central blood compartment, as shown in Fig. 4 [32]. The thermoregulation model is a negative feedback system that comprises controlling active and controlled passive sub-systems [33]. In the active system, the thermoafferent information is first transmitted by the skin thermoreceptors to the central nervous system where the information is integrated into thermoefferent signals (warms or colds). These signals are then forwarded by the autonomic nervous system to peripheral effectors that regulate cutaneous vasodilation and sweating (if warms) or cutaneous vasoconstriction and shivering (if colds). The effector responses together with the metabolic rate finally alter the balance between heat gain and heat loss within the body. In the passive system, heat flow between adjacent layers is by conduction, all layers exchange heat by convection with a central blood compartment, and heat exchange between the skin and the environment by convection, radiation, evaporation and respiration. The classical thermoregulatory model was based on set-point (fixed threshold) concept, i.e., the strength of the efferent signals depended on the comparison between the afferent signals and set-point temperatures. However, recent experimental studies have found that the threshold temperature is not fixed but varies within a zone [34]. Also, mounting evidence shows that dehydration not only decreases the skin blood flow by increasing the threshold for cutaneous vasodilation but also degrades the sweat response by increasing the threshold temperature and decreasing the sensitivity [35]. In order to predict the time course of the development of heatstroke during exposure to high temperature, Deng et al. improved the classical thermoregulation model by considering the effect of dehydration [32].

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Fig. 4 Multi-segment multi-node human thermoregulatory model [32]

3 Thermal Comfort Evaluation of Built Environments Thermal comfort in built environments is an everyday issue for architects and technicians. For a practical evaluation of a given environment’s internal thermal conditions, some helpful indices are generally used. These indices can be divided into two main categories: sensation indices and temperature indices. Sensation indices, based on a thermal sensation scale, predict the mean value of the votes expressed by a large group of people exposed to the same microclimatic conditions. Temperature indices express the thermal response of the subject in reference to so-called equivalent temperatures, which are the temperatures of imaginary environments in which the occupants would feel the same thermal sensation as in the real environments.

3.1 Sensation Indices 3.1.1

The Predicted Mean Vote (PMV)

The most widely applied index for the evaluation of indoor thermal conditions in moderate environments is the predicted mean vote (PMV), which has been incorporated into ISO 7730. This thermal sensation index, originally introduced by Fanger on the basis of a series of experimental studies, represents the mean sensation vote expressed by a large group of persons for any given combination of air temperature, mean radiant temperature, air velocity, humidity, activity and clothing [36]. These votes are linked to the environment on the basis of the ASHRAE thermal sensation scale. Fanger transposed the commonly used ASHRAE scale (numbered 1, cold, through 4, neutral; to 7, hot) to indicate the PMV from −3 through

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Table 2 Thermal sensation scale Sensation

Cold

Cool

Slightly cool

Neutral

Slightly warm

Warm

Hot

PMV

−3

−2

−1

0

+1

+2

+3

0 to +3. That is −3, cold; −2 cool; −1 slightly cool; 0, neutral; +1, slightly warm; That is −3, cold; −2 cool; −1 slightly cool; 0, neutral; +1, slightly warm; +2, warm; +3, hot, as indicated in Table 2 [36, 37]. When the comfort equation is satisfied, it would be expected that for a large group of people PMV = 0 (neutral). The PMV thus provides information about any deviation from the comfort equation, that is, the degree of thermal discomfort experienced by people in the assigned environment.

3.1.2

The Predicted Percentage of Dissatisfied (PPD)

The PMV represents the mean thermal sensation vote of a large group of people exposed to a given combination of the variables. However, there are biological differences between people, even though the mean vote would signal the achievement of thermal comfort conditions, some people may experience cold or hot sensations with respect to the thermal environment. In other words, although PMV indicates when the environment is optimum (PMV = 0, Neutral), and the degree of discomfort away from neutral, it does not indicate how much dissatisfaction there will be among the group for a given mean sensation vote. In order to take these deviations from the mean expected conditions into account, an additional index has been introduced, the predicted percentage of dissatisfied (PPD). A dissatisfied person is defined as a subject who, when exposed to a defined thermal load, would vote +3, +2, −2, or −3 on the previously defined thermal sensation scale. As people are not alike, there will be a variation in dissatisfaction across the group. This index assesses a quantitative prediction of the percentage of subjects who may declare themselves thermally dissatisfied with the indoor environment conditions. The PPD is therefore a function of the PMV and the relationship linking these two indices is graphically shown in Fig. 5. ISO recommends a PPD of less than 10% (i.e., PMV: −0.5 ~ +0.5) for acceptable thermal comfort.

3.1.3

The Adaptive Thermal Comfort

Though the PMV and PPD indices have been widely adopted internationally, they remain controversial, as the PMV and PPD indices are based on the theory of the neutral thermal state and the static thermal comfort models. The PMV and PPD indices fairly capable of describing the thermal sensations of subjects in indoor environments equipped with conditioning and ventilation systems. A subtle point is that it does account for the consequences of human behavior as if the environment to which people are exposed to change from one set of (steady state) conditions

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Fig. 5 The parabolic relationship between PMV and PPD

to another, when PMV will change. However, the PMV does not account for the disposition that if a person is uncomfortable, they will do something about it. If a person is cold, they will attempt to ‘warm up’. If they are hot, they will attempt to ‘cool down’. Good environmental design will provide opportunities to do so. For this reason, the adaptive thermal comfort model emerged as a result of the research studies by Nicol and Humphreys [38] and Humphreys [39], who detected that the thermal comfort models obtained in climate chambers were not adjusted to buildings that operated with natural ventilation. Users’ thermal adaptation capacity in buildings without HVAC systems was also detected when the external temperature varied, so the users’ temperature range is greater than that obtained by Fanger’s model. Also, the correspondence between the votes provided by the subjects and the indices are less obvious in naturally ventilated environments in which mechanical equipment is not present. It has been found that in such environments the occupants generally consider a wider range of thermal conditions acceptable and are inclined to prefer different indoor temperatures than those assumed by using the PMV index. The detailed contents on adaptive thermal comfort model can be seen in Chap. 2.

3.2 Temperature Indices 3.2.1

New Effective Temperature (ET*)

Effective temperature is a temperature index of the various combinations of dry-bulb temperature, humidity and air movement that induce the same thermal sensation. Effective temperature cannot be measured by a thermometer and can be obtained by human experiment. Effective temperature overestimates the effect of humidity on cool and comfortable conditions at low temperatures. Gagge et al. defined the New Effective Temperature (ET* ) as the temperature of a standard environment (ta = tr ; v < 0.15 m/s; RH = 50%) in which a person would have the same heat loss, at the

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same mean skin temperature and the same skin wettedness as he (or she) does in the actual environment [40]. The ET* was derived for use in the region of thermal comfort conditions for people in rooms conducting light work in light clothing.

3.2.2

Standard Effective Temperature (SET)

On the basis of the ET* , Gagge et al. also provided a more universal index that could be used for considerations of heat stress, cold stress and thermal comfort [41]. This is the Standard Effective Temperature (SET), which considers the combined effect of different activity levels and thermal resistance of clothing. The SET is defined as the temperature of an isothermal room (air temperature = average radiation temperature, still air; RH = 50%) in which a person with a standard level of clothing insulation would have the same heat loss, the same mean skin temperature and at the same skin wetness as he (or she) does in the actual environment and clothing insulation under consideration. As with the work of Fanger, it was recognized that the condition of the body for thermal comfort (or any sensation) will depend upon the activity level. So that the same SET value provided neutral comfort (24 °C, SET), the standard level of clothing used in the definition of the standard environment is modified as a function of activity level. The SET is an extension of the ET* , so that it can be used to assess not only moderate environments but also hot and cold environments. As the standard clothing varies with activity level, the SET value can be directly related to sensation and does not depend upon the metabolic rate.

3.2.3

Operative Temperature

Operative temperature is defined as a uniform temperature of a radiantly black enclosure in which an occupant would exchange the same amount of heat by radiation plus convection as in the actual non-uniform environment [1]. It is a synthetic temperature obtained by considering the influence of air temperature and average radiation temperature on human thermal sensation. It comprehensively considers the convection and radiation heat transfer between the environment and human body. In most practical cases where the relative air speed is small ( 0.05). In contrast, publication bias was found in PHD studies (see Fig. 4b), as evidenced by statistically significant Egger’s (p < 0.01) and Begg’s tests (p < 0.01). Cooling studies had low heterogeneity, as evidenced by I 2 of 38%, whereas heating studies has moderate heterogeneity, as evidenced by I 2 of 64%. As a result, the fixeffects and the random-effects models were used to calculate thermal comfort EFSs for PCDs and PHDs, respectively.

3.3 Effects of PCDs on Perceptual Responses and Power Consumption The I 2 statistic values for thermal sensation, thermal comfort and thermal acceptability are 45%, 38% and 0%, respectively. PCDs showed large effect in reducing thermal sensation [EFS = 1.23, 95% Confidence Intervals (CI): 1.13 to 1.32; n = 65 studies] and moderate effects on thermal comfort (EFS = 0.73, 95% CI: 0.63 to 0.83; n = 55) and thermal acceptability (EFS = 0.68, 95% CI: 0.52 to 0.84; n = 19).

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3.4 Effect of PCDs on Perceptual Responses Targeting Different Body Regions Figure 5 shows the effects of PCDs on thermal sensation, thermal comfort, and thermal acceptability targeting different body regions. Cooling the head/face demonstrated a large effect on improving thermal sensation (EFS = 1.07, 95% CI: 0.9 to 1.23, n = 20), and moderate effects in improving thermal comfort (EFS = 0.55, 95% CI: 0.36 to 0.74, n = 16) and thermal acceptability (EFS = 0.77, 95% CI: 0.35 to 1.18, n = 2). Cooling the torso had a large effect on thermal sensation improvement (EFS = 1.33, 95% CI: 1.06–1.59, n = 29), a moderate effect on thermal comfort (EFS = 0.71, 95% CI: 0.58–0.85, n = 29) and a small benefit on thermal acceptability (EFS = 0.47, 95% CI: 0.19–0.75, n = 9). One study found that cooling the extremities had a negligible effect on both thermal sensation (EFS = 0.20, 95% CI: −0.5–0.89, n = 1) and thermal acceptability (EFS = 0.19, 95% CI: −0.51–0.88, n = 1). Cooling more than two body regions manifested large effects in improving thermal sensation (EFS = 1.30, 95% CI: 1.11 to 1.49, n = 15), thermal comfort (EFS = 0.96,

Fig. 5 The hedge’s effect sizes of personal cooling devices on occupants’ overall thermal sensation vote (a), overall thermal comfort vote (b) and overall thermal acceptability vote (c) in warm environments related to subgroups based on cooled body regions

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95% CI: 0.6 to 0.8, n = 9) and thermal acceptability (EFS = 0.8, 95% CI: 0.57 to 1.02, n = 8).

3.5 Effect of Different Types of PCDs on Perceptual Responses Figure 6 displays the forest plot of the hedge’s effect sizes for the effect of different types of PCDs on thermal sensation, thermal comfort and thermal acceptability. CCPs showed a large effect on thermal sensation (EFS = 1.55, 95% CI: 1.19–1.91, n = 18) and thermal comfort (EFS = 0.95, 95% CI: 0.78–1.13, n = 18), but a small effect on thermal acceptability (EFS = 0.42, 95% CI: 0.06–0.79, n = 3). For the two types of CCPs, cooling chairs had a large effect on thermal sensation (EFS = 1.88) and thermal comfort (EFS = 1.17), while cooling pads had a large effect on thermal sensation (EFS = 1.13), and a moderate effect on thermal comfort (EFS = 0.66). The NCABs had large effects on thermal sensation (EFS = 1.14, 95% CI: 1.02– 1.26, n = 37), and moderate effects on thermal comfort (EFS = 0.66, 95% CI: 0.45 to 0.87, n = 31) and thermal acceptability (EFS = 0.65, 95% CI: 0.37 to 0.92, n = 9). For the various NCABs, desk fans, personalized ventilation, air nozzles and desk fans & air-cooling chairs demonstrated large effects in thermal sensation improvements (EFSs > 0.99), while air-cooling chairs demonstrated a moderate effect (EFS = 0.71). Only desk fans and air-cooling chairs had a large effect on thermal comfort improvement (EFS = 1.34), while the other types of NCABs showed moderate effects on thermal comfort (EFSs: from 0.5 to 0.59). The NCABs reported a moderate effect on thermal acceptability improvements (EFSs: from 0.54 to 0.79). CRTs had a large effect on thermal sensation improvement (EFS = 0.86, 95% CI: 0.14–1.31, n = 2), but a small benefit on improving thermal comfort (EFS = 0.34, 95% CI: 1.11–1.62, n = 1) and thermal acceptability (EFS = 0.42, 95% CI: 0.02– 1.18, n = 1). MCDs exhibited large effects on improving thermal sensation (EFS = 1.36, 95% CI: 1.11–1.62), thermal comfort (EFS = 0.91, 95% CI: 0.59–1.22) and thermal acceptability improvements (EFS = 0.87, 95% CI: 0.61–1.14). This type of device had a greater impact on improving perceptual sensations than other types of PCDs (p < 0.05).

3.6 Energy Performance of PCDs Thirty-one studies from eight publications reported the power consumption and energy performance of PCDs (Table S4). The various PCDs were discussed with air temperatures and relative humidity levels ranging from 28 to 32 °C and 40% to 60%, respectively. It should be noted that corrective power (CP) and corrective power efficiency (CEP) were included in the table, as well as the relationship between CEP.

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Fig. 6 The hedge’s effect sizes of personal cooling devices on occupants’ overall thermal sensation vote (a), overall thermal comfort vote (b) and overall thermal acceptability vote (c) in high temperature environments related to subgroups based on the type of PCDs

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Fig. 7 Relationship between corrective power values of personal cooling devices and air temperatures

It should also be noted the maximum input power values were presented in some papers and thus were listed as ranges. Corrective power values were calculated based on the thermal sensation values. The relationship between CEPs and air temperatures is depicted in Fig. 7. CEP values of CCPs are clearly seen to range from 11.4 to 47.6 W/K. It is also discovered that NCABs that use wind generated by mechanical fans produce much lower CEPs (i.e.,