Solar Energy Research Institute for India and the United States (SERIIUS): Lessons and Results from a Binational Consortium (Lecture Notes in Energy, 39) 3030331830, 9783030331832

Citation preview

Lecture Notes in Energy 39

David Ginley Kamanio Chattopadhyay   Editors

Solar Energy Research Institute for India and the United States (SERIIUS) Lessons and Results from a Binational Consortium

Lecture Notes in Energy

Lecture Notes in Energy (LNE) is a series that reports on new developments in the study of energy: from science and engineering to the analysis of energy policy. The series’ scope includes but is not limited to, renewable and green energy, nuclear, fossil fuels and carbon capture, energy systems, energy storage and harvesting, batteries and fuel cells, power systems, energy efficiency, energy in buildings, energy policy, as well as energy-related topics in economics, management and transportation. Books published in LNE are original and timely and bridge between advanced textbooks and the forefront of research. Readers of LNE include postgraduate students and non-specialist researchers wishing to gain an accessible introduction to a field of research as well as professionals and researchers with a need for an up-to-date reference book on a well-defined topic. The series publishes single- and multi-authored volumes as well as advanced textbooks. **Indexed in Scopus and EI Compendex** The Springer Energy board welcomes your book proposal. Please get in touch with the series via Anthony Doyle, Executive Editor, Springer ([email protected]) More information about this series at http://www.springer.com/series/8874

David Ginley  •  Kamanio Chattopadhyay Editors

Solar Energy Research Institute for India and the United States (SERIIUS) Lessons and Results from a Binational Consortium

Editors David Ginley Materials and Chemistry Science and Technology, National Renewable Energy Laboratory, Co-Director SERIIUS Golden, CO, USA

Kamanio Chattopadhyay SERB Distinguished Fellow and Honorary Professor, Co-Director SERIIUS Indian Institute of Science Bangalore, Karnataka, India

ISSN 2195-1284     ISSN 2195-1292 (electronic) Lecture Notes in Energy ISBN 978-3-030-33183-2    ISBN 978-3-030-33184-9 (eBook) https://doi.org/10.1007/978-3-030-33184-9 © Springer Nature Switzerland AG 2020 This work is subject to copyright. All rights are reserved 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, express 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 Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Preface

The energy landscape in India and the United States has changed and continues to do so very rapidly. The energy targets in both countries are increasingly ambitious as renewable energy becomes cost-effective and the need to minimize greenhouse gases is ever more critical. India is set to become a world leader in renewable energy. Since 2014, the Government of India has embarked on an ambitious course to increase renewable energy capacity fivefold by 2022—from the current 30 GW to a projected 175 GW (Renewable Energy Focus 16(4) 2015). The U.S. 2030 goal is 3 cents per kilowatt-­ hour (¢/kWh) for utility-scale photovoltaics (PV), 4 ¢/kWh for commercial rooftop PV, and 5 ¢/kWh for residential rooftop PV; the goal for dispatchable concentrated solar power is 5 ¢/kWh. Achieving these goals requires: (1) a deep understanding of the opportunity space in both countries, (2) the development of PV and solar thermal technologies with low capital-cost production, and (3) a demonstration of lifetime and bankability in deployable technologies. The precise needs and deployment strategies of each country are different, but the key research and technology needed to achieve them are similar. Thus, when the two countries signed an agreement to create binational centers in the areas of solar, buildings, and biofuel technologies, the hope was to capitalize on synergisms that could occur in true collaborative centers. The goal from the outset was to show research and deployable benefits in both India and the United States. In this book, we first discuss the origins and development of the solar consortium—the Solar Energy Research Institute for India and the United States (SERIIUS). Then, we highlight what it took for a successful proposal bid, with the proposal evaluated simultaneously in both countries. Finally, we describe how SERIIUS was managed and discuss some key technical and non-technical outcomes over the 5-year life of the consortium. Some of these outcomes include: developing close personal relationships with collaborators, both in-country and binationally; developing an efficient management structure; significantly engaging new partners over time, especially with active and increasing industry engagement on both sides; producing significant research v

vi

Preface

output with an expanding number of co-authored papers; developing an intern exchange program; and developing foundational scientific opportunities and defining needs in both countries. One key aspect of SERIIUS was its broad educational mission, which initially was internal to SERIIUS, but eventually expanded to be nationally and internationally active in creating an awareness of and opportunities in technologies that could be deployed and maintained in diverse climates throughout much of the world. This focus was especially for hot/dry as well as hot/wet climates, where problems encountered can limit the applicability of solar technologies. We are honored—and the rewards have been many—to have been able to lead such a talented group of scientists with a compelling shared mission. We hope that the lessons learned by SERIIUS will be foundational in developing other international collaborations to address the world’s increasing needs for energy and environmental sustainability. We thank the U.S.  Department of Energy, the Government of India, and the Indo-US Science and Technology Forum for their support and guidance through the lifetime of SERIIUS, and we hope that the shared interactions will benefit both countries in the long run. In addition, significant contributions from the following deputy directors, thrust leads, and coordinators are gratefully acknowledged: • • • • • • • • • •

Pradip Dutta—India Institute of Science—Bangalore (IISc) William Tumas—National Renewable Energy Laboratory (NREL) Juzer Vasi—India Institute of Technology—Bombay Maikel van Hest—NREL Clifford Ho—Sandia National Laboratories Parveen Kumar—Centre for the Study of Science, Technology and Policy (CSTEP) Thirumalai N. C. —CSTEP Aimee Curtright—RAND Corporation Hamsa Lakshmi—IISc Marisa Howe—NREL

Finally, we note that SERIIUS was formed with the leadership and inspiration of Dr. Larry Kazmerski who has remained active in SERIIUS even past retirement and that the content of this book could not have been assembled without the help of this entire team, and we appreciate their diligence in making it happen. We acknowledge the superb technical support especially from Don Gwinner and Alfred Hicks at NREL in the editing, assembling, and graphics for the final publication version. This, too, was a challenge in integrating content and figures from many groups in both countries. Golden, CO, USA Bangalore, India 

David Ginley Kamanio Chattopadhyay

Acknowledgements

This work was authored, in part, by the National Renewable Energy Laboratory, operated by Alliance for Sustainable Energy, LLC, for the U.S.  Department of Energy (DOE) under Contract No. DE-AC36-08GO28308. Funding provided by the DOE Office of Energy Efficiency and Renewable Energy, Solar Energy Technologies Office, and DOE Office of Science equally. The views expressed in the article do not necessarily represent the views of the DOE or the U.S. Government. The U.S. Government retains and the publisher, by accepting the article for publication, acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for U.S. Government purposes.

vii

Contents

1 Introduction������������������������������������������������������������������������������������������������    1 David Ginley and Kamanio Chattopadhyay 2 Sustainable Photovoltaics��������������������������������������������������������������������������   25 David Ginley, Joel Ager, Rakesh Agrawal, Muhammad A. Alam, Brij Mohan Arora, S. Avasthi, Durga Basak, Parag Bhargava, Pratim Biswas, Birinchi Bora, Wade A. Braunecker, Tonio Buonassisi, Sanjay Dhage, Neelkanth Dhere, Sean Garner, Xianyi Hu, Ashok Jhunjhunwala, Dinesh Kabra, Balasubramaniam Kavaipatti, Lawrence Kazmerski, Anil Kottantharayil, Rajesh Kumar, Cynthia Lo, Monto Mani, Pradeep R. Nair, Lakshmi Narsamma, Dana C. Olson, Amlan J. Pal, Srinivasan Raghavan, Praveen Ramamurthy, Bulusu Sarada, Shaibal Sarkar, O. S. Sastry, Harshid Sridhar, Govisami Tamizmani, Jeffrey Urban, Maikel van Hest, Juzer Vasi, Yanping Wang, and Yue Wu 3 Multiscale Concentrated Solar Power������������������������������������������������������   87 David Ginley, R. Aswathi, S. R. Atchuta, Bikramjiit Basu, Saptarshi Basu, Joshua M. Christian, Atasi Dan, Nikhil Dani, Rathindra Nath Das, Pradip Dutta, Scott M. Flueckiger, Suresh V. Garimella, Yogi Goswami, Clifford K. Ho, Shireesh Kedare, Sagar D. Khivsara, Pramod Kumar, C. D. Madhusoodana, B. Mallikarjun, Carolina Mira-Hernández, M. Orosz, Jesus D. Ortega, Dipti R. Parida, M. Shiva Prasad, K. Ramesh, S. Advaith, Sandip K. Saha, Shanmugasundaram Sakthivel, Sumit Sharma, P. Singh, Suneet Singh, Ojasve Srikanth, Vinod Srinivasan, Justin A. Weibel, and Tim Wendelin

ix

x

Contents

4 Solar Energy Integration��������������������������������������������������������������������������  133 David Ginley, Suhas Bannur, Mridula Dixit Bharadwaj, Aimee Curtwright, Vaishalee Dash, Rafiq Dossani, G. Srilakshmi, Praveen Kumar, Zhimin Mao, N. C. Thirumalai, Shanthi Nataraj, Oluwatobi Oluwatola, Badri S. Rao, Costa Samaras, Harshid Sridhar, Sara Turner, Bhupesh Verma, Henry Willis, and Rushil Zutshi 5 Summary����������������������������������������������������������������������������������������������������  153 David Ginley and Kamanio Chattopadhyay Index������������������������������������������������������������������������������������������������������������������  159

Contributors

S.  Advaith  Department of Mechanical Engineering, Interdisciplinary Centre for Energy Research, Indian Institute of Science-Bangalore, Bengaluru, India Joel Ager  Lawrence Berkeley National Laboratory, Berkeley, CA, USA Rakesh Agrawal  Purdue University, West Lafayette, IN, USA Muhammad A. Alam  School of Electrical and Computer Engineering, Hall for Discovery and Learning Research, Purdue University, West Lafayette, IN, USA Brij  Mohan  Arora  Department of Electrical Engineering, Indian Institute of Technology-Bombay, Mumbai, India R.  Aswathi  Corporate R&D Division, Ceramic Technological Institute, Bharat Heavy Electricals Limited Corporate R&D, Bangalore, India S. R. Atchuta  International Advanced Research Centre for Powder Metallurgy and New Materials, Hyderabad, Telangana, India S. Avasthi  Centre for Nano Science and Engineering (CeNSE), Indian Institute of Science, Bangalore, Karnataka, India Suhas Bannur  Center for Study of Science, Technology and Policy, Bengaluru, Karnataka, India Durga  Basak  Indian Association for the Cultivation of Science, Kolkata, West Bengal, India Bikramjiit Basu  Materials Research Centre, Indian Institute of Science-Bangalore, Bangalore, India Saptarshi Basu  Department of Mechanical Engineering, Interdisciplinary Centre for Energy Research, Indian Institute of Science-Bangalore, Bengaluru, India Mridula Dixit Bharadwaj  Center for Study of Science, Technology and Policy, Bengaluru, Karnataka, India xi

xii

Contributors

Parag Bhargava  Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Powai, Mumbai, India Pratim Biswas  Washington University in St. Louis, St. Louis, MO, USA Birinchi Bora  National Institute of Solar Energy, New Delhi, India Wade A. Braunecker  National Renewable Energy Laboratory, Golden, CO, USA Tonio Buonassisi  Massachusetts Institute of Technology, Cambridge, MA, USA Kamanio  Chattopadhyay  Division of Mechanical Engineering, Department of Materials Engineering, Indian Institute of Science-Bangalore, Bangalore, India Joshua M. Christian  Sandia National Laboratories, Albuquerque, NM, USA Aimee Curtwright  RAND Corporation, Pittsburgh, PA, USA Atasi  Dan  Materials Research Centre, Indian Institute of Science-Bangalore, Bangalore, India Nikhil Dani  Department of Mechanical Engineering, Interdisciplinary Centre for Energy Research, Indian Institute of Science-Bangalore, Bengaluru, India Rathindra Nath Das  Corporate R&D Division, Ceramic Technological Institute, Bharat Heavy Electricals Limited Corporate R&D, Bangalore, India Vaishalee Dash  Center for Study of Science, Technology and Policy, Bengaluru, Karnataka, India Sanjay  Dhage  International Advanced Research Centre for Powder Metallurgy and New Materials, Hyderabad, Telangana, India Neelkanth Dhere  Florida Solar Energy Center, Cocoa, FL, USA Rafiq Dossani  RAND Corporation, Santa Monica, CA, USA Pradip Dutta  Department of Mechanical Engineering, Indian Institute of Science-­ Bangalore, Bengaluru, India Scott  M.  Flueckiger  School of Electrical and Computer Engineering, Hall for Discovery and Learning Research, Purdue University, West Lafayette, IN, USA Suresh V. Garimella  School of Mechanical Engineering, Purdue University, West Lafayette, IN, USA Sean Garner  Corning Research & Development Corporation, Corning, NY, USA David Ginley  National Renewable Energy Laboratory, Golden, CO, USA Yogi Goswami  University of Southern Florida, Tampa, FL, USA Clifford K. Ho  Sandia National Laboratories, Albuquerque, NM, USA

Contributors

xiii

Xianyi Hu  School of Chemical Engineering, Forney Hall of Chemical Engineering, Purdue University, West Lafayette, IN, USA Ashok  Jhunjhunwala  Indian Institute of Technology-Madras, Chennai, Tamil Nadu, India Dinesh  Kabra  Department of Physics, Indian Institute of Technology-Bombay, Mumbai, India Balasubramaniam Kavaipatti  Department of Energy Science and Engineering, Indian Institute of Technology-Bombay, Mumbai, India Lawrence Kazmerski  National Renewable Energy Laboratory, Golden, CO, USA Shireesh Kedare  Department of Energy Science and Engineering, Indian Institute of Technology Bombay (IIT Bombay), Mumbai, India Sagar  D.  Khivsara  Department of Mechanical Engineering, Indian Institute of Science-Bangalore, Bengaluru, India Anil  Kottantharayil  Department of Electrical Engineering, Indian Institute of Technology-Bombay, Mumbai, India Pramod  Kumar  Department of Mechanical Engineering, Indian Institute of Science-Bangalore, Bengaluru, India Praveen Kumar  Center for Study of Science, Technology and Policy, Bengaluru, Karnataka, India Rajesh Kumar  Solar Energy Centre, MNRE, National Institute of Solar Energy, New Delhi, India Cynthia  Lo  Energy, Environmental and Chemical Engineering, Washington University in St. Louis, St. Louis, MO, USA C. D. Madhusoodana  Corporate R&D Division, Ceramic Technological Institute, Bharat Heavy Electricals Limited Corporate R&D, Bangalore, India B. Mallikarjun  International Advanced Research Centre for Powder Metallurgy and New Materials, Hyderabad, Telangana, India Monto  Mani  Centre for Sustainable Technologies, Indian Institute of Science-­ Bangalore, Bangalore, India Zhimin Mao  RAND Corporation, Santa Monica, CA, USA Carolina  Mira-Hernández  School of Chemical Engineering, Forney Hall of Chemical Engineering, Purdue University, West Lafayette, IN, USA Pradeep  R.  Nair  Department of Electrical Engineering, Indian Institute of Technology-Bombay, Mumbai, India

xiv

Contributors

Lakshmi  Narsamma  Indian Institute of Technology-Madras, Chennai, Tamil Nadu, India Shanthi Nataraj  RAND Corporation, Santa Monica, CA, USA Dana C. Olson  National Renewable Energy Laboratory, Golden, CO, USA Oluwatobi Oluwatola  RAND Corporation, Santa Monica, CA, USA M. Orosz  Massachusetts Institute of Technology, Cambridge, MA, USA Jesus D. Ortega  Sandia National Laboratories, Albuquerque, NM, USA Amlan  J.  Pal  Indian Association for the Cultivation of Science, Kolkata, West Bengal, India Dipti  R.  Parida  Department of Mechanical Engineering, Indian Institute of Science-Bangalore, Bengaluru, India M. Shiva Prasad  International Advanced Research Centre for Powder Metallurgy and New Materials, Hyderabad, Telangana, India Srinivasan Raghavan  Centre for Nano Science and Engineering (CeNSE), Indian Institute of Science, Bangalore, Karnataka, India Praveen  Ramamurthy  Centre for Nano Science and Engineering (CeNSE), Indian Institute of Science, Bangalore, Karnataka, India K. Ramesh  Corporate R&D Division, Hindustan Petroleum Corporation Limited Green R&D Center, Bangalore, Bangalore, India Badri  S.  Rao  Center for Study of Science, Technology and Policy, Bengaluru, Karnataka, India Sandip  K.  Saha  Department of Mechanical Engineering, Indian Institute of Technology-Bombay, Mumbai, Maharashtra, India Shanmugasundaram  Sakthivel  International Advanced Research Centre for Powder Metallurgy and New Materials, Hyderabad, Telangana, India Costa Samaras  Civil and Environmental Engineering, Carnegie Mellon University, Pittsburgh, PA, USA Bulusu  Sarada  International Advanced Research Centre for Powder Metallurgy and New Materials, Hyderabad, Telangana, India Shaibal Sarkar  Indian Institute of Technology-Bombay, Mumbai, India O. S. Sastry  Solar Energy Centre, MNRE, National Institute of Solar Energy, New Delhi, India Sumit Sharma  Department of Energy Science and Engineering, Indian Institute of Technology Bombay (IIT Bombay), Mumbai, India

Contributors

xv

P.  Singh  Department of Mechanical Engineering, Indian Institute of Science-­ Bangalore, Bengaluru, India Suneet Singh  Department of Energy Science and Engineering, Indian Institute of Technology Bombay (IIT Bombay), Mumbai, India Harshid Sridhar  Center for Study of Science, Technology and Policy, Bengaluru, Karnataka, India Ojasve  Srikanth  Corporate R&D Division, Ceramic Technological Institute, Bharat Heavy Electricals Limited Corporate R&D, Bangalore, India G.  Srilakshmi  Center for Study of Science, Technology and Policy, Bengaluru, Karnataka, India Vinod  Srinivasan  Department of Mechanical Engineering, Indian Institute of Science-Bangalore, Bengaluru, India Govisami Tamizmani  Arizona State University, Tempe, AZ, USA N. C. Thirumalai  Center for Study of Science, Technology and Policy, Bengaluru, Karnataka, India Sara Turner  RAND Corporation, Santa Monica, CA, USA Jeffrey Urban  Lawrence Berkeley National Laboratory, Berkeley, CA, USA Maikel van Hest  National Renewable Energy Laboratory, Golden, CO, USA Juzer Vasi  Department of Electrical Engineering, Indian Institute of Technology-­ Bombay, Mumbai, India Bhupesh Verma  Center for Study of Science, Technology and Policy, Bengaluru, Karnataka, India Yanping Wang  Solarmer Energy Inc., El Monte, CA, USA Justin  A.  Weibel  School of Mechanical Engineering, Purdue University, West Lafayette, IN, USA Tim Wendelin  National Renewable Energy Laboratory, Golden, CO, USA Henry Willis  RAND Corporation, Pittsburgh, PA, USA Yue Wu  Solarmer Energy Inc., El Monte, CA, USA Rushil Zutshi  RAND Corporation, Santa Monica, CA, USA

Chapter 1

Introduction David Ginley and Kamanio Chattopadhyay

Early in the twenty-first century, the governments of India and the United States realized that the partnering of the two countries could enhance both countries’ goals across many areas. Shared benefits were sought in the area of energy, and the pairing of the two countries’ renewable energy strategies served to nucleate four joint energy centers in solar, buildings, biomass, and grid science. Over their 5 years of existence, these centers aimed to address renewable energy cost goals for each country, ensure mutual energy security, and build a clean-energy economy to drive investment, job creation, and economic growth. India and the United States launched the U.S.–India Partnership to Advance Clean Energy (PACE) on November 24, 2009, under the U.S.–India Memorandum of Understanding to enhance cooperation on energy security, energy efficiency, clean energy, and climate change. As a priority initiative under the PACE umbrella, the U.S. Department of Energy (DOE) and the Government of India signed an agreement to establish the Joint Clean Energy Research and Development Center (JCERDC) on November 4, 2010. The JCERDC was designed to promote clean-­energy innovation by teams of scientists and engineers from India and the United States. In 2010, JCERDC issued a joint funding opportunity announcement (FOA)— The U.S.–India Joint Clean Energy Research and Development Center Funding Opportunity Number: DE-FOA-0000506. Announcement Type: Initial. CFDA Number: 81.087.

D. Ginley (*) National Renewable Energy Laboratory, Golden, CO, USA e-mail: [email protected] K. Chattopadhyay Division of Mechanical Engineering, Department of Materials Engineering, Indian Institute of Science-Bangalore, Bangalore, India e-mail: [email protected] © Springer Nature Switzerland AG 2020 D. Ginley, K. Chattopadhyay (eds.), Solar Energy Research Institute for India and the United States (SERIIUS), Lecture Notes in Energy 39, https://doi.org/10.1007/978-3-030-33184-9_1

1

2

D. Ginley and K. Chattopadhyay

The general philosophy of the FOA was stated as follows: “Energy cooperation is a central element of the U.S.–India Strategic Partnership. During President Obama’s November 2010 head of state visit to India, the U.S. Department of Energy (DOE) and the Government of India signed an Agreement to Establish a Joint Clean Energy Research and Development Center (JCERDC) designed to promote clean energy innovation by teams of scientists and engineers from India and the United States. Priority areas for cooperation include: solar energy, energy efficiency, smart grid, unconventional natural gas, and second-generation biofuels technologies. DOE and the Government of India intend to make funding awards under this Funding Opportunity Announcement (FOA) in three initial priority areas: 1 . Energy efficiency of buildings 2. Second-generation biofuels 3. Solar energy The work of the Center will be initiated by U.S.–India consortia with the knowledge and experience to undertake first-rate collaborative research programs. These consortia will help bring together top talent from both countries and are expected to generate key technological advancement through genuine collaboration between U.S. and Indian researchers. Funding will be competitively awarded on the basis of a joint U.S.–India merit review of the applications to ensure genuine collaboration and partnership of the awardees. To keep the focus on international collaborative research and development, management and administrative expenses will be kept to a minimum. New ‘brick-and-mortar’ facilities will not be supported.” This solar area specifically stated the following: Solar Energy: The objective is to contribute to dramatic improvements in solar energy technology, establishing the scientific basis needed to underpin the efficient capture, conversion, storage and utilization of solar energy for electricity generation in a cost-effective manner. The challenge in converting sunlight to electricity via photovoltaic cells is to reduce the cost/watt of delivered solar electricity through dramatic improvements in conversion efficiency. Devices that operate above the existing performance limit will require the development of new materials and new concepts for solar photoconversion. A description of the challenges and opportunities in this field can be found in the Workshop Report on Basic Research Needs for Solar Energy Utilization. Of high priority are new concepts and architectures in solar electricity production, including organic and hybrid organic/inorganic conversion systems, innovative nanoscale designs of interfaces and cells, and novel materials, as well as advanced theory, modeling and simulation of such systems. Additional topics include: advanced photovoltaic (PV) technologies (i.e. organic, crystalline, non-­ single crystal devices, photo-electrochemical, advanced multi-junction, low dimensional structures, optimized interfaces, and transport properties); concentrating solar power (CSP) technologies (e.g., thermal storage, advanced fluids, high temperature concepts and materials); integration in the electrical power grid (e.g., interconnection, intermittency, and balancing); low cost and environmentally safe manufacturing techniques to support investment decisions on solar applications; and solutions to PV and solar thermal component reliability issues. With respect to these technologies, expertise from both countries will be used to identify research gaps; prioritize research topics; and implement collaborative research teams focusing on innovations that are relevant to the Indian and/or U.S. energy ­frameworks. Of high priority are activities that align with Government of India and U.S. Department of Energy priorities.

1 Introduction

3

Underlying these overall goals and of specific importance was the establishment of bankability for solar technologies, in general, in India, as well as the approaches of SERIIUS, in general, in both India and the United States. This can be viewed as defining ways to improve the finance-ability and attractiveness of PV investments through common tools and best-practice guidelines for professional risk assessment. In part, this can serve to demonstrate reduced technical risks associated with investments in PV projects. And this work benefits the community and helps define realistic research priorities. As per the FOA, the funding level was to be $12.5 million over 5 years, to be split evenly by year and by country. There was also a 50% cost-share requirement for each side. Funding was prioritized across thrust areas based on the overall management team’s assessment for impact and could be adjusted using the 10-Point Plan. Overall government funding for the thrusts on the U.S. side was 67% PV, 19% CSP, and 14% SEI; on the Indian side, funding was 53% PV, 23% CSP, and 24% SEI. This plan aligned the desired research with established priorities in basic science and applied science set by the SunShot Initiative in the United States and with the overarching goals to develop and deploy solar on a large scale in India. It is worth noting that whereas the FOA established the overall definition of success for the consortia, the management of the consortia was actually across three DOE offices and three Indian agencies. Each had their own specific detailed objectives in addition to the more global ones. This required an increased level of communication with each sponsor to maintain a clear focus on the specific milestones for SERIIUS, which were captured in our 10-Point Plan discussed below.

1.1  Forming a Consortium The creation of a consortium between India and the United States focusing on solar energy began a year prior to the funding opportunity announcement in 2009 by developing a management structure. In the evolution of this structure, SERIIUS pursued the following actions: • An initial definition of preliminary work scope was developed early in the process, which allowed the delineation of an initial team that included academic institutions, national laboratories, and industry partners. • The initial set of partners convened several times to refine the thrusts of the consortium and to identify new partners who might be needed. For example, Sandia National Laboratories (Sandia) and the National Renewable Energy Laboratory (NREL) have had an extended partnership on solar technologies, leading to an obvious match in the CSP area, and both have had interactions over time with the Indian Institute of Science (IISc).

4

D. Ginley and K. Chattopadhyay

• The team then assessed the overall capabilities of the consortium to achieve the work outlined in the evolving work statement and began to define a management philosophy. This included defining a core group and key partners to lead the management of the consortium—in this case, NREL and IISc. • The next step was to codify the overall vision, activity directions, and how to manage intellectual interactions. –– SERIIUS developed a five-year vision of research goals that matched the overall goals of both the Indian and U.S. governments, incorporating sufficient flexibility to enable changes in direction as both the science and political environment changed. –– Research directions were further defined, and a thrust-based research structure was developed to enable the research in the specific areas of photovoltaics, solar thermal conversion, and solar energy analysis. –– Given the diverse nature of the consortium—including organizations, research philosophy, and international character of the consortium—it was important to develop an intellectual property (IP) framework that the whole consortium would concur with and operate under if funded. This helped generate the idea of open research projects and more industrially driven core projects—both with well-defined IP conventions. • Careful definition of detailed personnel capabilities enabled the development of a functional work statement. This required a complete assessment of the existing and proposed staff with their actual laboratory capabilities. • Given this definition of team members, goals, and capabilities, a management structure was then defined to synergize these three areas. In this case, we defined a core leadership team based on the thrust structure that was promulgated throughout the organizational structure. To ensure that the binational character of the consortium was preserved, binational leadership was paired at all levels. This structure was critical in defining both the proposal strategy and ultimately the definition of 10-Point Plan operating document under which the consortium operated. • The definition of a management structure then enabled the development of a communication plan for the consortium, funders, scientific community, and public. This led to creating a series of tools to manage the workflow of the consortium, ensuring the dissemination of the results of the consortium and the development of utility tools for use by the broader community. • An approach was developed to assess projects within the consortium, new research developments outside the consortium, and coherence to the overall goals of the consortium. This provided ongoing risk/reward assessment over time, which enhanced the relevancy of the consortium workflow.

1 Introduction

5

1.2  Realized Consortium Vision and Management The vision developed through the planning process resulted in SERIIUS’s vision “to create an environment for cooperation and innovation ‘without borders’ to develop and ready emerging and revolutionary solar electricity technologies toward the long-term success of India’s Jawaharlal Nehru National Solar Energy Mission and the U.S. Department of Energy SunShot Initiative.” To accomplish this, we created a structure to both enhance existing and develop new scientific and research collaborations in solar energy science and technology. These collaborations provided concerted, joint efforts that leveraged the extensive body of existing research programs, capabilities, experience, and knowledge within the large multidisciplinary team of world-renowned universities, premier national laboratories/institutes focusing on solar energy research and development (R&D), and leading companies in India and the United States that comprise SERIIUS. The overall goal of SERIIUS was “to accelerate the development of solar electric technologies by lowering the cost per watt of photovoltaics (PV) and concentrated solar power (CSP) through a binational consortium that will innovate, discover, and ready emerging, disruptive, and revolutionary solar technologies that span the gap between fundamental science and applied R&D, leading to eventual deployment by sustainable industries. SERIIUS will address critical issues in fundamental and applied research, analysis and assessment, outreach, and workforce development.” Here, “revolutionary” applies to the overall idea of new high-performance technologies based on new materials and processing approaches that significantly improve deployability and minimize capitalization costs, thus providing a platform for dramatic change in both the United States and India. The overall approach was based on understanding both the deployment options in the United States and India as well as the potential failure mechanisms in the diverse climate zones of India. This gave rise to a new specific approach to materials, processing, and integration using new “revolutionary” and deployable processes—in the context of creating dramatic change from conventional paradigms. Although the long-term approaches were based on achieving the overall goals above, it also became clear that there were a number of short-term approaches appropriate to the existing technologies and their reliability in the diverse Indian climate zones, the problems of soiling, and understanding the integration of solar technologies across the diverse length scales of potential deployment. This included the existing silicon solar technologies and solar thermal systems. SERIIUS work included research and technology development from nanoscale materials to lifetime of solar modules and thermal concentrator arrays, and from local village power to grid-based systems. The overall effort was development through new basic science of transferable technologies. Throughout this joint effort, a key element was engaging a significant base of Indian and U.S. industry dedicated to developing solar energy for both countries.

6

D. Ginley and K. Chattopadhyay

The specific objectives of SERIIUS were to: 1. Focus on high-impact fundamental and applied R&D to create disruptive technologies in PV and CSP. 2. Identify and quantify the critical technical, economic, and policy issues for solar energy development and deployment in India and the United States. 3. Overcome barriers to technology transfer by teaming research institutions and industry in an effective project structure—cutting the time from discovery to technology development and commercialization through effective coordination, communication, and IP management plans. 4. Create a new platform for binational collaboration using a formalized R&D project structure, along with effective management, coordination, and decision processes. 5. Create a sustainable network from which to build large collaborations and foster a collaborative culture and outreach programs, including the use of existing and new methodologies for collaboration based on advanced electronic and web-­ based communication to facilitate functional international focused teams. 6. Create a strong workforce development program in solar energy science and technology for both countries.

Projects

Activities

Thrusts

To address the priorities of both India and the United States in solar conversion, the consortium (from proposal to practice) at all levels (from director to project) managed each activity with a binational team and jointly coordinated activities. To accomplish this, we developed the thrust-based project structure shown in Fig. 1.1. The project tasks were highly integrated, which necessitated the thrust-based management. Details of these tasks are highlighted in Sect. 1.4. We note that the funding was not equal by thrust—with emphasis on PV, then on CSP, and finally, on SEI. The five operational years of SERIIUS brought substantial technical progress. But perhaps SERIIUS’ greatest impact was creating a new model for how to execute Sustainable Photovoltaics (PV)

Multiscale Concentrated Solar Power (CSP)

Earth-Abundant PV

High-T, Closed-Loop, Brayton Cycle

Advanced Process/Technology

Low-T Organic Rankine Cycle

Multiscale Modeling and Reliability

Thermal Storage & Hybridization

Consortium Projects

Consortium Projects

Consortium Projects

Core Projects

Core Projects

Core Projects

Fig. 1.1  Project structure of SERIIUS. (NREL, unpublished)

Solar Energy Integration (SEI) Roadmapping, Analysis and Assessment Grid Integration and Energy Storage

1 Introduction

7

truly collaborative research on an international scale. The communications tools implemented within SERIIUS and external to SERIIUS (see Sect. 1.3) integrated the research within SERIIUS, developing a shared vision and cooperation across and within research areas. They also engaged the international community through a series of workshops highlighting opportunities in solar R&D and its integration across a broad span of length scales—from grid-connected power to stand-alone village power. From the outset, it was realized that the specific projects and their focus—indeed, the management structure of SERIIUS—would need to be dynamic over the course of the 5 years. To this end, SERIIUS created the “10-Point Plan,” which was initially defined in the FOA to be the principal scope-of-work document for the consortium. It detailed the specific objectives by project and the associated milestones. To make this a working document, it was reviewed every 6 months, and any substantive changes were approved by both DOE and IUSSTF and their Program Management Committee (PMC). This included any rescoping of projects, new projects, or Principal Investigator changes—all of which did occur over the lifetime of SERIIUS.  In addition, the program was reviewed monthly by DOE and every 6 months by the PMC. This oversight was useful for providing the proper work scope, and also as a communication that kept both sides in sync. We note that this is a 96-page working document and cannot be included in this book. In part, the effectiveness of this integration can be judged by the number of journal publications (230), proceedings (142), presentations (488), IP disclosures/files patents (15), and workshops from the consortium. Of the total number of journal publications and conference proceedings, more than 45 were joint Indo-US-authored papers, which was one of SERIIUS’ greatest accomplishments. On a more fundamental level, the effectiveness can be judged by the number of interns (from both directions), number of exchanges, and other visits, and genuine collaboration in managing SERIIUS between two diverse countries. Another clear measure of success is the number of new partners who joined SERIIUS over the 5 years. Table 1.1 shows the starting and concluding number of partners for comparison as well as the actual partner names by country. We believe that the real-time communication between groups in SERIIUS— from the management to the individual researchers—functionally created an environment for cooperation and innovation “without borders.” It also began to develop and ready emerging and revolutionary solar electricity technologies, thereby meeting the promise of synergistically coupling the diverse R&D talents of some of the preeminent Indian and U.S. institutions to develop transformative and ultimately deployable solar technologies. The overall approach to defining the research strategies as well as the communication approach was effective for SERIIUS’ diverse institutions, which is difficult under normal circumstances and in some ways reflects the complex motivations for being part of such a center. We also believe that continuing dialogue of SERIIUS leadership with the sponsors on both sides served to accomplish the following: keep SERIIUS on track for evolving binational priorities, maintain an active dialogue on the research and changes to the 10-Point Plan, and identify opportunities for research and partnering

8

D. Ginley and K. Chattopadhyay

Table 1.1  Starting and concluding number of partners (upper section) and listing of actual partners by country (lower section) Partner country India U.S.

Initial 13 18

Collaborating Institutions, India

Collaborating Institutions, USA

Consortium leads •  Indian Institute of Science, Bangalore (IISc)

Final 15 23

• National Renewable Energy Laboratory (NREL) Research Thrust Leadership •  Indian Institute of Technology Bombay (IIT-B) •  Sandia National Laboratories • Center for the Study of Science, Technology and •  RAND Corporation Policy (CSTEP) Institutes and National Laboratories • International Advanced Research Centre for Power • Lawrence Berkeley National Metallurgy and New Materials (ARCI) Laboratory (LBNL) •  National Institute of Solar Energy (NISE) University partners •  Indian Institute of Technology Madras (IIT-M) •  Arizona State University • Indian Association for the Cultivation of Science •  Binghamton Universitya (IACS) •  Carnegie Mellon University •  Colorado School of Mines •  Colorado State Universitya • Massachusetts Institute of Technology •  Purdue University •  Stanford University •  University of Central Florida •  University of Colorado Bouldera •  University of South Florida • Washington University in St. Louis Industry Partners: Industry Partners: •  Bharat Heavy Electricals Ltd. •  Core Energy Works, LLCa •  Clique Developments Ltd. • Corning Research and •  GAIL (India) Limiteda Development Corp. •  Eastman Kodaka •  Hindustan Petroleum Corporation Ltd. •  Infosys Ltd.a •  Interphases Solara •  Moser Baer India Ltd. • Konarka •  Thermax Ltd. •  Semlux Technologies, Inc.a •  Wipro Ltd. •  Solarmer Energy, Inc. •  SunEdison, Inc. •  Underwriters Laboratoriesa •  Semlux Technologies, Inc.a •  Solarmer Energy, Inc. •  SunEdison, Inc. •  Underwriters Laboratoriesa Members who joined SERIIUS after the proposal phase

a

9

1 Introduction

on both sides that would not have been seen otherwise. In this sense, we are also indebted to the Technical Advisory Board. Their candid insights were especially valuable to focus on individual research projects and obtain value for both sides. And the selection of vocal experts was valuable in creating an outspoken—and therefore useful—Technical Advisory Board. Figures 1.2 and 1.3 indicate the initial and final configurations, respectively, of the SERIIUS management structure. Although the structure evolved over time and some positions changed one or more times, the overall vision of the management structure remained fixed. We adopted the management structure in Fig. 1.3 to ensure that research efforts were executed effectively. The co-directors and their deputies had biweekly calls with the thrust leaders for overall coordination. In addition, two meetings with the Technical Advisory Board obtained very applicable input. The Industry Board was planned to engage throughout the active execution of the work, but as time passed, this board was considered superfluous as industry partners became embedded in the consortium projects and their input was attained often. We also used any opportunities—for example,

US-India JCERDC Executive Oversight

NREL, IIT Bombay, IISc Bangalore, IACS, SNL, Purdue U., CSM, Washington U./SL, Stanford U., MIT, IIT Madras, Solar Energy Centre, CSTEP, ARCI

India-US Leadership and Coordination Co-Directors Industry Board

SERIIUS core industry partners

K. Chattopadhyay IISc Bangalore-India

D. Ginley NREL-US

Deputy Managing Co-Directors P. Dutta IISc Bangalore-India

W. Tumas NREL-US

SERIIUS Council SERIIUS Council: Internal governing board of consortium (core industry, university, SERIIUS leadership)

Research Thrusts

Sustainable PV

M. van Hest (NREL-US) J. Vasi (ITTP-India)

Multiscale CSP

S. Shinde (SNL-US) P. Dutta (IISc-India)

Competency Gateway

Solar Energy Integration

H. Willis (RAND-US) A. Bharadwaj (CSTEP-India)

Fig. 1.2  Early SERIIUS management structure. (NREL, unpublished)

Technical Advisory Board

10

D. Ginley and K. Chattopadhyay

US-India JCERDC Industry Board

SERIIUS core industry partners

India-US Leadership and Coordination Co-Directors

K. Chattopadhyay IISc Bangalore-India

D. Ginley NREL-US

Deputy Managing Co-Directors P. Dutta IISc Bangalore-India

W. Tumas NREL-US

Program Coordination H. Lakshmi IISc Bangalore-India

M. Howe NREL-US

SERIIUS Council SERIIUS Council: Internal governing board of consortium (core industry, university, SERIIUS leadership)

Executive Oversight

NREL, IIT Bombay, IISc Bangalore, IACS, SNL, Purdue U., CSM, Washington U./SL, Stanford U., MIT, IIT Madras, Solar Energy Centre, CSTEP, ARCI

Technical Advisory Board • Joan Brennecke

U. Notre Dame (US)

• Gautam Biswas

IIT-Guwahati (India)

• Viresh Dutta

Centre for Energy Studies, IIT-Delhi (India)

• Ravi Grover

Homi Bhabha Nat. Inst. (India)

• Vikram Kumar

Sustainable PV

Research Thrusts

M. van Hest (NREL-US) J. Vasi (IIT-B-India)

Multiscale CSP

C. Ho (SNL-US) P. Dutta (IISc-India)

National Physical Lab., IIT-Delhi (India)

Solar Energy Integration

A. Curtright (RAND-US) P. Kumar (CSTEP-India)

• Ajeet Rohatgi

Georgia Inst. of Tech. (US)

• Roland Winston

U. Cal., Merced (US)

Fig. 1.3  Final management structure. (Note: P. Kumar was replaced by Thirumalai N.C. during the final months of the project.) (NREL, unpublished)

the IEEE Photovoltaic Specialists (PVSC) Conference and the SolarPACES conference, as well as dedicated meetings—to continuously update progress and assess the overall quality of consortium operation. To facilitate a functional integration of the research efforts across the three thrusts, India and the United States took a new approach to managing the consortium by pairing scientists from the two countries to meet project objectives. Our underlying template for the work of the consortium became a living workflow document—specified by the proposal process and considered by us a necessity—termed the “10-Point Plan,” which was mandated by DOE and guided project execution. This living document was revised on a six-month basis from input of the management team and SERIIUS-wide meetings. Overall milestones changed only if both the Indian and U.S. governments agreed through a change-control process. Remarkably, SERIIUS met more than 95% of its milestones out of some 110 milestones as identified in the 10-Point Plan. The meaningful outputs as defined by the milestones are technical and represent an evolution of the thrusts over the period of the project. Also, we consider direct outputs to be papers, proceedings, presentations, SERIIUS-sponsored/organized meetings, number of interns and visits, and the general engagement of the solar energy community in India and the United States. The impact of this consortium measured by publications and IP, workforce development, binational interactions, outreach, and tools is summarized in Fig. 1.4. Table 1.2 lists the IP generated by the consortium. The consortium operated with a specific IP management plan that all partners initialed and subsequently signed,

1 Introduction

11

Fig. 1.4  Summary of achievements of SERIIUS. (NREL, unpublished)

and it formed a template for handling IP and distinguishing consortium and core projects. The IP plan helped to facilitate both the consortium and core projects by establishing a means for IP rights to be coordinated and for licensing to occur. In specific it delineated cases where exclusive or non-exclusive rights could be obtained.

1.3  Detailed Management Approach 1.3.1  Synergism and Programmatic Focus The consortium strove to ensure strong coupling both within SERIIUS and between SERIIUS and the appropriate communities in the United States and India, and we aimed at a worldwide scale by participating in and chairing meetings and communicating broadly through newsletters and publications. Our goal was to engage everyone within the consortium—from thrust leaders to new students—by providing broad opportunities to participate in SERIIUS meetings and workshops. Throughout its period of performance, SERIIUS maintained an active dialogue with both U.S. DOE and India’s Department of Science and Technology (DST) and the Project Monitoring Committee (PMC) in monthly conversations with DOE and

12

D. Ginley and K. Chattopadhyay

Table 1.2  Intellectual property generated by the consortium Title of invention disclosure/Filed patent (Institution) Optoelectronic Devices having Selective Contacts (Stanford) A Novel Electrochemical Method for Manufacturing CIGS Thin-Films Containing Nanomesh-Like Structures (ARCI) A Novel Method to Prevent Potential Induced Degradation (PID) of Photovoltaic Modules during Manufacturing or after Field Installation (ASU) Waterless Soiling Monitoring Stations for Solar Photovoltaic Plants (ASU) A Process for Preparation of Homogenous Mixture for Thermal Storage and Heat Transfer Applications (HPCL) Nanoparticle for Thermal Storage and Heat Transfer Applications (HPCL) A Method for Preparation of a Composition containing Nanoparticle for Thermal Storage and Heat Transfer Applications (HPCL) Heat Transfer Nanofluid Composition (HPCL) Heat Transfer Fluid Composition (HPCL) Oil Extract Containing Heat Transfer Fluid Composition (HPCL) A Method for Manufacturing Visible Transparent Conducting Material (IACS) Transparent Nanostructured Substrate for Use in Light Harvesting Devices (IISc) A Method for Manufacturing Visible Transparent Conducting Material (IACS) Reservoir Control Algorithm for Charge Control in During Startup and Shutdown in a s-CO2 Power Plant (IISc) Single Tank Thermocline based Optically Accessible Thermal Energy Storage Loop (IISc) Perovskite Solar Cell including Inorganic Oxide Electron Transport Material Deposited on Perovskite Absorber Layer (IIT-B including researchers from India and the US)

Related project objective Nanostructured Si and Si/Ge/ III-V Solar Cells (PV-3) Thin-Film Absorber Materials and Processing: Scalable Ink-Based CIGS (PV-1) Module Reliability Testing (PV-5) Reliability (PV-5) Low-Temperature ORC Storage and Hybridization (CSP-5)

Novel Materials for Intrinsic Stability in Harsh Environments (PV-6) Organic Photovoltaic Materials and Devices (PV-2) Novel Materials for Intrinsic Stability in Harsh Environments (PV-6) High Temperature Receiver for CO2 Cycle (CSP-1) Storage and Hybridization (CSP-5) Hybrid Solar Cells (PV-3)

reviews and discussions with the PMC and the constant active engagement of IUSSTF. The diverse perspectives from the Office of Science, the Office of Energy Efficiency and Renewable Energy, and the International Program Office in the United States and the PMC, DST, and IUSSTF in India were invaluable in tracking and evaluating the progress of SERIIUS against national priorities. The diversity of opinions between various funders could have created competing priorities. But the overall engagement of DOE International Affairs and the IUSSTF made sure that there was a continuing dialogue between the U.S. and Indian agencies.

1 Introduction

13

1.3.2  P  roject Management Structure: Internal Communications Focused on Research The organization and management structure was designed to facilitate the successful execution of the vision, objectives, and strategy. The underlying fundamental principle is that all work and responsibilities were co-shared by individuals and organizations from both India and the United States. Altogether, 24 projects under the three thrusts (PV, CSP, and SEI) were managed through a collaborative project management structure as shown in Fig.  1.5. Each thrust had one co-leader from India as well as one from the United States, and each project under a thrust had project co-leaders from both countries. Further, each project had one or more tasks with well-defined research objectives and deliverables, carried out by a team of researchers (e.g., students, postdoctoral researchers) led by a Principal Investigator (Task PI). Multiple institutions in both countries could contribute collaboratively toward a particular task, and the outcome of research from a particular task was integrated upward toward the overall goal of the respective Project. Hence, SERIIUS project management structure used a “bottom-up” approach, wherein the reporting began at the task level and was finally integrated at the thrust level before Quarterly/ Annual reports were prepared for the consortium.

1.3.3  Project Control SERIIUS used a number of tools to ensure adherence to the structure of the 10-Point Plan and also remain able to address opportunities when they arose.

Research Thrust (Thrust co-leaders)

Reporting: Quarterly reports Highlights Publications Patents

Project 1

Project 2

Project 3

Project co-leaders

Project co-leaders

Project co-leaders

Task 1 Task Pls from Institutions

Task 2 Task Pls from Institutions

Coordination: Bi-weekly Leadership Telecons Monthly Project Telecons SharePoint callaboration site SERIIUS.org website

Face-to-face meetings Short visits Exchange fellowships Sample exchanges

Fig. 1.5  SERIIUS project management structure showing binational collaboration at every level. (NREL, unpublished)

14

D. Ginley and K. Chattopadhyay

• To make significant changes in project scope or leadership, new ideas were vetted by the PMC and DOE and were then finalized through change-control documentation to all sponsors. Changes in staffing below the PI level and minor changes in milestones were related via the 10-Point Plan project updated twice a year. The following list highlights the most significant changes made to the 10-Point Plan: (1) Phase-out of dye-sensitized solar cells; (2) Development of a perovskite program; (3) Greater emphasis on reliability; and (4) Redefined scope of the Solar Energy Integration activity. • Another way SERIIUS managed technical risks was through advisement from the Technical Advisory Board, composed of independent research authorities who provided technical review and guidance to the Leadership team. Specific recommendations were made by the Technical Advisory Board members regarding possible course corrections in some of the projects and the way forward for SERIIUS.

1.3.4  C  ommunication: Bilateral Face-to-Face Collaborative Meetings Inter-SERIIUS-partner organization visits were key to ensuring research interactions, sharing of results, fostering relationships, and maintaining the enthusiasm for SERIIUS among members. More than 60 visits/exchanges occurred each year. The first major bilateral meeting of the entire consortium leadership team was held in September 2012 in Bangalore, primarily to formulate the first draft of the 10-Point Plan with all the milestones in each project—a document that continuously served as a guideline for SERIIUS project administration and that recorded all course corrections. The PV groups of all projects met every year during the annual IEEE PVSC meetings (e.g., Tampa, Florida, 2013; Denver, Colorado, 2014; New Orleans, Louisiana, 2015; Portland, Oregon, 2016; and Washington DC, 2017), whereas the CSP groups usually met during the annual ASME Energy Sustainability Conferences (e.g., at Boston, Massachusetts, in June 2014) and during the annual SolarPACES conferences. Internal review meetings with the leadership team members along with several project co-leaders from both countries were held periodically (e.g., in Breckenridge, Colorado, in June 2014; in Bangalore during March 2014 and February 2016; and in Hyderabad during March 2015). During these internal review meetings, issues regarding SERIIUS governance and planning were discussed, and all projects were critically assessed internally with respect to actions taken based on PMC and SERIIUS Technical Advisory Board recommendations. Other tools to facilitate communication included the following: • Telecons over the life of the project: ~140 management and ~600 project calls. • Development of reports: Researchers and leaders coordinated PMC biannual reports, and quarterly and annual reports, and the dispersal of reports allowed

1 Introduction

15

SERIIUS members to learn about SERIIUS activities. SERIIUS had about 20 quarterly reports. • SERIIUS had a Fellows and Scholars Program that involved exchange of students and postdoctoral researchers among SERIIUS partners. The Fellowship program was funded by cost share from SunEdison, Sigma Aldrich, and Core Energy Works, LLC, administered through Washington University at St. Louis and NREL. SERIIUS funded 29 ten-week exchanges for graduate students and early-career scientists. In addition to mentoring future researchers, this program nucleated research groups together and functioned as a glue for research relationships within our consortium. • Coordination and development of publications occurred at all levels of interaction: task leaders, project leaders, thrust leaders, and the management team. Each publication had a one-page highlight that was posted online and used to summarize results to sponsors. SERIIUS had more than 45 joint Indo-US publications. • Inter-SERIIUS-partner organization visits were key to ensuring research interactions, sharing of results, fostering relationships, and maintaining the enthusiasm for SERIIUS among members. SERIIUS had more than 270 bilateral individual researcher visits.

1.3.5  External Coupling to the Wider Solar Community As part of its outreach activities, SERIIUS held an annual Reception and Business Meeting at the Annual IEEE PVSC meeting, reaching out to all IEEE PVSC attendees including non-members of SERIIUS. The SERIIUS Leadership Team made presentations about the vision and objectives of the consortium, R&D activities under the three research thrusts, levels of industry membership, and modalities of joining SERIIUS. Posters showing activities in various projects were also displayed during the reception. Another avenue for outreach was SERIIUS-led workshops (Table 1.3). Other outreach mechanisms included: • Webpage: SERIIUS Web Gateway (www.SERIIUS.org) was the main website, providing information about the consortium to the public and to SERIIUS members. • Newsletters: To keep SERIIUS stakeholders informed about the progress within SERIIUS, CSTEP published six SERIIUS newsletters featuring project highlights, relevant solar news, interviews with leaders in the field (including leaders from the public and private sectors), and summaries of the SERIIUS-MAGEEP Fellows and Scholars program. The IUSSTF newsletter CONNECT also featured all of the JCERDC collaborations, including SERIIUS. • Social media presence: We had two Facebook pages—a general page and a student page—that featured monthly posts about new publications and articles in

16

D. Ginley and K. Chattopadhyay

Table 1.3  Sampling of SERIIUS-led workshops SERIIUS-led outreach workshops 1st SERIIUS Annual Conference at IISc Bangalore, April 2014

NCPRE-SERIIUS Module Reliability Workshop at IIT-B, May 2014 CSP workshop in Bangalore under the DST-SERI umbrella, April 2015 PV Module Reliability in Hot Climates in collaboration with NCPRE, NISE, and NREL at IIT-B, October 2015

Purpose and outcome As part of its outreach activities, the SERIIUS team conducted its first series/symposium on research directions in solar energy, during April 1–2, 2014. The main objective of the 2-day symposium was to provide an exclusive forum on the recent developments of cutting-edge technologies in PV and CSP. This 2-day workshop focused on module reliability issues and was attended by most of the leading solar PV companies in India. The purpose was for brainstorming ideas on new-generation CSP technologies, in which many researchers across India were invited. The outcome of the workshop was a set of recommendations to DST-SERI on topics for calling new proposals under CSP. This 3-day workshop was organized as a follow-up of the 2014 workshop, with a special emphasis on reliability and durability in hot climates. Besides several SERIIUS participants from the U.S. and India, the workshop also had inputs from representatives of other hot countries such as the United Arab Emirates, Saudi Arabia, and Qatar.

the MIT Technology Review, SPAN Magazine, and even in Nature. Additionally, we had a Twitter account that was less active than Facebook. • Synergistic fellowships: U.S. entities in SERIIUS experienced extensive interest from applicants to external-to-SERIIUS fellowships such as the Indian-led Bhaskara and Raman fellowships. At the SERIIUS U.S. institutions, there were at least four fellowships from other programs that were synergistic to that of SERIIUS.

1.3.6  Communication with the Sponsors • DOE conducted monthly calls with NREL, and in November 2014, SERIIUS leadership from NREL provided a briefing to all DOE sponsors. • SERIIUS participated in the India–U.S. Joint Energy Dialogue on two occasions (January 2014 and September 2015) by providing overviews of the program to government officials. • The DOE and PMC conducted a mid-term review in 2016. Our overall success depended on our ability to communicate effectively to the diverse parties that were internal and external to SERIIUS. The communication tools included, as above, a diverse set of channels and active dissemination of information through the internet, meetings, publications, and dedicated communications.

1 Introduction

17

1.4  SERIIUS Research Thrusts The following summaries briefly review the state of the art in the three thrust areas for SERIIUS.  The discussion provides motivation based on the goals of the two governments and the continuously evolving technical landscape. India’s National Solar Mission target was for large-scale deployment of grid-connected as well as off-grid distributed systems supplying stable/continuous power at a low cost, and the U.S.’s SunShot Initiative target demanded large-scale CSP plants with high efficiency and low production costs as well as significant reductions in the cost of photovoltaics. Therefore, the ultimate aim of SERIIUS was to choose and develop cutting-edge technologies and establish and accomplish roadmaps specific to the needs, conditions, and resources available in India and the United States—with an ultimate goal of significantly reducing the solar levelized cost of electricity (LCOE) across a broad range of application length scales. The following description sets the context for each area, and the stories covered in the discussion are not all-inclusive but chosen to illustrate both the long-term revolutionary and shorter-term evolutionary character and successes of the projects against the overall goals of SERIIUS and the progress in the field. The stories illustrate both some of our most significant results and those of the best integration across SERIIUS; the same is true for all three thrust areas.

1.4.1  Earth-Abundant Photovoltaics Solar photovoltaics have made enormous advances in recent years in terms of new materials, new devices, increased deployment, and reduced cost. During the course of the program, DOE’s SunShot Initiative [1] was on its way to achieving the $1/ watt target. India announced a major new goal for its solar energy program—100 GW by 2022 [2]. These ambitious targets can only be met with continued advances in key research areas. The SERIIUS PV activities focused specifically on selected opportunities to achieve these goals. PV is a rapidly evolving research area and SERIIUS evolved its 10-Point Plan to ensure the relevancy of the research. We note that our basic materials set was based on the priorities in both countries; thus, CdTe was not considered—it was not a priority in India because India does not have the elemental resources in this area. New materials continue to enable low-cost PV.  SERIIUS focused on Earth-­ abundant materials that have the potential for low-cost roll-to-roll processing on flexible substrates. Therefore, we continued to look primarily at thin-film PV, of which copper indium gallium diselenide (CIGS) was the primary focus, with efficiencies of more than 20% for cells and 17% for modules [3, 4]. These results were obtained primarily from physical vapor deposition-based approaches, which only recently demonstrated scalability. The SERIIUS CIGS activity focused on ink-­based techniques suitable for roll-to-roll processing. The kesterite material copper zinc tin

18

D. Ginley and K. Chattopadhyay

diselenide (CZTS) was originally thought to be an Earth-abundant alternative and had shown great potential, with CZTSSe achieving an efficiency of >12% [5]. But recent research showed that difficulties in surmounting some intrinsic issues might limit its potential [6]. Our research activities, bridging two projects in SERIIUS, identified selection criteria that can be used to discover materials such as CZTS that overcome these limitations and to identify limitations in existing materials. Organic PV (OPV) has advanced remarkably in recent years, with modules reaching about 10% [7], and these modules have the potential for low-cost processing and have demonstrated deployable lifetimes [8]. Due to its low-cost processing advantage, SERIIUS worked on new OPV cells and modules. The recent discovery of perovskite-based solar cells [9, 10] emphasized the need for continued flexibility in research directions for new PV materials, which can disruptively bring down the cost. Efficiencies of perovskite-based solar cells have reached an impressive 20% [11], but further work on ensuring stability and scale-up are key unresolved issues. Recently, SERIIUS took up some of these issues and also worked on roll-to-roll processing of perovskites on Corning® Willow® glass. Even as new materials come on the scene, the workhorse of today’s PV technology continues to be silicon (Si). Here, too, significant strides have been made in recent years in manufacturing, with the use of new device structures for improved efficiency and lower cost. As worldwide deployment of solar PV nears 1000 GW by 2025 [12], the cost of Si wafers (in terms of dollars as well as energy) can be brought down by appropriate techniques for wafer production and kerf recovery [13]. Energy-conserving non-Siemens-based processes such as the fluidized-bed reactor process [14] are likely to become more widespread, and research is ongoing. Also, non-conventional Si technologies such as the HIT cell are forecast to grow in importance in the future, particularly because the Si HIT cell holds the record of 25.6% efficiency for any single-junction solar cell [15]. SERIIUS core projects, in collaboration with industry, worked on several of these issues related to silicon. Hand-in-hand with the development of new/improved module-based technologies is the need for a focused understanding of deployment issues versus location. One of the most important issues is the reliability and lifetime prediction for PV modules based on understanding degradation in the field [16]. Recent work [17, 18] shows that degradation in hot climates is faster than in temperate climates, and because many new PV installations will be in hot climates, a good understanding is needed of the degradation mechanisms. Similarly, improved encapsulation can extend the lifetime of modules in most climatic zones, and this is an important “back-end” area of research [19]. Deployment can be enabled by developing good accelerated lifetime models that provide accurate predictions of lifetimes of modules, going beyond the traditional certification testing [20]. SERIIUS work addressed the reliability issues by collecting and analyzing field data in both countries, especially in hot climates. Soiling is another issue that can significantly reduce output power and increase operations and maintenance costs [21, 22]. SERIIUS research included developing dust-resistant coatings and determining the influence of various cleaning cycles for different dust samples collected from the field.

1 Introduction

19

Modeling and understanding the performance of PV materials, devices, and modules across all length scales is critical to advancing the state of the art. These are useful for understanding and even designing the properties of materials used in PV, to undertake “end-to-end” modeling that connects the effects of material defects to device behavior to module performance [23]. Modeling can also evaluate the limits to the performance of various PV cells and structures. A recent example is the work that shows how to reach the highest efficiencies for perovskite solar cells [24]. Similarly, modeling allows exploration of ideas on new structures, for example, for tandem solar cells. Finally, when coupled with field data, modeling provides an excellent understanding and prediction of reliability. The modeling activity in SERIIUS addressed many of these concerns and also provided modeling and simulation assistance to all the other PV projects, as required.

1.4.2  Multiscale Concentrated Solar Power The ultimate goal was to achieve large-scale deployment of CSP plants across a broad range of plant sizes. The consortium did not aim to replicate existing utility-­ scale CSP technologies or to pursue research for incremental improvement in performance of existing CSP systems. To achieve Indian and U.S. goals, SERIIUS chose high-efficiency, scalable, distributed solar thermal power as the focus. The basic approach was to first identify appropriate power cycles, address technology gaps to develop the components through cutting-edge technology, and follow a systems-­ level approach toward developing innovative laboratory-scale technologies. As a first step, the SERIIUS team recognized that the conventional steam-based Rankine cycle cannot be used to meet all the above objectives because of the limitation of efficiency and severe scale-down penalty. Understanding the need for new power cycles that can be efficient as well as cost-effective at the medium scale, SERIIUS focused on the closed-cycle supercritical CO2 (s-CO2) Brayton cycle because it has the potential to achieve nearly 50% cycle efficiency even at a receiver temperature of 700 °C. The concept of using closed-cycle CO2 for generating power dates back to 1950 [25], and these cycles were analyzed in some detail in the 1960s [26–28]. Also, use of s-CO2 for nuclear power generation has been studied [29, 30], where heat-source temperatures are in the range of 600–700 °C, which suggests that the cycle could be suitably adopted for CSP applications. SERIIUS partner Sandia National Laboratories had already acquired synergistic experience in developing the first s-CO2 test loop at the scale of 200-kW power input [29, 30]. However, a s-CO2-based power plant is far from being a mature technology because several technological challenges need to be overcome—especially the development of critical components such as turbines, compressors, and heat exchangers for the prescribed pressure and temperature range. Also, adaptation of this cycle for CSP applications is not straightforward, and it requires overcoming several challenges in the form of integration of solar tower and solar receivers, and development of special heat exchangers to realize the potential of high thermody-

20

D. Ginley and K. Chattopadhyay

namic efficiency of the Brayton power cycle. The projects in SERIIUS under the “high-pressure high-temperature” track were designed to address these challenges. One project involved developing new solar receivers for s-CO2 and relevant test loops, and another one focused on cost-effective small-footprint heliostat systems ideal for distributed-scale operations. For deployability in the Indian context, low- and medium-temperature organic Rankine cycles (ORCs) are more suitable than high-temperature cycles, specifically in large parts of India receiving moderate solar direct normal irradiance (DNI) [31]. High DNI regions are restricted to only about 10% of India [31]. ORC-based power plants are fairly well established at the 500-kW to 1-MW level [32–38]. The main advantage of these cycles at this scale is that isentropic efficiencies of turbines for moderate-scale plants (few hundred kW level) are fairly high (~70–80%)—in contrast to steam cycles, which have such high efficiencies only at tens of MW or higher scales. However, even for ORC, the turbine isentropic efficiency drops drastically and demands high RPM at smaller scales, which makes small-scale ORC economically nonviable. For distributed ORC, a huge market exists in the range of 10–500 kW because of the ease of land availability at that scale and the nature of electricity needs at the village/district level. Hence, a major focus of this consortium under the “low/medium-temperature” track was to develop efficient small-scale expanders for ORC, and also, to develop appropriate fluids to enhance efficiency at small scale. Although stand-alone ORC systems cannot be very efficient thermodynamically (because of low operating temperatures), cost-effective collector/tracking systems and associated absorber/reflector materials can be developed for this temperature range to bring down the LCOE significantly. One of the objectives of SERIIUS for distributed CSP was to supply stable power. Hence, a third line of activity by the SERIIUS CSP team was to develop auxiliary heating systems (hybridization) and effective thermal storage systems for Brayton cycle and ORC plants. The development of high-temperature heat-transfer fluid and storage material is critical for the viability of s-CO2 Brayton-cycle-based systems. Therefore, SERIIUS undertook the task of developing new molten salts and thermic fluids as well as test loops to study the dynamics of charge/discharge operations. In the case of ORC systems, low-cost storage was the focus to minimize the exergy loss due to hybridization with high-temperature heat sources (such as biomass or fossil fuels), with an aim to increase capacity utilization and further reduce LCOE.

1.4.3  Solar Energy Integration India’s Jawaharlal Nehru National Solar Mission set a target of 100 GW of solar power by 2022. Around 2015, solar was only about 4.5 GW in India’s energy mix, so this is a very ambitious target. This target couples well with the U.S. DOE SunShot Initiative to achieve grid parity for solar. The role of the Solar Energy Integration (SEI) thrust was to identify and analyze the critical technical, economic,

1 Introduction

21

environmental, and policy issues for solar energy development and deployment in India and also evaluate the U.S.-specific scenarios and barriers in a synergistic fashion to what is being done in both countries currently. Thus, the SEI thrust connected SERIIUS to the bigger picture and provided guidance on research priorities to the PV and CSP thrusts of SERIIUS, which focused on high-impact fundamental and applied R&D to create disruptive solar technologies. The thrust aimed to achieve all this by leveraging existing technical, computational, and policy framework analysis capabilities of its Indian and U.S. partners, and also by interfacing with Indian and U.S. solar energy deployment initiatives through engagements with stakeholders. The thrust thus expected to contribute to the overall objective of the JCERDC to promote clean-energy innovation by joint teams of scientists and engineers from India and the United States. The ambitious solar deployment goals in India have brought successes in deployment, but have also been marked by limitations [39]. The U.S. and Indian governments will need to make difficult choices in allocating finite financial resources—whether these investments are in applied R&D or in deployment and industrial-policy incentives. A number of recent studies have explored the policy tradeoffs associated with making different renewable-energy investment choices [40–42]. Recent literature suggests that efficient policy designs can increase the feasibility of renewable technology deployment and reduce costs, which could significantly increase technology uptake and lead to increased access with substantial climate change mitigation and air-quality benefits [43, 44]. SEI aimed to help SERIIUS contribute to the goals of maximizing the social benefits of solar technology deployment and of minimizing the costs of doing so at large scale. Activities in the SEI research thrust were designed to integrate, coordinate, and generally advance the activities of the PV and CSP research thrusts. The three SEI projects did this in different and complementary ways, but in sum, they aimed to enable PV and CSP researchers to better understand the technical, economic, environmental, and policy landscape in which their technologies must ultimately be deployed. This understanding allowed PV and CSP leadership to fine-tune their research activities so that the technologies they developed are most likely to be successful at large scales of deployment, and, most likely, to lead to market success in India and the United States. It is only by understanding the integration landscape that large-scale market goals of both the U.S. and Indian governments can be met. Figure 1.6 shows the direct linkages that were established between the SEI projects and the PV and CSP activities, and how these logically flowed into supporting the goals of the U.S.’s SunShot Initiative and the Indian Government’s 100 GW goal. SEI activities directly supported and informed PV and CSP in a number of ways, including: bigger-picture studies and reports (e.g., technology roadmapping, Indian solar resource assessments) as well as market-specific analysis (e.g., financial and market analysis for low-cost polycrystalline Si and CIGS, industrial base analyses of the crystalline-Si and thin-film industries in India) [45]. SERIIUS developed a number of its own techno-economic models for both CSP and PV that are in various stages of maturity [46]. SERIIUS collaborated with NREL to leverage existing NREL tools for techno-economic modeling in the Indian context; this tool is able to

22 Fig. 1.6  SEI task organization linking the national mission. (NREL, unpublished)

D. Ginley and K. Chattopadhyay

CSP

India-US JCERDC

PV

sCO2 Brayton Reliability (NISE, IIT-B) Low Cost Poly Si & CIGS (Sandia, IISc,Thermax) (SunEdison, IIT-B, NREL,ARCI, Purdue) ORC (MIT, IISc) Pervoskites (WUSTL, NREL, IIT-B, IISc) Roadmapping, Bankability, Engineering Economics, Computational Tools R&D Advisory (CSTEP, RAND, NREL)

SEI Tech to Market

Policy Landscape System Integration Grid Studies Energy Storage (CMU, IITB, IISc, NISE, WUSTL, CSTEP, RAND, HPCL, WIPRO) India & US

NSM – 100 GW Solar

SunShot Initiative

reach a global community of researchers and decision-makers who rely on NREL’s tools. SEI also addressed the integration of solar technologies into the existing electricity grid, with an emphasis on integration and storage that supports PV or CSP deployment. This work included statistical analysis of integrating intermittent PV into a centralized, utility-scale grid [47] as well as modeling storage requirements for PV integration into microgrids [48, 49]. These areas are discussed in substantially greater detail in the chapters dedicated to the specific outcomes of each research thrust.

References 1. http://energy.gov/eere/sunshot/about-sunshot-initiative. Accessed January 2016 2. http://pib.nic.in/newsite/PrintRelease.aspx?relid=122566 (2015) 3. P.  Jackson et  al., Properties of cu(in,Ga)Se2 solar cells with new record efficiencies up to 21.7%. Phys. Status Solidi RRL 9, 28–31 (2015) 4. A. Colthorpe, Thin-film competition hots up as TSMC Solar breaks day-old CIGS efficiency record. PVTech, http://www.pv-tech.org/news/thin_film_competition_hots_up_as_tsmc_producer_breaks_day_old_cigs_efficien (2015) 5. W. Wang et al., Device characteristics of CZTSSe thin-film solar cells with 12.6% efficiency. Adv. Energy Mater. 4, 1301456 (2014)

1 Introduction

23

6. T. Gokmen et al., Band tailing and efficiency limitation in kesterite solar cells. Appl. Phys. Lett. 103, 103506 (2013) 7. M. Hosoya, et al., Organic thin-film photovoltaic modules, in Proc. 93rd Annual Meeting of the Chemical Society of Japan, 2C-5O-08 (2013) 8. C.H.  Peters et  al., High efficiency polymer solar cells with long operating lifetimes. Adv. Energy Mater. 1, 491–494 (2011) 9. H.J. Snaith, Perovskites: The emergence of a new era for low-cost, high-efficiency solar cells. J. Phys. Chem. Lett. 4, 3623–3630 (2013) 10. G.  Hodes, D.  Cahen, Photovoltaic perovskite cells roll forward. Nat. Photonics 8, 87–88 (2014) 11. J.H.  Noh et  al., Chemical management for colorful, efficient, and stable inorganic-organic hybrid nanostructured solar cells. Nano Lett. 13, 1764–1769 (2013) 12. International Energy Agency, Technology Roadmap: Solar Photovoltaic Energy (2014 Edition). https://www.iea.org/publications/freepublications/publication/TechnologyRoadmap SolarPhotovoltaicEnergy_2014edition.pdf (2014) 13. N. Drouiche et al., Recovery of solar grade silicon from kerf loss slurry waste. Renew. Sust. Energ. Rev. 36, 936–943 (2014) 14. W.O. Filtvedt et al., Development of fluidized bed reactors for silicon production. Sol. Energy Mater. Sol. Cells 94, 1980–1995 (2010) 15. K. Masuko et al., Achievement of more than 25% conversion efficiency with crystalline silicon heterojunction solar cell. IEEE J. Photovoltaics 4, 1433–1435 (2014) 16. M.  Kontges, et  al., Performance and reliability of photovoltaic systems: Review of failures of photovoltaic modules. IEA Report IEA-PVPS T13-01:2014, http://iea-pvps.org/index. php?id=275 (2014) 17. S.  Chattopadhyay et  al., Visual degradation in field-aged crystalline silicon PV modules in India and correlation with electrical degradation. IEEE J. Photovolt. 4, 1470–−1475 (2014) 18. D.C. Jordan, et al., Technology and climate trends in PV module degradation. Presented at 27th European PV Solar Energy Conference and Exhibition (2012) 19. M. Kempe, Overview of scientific issues involved in selection of polymers for PV applications. Presented at 37th IEEE Photovoltaic Specialists Conference (2011) 20. S. Kurtz, et al., Photovoltaic module qualification plus testing. NREL Technical Report NREL/ TP-5200-60950, http://www.nrel.gov/docs/fy14osti/60950.pdf (2013) 21. W. Herrmann, et al., Soiling and self-cleaning of PV modules under the weather conditions of two locations in Arizona and South-East India. Presented at 42nd IEEE Photovoltaic Specialists Conference (2015) 22. T.  Sarver et  al., A comprehensive review of the impact of dust on the use of solar energy: History, investigations, results, literature, and mitigation approaches. Renew. Sust. Energ. Rev. 22, 698–733 (2013) 23. E.S. Mungan et al., From process to modules: End-to-end modeling of CSS-deposited CdTe solar cells. IEEE J. Photovolt. 4, 954–−961 (2014) 24. S. Agarwal, P. Nair, Device engineering of perovskite solar cells to achieve near ideal efficiency. Appl. Phys. Lett. 107, 123901 (2015) 25. G. Sulzer, Verfahren zur Erzeugung von Arbeit aus Warme, Switzerland Patent, 1950 26. E.G. Feher, The supercritical thermodynamic power cycle. Douglas Paper No. 4348. Presented at the Intersociety Energy Conversion Engineering Conference, Miami Beach, FL, 13–17 August 1967 27. E.G.  Feher, Investigation of Supercritical Cycle. Astropower Laboratory, Missile & Space Systems Division (1968) 28. J.R. Hoffmann, E.G. Feher, 150 kWe Supercritical Closed Cycle System. ASME Paper (1970) 29. V.  Dostal, P.  Hejzlar, M.J.  Driscoll, N.E.  Todreas, A supercritical CO2 Brayton cycle for advanced reactor applications. Trans. Am. Nucl. Soc. 85, 110–111 (2001) 30. V. Dostal, A supercritical carbon dioxide cycle for next generation nuclear reactors. Ph.D., MIT (2004)

24

D. Ginley and K. Chattopadhyay

31. B. Bandyopadhyay, C.S., A. Datta, India Solar Resource Maps. http://mnre.gov.in/sec/DNI_ Annual.jpg (2013) 32. H.M.  Curran, Use of organic working fluids in Rankine engines. J.  Energy 5(4), 218–223 (1981) 33. H.D. Madhawa Hettiarachchi, M. Golubovic, W.M. Worek, Y. Ikegami, Optimum design criteria for an organic Rankine cycle using low-temperature geothermal heat sources. Energy 32, 1698–1706 (2007) 34. V.  Maizza, A.  Maizza, Unconventional working fluids in organic Rankine-cycles for waste energy recovery systems. Appl. Therm. Eng. 21, 381–390 (2001) 35. P.J. Mago, L.M. Chamra, K. Srinivasan, C. Somayaji, An examination of regenerative organic Rankine cycles using dry fluids. Appl. Therm. Eng. 28, 998–1007 (2008) 36. B.F. Tchanche, G. Lambrinos, A. Frangoudakis, G. Papadakis, Low-grade heat conversion into power using organic Rankine cycles – A review of various applications. Renew. Sust. Energ. Rev. 15, 3963–3979 (2011) 37. E.H. Wanga, H.G. Zhanga, B.Y. Fana, M.G. Ouyangb, Y. Zhaoc, Q.H. Muc, Study of working fluid selection of organic Rankine cycle (ORC) for engine waste heat recovery. Energy 36, 3406–3418 (2011) 38. H. Chen, D.Y. Goswami, E.K. Stefanakos, A review of thermodynamic cycles and working fluids for the conversion of low-grade heat. Renew. Sust. Energ. Rev. 14, 3059–3067 (2010) 39. G. Shrimali, S. Rohra, India’s solar mission: A review. Renew. Sust. Energ. Rev. 16, 6317– 6332 (2012) 40. G.R. Timilsina, L. Kurdgelashvili, P.A. Narbel, Solar energy: markets, economics and policies. Renew. Sust. Energ. Rev. 16, 449–465 (2012) 41. G.  Shrimali, D.  Nelson, S.  Goel, C.  Konda, R.  Kumar, Renewable deployment in India: Financing costs and implications for policy. Energy Policy 62, 28–43 (2013) 42. Bridge to India, India Solar Handbook, 2015 Edition (2015) 43. C.  Cameron, S.  Pachauri, N.D.  Rao, D.  McCollum, J.  Rogelj, K.  Riahi, Policy trade-offs between climate mitigation and clean cook-stove access in South Asia. Nature Energy 1, 15010 (2016) 44. U.N.E.P., Green Energy Choices: The Benefits, Risks, and Trade-Offs of Low-Carbon Technologies for Electricity Production (2016) 45. CSTEP, Solar Resource Assessment and Technology Roadmap for India. CSTEP SEI-1 Report (2013) 46. N.S. Suresh, N.C. Thirumalai, B.S. Rao, M.A. Ramaswamy, Methodology for sizing the solar field for parabolic trough technology with thermal storage and hybridization. Solar Energy 110, 247–259 (2014) 47. K. Klima, J. Apt, Geographic smoothing of solar PV: Results from Gujarat. Environ. Res. Lett. 10(1–7) (2015) 48. M.T. Lawder, V. Viswanathan, V.R. Subramanian, Balancing Autonomy and utilization of solar power and battery storage for demand based microgrids. J. Power Source 279, 645–655 (2015) 49. M.T. Lawder, B. Suthar, P.W.C. Northrop, S. De, C.M. Hoff, O. Leitermann, et al., Battery energy storage system (BESS) and battery management system (BMS) for grid-scale applications. Proc. IEEE 102, 1014–1030 (2014)

Chapter 2

Sustainable Photovoltaics David Ginley, Joel Ager, Rakesh Agrawal, Muhammad A. Alam, Brij Mohan Arora, S. Avasthi, Durga Basak, Parag Bhargava, Pratim Biswas, Birinchi Bora, Wade A. Braunecker, Tonio Buonassisi, Sanjay Dhage, Neelkanth Dhere, Sean Garner, Xianyi Hu, Ashok Jhunjhunwala, Dinesh Kabra, Balasubramaniam Kavaipatti, Lawrence Kazmerski, Anil Kottantharayil, Rajesh Kumar, Cynthia Lo, Monto Mani, Pradeep R. Nair, Lakshmi Narsamma, Dana C. Olson, Amlan J. Pal, Srinivasan Raghavan, Praveen Ramamurthy, Bulusu Sarada, Shaibal Sarkar, O. S. Sastry, Harshid Sridhar, Govisami Tamizmani, Jeffrey Urban, Maikel van Hest, Juzer Vasi, Yanping Wang, and Yue Wu

Photovoltaics was the largest of the three thrust areas of SERIIUS and included 10 distinct projects—ranging from materials (CIGS, CZTS, OPV perovskites) to devices (Si HIT) to deployment (reliability, applications). Each project had two project co-leads—one from each country—who coordinated the activities of tens of PIs and students scattered over different organizations. A description of some representative activities is given in the sections that follow. Many of the successes could be attributed to the collaborative nature of the work, which used the diverse expertise and perspectives present in the two countries. D. Ginley (*) · W. A. Braunecker · L. Kazmerski · D. C. Olson · M. van Hest National Renewable Energy Laboratory, Golden, CO, USA e-mail: [email protected]; [email protected]; [email protected]; [email protected] J. Ager · J. Urban Lawrence Berkeley National Laboratory, Berkeley, CA, USA e-mail: [email protected]; [email protected] R. Agrawal Purdue University, West Lafayette, IN, USA e-mail: [email protected] M. A. Alam School of Electrical and Computer Engineering, Hall for Discovery and Learning Research, Purdue University, West Lafayette, IN, USA e-mail: [email protected] B. M. Arora · A. Kottantharayil · P. R. Nair · J. Vasi Department of Electrical Engineering, Indian Institute of Technology-Bombay, Mumbai, India e-mail: [email protected]; [email protected]; [email protected] © Springer Nature Switzerland AG 2020 D. Ginley, K. Chattopadhyay (eds.), Solar Energy Research Institute for India and the United States (SERIIUS), Lecture Notes in Energy 39, https://doi.org/10.1007/978-3-030-33184-9_2

25

26

D. Ginley et al.

2.1  Solution-Processing Approaches 2.1.1  S  olution Processing of Thin-Film Solar Cells Based on Cu2ZnSn(S,Se)4 and Cu(In, Ga)Se2 Silicon-based solar cells are the current workhorse of the photovoltaic (PV) industry. However, thin films made from compound semiconductors hold promise in providing highly efficient and versatile solar cells for PV applications. In particular, solution-processed, roll-to-roll-manufactured solar cells on flexible supports can yield low-cost, lightweight PV modules. This will reduce the material used in the manufacture of solar cells, but will also shift the balance-of-system cost. Because of this great potential, scientists and engineers in both India and the United States joined forces under the SERIIUS project and embarked on developing methods for fabricating chalcogenide thin-film solar cells. S. Avasthi · S. Raghavan · P. Ramamurthy Centre for Nano Science and Engineering (CeNSE), Indian Institute of Science, Bangalore, Karnataka, India e-mail: [email protected]; [email protected]; [email protected] D. Basak · A. J. Pal Indian Association for the Cultivation of Science, Kolkata, West Bengal, India e-mail: [email protected]; [email protected] P. Bhargava Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Powai, Mumbai, India e-mail: [email protected] P. Biswas Washington University in St. Louis, St. Louis, MO, USA e-mail: [email protected] B. Bora National Institute of Solar Energy, New Delhi, India T. Buonassisi Massachusetts Institute of Technology, Cambridge, MA, USA e-mail: [email protected] S. Dhage · B. Sarada International Advanced Research Centre for Powder Metallurgy and New Materials, Hyderabad, Telangana, India e-mail: [email protected]; [email protected] N. Dhere Florida Solar Energy Center, Cocoa, FL, USA e-mail: [email protected] S. Garner Corning Research & Development Corporation, Corning, NY, USA e-mail: [email protected]

2  Sustainable Photovoltaics

27

The first material system considered by the team is Cu2ZnSn(S,Se)4 (CZTSSe) and its derivatives. CZTSSe solar cells have drawn much attention because of their high absorption coefficient, low toxicity, and Earth-abundant constituent elements. The record-efficiency CZTSSe solar cell was created using a hydrazine-based slurry coating process [1–4]. Hydrazine is extremely explosive and toxic, so the team focused on developing a more benign solvent system to achieve scalable fabrication of solution-processed CZTSSe thin films. Additionally, the SERIIUS project focused on understanding the growth mechanism of CZTSSe grains in thin films as well as manipulating their material/optoelectronic properties to develop high-­ efficiency solar cells. X. Hu School of Chemical Engineering, Forney Hall of Chemical Engineering, Purdue University, West Lafayette, IN, USA e-mail: [email protected] A. Jhunjhunwala · L. Narsamma Indian Institute of Technology-Madras, Chennai, Tamil Nadu, India e-mail: [email protected]; [email protected] D. Kabra Department of Physics, Indian Institute of Technology-Bombay, Mumbai, India e-mail: [email protected] B. Kavaipatti Department of Energy Science and Engineering, Indian Institute of Technology-Bombay, Mumbai, India e-mail: [email protected] R. Kumar · O. S. Sastry Solar Energy Centre, MNRE, National Institute of Solar Energy, New Delhi, India C. Lo Energy, Environmental and Chemical Engineering, Washington University in St. Louis, St. Louis, MO, USA e-mail: [email protected] M. Mani Centre for Sustainable Technologies, Indian Institute of Science-Bangalore, Bangalore, India e-mail: [email protected] S. Sarkar Indian Institute of Technology-Bombay, Mumbai, India e-mail: [email protected] H. Sridhar Center for Study of Science, Technology and Policy, Bengaluru, Karnataka, India e-mail: [email protected] G. Tamizmani Arizona State University, Tempe, AZ, USA e-mail: [email protected] Y. Wang · Y. Wu Solarmer Energy Inc., El Monte, CA, USA e-mail: [email protected]; [email protected]

28

D. Ginley et al.

Another material system considered is Cu(In,Ga)Se2 (CIGSe). CIGSe has been extensively studied because of its extraordinary performance as a light-absorber material in thin-film solar cells. The record single-junction power conversion efficiency (PCE) for CIGSe solar cells has reached 22.6% at lab scale [5]. However, most high-efficiency CIGSe solar cells are fabricated by vacuum-based techniques. Solution-processed CIGSe thin films are desirable for fast throughput, better material utilization, and improved compositional uniformity of the film. In this project, we focused on developing molecular and nanocrystal precursors as well as processing CIGSe thin films on flexible glass substrates. 2.1.1.1  S  olution-Processed Thin-Film CZTSSe Solar Cells: Molecular Precursor and Nanocrystal Inks This project included three major aspects: establishing a precursor, exploring materials science, and fabricating high-performance CZTSSe solar cells by non-­ hydrazine methods. The team worked on two solution-processed deposition methods: molecular-precursor deposition and colloidal nanocrystal-ink deposition. Similar to hydrazine-based slurry coating, the molecular-precursor route requires one to identify molecular precursors and a solvent system for dissolution. In our study, we established a new versatile solvent system using a mixture of (di)amines and (di) thiols. This alkahest solvent system is capable of dissolving a series of anion (sulfur, selenium, tellurium) and cation (metal, metal salts, metal chalcogenides) sources as shown in Fig. 2.1 [6–10]. With the molecular-precursor route, we expect to achieve better control of composition and uniformity by independently adjusting different cation/anion ratios. This route is very promising for industrial-scale production. In the work by Zhang et al., CZTSSe solar cells were first fabricated by spin-­ coating of an amine-thiol solution [6]. In this particular case, relatively low-boiling-­ point amines/thiols are generally preferred to eliminate the carbon-rich fine-grain layer. On the other hand, if the solvent is too volatile, it may result in film cracks during annealing and affect the final film morphology. Considering the viscosity as well as evaporation rate, a mixture of hexylamine-propanethiol was used as the solvent. CuCl, ZnCl2, and SnCl2 were dissolved as cation sources, and a mixture of S and Se was used as the anion source at a total concentration of 2 M. The cation and anion solutions were mixed prior to spin coating. After each coating, the film was annealed at 250 °C to break the organometallic complexes and form a CZTSSe precursor film. These steps were typically repeated eight times to obtain a total thickness of 800 nm. The precursor films showed reflective, smooth surface morphology and long-range compositional uniformity as confirmed by energy-dispersive spectroscopy (EDS) under a scanning electron microscope (SEM) and scanning transmission electron microscope (STEM). After annealing in Se vapor (selenization), these films transformed into CZTSSe with no detectable Cl signal by EDS. The PV device was fabricated by deposition of CdS/ ZnS/ITO/Al/Ni grids in sequence, resulting in a PCE of 7.86% with a total area of 0.47 cm2 (8.09% for 0.456-cm2 active area) under AM1.5 illumination.

2  Sustainable Photovoltaics

29

Fig. 2.1  Solution prepared by dissolving (a) Cu, Sn, Zn, Cu2S, CuS, SnS, SnSe, Cu2Se, and CuSe in solvent mixtures of HA-EDT; (b) gallium, indium, and copper salts in HA-EDT; (c) gallium, indium, and copper salts in HA-PT; and d) copper, zinc, tin salts, or oxides in HA-PT. (a) Reprinted with permission from [7]. Copyright ©2016 The Royal Society of Chemistry. (b, c) Reprinted with permission from [8]. Copyright ©2016 The Royal Society of Chemistry. (d) Reprinted with permission from [7]. Copyright ©2015 American Chemical Society

In a second approach, nanocrystal inks were used for film deposition. This approach typically started by synthesizing CZTS nanocrystals and was followed by dispersing these particles in solvent(s) to form a nanocrystal ink. The ink was coated on a substrate and then baked at an elevated temperature to obtain the sulfide nanoparticle precursor film. This nanoparticle film was then selenized to form CZTSSe as the absorber layer in solar cells. Compared with other methods, nanocrystal-­ink deposition is attractive because of its robust and relatively low-­ toxicity processing. CZTS nanocrystals were first synthesized by Guo et al. at Purdue University and successfully fabricated into solar cells in 2009. The efficiency of these cells was improved to 7.2% in the following year, demonstrating the great potential of this method [11, 12]. Based on previous work, Miskin et al. modified the nanoparticle synthesis procedure, i.e., changed the synthesis method to a hot-injection method, altered the reaction time and temperature, and adjusted the nanocrystal washing procedure [13]. By hot injection, one is able to ensure that the sulfur source is fully

30

D. Ginley et al.

dispersed, and nucleation can occur and proceed in a uniform sulfur-rich environment, which minimizes the inhomogeneity of the composition and structure of the nanocrystals [13, 14]. Through the new route, we achieved 9.0% PCE for the CZTSSe thin-film PV devices with an active area of 0.47 cm2, which was the record efficiency for nanocrystal-ink-based, solution-processed CZTSSe thin-film solar cells and met the SERIIUS task to demonstrate >8%-efficient CZTSSe solar cells from a non-hydrazine-based solution route. Using this nanocrystal ink, a CZTSSe thin film was grown on 100-μm-thick Corning® Willow® Glass substrate and a PCE of 6.9% was obtained, demonstrating the versatility of this solution route. In addition to nanocrystal synthesis, the selenization process was also studied to pursue high-PCE devices. During selenization, the precursor film grows into a layer of micron-sized, continuous, dense grains [15]. The phase purity and compositional uniformity of this large-grain layer is essential for good optoelectronic properties of the absorber layer and the resulting devices. Aiming at the SERIIUS objective to achieve 15%-efficient CZTSSe cells and provide a materials science basis for high-­ efficiency CZTSSe research-scale cells, a multi-zone rapid-thermal-processing (RTP) furnace was used to understand the grain-growth mechanisms and to optimize the morphology as well as the optoelectronic properties of the CZTSSe absorber layer. In this project, the nanoparticle film and Se source were placed in different zones with separate control of temperature in the RTP. By manipulating the temperature difference between the Se source and nanoparticle film, we can vary the amount of Se delivered on the surface of the nanoparticle film; therefore, we can study how the phase/vapor pressure of Se will affect the grain growth of CZTSe during selenization. By keeping the Se source temperature higher than the nanoparticle film, we can create a saturated vapor pressure on top of the nanoparticle film, which forces the formation of liquid-phase Se via condensation on the film. Liquid selenium can dissolve Cu, Zn, and Sn from nanoparticles as reaction media at elevated temperatures (Tfilm > 300 °C) and Cu2-xSe nucleates from the liquid, followed by the incorporation of Sn and Zn sequentially to form CZTSe grains [16, 17]. Also, we demonstrated that the morphology of the large grains in the absorber layer is highly related to the supply of Se [15]. It shows a more uniform nucleation process when the nanoparticle film is annealed in a saturated Se atmosphere or a Se layer is deposited directly on top of the nanoparticle layer prior to selenization. The uniform nucleation process shows highly faceted grains because of a reaction-controlled mechanism in which low-surface-energy planes are preferred during nucleation and grain growth. On the contrary, if the Se vapor pressure is unsaturated, then round, dense grains with a curved surface are generated instead due to a diffusion-­controlled grain-growth process. In addition to the liquid Se-assisted grain-growth mechanism, the role of sodium dopant during selenization was also pursued. Traditionally, CZTS nanoparticles are deposited on Mo-coated soda-lime glass (SLG), and an appreciable amount of Na can diffuse from the SLG into the CZTS layer and affect grain growth during selenization [18–21]. In contrast, we conducted film-growth experiments with borosilicate glass (BSG) because very little Na is present in this substrate [15]. Based on the liquid Se-assisted theory that we developed, we found that if saturated Se vapor pressure is provided, then the CZTSSe film grown on BSG with no Na addition has

2  Sustainable Photovoltaics

31

a morphology similar to the film grown on SLG. The role of Na is significant only if the Se supply is insufficient. In this case, Na will diffuse at the surface of particles and form Na2Sey with Se, which helps with the dissolution and sequential precipitation of Cu2-xSe as well as the subsequent formation of CZTSe. The final device performances were examined for the CZTSSe thin-film solar cells fabricated with different selenization conditions. Notably, the films consisting of round grains (diffusion-controlled grain growth) had a higher PCE compared with the films with highly faceted grains (reaction-controlled grain growth). This efficiency difference is attributed mostly to the open-circuit voltage (Voc), which is limited by recombination at surface/interface defects from the faceted films. To eliminate these surface/interface defects, a low-temperature annealing was applied in inert gas after selenization. This significantly improved the Voc and helped the faceted film devices to achieve PCE values comparable to the round-grain thin-film devices. Lastly, a champion device with 9.3% efficiency was fabricated for a total area of 0.47 cm2 [15]. In summary, under the SERIIUS project, we investigated the role of Se atmosphere during the selenization of nanocrystal-ink-based thin films. We elucidated the liquid Se-assisted mechanism for nucleation and grain growth and provided understanding and gained insights for the future fabrication of high-PCE CZTSSe thin-film solar cells. 2.1.1.2  CZTS Synthesis from Electrodeposited Metal Precursors As an alternative to the direct ink-precursor solution approach, we also studied the ability to electrodeposit metal layers and subsequently sulfurize or selenize those to create the CZTS or CZTSe absorber layer. Here, we provide a kinetic and thermodynamics basis for phase/alloy development under different thin-film deposition scenarios. We have studied the phase evolution and origin of impurity-phase formation of CZTS on FTO-coated glass substrates, because this is the plausible substrate for thin-film PV devices based on CZTS absorber layers. The electrodeposition sequence of Cu(A)-Sn-Cu(B)-Zn (CTCZ) precursors with variations in the molar ratio of the two Cu layers—Cu(A)/Cu(B) = 0.33 and 3—and subsequent sulfurization to yield CZTS has been studied. The sequence of ­electrodeposition of metal precursor layers is chosen based on the standard reduction potentials of Cu, Sn, and Zn; the second Cu layer (Cu(B)) is necessary to enable Cu-Zn intermetallic-phase growth and avoid hydrogen evolution during the electrodeposition of the Zn layer. Based on the phase-field position of the Cu(A)-Sn couple in the Cu-Sn phase diagram, two different sequences of the metallic layers are investigated. CTCZ-0.33 places the Cu(A)-Sn couple in the Sn-Cu6Sn5 phase field and CTCZ-3 places the Cu(A)-Sn couple in the Cu6Sn5-Cu3Sn phase field. For best results, the metal precursor films were sulfurized at 550 °C for 2 h, and structural (XRD and Raman) analysis revealed the presence of Cu2S inclusions in the CZTS thin film obtained from the CTCZ-0.33 metal precursor. Such Cu2S inclusions are not present in the CZTS film obtained from the CTCZ-3 precursor. Further, Raman mapping was done on the films obtained from both the precursors to analyze

32

D. Ginley et al.

Fig. 2.2  Raman mapping of the sulfurized thin films obtained over an area of 10 × 10 μm2. The Raman peak corresponding to CZTS at 337  cm−1 is mapped for CTCZ–0.33 (a) and CTCZ–3 precursors (b). Similar mapping of the Raman peak corresponding to the Cu2S phase at 475 cm−1 is shown in (c) and (d) for both samples. (Unpublished)

the homogeneity of CZTS and the extent of Cu2S impurities present. Figure 2.2a,b show the Raman maps of the CZTS peak at 337 cm−1 over the 10 × 10 μm2 region of the film. The color scale of the mapped images varies from black to bright yellow. The bright yellow corresponds to the CZTS phase and black to the region devoid of the CZTS phase. The Raman mapping clearly indicates that phase-pure CZTS is obtained only with CTCZ-3 whereas CTCZ-0.33 precursor yields a mixture of CZTS and possibly Cu2S. This is confirmed by the Cu2S Raman (475 cm−1) mapping shown in Fig. 2.2c,d, where the bright yellow region corresponds to Cu2S and the black region to phases other than Cu2S. Sulfurization of the CTCZ-3 precursor does not yield Cu2S impurity phase, whereas CTCZ-0.33 forms a Cu2S phase indicated by the presence of a bright yellow spot (Fig. 2.2c). 2.1.1.3  Defect Study for CZTSSe Solar Cells After extensive optimization regarding the light-absorber layer in CZTSSe solar cells, the SERIIUS record PCE met its bottleneck at 9.3%. Therefore, significant effort was devoted toward studying the optoelectronic properties of this absorber material to explore the potential limiting intrinsic defects within the CZTSSe material. The researchers at Purdue established a generalized model for quantum efficiency (QE) analysis of CZTSSe solar cells, which is also applicable to the analysis

2  Sustainable Photovoltaics

33

of other nonideal heterojunction thin-film solar cells [22]. Additionally, different optoelectronic parameters such as absorption coefficient, diffusion length, and mobility can be extracted from this model. The contribution from diffusion and recombination is separately extracted from wavelength- and voltage-dependent QE data. It was found that the diffusion length is not the limiting issue for CZTSSe solar cells. Based on the voltage-dependent QE data, the recombination resulting from tail states or potential fluctuation at the band edge is responsible for limiting the carrier collection and thus the performance of the solar cell. To further explore the source of tail states, researchers at NREL and Purdue completed voltage-dependent admittance spectroscopy (AS) with carrier-injection pretreatment (CIP) at different applied biases [23]. By measuring the admittance under AC signal with different frequencies at low temperature in the dark, we were able to extract the activation energies from the admittance signatures derived from capacitance-temperature curves. It was observed that an additional activation energy shows up for the CIP devices compared to relaxed devices, which demonstrates the existence of metastable defects generated from carrier injection. The voltage-­ dependent AS was also conducted, showing that activation energies from metastable defects are independent of applied bias, which corresponds to defects from the bulk absorber instead of surface or interfacial defects between CZTSSe and CdS. From this work, we concluded that the defects in CZTSSe that significantly limit the Voc of the device are bulk defects. Further improvement of CZTSSe solar cells can hardly be achieved without solving this intrinsic problem of this material. 2.1.1.4  S  ubstitution of Cu with Ag to Eliminate I-II Antisite Defects in CZTSSe Having proved that bulk defects are the limiting issue for CZTSSe thin-film solar cells, we revised our plan to modify CZTSSe solar cells with substitutional elements to meet the SERIIUS goal for high-performance solution-processed solar cells. From theory calculation [24, 25] and experimental observation [26–28], both have demonstrated that the most common bulk defects in CZTSSe are Cu-Zn antisite defects. Because of the similar sizes between Cu+ and Zn2+ as well as the dual valences of Cu ions, the formation energy for Cu-Zn antisite defects is very low in CZTSSe. These defects create defect states near the band edges in CZTSSe (CnZn state ~0.15 eV above valence band and ZnCu ~0.1 eV below conduction band) and significantly decrease the Voc in CZTSSe devices [25]. Researchers have proposed replacing Cu or Zn by other elements to reduce/eliminate the I-II antisite defect density. Ag substitution for Cu is one such successful example. Due to the significant size difference with Zn2+ and the single valence property of Ag+, it has been calculated that the formation energy of AgZn + ZnAg pairs is significantly higher than that of CuZn + ZnCu in CZTSSe [29–31]. The SERIIUS team was the first to explore and report the optoelectronic properties of Ag-alloyed CZTSSe absorbers [32]. Ag was incorporated into the CZTS system from nanoparticles at

34

D. Ginley et al.

Fig. 2.3  Cross section (top) and plan view (bottom) of SEM images for (a) CZTSe, (b) 5%-ACZTSe, and (c) 50%-ACZTSe selenized absorbers. Reprinted with permission from [32]. Copyright ©2015 Elsevier Ltd

different [Ag]/([Ag]  +  [Cu]) ratios. Nanoparticles were synthesized starting with the [Ag]/([Ag] + [Cu]) ratio at 0%, 5%, and 50% (50% nanoparticles were a mixture of ACZTS and SnS-Ag2S binary nanoparticles). After selenization in the RTP, the nanoparticles were fully selenized into ACZTSe large grains without detectable secondary phases for all different Ag concentrations up to 50%. It is observed that less-­faceted grains with fewer voids are present with increased Ag content, which is the preferred morphology for light-absorber layers in thin-film solar cells (shown in Fig. 2.3). The enhanced grain growth may be attributed to the Ag-incorporated liquid Se-assisted sintering due to the low eutectic temperature of the Ag-Cu-Sn ternary system [33, 34]. In addition to investigating film morphology related to Ag concentration, the optoelectronic properties of Ag-alloyed CZTSSe were studied with regard to minority-­ carrier lifetime by using time-resolved photoluminescence (TRPL). Results showed that the minority-carrier lifetime of 5%-ACZTSe is 50% longer than that of CZTSe and that 50%-ACZTSe has about 10  ns of minority-carrier ­lifetime, which is 4 times that of CZTSe. The extended carrier lifetime is not only due to the enlarged grains size but is also related to the low defect density resulting from the Ag alloying. The CZTSe and ACZTSe thin films were fabricated into solar cells. 5%-ACZTSe solar cells have the highest average PCE among 0%, 5%, and 50% Ag-incorporated devices, which is ~10% higher than the CZTSe case. External quantum efficiency (EQE) analysis was also conducted for CZTSe and ACZTSe solar cells. A slight increase in bandgap—which reduces the current at long wavelength range—is observed in 5%-ACZTSe compared with CZTSe. But based on EQE measurement, the Urbach energy was found to decrease substantially with Ag incorporation, demonstrating an elimination of defect density. 50%-ACZTSe has relatively low efficiency despite having the longest carrier lifetime and lowest Urbach energy. This is probably because of the spike between the conduction bands at the ACZTSe/CdS interface. Higher PCE is expected with further optimization regarding the band alignment.

2  Sustainable Photovoltaics

35

Based on this work, we conclude that Ag is able to suppress the band tailing, but it also enhances grain growth in ACZTSe thin films [32, 35, 36]. Both benefits help improve the optoelectronic properties of the kesterite absorber layer, as demonstrated by the increase in the minority-carrier lifetime. With further optimization on band alignment at the heterogeneous junction, high-performance Ag-alloyed ACZTSe PV devices are expected in the future. 2.1.1.5  Development of CIGSe Solar Cells from Molecular Precursors Similar to the case of CZTSSe, the nanocrystal-ink and molecular-precursor routes are two major approaches for solution-processed thin-film CIGSe. For the nanoparticle route, the Purdue group established a method for nanocrystal-based ink deposition with a subsequent selenization process and achieved 15% PCE through this method [37–39]. The nanocrystal-ink method has also proven to be universal for flexible substrates under the SERIIUS project. The nanocrystal film was deposited on 100-μm-thick Corning® Willow® Glass and selenized in the RTP, yielding a PCE of 7.5% for the champion device. More optimization and modifications are needed to further improve the PV performance of this flexible device. Under this project, we primarily worked on developing a universal solution system for CIGSe and solar cell fabrication. Following the previous discovery of the dissolution ability of amine-thiol mixtures [6, 7, 10, 40–42], Zhao et al. from the SERIIUS team explored the potential of a monoamine and dithiol mixture, which is less aggressive than either hydrazine or diamine-dithiol mixtures [8]. Metal chalcogenides, metal salts, and elemental chalcogens can be dissolved in this system at an appreciable concentration at room temperature and ambient pressure. For fabricating CIGSe solar cells, Cu2Se, In(C2H3O2)3, Ga(C5H7O2)3, and Se were dissolved in a hexylamine (HA)-1, 2-ethanedithiol (EDT) mixture with a cation ratio of [Cu]:[In]:[Ga] = 0.95:0.7:0.3. During the dissolution study, it was observed that use of chloride salts for any of the elements as molecular precursor was unsuitable for this system. If a chloride salt is used, GaCl3 forms and evaporates during annealing at elevated temperatures, resulting in Ga loss during the process [8]. The amine-dithiol solution was spin coated on Mo-coated glass and the deposited film was annealed on a hot plate at 325 °C to get rid of excess solvent and stabilize the precursor film. This coating and annealing procedure was repeated eight times to get a sulfide film with a thickness of about 560 nm. This annealed precursor film consisted of Cu(In,Ga)S2 nanoparticles with diameters of ~2.3 nm estimated from the full-width at half-maximum (FWHM) of the X-ray diffraction (XRD) pattern. These ultrafine nanocrystals also benefit the subsequent selenization process by depleting the sulfur. A major challenge with the selenization of thin films from amine-thiol molecular precursors is that large grains could not be grown throughout a film with thicknesses greater than ~500 nm. The ultra-thin thickness of ~500 nm is not enough to absorb most of the incident light, and therefore, it limits the current collection. The solar cell with an ultrathin absorber layer only had a PCE of 10.3%. An attempt to increase

36

D. Ginley et al.

Fig. 2.4  Cross-section SEM image of (a) ultrathin CIGSe solar cell, (b) a typical sandwich-­ structure CIGSe solar cell, and (c) modified CIGSe/Mo/glass thin film after soaking and selenization. Reprinted with permission from [8]. Copyright ©2016 The Royal Society of Chemistry

the film thickness led to a much thicker fine-grain layer at the bottom of the film near the Mo interface. Occasionally, with thicker films, a sandwich layer with large grains at the top and bottom and a fine-grain layer in between was observed after selenization, which led to low-PCE solar cells (as shown in Fig. 2.4b). To solve this problem, the thicker precursor films were soaked in the cation molecular precursor with only 0.05 M cation concentration and no added chalcogen. After this soaking procedure, the films were subjected to the standard selenization process and no sandwich structure (or bottom fine-grain layer thicker than 100 nm) was generated up to a total thickness of 1.1 μm , as shown in Fig. 2.4c. Our hypothesis is that the soaking benefits the selenization of thick films through two aspects. On one hand, it reduces the porosity of this film, thereby preventing Se from getting to the bottom of the film at the onset of selenization. On the other hand, the additional cation supply from soaking helps with the coarsening and grain growth. After modification, the fabricated CIGSe solar cells achieved PCEs up to 12.2% with a total area of 0.49 cm2. The PCE met the SERIIUS goal of >12% CIGSe solar cells [8]. This study establishes a robust method for solution-processed fabrication of CIGSe thin-film solar cells based on a monoamine-dithiol solution system. Furthermore, the strategies developed under the SERIIUS project to help eliminate elemental loss as well as enhance grain growth will be useful for the future fabrication of molecular-precursor ink-based CIGSe thin films. 2.1.1.6  C  IGSe Thin-Film Absorbers by Non-vacuum Inkjet-Printing Route In line with the objectives of the PV project to develop the inks, precursors, and process chemistries to enable precursor-based atmospheric processing for CIGS absorber layer, ARCI investigated a non-vacuum route using inkjet printing and an atmospheric-pressure selenization process. For making the CIGS absorber layer via inkjet printing, the most challenging problems addressed were formulation of long-­ term stable precursor ink, systematic optimization of jetting and printing parameters [43, 44], and atmospheric-pressure process selenization using elemental selenium as a source. At first, a particle-based ink and photonic sintering using intense pulsed light (IPL)/laser was initiated to make the CIGS absorber. The devices fabricated

2  Sustainable Photovoltaics 6

(a) Current (mA)

Piezo Ink printing on substrate

4 2 0

37

(b)

Efficiency - 3.4% Voc - 330 mV

Dark

-2 -4

(c)

Jsc - 24 mA/cm 2 FF - 39%

AM 1.5

-6

Crystalline Fine grain

CIGS Mo

200 nm

-0.30 -0.15 0.00 0.15 0.30 0.45 0.60 Voltage(V)

Fig. 2.5 (a) Schematic representation of inkjet printing of formulated CIG precursor ink on substrate, (b) dark and photocurrent characteristic of CIGS thin-film solar cell, and (c) cross-section field-effect SEM of CIGS by inkjet printing and selenization. (Unpublished)

using the above-mentioned methodology have demonstrated a PCE of 1.6% despite the limitations of the particle-based ink, which has shown difficulty to inkjet print, resulting in compositional and thickness nonuniformity. A quality CIGS absorber with chalcopyrite phase could be made by simply using the IPL/laser treatment, but it turned out to be difficult to achieve grain growth and to attain a decent CIGS/Mo interface, which is essential for effective charge-carrier transport. In view of this, a precursor-based ink approach for the absorber was adapted to overcome the limitations. Nitrates of copper (II), indium (III), and gallium (III) were used in precursor (Cu/ (In + Ga)) ratio of 0.95 and (Ga/(In + Ga)) ratio of 0.3 in aqueous medium. The viscosity and other desired properties of the formulation were adjusted using polyethylene glycol to make the ink suitable for inkjet printing. A high-resolution piezoelectric-­based drop-on-demand (DOD) Ceraprinter (Ceradrop) was used for printing patterns, shown schematically in Fig. 2.5a. The characteristics and ejection properties of droplets were analyzed and adjusted to get the desired high-quality droplet. A multiple-layers printing approach was adapted to print about 500-nm CIG precursor films on a Mo-coated soda-lime glass (SLG) substrate using optimized jetting parameters. Further, selenization of the inkjet-printed CIG precursor film was done using an atmospheric-pressure rapid thermal heating process to make the CIGSe2 absorber. Full-device fabrication was done using sequential deposition of 80-nm n-type CdS, 30-nm i-ZnO, and 950-nm Al:ZnO top contact on the CdS/ CIGS/Mo stack. The fabricated device exhibited a PCE of 3.4% on an active device area of 16 mm2 (Fig. 2.5b). It is presumed that numerous factors are responsible for the low performance of a device by inkjet printing—mainly, the bilayer structure of the CIGSe2 absorber, top large-grain layer of ~350 nm, and bottom fine-grain layer of ~250 nm with moderate MoSe2 formation at the Mo/CIGS interface (Fig. 2.5c). The bottom fine-grained layer has a detrimental effect because of the high recombination rate, poor light absorption, and low diffusion length resulting in low Voc and short-circuit current density (Jsc). The reason for the bottom fine-grain layer could be uneven distribution of Se vapors during the selenization process. The exposure of precursor film to the selenium vapor would initiate selenization from the top surface

38

D. Ginley et al.

to the bottom. Further, a highly dense, fully selenized top layer can act as a hindrance to effective selenization at the bottom. Moreover, the uneven distribution of Se vapors in some areas causes Se to react with Mo and form thin MoSe2 at the Mo/ CIGS interface, responsible for an increase in series resistance. Despite the inadequate selenization process, the inkjet printing of aqueous-based ink has demonstrated reasonable PCE; however, more optimization and fine-tuning are needed to further improve the PCE. 2.1.1.7  Summary In this SERIIUS project, we were able to obtain CZTSSe solar cells with >8% PCE by a non-hydrazine-based solution route. We investigated nanocrystal and molecular-­ precursor ink approaches toward phase-pure CZTSSe thin films, with noteworthy PV performance for the fabricated thin-film solar cells. The material and optoelectronic properties of this material were systematically explored in this project through different characterization methods. Based on these studies, we conclude that CZTSSe—the kesterite thin-film material—has bulk defects that significantly limit the open-circuit voltage of the PV devices and prevent the realization of 20% PCE solar cells. Subsequently, the SERIIUS team published the first results on the use of Ag partially substituted in Cu to minimize/eliminate the formation of these defects. Morphological and optoelectronic properties of the films were both improved after Ag incorporation. For solution-processed CIGSe solar cells, we discovered a monoamine-dithiol solvent system to dissolve cation/anion sources and produce a versatile, stable molecular precursor for CIGSe thin-film deposition. This system is more benign compared with the hydrazine system, and this paves a path forward toward scalable, roll-to-roll production of CIGSe thin-film solar cells. Throughout the whole project, the SERIIUS team at Purdue University, NREL, IIT Bombay, and ARCI Hyderabad worked in close collaboration. Due to this cooperation, Dr. Sreekanth Mandati visited Purdue for 6 months to explore the difference between vacuum-deposited and solution-processed CIGSe thin films. Mr. Ashish Singh from IIT Bombay spent 3 months at NREL and visited Purdue University to study the Mo sputtering on glass that is used as a back contact for both CZTSSe and CIGSe solar cells. Graduate students from Purdue visited NREL to coat CIGSe films on flexible Willow Glass and made flexible thin-film solar cells. Well-organized monthly telephone meetings offered frequent discussion and exchange of ideas. Based on the outcome of the SERIIUS project, near-term future work at Purdue University will focus on two projects: Ag-substituted CZTSSe solar cells and understanding amine-thiol dissolution chemistry. For Ag-substituted CZTS, we are looking into the effect of Ag incorporation into the kesterite system. Solar cells fabricated with Ag-incorporated kesterite absorbers are anticipated to have better PV performance, especially higher Voc, compared with CZTSSe solar cells. For the molecular-­ precursor route to CIGSe thin films, the mechanism of amine-thiol mixture dissolution for all kinds of metals, metal salts, and metal chalcogenides is being

2  Sustainable Photovoltaics

39

studied by mass spectroscopy, proton nuclear magnetic resonance, and X-ray absorption spectroscopy. This work is expected to shed light on the fundamental chemistry of dissolution for these mixtures, leading to broad applications of this approach for other semiconducting thin-film depositions.

2.1.2  Organic Photovoltaic Materials and Devices 2.1.2.1  Background and Motivation When the SERIIUS effort commenced in 2012, the first few reports of organic photovoltaic (OPV) cells reaching new world-record efficiencies around 10% had only recently begun circulating [45]. Efficiencies in the field had more than doubled in just a few short years, which generated much enthusiasm in the OPV community [46, 47]. Outside the field, the hype was not always fully appreciated because efficiencies for inorganic PV technologies were 3–4 times that of organic solar cells [48]. However, although crystalline inorganic PV is perfect for various rooftop and solar array applications, OPV still has its place. For example, the lightweight and flexible nature of OPV gives it many niche applications [49]. The transparency of OPV films can allow all the windows of the world’s tallest skyscrapers to harvest energy. OPV can also be manufactured on a large scale relatively inexpensively using roll-to-roll technology—a technology with start-up costs orders of magnitude lower than some other PV technologies [50]. Indeed, this latter aspect made OPV technology attractive for the developing regions of the world. And for this reason, OPV was particularly well received by the funding agencies on the Indian side of this partnership. The SERIIUS team initially set out to develop new high-performance absorbers largely by optimizing their optoelectronic properties. In the early part of this decade, the morphology of donor–acceptor bulk heterojunctions was certainly recognized as important in influencing device performance. But the majority of research in the field focused on bandgap engineering, because this was the primary way most recent efficiency gains in the field had been made [51]. Milestones in the SERIIUS OPV effort were structured around identifying and synthesizing absorbers with optimal energy levels to push device efficiencies beyond existing world records (with initial milestones of 10%-efficient single-junction devices >1 cm2 and 13%-efficient tandem devices). The collective strategy sought to address two main bottlenecks in developing OPV materials: synthesis and device optimization. The assembled team possessed unique and diverse skillsets that, in principle, allowed them to efficiently address these bottlenecks with a strategy that looked something like the following: (1) tens of thousands of new absorber materials would be screened for optimal optoelectronic properties using the National Renewable Energy Laboratory’s (NREL’s) high-performance computing center; (2) the best candidates would then be synthesized by the Indian Institute of Science’s (IISc’s) organic nanoelectronics group; (3) those materials would then be further screened

40

D. Ginley et al.

for some critical photophysical properties using a rather unique and powerful tool developed at NREL known as time-resolved microwave conductivity; (4) the most-­ promising materials would then be sent for device optimization at Solarmer; and (5) fundamental information about how morphology affected performance in these systems would be gleaned with powerful imaging tools at the Indian Association for the Cultivation of Science (IACS). At the outset of the project, the plan seemed promising and success imminent with such a talented team. However, 2 years in—and many dozens of new materials later—not a single new SERIIUS material had been developed with this strategy that was even close to challenging existing efficiency records. At the same time, there was another growing realization in the field: energy-level positioning of OPV components does, in principle, provide a theoretical maximum device efficiency; however, it is not actually a great predictor of efficiency. A 2015 review of 150 bulk heterojunction devices demonstrated what many in the field had already known or suspected for several years—that, in reality, measured energy levels show little correlation with power conversion efficiencies (PCEs) in published device work [52]. So, it became evident that microstructural and morphological controls in the active-­ layer components were more serious practical obstacles than once thought. Furthermore, if the SERIIUS team’s research was going to remain relevant, the overarching strategy would need to be refocused. A face-to-face meeting was held in New Orleans in the summer of 2015, at about the half-way point of the project. While many other SERIIUS projects were celebrating their successes at this point, the OPV effort was largely reevaluating. As the state of the OPV field was assessed, we identified a rather substantial hole in the focus of the vast majority of OPV research—one that could be uniquely addressed by the strengths of the team: while many in the field were chasing higher and higher efficiencies, little attention was being devoted to the rather critical issue of l­ ong-­term stability of the materials under operating conditions. In fact, certain additives (such as diiodooctane, or DIO) were now being employed routinely to improve device efficiencies, but the fact that their use would then dramatically decrease device stability was often “swept under the rug” [53]. Very few studies provided any kind of guidance for designing OPV absorbers with improved intrinsic stability [54], despite the desperate need for such knowledge that could dramatically improve the industrial viability of the technology. With the freedom and permission from the SERIIUS directors and the program sponsors, the co-PIs rewrote the milestones to focus on developing protocols for improving active-layer stability—thereby initiating a major pivot that would ultimately make or break the project. 2.1.2.2  Results New materials would still be designed with energy-level alignment in mind using NREL’s computational resources. However, the team would employ additional design criteria to target materials with improved oxidative stability, i.e., fluorinated materials, including both fluorinated donors (polymers and small molecules) and

2  Sustainable Photovoltaics

41

acceptors (fluorinated fullerenes). Preliminary results at NREL indicated that in some cases fluorinated materials were dramatically more stable than nonfluorinated analogues; however, generally speaking, as the fluorine content was increased in these materials, dramatic phase separation of the donor and acceptor components would typically ensue. Work at IACS involving density-of-states band mapping and scanning tunneling microscopy (STM) imaging—to investigate the morphology and energetic structure of OPV devices—would now greatly assist the team in their effort to develop protocols for improving the compatibility of different fluorinated components. A new automated photobleaching station at NREL would be employed to evaluate photodegradation of active-layer blends. Close collaboration between IISc, NREL, and IACS would now be critical to the success of the effort as a whole. Additionally, Colorado State University (CSU) was recruited as a member of SERIIUS for its expertise in perfluoroalkylfullerenes, which would become a key component of the new stability effort. The key accomplishments in this effort are best represented by the four joint papers that resulted from the collaboration. The first joint manuscript published in ACS Energy Lett. was a collaboration between IACS and NREL [55]. A powerful new combination of techniques (STM and time-resolved microwave conductivity) was employed to understand how OPV active-layer morphology influenced device performance metrics. Ultimately, this combination of techniques will aid in developing OPV morphologies with longer-lived charge carriers that will, in turn, be more amenable to thick active layers and large-scale processing. Charge-carrier dynamics and device metrics of high-performance PCE10 Polymer:PC70BM fullerene blends were measured at NREL, and the imaging of the blends and their density of states were measured at IISc (Fig. 2.6). Performance metrics were correlated with morphology in this manuscript, and certain characteristics of morphologies can now be targeted to enhance the life of charge carriers, which, in principle, will allow the use of thicker active layers more amenable to roll-to-roll processing.

Fig. 2.6  STM dI/dV images of PCE10 polymer:PC70BM fullerene blends. DIO additive significantly reduces domain size. Green and red regions correspond to PCE10-rich and PC70BM-rich domains, respectively. Reprinted with permission from [55]. Copyright ©2017 American Chemical Society

42

D. Ginley et al.

C60(CF3)2

Fig. 2.7  (Left) Polymer and perfluoroalkylfullerene employed in this work. (Center) More than 575 spectra of unencapsulated P1:C60(CF3)2 auto-recorded over 96 h during continuous illumination. (Right) Photobleaching kinetics reveal how C60(CF3)2 stabilizes the blend relative to PC70BM. Reprinted with permission from [56]. Copyright ©2018 Royal Society of Chemistry

A second joint manuscript was published in J. Mater. Chem. A by IISc, NREL, and CSU [56]. In this work, a perfluoroalkylfullerene was employed to stabilize the photobleaching rate of two organic donor molecules by a factor of 15 over blends with a traditional fullerene in unencapsulated thin films (Fig. 2.7). The study provided guidance for designing OPV active-layer components with improved intrinsic stability, which will ultimately help improve the industrial viability of the technology. The photobleaching dynamics of two organic donors studied under white-light illumination in air with blends of PC70BM and C60(CF3)2 were recorded at NREL. Perfluoroalkylfullerenes were developed and supplied by CSU. An IISc student correlated changes in bleaching rates during the experiment with changes in morphology using photoluminescence quenching measurements. Initial design rules (in terms of fullerene electron affinity and miscibility of the donor–acceptor components) were developed for successfully using C60(CF3)2 to stabilize OPV donor materials. A third joint manuscript has been submitted that was coauthored by IISc, NREL, and CSU [57]. This work expanded upon the range of perfluoroalkylfullerenes. A series of strategically fluorinated OPV donor polymers were designed to improve miscibility with perfluoroalkylfullerenes that have relatively large electron affinities and that can stabilize the donors toward photooxidation. After probing their miscibility with photoluminescence quenching measurements, the photobleaching dynamics for 15 donor–acceptor combinations were investigated. One particular fluorinated donor–acceptor blend required a remarkable 150-fold increase in the dosing of photons to bleach to 80% of its initial optical density than an analogous blend of PC60BM and nonfluorinated donor polymer. Finally, a fourth joint manuscript is in preparation, coauthored by IISc, NREL, IACS, and CSU [58]. This work represents the culmination of the project and highlights the collaboration between key members of the partnership. High-performance fluorinated OPV polymers were strategically combined with perfluoroalkylfullerenes using knowledge generated from the aforementioned studies, and the morphology was studied using STM imaging techniques. The work provides excellent

2  Sustainable Photovoltaics

43

guidelines for appropriate fluorination strategies that can be applied to both the donor and acceptor to improve morphology as well as stability, and it further suggests that this approach is a viable route toward a new paradigm of intrinsically photo- and phase-stable OPV active layers. Other notable outcomes include the strengthening of local partnerships (e.g., NREL/CSU and NREL/Sigma-Aldrich). Sigma-Aldrich sponsored one of the visiting graduate students from IISc who was hosted by NREL in an effort to help identify key fluorinated organic components that would both improve the photostability of OPV active-layer blends and that could potentially be commercialized. Beyond SERIIUS, this partnership lived on in a collaborative effort between NREL and Sigma-Aldrich for a DOE Advanced Manufacturing Office funding opportunity concerning the development of new ionic liquids. Knowledge developed concerning fluorinated organic components will also propagate into other areas of DOE Office of Energy Efficiency and Renewable Energy funding. 2.1.2.3  Assessment The science aside, two main issues proved critical to the success of this collaborative OPV effort. The first key to success was the freedom provided by the program managers and directors to pivot the focus of the project from pursuing high-­efficiency devices to highly stable materials. Writing 5-year milestones in a fast-­moving field is challenging at best and often not even practical, and this certainly proved true for OPV. But when a solid justification was provided by the co-PIs for tuning the milestones and pivoting the project, a go-ahead was given almost immediately. Had it been delayed even 6 months, the effort would not have been as ­successful—with three of the four final joint publications that ultimately defined the success of the project coming from research conducted in the final months of the program. The second key to the success of the OPV effort was the exchange of graduate students, particularly between India and the United States (Fig.  2.8). The extra Fig. 2.8  Wade Braunecker (Co-PI, NREL) and Vinila Nellissery Viswanathan (visiting graduate student, IISc) working together at NREL. (NREL, unpublished)

44

D. Ginley et al.

hands in the lab at NREL from a local university (CSU) leveraged the NREL labor force and significantly benefited the students. Sharing a graduate student who physically spent several months at a visiting institution proved to be a highly effective means to keep both home and host institutions on task and collaborating—much more so than monthly conference calls and yearly face-to-face visits. Future Work/Long-Term Developments: In general, the field of OPV was advanced by this collaborative work, particularly on the front of developing design rules for improving the intrinsic stability of active-layer components and demonstrations of scalability. To some extent, all of the partners will be able to build from this success in their own continued work. The success of the effort in OPV research may stimulate a re-evaluation of the potential of OPV and of OPV components in other scalable solution-based technologies such as perovskites. For example, the knowledge developed concerning fluorinated organic components will also live on in other areas of EERE funding. For example, Braunecker (NREL) recently incorporated fluorinated organic components into covalent organic frameworks to improve their potential for gas storage and separation applications in an EERE Fuel Cell Technologies Office-funded effort [59]. This concept is also being employed in laboratory-funded work developing these framework materials for catalysis applications with NREL’s National Bioenergy Center. Thus, the implications of this research will have far-reaching ripple effects into several other areas of renewable energy research beyond simply solar.

2.1.3  T  oward Large-Area Ambient-Processable Perovskite Solar Cells 2.1.3.1  Motivation The goal of the Perovskite-Based Solar Cells (PV-3) group was to deliver Earth-­ abundant, low-cost, and reliable thin-film photovoltaics. The motivation for this goal is obvious: an energy source must be cheap, available, and efficient to be competitive. This was especially true of photovoltaics in 2011, when the high price of silicon wafers was an issue. In the Indian scenario, the cost was to some extent even more important because the purchasing power of the average Indian customer is low. Technology and infrastructure to manufacture silicon wafers and solar cells indigenously in India was poor to nonexistent. India also had low power penetration and poor transmission infrastructure. Therefore, it was reasoned that low-cost, efficient, thin-film PV could potentially reduce $/W, provide an indigenous non-silicon PV technology, and allow a distributed power infrastructure that could revolutionize rural India. When SERIIUS was originally launched, the consensus was that dye-sensitized solar cells (DSSCs) represented the best way forward. A critical mass of researchers and capabilities in both the United States and India worked on DSSCs,

2  Sustainable Photovoltaics

45

and the technical capability of these researchers was complementary. The Indian side of SERIIUS focused primarily on material development and fundamental scientific measurements, whereas the U.S. side focused on device development and scale-up. The research direction significantly changed with the rapid development of perovskite solar cells, which showed far higher efficiency than DSSCs. Despite being one of the largest collaborations in SERIIUS, PV-3 was agile enough to realize and adapt to the shifting landscape. In 2015, the group petitioned to drop the DSSC deliverables and proposed new ones based on perovskite solar cells. The goal of the project was to demonstrate a 14%-efficient perovskite cell (>20  cm2) using an alternative deposition method with T80 greater than 2500  h (where T80 is the time of operation until the device performance, i.e., efficiency, declines to 80% of its initial value). At the time, the state-of-the-art perovskite efficiency was >20% on a small area; however, large-area deposition with stability was an open question. 2.1.3.2  What Was Done 2.1.3.2.1  The Chosen Science Approach The initial goal was to develop a 12%-efficient dye-sensitized solar cell, to be achieved by using (1) nanostructured donors, (2) Pt-free counter electrodes with graphene oxide, and (3) novel Bodipy dyes, and by (4) better understanding the interface charge process. By early 2014, the two-year goal of the project was met. Although scientifically interesting, the goal of 12%-efficient DSSC was simply not challenging enough. Several groups in SERIIUS were already getting higher efficiency in a perovskite architecture. Sometime in 2015, focus shifted from the traditional DSSC architecture to perovskites solar cells. Two main challenges were targeted in perovskite PV: stability and scalability. It was known that organic/inorganic hybrid perovskite is very sensitive to oxygen and water. One way to improve the stability was to encapsulate perovskite solar cells using encapsulants such as atomic layer deposited (ALD) oxides, graphene/polymer composite, and flexible glass. The latter two approaches were synergistic with other SERIIUS tasks: graphene/polymer composite was being developed in the Organic PV Materials and Devices (PV-2) work for organic solar cells, and use of flexible 100-μm-thick Corning® Willow® Glass was being pursued as a Core PV project. Scalability is a broad problem that includes challenges in run-to-run and device-­ to-­device repeatability, ambient processing, and large-area device fabrication. These challenges were tackled by developing: (1) novel vapor annealing techniques to ensure a uniform and repeatable perovskite film, (2) additives, (3) novel deposition methods such as electrodeposition, and (4) roll-to-roll (R2R) device fabrication.

46

D. Ginley et al.

2.1.3.2.2  Key Results Significant progress was made in developing novel deposition processes and scale­up of perovskite solar cells. The goal of 14% efficiency with alternative deposition methodologies was not only met but was exceeded. The group at the National Renewable Energy Laboratory, for the first time in the world, demonstrated a complete R2R fabrication process of perovskite solar cells on flexible Corning® Willow® Glass with efficiencies of 18–19% [60, 61]. The breakthrough was achieved by solving several technological challenges, such as formulating stable ink using ball milling, optimizing perovskite stoichiometry, and exploring mixed-cation composition (Fig. 2.9). Washington University in St. Louis (WUStL) developed a novel electrospray deposition technique to demonstrate 12%-efficient solar cells in ambient conditions (no glove box) (Fig. 2.10) [62]. The key mechanism behind the improved stability is precisely controlling the reaction between the two precursors by gradually supplying MAI in nanoparticle form onto the PbI2 layer. The Indian Institute of Technology Bombay developed novel inorganic transport layers that can replace the traditional organic transport layers. Using a novel N2-plasma SnO2 electron-­transport layer (ETL) that can be deposited at room temperature, researchers demonstrated devices with 20% efficiency on glass and 18.1% efficiency on PET substrates (Fig. 2.11) [63]. The Indian Institute of Science focused on developing a post-deposition vapor anneal process to optimize the perovskite morphology and reduce defects at the ETL/perovskite interface [64, 65]. The anneal allows perovskite layers with better morphology—and more importantly, better repeatability. The process leads to perovskite devices with an average efficiency of 17% with a standard deviation of just 2%. The consortia demonstrated several competing approaches to achieve devices with stability of >2500 h. The electrodeposited spray-coating technique pioneered by WUStL yields smoother perovskite films that are stable, enabling ~12%-efficient devices with T80 > 4000 h [62]. Using hydrophobic TEOS as an in-situ protective layer and a P3HT transport layer, WUStL also demonstrated 12%-efficient perovskite with T80 of 1200 h and an implied T80 of 2500 h (Fig. 2.12) [66]. IISc leveraged the excellent encapsulation properties of Corning® Willow® Glass to demonstrate flexible 12%-efficient perovskite devices with T80 of 200 h and T60 of 2000 h [67]. IITB demonstrated novel ALD encapsulants that—amazingly—protect perovskite layers even when immersed in water. Such “deposited” encapsulants were possible due to advances in low-temperature ALD and precursors. Notable progress beyond the stated deliverables was also achieved in areas of perovskite PV technology. Several successful efforts were made to replace the organic transport layers of the conventional perovskite structure with inorganic alternatives, e.g., Cu2O as the hole-transport layer (HTL) by the Indian Association for the Cultivation of Science [68], NiO as the HTL by IISc [69], ALD TiO2 by IISc, and SnO2 as the ETL by IITB [63]. Such approaches not only increase the stability of the devices but also remove the most expensive component of the perovskite device stack. Alternative perovskite absorbers, such as FAPbBr3 [70], MAPbI3/PbS bilayer [71], MASnI3 [72], and Sb-substituted perovskite, were also explored. The discovery of FAPbBr3 led to a joint Indo-U.S. patent owned by IITB and NREL.

-24

-20

-16

-12

-8

-4

0

4

Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Voltage (V)

All-blade coated 2 Jsc= 22.86 mA/cm Voc= 1.124 V FF=76.17 % PCE= 19.6 %

MABr, PbI2, PbBr2, FAI, CsI

Intensity (a.u.) 11.0

11.5 2θ (°)

Day 0 Day 3 Day 5 Day 6 Day 10 Day 12 Day 26 Day 30

12.0

Fig. 2.9 (a) Flexible perovskite solar cells, (b) X-ray diffraction showing limited shelf life for perovskite inks, (c) ball milling of perovskite salts results in a stable precursor for perovskites, (d) large-area fully blade-coated perovskite solar cells and slot-die-coated perovskite films. Reprinted with permission from [60, 61]. Copyright ©2017 American Chemical Society

d

c.

-2

b.

Current Density (mA/cm )

a.

2  Sustainable Photovoltaics 47

48

D. Ginley et al.

Fig. 2.10 (a) Schematic of electrospray deposition of MAI precursor on the PbI2/mp-TiO2/dense-­ TiO2/FTO substrate. (b) Time evolution of the efficiency of the perovskite solar cells fabricated using electrospray at various conditions. Reprinted with permission from [62]. Copyright ©2017 John Wiley and Sons

Fig. 2.11  (left) Light current density-voltage (JV) characteristics of highly efficient (η~20.3%) triple-cation perovskite devices using modified low-temperature SnO2 layer as ETL. (right) Record-efficient flexible perovskite devices (ITO-PET) obtained using low-temperature SnO2 ETL. Reprinted with permission from [63]. Copyright ©2018 American Chemical Society.

Fig. 2.12 (a) Long-term stability comparison among the TEOS-doped best device, undoped best device, and the spiro-MeOTAD reference device [66]. (b) Aging studies of perovskite device on Willow Glass, encapsulated with Willow Glass using epoxy glue [67]. (Unpublished)

2  Sustainable Photovoltaics

49

2.1.3.2.3  Comparison to the Initial Project Goals The goal of the project was to demonstrate a 14%-efficient perovskite cell (>20 cm2) using an alternative deposition method with T80 greater than 2500 hours. Although the efficiency and stability goals were exceeded, 20-cm2 devices were not demonstrated. Perovskite and transport layers were successfully deposited over areas greater than 20 cm2 (the R2R process generates films >500,000 cm2, with an active-­ area efficiency >14%). However, series-resistance losses in the transparent conductive oxide limited the efficiency of large-area devices to only 8% [73]. Large-area devices generate high currents that can only be handled by mini-modules having patterned electrodes. Due to the lack of a laser scriber, patterned mini-modules could not be fabricated. However, this was a logistical not a technical bottleneck. Laser patterning is a well-known technology that can easily be incorporated with perovskite to showcase >14%-efficient 20-cm2 devices. 2.1.3.2.4  Key Accomplishments in the Project Some key research accomplishments were the following: 1. Demonstrated a 14%-efficient R2R-printed perovskite solar cell, the first such demonstration in the world. 2. Demonstrated a 14%-efficient perovskite solar cell fabricated in ambient condition (no glove box) using electrodeposition. 3. Demonstrated that 100-μm-thick Corning® Willow® Glass can be used as both a flexible substrate and an encapsulate for perovskite solar cells. Flexible solar cells were demonstrated with an efficiency of 12% and T80 of 200 h and T60 of 2500 h. 4. Demonstrated 20%-efficient perovskite solar cells using SnO2 as a transport layer that are stable and resistant to repeated bending. 5. Demonstrated 18%-efficient perovskite solar cells at both NREL and IITB. 6. Demonstrated Pb-free and reduced-Pb perovskite absorbers such as MASnX3 and MA(Sb)PbX3. 7. Demonstrated a wide-bandgap FAPbBr3 absorber with enhanced stability. 2.1.3.2.5  Other Outcomes The effort led to synergies and partnerships with several different areas. Perhaps the most productive was the close interaction with the team at Corning Research & Development Corporation (CRDC). Perovskite solar cells were demonstrated on Willow Glass for the first time at small scale and at large scale using R2R fabrication. Enabling this required several process optimizations, e.g., large-scale deposition of ITO on Willow Glass rolls, which required interaction with other tasks within SERIIUS and beyond. The collaboration showed that Corning® Willow® Glass is a promising substrate and encapsulant for perovskite solar cells. The partnership with CRDC continues beyond SERIIUS.

50

D. Ginley et al.

The project also enabled very close interaction between NREL and IISc-­ IITB. Obvious outcomes were student and staff exchanges that led to joint papers, patents, and continued interaction. More interesting was the transfer of critical technical knowhow. NREL’s experience and technical support was crucial in defining the hardware for a R2R setup to be installed in India. The proposal for this work is currently being considered by funding agencies in India. 2.1.3.3  Assessment 2.1.3.3.1  Key Takeaways The project proved that high-throughput fabrication of high-efficiency perovskite is feasible. Furthermore, the project showed that perovskite can be encapsulated to nearly eliminate the extrinsic stability issues. However, significant challenges still exist. We showed that even with near-ideal encapsulation, stability is a concern due to intrinsic effects such as ion migration. The project attempted to provide solutions to this issue with integration of inorganic transport layers and novel absorbers, such as FAPbBr3, mixed-cation compositions, and Pb-free/Pb-reduced perovskites. Beyond the technical issues, we also learned how to administer a large international collaboration. Even in SERIIUS, PV-3 was one of the larger groups. The “software” to manage resources, priorities, and people was not trivial and had to be learned. It took effort and time to develop relationships to the point where ideas and unpublished results could be shared freely. Transitioning from DSSC to perovskite proved to be another important milestone in project management. Large collaborations tend to be very rigid. In contrast, SERIIUS PV-3 was remarkably agile. The SERIIUS PV-3 groups provide a case study on how to implement “sunset” and “sunrise” clauses—where tasks with poor results are replaced with more promising ones—with continuing consultation among the various shareholders (investigator, project leads, reviewers, and two different funding agencies). SERIIUS was able to pivot from DSSC to perovskite midway through the project. This transition occurred over a period of just 6 months, without requiring additional funding, without affecting students’ graduation timelines, and within the project timeline. The pivots did not just end there. Close collaboration revealed new avenues that were not even part of the deliverables, e.g., the use of Willow Glass as a flexible substrate. Such a course correction allowed SERIIUS, a 5-year project, to remain relevant in a field that changes continuously. 2.1.3.3.2  Importance of International Collaboration International collaboration was critical in all stages of the project. Given the integrated nature of the deliverables, the exchange of ideas and technical knowhow was mandatory. A result of the close international collaboration was that the knowhow to fabricate efficient perovskite solar cells was transferred and assimilated by so many research groups in just a few months. Collaboration was also critical in devel-

2  Sustainable Photovoltaics

51

oping perovskite solar cells on Willow Glass. The CRDC team organized several face-to-face training and hands-on demonstrations to educate the partners on using and handling Willow Glass. Such interaction would have been impossible without the existence of SERIIUS. 2.1.3.3.3  Engagement of the Joint Work Force The primary driver for engagement was regular meetings, both online and face-to-­ face. The importance of the latter cannot be overstated. Travel in long-distance collaborations is often a bottleneck because that activity is always underfunded. But SERIIUS was different. Generous travel funding and a strict reporting schedule— with quarterly reports and biyearly reviews, and an annual get-together at the IEEE Photovoltaic Specialists Conference—meant that the SERIIUS team met several times a year. Such constant engagement was the only way to achieve the milestones and create camaraderie that lasts beyond the project tenure. 2.1.3.3.4  Near-Term Future Work In the near term, the engagement between NREL-IISc and NREL-IITB is expected to continue. NREL is helping the IISc-IITB team define the hardware for a R2R setup that can be installed in India. Student and staff exchanges are also being discussed. 2.1.3.3.5  Long-Term Anticipated Developments from the Project Demonstration of R2R perovskite manufacturing is a significant achievement that provides a path toward commercialization of perovskite solar cells. SERIIUS has provided a platform where the best researchers in United States and India could work together to deliver scientific advances. In the long term, we hope that this collaboration will translate into actual technology that can be commercialized.

2.2  Roll-to-Roll Processing 2.2.1  F  lexible Glass Substrates for Solution-Based Photovoltaics 2.2.1.1  Motivation Significant progress has recently been made toward achieving photovoltaic technologies fabricated on flexible glass substrates [74–81]. Similar to PV modules currently fabricated on rigid glass substrates, these early-stage flexible glass

52

D. Ginley et al.

demonstrations have mainly been based on processing discrete cells and panels, essentially one at a time, while using predominantly vacuum-based processing. A significant disruption to current vacuum-based processing is the emergence of solution-­based atmospheric processing, which is compatible with high-throughput manufacturing methods such as roll-to-roll processes. This approach requires web-­ based substrates optimized for the thermal, chemical, and mechanical requirements of these high-throughput manufacturing methods as well as meeting requirements for device performance and reliability. This work describes the evaluation of flexible glass substrates and superstrates for high-throughput, solution-based fabrication of PV devices. Examples of these devices include solution-based CIGS, CZTS, organic, and perovskite PV. In addition to enabling PV fabrication, flexible glass substrates can also be used for depositing high-quality reflectors for concentrated solar power (CSP) systems, mitigating potential-induced degradation (PID) in silicon modules [82], and fabricating electrodynamic soiling mitigation [83]. The focus of the following description, however, is specifically on PV applications. 2.2.1.2  Flexible Glass In general, flexible glass substrates enable high-performance PV modules for thin, lightweight, and curved applications. In addition to contributing to the final module form factor, its unique attributes also enable high-throughput manufacturing methods such as R2R processing. The overall properties of flexible glass substrates for electronic applications have been well documented [84, 85], so the focus of this short summary is on attributes most applicable to PV. An example of flexible glass optimized for electronic device applications is Corning® Willow® Glass, which is manufactured in a continuous process at a width of >1 m, thickness of ≤200 μm, and lengths of 300 m wound onto spools. The flexible glass can be cut into discrete substrates for sheet handling—either with or without a processing carrier—or it can be slit to appropriate widths for R2R device manufacturing. Figure 2.13 shows a manufacturing-scale spool of flexible Willow Glass compatible with roller conveyance and R2R device manufacturing. The 100-μm-thick Willow Glass substrates provided in this study targeted improved device designs, materials, performance, lifetime, and processes. These improvements are based on the glass’ unique attributes of high-quality surface, optical transmission, hermeticity, high thermal capability, dimensional stability, and chemical compatibility [84, 85]. Table 2.1 highlights the range of material properties available by optimizing the alkali-free borosilicate composition and the forming process to meet specific application requirements. In terms of flexibility, the stiffness (flexural rigidity) of substrates depends linearly on thickness and on the cube of Young’s Modulus. Figure 2.14 compares the relative stiffness of glass, stainless steel, and polyimide as a function of thickness. The focus of this study has been on a 100-μm glass thickness to enable thin and lightweight devices fabricated on substrates compatible with conveyance through roller systems.

Fig. 2.13  Corning® Willow® Glass compatible with R2R processing manufactured at a width of >1 m, thickness of ≤200 μm, and lengths of 300 m. (Image provided courtesy of Corning Incorporated) Table 2.1  Corning® Willow® Glass properties Bulk Properties Density Coefficient of thermal expansion (0–300 °C) Young’s Modulus Poisson ratio Strain point Dielectric constant (k = E0/E) Surface roughness Water vapor transmission rate

Metric Unit g/cc ppm/°C GPa – °C – Ra (nm) g/m2/day

Nominal Values 2.38–2.6 3.2–3.5 73–83 0.23–0.25 669–758 5.27–6.18 5 >6 >8 >18

for initial encapsulation process trials that included lamination, thin-film deposition, and frit sealing. Results based on discrete flexible glass substrates are described in much more detail in other sections, but Table 2.2 highlights some key demonstrations. Based on this learning, activities for these solution-based devices proceeded next to R2R processing trials. 2.2.1.4  R2R Processing The R2R processing evaluations performed in this study were based on 330-mm web widths and 20-m web lengths for use in research-scale processing equipment. This work enabled critical learnings on PV cell materials and continuous fabrication processes. It also enabled planning for the next steps of scaling the substrate width, length, and module fabrication. The specific R2R processing performed in these trials included vacuum deposition of conductor layers (ITO, IZO, Mo) and solution-­ based coatings of the PV, hole-transport, and electron-transport layers. The vacuum deposition was performed by sputtering in collaboration with Binghamton University, State University of New York; solution coating was performed by either slot-die or microgravure coating techniques in collaboration with NREL. The patterned top contacts in these demonstrations were vacuum deposited after the flexible glass was singulated into discrete coupons. Beyond the scope of these trials, the capability for R2R processing a flexible glass web has been demonstrated for a wide variety of processes that include: vacuum deposition, lamination, laser patterning, printing, photolithography, wet etching, and microreplication. Results based on R2R PV fabrication on flexible glass web are described in much more detail in other sections. As examples, R2R processing was pursued for solution-­based organic, CIGS, and perovskite devices. Figure  2.16 illustrates the flexible glass web being conveyed through a laboratory-scale R2R solution coating system. Figure  2.17 highlights solution-based coatings applied to the flexible glass web. 2.2.1.5  Summary Flexible glass substrates offer several advantages for thin, lightweight, and curved PV device design, performance, and fabrication. These advantages include surface quality, optical properties, hermiticity, and thermal and chemical compatibility. In addition, a flexible glass web is compatible with high-throughput manufacturing

56

D. Ginley et al.

Fig. 2.16  Conveyance of flexible glass web through laboratory-scale R2R solution coating system. (NREL, unpublished)

such as R2R processes that could significantly enable scaling of PV fabrication. Going forward, potential focus areas include module fabrication and performance, process scaling, and packaging and device reliability.

2.3  Reliability The durability and reliability of photovoltaic modules and systems are of great importance, given that these systems are expected to work for 25 years in the field, while degrading by no more than 20% in power. The ability to last for so long depends on several factors, including technology type, climatic conditions, type of mounting, and quality of production. The binational scope of activities in SERIIUS provided a unique opportunity to study the comparative reliability of PV in two countries, with lessons for future widespread deployment in India, the United States, and indeed, across the globe. This joint work—one of the most exhaustive field studies of reliability undertaken—is an excellent example of how the two countries, working together, produced detailed results that could not have been done by one country alone. The SERIIUS study looked at several aspects of PV reliability: (1) the climatic dependence of degradation rates of PV modules, with special emphasis on the effect of hot and humid climates; (2) implications for standards and certifying procedures for areas with harsh climates; (3) performance of different PV systems (besides modules); and (4) the effect of soiling on performance at different locations. The methodology used for most of the work was a combination of field and laboratory work, supported by some modeling. For example, while studying the durability of PV modules, the team undertook extensive field surveys in India and the United States, covering several thousand modules in different climatic zones. In addition,

2  Sustainable Photovoltaics

57

Fig. 2.17 (a) Coating of PEDOT:PSS; (b–d) Coating of perovskite layers on IZO-coated flexible glass. (NREL, unpublished)

laboratory experiments were conducted to see how different materials and modules behave under accelerated test conditions, and these results, together with modeling, were used to better understand the field data.

2.3.1  Climatic Dependence of Degradation Rates The field surveys of PV module reliability were performed in both the United States and India, and they encompassed different climatic zones in both countries. In India, two separate “All-India Surveys”’ were undertaken in 2014 [88, 89] and 2016 [90,

58

D. Ginley et al.

91] by a team from the Indian Institute of Technology Bombay (IITB) and National Institute of Solar Energy (NISE). (The IITB work was also partly supported by the National Center for Photovoltaic Research and Education, funded by the Ministry of New and Renewable Energy.) Each of these surveys covered about 1000 modules of different technologies and covered six climatic zones of India. In the United States, a team from Arizona State University (ASU) undertook several surveys [92– 94] and recorded the degradation of more than 50,000 modules covering four ­climatic zones. In addition, the Florida Solar Energy Center (FSEC) team assessed performance of modules in the humid subtropical climate of Florida [95]. Most of the modules used silicon technology, but several other technologies were also represented. Figure 2.18a,b show the India and U.S. teams, respectively, in the field. Figure 2.19a,b show, respectively, some data of the climatic zone variation in India and the United States. The annual degradation rate in power is defined as the average percentage loss of power per year: Degradation Rate =

Pinitial − Pmeasured % / year Pinitial × ( Age of Module )

(2.1)

The international benchmark value of the average degradation rate is about 0.8%/ year, corresponding to a loss of 20% of the initial power over a 25-year period. It can be seen that whereas some of the modules do indeed satisfy this requirement, the degradation rates of many modules are significantly higher. A joint paper [96] that compared the extensive set of data obtained by us in the two countries clearly showed (see Table 2.3) that, for both countries, modules in the “hot” climatic zones performed worse. A detailed analysis yielded two major reasons for poor performance in the hot climates: a higher discoloration of the encapsulant (ethylene vinyl acetate, or EVA), which reduces short-circuit current Isc; and increased solder-bond fatigue and corrosion, which results in a loss of fill factor FF; both of these factors reduce power. It is well known that encapsulant discoloration is accelerated at high temperatures, and because many hot tropical climates are also humid, water ingress and corrosion are also enhanced, along with solder-bond fatigue due to thermal cycling (daily, seasonal, and cloud cycles). In addition, two other points were also observed in hot climates that may lead to enhanced degradation: increased incidence of delamination, and the existence of more and hotter “hot spots.” Our study points to the need for special tests for certification of modules to be deployed in “hot” climates. When looking at the climatic variations, an important question that arises is: which degradation or failure mode is the most relevant for a particular climate? SERIIUS researchers have used the “Risk Priority Number” (RPN) approach to classify the most vulnerable mode [97]. Figure  2.20 shows a typical result for a specific module design, indicating that in the cold-dry region of the United States, delamination is the most likely cause of performance degradation and bypass diode failure is the most likely safety risk.

2  Sustainable Photovoltaics

59

Fig. 2.18  Field survey teams in (a) India and (b) United States. (Arizona State University, unpublished)

Another important finding from the survey, especially in India, was that many modules, especially those on rooftops, showed enhanced presence of cracks and microcracks as measured by onsite electroluminescence (EL), shown in Fig. 2.21a. This survey represented one of the most extensive EL studies performed on fielded PV modules [98]. These cracks can cause a reduction in FF and power, and this correlation between number of cracks and power degradation is shown in Fig. 2.21b. The procedures for installing modules on rooftops are believed to be inadequate, and mishandling causes cracks to develop. A comparison of the degradation rates

60

D. Ginley et al.

Fig. 2.19  Typical results of power degradation rates (%/year) for different climatic zones in (a) India and (b) the United States. (Courtesy of Arizona State University)

2  Sustainable Photovoltaics

61

Table 2.3  Comparison of degradation rates in “Hot” and “Non-hot” climates in India and the United States Country United States India

Hot zones Hot & Dry [%/y] 1.20 1.93

Hot & Humid [%/y] 1.06 1.38

Non-hot zones Temperate [%/y] Cold & Dry [%/y] 0.15 0.72 1.27 0.80

Fig. 2.20  Module-level RPN and frequency for various defects in cold-dry climate for frameless modules. (Courtesy of Arizona State University)

for rooftop rack-mounted modules and ground rack-mounted modules is shown in Fig. 2.21c, indicating that rooftop modules degrade faster on average. This finding has important implications for India’s target for 40 GW to be deployed on rooftops by 2022. The observation of widespread encapsulation discoloration and generation of cracks led us to perform laboratory studies to better understand these phenomena. Special test setups were developed: (1) a chamber to study accelerated discoloration of different types of encapsulant by going to higher temperatures and ultraviolet exposures [99], and (2) a dynamic mechanical-load (DML) system to study crack generation and propagation; these are shown in Fig. 2.22a,b, respectively. The first of these was developed at ASU, and then replicated (with modifications) at IITB; the second was originally developed at NREL with the help of a SERIIUS exchange student, and later expanded at IITB.

2.3.2  Need for New Certification Protocols for Hot Climates During the course of the SERIIUS work on reliability, it became clear that hot climates present a challenging environment for PV modules and that degradation rates of standard modules may be adversely affected [89]. To address this problem,

62

D. Ginley et al.

Fig. 2.21 (a) Typical EL image of a module showing cracks; (b) Correlation of power loss with number of cracks; (c) Comparison of degradation rates for roof- and ground-mounted modules. (Courtesy of Arizona State University)

SERIIUS coorganized a workshop in Mumbai, India, in October 2015, where reliability experts met from India and the United States, together with researchers from the United Arab Emirates, Saudi Arabia, Qatar, and Japan (see Fig.  2.23). The ­consensus of this workshop was that more stringent protocols were needed to certify panels to be used in hot climates, whether in India, the Southwest United States, the

2  Sustainable Photovoltaics

63

Fig. 2.22 Systems developed for accelerated testing: (a) Chamber for accelerated testing of encapsulant materials at ASU; (b) Dynamic Mechanical Loading system (background) with controller (foreground) for artificially generating cracks at IITB. (Courtesy of Arizona State University)

Fig. 2.23  Participants at the 2015 Workshop on “PV Module Reliability in Hot Climates.” (Unpublished)

64

D. Ginley et al.

Middle East and North Africa countries, or other tropical countries where solar PV is increasingly being deployed. For example, it was suggested that in the thermal-­ cycling as well as damp-heat tests of the IEC 61215 certification protocol, the ­temperature of testing be increased from 85 to 95 °C. Arising partly out of the work reported by SERIIUS investigators, as well as others (e.g., an NREL presentation in 2016), the IEC Technical Committee is currently working on new standards for hot climates, with the proposition that “one size does not fit all.”

2.3.3  Performance of PV Systems in the Field In addition to studying the reliability of the modules, the SERIIUS team studied the performance and reliability of specific PV systems. This included PV solar pumps, inverters for rural applications, and solar PV systems for home and commercial use. The use of PV power for solar pumps is an important application in India, where availability of electricity for irrigation purposes in remote areas is either nonexistent or unreliable. The research focused primarily on higher-efficiency water-pumping systems using sine-wave pump controllers (SPCM) with maximum power-point tracking (MPPT) and variable-frequency drive (VFD) controllers [100]. The key results obtained in PV water-pumping systems are shown in Fig. 2.24. The Indian Institute of Technology Madras (IITM) has come out with solar PV solutions using only direct-current (DC) power called Green Offices and Apartments

SPCM 20 m VFD 20 m SPCM 18 m VFD 18 m SPCM 10 m VFD 10 m

500 450 400

Water output (l)

350 300 250 200 150 100 50 0

0

25

0

50

0

75

00

10

50 000 50 500 17 12 2 1 Input Power (W)

50

22

00

25

Fig. 2.24  Water-pumping system at different head for SPCM and VFD controllers. Reprinted with permission from [100]. Copyright ©2015 Elsevier Ltd

2  Sustainable Photovoltaics

65 Solar panel

440V threephase 11KV

D T

RBM D P U

43V DC

ST5

DRAU

Multi-storied Apartments

battery AC

Brown-out control using GPRS

RBM

Providing 100W to 400W for each flat DPU: DC Power Unit; DPAU: DC Power Augmentation Unit; STB: Solar Terminal Block; RBM: Reset, Buzzer and Metering

Fig. 2.25  Solar-powered GOA system for “Green Offices and Apartments.” (Unpublished)

Fig. 2.26  Prototype of Solar-Powered Portable Culture Incubator has a basic charge controller, 12-V and 7-Ah battery, automatic temperature controller, solid-state relay, and 10-W panel. (Unpublished)

(GOA) for commercial and urban areas, and Off-Grid Homes (OGH) for rural areas with minimal or no grid. The GOA solution shown in Fig. 2.25 is an innovative solution that provides a highly energy-efficient power distribution system, targeted for next-generation GOAs. It is a modular, scalable solution that enables expansion of the energy requirements. The GOA solution provides 48-VDC as a new power line. This line enables use of energy-efficient DC appliances such as brushless DC (BLDC) fans and light-emitting diode (LED) lights, resulting in reduced energy consumption [101]. A modified version of this is also available for smaller off-grid rural homes [102]. A final example of a solar-powered device meant specifically for rural areas is the development of a portable culture incubator (Fig.  2.26), designed and tested with the help of the Indian Council of Medical Research, India. The Solar-Powered Portable Culture Incubator will help in establishing mobile culture facilities at the district level or the sub-district level in rural areas [103].

66

D. Ginley et al.

2.3.4  Soiling Soiling of PV panels is an important concern in many locations, especially dry areas. This is certainly true in parts of India (Rajasthan) and the United States (Southwest), as well as other areas such as the Gulf countries, where solar deployments are occurring. Cleaning of soiled panels can add significantly to running costs and can also use large quantities of water, which is often scarce in these areas. The SERIIUS program included several aspects of soiling: assessing loss of performance due to soiling, analyzing the effect of soil composition from different areas on performance, the interplay of dust and wind, and strategies for mitigation. The amount of soiling depends on location and can result in loss of power (measured in %/day) due to less sunlight reaching the cells. We set up soiling stations that measured this loss by comparing the power output of a soiled panel (that is not cleaned) with that of a panel cleaned every day. Two examples of such soiling stations are shown in Fig.  2.27a,b at ASU and IITB, respectively. For example, the soiling rate at IITB, an urban area in the warm and humid climatic zone, was found Fig. 2.27  Soiling stations at (a) ASU and (b) IITB. (Unpublished)

2  Sustainable Photovoltaics

67

to be 0.4%/day. The angle of incidence [104, 105] also plays an important role in the power loss because of different amounts of reflection and scattering. The chemical composition and morphology of the dust also determines the power loss (especially the spectral dependence) and informs the cleaning strategy. The type of dust varies widely from location to location and also with the season [106]. To evaluate the effect of different types of dust in a laboratory environment, SERIIUS researchers have developed “artificial dust deposition” apparatuses [107]. Water-based and dry deposition systems, respectively, simulate the wet and dry climates, which create very different conditions for adherence of dust particles to the glass. The dry-dust deposition system is shown in Fig. 2.28. Figure 2.29 shows the quantum efficiency curves for dust collected from different parts of India [108], measured in the laboratory using the artificial deposition system. Fig. 2.28 Indoor (laboratory) artificial dust deposition system for dry dust. (Unpublished)

Fig. 2.29  Comparison of quantum efficiency (QE) curve of multicrystalline silicon solar cell with dust samples from different parts of India (viz., Jodhpur, Hanle, Gurgaon, and Agra). (Unpublished)

68

D. Ginley et al.

Fig. 2.30  Scatter between power output from the three modules and solar insolation. (Unpublished)

An outdoor experiment was set up at IISc to investigate the interplay between temperature, dust deposition, and performance of solar PV with regulated wind. The setup consisted of three identical PV modules (37 W each) at a tilt angle of 13° facing south. Modules 1 and 3 were cleaned on the first day of the month and left uncleaned thereafter, whereas Module 2 was cleaned every day. Modules 2 and 3 were artificially cooled using fans, which maintained uniform wind speed throughout the day. This setup allowed us to assess the impact of module temperature and wind speed for different insolation conditions, and the results are shown in Fig. 2.30, which indicates that Module 2 (cleaned and cooled) performed best [109]. Several strategies for mitigation of soiling were explored in SERIIUS. An innovative method was to use vertically mounted bifacial modules that do not collect dust [110]. This is shown in Fig. 2.31, where we observe that the soiling rate for the vertically mounted bifacial module was close to zero, whereas that of the conventionally mounted modules was ~ 0.43%/day. The vertical bifacial modules showed a double-humped power output curve, with peaks in the mornings and afternoons. Although the integrated power output for the vertical modules was less than that for a conventionally mounted clean module, the reverse is true under heavy soiling conditions.

2.3.5  Improved Encapsulants and Transparent Conductors An important aspect of reliability, especially in humid climates, is to develop encapsulants with very low permeability of moisture—that is, low water vapor transmission rate (WVTR). We successfully designed and synthesized polymer films that exhibit ultra-low water permeability representing an improvement by a factor of ~106 compared to the neat polymer (Fig. 2.32). Organic PV devices encapsulated using these new materials have shown lifetimes of more than 10,000 h. Significant work was also done on high-performance p-type transparent conductors (CuZnS) suitable for harsh operating conditions.

2  Sustainable Photovoltaics

69

30 Monofacial Bifacial 90 degree Bifacial 19 degree

Soiling Loss (%) based on Energy

25

05/15/17 - Rain

20

0.43%/day

15

0.43%/day

10 5

0.002%/day 6/6/2017

5/30/2017

5/23/2017

5/16/2017

5/9/2017

5/2/2017

4/25/2017

4/18/2017

4/11/2017

4/4/2017

3/28/2017

3/21/2017

0

Date (mm/dd/yy) Fig. 2.31  Soiling loss for conventionally mounted monofacial and bifacial modules, and also a vertically mounted bifacial module. Reprinted with permission from [110]. Copyright ©2019 Institute of Electrical and Electronics Engineers 10 1 WVTR (g m-2 day -1)

Fig. 2.32 Million-fold decrease in WVTR of polymer films for encapsulation. (Unpublished)

Surlyn

PE

Saran

0.1 4 orders

0.01 1E-3 1E-4

6 orders

5 orders

1E-5 1E-6 1E-7

N=5 Graphene/Surlyn Graphene/PE Graphene/Saran

2.3.6  Awards Several of the papers presented by members of the SERIIUS Reliability group received “Best Student Paper” and “Best Poster” awards at leading conferences, which include the following: • J. Oh, S. Bowden, and G. TamizhMani, “Application of Reverse Bias Recovery Technique to Address PID Issue: Incompleteness of Shunt Resistance and Quantum Efficiency Recovery,” 40th IEEE Photovoltaic Specialists Conference, Denver, USA (2014).

70

D. Ginley et al.

Fig. 2.33  Student awardees of a best poster award (L to R): Sachin Zachariah, Sonali Bhadure, Shashwata Chattopadhyay, and Rajiv Dubey. (Unpublished)

• R.  Dubey, S.  Chattopadhyay, V.  Kuthanazhi, J.  J. John, C.  S. Solanki, A. Kottantharayil, B. M. Arora, K. L. Narasimhan, J. Vasi, B. Bora, Y. K. Singh, and O. S. Sastry, “Correlation of electrical and visual degradation seen in field survey in India,” 43rd IEEE Photovoltaic Specialists Conference, Denver, USA (2014). • S. Chattopadhyay, R. Dubey, V. Kuthanazhi, J. J. John, J. Vasi, A. Kottantharayil, B.  M. Arora, K.  L. Narsimhan, C.  S. Solanki, B.  Bora, Y.  K. Singh, and O.S.  Sastry, “Effect of Hot Cells on Electrical Degradation of PV Modules,” Photovoltaic Reliability Workshop, Lakewood, CO, USA (2016). • S.  Bhaduri, S.  Chattopadhyay, R.  Dubey, S.  Zachariah, V.  Kuthanazhi, C.  S. Solanki, B.  M. Arora, K.  L. Narsimhan, A.  Kottantharayil, and J.  Vasi, “Correlating Infra-Red Thermography with Electrical Degradation of Modules Inspected in All India Survey of Photovoltaic Module Reliability 2016,” Photovoltaic Reliability Workshop, Lakewood, CO, USA (2017). (See Fig. 2.33.) • Tummala, Jaewon Oh, S. Tatapudi, and G. TamizhMani, “Degradation of Solder Bonds in Field Aged PV Modules: Correlation with Series Resistance Increase,” 44th IEEE Photovoltaic Specialists Conference, Washington DC (2017). In addition, IITM received the IEEE Spectrum “Technology in the Service of Society” award for their work (co-funded by SERIIUS) on innovative DC microgrids, led by Ashok Jhunjhunwala.

2.3.7  Conclusions, Assessment, and Future Work The work on reliability was extremely successful, with researchers from the two countries actively working together. In addition to a great amount of exchange of know-how and information, the coordinated work resulted in a good understanding

2  Sustainable Photovoltaics

71

of the important issues facing both countries. An important example of this was the realization that hot climates in both countries cause higher degradation rates; therefore, more stringent certification is necessary for such climatic conditions. The researchers met several times every year, and there were also internships of students from India to three laboratories in the United States: ASU, NREL, and FSEC. Arising out of the work done in SERIIUS, several interesting and important research projects can be undertaken in future. These include: (1) establishing new protocols for testing and certification of PV modules to be used in hot climates; (2) accelerated test models and experiments for better prediction of life of modules, and the ability to differentiate high-quality modules from mediocre-quality modules; (3) development of new materials for improved reliability; and (4) development of anti-­ soiling coatings and other soiling-mitigation strategies.

2.4  Multiscale Modeling Modeling is an important part of any scientific endeavor and this is so for the PV thrust area. One of challenges of modeling for PV is to be able to perform a “multiscale”’ or “‘atom-to-system” modeling of PV and to see how changes in the fabrication process affect device characteristics, which, in turn, affect module and system performance. This challenge was taken up by the SERIIUS team, which produced some pioneering results. Another important task of the SERIIUS PV modeling team was to connect up with the other areas of the thrust area and to provide theoretical and conceptual guidance and backing to their work. Both these aspects are described below.

2.4.1  Motivation What was the overall project goal in the context of the broader needs of India and the United States and the state of the art when the project was proposed? Although solar modules are produced globally, the two factors that determine the cost of solar energy (i.e., energy yield and the lifetime) depend on local weather conditions. Initially, U.S. solar cell development focused on space applications where cost was not an important consideration. Today, however, the initial cost of grid-connected solar farms as well as rooftop applications in both the United States and India must be low enough to compete with other sources of energy, such as oil. In addition, the Jet Propulsion Lab block-buy program in the 1970s helped structure the U.S. PV reliability program and improved design and established qualification protocols to ensure a 25-year lifetime. India is beginning to develop a similar comprehensive reliability program so that solar energy can be economically and reliably harvested regardless the climatic zone. As a testbed, India offers valuable lessons for U.S. manufactures about module design in a variety of climatic conditions and developing lower-cost alternatives for broader access. In turn, the program allowed Indian

72

D. Ginley et al.

researchers access to the U.S. PV knowledge base. An end-to-end modeling effort, connecting local performance and reliability and serving as a knowledge repository for the research undertaken, will contribute to the further development of PV manufacturing and deployment in both countries. The SERIIUS project brought together theorists, experimentalists, and technologists from multiple disciplines who could cover the entire length and timescale of the relevant problems. An end-to-end modeling framework captured the data and insights generated in various thrusts and allowed one to see how a new result from one thrust has cost and reliability implications for other thrusts. The project-specific goals as summarized in the 10-Point Plan were to develop a technology-agnostic, “end-to-end” modeling framework for assessing both commercial, as well as new, early-stage, PV concepts, which were made available through the SERIIUS Web Gateway. The development of the framework was driven by four promising technology concepts: (1) intrinsic reliability of commercial/ early-stage technology, (2) novel optics for organic solar cells, (3) perovskite-based solar cells, and (4) technology-agnostic interpretation of the cell-module efficiency gap. Expected outcomes were specific technical advances in the proposed topics for the explicit benefit of the PV community in India and the United States. More broadly, the work was to create a transformative, physics-based, end-to-end modeling framework serving as a bridge between basic PV science, applied science and engineering, and PV technology. The work had an impact on the broader Indo-­U.S. PV research community by integrating various threads of research within an integrated conceptual framework and captured simulation tools through the “nanoHUB.org” Science Gateway and the PVHub at Purdue University.

2.4.2  What Was Done 2.4.2.1  Science Approach The modeling effort supported a variety of technologies at various stages of development; therefore, we adopted multiscale modeling concept validation by experiments. This approach was essential to integrate the multiple threads of research within SERIIUS.  For example, Indian industrial partner Moser-Baer focused on mature, high-performance silicon heterojunction technology requiring sophisticated characterization, whereas others focused on other thin-film options, such as SnS (MIT), CZTS (Purdue), perovskites (IIT, IISc, Purdue), and organic PV (IIT, IISc), as well as n- and p-type transparent contacts (IISc). A variety of reliability research was undertaken: from fundamental issues of soiling and potential-induced degradation in modules installed at test sites (ASU) to in-depth degradation studies of fielded modules in solar farms (All Indian Survey, IIT). Only an end-to-end modeling framework could capture this atom-to-farm research within the SERIIUS consortium. The approach consisted of four steps: (a) in-depth analysis of the underlying physics problem; (b) corresponding experimental characterization for parameter

2  Sustainable Photovoltaics

73

extraction; (c) developing and web-enabling a software tool and associated tutorial to make the tool accessible to the SERIIUS community; and (d) integrating the tool within an end-to-end modeling framework. With this framework, one can interpret system and cost implications of improvement and/or device levels. 2.4.2.1.1  Key Results Thermodynamics of individual and tandem cells PV. As a potential performance guide for various materials and cell architectures developed within SERIIUS program, the modeling group developed a simple but powerful model to calculate the thermodynamic performance limit of solar cells [111]. They considered single-­ junction, tandem-tandem, and bifacial-tandem technologies. A number of publications and the web-enabled tool (PVLimits) were made available to all the SERIIUS members. The unique approach to thermodynamics (e.g., “Shockley-Queisser Triangle”) developed in this effort formed the basis of the book “Physics of Solar Cells: An Atom-to-Farm Perspective” (https://nanohub.org/groups/etsc). Defect calculation by Washington State at St. Louis and MIT groups. At the atomistic level, first-principle calculations by Washington University researchers were critically important for the SERIIUS researchers, especially material scientists. The first-principle calculations answered two classes of related questions: (a) midgap defect states in new materials that may shorten the carrier lifetime in new materials being considered, and (b) narrowing the choice of materials being considered as passivation layers or transparent contacts. Regarding the first class of problems, first-principle calculation helped identify the defects and their formation energy in SnS, a new material being developed at MIT. Based on the insights, the MIT group optimized the process technology to reduce the midgap defects to improve carrier lifetime [112]. Specifically, a close-spaced sublimation (CSS) furnace was developed for SnS growth at high homologous temperature (up to 450 °C vs. previous 240 °C), low metal content (only high-purity graphite and quartz parts near the hot zone), and high sulfur content during growth (H2S incorporation during growth). Resulting films from CSS showed very high minority-carrier lifetime well in excess of 1 ns. Device modeling indicated that a lifetime of 1 ns is necessary for >10%-efficient devices. In a related work, the first-principle approach also identified impurity defects in solar-grade silicon and suggested thermodynamic routes to suppress them. As a second contribution, the first-principle calculation narrowed the material options for n-type transparent conducting oxides (TCOs). In addition to being highly conductive (with high mobility) and transparent, the materials should be stable and low cost so that cell reliability is not compromised. In this effort, the group screened ~100 promising candidates by automating the formation-energy calculation to predict n−/p-type conductivity. Some results are shown in Fig. 2.34. The effort guided multiple SERIIUS groups involved in TCO development [113]. A related software called aMoBT (https://nanohub.org/tools/amobt) was posted for use by Indian and U.S. researchers.

74

D. Ginley et al.

Fig. 2.34  First-principle calculations were used to identify defect levels and defect formation energy in various materials (left). Given this information, one can calculate the transport parameters such as mobility (right). Reprinted with permission from [113]. Copyright ©2015 American Physical Society

Physics/reliability of perovskite cells. The development of perovskite solar cells as a low-cost thin-film technology excited researchers across the world, including several groups within SERIIUS.  Initially, the high performance could only be obtained by careful empirical tuning of the process parameters because the optoelectronic operation of the solar cells was not fully understood. The IIT Bombay group supported the SERIIUS effort by developing physics-based models to (a) interpret dark I-V characteristics [114], (b) estimate practical efficiency limits, (c) optimize the design space to further improve the state of the art [115], and (d) estimate the performance of large-area cells through physics-based compact models [116, 117]. In this theme, the SERIIUS researchers also addressed emerging reliability aspects of perovskite solar cells. Specifically, the issue of ion-migration-­ induced performance degradation [118] and aspects were addressed related to process-induced pinholes [119] and the subsequent efficiency degradation. Some of these results are shown in Fig. 2.35. Complementary work on light-activated photocurrent degradation and self-heating of 2D Ruddlesden-Popper perovskite solar cells were reported by the Purdue groups [120, 121]. End-to-end modeling of passivated contact silicon heterojunction cell. During the planning phase of SERIIUS, our industrial partner Moser Baer highlighted the importance of developing a foundational modeling framework for the “passivating-­ contact” solar cell technology that was becoming prominent for its world-record performance. Although passivating-contact solar cells are remarkably efficient, their underlying device physics is not yet completely understood, not in the least because they are constructed from diverse materials that may introduce electronic barriers in the current flow. To bridge this gap in understanding, the IIT and Purdue groups explored the device physics of passivating contact silicon heterojunction (SHJ) solar cells and identified the key properties of heterojunctions that affect cell efficiency, analyzed the dependence of key heterojunction properties on carrier

2  Sustainable Photovoltaics

75

Fig. 2.35  The left panel shows performance optimization of perovskite solar cells [5] through improvement in parameters such as optical generation rate (G), carrier lifetime (τ), and contact-­ layer doping (N). The right panel shows the effect of ion migration in perovskite solar cells. Here, we observe a simultaneous improvement in Voc and degradation in Jsc. The inset shows experimental trends from literature. Reprinted with permission from [118]. Copyright ©2017 American Chemical Society

transport under light and dark conditions, provided a self-consistent multiprobe approach to extract heterojunction parameters using several characterization techniques (including dark J-V, light J-V, C-V, admittance spectroscopy, and Suns-Voc), proposed design guidelines to address bottlenecks in energy production in SHJ cells, and developed a process-to-module modeling framework to establish the module’s performance limits. This is shown schematically in Fig. 2.36. Today, this SERIIUS-initiated effort defines the most sophisticated framework for understanding of these devices [122, 123]. Modeling of monofacial and bifacial solar farms. Initially, the SERIIUS end-to-­ end modeling effort was confined to cell and module modeling. The rapidly falling prices of silicon modules transformed the scientific and economic landscape, so it became important to consider the system implications (e.g., farm configuration and energy yield, storage, lifetime) in the context of India and the United States. The SERIIUS modeling group made important contributions regarding the design of silicon-based bifacial solar modules, an exciting technology for vertical integration with buildings, bridges, and more [124–126]. A vertical solar farm could be a transformative technology to address energy loss due to soiling. The group integrated the global satellite-derived insolation data from NASA with the clear-sky model from Sandia to estimate hourly insolation information. They also analyzed the effects of nonuniform illumination on the electrical energy output of the solar farm. Global optimization of a vertical bifacial solar farm demonstrates that although vertical bifacial modules produce lower energy, the uniformity of energy production and reduced cleaning cost make the technology economically viable for many locations in the world. See Fig. 2.37.

76

D. Ginley et al.

Fig. 2.36  Among the various contributions to SHJ modeling, the most significant was the development of a multiprobe approach where an array of quasi-orthogonal characterization methods are used to study the SHJ solar cell. (Unpublished)

Fig. 2.37  (top, left) A vertical bifacial solar farm provides a new system design option in regions with high albedo and soiling, as in the deserts of the Middle East. (top, right) The global energy map of the energy output of vertical solar farms, based on end-to-end modeling. (bottom, right) Integrated modeling of HIT solar cells explains the cell-to-module efficiency gap in modern solar cells. (Unpublished)

2  Sustainable Photovoltaics

77

2.4.2.1.2  Comparison to Initial Project Goals SERIIUS wanted to develop a technology-agnostic, end-to-end modeling framework for both commercial and emerging PV. Indeed, the key results listed above clearly indicate that the team has been broadly successful in this aspect. For e­ xample, the SERIIUS team reported the first-ever physics-based process-panel modeling of silicon heterojunction cells—a commercial technology. Similarly, the device physics and performance optimization were addressed for perovskite solar cells—an emerging technology. Both of these activities also gave vital support and informed the research plan for related PV projects in SERIIUS. These, along with the detailed modeling on design considerations of solar farms and power-output variability with weather, show that the SERIIUS modeling program was very ambitious and achieved results beyond the targets set by initial project goals. 2.4.2.1.3  Key Accomplishments • Thermodynamic limits for single-junction, tandem, and bifacial-tandem solar cells. The approach is also used to calculate fundamental limits of energy storage efficiency in electrochemical cells. • A first-principle model to calculate defect levels and defect formation energy in various semiconductors. • A new multiprobe characterization protocol for silicon heterojunction solar cells. • An end-to-end performance and reliability model for silicon heterojunction and perovskite solar cells. • An experimentally validated model for bifacial solar farms. 2.4.2.1.4  Other Outcomes A strong connection to U.S. national laboratories (NREL, Sandia, and Los Alamos) has been established. Also, the following software have been deployed, which are widely available to the solar PV community worldwide: • aMoBT: ab initio model for calculating mobility and Seebeck coefficient in Boltzmann transport framework (https://nanohub.org/tools/amobt), • PVLimits (https://nanohub.org/resources/pvlimits) defines the limiting performance of variety of solar cells. • Adept 2.0 (https://nanohub.org/resources/adeptnpt) now includes the frozen potential approach and a web interface. • TAG defines a compact model for p-i-n and perovskite PV (https://nanohub.org/ publications/20/1). • PUMET integrates information from ground- and satellite-based databases to calculate solar irradiance at various parts of the world. • PUB can calculate the energy output from bifacial solar farms.

78

D. Ginley et al.

2.4.3  Assessment 2.4.3.1  Key Takeaways The standout aspect of the modeling research has been that the team was able to address timely system-level problems of a fundamental nature. Indeed, this was possible, in part, due to the flexibility allowed at the initial-level definition of project goals and targets, but also, due to the freedom allowed at the later stages to update the targets. As such, the project goals evolved as the project progressed, and this allowed the team to address many critical as well as timely problems. 2.4.3.2  Importance of the International Collaboration The joint work progressed through frequent interactions—monthly conference calls, in-person meetings over conferences, visits to other research labs, and more. Among the above, the regular monthly conference calls made the most-telling impact because this resulted in sharing and defining of problem statements, getting feedback, and leveraging expertise from multiple domain experts to arrive at optimal solutions.

2.4.4  Near-Term Future Work The Washington group continues to develop the first-principle models. The Purdue group focuses on bifacial solar farms (especially with ground-sculpting), development of bifacial silicon-heterojunction technology with tunnel/interdigitated back-­ contact topology, and perovskite-SHJ tandem technology. The group also wants to focus on developing the framework for physics-based reliability modeling of corrosion and PID. The IIT Bombay group continues to contribute to developing design principles of perovskite technology.

References 1. W. Wang, M.T. Winkler, O. Gunawan, T. Gokmen, T.K. Todorov, Y. Zhu, D.B. Mitzi, Device characteristics of CZTSSe thin-film solar cells with 12.6% efficiency. Adv. Energy Mater. 4, 1301465–1301465 (2013) 2. T.K. Todorov, K.B. Reuter, D.B. Mitzi, High-efficiency solar cell with earth-abundant liquid-­ processed absorber. Adv. Mater. 22, E156–E159 (2010) 3. D.A.R. Barkhouse, O. Gunawan, T. Gokmen, T.K. Todorov, D.B. Mitzi, Device characteristics of a 10.1% hydrazine-processed Cu2ZnSn(se,S)4 solar cell. Prog. Photovolt. Res. Appl. 20, 6–11 (2011)

2  Sustainable Photovoltaics

79

4. T.K. Todorov, J. Tang, S. Bag, O. Gunawan, T. Gokmen, Y. Zhu, D.B. Mitzi, Beyond 11% efficiency: characteristics of state-of-the-art Cu2ZnSn(S,se)4 solar cells. Adv. Energy Mater. 3, 34–38 (2012) 5. P. Jackson, R. Wuerz, D. Hariskos, E. Lotter, W. Witte, M. Powalla, Effects of heavy alkali elements in cu(in,Ga)Se2 solar cells with efficiencies up to 22.6%. Phys. Status Solidi RRL 10, 583–586 (2016) 6. R. Zhang, S.M. Szczepaniak, N.J. Carter, C.A. Handwerker, R. Agrawal, A versatile solution route to efficient Cu2ZnSn(S,se)4 thin-film solar cells. Chem. Mater. 27, 2114–2120 (2015) 7. R. Zhang, S. Cho, D.G. Lim, X. Xianyi, E.A. Stach, C.A. Handwerker, R. Agrawal, Metal and metal chalcogenide molecular precursors to binary, ternary, and quaternary metal chalcogenide thin films for electronic devices. Chem. Commun. 52, 5007–5010 (2016) 8. X. Zhao, M. Lu, M.J. Koeper, R. Agrawal, Solution-processed sulfur depleted cu(in, Ga)Se2 solar cells synthesized from a monoamine–dithiol solvent mixture. J. Mater. Chem. A Mater. Energy Sustain. 4, 7390–7397 (2016) 9. C.K.  Miskin, A.  Dubois-Camacho, M.O.  Reese, R.  Agrawal, A direct solution deposition approach to CdTe thin films. J. Mater. Chem. C 4, 9167–9171 (2016) 10. B.C. Walker, R. Agrawal, Contamination-free solutions of selenium in amines for nanoparticle synthesis. Chem. Commun. 50, 8331–8334 (2014) 11. Q. Guo, H.W. Hillhouse, R. Agrawal, Synthesis of Cu2ZnSnS4 nanocrystal ink and its use for solar cells. J. Am. Chem. Soc. 131, 11672–11673 (2009) 12. Q.  Guo, G.M.  Ford, W.-C.  Yang, B.C.  Walker, E.A.  Stach, H.W.  Hillhouse, R.  Agrawal, Fabrication of 7.2% efficient CZTSSe solar cells using CZTS nanocrystals. J. Am. Chem. Soc. 132, 17384–17386 (2010) 13. C.K. Miskin, W.-C. Yang, C.J. Hages, N.J. Carter, C.S. Joglekar, E.A. Stach, R. Agrawal, 9.0% efficient Cu2ZnSn(S,Se)4 solar cells from selenized nanoparticle inks. Prog. Photovolt: Res. Appl. 23, 654–659 (2014) 14. W.-C. Yang, C.K. Miskin, N.J. Carter, R. Agrawal, E.A. Stach, Compositional inhomogeneity of multinary semiconductor nanoparticles: a case study of Cu2ZnSnS4. Chem. Mater. 26, 6955–6962 (2014) 15. C.J.  Hages, M.J.  Koeper, C.K.  Miskin, K.W.  Brew, R.  Agrawal, Controlled grain growth for high performance nanoparticle-based kesterite solar cells. Chem. Mater. 28, 7703–7714 (2016) 16. N.J. Carter, R. Mainz, B.C. Walker, C.J. Hages, J. Just, M. Klaus, S.S. Schmidt, A. Weber, W.-C.D.  Yang, O.  Zander, et  al., The role of interparticle heterogeneities in the selenization pathway of cu-Zn-Sn-S nanoparticle thin films: a real-time study. J. Mater. Chem. C 3, 7128–7134 (2015) 17. R. Mainz, B.C. Walker, S.S. Schmidt, O. Zander, A. Weber, H. Rodriguez-Alvarez, J. Just, M. Klaus, R. Agrawal, T. Unold, Real-time observation of Cu2ZnSn(S,se)4 solar cell absorber layer formation from nanoparticle precursors. Phys. Chem. Chem. Phys. 15, 18281 (2013) 18. T. Prabhakar, N. Jampana, Effect of sodium diffusion on the structural and electrical properties of Cu2ZnSnS4 thin films. Sol. Energy Mater. Sol. Cells 95, 1001–1004 (2011) 19. C.-Y.  Liu, Z.-M.  Li, H.-Y.  Gu, S.-Y.  Chen, H.  Xiang, X.-G.  Gong, Sodium passivation of the grain boundaries in CuInSe2 and Cu2ZnSnS4 for high-efficiency solar cells. Adv. Energy Mater. 7, 1601457–1601458 (2016) 20. A.  Mule, B.  Vermang, M.  Sylvester, G.  Brammertz, S.  Ranjbar, T.  Schnabel, N.  Gampa, M. Meuris, J. Poortmans, Effect of different alkali (Li, Na, K, Rb, Cs) metals on Cu2ZnSnSe4 solar cells. Thin Solid Films 633, 156–161 (2017) 21. Y. Li, H. Yu, X. Huang, Z. Wu, H. Xu, Improved performance for polymer solar cells using CTAB-modified MoO3 as an anode buffer layer. Sol. Energy Mater. Sol. Cells 171, 72–84 (2017) 22. C.J. Hages, N.J. Carter, R. Agrawal, Generalized quantum efficiency analysis for non-ideal solar cells: case of Cu2ZnSnSe4. J. Appl. Phys. 119, 014505–014518 (2016) 23. M.J. Koeper, C.J. Hages, J.V. Li, D. Levi, R. Agrawal, Metastable defect response in CZTSSe from admittance spectroscopy. Appl. Phys. Lett. 111, 142105 (2017)

80

D. Ginley et al.

24. Q. Liu, Z. Cai, D. Han, S. Chen, Natural intermediate band in I2-II-IV-VI4 quaternary chalcogenide semiconductors. Sci. Rep. 8, 1–8 (2018) 25. S. Chen, J.-H. Yang, X.G. Gong, A. Walsh, S.-H. Wei, Intrinsic point defects and complexes in the quaternary kesterite semiconductor Cu2ZnSnS4. Phys. Rev. B 81, 245204–245210 (2010) 26. B.G.  Mendis, M.D.  Shannon, M.C.  Goodman, J.D.  Major, R.  Claridge, D.P.  Halliday, K.  Durose, Direct observation of cu, Zn cation disorder in Cu2ZnSnS4 solar cell absorber material using aberration corrected scanning transmission electron microscopy. Prog. Photovolt. Res. Appl. 22, 24–34 (2012) 27. J.J.S. Scragg, L. Choubrac, A. Lafond, T. Ericson, C. Platzer-Björkman, A low-temperature order-disorder transition in Cu2ZnSnS4 thin films. Appl. Phys. Lett. 104, 041911–041915 (2014) 28. P.  Bais, M.T.  Caldes, M.  Paris, C.  Guillot-Deudon, P.  Fertey, B.  Domengès, A.  Lafond, Cationic and anionic disorder in CZTSSe kesterite compounds: a chemical crystallography study. Inorg. Chem. 56, 11779–11786 (2017) 29. E. Chagarov, K. Sardashti, A.C. Kummel, Y.S. Lee, R. Haight, T.S. Gershon, Ag2ZnSn(S,se)4: a highly promising absorber for thin film photovoltaics. J. Chem. Phys. 144, 104704–104712 (2016) 30. Z.-K.  Yuan, S.  Chen, H.  Xiang, X.-G.  Gong, A.  Walsh, J.-S.  Park, I.  Repins, S.-H.  Wei, Engineering solar cell absorbers by exploring the band alignment and defect disparity: the case of cu- and Ag-based kesterite compounds. Adv. Funct. Mater. 25, 6733–6743 (2015) 31. S. Chen, A. Walsh, X.-G. Gong, S.-H. Wei, Classification of lattice defects in the kesterite Cu2ZnSnS4 and Cu2ZnSnSe4 earth-abundant solar cell absorbers. Adv. Mater. 25, 1522–1539 (2013) 32. C.J. Hages, M.J. Koeper, R. Agrawal, Optoelectronic and material properties of nanocrystal-­ based CZTSe absorbers with Ag-alloying. Sol. Energy Mater. Sol. Cells 145, 342–348 (2016) 33. J. Kumar, S. Ingole, Structural and optical properties of (AgxCu1-x)2ZnSnS4 thin films synthesised via solution route. J. Alloys Compd. 727, 1–24 (2017) 34. I.  Ohnuma, Phase equilibria and the related properties of Sn-Ag-cu based Pb-free solder alloys. J. Electron. Mater. 29(10), 1137–1144 (2000) 35. T.  Gershon, Y.S.  Lee, P.  Antunez, R.  Mankad, S.  Singh, D.  Bishop, O.  Gunawan, M. Hopstaken, R. Haight, Photovoltaic materials and devices based on the alloyed kesterite absorber (Agx Cu1-x)2ZnSnSe4. Adv. Energy Mater. 6, 1502468–1502467 (2016) 36. W. Li, X. Liu, H. Cui, S. Huang, X. Hao, The role of Ag in (Ag,cu)2ZnSnS4 thin film for solar cell application. J. Alloys Compd. 625, 277–283 (2015) 37. Q. Guo, G.M. Ford, H.W. Hillhouse, R. Agrawal, Sulfide nanocrystal inks for dense cu(In1− xGax)(S1− ySey)2 absorber films and their photovoltaic performance. Nano Lett. 9, 3060–3065 (2009) 38. Q. Guo, G.M. Ford, R. Agrawal, H.W. Hillhouse, Ink formulation and low-temperature incorporation of sodium to yield 12% efficient cu(in,Ga)(S,se)2 solar cells from sulfide nanocrystal inks. Prog. Photovolt. Res. Appl. 21, 64–71 (2012) 39. S.M. McLeod, C.J. Hages, N.J. Carter, R. Agrawal, Synthesis and characterization of 15% efficient CIGSSe solar cells from nanoparticle inks. Prog. Photovolt: Res. Appl. 23, 1550– 1556 (2015) 40. Y. Liu, D. Yao, L. Shen, H. Zhang, X. Zhang, B. Yang, Alkylthiol-enabled se powder dissolution in oleylamine at room temperature for the phosphine-free synthesis of copper-based quaternary selenide nanocrystals. J. Am. Chem. Soc. 134, 7207–7210 (2012) 41. D.H.  Webber, R.L.  Brutchey, Alkahest for V2VI3 chalcogenides: dissolution of nine bulk semiconductors in a diamine-dithiol solvent mixture. J. Am. Chem. Soc. 135, 15722–15725 (2013) 42. C.L. McCarthy, D.H. Webber, E.C. Schueller, R.L. Brutchey, Solution-phase conversion of bulk metal oxides to metal chalcogenides using a simple thiol-amine solvent mixture. Angew. Chem. 127, 8498–8501 (2015)

2  Sustainable Photovoltaics

81

43. W.  Wang, Y.-W.  Su, C.  Chang, Inkjet printed chalcopyrite CuInxGa1−xSe2 thin film solar cells. Sol. Energy Mater. Sol. Cells 95, 2616–2620 (2011). https://doi.org/10.1016/j. solmat.2011.05.011 44. X. Lin, R. Klenk, L. Wang, T. Köhler, J. Albert, S. Fiechter, A. Ennaoui, M.C. Lux-Steiner, 11.3% Efficiency Cu(In,Ga)(S,Se)2 thin film solar cells via drop-on-demand inkjet printing. Energy Environ. Sci. 9, 2037–2043 (2016). https://doi.org/10.1039/C6EE00587J 45. http://Optics.Org/Indepth/2/12/2. Accessed June 2018 46. G.J. Hedley, A. Ruseckas, I.D.W. Samuel, Light harvesting for organic photovoltaics. Chem. Rev. 117, 796–837 (2017) 47. L. Lu, T. Zheng, Q. Wu, A.M. Schneider, D. Zhao, L. Yu, Recent advances in bulk heterojunction polymer solar cells. Chem. Rev. 115, 12666–12731 (2015) 48. https://www.Nrel.Gov/Pv/Assets/Images/Efficiency-Chart.Png. Accessed June 2018 49. Y.-W. Su, S.-C. Lan, K.-H. Wei, Organic photovoltaics. Mater. Today 15, 554–562 (2012) 50. Y. Galagan et al., Roll-to-roll slot-die coated organic photovoltaic (OPV) modules with high geometrical fill factors. Energy Technol. 3, 834–842 (2015) 51. P.L.T. Boudreault, A. Najari, M. Leclerc, Processable low-bandgap polymers for photovoltaic applications. Chem. Mater. 23, 456–469 (2011) 52. N.E. Jackson, B.M. Savoie, T.J. Marks, L.X. Chen, M.A. Ratner, The next breakthrough for organic photovoltaics? J. Phys. Chem. Lett. 6, 77–84 (2015) 53. B.J.  Tremolet de Villers, K.A.  O'Hara, D.P.  Ostrowski, P.H.  Biddle, S.E.  Shaheen, M.L.  Chabinyc, D.C.  Olson, N.  Kopidakis, Removal of residual diiodooctane improves photostability of high-performance organic solar cell polymers. Chem. Mater. 28, 876–884 (2016) 54. E.T.  Hoke, I.T.  Sachs-Quintana, M.T.  Lloyd, I.  Kauvar, W.R.  Mateker, A.M.  Nardes, C.H.  Peters, N.  Kopidakis, M.D.  McGehee, The role of electron affinity in determining whether fullerenes catalyze or inhibit photooxidation of polymers for solar cells. Adv. Energy Mater. 2, 1351–1357 (2012) 55. L.E. Garner, A. Bera, B.W. Larson, D.P. Ostrowski, A.J. Pal, W.A. Braunecker, Promoting morphology with a favorable density of states using diiodooctane to improve organic photovoltaic device efficiency and charge carrier lifetimes. ACS Energy Lett. 2(7), 1556–1563 (2017) 56. L.E. Garner et al., Photobleaching dynamics in small molecule vs. polymer organic photovoltaic blends with 1,7-bis-trifluoromethylfullerene. J. Mater. Chem. A 6, 4623–4628 (2018) 57. V.N. Viswanathan, A.J. Ferguson, J.R. Pfeilsticker, B.W. Larson, L.E. Garner, C.P. Brook, S.H.  Strauss, O.V.  Boltalina, P.C.  Ramamurthy, W.A.  Braunecker, Strategic fluorination of polymers and fullerenes improves photostability of organic photovoltaic blends. Org. Electron. submitted (2018) 58. C.P.  Brook, V.N.  Viswanathan, G.  Paul, B.W.  Larson, A.J.  Ferguson, S.H.  Strauss, O.V. Boltalina, P.C. Ramamurthy, A.J. Pal, W.A. Braunecker, Stabilization of organic photovoltaic donor materials toward photodegradation with 1,7-bis-trifluoromethylfullerene: role of electron affinity and intimate mixing in the blend. In Preparation (2018) 59. W.A.  Braunecker, K.E.  Hurst, K.G.  Ray, Z.R.  Owczarczyk, M.B.  Martinez, N.  Leick, A. Keuhlen, A. Sellinger, J.C. Johnson, Phenyl/perfluorophenyl stacking interactions enhance structural order in two-dimensional covalent organic frameworks. Cryst. Growth Des. in print (2018) 60. B. Dou et al., High-performance flexible perovskite solar cells on ultrathin glass: implications of the TCO. J. Phys. Chem. Lett. 8(19), 4960–4966 (2017) 61. B. Dou, et al., Nature nanotechnology, In Preparation (2018) 62. S. Kavadiya, et al., Electrospray-assisted fabrication of moisture-resistant and highly stable perovskite solar cells at ambient conditions. Advanced Energy Materials 1700210 (2017) 63. S. Subbiah et al., Novel plasma-assisted low-temperature-processed SnO2 thin films for efficient flexible perovskite photovoltaics. ACS Energy Lett 3, 1482–1491 (2018) 64. A.S. Chouhan et al., Large grained and high charge carrier lifetime CH3NH3PbI3 thin-films: implications for perovskite solar cells. Curr. Appl. Phys. 17, 1335 (2017)

82

D. Ginley et al.

65. A.S. Chouhan, et al., Materials Letters, Accepted (2018) 66. A.A. de Carvalho et al., Highly stable perovskite solar cells fabricated under humid ambient conditions. IEEE J. Photovolt. 7(2), 532–538 (2017) 67. A.D. Rao, et al., Advanced materials and interfaces, Submitted (2018) 68. S. Chatterjee, A.J. Pal, Introducing Cu2O thin films as a hole-transport layer in efficient planar perovskite solar cell structures. J. Phys. Chem. C 120(3), 1428–1437 (2016) 69. T. Prasanti, et al., WCPEC-7, Submitted (2018) 70. U. Dasgupta et al., Band-engineered PbS nanoparticles in CH3NH3PbI3 solar cells to extend activity toward near-infrared region. Solar RRL 2, 1800012 (2018) 71. A.S. Subbiah et al., Towards all-inorganic transport layers for wide-band-gap formamidinium lead bromide-based planar photovoltaics. Energ. Technol. 5(10), 1800–1806 (2017) 72. A. Singh, et al., Highly crystalline methylammonium tin iodide perovskite films using post-­ deposition methylamine vapor annealing for improved grain size. Presented at spring MRS, April 2–6, 2018, Phoenix, Arizona (2018) 73. S.  Chouhan, et  al., Compact-TiO2 deposited via ALD for highly-repeatable and low-­ hysteresis perovskite solar cells. Presented at 2017 MRS spring meeting, Phoenix, AZ, April 2017 (2017) 74. M.O.  Reese, T.M.  Barnes, Flexible glass in thin film photovoltaics, in S.M.  Garner (Ed.), Flexible Glass: Enabling Thin, Lightweight, and Flexible Electronics (pp.  213–246). Hoboken, NJ: Wiley-Scrivener (2017) 75. H.P. Mahabaduge, W.L. Rance, J.M. Burst, M.O. Reese, D.M. Meysing, C.A. Wolden, J. Li, J.D. Beach, T.A. Gessert, W.K. Metzger, S. Garner, T.M. Barnes, High-efficiency, flexible CdTe solar cells on ultra-thin glass substrates. Appl. Phys. Lett. 133501, 106 (2015) 76. S.P.  Adusumilli, J.M.  Dederick, I.-T.  Bae, S.M.  Garner, A.  Sharma, C.R.  Westgate, T.P. Dhakal, Iron pyrite thin films grown through a one-step annealing of iron oxide using sulfur sources, tert-butyl disulfide and H2S. Thin Solid Films 615, 271–280 (2016) 77. C.-Y.  Peng, T.P.  Dhakal, S.M.  Garner, P.  Cimo, S.  Lu, C.R.  Westgate, Strained growth of aluminum-doped zinc oxide on flexible glass substrate and degradation studies under cyclic bending. IEEE Trans. Device Mater. Reliab. 14, 121–126 (2014) 78. C.-Y. Peng, T.P. Dhakal, S. Garner, P. Cimo, S. Lu, C.R. Westgate, Fabrication of Cu2ZnSnS4 solar cell on a flexible glass substrate. Thin Solid Films 562, 574–577 (2014) 79. S. Sheehan, P.K. Surolia, O. Byrne, S. Garner, P. Cimo, X. Li, D.P. Dowling, K.R. Thampi, Flexible glass substrate based dye sensitized solar cells. Sol. Energy Mater. Sol. Cells 132, 237–244 (2015) 80. S.J.C. Irvine, D.A. Lamb, A.J. Clayton, G. Kartopu, V. Barrioz, Cadmium telluride solar cells on ultrathin glass for space applications. J. Elec. Mater. 43, 2818 (2014) 81. A. Mandal, A. Kole, S.M. Garner, P. Chaudhuri, Study of the VHF plasma etching of micro/ nano patterned PMMA coated on ultra-thin flexible glass substrates. Plasma Process. Polym. 13(10), 990–996 (2016). https://doi.org/10.1002/ppap.201600016 82. J.  Oh, G.  TamizhMani, S.  Bowden, S.  Garner, Surface disruption method with flexible glass to prevent potential-induced degradation of the shunting type in PV modules. IEEE J. Photovolt. 7, 62–67 (2017) 83. M.K. Mazumder, M.N. Horenstein, N.R. Joglekar, A. Sayyah, J.W. Stark, A.A.R. Bernard, S.M.  Garner, J.E.  Yellowhair, H.Y.  Lin, R.S.  Eriksen, A.C.  Griffin, Y.  Gao, R.  La Centra, A.H. Lloyd, Mitigation of dust impact on solar collectors by water-free cleaning with transparent electrodynamic films: progress and challenges. IEEE J.  Photovolt. 7, 1342–1353 (2017) 84. S.M. Garner, X. Li, M.-H. Huang, Introduction to flexible glass substrates, in Flexible Glass: Enabling Thin, Lightweight, and Flexible Electronics, ed. by S. M. Garner, (Wiley-Scrivener, Hoboken, NJ, 2017), pp. 3–34 85. S. Garner, S. Glaesemann, X. Li, Ultra-slim flexible glass for roll-to-roll electronic device fabrication. Appl. Phys. A Mater. Sci. Process. 116, 403–407 (2014) 86. B.  Dou, E.M.  Miller, J.A.  Christians, E.M.  Sanehira, T.  Klein, F.  Barnes, S.E.  Shaheen, S. Garner, S. Ghosh, A. Mallick, D. Basak, M. van Hest, High-performance flexible perovskite

2  Sustainable Photovoltaics

83

solar cells on ultrathin glass: implications of the TCO. J. Phys. Chem. Lett. 8, 4960–−4966 (2017) 87. K.W. Brew, S.M. McLeod, S.M. Garner, R. Agrawal, Improving efficiencies of Cu2ZnSnS4 nanoparticle based solar cells on flexible glass substrates. Thin Solid Films 642, 110–116 (2017) 88. S.  Chattopadhyay, R.  Dubey, V.  Kuthanazhi, J.J.  John, C.S.  Solanki, A.  Kottantharayil, B.M. Arora, K.L. Narasimhan, J. Vasi, B. Bora, Y.K. Singh, O.S. Sastry, All India Survey of Photovoltaic Module Degradation 2014: Survey Methodology and Statistics. Presented at 42nd IEEE Photovoltaic Specialists Conference, New Orleans, USA (2015) 89. R.  Dubey, S.  Chattopadhyay, V.  Kuthanazhi, A.  Kottantharayil, C.S.  Solanki, B.M.  Arora, K.L. Narasimhan, J. Vasi, B. Bora, Y.K. Singh, O.S. Sastry, Comprehensive study of performance degradation of field-mounted PV modules in India. Energy Sci. Eng. 5, 51 (2017) 90. R.  Dubey, S.  Zachariah, S.  Chattopadhyay, V.  Kuthanazhi, S.  Rambabu, S.  Bhaduri, H.K. Singh, A. Sinha, C.S. Solanki, A. Kottantharayil, B.M. Arora, K.L. Narasimhan, J. Vasi, B. Bora, R. Kumar, O.S. Sastry, Performance of field-aged PV modules in India: Results from 2016 all India survey of PV module reliability. Presented at 44th IEEE photovoltaic specialists conference, Washington DC, USA (2017) 91. S.  Bhaduri, S.  Chattopadhyay, R.  Dubey, S.  Zachariah, V.  Kuthanazhi, C.S.  Solanki, B.M. Arora, K.L. Narsimhan, A. Kottantharayil, J. Vasi, Correlating Infra-Red Thermography with Electrical Degradation of Modules Inspected in all India Survey of Photovoltaic Module Reliability 2016. Presented at Photovoltaic Reliability Workshop, Lakewood, CO, USA (2017) 92. K.  Yedidi, J.  Mallineni, B.  Knisely, S.  Tatapudi, J.  Kuitche, G.  TamizhMani, Failure and Degradation Modes and Rates of PV Modules in a Hot-Dry Climate: Results after 16 Years of Field Exposure. Presented at 40th IEEE Photovoltaic Specialists Conference, Denver, USA (2014) 93. J. Kuitche, R. Pan, G. TamizhMani, Investigation of dominant failure mode(s) for field-aged crystalline silicon PV modules under desert climatic conditions. IEEE J. Photovolt. 4, 814 (2014) 94. C.  Raupp, C.  Libby, S.  Tatapudi, D.  Srinivasan, J.  Kuitche, B.  Bicer, G.  TamizhMani, Degradation rate evaluation of multiple PV technologies from 59,000 modules representing 252,000 modules in four climatic regions of the United States. Presented at 43rd IEEE photovoltaic specialists conference, Portland, OR, USA (2016) 95. N.G. Dhere, E. Schneller, W.R. Martin, R.G. Dhere, Study of 1 MW PV array at the Kennedy Space Center, in Proc. SPIE 9938, Reliability of Photovoltaic Cells, Modules, Components, and Systems IX, 99380K (2016) 96. G.  TamizhMani, S.  Tatapudi, R.  Dubey, S.  Chattopadhyay, C.  Solanki, J.  Vasi, B.  Bora, O.S. Sastry, A. Kottantharayil, Comparative study of performance of fielded PV modules in two countries. Presented at 26th international photovoltaic science and engineering conference, Dermatol. Sin. (2016) 97. M. Moorthy, G. TamizhMani, Automation of risk priority number (RPN) calculation for photovoltaic modules in power plant. Presented at 43rd IEEE photovoltaic specialists conference, Portland, OR, USA (2016) 98. R.  Dubey, S.  Chattopadhyay, S.  Zachariah, S.  Rambabu, H.K.  Singh, A.  Kottantharayil, B.M.  Arora, K.L.  Narasimhan, N.  Shiradkar, J.  Vasi, On-site electroluminescence study of field-aged PV modules. Presented at 7th World Conference on Photovoltaic Energy Conversion and 45th IEEE Photovoltaic Specialists Conference, Waikoloa, Hawaii, USA (2018) 99. K.  Dolia, A.  Sinha, S.  Tatapudi, J.  Oh, G.  TamizhMani, Early Detection of Encapsulant Discoloration by UV Fluorescence Imaging and Yellowness Index Measurements. Presented at 7th World Conference on Photovoltaic Energy Conversion and 45th IEEE Photovoltaic Specialists Conference, Waikoloa, Hawaii, USA (2018) 100. K.  Yadav, O.S.  Sastry, R.  Wandhare, N.  Sheth, M.  Kumar, B.  Bora, R.  Singh, R.  Dahiya, A. Kumar, Performance comparison of controllers for solar PV water pumping applications. Sol. Energy 119, 195 (2015)

84

D. Ginley et al.

101. P.  Kaur, S.  Jain, A.  Jhunjhunwala, Solar-DC Deployment Experience in off-Grid and near off-Grid Homes: Economics, Technology and Policy Analysis. Presented at IEEE First International Conference on DC Microgrids, Atlanta USA (2015) 102. A.  Jhunjhunwala, Innovative Direct-Current Microgrids to Solve India’s Power Woes. IEEE Spectrum, January (2017). https://spectrum.ieee.org/energy/renewables/innovativedirect-current-microgrids-to-solve-indias-power-woes 103. V. Thavaraj, B. Vashishth, O.S. Sastry, A. Kapil, N. Kapoor, Solar powered portable culture incubator. Ann. Pediatr. Child Health 3, 1063 (2015) 104. J. Cano, J.J. John, S. Tatapudi, G. TamizhMani, Effect of Tilt Angle on Soiling of Photovoltaic Modules. Presented at 40th IEEE Photovoltaic Specialists Conference, Denver, USA (2014) 105. J.J. John, V. Rajasekar, S. Boppana, S. Chattopadhyay, A. Kottantharayil, G. TamizhMani, Quantification and modeling of spectral and angular losses of naturally soiled PV modules. IEEE J. Photovolt. 5, 1727 (2015) 106. J. Mallineni, K. Yedidi, S. Shrestha, S. Tatapudi, Brett Knisely, J. Kuitche, G. TamizhMani, Soiling Losses of Utility-Scale PV Systems in Hot-Dry Desert Climates: Results from Four 4–16 Years Old Power Plants. Presented at 40th IEEE Photovoltaic Specialists Conference, Denver, USA (2014) 107. S.  Bopanna, V.  Rajashekhar, G.  TamizhMani, Working towards the Development of a Standardized Artificial Soiling Method. Presented at 42nd IEEE Photovoltaic Specialists Conference, New Orleans, USA (2015) 108. J.J. John, S. Warade, G. TamizhMani, A. Kottantharayil, Study of soiling loss on photovoltaic modules with artificially deposited dust of different gravimetric densities and compositions collected from different locations in India. IEEE J. Photovolt. 6, 236 (2016) 109. K.K. Khanum, A. Rao, N.C. Balaji, M. Mani, P. Ramamurthy, Performance Evaluation for PV Systems to Synergistic Influences of Dust, Wind and Panel Temperatures. Presented at 43rd IEEE Photovoltaic Specialists Conference, Portland, USA (2016) 110. S. Bhaduri, B. Kavaipatti, A. Kottantharayil, Mitigation of soiling by vertical mounting of bifacial modules. Presented at 7th World Conference on Photovoltaic Energy Conversion and 45th IEEE Photovoltaic Specialists Conference, Waikoloa, Hawaii, USA (2018) 111. M.A.  Alam, R.  Khan, Thermodynamic efficiency limits of classical and bifacial multi-­ junction tandem solar cells: an analytical approach. Appl. Phys. Lett. 109(17), 173504 (2016) 112. A.  Polizzotti, A.  Faghaninia, J.R.  Poindexter, L.  Nienhaus, V.  Steinmann, R.L.  Hoye, A.  Felten, A.  Deyine, N.M.  Mangan, J.P.  Correa-Baena, S.S.  Shin, Improving the carrier lifetime of tin sulfide via prediction and mitigation of harmful point defects. J. Phys. Chem. Lett. 8(15), 3661–3667 (2017) 113. A. Faghaninia, J.W. Ager III, C.S. Lo, Ab initio electronic transport model with explicit solution to the linearized Boltzmann transport equation. Phys. Rev. B 91, 235123 (2015) 114. S.  Agarwal, M.  Seetharaman, M.  Kumawat, A.  Subbiah, S.  Sarkar, D.  Kabra, M.A.G. Namboothiry, P.R. Nair, On the uniqueness of ideality factor and voltage exponent of perovskite-based solar cells. J. Phys. Chem. Lett. 5, 4115–4121 (2014) 115. S. Agarwal, P.R. Nair, Device engineering of perovskite solar cells to achieve near ideal efficiency. Appl. Phys. Lett. 107, 123901 (2015) 116. X. Sun, R. Asadpour, W. Nie, A.D. Mohite, M.A. Alam, A physics-based analytical model for perovskite solar cells. IEEE J. Photovolt. 5(5), 1389–1394 (2015) 117. Y. Raote, H. Choubisa, P.R. Nair, Performance Prediction for Large Area Perovskite Solar Cells. Presented at the IEEE PV Specialists Conference, Washington DC (2017) 118. V. Nandal, P.R. Nair, Predictive modeling of ion migration induced degradation in perovskite solar cells. ACS Nano 11(11), 11505–11512 (2017) 119. S. Agarwal, P.R. Nair, Pinhole induced efficiency variation in perovskite solar cells. J. Appl. Phys. 122, 163104 (2017) 120. W.  Nie, H.  Tsai, R.  Asadpour, J.-C.  Blancon, A.J.  Neukirch, G.  Gupta, J.  Crochet, M. Chhowalla, S. Tretiak, M.A. Alam, H.-L. Wang, A.D. Mohite, High efficiency solution-­ processed perovskite solar cells with millimeter-scale grains. Science 347(6221), 522 (2015)

2  Sustainable Photovoltaics

85

121. H. Tsai, W. Nie, J.-C. Blancon, C.C. Stoumpos, R. Asadpour, B. Harutyunyan, A.J. Neukirch, R. Verduzco, J.J. Crochet, S. Tretiak, L. Pedesseau, J. Even, M.A. Alam, G. Gupta, J. Lou, P.M. Ajayan, M.J. Bedzyk, M.G. Kanatzidis, A.D. Mohite, High-efficiency two-dimensional Ruddlesden-popper perovskite solar cells. Nature 536(7616), 312–316 (2016) 122. R.V.K. Chavali, J.V. Li, C. Battaglia, S.D. Wolf, J.L. Gray, M.A. Alam, A generalized theory explains anomalous suns-Voc response of Si heterojunction solar cells. IEEE J.Photovolt. 7(1), 169–176 (2017) 123. R. Chavali, S. De Wolf, M.A. Alam, Device physics underlying silicon heterojunction solar cells: a topical review. Prog. Photovolt. 26(4), 241–260 (2017) 124. R.  Asadpour, R.V.K.  Chavali, M.R.  Khan, M.A.  Alam, Bifacial HIT-perovskite organic-­ inorganic tandem to produce highly efficient solar cell. Appl. Phys. Lett. 106(24), 243902 (2015) 125. M.R. Khan, A. Hanna, X. Sun, M.A. Alam, Vertical bifacial solar farms: physics, design and global optimization. Appl. Energy 206, 240–248 (2017) 126. X.  Sun, M.R.  Khan, C.  Deline, M.A.  Alam, Optimization and performance of bifacial solar modules: a global perspective. Appl. Energy 212(C), 1601–1610 (2018). https://doi. org/10.1016/j.apenergy.2017.12.041

Chapter 3

Multiscale Concentrated Solar Power David Ginley, R. Aswathi, S. R. Atchuta, Bikramjiit Basu, Saptarshi Basu, Joshua M. Christian, Atasi Dan, Nikhil Dani, Rathindra Nath Das, Pradip Dutta, Scott M. Flueckiger, Suresh V. Garimella, Yogi Goswami, Clifford K. Ho, Shireesh Kedare, Sagar D. Khivsara, Pramod Kumar, C. D. Madhusoodana, B. Mallikarjun, Carolina Mira-Hernández, M. Orosz, Jesus D. Ortega, Dipti R. Parida, M. Shiva Prasad, K. Ramesh, S. Advaith, Sandip K. Saha, Shanmugasundaram Sakthivel, Sumit Sharma, P. Singh, Suneet Singh, Ojasve Srikanth, Vinod Srinivasan, Justin A. Weibel, and Tim Wendelin The goal of the concentrating solar power (CSP) thrust was to develop cost-­effective, multiscale, dispatchable CSP generation. SERIIUS chose to focus on a high-­ temperature closed-loop supercritical carbon dioxide (s-CO2) Brayton cycle, which is efficient and scalable for distributed operation. Technologies developed with respect to s-CO2 Brayton cycles are addressed in Sect. 3.1. For regions with moderate direct normal radiation (DNI), technologies based on low- and medium-­ temperature organic Rankine cycles (ORCs) were also developed under SERIIUS, as described in Sect. 3.2. Section 3.3 describes the activities related to thermal storage and hybridization associated with high- and low-temperature cycles. D. Ginley (*) · T. Wendelin National Renewable Energy Laboratory, Golden, CO, USA e-mail: [email protected]; [email protected] R. Aswathi · R. N. Das · C. D. Madhusoodana · O. Srikanth Corporate R&D Division, Ceramic Technological Institute, Bharat Heavy Electricals Limited Corporate R&D, Bangalore, India e-mail: [email protected] S. R. Atchuta · B. Mallikarjun · M. S. Prasad · S. Sakthivel International Advanced Research Centre for Powder Metallurgy and New Materials, Hyderabad, Telangana, India e-mail: [email protected]; [email protected]; [email protected]; [email protected] B. Basu · A. Dan Materials Research Centre, Indian Institute of Science-Bangalore, Bangalore, India S. Basu · N. Dani · S. Advaith Department of Mechanical Engineering, Interdisciplinary Centre for Energy Research, Indian Institute of Science-Bangalore, Bengaluru, India e-mail: [email protected] © Springer Nature Switzerland AG 2020 D. Ginley, K. Chattopadhyay (eds.), Solar Energy Research Institute for India and the United States (SERIIUS), Lecture Notes in Energy 39, https://doi.org/10.1007/978-3-030-33184-9_3

87

88

D. Ginley et al.

3.1  CSP for High-Temperature s-CO2 Brayton Cycle Advanced power cycles operating at higher temperatures than in conventional Rankine cycles are being studied to increase the thermal-to-electric efficiency and to reduce the levelized cost of energy from CSP systems [1]. Supercritical CO2 closed-loop Brayton cycles with recuperation and recompression can achieve ~50% thermal-to-electric efficiency at turbine inlet temperatures above ~700  °C [2]. Implementing s-CO2 Brayton cycles with CSP is projected to improve overall CSP plant economics due to their high power-conversion efficiency and compact system J. M. Christian · C. K. Ho · J. D. Ortega Sandia National Laboratories, Albuquerque, NM, USA e-mail: [email protected]; [email protected]; [email protected] P. Dutta · S. D. Khivsara · P. Kumar · D. R. Parida · P. Singh · V. Srinivasan Department of Mechanical Engineering, Indian Institute of Science-Bangalore, Bengaluru, India e-mail: [email protected]; [email protected]; [email protected]; [email protected] S. M. Flueckiger School of Electrical and Computer Engineering, Hall for Discovery and Learning Research, Purdue University, West Lafayette, IN, USA S. V. Garimella · J. A. Weibel School of Mechanical Engineering, Purdue University, West Lafayette, IN, USA e-mail: [email protected]; [email protected] Y. Goswami University of Southern Florida, Tampa, FL, USA e-mail: [email protected] S. Kedare · S. Sharma · S. Singh Department of Energy Science and Engineering, Indian Institute of Technology Bombay (IIT Bombay), Mumbai, India e-mail: [email protected]; [email protected] C. Mira-Hernández School of Chemical Engineering, Forney Hall of Chemical Engineering, Purdue University, West Lafayette, IN, USA e-mail: [email protected] M. Orosz Massachusetts Institute of Technology, Cambridge, MA, USA e-mail: [email protected] K. Ramesh Corporate R&D Division, Hindustan Petroleum Corporation Limited Green R&D Center, Bangalore, Bangalore, India e-mail: [email protected] S. K. Saha Department of Mechanical Engineering, Indian Institute of Technology-Bombay, Mumbai, Maharashtra, India e-mail: [email protected]

3  Multiscale Concentrated Solar Power

89

layout, which will reduce installed costs relative to conventional alternatives including steam turbines and open-cycle gas turbines [2–4]. The high cycle thermal efficiency is achieved by significantly reducing the compressor work of pumping CO2 in its high-pressure supercritical dense phase (at 7.5 MPa, 31 °C). Recuperative heat transfer is also used to preheat the s-CO2 before it is expanded through the turbine at high temperature and pressure (~20  MPa, 550–700 °C). High fluid densities at the compressor and turbine (~500 kg/m3 and 100  kg/m3, respectively) result in an anticipated order-of-magnitude reduction in turbomachinery size as compared with steam plants. Previous studies have investigated the performance of the s-CO2 cycle at transient conditions characteristic of a directly heated solar s-CO2 receiver [3]. Sandia’s s-CO2 Brayton cycle was brought to its design conditions; then the thermal input was reduced to 50% and 0% for several minutes before being restored. Pressure, temperature, and electricity-generation transients were recorded. Results of the tests showed that the overall system’s large thermal mass enabled the cycle to continue operation with relatively little impact in each case. Thermal storage with indirect solar receivers can also be implemented to eliminate the transient response altogether. The focus of SERIIUS was on developing multiscale CSP systems, with an emphasis on smaller-scale systems (~0.1–1  MWe) for India. Previous studies focused on larger commercial capacities of ~100 MWe [2]. Implementing CSP technologies at significantly smaller scales than conventional commercial CSP systems requires highly efficient and low-cost receivers and heliostat fields. The following sections describe efforts within SERIIUS to develop and enhance technologies for highly efficient receivers and low-cost heliostats.

3.1.1  High-Temperature Receivers for s-CO2 Alternative designs for high-temperature receivers are being studied to enable higher-efficiency power cycles that operate at higher temperatures (>700 °C) than conventional Rankine cycles [5]. Unique challenges associated with high-­ temperature receivers include the development of geometric designs (e.g., dimensions, configurations), materials, heat-transfer fluids, and processes that maximize solar irradiance and absorptance, minimize heat loss, and have high reliability at high temperatures over thousands of thermal cycles. In addition, consideration must be given to advantages and disadvantages of direct versus indirect heating of the power-cycle working fluid. For example, advantages of direct heating of the working fluid include reduced exergetic losses through intermediate heat exchange. Advantages of indirect heating include the ability to store the heat-transfer media (e.g., molten salt, solid particles) for energy production during nonsolar hours. In both direct- and indirect-heating receivers, the heat addition to the receiver media can be done directly (e.g., exposed liquid films or solid particles) or indirectly (e.g., fluid flowing through tubular receivers).

90

D. Ginley et al.

For the small-scale CSP systems evaluated in SERIIUS, direct-heating receivers were considered due to their simplicity and potential for smaller footprints and CSP plants. Although thermal storage is an inherent value proposition for CSP systems, we desired to initially focus on direct-heating systems that did not require additional heat exchangers and storage systems between the receiver and power block. The following sections describe novel high-temperature receiver designs that are expected to yield higher efficiencies than conventional tubular panel receiver designs. 3.1.1.1  Bladed Receiver As part of the SERIIUS project, Sandia and IISc investigated alternative receiver geometries to achieve higher efficiencies at high temperatures by trapping light and reducing radiative losses. Conventional receiver designs employ flat panels of tubes through which the heat-transfer fluid flows; any light or thermal radiation reflected or emitted from its surface is lost to the environment. Initially, flat-panel designs were considered within SERIIUS [6]. Later, novel designs with volumetric features were investigated to employ geometric configurations that could increase the amount of absorbed solar radiation by trapping the incoming rays [7–9]. In addition, the flow pattern was designed such that the hottest region of the receiver was toward the interior, resulting in reduced radiative thermal losses. Christian et al. [7, 8] concluded that the bladed-receiver geometry configuration exhibited the best performance relative to the various geometric options considered. An optimized design, along with detailed dimensions for the bladed receiver, was presented by Ortega et al. [10]. On-sun testing of the bladed-receiver design was performed using different gases. In addition, coupled optical/fluid/thermal/structural modeling of the bladed receiver was performed [11, 12]. 3.1.1.1.1  Design and Analysis of Bladed Receiver Ray-tracing analyses using SolTrace were performed to understand the light-­ trapping effects of the bladed receivers, which enable re-reflections between the fins that enhance the effective solar absorptance. A parametric optimization study was performed to determine the best possible configuration with a fixed intrinsic absorptivity of 0.9 and exposed surface area of 1 m2. The resulting design consisted of three vertical panels 0.584 m long and 0.508 m wide and three blades 0.508 m long and 0.229  m wide, with a downward tilt of 50° from the horizontal. Each blade consisted of two panels that were placed in front of the three vertical panels. The receiver was tested at Sandia’s National Solar Thermal Test Facility (NSTTF) using pressurized air. However, the receiver was designed to operate using s-CO2 at 650 °C and 15 MPa for 100,000 h following the ASME Boiler and Pressure Vessel Code Section VIII Division 1. The air first flowed through the leading panel of the blade, and then recirculated toward the back panel of the blade before flowing through one of the vertical back panels (Fig. 3.1).

3  Multiscale Concentrated Solar Power

91

Fig. 3.1  Design and flow pattern for bladed receiver. Reprinted with permission from [10]. Copyright ©2017, American Chemical Society

The receiver design consists of three vertical panels (13 tubes per panel) at the back with three blades aligned at an angle of 50° to the vertical. Each blade consists of two panels (9 tubes per panel). Each tube had an outer diameter of 12.7 mm and a thickness of 1.65 mm. To obtain the heat flux to be used for the thermal fluid modeling, we used SolTrace, ray-tracing-based optical modeling software developed by NREL. The heliostat field at the NSTTF was modeled in SolTrace, and the receiver geometry was created in SOLIDWORKS for import into SolTrace. The results obtained in SolTrace were coupled with ANSYS Fluent using a MATLAB code developed that generates a file that can be used as a boundary condition in ANSYS Fluent. ANSYS Fluent was used for detailed investigation of the thermal fluid performance of the receiver. Three different power levels were simulated, corresponding to peak fluxes used in the on-sun tests of 90, 120 and 150 kW/m2. The power imposed on the receiver surface during the flow time of up to 900  s was about 23 kW, 32 Kw, and 43 kW for the three cases, respectively. 3.1.1.1.2  Fabrication and On-Sun Testing The tubes and headers, made from Inconel 625, were designed and welded according to ASME Boiler and Pressure Vessel Code Section VIII. The design criterion was 100,000 h of operation at 20 MPa and 700 °C. The tubes were oxidized in a furnace at 800 °C, which increased the solar absorptance from ~0.5 to ~0.9 without the use of paints or coatings. The leading tube of each bladed panel was painted white to enable a similar amount of absorbed flux on the leading tube as that of other tubes receiving less irradiance. Figure 3.2 shows the assembled receiver. Both flat- and bladed-receiver configurations were evaluated. For the flat configuration, only the back vertical panel was used.

92

D. Ginley et al.

Fig. 3.2  Bladed-receiver panels prior to final insulation (left) and fully assembled (right). Reprinted with permission from [10]. Copyright ©2017, American Chemical Society

Fig. 3.3  Simulated and measured receiver efficiencies using air as the heat-transfer fluid for both bladed- and flat-receiver configurations. Reprinted with permission from [10]. Copyright ©2017, American Chemical Society

On-sun test results showed that the air-temperature rise in the bladed-receiver panel was 200–300  °C higher than in the flat panel, and the thermal efficiencies were ~5% points higher. Figure 3.3 shows that the modeled results were generally within the error bars of the measured values of receiver efficiency for the majority of tests using air. After the prototype tests, studies were performed to refine the gas mixture within the receiver to maximize the heat-transfer coefficient and thermal efficiency. Challenges included the need to use gas bottles to accommodate the mixtures (rather

3  Multiscale Concentrated Solar Power

93

than using a pump for air) and an analytical method to account for the transients during the tests. Models and tests were performed with air, argon, and a mixture of helium and nitrogen. Results showed that the thermal efficiency was directly ­correlated to the heat-transfer coefficient and that the mixture using helium and nitrogen had the best thermal efficiency, whereas argon yielded the lowest heattransfer coefficient and thermal efficiency. The bladed-receiver design demonstrated that it can be used for direct heating of gases for an s-CO2 Brayton cycle. 3.1.1.1.3  Conclusions A novel high-temperature receiver configuration was designed, analyzed, and tested as part of the SERIIUS project to evaluate small-scale CSP systems implementing solarized s-CO2 Brayton cycles. A bladed-receiver design consisting of louvered panels of tubes evolved from designs and analyses comparing flat panels with bladed designs. The spacing and pitch of the blades were optimized, and a receiver was constructed and tested on sun. Results showed that the bladed configuration yielded receiver thermal efficiencies that were ~5 percentage points higher than the flat-panel receiver configuration when air was used. Tests with different gases revealed that the thermal efficiency was correlated to the heat-transfer coefficient of the gas, and the use of a mixture of helium and nitrogen yielded the highest thermal efficiency. 3.1.1.2  Spiral-Wound Tubular Solar Receiver for s-CO2 Brayton Cycle The s-CO2 Brayton cycle has received considerable attention in the past decade, with research focused on developing efficient high-temperature power cycles. This cycle exhibits efficiencies of ~50 percentage at turbine inlet temperatures of ~700 °C (at 200 bar) or 650 °C (at 250 bar), which is considerably lower temperature than the air-Brayton cycle (~1000 °C) for the same cycle efficiency [13]. The thermophysical properties of s-CO2 render the equipment such as receivers, turbine, and heat exchangers to be very compact and efficient [14, 15]. This cycle was initially proposed with nuclear energy as the heat source but recently received much attention with solar energy as the heat source [16, 17]. Under the High-Temperature Receiver for CO2 Cycle project, a spiral-coiled tubular solar-receiver design (Fig. 3.4) was proposed, numerically evaluated, and tested for application in a solarpowered s-CO2 Brayton cycle. 3.1.1.2.1  Coupled Optical/Thermal/Fluid Modeling Detailed optical-thermal-fluid assessment of the proposed design was performed for the design shown in Fig. 3.4. For the thermal-fluid evaluation, the actual heat-flux distribution was obtained by detailed optical modeling of the concentrator–receiver arrangement, rather than using a constant heat flux or a constant-temperature bound-

94

D. Ginley et al.

Fig. 3.4  Proposed geometry of the coiled receiver for s-CO2. (Courtesy of Indian Institute of Science, Bangalore)

ary condition on the irradiated coils. The receiver geometry was first modeled in SOLIDWORKS, and a code scripted in MATLAB processed the geometric data to prepare the appropriate input for optical modeling in SolTrace (called as a Stage file). SolTrace is a ray-tracing tool used to model the optical arrangement and obtain the ray intersection data. These data were then processed using another MATLAB script to obtain a detailed heat-flux map. The heat flux was then modified to obtain a heat-generation profile boundary-condition file, which was finally used in ANSYS Fluent for thermal-fluid evaluation of the receiver. This modeling methodology is described in more detail by Ortega et al. [18]. Figure 3.5 shows the exchange of data between modeling tools, along with a representative ray-intersection map obtained in SolTrace. The concentrator arrangement (Fig. 3.6) used for testing the receiver consisted of a 1-m-diameter Fresnel lens with single-axis manual tracking, and it was connected to a compressed-air loop. The receiver was designed and manufactured, and the modeling was validated by testing with air as the heat-transfer fluid. The design was also analyzed for s-CO2 as the working fluid. Figure  3.7 shows the manufactured receiver and assembly in progress, and Fig. 3.8 shows the irradiated receiver. 3.1.1.2.2  Important Modeling and Test Results Some of the interesting and important results are summarized here. • Effect of mass flow rate: Receiver surface temperature decreases with an increase in mass flow rate; hence, an increase in the thermal efficiency was

3  Multiscale Concentrated Solar Power

95

Fig. 3.5  Data exchange for coupled modeling and representative ray-intersection map from SolTrace. (Courtesy of Indian Institute of Science, Bangalore) Fig. 3.6  Fresnel lens-­ based concentrator. (Courtesy of Indian Institute of Science, Bangalore)

predicted with increasing mass flow rate (Fig. 3.9). Small flow rates of air and s-CO2 due to the smaller scale of the helical receiver result in negligible pressure drop for both s-CO2 and air. Due to the small scale of the receiver (~1 kWth), s-CO2 performed only marginally better than air for similar flow rates.

96

D. Ginley et al.

Fig. 3.7  Helical receiver and placement of back reflector. (Courtesy of Indian Institute of Science, Bangalore) Fig. 3.8  Irradiated helical receiver. (Courtesy of Indian Institute of Science, Bangalore)

Fig. 3.9  Predicted receiver thermal efficiency. (Courtesy of Indian Institute of Science, Bangalore)

3  Multiscale Concentrated Solar Power

97

• Effect of change in receiver aperture position: Even a minor change in the axial position of the receiver with reference to the focal plane of the concentrator resulted in a drastic change in the flux distribution of the receiver, as seen in Fig. 3.10. • Effect of interchanging inlet–outlet: In the case of the inlet at the periphery, comparatively colder fluid comes in contact with the high-intensity irradiation zone of the helical receiver. In this case, the efficiency was higher by about 1.5% compared with the central-inlet/peripheral-outlet case. The temperature distributions for the two cases are shown in Fig. 3.11.

Fig. 3.10  Surface heat flux for different positions of the receiver with respect to the focal plane. (Courtesy of Indian Institute of Science, Bangalore)

Fig. 3.11  Temperature distribution, showing the effect of swapping the inlet and outlet. Figure on the left corresponds to air inlet at the bottom of the spiral, and the one on the right corresponds to air inlet at the top of the spiral. (Courtesy of Indian Institute of Science, Bangalore)

98

D. Ginley et al.

• Influence of conical back reflector: The cone serves a dual purpose. It restricts any irradiation loss from the bottom of the receiver and distributes the irradiation in the lower region of the cavity. Our numerical study showed that its effect on the receiver thermal efficiency is negligible, whereas the optical efficiency was drastically affected by the presence of a back reflector. At higher thermal inputs and larger geometric scales of the receiver, proper distribution of irradiation inside the cavity must be ensured, and the conical back reflector will play a significant role. • Air versus s-CO2 receiver performance comparison: The volume flow rate (and velocities) for air was drastically larger than that of s-CO2 due to the high density and heat capacity of s-CO2. A hotspot was observed for the air receiver operating at the same outlet temperature as s-CO2 due to the desirable thermal properties of s-CO2, whereas the receiver thermal efficiency was higher for s-CO2. • S-CO2 operating conditions effect: For a receiver outlet temperature of 700 °C (at 200 bar), the receiver thermal efficiency was slightly lower than that for a receiver outlet temperature of 650 °C (at 250 bar), but the effect was negligible. 3.1.1.3  Spectrally Selective Absorbers One of the major components of a solar thermal system is the receiver, which plays an important role in enhancing the photothermal efficiency by absorbing a maximum amount of solar radiation with minimum heat loss. In this regard, our objective was to fabricate spectrally selective absorbers for receivers that should have a high absorptance of ≥0.95 in the solar spectrum (0.25–2.5 μm) and a low thermal emittance of ≤0.05 in the infrared region (2.5–25 μm). In addition, while considering real field applications, these coatings should exhibit exceptional thermal (>450 °C) and environmental stability in different operational conditions [19, 20]. 3.1.1.3.1  Development of Spectrally Selective Absorber Investigation on TiB2/TiB(N)/Si3N4 film: Our research group aimed to explore the spectral selectivity of TiB2-based multilayer thin films by tuning different deposition conditions. Ultrahigh-temperature ceramics have outstanding thermochemical and thermophysical stability. But there are only a few reports on the spectral selectivity of these materials. Therefore, in the first part of our research, we attempted to fabricate TiB2/TiB(N)/Si3N4-based multilayer thin films using a DC and RF magnetron sputtering system. We conducted a systematic investigation to understand the influence of various deposition parameters—including target power, deposition time, and reactive gas flow—on the spectral selectivity of the film. The optimal process parameters led to a high absorptance of 0.964 and an emittance of 0.18. Further investigation was performed to explore the underlying mechanism of shifting the absorption edge toward a longer- wavelength regime to achieve the desired

3  Multiscale Concentrated Solar Power

99

Fig. 3.12  Optimization process of SS/TiB2/TiB(N)/Si3N4-based spectrally selective absorber. Reprinted with permission from [21]. Copyright ©2018, Elsevier Ltd

spectral selectivity. Figure 3.12 illustrates a portion of the optimization process of target power, deposition power, and reactive gas flow for developing a multilayer stack of TiB2/TiB(N)/Si3N4 absorber. Assessment of W/WAlN/WAlON/Al2O3 as spectrally selective absorber: In the next part of our study, we developed a W/WAlN/WAlON/Al2O3-based multifunctional novel coating using magnetron sputtering. The rationale of this specific coating design was based on good optical properties and a larger barrier toward diffusion of transition-metal oxides and oxynitrides. The optimally fabricated coating possessed a superior spectral selectivity with a maximum absorptance of 0.958 and a low emittance of 0.08 [21]. Based on extensive analysis using transmission electron microscopy (TEM), phase-modulated spectroscopic ellipsometry, along with computational analysis, we determined that the optical constants of each layer decrease from substrate to surface of coating. This could be exploited to trap solar energy. We developed a significant understanding of how the spectral selectivity of the coating is enhanced by the gradual transformation of metallic properties from substrate to surface and a graded layer-by-layer structure stack. Using a host of theoretical models (e.g., Tauc-Lorentz, Cauchy) and ellipsometry measurements, we predicted the presence of 26% WAlN—74% WAlON at the WAlN/WAlON interface and 60% WAlON—40% Al2O3 at the WAlON/Al2O3 interface using the Bruggeman effective medium approximation, which provides an effective guideline to understand the underlying physics of newly developed absorber coatings [22]. The stability of the coating at elevated temperature is of great importance so as to maintain the efficiency of the photothermal conversion system. A prolonged ­thermal annealing test established that the spectral properties of the coating were retained after heat treatment at 350  °C in air for 1000  h [23, 24]. Note that the

100

D. Ginley et al.

Fig. 3.13  Environmental and thermal stability of SS/W/WAlN/WAlON/Al2O3 thin film. Reprinted with permission from [25]. Copyright ©2018, Elsevier Ltd

recently conducted high-temperature testing (30 cycles at 450 °C) in a simulated solar thermal environment at Sandia National Laboratories established the excellent thermal shock resistance of the coating. Additionally, several experiments in various environmental conditions over several months proved the properties of corrosion resistance, appreciable mechanical stability, hydrophobicity, and moisture resistance of the coating (Fig. 3.13) [25]. Also, directional and hemispherical emissivity were not compromised during annealing at 500 °C in air, as well as in vacuum for 12 h. The resulting photothermal conversion efficiency of W/WAlN/WAlON/Al2O3 was around 87%, which remained almost constant even after aging at 400 °C in air up to 309 h. In summary, two multilayer absorbers—TiB2/TiB(N)/Si3N4 and W/WAlN/ WAlON/Al2O—were investigated to demonstrate the underlying physics that explains the origin of spectral selectivity of these absorbers. Both the coatings in the present study possess excellent spectrally selective properties and have potential application in solar photothermal conversion technology. 3.1.1.3.2  Future Perspective Magnetron sputtering is preferable for commercialization of different coatings because sputtering is a superior physical vapor deposition technique that offers strongly adhesive as well as uniform films for high-temperature applications. Therefore, for the last two decades, fabricating multilayer stacks using sputtering has been employed by industry [26]. However, the process parameters in sputtering are numerous and complicated, so fabricating multilayer structures is a time-­ consuming and expensive procedure. The ability to control deposition parameters is extremely crucial, helping to avoid trial-and-error methods for achieving highest selectivity with a remarkably steep absorption edge and to obtain reproducible per-

3  Multiscale Concentrated Solar Power

101

formance. Therefore, there is an urgent need for an effective and systematic guideline to better understand the influence of optimal deposition conditions on spectral selectivity of the film. With this in view, we illustrate here how a materials genome approach can help find absorber materials with high photothermal conversion efficiency [27]. The Materials Genome Initiative (MGI), introduced in 2011 in the United States, is a large, scalable, collaborative attempt by scientists (both experimentalists and theorists) with materials and computer backgrounds to exploit proven computational methodologies to predict and optimize materials as well as processing parameters at an unparalleled scale and rate [28, 29]. Traditionally, deposition conditions are parameterized by conducting characterization of a series of samples using various techniques such as scanning electron microscopy, atomic force microscopy, or optical profilometry to determine the empirical relationship between the deposition parameters and deposition rate. Computer modeling can expediate the optimization process by drastically reducing the number of exploratory experiments required to determine an appropriate deposition condition. A semiempirical model—based on the relationship between inputs (parameters) and outputs (absorptance–emittance)—should be constructed to predict the deposition rates of materials deposited by DC and RF magnetron sputtering, which can be used as a preliminary screen of fabrication parameters for estimating composition and thickness of the film. Subsequent deposition experiments would be carried out for final optimization. This virtual approach will undoubtedly reduce the number of experiments for determining the optimized deposition condition. The semiempirical model will account for varying factors such as target power, deposition time, and reactive gas flow rate. This method can also be employed to rapidly explore PSP (process → structure → properties) linkages of thin-film materials. Using the MGI approach, we may also be able to determine the relation between structural characteristics and performance of spectrally selective absorbers. MGI can drastically shorten the materials research cycle by reducing the burden of multiple experiments and by using data visualization and implementing different models [30]. We believe that these ideas and methodologies will be beneficial for future studies of solar absorber coatings. 3.1.1.4  Volumetric Ceramic Receiver and Thermal Storage 3.1.1.4.1  High-Temperature Volumetric Receiver and Participating Nature of s-CO2 In a typical volumetric receiver, solar radiation from the concentrator penetrates the volume of a porous absorber, which is typically made of metallic wires, foam, or a ceramic matrix. Simultaneously, the working fluid flows through the absorber and carries the heat from the absorber (Fig. 3.14). Volumetric receivers are an attractive alternative to tubular receivers due to their geometry, functionality, and reduced thermal losses.

102

D. Ginley et al.

Fig. 3.14  Schematic of a basic element of a volumetric receiver. Reprinted with permission from [31]. Copyright ©2017, Elsevier Ltd

If pressurized, volumetric receivers need a transparent window that allows solar radiation to enter the receiver and impinge on the porous matrix, while separating the working fluid from the atmosphere. Pressurized volumetric receivers are reported to be capable of sustaining high incident fluxes compared with other receiver designs. However, the sealing and cooling of the window for high-pressure receivers pose a major engineering challenge in designing and developing a volumetric receiver. Another important concern for the volumetric receiver is the metallurgical and mechanical failure of the receiver due to cracking of the absorber and the development of hotspots on its surface. Direct heating of s-CO2 using a volumetric receiver is not an easy prospect because of questions concerning the integrity of the receiver window subjected to high pressure and temperature. However, a volumetric receiver can certainly be used for obtaining high-temperature air as the secondary fluid for indirect heating of s-CO2. If the engineering challenges of making a high-temperature sealing and a pressurized window can be addressed, then direct heating of s-CO2 in a volumetric receiver is a very attractive prospect. In this work, we have proposed and analyzed a concept of a miniature, ceramic, pressurized volumetric receiver for s-CO2 (Fig. 3.15). Small quartz windows may be able to sustain the pressure differential between high-pressure s-CO2 and surrounding air. We carried out a computational fluid dynamics (CFD) analysis, along with a discrete ordinate method (DOM) radiation heat-transfer model, and we present the results for temperature distribution in the receiver and the resulting thermal efficiency. The effect of mass flow rate, geometric parameters of absorber, flow con-

3  Multiscale Concentrated Solar Power

103

Fig. 3.15  Schematic and dimensions of proposed miniature volumetric receiver. (Courtesy of Indian Institute of Science, Bangalore)

Fig. 3.16  Effect of absorption of radiation by s-CO2 on efficiency and peak absorber temperature. (Courtesy of Indian Institute of Science, Bangalore)

figurations, absorber optical properties, and direct absorption of long-wavelength radiation by s-CO2 were evaluated in this work. This study was one of the first investigations to address the issue of heat transfer to s-CO2 by direct absorption of emitted radiation from receiver walls to the fluid. We discuss issues regarding material selection for the absorber structure, window, coating, receiver body, and insulation. We also analyzed a modular small-scale prototype with 0.5-kWth solar-heat input. In the case of a volumetric receiver, an error greater than 150 K in predicting the peak temperature of the absorber may be incurred if one neglects absorption by s-CO2 (Fig.  3.16). Another important outcome of the analysis is that the optical

104

D. Ginley et al.

Fig. 3.17  Temperature contours for absorber surface. (Courtesy of Indian Institute of Science, Bangalore)

properties of the absorber can be optimized to control the temperature distribution within the absorber and prevent the formation of hotspots. This increases the durability of the receiver and results in high efficiency due to the presence of high temperatures in the interior of the absorber (Fig. 3.17). 3.1.1.4.2  Ceramic-Based Thermal Storage Thermal energy storage (TES) at high temperature is a challenging research area with typical applications such as regenerative heating in steel production plants and auxiliary energy sources in solar thermal plants. Honeycomb structures made of ceramics are used as high-temperature TES units because of their large heat-transfer surface area per unit volume, large thermal capacity, and good thermal shock resistance. The material properties and geometric parameters of these units determine the storage capacity and heat addition/retrieval rate. A thorough understanding of the thermal response of the storage unit at different process conditions is crucial for designing the system. In this work, we developed a new composition of mullite- and chromite-based ceramic honeycombs for high-temperature thermal storage application (Fig. 3.18). An experiment was designed to evaluate the performance of the ceramic honeycomb in the temperature range of 773–1273  K by studying the storage and discharge characteristics in a cyclic mode. Numerical studies using ANSYS Fluent predicted the effect of honeycomb design, material properties, and flow rates on thermal energy storage and heat-transfer characteristics. These data were used to validate the experimental results and design an optimum TES system. We built an experimental setup, shown schematically in Fig.  3.19, capable of evaluating the heat-regenerative capacity of the ceramic honeycombs in the temperature range of 773–1273  K.  The test section, which housed the sample, was 70 × 70 × 100 mm3. Four K-type thermocouples, mounted as shown in Fig. 3.19, were used to measure the temperature. Thermocouples T2 and T3 were placed in contact with the ceramic honeycomb to measure the surface temperature of the sample, whereas T1 and T4 recorded the temperatures of air at inlet/outlet.

3  Multiscale Concentrated Solar Power

105

Fig. 3.18 Honeycomb sample. (Courtesy of Indian Institute of Science, Bangalore)

Fig. 3.19  Experiment schematic. Reprinted with permission from [32]. Copyright ©2017, Taylor & Francis

ANSYS Fluent was used for numerical simulations to predict the effect of honeycomb dimensions and shape, material properties, and flow rates on the storage and heat-transfer performance. Mullite- and chromite-based ceramics were both analyzed, and the initial temperature of the ceramic was 300 K and 1273 K for the charging and discharging process, respectively. Sample results from the numerical analysis and the experiment are show in Figs. 3.20 and 3.21, which depict the transient charging and discharging response of the samples. From the numerical simulations, we observed that there is no channel-shape effect if the area of the channels was kept constant, whereas the mass flow rate had significant influence on charging and discharging of the blocks. Ceramic honey-

106

D. Ginley et al.

Fig. 3.20 Thermal response from numerical study. Reprinted with permission from [32]. Copyright ©2017, Taylor & Francis

Fig. 3.21  Experimental cycle performance of chromite-based ceramic honeycomb. Reprinted with permission from [32]. Copyright ©2017, Taylor & Francis

comb structure with an optimum area of contact for heat transfer, i.e., with an appropriate channel size for a given flow rate, can store thermal energy in a short time. Experimental evaluation of thermal energy storage and regeneration performance using hot air input at 1073  K was conducted in cyclic mode. Both mullite- and chromite-based ceramic honeycombs exhibited high-temperature thermal storage

3  Multiscale Concentrated Solar Power

107

that discharged to regenerate hot air at temperatures more than 773  K.  The estimated heat storage in mullite and chromite samples was 549 MJ/m3 and 925 MJ/m3, respectively. The heat energy storage and regeneration cycle time of mullite was lower than that of chromite. The simulation data also showed this trend. The simulation and experimental data generated in these experiments showed suitability of ceramic honeycombs for TES at high temperature, and they provide data for designing ceramic-based optimum TES systems.

3.1.2  S  upercritical Carbon Dioxide Test Loop Facility at Indian Institute of Science, Bangalore Several studies have revealed that s-CO2 power cycles have the potential to attain higher cycle efficiencies than conventional steam-Rankine or air-Brayton power cycles. s-CO2 Brayton cycles have a smaller footprint and offer a simple layout with compact turbomachinery and heat exchangers. Rankine-based steam or gas-turbine Brayton systems operate as open cycles considering the timescales governing the thermophysical processes. In contrast, the s-CO2 system inherently operates in a closed cycle with finite timescales for heat transfer. This is one important factor that makes the s-CO2 power plant highly susceptible to variations in ambient conditions. Therefore, designing a suitable control system for stable operation of an s-CO2 Brayton system is extremely complex and challenging. An s-CO2 Brayton power plant in its simplest form employs a single recuperator, gas cooler, and a single compressor, as shown schematically in Fig. 3.22. The corresponding thermodynamic cycle is represented in Fig. 3.23. The compressor in the simple recuperated system pressurizes CO2 to a pressure above the critical pressure, usually in the range of 140–800 bar. The compressor discharge pressure is governed by a range of operating variables such as available heat-source temperature, type of turbomachinery (e.g., single-stage or multistage, Fig. 3.22  Schematic of a simple recuperated s-CO2 Brayton cycle. (Courtesy of Indian Institute of Science, Bangalore)

Heat Exchanger Turbine

4

5 oHTF

Regenerator

6

1 Gas cooler

3

2

Compressor

iHTF

108

D. Ginley et al. 600

2

5

3

100 1 6

4 7

Temperature (ºC)

Pressure (bar)

1000

500 5

400 300 200 2

100 10

0

500 1000 Enthalpy (kJ/kg)

(a)

3 7 4

0 1.0

1 1.5

6

2.0 2.5 Entropy (kJ/kgK)

3.0

(b)

Fig. 3.23  Representation of a simple recuperated s-CO2 Brayton cycle on (a) p-h diagram and (b) T-s diagram. (Courtesy of Indian Institute of Science, Bangalore)

axial or radial), power-generation capacity, control philosophy [2] (pressure ratio or source temperature–based control), and finally, thermodynamic aspects such as desired efficiency and specific work output—all of which define the operating pressure ratio and the corresponding turbine exit pressure. The highly dense CO2 discharged from the compressor (state 2  in Fig.  3.23) is partially heated in the recuperator (state 5) before passing through the secondary heater (electrical heater 2), where it is heated to the desired turbine inlet temperature (state 3). The hot CO2 stream leaving the heater expands through the turbine to generate mechanical work, which eventually is converted to electrical power using a generator. The hot low-­ pressure CO2 discharged from turbine exit (at state 4) transfers heat to the cold CO2 discharged from the compressor exit in the recuperator (state 6) before rejecting heat to the ambient in the gas cooler. The compressor compresses the cold CO2 stream leaving the gas cooler (state 1) to the desired high pressure, thus completing the cycle. One of the unique features of an s-CO2 Brayton cycle is the minimal temperature drop in the working fluid (CO2) across the turbine, thus providing ample opportunity for heat recovery. Unlike a steam-based Rankine cycle, where steam is expanded to near-ambient temperatures, the drop in CO2 temperature across the turbine is only about 80  °C for a turbine inlet temperature of 535  °C and a pressure of 135  bar expanding to a pressure of 76 bar [33]. Apart from the benefits of better heat-­transfer properties of CO2, recuperation is the key reason for higher thermodynamic-cycle efficiency compared with steam- or air-based gas-turbine cycles [33]. Another novelty of a CO2 power plant is the compact equipment resulting from high densities of CO2. The lowest density of CO2 in the simple recuperated cycle is about 55 kg/m3, which is about 300 times denser than a corresponding steam-based Rankine cycle operating under identical turbine inlet conditions. Another interesting aspect of the CO2 cycle is the high discharge temperature from the recuperator (state 6) entering the gas cooler. The temperature can be as high as 170 °C, which makes it possible to use dry cooling, thus saving precious water resources.

3  Multiscale Concentrated Solar Power

109

Irrefutably, operating at higher ambient comes at a penalty in thermodynamic efficiency. Nevertheless, the flexibility of using dry cooling not only makes it viable for CSP applications but also makes it a potential candidate as a gas-turbine bottoming cycle. To realize an s-CO2 cycle in practice at commercial scale, one needs to establish detailed experimental data such as heat-transfer coefficient correlations in the supercritical regime, robust control strategies, and material compatibility issues. With this endeavor, the Indian Institute of Science, Bangalore, in collaboration with Sandia National Laboratories, developed an s-CO2 test loop facility under the SERIIUS consortium. Although the primary aim of the loop was to generate experimental test data, the loop was also used for component endurance tests and to demonstrate stable operations for extended durations at operating pressures and temperatures. In addition, the loop also served to establish the efficacy of the control system and to understand dynamics of what-if scenarios by occasional offshoots outside the designed operating regime. Figure 3.24 shows the basic components of the test loop. The photograph of the actual installation at IISc is shown in Fig. 3.25. The test loop was designed to simulate the simple recuperated s-CO2 cycle generating 20 kW of mechanical turbine power. The maximum operating pressure and temperature was restricted to 140 bar and 540 °C to facilitate use of standard components and materials. The CO2 flow path in the test loop was constructed using 316-grade stainless-steel seamless tubing. This restricted the maximum temperature to 540 °C, which is close to the creep limit of SS316. A standard reciprocating compressor used in transcritical refrigeraELECTRICAL HEATER-1

DISCHARGE PRESSURE CONTROL VALVE

3 WAY VALVE

TURBINE SIMULATOR

100KW 350°C

ELECTRICAL HEATER-2

RECUPERATOR

41KW 535°C

COMPRESSOR

3 WAY VALVE

COMPRESSED AIR OUT

AIR GAS COOLER

THROTTLE VALVE

3 WAY VALVE

3 WAY VALVE 3 WAY VALVE

3 WAY VALVE

CO2 SUPPLY TUBE

COMPRESSED AIR IN

VACUUM PUMP

WATER IN GAS COOLER

WATER OUT

WATER DRY COOLER

Fig. 3.24  Schematic of s-CO2 test loop showing the various components. (Courtesy of Indian Institute of Science, Bangalore)

110

D. Ginley et al.

Fig. 3.25  Photo of s-CO2 test loop at IISc Bangalore. (Courtesy of Indian Institute of Science, Bangalore)

tion was employed to pressurize CO2 to 140 bar. Due to the unavailability of a standard turbine, a turbine simulator comprising a combination of a throttling device and an air-cooled heat exchanger was specifically designed to mimic the operating conditions across the turbine. Thermodynamically, these processes are identified as isenthalpic (processes 3–7) and isobaric (processes 7–4) on the p-h and T-s diagrams shown in Fig. 3.23. Two compact printed-circuit-type heat exchangers served as a recuperator and a gas cooler in the test loop. The gas cooler was connected to a dual circuit plate heat exchanger, which eventually rejected heat to the ambient via a 100-kW dry cooler shown in Fig.  3.24. Water at a pressure of 4  bar was used as the intermediate ­heat-­exchange fluid between air and CO2 in the gas cooler. Maintaining the water pressure higher than atmospheric pressure ensured single-phase heat transfer for CO2 discharge temperatures beyond 100 °C. In addition, an intermediate heat exchanger in the arrangement shown in Fig. 3.24 provided flexibility to simulate a wide range of ambient conditions and also lowered the high-pressure CO2 inventory in the test loop. Heat input to the loop was facilitated from a variety of heat sources such as concentrated solar, a low-emission lean-­ burn syngas combustor, or a set of electrical heaters. The control system provided flexibility to control the temperature and quantum of heat addition from the above sources to the CO2 in the test loop. However, for steady-state experiments, primary

3  Multiscale Concentrated Solar Power

111

ELECTRICAL HEATER-1

100KW 350°C

2

RECUPERATOR

COMPRESSED AIR IN

ELECTRICAL HEATER-2

5 3 WAY VALVE

DISCHARGE PRESSURE CONTROL VALVE

TURBINE SIMULATOR COMPRESSED AIR OUT

3

41KW 535°C

3 WAY VALVE

THROTTLE VALVE

7

AIR GAS COOLER

4

3 WAY VALVE

6

COMPRESSOR

1

3 WAY VALVE

CO2 SUPPLY TUBE

3 WAY VALVE

3 WAY VALVE

VACUUM PUMP

WATER WATER OUT IN GAS COOLER

WATER DRY COOLER

Fig. 3.26  Schematic of s-CO2 test loop operation sequence during startup. (Courtesy of Indian Institute of Science, Bangalore)

heat was derived from a set of skid-mounted electrical heaters, as shown in Fig. 3.25. The heaters provided a total heat input of 140 kW to CO2 circulating in the test loop. The two operation modes are explained through the schematics shown in Figs. 3.26 and 3.27. 3.1.2.1  Startup Operation The startup sequence is shown in Fig. 3.26. In the absence of recuperation during the startup, both CO2 heaters were powered to add a total of 140 kW to the CO2 circulating in the loop. The recuperator was bypassed using a set of three-way valves shown in Fig. 3.26. The system continued to receive heat until prescribed conditions of both pressure and temperature were achieved at various locations in the test loop. 3.1.2.2  Steady-State Operation The steady-state operation is shown in Fig. 3.27. Once the desired turbine inlet and outlet temperature and pressure were achieved, the recuperator kicked in; thus, it eliminated the need for the first heater (electrical heater 1), which was subsequently powered off. The three-way valves shown in Fig. 3.27 bypassed the electrical heater 1 to eliminate an additional pressure drop due to the flow passing through the heater.

112

D. Ginley et al. ELECTRICAL HEATER-1

TURBINE SIMULATOR

100KW 350°C

2 DISCHARGE PRESSURE CONTROL VALVE

5 3 WAY VALVE

RECUPERATOR

COMPRESSOR

1

ELECTRICAL HEATER-2

COMPRESSED AIR OUT

3

41KW 535°C

3 WAY VALVE

THROTTLE VALVE

7

AIR GAS COOLER

4 3 WAY VALVE

3 WAY VALVE 3 WAY VALVE

3 WAY VALVE

CO2 SUPPLY TUBE

COMPRESSED AIR IN

6 WATER WATER OUT IN GAS COOLER

VACUUM PUMP

WATER DRY COOLER

Fig. 3.27  Schematic of s-CO2 test loop operation sequence at steady state. (Courtesy of Indian Institute of Science, Bangalore)

High-temperature heat input to CO2 circulating in the loop was provided by electrical heater 2 to maintain the setpoint temperature at the turbine inlet (state 3 shown in Fig. 3.23).

3.2  CSP for Low-Temperature ORC 3.2.1  Cost-Efficient Solar Receiver Tube High-temperature solar collectors are the best option to obtain high efficiency in concentrating solar thermal (CST) systems. But employing these collectors often leads to high heat loss, and also the generation of power is not economical through convention cycles. In this connection, a low-cost solar collector coupled with an ORC system was investigated by the International Advanced Research Centre for Powder Metallurgy and New Materials (ARCI), Thermax, and the Indian Institute of Science within the SERIIUS project. As a best alternative, this project considered reducing the heat losses at medium-temperature operation, which, in turn, provides good efficiency for producing economic power. The project targets are cost of parabolic collector 70%; temperature of operation of 200–250 °C; and thermal loss  0.95 and emittance (ε) 70% using positive-displacement expanders. Dr. M. Orosz at MIT led the project through integrating modeling, mechanical design, prototyping, and testing, along with Dr. P. Singh of IISc and Dr. Y. Goswami and his team at University of Southern Florida. Figures 3.30 and 3.31 are photos of the prototype scroll assembly at Florida and at IISc that produced data on the performance of the scroll expanders for low-temperature ORC systems.

Fig. 3.30  Scroll assembly at MIT and testing at Florida. (Courtesy of Indian Institute of Science, Bangalore)

Fig. 3.31  Prototyping of scroll expander and test rig at IISc. (Courtesy of Indian Institute of Science, Bangalore)

116

D. Ginley et al.

Fig. 3.32  Variation of LCOE in (USD/kWhe) with solar field and storage. (Courtesy of Indian Institute of Science, Bangalore)

A detailed methodology was also developed for technical assessment of ORC systems using scroll expanders. A 50-kWe solar ORC system (using parabolic trough solar collectors and scroll expander) was analyzed for its performance and cost. Figure 3.32 shows the variation of levelized cost of electricity (LCOE) in the form of isocontours over a two-dimensional space (solar-field area and storage hours) for a hypothetical plant located in Ahmedabad. The analyses suggest that such systems mandate the addition of thermal storage for optimal performance and economic viability. The LCOE of these systems is comparable with that of a PV-battery system but with relatively higher capacity utilization factors.

3.3  Storage and Hybridization 3.3.1  H  igh-Temperature Molten-Salt Storage and Hybridization for Brayton Cycles A single-tank thermocline-based thermal energy storage system was jointly investigated by the Indian Institute of Science and Purdue University as part of the SERIIUS project under India’s Jawaharlal Nehru Solar National Energy Mission and DOE’s SunShot Initiative. The TES loop—designed and fabricated at the Interdisciplinary Centre for Energy Research (ICER), IISc Bangalore—was first made suitable for commercial heat-transfer fluid (oil), then extended to molten salt. The purpose of this thermal loop was to characterize various heat-storage media and

3  Multiscale Concentrated Solar Power

117

to predict the thermal cyclic behavior because it is a cost-effective [43] and energy-­ efficient alternative to two-tank TES systems. The advantages of molten salt over oil are high operating temperature and high energy density [44, 45]. However, key challenges are high liquidus temperature, corrosion potential, and stability as well as sustainability of thermocline. Previous studies explained the effect on thermal stratification of flow disturbances, if present, during charging and discharging processes [46, 47]. Manu et al. studied the Rayleigh–Taylor instability and K-H instability for thermocline distortion. Important studies in this area include stability analysis of oil [48], thermocline disruption [49], and porous-layer instabilities [50]. Our study was tailored toward examining the stability and sustainability issues of a thermocline storage system. Numerical simulations were performed to determine the sensitivity of thermal stratification subjected to real-time flow disturbances [51]; the dimensions of the computational domain were kept the same as the fabricated TES tank. In addition, experimental investigations explored the degradation of the thermocline over time. 3.3.1.1  Design of Lab-Scale Thermal Storage Loop The thermocline-based single-tank TES loop is shown in Fig. 3.33a. The thermal loop consisted of a TES tank, inventory tank, heater (35 kW), flow meter (Coriolis type), 3-way valve (pneumatic), pump (7.5 HP), and thermocouples (K type). The TES tank was made optically accessible, which enabled us to study fluid dynamics and heat-transfer characteristics of single-tank TES in depth. The primary purpose was to test the applicability of various energy storage media for concentrated solar power plant applications. In the lab-scale loop, solar energy was mimicked by a two-stage heating system. The first stage of heating was done in the inventory tank using three immersion heaters arranged at about 120° to each other with a power capacity 3.3  kW per heater. Subsequently, the heat-transfer fluid was pumped via an Inconel-made vertical-­axis centrifugal pump to a heat receiver (heater) for a second level of ­heating. The entire facility was designed to withstand high temperature (up to 800°C) and was made up of highly corrosion-resistant material (Inconel 600). The loop operation was started with an initial heating of the heat-transfer fluid to a minimum temperature TC (cold-fluid temperature) in the inventory tank, followed by filling up the entire loop. Meanwhile, the second-level heating in the heat receiver raised the temperature to a maximum limit TH (hot-fluid temperature). A ­variable-­frequency drive-controlled pump, coupled with the flow meter, ensured precise control over hot-fluid addition to the TES tank. Finally, relatively hotter fluid (lighter fluid at temperature TH) was made to reside on top of relatively colder fluid (denser fluid at temperature TC). These two fluid layers were separated by a sharp temperature gradient called a “thermocline” [52]. This process of establishing the thermocline is termed “charging” (Fig.  3.33b). This stored energy can be extracted during off-peak hours per application requirements. This process, called

118

D. Ginley et al.

Fig. 3.33  Lab-scale thermal energy storage loop: (a) experimental setup and (b) process flow diagram. (Courtesy of Indian Institute of Science, Bangalore)

“discharging,” was achieved by passing the stored hot fluid through a heat exchanger. Both charging and discharging processes are shown in Fig. 3.33b. The effectiveness of this single-tank thermal storage unit depends on the stability and sustainability of the thermocline over long hours. Stability ensures the thermal stratification during dynamic charging and discharging operation, whereas sustainability ensures the same during storage.

3  Multiscale Concentrated Solar Power

119

3.3.1.2  Sensitivity Analysis of Thermocline TES System In a real scenario, the TES operation of charging and discharging might include temperature fluctuations in the incoming fluid (disturbance fluid). The effect of such irregularities on a preestablished thermocline was studied to quantify the spatiotemporal dynamics. A two-dimensional model of the TES tank was built in ANSYS DesignModeler. Model dimensions were those of the fabricated lab-scale TES. Temperature-dependent molten-salt (Solar Salt) properties were assigned to the computational domain with the boundary conditions as shown in Fig. 3.34. The flow condition was taken as laminar with no viscous dissipation and solved in ANSYS Fluent using the PISO Algorithm for pressure–velocity coupling through a second-order upwind scheme for momentum and energy equations. Finally, the solution domain was discretized with rectangular cells, and grid independence was ensured prior to solution. The strengths of the thermal stratification and disturbance were quantified in terms of Atwood numbers At ((ρC − ρH)/(ρC + ρH)) and Atd ((ρd − ρH)/(ρd + ρH)), where, ρC, ρH, and ρd were the densities of cold, hot, and disturbance fluids, respectively. Figure  3.35 represents the time evolution of disturbance at selected time instants. These temperature contours depict the effects of parabolic disturbance (Atwood number 0.00085) on low-strength thermal stratification (relatively small Atwood number, 0.00167). This low Atwood number disturbance was observed to form a toroidal vortex near the inlet on account of gravity (Rayleigh Taylor instability). Later, the toroidal

Fig. 3.34  Boundary-conditions schematic diagrams: (a) charging process, (b) introduction and evolution of a disturbance for the short-term analysis, and (c) charging–discharging cycles without disturbance. Reprinted with permission from [51]. Copyright ©2016, Elsevier Ltd

120

D. Ginley et al.

Fig. 3.35  Temperature contours showing the time evolution of the disturbance at selected time instants: At = 0.00167. Reprinted with permission from [51]. Copyright ©2016, Elsevier Ltd

vortex became larger with time and finally diffused near the top of the TES tank. However, the leading end took the shape of a sphere and advected toward the thermocline. Strong resistance offered by the stratified region (thermocline) decelerated the disturbance, resulting in a bounce back. As a result, the disturbance caused the mixing in the thermal layer, leading to an increase in the stratified thickness. Complete penetration of the disturbance (vortex) was noticed for this weak (low At number) thermal stratification (see Fig. 3.35). 3.3.1.3  Temporal Degradation of Thermocline in the Experimental Loop Temporal degradation is simply the widening of the thermocline over time due to radial heat loss through external walls and axial heat exchange between hot and cold fluid across the TES. Experiments were performed with HYTHERM 600 (provided by HPCL, a partner industry) in the thermal loop. To capture the real-time variations in axial temperature, the TES was equipped with a longitudinal array of thermocouples (K type). The nondimensionalized temperature profile across TES is shown in Fig. 3.36 for a storage duration of 8 h. The study of thermocline degradation seemed to be significant after 1 h of filling because the oscillations during the initial charging took time to dissipate. The observed distortion was due to the combined effect of inlet-flow-induced mixing, conductive heat loss through the wall, and thermal diffusion. A relatively higher drop in temperature in hot-fluid region signifies major heat loss to the surroundings. A sharp and sustainable thermocline was established by mitigating the above-­ mentioned effects by modifications such as charging via novel distributor head

3  Multiscale Concentrated Solar Power

121

60min 120min 180min 240min 300min 360min 420min 480min

(T-TC)/(TH-TC)

0.6

0.4

0.2

0 0

0.2

0.4

h/H

0.6

0.8

1

Fig. 3.36  Nondimensionalized axial temperature variation across TES during storage. (Courtesy of Indian Institute of Science, Bangalore)

Table 3.1  Property comparison of developed molten salts with existing HITEC salt Materials Melting point (°C) Thermal stability (°C) Thermal conductivity (W/mK) Specific heat at 300 °C (kJ/kgK)

HITEC 142 ~565 0.5028 1.61

HP MS-1 135 ~600 0.5063 2.08

HP NanoMS-1 136 ~600 0.5921 2.38

(which minimizes inflow induced mixing), thick glass–wool insulation (which ­minimizes ambient heat loss), and nonconductive ceramic coating of internal TES walls (which minimizes heat loss along the wall). 3.3.1.4  Development of Novel Heat-Storage Media At present, an efficient heat-storage system needs to be developed that can overcome the aforementioned issues such as energy density. Under the current initiative, HPCL (partner industry) developed molten-salt-based materials for high-­ temperature thermal-storage applications (Table  3.1). The nitrate-based ternary molten salts as well as nano-incorporated molten salts exhibited good thermal stability and high specific heat capacity compared with the commercially available molten salts [53, 54]. The materials were tested for cyclic stability and heat-­ exchanging capability using an in-house high-temperature solar test loop at HPCL. These materials and the high-temperature loop were patented [55].

122

D. Ginley et al.

3.3.1.5  Conclusions A single-tank, thermocline-based, high-temperature molten-salt TES test facility was established at the thermal energy storage laboratory ICER, IISc Bangalore, as a part of SERIIUS project to form a stable and sustainable means of TES for CSP applications. CFD analyses were performed to explore the thermocline stability subjected to flow disturbances. Results showed that the stability of the thermal storage depends on the strength of the stratification. A lower ratio of stratification to disturbance strength yields greater possibilities of thermocline distortion, leading to ineffective thermal storage. Results from experimental observations revealed that the thermal stratification can be sustained for a longer duration (8 h). It justifies that the single-tank thermal loop can be a potential candidate for sensible thermal-­ storage applications across multiple domains.

3.3.2  M  ultiscale Analysis of Molten-Salt Thermocline Tank Performance for Thermal Energy Storage in CSP Power Plants Concentrating solar power plants require cost-effective thermal energy storage to overcome intrinsic variability in solar energy availability and to maintain steady electricity generation. TES using a thermocline tank is an attractive alternative to two-tank storage systems that offers cost savings by storing both cold and hot reserves of heat-transfer fluid in a single container. Stable thermal stratification inside the tank due to large buoyancy forces enables the coexistence of isothermal hot-fluid and cold-fluid regions separated by a “thermocline region” of temperature gradient. Additionally, thermocline tanks offer the possibility of using a low-cost filler material as the energy-storage medium. The thermal performance and behavior of thermocline tanks must be assessed, typically through modeling efforts, prior to considering their adoption as the energy storage technology in CSP plants. Prior work has primarily focused on the impact of design parameters and operating conditions on the development of the thermocline region and the ensuing exergy destruction during a single charge process or under time-periodic operation [56]. Under the SERIIUS collaboration, computational studies for high-temperature molten-salt storage were performed that significantly advanced the understanding of thermocline tanks as a TES device for CSP plants. These modeling studies addressed critical aspects of TES using thermocline tanks, including the long-term performance and stability under realistic operating conditions, incorporation of phase-change materials, trade-offs between thermocline designs with or without filler materials, and economic viability of CSP plants with thermocline energy storage. The major findings of these studies are summarized below. A reduced-order model of a thermocline tank for TES was developed and validated against high-fidelity simulations to enable the analysis of thermal perfor-

3  Multiscale Concentrated Solar Power

123

mance under realistic and long-term operating conditions. The reduced-order model for thermocline tanks was incorporated into a system-level model of a CSP plant to quantify the benefits of this TES technology and verify the stability of the stratification inside the thermocline tank over long-term operating conditions [57]. The system-­level model considered the example case of yearlong simulations for a 100-­ MW plant operating at Barstow, California, in 1977. Energy-collection data were obtained through the transient simulation of the receiver with SOLERGY (Sandia National Laboratories) and was used as an input to the system-level model. Selected simulation results for single-day and yearlong operation are presented in Fig. 3.37. Simulation results showed that the tank effectiveness remains above 99% throughout the year, verifying that a thermocline tank is indeed a reliable storage concept. Also, the incorporation of a thermocline tank for TES doubled the capacity factor of the power plant when compared with an alternative system without any storage. Economic optimization of a CSP plant with molten-salt thermocline energy storage was performed using the developed system-level model [58]. The location of the plant and electric power capacity were kept constant (namely, Barstow, CA, and 100 MW). The effects of solar multiple (varied from 1 to 4) and thermocline tank storage capacity (varied from 6 to 20 h) were considered, and the economic performance was characterized using an LCOE metric. For each value of solar multiple greater than unity, a value of energy storage capacity that minimizes the LCOE of electricity is found, above which any added storage capacity is poorly utilized due to restricted solar energy collection. Hence, for plants with a larger solar multiple, a larger storage capacity provides the minimum LCOE. The minimum LCOE initially decreases with increasing solar multiple, but the trend is reversed for larger solar multiple due to the added cost of heliostats. A global optimum is found for a solar multiple of 3 and a thermocline energy capacity of 16 h. For this plant design, the LCOE is 12.2 ₵/kWh, which still exceeds the cost required to achieve domestic grid parity with fossil fuels. Heliostats incur the largest plant capital cost for the CSP

Fig. 3.37  Simulated molten-salt power-tower plant performance for (a) consecutive days around solar solstice and (b) yearlong operation. Reprinted with permission from [57]. Copyright ©2013, Elsevier Ltd

124

D. Ginley et al.

plant and can be a subsystem for which cost reductions should be targeted. The developed methodology for economic analysis of CSP plants can be generally applied to different locations and electric power capacities to evaluate the viability of this technology. Alternative novel design concepts for thermocline energy storage were also assessed. The reduced-order model for TES in a thermocline tank was expanded to parametrically evaluate the inclusion of hypothetical phase-change materials of different melting temperature and heat of fusion [59]. The annual performance for a CSP plant with phase-change energy storage is presented in Fig.  3.38. A ­nonmonotonic trend in the thermal performance is observed with respect to the melting temperature of the phase-change material that replaces the solid filler inside the thermocline tank (Fig. 3.38a). Materials with low melting temperature marginally increase the performance, and their latent heat is well utilized. Materials with intermediate melting temperatures significantly worsen the performance due to an amplified effect of the deconstruction of the thermocline region inside the tank between sensible and latent heat transfer. Materials with high melting temperature also produce a marginal improvement in performance, but their latent heat is poorly utilized. Based on these observations, a cascade structure of phase-change materials having different melting temperature was proposed that offers significant gains in performance (Fig. 3.38b). However, the benefits of this cascaded design are highly sensitive to the melting temperatures, and its viability depends on developing robust and compatible phase-change materials having these properties. A comparison was also performed between the thermal performance of a dual-­ media thermocline tank with molten-salt and quartzite-rock filler material, and a single-medium thermocline tank with only molten salt [60]. Each thermocline tank design offers different benefits that were assessed in this study. The inclusion of a

Fig. 3.38  Annual CSP performance with thermocline-tank energy storage with (a) single phase-­ change material and (b) cascade structure of three phase-change materials with different melting temperature. Reprinted with permission from [59] with modifications. Copyright ©2013, Elsevier Ltd

3  Multiscale Concentrated Solar Power

125

Fig. 3.39  Instantaneous temperature and velocity field inside (a) dual-media and (b) single-­ medium thermocline tank designs. The white dashed lines represent the limits of the thermocline region. Reprinted with permission from [60]. Copyright ©2015, American Society of Mechanical Engineers

solid filler material provided additional thermal capacity at low cost. However, the single-medium thermocline tank design has a simpler design and construction, and it is not susceptible to mechanical failure by thermal ratcheting. Detailed finite-­ volume numerical simulations were performed that consider the two-dimensional heat-transfer effects of heat loss through the tank wall. Instantaneous molten-salt temperature and velocity fields for both thermocline tank designs are presented in Fig. 3.39. A thicker thermocline region developed inside the dual-media thermocline tank due to the higher thermal conductivity of the quartzite rock (Fig. 3.39a). Flow disturbances due to heat loss through the tank wall were amplified inside the single-medium thermocline tank, increasing the convective mixing (Fig.  3.39b). Both thermocline tank designs exhibited excellent thermal performance, but the trade-off between reduced thermal diffusion versus convective mixing favors the single-medium thermocline tank for the cases considered.

3.3.3  Low-Temperature ORC Storage Within the SERIIUS project, the Indian Institute of Technology Bombay investigated the latent-heat thermal energy storage for low- and medium-temperature (~200 °C) solar thermal power plants with an objective to mitigate the variability in solar radiation during the daytime. Based on the application, a commercial-grade organic phase-change material (PCM), A164, was selected, which has a melting temperature of 168.7  °C.  The heat-transfer fluid (HTF) was thermic oil-based Hytherm 600 and the container material was SS316. The PCM and HTF were experimentally characterized to determine their thermophysical properties and

126

D. Ginley et al.

evaluate the energy and exergy efficiencies of the storage. The thermal conductivity of PCM was low, which slowed the heat transfer in PCM. Therefore, three types of thermal conductivity enhancers were considered: fin [61, 62], matrix [63–67], and encapsulated PCM [68]. The flow through the HTF tubes was laminar; hence, the improvement in heat transfer between PCM and HTF was achieved by introducing fins inside the HTF tube. In the case of fin and matrix, a multitube tube-and-shell-­ based TES system was chosen, whereas encapsulated PCMs were filled in the cylindrical container in the third case. An experimental setup of multitube tubeand-shell-based TES system was developed to investigate the effects of several operating parameters, such as the effect of flow rate, inlet HTF temperature, and initial PCM temperature on the thermal performance. Further, the procedure for optimizing the TES with fins is presented, with the objective of determining the best configuration [69]. 3.3.3.1  Design and Analysis of Latent-Heat TES The preliminary design of latent-heat TES for the charging period was performed by considering one-dimensional steady-state conduction heat transfer. In designing the TES unit, the PCM was initially considered at the melting temperature, although in a solid state. In applying heat to the TES system during charging, the PCM starts melting isothermally and changes to a liquid state. During phase change, the PCM maintains the fluctuating temperature of the incoming HTF at a certain level. The cylindrical configuration was chosen as a basic configuration of the TES. In the case of shell-and-tube-based TES, the inner diameter of the HTF pipe was 10.4 mm and the outer diameter was 13.7 mm. The total number of fins inside the HTF tube was six, and the thickness of a fin was 1 mm. For a multipass of HTF pipes in TES, the length of the TES was estimated as 0.8 m for seven parallel HTF flowing tubes and the outer diameter of the shell was found to be 68 mm. Figure 3.40 shows the schematic diagram of the TES and an enlarged view of an HTF pipe with six fins.

Fig. 3.40  Schematic diagram of (a) front view and (b) side view of the latent-heat thermal energy storage with fins inside the HTF tubes (all dimensions are in mm). (Courtesy of Indian Institute of Technology Bombay)

3  Multiscale Concentrated Solar Power

127

Fig. 3.41 (a) Optimization of fin-based TES [62] and (b) effectiveness of encapsulated-PCMs-­ based TES. Reprinted with permission from [68]. Copyright ©2016, Elsevier Ltd

3.3.3.2  Numerical Simulations We developed a numerical model, coupled with the enthalpy method, to account for solid–liquid phase change in PCM, to investigate the fluid flow and heat-transfer behaviors in the three types of TES systems. Several hydrothermal factors, such as pressure drop, energy stored and released, first- and second-law efficiencies, and thermal performance index (TPI) were estimated for different operating conditions and geometrical parameters of thermal-conductivity enhancers. Figure 3.41a shows the optimization of fin-type multitube shell-and-tube-based TES [62]. We found that the thermal performance of TES with 24 fins and a fin thickness of 0.6 mm was better than any other configuration. The metal-matrix-based TES with variz  able  porosity with a relation of ε ( z ) = 0.95 −  0.125  was proposed [64]. We L   found that the volume of PCM with metal matrix can be reduced in size by 5.6% compared to that with the constant porosity (ε = 0.9) metal-matrix-based TES for the latent-heat storage capacity of 286  W.  Figure  3.41b shows the effectiveness   p,l ( Tin − Tout ( t ) )  mc ∑   tc εo =  of encapsulated-PCMs-based TES for differ p,l ( Tout ( t ) − Tin )  Er = ∑mc  td   ent HTF inlet temperatures. 3.3.3.3  Experiments A lab-scale setup of TES of multitube shell-and-tube type having latent heat capacity of ~300 W was designed and manufactured for the experimental study and is shown in Fig. 3.42a,b. It consists of latent-heat TES with and expander and nozzle, a canned motor pump for circulation of HTF, HTF reservoir, 22 electric heaters (each 500 W), a cooler, and 3-way valve. An electro-discharge machining process (EDM) was used to fabricate the fins on the inner diameter of seven HTF tubes in

128

D. Ginley et al.

Fig. 3.42 (a) Schematic diagram of the experimental setup, (b) photograph of the experimental setup, and (c) validation of the present numerical model with experiments. Reprinted with permission from [62]. Copyright ©2018, Elsevier Ltd

Fig. 3.43  Temperature contours in PCM at different time during (a) charging and (b) discharging periods [69]. (Courtesy of Indian Institute of Technology Bombay)

the TES. The expander and nozzle are in the form of a truncated cone with dimensions of 12.7 (top surface inner radius) × 50 (base inner radius) × 300 (height) mm. Figure 3.42c depicts the validation of the presently developed numerical model with the experimental results for the inlet temperature of HTF of 180 °C and HTF flow rate of 8.2 liters per minute (Lmp), which shows good agreement with a maximum error of 8 °C. Figure 3.43 shows the PCM temperature contours during charging and discharging periods for the HTF flow rate of 9.1 Lpm and the HTF inlet temperature of 200 °C. 3.3.3.4  Conclusions This work studied the design and development of PCM-based thermal storage technologies—such as fins, metal matrix, and encapsulated PCM—for medium-­ temperature solar thermal applications (~ 200 °C). A suitable PCM was selected and characterized thoroughly before use in the storage system. Three types of technolo-

3  Multiscale Concentrated Solar Power

129

gies to enhance thermal conductivity were analyzed numerically: fin-type, matrix-­ type, and encapsulated PCMs. First-law efficiency and entropy generation leading to second-law efficiency of TES systems were calculated under different conditions. A lab-scale experimental setup of multitube shell-and-tube-type TES systems was designed and manufactured to evaluate the thermal performance of the TES with different operating parameters, such as operating temperature and flow rate of HTF. The experimental results were used to validate the numerical model, and good agreement was found. The optimized design of a multitube shell-and-tube-type TES system with fins was determined. A novel TES with variable metal matrix was proposed and analyzed. The encapsulated-PCMs-based TES was found to have a high thermal-performance index with reasonable pressure drop compared with the other two types of TES.

References 1. C.K.  Ho, M.  Carlson, P.  Garg, P.  Kumar, Technoeconomic analysis of alternative solarized s-CO2 Brayton cycle configurations. J. Sol. Energy Eng. Trans. ASME 138(5), 051008 (2016) 2. C.S. Turchi, Z.W. Ma, T.W. Neises, M.J. Wagner, Thermodynamic study of advanced supercritical carbon dioxide power cycles for concentrating solar power systems. J.  Sol. Energy Eng. Trans. ASME 135(4), 041007 (2013) 3. B.D. Iverson, T.M. Conboy, J.J. Pasch, A.M. Kruizenga, Supercritical CO2 Brayton cycles for solar-thermal energy. Appl. Energy 111, 957–970 (2013) 4. S.A. Wright, P.S. Pickard, R. Fuller, R.F. Radel, M.E. Vernon, Supercritical CO2 Brayton cycle power generation development program and initial test results. ASME Power Conference, 573–583 (2009). 2009 5. C.K. Ho, B.D. Iverson, Review of high-temperature central receiver designs for concentrating solar power. Renew. Sustain. Energy Rev. 29, 835–846 (2014) 6. J.D. Ortega, A. Castellanos, S. Afrin, V. Kumar, C.K. Ho, Computational modeling of a high temperature direct tubular receiver design for supercritical CO2 Brayton cycle. Presented at ASME 2014 4th Joint US-European Fluids Engineering Division Summer Meeting, Chicago, IL, August 3–7, 2014 (2014) 7. J.M. Christian, J.D. Ortega, C.K. Ho, Novel tubular receiver panel configurations for increased efficiency of high-temperature solar receivers, in ASME Proceedings of the 9th International Conference on Energy Sustainability, San Diego, CA, June 28–July 2, 2015 (2015) 8. J.M. Christian, J.D. Ortega, C.K. Ho, J. Yellowhair, Design and modeling of light-trapping tubular receiver panels, in Proceedings of the ASME 2016 Power and Energy Conference, PowerEnergy2016-59158, Charlotte, North Carolina, June 26–30, 2016 (2016) 9. C.K. Ho, J.M. Christian, J.D. Ortega, J. Yellowhair, M.J. Mosquera, C.E. Andraka, Reduction of radiative heat losses for solar thermal receivers, in Proceedings of the SPIE Optics+Photonics Solar Energy+Technology High and Low Concentrator Systems for Solar Energy Applications IX, San Diego, August 17–21, 2014 (2014) 10. J.D.  Ortega, J.M.  Christian, C.K.  Ho, Design and testing of a novel bladed receiver, in Proceedings of the ASME 11th International Conference on Energy Sustainability (2017) 11. J. Ortega, S. Khivsara, J. Christian, C. Ho, P. Dutta, Coupled modeling of a directly heated tubular solar receiver for supercritical carbon dioxide Brayton cycle: Structural and creep-­ fatigue evaluation. Appl. Therm. Eng. 109(B), 979–987 (2016). https://doi.org/10.1016/j. applthermaleng.2016.06.031 12. J. Ortega, S. Khivsara, J. Christian, C. Ho, J. Yellowhair, Coupled modeling of a directly heated tubular solar receiver for supercritical carbon dioxide Brayton cycle: Optical and thermal-­

130

D. Ginley et al.

fluid evaluation. Appl. Therm. Eng. 109(B), 970–978 (2016). https://doi.org/10.1016/j. applthermaleng.2016.05.178 13. P. Garg, P. Kumar, K. Srinivasan, Supercritical carbon dioxide Brayton cycle for concentrated solar power. J. Supercrit. Fluids 76, 54–60 (2013) 14. E.G. Feher, The supercritical thermodynamic power cycle, in Proceedings of the Intersociety Energy Conversion Engineering Conference, Douglas Paper No. 4348 (1967) 15. E.G.  Feher, Investigation of supercritical cycle. AstroPower laboratory, Missile & Space Systems Division, Technical Report AFAPL-TR-68-100, AD843063 (1968) 16. V.  Dostal, P.  Hejzlar, M.J.  Driscoll, N.E.  Todreas, A supercritical CO2 Brayton cycle for advanced reactor applications. Trans. Am. Nucl. Soc. 85, 110–111 (2001) 17. V.  Dostal, A supercritical carbon dioxide cycle for next generation nuclear reactors. Ph.D. thesis, MIT-ANP-TR-100 (2004) 18. J.D.  Ortega, J.M.  Christian, J.E.  Yellowhair, C.K.  Ho, Coupled Optical-Thermal-Fluid Modeling of Novel Light-Trapping Tubular Panels for Concentrating Solar Power Receivers, in Proceedings of the 9th International Conference on Energy Sustainability (ES2015), San Diego, CA, June 28–July 2, 2015 (2015) 19. A.  Dan, H.C.  Barshilia, K.  Chattopadhyay, B.  Basu, Solar energy absorption mediated by surface plasma polaritons in spectrally selective dielectric-metal-dielectric coatings: A critical review. Renew. Sust. Energ. Rev. 79, 1050–1077 (2017) 20. C.E.  Kennedy, Review of Mid- to High-Temperature Solar Selective Absorber Materials. NREL/TP-520-31267, National Renewable Energy Laboratory, Golden, Colorado (2002) 21. A. Dan, K. Chattopadhyay, H.C. Barshilia, B. Basu, Shifting of the absorption edge in TiB2/ TiB(N)/Si3N4 solar selective coating for enhanced photothermal conversion. Sol. Energy 173, 192–200 (2018) 22. A.  Dan, A.  Biswas, P.  Sarkar, S.  Kashyap, K.  Chattopadhyay, H.C.  Barshilia, B.  Basu, Enhancing spectrally selective response of W/WAlN/WAlON/Al2O3–based nanostructured multilayer absorber coating through graded optical constants. Sol. Energy Mater. Sol. Cells 176, 157–166 (2018) 23. A. Dan, K. Chattopadhyay, H.C. Barshilia, B. Basu, Angular solar absorptance and thermal stability of W/WAlN/WAlON/Al2O3-based solar selective absorber coating. Appl. Therm. Eng. 109, 997–1002 (2016) 24. A.  Dan, K.  Chattopadhyay, H.C.  Barshilia, B.  Basu, Thermal stability of WAlN/WAlON/ Al2O3-based solar selective absorber coating. MRS Advances 1(41), 2807–2813 (2016) 25. A. Dan, B. Basu, T. Echániz, I.G. de Arrieta, G.A. López, H.C. Barshilia, Effects of environmental and operational variability on the spectrally selective properties of W/WAlN/WAlON/ Al2O3-based solar absorber coating. Sol. Energy Mater. Sol. Cells 185, 342–350 (2018) 26. N. Selvakumar, H.C. Barshilia, Review of physical vapor deposited (PVD) spectrally selective coatings for mid- and high-temperature solar thermal applications. Sol. Energy Mater. Sol. Cells 98, 1–23 (2012) 27. D.L. McDowell, S.R. Kalidindi, The materials innovation ecosystem: A key enabler for the materials genome initiative. MRS Bull. 41, 326–337 (2016) 28. Q. Yan, J. Yu, S.K. Suram, L. Zhou, A. Shinde, P.F. Newhouse, W. Chen, G. Li, K.A. Persson, J.M.  Gregoire, Solar fuels photoanode materials discovery by integrating high-throughput theory and experiment. Proc. Natl. Acad. Sci. 114, 3040–3043 (2017) 29. M.L. Green, C.L. Choi, J.R. Hattrick-Simpers, A.M. Joshi, I. Takeuchi, S.C. Barron, E. Campo, T. Chiang, S. Empedocles, J.M. Gregoire, Fulfilling the promise of the materials genome initiative with high-throughput experimental methodologies. Appl. Phys. Rev. 4, 011105 (2017) 30. A. Yosipof, O.E. Nahum, A.Y. Anderson, H.N. Barad, A. Zaban, H. Senderowitz, Data mining and machine learning tools for combinatorial material science of all-oxide photovoltaic cells. Mol. Inform. 34, 367–379 (2015) 31. P. Dutta, High temperature solar receiver and thermal storage systems. Appl. Thermal Eng. 124, 624–632 (2017) 32. O. Srikanth, S. Khivsara, R. Aswathi, C.D. Madhusoodana, Numerical and experimental evaluation of ceramic honeycombs for thermal energy storage. Transact. Indian Cer. Soc. 76(2), 1–6 (2017)

3  Multiscale Concentrated Solar Power

131

33. P.  Garg, P.  Kumar, P.  Dutta, T.  Conboy, C.  Ho, Design of an experimental test facility for supercritical CO2 Brayton cycle. Presented at ASME 8th International Conference on Energy Sustainability, June 30–July 2, 2014, Boston, MA (2014) 34. O.P. Agnihotri, B.K. Gupta, Solar Selective Surfaces, 1st edn. (Wiley-Interscience Publication, New York, 1981) 35. C.E. Kennedy, Review of mid- to high-temperature solar selective absorber materials. NREL tech report, NREL/TP-520-31267 (2002) 36. K.R. Newby, Functional chromium plating. Met. Finish. 97, 223–247 (1999) 37. V. Moise, R. Cloots, A. Rulmont, Study of the electrochemical synthesis of selective black coatings absorbing solar energy. Int. J. Inorg. Mater. 3, 1323–1329 (2001) 38. K.D. Lee, Preparation and characterization of black chrome solar selective coatings. J. Korean Phys. Soc. 51, 135–144 (2007) 39. Z. Zeng, A. Liang, J. Zhang, A review of recent patents on trivalent chromium plating. Recent Patents Mater. Sci. 2, 50–57 (2009) 40. M. Shiva Prasad, K. Chandra Sekhar Reddy, S. Sakthivel, Development of cost efficient solar receiver tube with a novel tandem absorber system. Appl. Therm. Eng. 109, 988–996 (2016) 41. S. Sakthivel, M. Shiva Prasad, B. Mallikarjun, S.V. Joshi, An improved performance of nanocomposite oxide selective absorber coating with excellent optical and thermal resistant properties and method of manufacturing the same. Indian Patent Application No.1111/DEL/2015 (2015) 42. A. Srinivasa Rao, S. Sakthivel, A highly thermally stable Mn–Cu–Fe composite oxide based solar selective absorber layer with low thermal loss at high temperature. J. Alloys Compd. 644, 906–915 (2015) 43. F.G.F. Qin, X. Yang, Z. Ding, Y. Zuo, Y. Shao, R. Jiang, Thermocline stability criterions in single-tanks of molten salt thermal energy storage. Appl. Energy 97, 816–821 (2012) 44. Z. Yang, Thermal analysis of solar thermal energy storage in a molten-salt thermocline. Sol. Energy 84, 974–985 (2010) 45. T.  Bauer, N.  Breidenbach, M.  Eck, Overview of molten salt storage systems and material development for solar thermal power plants. World Renewable Energy Forum 2012, 1–8 (2012) 46. K.V. Manu, P. Anand, U.K. Chetia, S. Basu, Effects of instabilities and coherent structures on the performance of a thermocline based thermal energy storage. Appl. Therm. Eng. 87, 768–778 (2015) 47. S. Advaith, K.V. Manu, A. Tinaikar, U.K. Chetia, S. Basu, Interaction of vortex ring with a stratified finite thickness interface. Phys. Fluids 29(9), 093602 (2017) 48. S. Advaith, A. Punia, U. Kumar Chetia, N. Dani, K. Chattopadhyay, S. Basu, Stability Analysis of an Oil Based Thermocline Energy Storage System, in Proceedings of the International Conference on Sustainable Energy and Environmental Challenges (SEEC) (2018) 49. A. Tinaikar, S. Advaith, U.K. Chetia, K.V. Manu, S. Basu, Spatio-temporal disruption of thermocline by successive laminar vortex pairs in a single tank thermal energy storage. Appl. Therm. Eng. 109, 924–935 (2016) 50. P. Deepu, S. Dawande, S. Basu, Instabilities in a fluid overlying an inclined anisotropic and inhomogeneous porous layer. J. Fluid Mech. 762, R2 (2015) 51. S. Hatte et al., Short and long-term sensitivity of lab-scale thermocline based thermal storage to flow disturbances. Appl. Therm. Eng. 109, 936–948 (2016) 52. S.V.  Garimella, Review of molten-salt thermocline tank modeling for solar thermal energy storage. Heat Transf. Eng. 34(10), 787–800 (2013) 53. P.K. Madathil et al., Preparation and characterization of molten salt based nanothermic fluids with enhanced thermal properties for solar thermal applications. Appl. Therm. Eng. 109, 901–905 (2016) 54. K.M. Pramod, P.V.C. Rao, N.V. Choudary, K. Ramesh, Novel methodology to prepare homogenous ternary molten salts for concentrated solar power applications and their thermo-physical characterization. Appl. Therm. Eng. 109, 906–910 (2016)

132

D. Ginley et al.

55. K.  Ramesh, K.M.  Pramod, P.V.C.  Rao, N.V.  Choudary, A Process for Preparation of Homogenous Mixture for Thermal Storage and Heat Transfer Applications. Google Patents, 2017 56. S.M. Flueckiger, Z. Yang, S.V. Garimella, Review of molten-salt thermocline tank modeling for solar thermal energy storage. Heat Transf. Eng. 34, 787–800 (2013) 57. S.M.  Flueckiger, B.D.  Iverson, S.V.  Garimella, J.E.  Pacheco, System-level simulation of a solar power tower plant with thermocline thermal energy storage. Appl. Energy 113, 86–96 (2014) 58. S.M.  Flueckiger, B.D.  Iverson, S.V.  Garimella, Economic optimization of a concentrating solar power plant with molten-salt thermocline storage. J. Sol. Energy Eng. 136(1), 011015 (2013) 59. S.M. Flueckiger, S.V. Garimella, Latent heat augmentation of thermocline energy storage for concentrating solar power – a system-level assessment. Appl. Energy 116, 278–287 (2014) 60. C.  Mira-Hernández, S.M.  Flueckiger, S.V.  Garimella, Comparative analysis of single- and dual-media thermocline tanks for thermal energy storage in concentrating solar power plants. J. Sol. Energy Eng. 137(3), 031012 (2015) 61. M. Prabhakar, S. Singh, S.K. Saha, Heat transfer characterization of latent heat thermal energy storage system using fins in solar thermal power plant. Presented at Third Southern African Solar Energy conference, SASEC 2015 11−13, 2015 (2015) 62. K. Bhagat, M. Prabhakar, S.K. Saha, Estimation of thermal performance and design optimization of finned multitube latent heat thermal energy storage. J.  Energy Storage 19, 135–144 (2018). https://doi.org/10.1016/j.est.2018.06.014 63. A. Kumar, S.K. Saha, Energy and exergy analyses of medium temperature latent heat thermal storage with high porosity metal matrix. Appl. Therm. Eng. 109(B), 911–923 (2016). https:// doi.org/10.1016/j.applthermaleng.2016.04.161 64. A. Kumar, S.K. Saha, Latent heat thermal storage with variable porosity metal matrix: A numerical study. Renew. Energy 125, 962–973 (2018). https://doi.org/10.1016/j.renene.2018.03.030 65. A. Kumar, S.K. Saha, Second law analysis of latent heat thermal storage for ORC-based solar thermal power plant. Presented at International Conference on Advances in Energy Research, December 15–17, 2015, IIT Bombay (2015) 66. S. Halder, S. Singh, S.K. Saha, Numerical simulation of latent heat thermal storage using metal matrix and foam for solar thermal power plant. Presented at third southern African solar energy conference, SASEC 2015, May 11−13, 2015 (2015) 67. R. Shah, S.K. Saha, Thermal performance of multitube latent heat thermal storage using metal matrix for solar applications: A numerical study. Heat Transf. Res. 50, 545 (2018). https://doi. org/10.1615/HeatTransRes.2018024568 68. K. Bhagat, S.K. Saha, Numerical analysis of latent heat thermal energy storage using encapsulated phase change material for solar thermal power plant. Renew. Energy 95, 323–336 (2016). https://doi.org/10.1016/j.renene.2016.04.018 69. A. Kumar, P. Shahi, S.K. Saha, Experimental study of latent heat thermal energy storage system for medium temperature solar applications. In Proceedings of the 4th World Congress on Mechanical, Chemical, and Material Engineering (MCM'18), Madrid, Spain, Paper No. HTFF 152, August 16–18, 2018 (2018)

Chapter 4

Solar Energy Integration David Ginley, Suhas Bannur, Mridula Dixit Bharadwaj, Aimee Curtwright, Vaishalee Dash, Rafiq Dossani, G. Srilakshmi, Praveen Kumar, Zhimin Mao, N. C. Thirumalai, Shanthi Nataraj, Oluwatobi Oluwatola, Badri S. Rao, Costa Samaras, Harshid Sridhar, Sara Turner, Bhupesh Verma, Henry Willis, and Rushil Zutshi

Both India and the United States have set ambitious solar energy targets as part of their clean-energy missions. This implies that massive deployment of solar energy technologies will occur at both grid and off-grid scales. To enable this transition and to identify best-possible pathways for adopting solar power, the Integration and Energy Storage project (SEI-3) undertook crucial studies to address the challenges associated with solar integration. The following sections discuss some of the key studies undertaken.

D. Ginley (*) National Renewable Energy Laboratory, Golden, CO, USA e-mail: [email protected] S. Bannur · M. D. Bharadwaj · V. Dash · G. Srilakshmi · P. Kumar · N. C. Thirumalai B. S. Rao · H. Sridhar · B. Verma Center for Study of Science, Technology and Policy, Bengaluru, Karnataka, India e-mail: [email protected]; [email protected]; [email protected]; [email protected]; [email protected] A. Curtwright · H. Willis RAND Corporation, Pittsburgh, PA, USA e-mail: [email protected]; [email protected] R. Dossani · Z. Mao · S. Nataraj · O. Oluwatola · S. Turner · R. Zutshi RAND Corporation, Santa Monica, CA, USA e-mail: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; [email protected] C. Samaras Civil and Environmental Engineering, Carnegie Mellon University, Pittsburgh, PA, USA e-mail: [email protected] © Springer Nature Switzerland AG 2020 D. Ginley, K. Chattopadhyay (eds.), Solar Energy Research Institute for India and the United States (SERIIUS), Lecture Notes in Energy 39, https://doi.org/10.1007/978-3-030-33184-9_4

133

134

D. Ginley et al.

4.1  Resource Assessment and Technology Roadmap India’s electricity demand is estimated to increase from about one trillion kilowatt-­ hours (kWh) in 2015–2016 to roughly three trillion kWh in 2030 [1]. This huge increase in demand requires significant planning with respect to adding adequate generation capacity well ahead of time. Conventional thermal plants, in addition to being highly polluting, are also subject to fluctuating coal prices and water shortages. The comparatively clean hydro plants are constrained by geography, and also, they have several social and ecological barriers to widespread adoption. To address these concerns, both the United States and India want to significantly reduce the price of solar-generated electricity as part of their overall energy-generation mix. This led to the formation of the U.S. SunShot Initiative and India’s National Solar Mission. To achieve the targets set by these ambitious plans, it was crucial to conduct a solar resource assessment and develop the necessary technology and policy roadmaps. In this context, the Solar Energy Integration (SEI) team led by the Center for Study of Science, Technology and Policy (CSTEP) (India) and RAND Corporation (USA) conducted various analyses to show pathways to increase the adoption of solar energy. CSTEP developed a framework for solar resource assessment to determine the net available potential (in megawatts) for solar energy deployment in the country. A block schematic of the framework is shown in Fig. 4.1. An overlap of land and solar resource data was used for a geographical information systems (GIS) analysis to arrive at total potential numbers. A case study for the state of Karnataka was also presented in the report [2]. Suitable wastelands in Karnataka were i­ dentified Crystalline

PV

Parabolic Trough

Technology

Thin Film

CSP

Annual Average DNI/GHI

Solar to Energy Conversion Efficiency

Solar Tower Linear Fresnel

Methodology for Potential Estimation

Capacity Factor

Available Suitable Area

Annual Energy

Potential Per Unit Area

Total Potential (MW)

Fig. 4.1  Methodology to estimate solar potential in Karnataka. (Courtesy of Center for Study of Science, Technology and Policy (CSTEP))

4  Solar Energy Integration

135

based on their proximity to the nearest substation, road and rail networks, and water bodies. The study included a comprehensive review of policies pertaining to adoption or deployment of solar power (for both grid and off-grid applications) in India. As part of the study, techno-economic assessments were also carried out for large ground-­ mounted solar plants, rooftop solar, and solar process heating in industries. The study emphasized the significant potential to reduce emissions offered by solar process heating in industries. In addition, the absence of adequate policies to promote such applications and possible ways to address this were discussed in the study. For example, the need was highlighted to incentivize rooftop PV installation with schemes such as feed-in tariffs and renewable energy certificates.

4.1.1  Site Options for Solar Deployment Solar PV plant siting requires extensive planning and close-to-accurate prediction of solar power output. Carnegie Mellon University (CMU) and CSTEP jointly undertook a performance comparison study to evaluate the efficacy of existing PV modeling approaches that help in plant siting or pre-feasibility studies. The objective of this comparison was to determine the most-fitting approach that provides close-to-actual generation values. This was critical in view of both the U.S. SunShot Initiative (to reduce solar costs) and India’s National Solar Mission (with 100 GW solar energy targets). The study compared results from a detailed power-plant model using local weather data and plant details with other models such as the Bird’s Clear Sky model and the National Solar Radiation Database (NSRDB)—a satellite irradiance model and CSTEM [3] (mode developed in-house). The PV generation data from four solar plants in Gujarat were used for comparison with the modeled PV generation. Figure 4.2 shows the actual (observed) PV generation and outputs from four PV models. The difference between the modeling approaches can be seen both on a relatively clear day (left) and a cloudy day (right). Models 1, 2, 3, and 4 represent

Fig. 4.2  Results of PV model comparison with observed generation for a sample PV plant. (Courtesy of Center for Study of Science, Technology and Policy (CSTEP))

136

D. Ginley et al.

the outputs from Bird’s Clear Sky index, scaled NSRDB gridded satellite data, scaled SRRA (India) local solar data, and a detailed power-plant model. The study showed that using a detailed power-plant model, which incorporates local—the data were not 100% spatially coincident—solar resource and temperature data, provides a better match with actual PV power output than the other methods. However, in the absence of coincident weather data, the NSRDB satellite data can be the next-best alternative for siting analysis.

4.1.2  Tech-to-Market Analysis of SERIIUS Technologies Crystalline silicon (c-Si) and thin film are the two popular types of PV technologies. Globally, c-Si dominates the market and is used in more than 90% of all installations. In India, too, c-Si has played a dominant role, being used in more than 70% of total installations [4]. India has a reliable manufacturing base for modules, whereas other upstream supply-chain components (polysilicon/ingot/wafer) are missing. The current module manufacturing capacity stands at ~8400 MW per annum, which is sufficient to cater to the domestic demand [5]. However, the capacity utilization is below par [4, 5], which is due to a high cost difference (~10–12% higher) between Indian and imported modules. Domestic modules have turned price incompetent because of various reasons: lack of economies of scale, lack of domestic upstream sector, limited balance-of-module supply chain, and inefficient manufacturing processes. Therefore, the study analyzed the impact of policy incentives provided at state levels on module manufacturing costs. Several important states were identified based on their solar potential (from Ministry of New and Renewable Energy [MNRE] data), relative ease of doing business, installed solar PV capacity, and level of activity in promoting solar. Subsequently, we evaluated the policies in each state, such as capital and interest rate subsidies, tax breaks, and other incentives and exemptions. Next, the study detailed the essentials of a module manufacturing facility, which was modeled and analyzed. A financial model, given the current policy scenario, was also created to evaluate the situation of module manufacturing in India. The framework in Fig. 4.3 was followed for state selection and financial assessment of module manufacturing in the select states. Table 4.1 lists the input parameters assumed for the financial model. Manufacturing costs were calculated based on the above-mentioned assumptions. The biggest cost component in module manufacturing is raw material cost, which makes up about 90% of the cost share. Subsequently, cost related to interest rate is the second largest cost component, especially interest on working capital. Figure 4.4 shows the share of various cost components in a module. After estimating that raw materials (80–90%) and the working capital (12–15%) comprise the majority of manufacturing costs, this financial analysis suggested that

4  Solar Energy Integration

Identification of top 10 states

Analysis of fiscal incentives/subsidies

137

• Ease of doing business • Solar PV installed generation capacity • Solar resource potential by MNRE • Capital subsidy • Interest rate subsidy • Taxes • Other exemptions/subsidies

Module manufacturing facility

• Processes, raw material, energy, and other requirements

Financial model

• Estimation of indigenous cost of module manufacturing (with current incentives)

Sensitivity analysis

• Impact of various inputs on financial viability • Effects of fiscal incentives and policies on viability outcomes • Suggest new incentive policies

Fig. 4.3  Framework for assessing module manufacturing in select states in India. (Courtesy of Center for Study of Science, Technology and Policy (CSTEP))

Table 4.1  Assumptions for financial modeling Plant Type Capacity Construction period Life of plant Plant and machinery cost Debt-to-equity ratio Electricity cost Labor Land required Income tax rate a

Semiautomatic Module Manufacturing Multicrystalline Silicon PV 200 MWp/annum 9 months 15 years INRa 2/Wp 70:30 INR 0.275/Wp INR 0.50/Wp 1 acre 34%

INR = Indian Rupee

incentives given on capital investment were not very helpful in lowering the cost of manufacturing. This analysis identified major challenges that need to be addressed to make module manufacturing competitive in India:

138

D. Ginley et al. Raw material 0.86%

Energy

1.56%

Labor O&M Insurance Costs

89.76%

1.26%

Interest on Term Loans

5.56%

Interest on Working Capital

0.83%

Depreciation

Fig. 4.4  Cost share of various components in a module. (Courtesy of Center for Study of Science, Technology and Policy (CSTEP))

• Higher working-capital (WC) needs are the hallmark of module manufacturing. Non-availability of WC makes it difficult to compete against firms from China/ South East Asia, who offer better terms. • Interest rate is one of the major cost components affecting the cost of module manufacturing. The current interest rates are in the range of 12–15%, which is high. • Higher inventory levels of raw materials and finished modules raise the manufacturing costs. • Falling prices also make inventories extremely costly. • A low utilization factor increases the cost of manufacturing. A sensitivity analysis was performed to see the impact of above-mentioned challenges on the final manufacturing cost of a module. The analysis indicated that a high capacity factor and low interest rates help to reduce manufacturing costs by 4.57% and 3.22%, respectively. Table 4.2 shows the impact of these parameters on the cost of module manufacturing. Further, a study was performed to determine optimal requirements (e.g., of plant size, manufacturing processes, raw material procurement, financial incentives) to attain economies of scale and become price competitive for various supply-chain components (polysilicon, wafer, cell, and module). The analysis was based on the conditions of a stand-alone and a vertically integrated module manufacturing plant.

4  Solar Energy Integration

139

Table 4.2  Module manufacturing scenarios

Scenarios Base case Goods and service tax 5% 18% Low inventory and high capacity utilization factors High inventory and low capacity utilization factors Low interest rate Lowest interest rate

Table 4.3  Assumptions used in manufacturing-cost estimations for a vertically integrated manufacturing facility

Module Price (INR/Wp) 31.97 33.45 37.30 30.51

Module Price Capital Cost LCOE (INR/Wp) (Crore INR/MW) (INR/kWh) – 5.89 5.65 % Change in values over base case above −4.63% −2.57% −2.68% −16.67% −9.09% −10.20% 4.57% 2.40% 2.86%

33.95

−6.19%

−3.43%

−3.76%

31.23 30.94

2.31% 3.22%

1.20% 1.72%

1.43% 1.97%

Category Manufacturing capacity Land required Capital cost O&M costs Energy costs Interest rate Return on equity Debt to equity Life of plant Capacity utilization

Amount 2000 MW/annum 400 acres ~INR 9500 crore 1% of capital cost 3.6 INR/kWh 9% 13% 70:30 15 years 90%

4.1.2.1  Feasibility Analysis of c-Si PV Manufacturing in India The financial model was used to perform a feasibility analysis for c-Si PV module manufacturing in a stand-alone facility (polysilicon, wafer, cell, and module) and in a vertically integrated facility in India. Based on extensive literature review and discussions with industry experts, capacity, costs, and other parameters were chosen for the analysis. In the case of stand-alone facilities, the manufacturing costs for polysilicon, wafer, cell, and module were calculated to be around INR 823/kg, INR 8.50/Wp, INR 14.96/Wp, and INR 20.94/Wp, respectively. In the vertically integrated manufacturing facility, the estimated module manufacturing cost was INR 18.94/Wp. The assumptions made are given in Table 4.3, and the cost share is shown in Fig. 4.5.

140

D. Ginley et al.

1% 3% 1%

11% Raw material Energy

2%

Other Materials Labor

12%

O&M, Insurance S&G, Overheads, R&D

1% 2% 67%

Interest Term Loan

Fig. 4.5  Cost share of various components for a vertically integrated manufacturing facility

Depreciation

32.0

Interest Working Capital

28.0

INR/Wp

24.0

23.0

Interest Term Loan S&G,Overheads, R&D

20.9 18.9

20.0

17.6

16.0

O&M, Insurance Labor Energy

12.0

Raw Material Total

8.0 4.0 0.0

DCR

SA

VI

Chinese

Fig. 4.6  Cost comparison of domestic modules under various scenarios. (Courtesy of Center for Study of Science, Technology and Policy (CSTEP))

An overall reduction of INR 2/Wp was seen in a module manufactured in a vertically integrated plant (compared to one manufactured in a stand-alone plant). (Courtesy of Center for Study of Science, Technology and Policy (CSTEP)). Three different cases were analyzed to understand the impact of various parameters on manufacturing costs. These cases examined the cost of modules under Domestic Content Requirement (DCR) in stand-alone and vertically integrated facilities, and the costs are further compared with the cost of Chinese modules. Figure 4.6 provides the cost comparison of these cases.

4  Solar Energy Integration

141

4.1.3  Policy Analysis for Solar Deployment The objective of this study was to identify the most suitable consumer category for rooftop solar and to calculate the gap between the existing and viable tariffs for rooftop solar. Industrial and commercial rooftops were found to have lucrative business cases; viable business cases were also observed for domestic consumers with a large rooftop area and low self-consumption. Off-grid solar rooftop systems were not viable because of higher costs of storage [6].  This study examined the techno-economics of a rooftop photovoltaic (RTPV) system in Bengaluru (Karnataka), considering the existing policy framework [7]. The approach involved field visits to some of the identified locations. Monthly and annual electricity bills paid to Bangalore Electricity Supply Company Limited (BESCOM), the local electricity distribution company, were obtained and analyzed to identify the electrical energy demand. The daily load profiles were scrutinized to identify periods of peak demand. Monthly diesel consumption and capacity of diesel generators (for backup power) were obtained. Due to unreliable power supply, diesel generators had to be operated for about 2 h a day. The price at which industries purchase electricity from BESCOM is in the range of INR 4.5–5.5/kWh. The residential areas in this study were found to operate diesel generators for around 1 h a day. The price paid by residential customers to BESCOM is in the range of INR 2.2–5.5/kWh. The capacity and type of RTPV systems to be installed on each rooftop were calculated. Each of the case studies was analyzed in terms of financial feasibility for the consumer as well as the financial implications for the utility or the state/central government institution. The important conclusions from this study are mentioned below: • The most profitable of all business cases in this analysis was an RTPV system supplying power to a private industry. • The economics of an RTPV system with battery, in terms of payback period and internal rate of return (IRR), are not viable. • An upper limit of 75% of rated load does not make the business case viable for RTPV. • Off-grid RTPV systems are still expensive, and proper microfinancing models are needed to make these systems viable.

4.1.4  D  eveloping an Approach to Feed Analysis Back to Technology Decisions Under SEI, a suite of tools was developed for resource assessment and techno-­ economic assessment for solar deployment. Figure 4.7 shows the schematic of the tools and the sequence of tool usage for decision-making. The RE Atlas tool from CSTEP [8] can help to identify a suitable location for solar deployment, whereas CSTEP’s Solar Techno Economic Model

142

D. Ginley et al.

RE Technology

RE Atlas Tool

CSTEM Tool

PV/CSP

Suitable Location

Performance & Financial Viability

Policy & Technology Roadmap

Fig. 4.7  Schematic of tools and the corresponding decision-making areas. (Courtesy of Center for Study of Science, Technology and Policy (CSTEP))

(CSTEM) and NREL’s System Advisory Model (SAM) for India tools can help in the techno-economic assessment of the selected solar technology. The combination of RE Atlas and CSTEM tools can help to perform a pre-feasibility analysis, which will help in framing the policy roadmap and the technology roadmap. Researchers can see the impact of improvement via technological innovation on overall cost, whereas policymakers and industry stakeholders can identify the suitable policy interventions as well as financial incentives to enhance the economic viability of the solar technologies. These tools can help explore the economic feasibility of the selected technology or major bottleneck in commercial viability.

4.2  Computational Tools for Techno-Economic Assessment of Solar PV and CSP Technologies One of the important tasks of the SEI thrust was to develop a suite of computational tools for pre-feasibility/potential assessment and to determine financial viability of various solar technology options for grid and select off-grid applications. In this pursuit, CSTEP developed the CSTEM tool for CSP [3] and for solar PV technologies (monocrystalline, multicrystalline, and thin film). Also, the well-established SAM developed by NREL was modified to cater to Indian criteria with collaborative efforts between CSTEP, RAND, and NREL. CSTEM is an open-access computational tool that can facilitate analyses of grid-­ connected solar power plants. The technologies covered by the tool are CSP and PV. The tool can be used to estimate the performance of a solar power plant and the cost of electrical energy generation. This tool blends technology-centered engineering analysis with financial models. The technical model has been developed as per scientific and industrial literature. The financial model is based on the norms specified by the Central Electricity Regulatory Commission (CERC) [9] of India. However, there is a provision for user-specified or customized inputs in both models. CSTEM facilitates analyses such as sensitivity studies, risk assessment, economic viability analyses, and comparison of different plant configurations in pursuit of building competitiveness with traditional energy sources. CSTEM can serve as a benchmark for optimizing (in terms of installed capacity and land-area coverage)

4  Solar Energy Integration

143

the size of plants to be established in a given location. On a broader perspective, this model can serve as a tool to aid in setting realistic targets in a planned fashion to realize the long-term national renewable energy goals. CSTEM is designed to cater to a broad audience—ranging from policy analysts and researchers to technology developers and investors. It considers an engineering-­ based approach to capture the technical details of a prospective plant that considers the interest of technology developers and researchers. By offering the twin perspectives of potential energy and cost metrics, it can serve as a tool for policy analysts and investors in their decision-making process. CSTEM and SAM for India tools are available for download at: http://www. seriius.org/modeling.html.

4.2.1  S  AM for India: A Tool for Understanding Solar Deployment Barriers and Costs The System Advisor Model (SAM) developed by NREL was modified to take into account Indian criteria via a collaborative effort among CSTEP, RAND, and NREL [10]. “SAM for India” built on the existing, well-known SAM platform and provided a practical financial modeling tool for the Indian context. The SAM for India tool enables researchers, policymakers, and solar project developers in India to understand site-specific costs for solar technology deployment. The tool can now be used to model tradeoffs in R&D investments, technology choices, and policy scenarios. The tool also provides users the ability to conduct stochastic and parametric assessments of power generation and cost, includes broad representation of technologies and financing models, and was developed in a way that allows users to develop custom calculations within its structure. Created in consultation with Indian project developers, the extended SAM tool is intended as a baseline product that can be expanded in the future to continue to represent Indian user community needs.

4.2.2  CSTEM for PV and CSP 4.2.2.1  CSTEM-PV and RTPV Tool This is an open-access tool, built on publicly accessible data, that can estimate the performance of utility-scale and rooftop-based solar photovoltaic systems. Figure  4.8 presents an infographic showing the features of CSTEM-PV and RTPV. The financial model was built based on the regulatory norms established by CERC. The tool was built in a modular fashion such that it can incorporate technological and regulatory interventions. Researchers and policymakers alike can use the tool to perform analyses such as:

144

D. Ginley et al.

Fig. 4.8  Features of CSTEM-PV tool. (Courtesy of Center for Study of Science, Technology and Policy (CSTEP))

• Performance comparison of a particular solar PV module technology across different locations. • The effect of module degradation rates and cleaning cycle on a plant’s financial viability. • Performance comparison of different module options at a chosen location. • Comparison of various tariff schemes for RTPV systems. In collaboration with the Indian Institute of Technology Bombay team, the tool was used to assess the effect of module degradation rates and cleaning cycle via soiling losses on the plant’s financial viability in terms of the LCOE. 4.2.2.2  CSTEM-CSP Tool This tool allows analysis of parabolic trough (PT) and solar tower (ST) systems. It facilitates analyses such as sensitivity studies, risk assessment, economic viability analysis, and comparison of different plant configurations in pursuit of building competitiveness with traditional energy sources. The PT component of CSTEM-­ CSP allows the stakeholder to perform a techno-economic analysis for various locations in India having suitable solar radiation. An infographic showing the features of CSTEM-CSP is presented in Fig. 4.9. CSTEM-PT was used to perform an analysis for a plant with a capacity of 50 MWe located in Jodhpur. The influence of thermal storage and hybridization on

4  Solar Energy Integration

145

Fig. 4.9  Features of CSTEM-CSP tool. (Courtesy of Center for Study of Science, Technology and Policy (CSTEP))

Fig. 4.10  Variations in capacity factor with solar multiple for a thermal storage of 6 h. (Courtesy of Center for Study of Science, Technology and Policy (CSTEP))

the performance of the plant was examined. As seen from Fig. 4.10, the capacity factor increased with solar multiple (SM) and also with hybridization. CSTEM-ST is a part of CSTEM-CSP; this tool can be used to analyze technical and economic performance for a given plant capacity and location. The salient features in the technical model of CSTEM-ST are given below: • A detailed engineering model for the reflected image from the heliostat was developed for CSTEM Solar Tower [3].

146

D. Ginley et al.

Tower Height (m)

250

fhb= 0

ts=0, SM=1.4 ts=6, SM=2.1

200

ts=15, SM=3.2

150 100 50 0

0

10

20

30

40

50

Capacity (MW) Fig. 4.11  Variation of tower height with capacity at optimum solar multiple (SM). (Courtesy of Center for Study of Science, Technology and Policy (CSTEP))

• A detailed model for assessing the annual efficiency factor of the heliostat field was developed considering an external cylindrical receiver. This factor is based on the cosine losses, shadowing, and blocking effects of each heliostat. • For any heliostat size and position (with respect to the tower), the model can determine the hourly, daily, and annual field efficiencies. The CSTEM-ST tool was used to perform case studies and sensitivity analyses for different plant configurations and locations. This was done to understand and identify parameters that drastically impact the performance and costs. Tower height is one such important parameter, and its variation with SM is shown in Fig. 4.11. Tower height also increases with increasing capacity and optimal SM (for different thermal energy storage). This is expected because the field size increases; hence, to capture all the reflected solar radiation, the tower height must increase.

4.3  Solar Energy Integration Assessment The Integration and Energy Storage (SEI-3) objective can be broadly classified into two categories. The first was to understand the issues associated with grid integration of solar and quantify them as much as possible. Analyzing these issues in detail—from both technical and policy perspectives—would provide options to tackle them and suggest appropriate roadmaps for increasing grid penetration of solar. The second category was to help or enable adoption of solar through flexible resources such as energy storage technologies. In addition, SEI-3 also focused on evaluating options for providing power through off-grid solar technology.

4  Solar Energy Integration

147

The three tasks are briefly described below. Task 1 mainly dealt with understanding issues relating to grid intermittency and variability. It also looked at approaches in quantifying the challenges and suggested options to handle them through using geographic sites for smoothing or adding storage options. Task 2 involved exploring and analyzing various battery technologies using system-­level and multiscale modeling approaches. Studies were carried out to understand pathways to help lower storage integration costs. Task 3 assessed options to use storage-backed solar technologies for powering rural regions. Techno-economic and feasibility studies were conducted to understand the true cost of delivering power to off-grid sites through solar.

4.3.1  M  ultiple-Site Geographical Smoothing of Solar PV: Quantifying the Reduction in Solar Generation Variability Through Interconnected PV For large-scale grid application of solar, CMU conducted analysis to understand the effect of geographic smoothing for interconnected solar PV plants in Gujarat. They examined the potential for geographic smoothing of solar PV electricity generation using 13 months of observed power production from utility-scale plants in Gujarat, India. Collection and analysis of more than 1 year of observed power production data from 50 utility-scale solar plants in the state of Gujarat showed that interconnecting as few as 12 PV plants achieved the majority of the reduction of variability. This is the first analysis of geographic smoothing of solar PV using actual generation data at high time resolution from utility-scale PV plants. Interconnecting 20 Gujarat plants reduced fluctuations at frequencies corresponding to 6 h and 1 h by 23% and 45%, respectively. Half of this smoothing could be obtained through connecting two plants; the point of diminishing returns occurred at 10–12 plants [11] (Fig. 4.12).

4.3.2  Storage Sizing for Solar PV Applications This effort was explored in detail for more than 5 years, with several analyses conducted under its umbrella. Initial work focused on a detailed literature review of policies supporting decentralized applications of solar PV. A techno-economic analysis followed to evaluate off-grid and hybrid solar PV technologies for providing electricity to remote rural regions in India.

148

D. Ginley et al.

Fig. 4.12  Fraction of the spectrum of a single plant retained vs. number of interconnected solar plants normalized at f = 1/24 h at different timescales. (Courtesy of Center for Study of Science, Technology and Policy (CSTEP))

4.3.2.1  Battery Utilization and Autonomy Washington University in St. Louis conducted a study to understand the correlation between battery autonomy and battery use when combined with solar PV systems. Adding batteries improved system autonomy from an average of 40.9 to 89.7% [12]. Autonomy and utilization of a battery were found to correlate with the site irradiance, but they also depended on the temporal distribution of solar energy at the site. The lifetime economic value of adding batteries was evaluated with respect to time-of-use (TOU) pricing of utilities and was found to be sensitive to TOU price. The break-even costs for adding storage is a function of peak to off-peak price difference and battery life, as shown in Fig. 4.13. 4.3.2.2  T  echno-Economic Analysis of Solar PV and Battery Microgrids for Decentralized and Rural Solar Applications The direct link between energy use and economic growth is well-established. Rural electrification has always been a top priority for the Government of India and a significant story in India’s developmental trajectory. Recently, the Government of India declared that electricity has reached every village in India, although connections to each household are still pending [13].

4  Solar Energy Integration

149

Fig. 4.13  Break-even cost for battery installation in 2011 under TOU pricing at Lowry Range (USA). (Courtesy of Center for Study of Science, Technology and Policy (CSTEP))

Solar microgrids offer significant potential to electrify remote villages. In addition, they can use the decreasing solar and battery costs to provide affordable power. A study initiated under the Integration and Energy Storage project, supported by industrial partner WIPRO, evaluated the techno-economic feasibility of solar microgrids for villages in India. The objective of this analysis was to understand the plant capacity needed for such installations, considering sample villages (in Karnataka), and to evaluate the system capital and operating costs. Some important aspects of the analysis included: (1) modeling the solar PV and battery system performance to consider dynamic variations in load and solar irradiance values, (2) modeling different scenarios by varying the system dispatch, and (3) conducting a detailed analysis to evaluate a techno-economically viable system configuration. Battery-life calculations were done based on a cycle-counting algorithm, and the outputs were used in cost calculations. A financial model (based on CERC, India regulatory norms) was developed specifically to evaluate the system cost for such off-grid systems. This detailed sizing study helped determine the appropriate system size by necessary trade-offs between system cost and reliability of supply. In addition, we performed sensitivity analyses to understand the scope of further cost reduction. Scenario 1 (Fig. 4.14) showed the reduction in unmet load (or improvement in system reliability) with an increase in PV and battery size for 24 h of discharge. The trend lines showed the dependence of unmet load on PV and battery size. Scenario 2 (Fig. 4.15) was the most suitable in providing an appropriate system size that was well-balanced in terms of both cost and reliability. The study helped to develop a methodology for selecting techno-economically feasible solar PV microgrids. This approach could be extended to evaluate solar

150

Unmet Load (%)

D. Ginley et al. 60

PV=25 kW

50

PV=30 kW PV=35 kW

40

PV=40 kW PV=45 kW

30 20 10 0

0

100

200 300 Battery Capacity (kWh)

400

Fig. 4.14  Variations of unmet load with battery capacity for 24  h of discharge (Scenario 1). (Courtesy of Center for Study of Science, Technology and Policy (CSTEP))

60

PV=25 kW PV=30 kW

50 Unmet Load (%)

Fig. 4.15  Variations of unmet load with battery capacity for 12 h of discharge (Scenario 2). (Courtesy of Center for Study of Science, Technology and Policy (CSTEP))

PV=35 kW

40

PV=40 kW

30

PV=45 kW

20 10 0

0

100

300 200 Battery Capacity (kWh)

400

microgrids in any location in India. The analysis conducted showed the importance of dispatch strategy in optimizing battery size and system reliability. It also emphasized the significance of performing minute-wise simulations for a detailed understanding of system operation.

References 1. A.K. Saxena, et al., Transition in Indian electricity sector 2017–2030. The Energy and Resource Institute (TERI), 28p. (2017). http://www.teriin.org/files/transition-report/files/downloads/ Transitions-in-Indian-Electricity-Sector_Report.pdf 2. S. Ghosh, et al., Solar Resource Assessment & Technology Roadmap Report: SEI-1. Center for Study of science, technology and policy (CSTEP), 111p., August 2014 (2014). https:// www.seriius.org/pdfs/sei_cstep_resource-assessment_2014.pdf

4  Solar Energy Integration

151

3. http://cstem.cstep.in/cstem/ 4. B. Verma, G. Pavaskar, T. Oluwatola, A. Curtright, state-level policy analysis for PV module manufacturing in India. Center for Study of science, technology and policy (CSTEP), 29 p. (2017). https://www.seriius.org/pdfs/policy-mapping-study-201710-cstep.pdf 5. B. Verma, P. Kumar, Feasibility analysis for c-Si PV manufacturing in India. Center for Study of Science, Technology and Policy (CSTEP), CSTEP-SP-2018-01, 9p. (2018). http://cstep.in/ uploads/default/files/publications/stuff/WP_SiPV_Manufacturing_Final_26-02-18.pdf 6. CSTEP internal study 7. S. Ghosh, A. Nair, S.S. Krishnan, Techno-economic review of rooftop photovoltaic systems: Case studies of industrial, residential and off-grid rooftops in Bangalore, Karnataka. Renew. Sustain. Energy Rev. 42, 1132–1142 (2015). https://doi.org/10.1016/j.rser.2014.10.094 8. CSTEP, Renewable Energy Atlas of India (2016). http://darpan.cstep.in/reatlas/explore 9. S.G.B.  Pradhan, Central electricity regulatory commission, New Delhi, petition no. SM/03/2016 (Suo-Motu), March 30, 2016 (2016). http://cercind.gov.in/2016/orders/sm_3.pdf 10. N. Blair, N. DiOrio, J. Freeman, P. Gilman, S. Janzou, T. Neises, M. Wagner, System advisor model (SAM) general description (version 2017.9.5). Golden, CO: National Renewable Energy Laboratory. NREL/TP-6A20-70414. (2018). https://www.nrel.gov/docs/fy18osti/70414.pdf. Webinar to introduce system advisor model (SAM) modifications for Indian users. Presented by a. Curtright, N. Blair, S. turner, May 25, 2017. https://www.youtube.com/ watch?v=TkwerT5X63A 11. K. Klima, J. Apt, Geographic smoothing of solar PV: Results from Gujarat. Environ. Res. Lett. 10, 104001 (2015). https://doi.org/10.1088/1748-9326/10/10/104001 12. M.T.  Lawder, V.  Viswanathan, V.R.  Subramanian, Balancing autonomy and utilization of solar power and battery storage for demand based microgrids. J. Power Sources 279, 645–655 (2015). https://doi.org/10.1016/jpowsour.2015.01.015 13. Ministry of Power. http://saubhagya.gov.in/

Chapter 5

Summary David Ginley and Kamanio Chattopadhyay

SERIIUS has clearly demonstrated that the different approaches and philosophies of two countries can be leveraged to form an organization that can make unique contributions to the research landscape. The very structure of combining national laboratories, academic institutions, and industry from both countries has produced opportunities not otherwise achievable and created associations that will significantly outlast the organization itself. This book has presented how we developed the team, managed the consortium and research across diverse thrusts and projects, and broadly engaged educational activities internal and external to SERIIUS. Some key lessons learned include the following: • A management structure was developed to be effective across the complex organizational and research structure as well as SERIIUS’ many participants, specifically by active, engaged, continuous communication. • In particular, monthly calls were typically very effective. For joint activities, the discussions ensured that the activities were well synchronized. For Ph.D. students, the opportunity to talk monthly with experts was valuable, and these senior researchers were extremely generous with their time and advice. This connection was amplified by opportunities for face-to-face contact at SERIIUS-­specific allhands meetings and SERIIUS meetings at other technical meetings such as the IEEE Photovoltaic Specialists Conference. More than anything else, this constant

D. Ginley (*) National Renewable Energy Laboratory, Golden, CO, USA e-mail: [email protected] K. Chattopadhyay Division of Mechanical Engineering, Department of Materials Engineering, Indian Institute of Science-Bangalore, Bangalore, India e-mail: [email protected] © Springer Nature Switzerland AG 2020 D. Ginley, K. Chattopadhyay (eds.), Solar Energy Research Institute for India and the United States (SERIIUS), Lecture Notes in Energy 39, https://doi.org/10.1007/978-3-030-33184-9_5

153

154

•

•

•

•

D. Ginley and K. Chattopadhyay

integration of communication, especially between active collaborating researchers, facilitated meaningful teaming ultimately reflected in joint publications. Internships in both directions under the MAGEEP Fellowships served as another kind of active integration of the research agenda and to give interns invaluable multicultural experience. This program could have been useful even at a much larger scale, and we are working to see it sustained because of the lasting value for students on both sides. In many cases, the joint projects used complementary facilities and expertise of the partner institutions and resulted in research that could not have been conducted otherwise. During the project, this was increasingly true as members became more aware of the diverse capabilities across the consortium. We note that this increasing integration occurred within projects in a specific thrust—but also, across the thrusts. For example, the PV thrust benefited in terms of defining its agenda from the SEI thrust and learned about optics and materials from the CSP thrust. Industry members played a critical role in this integration because of their multifaceted interests. SERIIUS also took an active role in integrating across India and the entire global hot-climate region in the CSP, PV, and SEI areas. This helped to articulate the value proposition and broad challenges for the CSP and PV technologies, and it also educated the technical and investment communities. In addition, the online tools developed for all technology areas served as key resources for the community, especially those tools—such as the Solar Advisor Model for India—developed to analyze technology deployment options. This, together with the intern programs and significant graduate student and postdoctoral involvement, created a lasting community supporting the solar initiatives in both countries. The consortium, by virtue of integration priorities binationally, allowed for some unique work to be done that has potentially significant implications for the following: the deployment of solar in both countries, the ultimate bankability of solar in India, and the problems faced worldwide for large solar penetration in harsh climates. Some key observations in these areas are as follows:

• For PV: –– The potential of low-capital-cost, solution-based processing for high-quality PV materials. This has been demonstrated for inorganic semiconductors such as CIGS, organic semiconductors, and hybrid materials such as perovskites. These approaches have direct applicability in the United States and where low-capital-cost manufacturing has considerable potential for direct production and integration of thin films in tandem structures. –– The use of multiscale modeling to establish correlations across length scales— from atomic to system—and the identification of research opportunities even in existing crystalline-silicon technology. –– The development of new materials and device configurations to produce very high-performing devices with long lifetimes in both organic PV and perovskite PV based on new basic understandings and modeling.

5 Summary

155

–– The impact of soiling and harsh climates on PV production and reliability and potential approaches for mitigation based on the chemistry of contaminants. –– The development of new materials and approaches for low-cost, flexible encapsulation to improve lifetimes across a wide range of technologies. –– An integrated research strategy for developing and deploying low-cost flexible PV using Corning® Willow® glass and polymer substrates. • For CSP: –– The realization of the potential of high-temperature supercritical-CO2 solar thermal conversion. Substantial cost share demonstrated a pilot system and developed a matched phase-change-based storage system. This work was a harbinger to the directions that DOE would choose to go in its solar thermal program. –– The development of low-cost heliostats meeting both Indian and DOE cost targets. This was associated with developing low-cost, Earth-abundant, solution-­processible reflection and absorber coatings, as well as new solar collectors for concentrated solar applications. –– The development of new low-cost, molten-salt heat-storage systems with high-­temperature capabilities up to 400 °C. –– Field testing of Rankine solar thermal systems employing the low-cost reflectors and absorber on low-capitalization parabolic collectors. • For SEI: –– Development of an understanding of deployment options for solar conversion in India as a function of region and scale. This included an initial analysis of the potential role of storage integration across these same length scales—from village to grid. –– Modeling of potential policy incentives and their potential implications in increasing solar penetration. –– Creation of online modeling resources to assess solar deployability, including the development of SAM for Indian, which ported the U.S.-developed SAM tool to India using India-derived data. This helps to establish SAM as a viable international platform when incorporating validated location-specific data, and it broadens the overall applicability of SAM. –– Creation of technology-to-market tools specifically aimed at deployable technologies being developed by SERIIUS to assess their potential in both the United States and India and to provide feedback on the most-efficacious research directions. –– The ability to articulate the bankability of solar in different deployment options was of direct benefit to both countries including helping to establish the viability of solar in India as an investment option. Similarly, these studies helped to inform U.S. companies of the opportunities for investment in solar technology in India. • SERIIUS built lasting friendships between Indian and U.S. researchers that will be leveraged for a considerable time. Many researchers continue to participate

156

•

• •

• •

D. Ginley and K. Chattopadhyay

with their partners, and research continues to be productive (the latter continues to be supported by the Bhaskara and Fulbright Scholarships). Although many researchers would like to see more sustainable approaches to maintaining research engagement, the foundation of these interactions continues to facilitate binational connections. Both sides learned much—both technically and culturally—through the close collaboration, which was a hallmark of this project, leading to a lasting commitment by many SERIIUS participants to solve critical world problems through international efforts. Administrative and other supports were outstanding, which is critical for a project of this magnitude. Many of the researchers enjoyed the opportunity to work on a program with metrics and goals based on international priorities and serving both countries’ ultimate goals—e.g., SERIIUS’ focus on smaller-scale and multiscale CSP, and solution-processed PV derived from different opportunities to advance renewable science in both countries. The professional level of interactions was outstanding throughout the life of the consortium. The research outputs of the consortium—especially the joint international papers and multi-institutional papers, patents, and presentations—created a substantial legacy for SERIIUS.

We also highlight some areas that could have been improved: • SERIIUS had many partners and participants, and overall, this was an asset and added very positive diversity to SERIIUS.  However, it was their passion for SERIIUS and its mission that maintained their engagement. This resulted in the research often depending on the loyalty and vision of the individual investigators. Future efforts and understanding of the funding mechanisms in both countries could result in a tuning of the 10-Point Plan to help ensure full engagement. • Overall, the collaborations were sometimes difficult to maintain, and not all were productive. This outcome was reflected in a rather lengthy introductory period for some of the interactions. In cases where collaborations worked, the successes were great, but many barriers had to be overcome to achieve fully functional international collaboration. In many ways, the culture of SERIIUS had to supersede that of the individual institutions—and even of the two countries. • The positive rapport developed during these 5 years created a lasting partnership between the two countries. The lessons learned continue to enhance these interactions. In general, SERIIUS received great support from both countries, and many of the sponsors and partners would like to pursue the potentially deployable outcomes. • SERIIUS was indeed a partnership of some of the best organizations from both countries. Some significant breakthroughs were hoped for, but only time will prove whether the substantial technical output of SERIIUS will live up to this desired impact.

5 Summary

157

• Collaboration between the thrusts could have been better, and it took substantial time to foster that kind of interaction. This is especially true of the interactions with the systems integration team, which took a long time to mature. More crosscutting workshops may have helped. In summary, the question could be asked: Did SERIIUS achieve its goals? The overall goal of SERIIUS was “to accelerate the development of solar electric technologies by lowering the cost per watt of photovoltaics (PV) and concentrated solar power (CSP) through a binational consortium that will innovate, discover, and ready emerging, disruptive, and revolutionary solar technologies that span the gap between fundamental science and applied R&D, leading to eventual deployment by sustainable industries.” This effort was built on a foundation defined by the following objectives, where we have provided our assessment of what SERIIUS attained for each: 1. Focus on high-impact fundamental and applied R&D to create disruptive technologies in PV and CSP. The output of a research consortium can be measured by its technical output and the achievements of its personnel. SERIIUS had an outstanding production of papers, patents, and presentations; in addition, the program served to educate a broad range of students, staff, and the solar community in India. This body of work could not have existed without the collaborative integration of SERIIUS, and the publications that are truly joint exist because of the unique set of complementary skills non-existent in each country alone. 2. Identify and quantify the critical technical, economic, and policy issues for solar energy development and deployment in India. SERIIUS researchers, through a series of analyses and reports, addressed the scope of potential applicability of scalable solar systems and the cost analysis versus configuration for systems. They also assessed the potential impact of storage versus implementation approach. And they modeled the potential impact of new policy approaches. These analyses were coupled with tech-to-market analysis to help inform the specific research agenda. 3. Overcome barriers to technology transfer by teaming research institutions and industry in an effective project structure—cutting the time from discovery to technology development and commercialization through effective coordination, communication, and intellectual property management plans. The functional integration of industry, academia, and national laboratories created teams with broad research and technical knowledge that helped define and enable the research agenda. The unique nature of the core projects—which were industry-driven, but consortium-enabled—facilitated an acceleration of industry-relevant and deployable research that provided a positive feedback mechanism with the consortium’s more R&D-focused projects. This complementary effect combined to significantly accelerate the rate of technical progress.

158

D. Ginley and K. Chattopadhyay

4. Create a new platform for binational collaboration using a formalized R&D project structure, along with effective management, coordination, and decision processes. As discussed above, we believe that the structure and management approach of SERIIUS enabled facilitating and executing the complex research agenda of this binational consortium. We learned many lessons on how to facilitate a functional binational research team and keep it on track over the course of a 5-year project. 5. Create a sustainable network from which to build large collaborations and foster a collaborative culture and outreach programs, including the use of existing and new methodologies for collaboration based on advanced electronic and web-­ based communication to facilitate functional international focused teams. SERIIUS served as a very effective convener in India, the United States, and much of the surrounding countries. It ran workshops and meetings focused on the R&D of SERIIUS and its relationship to the rest of R&D in India; the overall potential of renewable energy across length scales and application space; the nature of reliability and lifecycle across the diverse climate zones of India, and more broadly, hot and dry as well as hot and wet climates; and basic system-level analysis of renewable solar energy systems. In addition, SERIIUS created a number of online tools to analyze and model solar materials and systems. SERIIUS and its staff have participated broadly in national and international meetings and dialogues to help integrate the community. 6. Create a strong workforce development program in solar energy science and technology. SERIIUS focused on developing and placing students in a broad range of SERIIUS institutions. This has resulted in students graduating and moving into the renewables workforce in all of the thrust areas. In addition, SERIIUS actively engaged with a wide range of other students and participated in international fellowships such as the Bhaskara, Raman, and Fulbright Fellowships. Overall, SERIIUS has broadly achieved its objectives, built long-term research partnerships, transitioned research results, and created lasting friendships. It has been a great pleasure to work with the dedicated team comprising SERIIUS and a privilege for all of us to be engaged in the consortium.

Index

A Academic institutions, 3 Active-layer components, 44 ACZTSe, 34 Admittance spectroscopy (AS), 33 Alkahest solvent system, 28 Amine-dithiol solution, 35 Amine-thiol molecular precursors, 35 Amine-thiol solution, 28 aMoBT, 73 Annual degradation rate, 58 ANSYS DesignModeler, 119 ANSYS Fluent, 119 Antisite defects, 33–35 Artificial dust deposition, 67 Atmospheric-pressure selenization process, 36 Atomic layer deposited (ALD) oxides, 45 Atom-to-system modeling, 71 Autonomy, 148 Average degradation rate, 58, 61 Awards, 69, 70 B Bandgap engineering, 39 Bangalore Electricity Supply Company Limited (BESCOM), 141 Bankability, 3, 155 Bifacial modules, 68, 69 Binational collaboration, 158 Bladed receiver conventional receiver designs, 90 design and analysis, 90, 91 fabrication, 91–93

flat-panel designs, 90 geometries, 90 on-sun testing, 91–93 panels prior, final insulation, 92 simulated and measured receiver efficiencies, 92 small-scale CSP systems, 93 spacing and pitch, 93 thermal efficiencies, 93 Borosilicate glass (BSG), 30 Brayton cycle, 87 Brushless DC (BLDC) fans, 65 C Carrier-injection pretreatment (CIP), 33 Center for Study of Science, Technology and Policy (CSTEP), 134, 135, 141–146, 148–150 Central Electricity Regulatory Commission (CERC), 142, 143, 149 Ceramic-based thermal storage ANSYS Fluent, 105 heat-regenerative capacity, 104 honeycombs, 104, 105 numerical simulations, 105 TES, 104 CIGSe solar cells development, 35–36 light-absorber material, 28 molecular precursors, 35–36 PCE, 28 thin-film absorbers, 36–38 CIGSe thin-film absorbers, 36–38

© Springer Nature Switzerland AG 2020 D. Ginley, K. Chattopadhyay (eds.), Solar Energy Research Institute for India and the United States (SERIIUS), Lecture Notes in Energy 39, https://doi.org/10.1007/978-3-030-33184-9

159

160 Climatic dependence accelerated testing, 61, 63 “All-India Surveys”, 57 degradation rates, 58, 60, 61 EL, 59, 62 RPN approach, 58, 61 Climatic variations, 58 Climatic zones, 56–58, 60, 66, 71 Close-spaced sublimation (CSS), 73 Coating, 99 Compressor discharge pressure, 107 Computational fluid dynamics (CFD), 102 Concentrating solar power (CSP), 2, 3, 5, 6, 13, 14, 16, 17, 19–22, 155–157 cost-effective, 87 high-temperature s-CO2 Brayton cycle (see High-temperature s-CO2 Brayton cycle) low-temperature ORC (see Low-­ temperature ORC, CSP) multiscale, 87 storage and hybridization (see Storage and hybridization, CSP) Consortium, 153, 154, 156–158 Conventional thermal plants, 134 Copper indium gallium diselenide (CIGS), 17, 21 Copper zinc tin diselenide (CZTS), 17–18 Corning Research & Development Corporation (CRDC), 49 Corning® Willow® Glass, 30, 35, 45, 46, 49, 52, 53 Crosscutting workshops, 157 Crystalline silicon (c-Si), 136 CSTEM-CSP tool, 144–146 CSTEM-PT, 144 Cu(A)-Sn-Cu(B)-Zn (CTCZ) precursors electrodeposited metal, 31, 32 Cu-Zn antisite defects, 33 CZTS nanocrystals, 29 CZTSSe solar cells antisite defects, 33–35 defect study, 32, 33 growth mechanism, 27 hydrazine, 27 light-absorber layer, 32 molecular precursor, 28–31 nanocrystal inks, 28–31 QE analysis, 32 record-efficiency, 27 substitutional elements, 33–35

Index D Defect calculation, 73, 74 Degradation rates climatic dependence, 56–63 PV modules, 56 rooftop rack-mounted modules, 61 Department of Energy (DOE), 1–3, 7, 10, 11, 14, 16, 155 Department of Science and Technology (DST), 11 Deployable processes, 5 Diffusion-controlled grain growth, 31 Diiodooctane (DIO), 40 Direct normal radiation (DNI), 87 Discrete ordinate method (DOM), 102 DOE SunShot Initiative, 17, 20 Domestic Content Requirement (DCR), 140 Domestic modules, 136 Drop-on-demand (DOD) Ceraprinter (Ceradrop), 37 Dry deposition systems, 67 Dust varies, 67 Dye-sensitized solar cells (DSSCs), 44, 45, 50 Dynamic mechanical-load (DML) system, 61, 63 E Earth-abundant photovoltaics CZTS, 18 deployment, 18 DOE SunShot Initiative, 17 energy-conserving non-Siemens-based processes, 18 low-cost roll-to-roll process, 17 modeling, 19 OPV, 18 perovskite-based solar cells, 18 PV, 17, 18 Earth-abundant thin-film solar cells, 44 EERE funding, 44 Electrodeposited metal precursors, 31, 32 Electrodeposited spray-coating technique, 46 Electrodeposition Cu(A)-Sn-Cu(B)-Zn (CTCZ) precursors, 31 spray-coating technique, 46 Electro-discharge machining process (EDM), 127 Electroluminescence (EL), 59, 62 Electrospray deposition MAI precursor, 46, 48

Index Encapsulants, 45, 46, 49, 58, 61, 63, 68, 69 End-to-end modeling passivated contact silicon heterojunction cell, 74, 75 Energy-conserving non-Siemens-based processes, 18 Energy-dispersive spectroscopy (EDS), 28 Energy-efficient DC appliances, 65 External quantum efficiency (EQE) analysis, 34 F Face-to-face content, 153 Face-to-face meeting, 40 Film deposition, 29 Financial modeling, 137, 143 Flexible glass substrates Corning® Willow® Glass properties, 52, 53 device testing, 54, 55 evaluation, 52 flexural rigidity, 52, 53 optical transmission, 54 properties, 52 PV modules, 51, 52 R2R processing, 55–57 solution-processing devices, 55 vacuum-based processing, 52 web-based substrates, 52 Flexural rigidity, 52, 53 Florida Solar Energy Center (FSEC) team, 58 Fluorinated materials, 40 Fullerenes, 41, 42 Full-width at half-maximum (FWHM), 35 Funding opportunity announcement (FOA), 1–3, 7 G Geographical information systems (GIS), 134 Grid penetration, 146 H Heat-storage system, 121 Heat-transfer fluid (HTF), 89, 90, 92, 94, 116, 117, 122, 125, 126 Heliostat field, 146 High-performance fluorinated OPV polymers, 42 High-temperature receivers, s-CO2 advantages, 89 bladed receiver, 90–93

161 direct-heating systems, 90 geometric designs, 89 small-scale CSP systems, 90 High-temperature s-CO2 Brayton cycle characteristics, 89 conventional Rankine cycles, 88 CSP plant economics, 88 high cycle thermal efficiency, 89 high-temperature receivers (see High-­temperature receivers, s-CO2) loop facility (see Supercritical carbon dioxide test loop) multiscale CSP systems, 89 recuperative heat transfer, 89 spiral-wound tubular solar receiver (see Spiral-wound tubular solar receiver, s-CO2 Brayton cycle) thermal storage, 89 Hole-transport layer (HTL), 46 Hot climates, 61, 63, 64 Hot and humid climates, 56, 58, 61, 71 Hot-injection method, 29 Hydrazine, 27 Hydro plants, 134 Hydrophobic TEOS, 46, 48 I Indian Association for the Cultivation of Science (IACS), 40 Indian Institute of Science (IISc), 3 Indian Institute of Technology Bombay (IITB), 58 India’s electricity demand, 134 India’s National Solar Mission, 17 Individual and tandem cells PV, 73 Indoor (laboratory) artificial dust deposition system, 67 Industry Board, 9 Industry partners, 3, 8, 9 Inkjet printing, 36–38 Intellectual property (IP), 4 Intense pulsed light (IPL)/laser, 36, 37 Interdisciplinary Centre for Energy Research (ICER), 116 Internal rate of return (IRR), 141 International Advanced Research Centre for Powder Metallurgy and New Materials (ARCI), 112 International collaboration, 50, 78 Inter-SERIIUS-partner organization, 14, 15 Intrinsic stability, 44

Index

162 J Jawaharlal Nehru National Solar Mission, 5, 20 Joint Clean Energy Research and Development Center (JCERDC), 1, 2, 15, 21 K Kesterite thin-film material, 38 L Laboratory-scale R2R solution coating system, 55, 56 Lab-scale thermal energy storage loop, 117, 118 Levelized cost of electricity (LCOE), 17, 20, 116 Levelized cost of energy, 88 Light-absorber layers, 34 Light-emitting diode (LED) lights, 65 Low-temperature ORC, CSP cost-efficient solar receiver tube ARCI, 112 coating, 114 flexible reflector, 112 high-performance absorber coating, 113, 114 prototype receiver tube, 114 small-scale positive-displacement, 115, 116 Low-temperature ORC storage experiments, 127 HTF, 125, 126 latent-heat TES, 126 numerical simulation, 127 PCM, 125, 126 Low-toxicity processing, 29 M MAGEEP Fellowship, 154 Magnetron sputtering, 100 Management structure, 4, 153 Manufacturing costs, 136 Mass flow rate, 94, 102, 105 Materials genome, 101 Materials Genome Initiative (MGI), 101 Maximum power-point tracking (MPPT), 64 Metal precursor films, 31 Metrics, 156 Microfinancing model, 141 Mini-modules, 49 Mo-coated soda-lime glass (SLG), 37

Module manufacturing, 136–139 Molecular precursors CIGSe solar cells, 35–36 CZTSSe solar cells, 28–31 Molten-salt storage, 122 Monofacial and bifacial solar farms, 75, 76 Multiple-layers printing approach, 37 Multiscale concentrated solar power, 19, 20 Multiscale modeling, 154 accomplishments, 77 assessment, 78 concept validation, 72 defect calculation, 73, 74 end-to-end modeling, 72, 74, 75 framework, 72 monofacial and bifacial solar farms, 75, 76 outcomes, 77 perovskite cells, 74–76 project goals, 72, 77 reliability research, 72 solar modules, 71 space applications, 71 TCOs, 73 thermodynamics, 73 U.S. PV reliability program, 71 N Nanocrystal-ink method, 28–31, 35 National Institute of Solar Energy (NISE), 58 National laboratories, 3, 5, 19 National Renewable Energy Laboratory (NREL), 3, 8, 9, 11, 13, 15, 16, 21, 39, 40 National Solar Mission, 134, 135 National Solar Radiation Database (NSRDB), 135, 136 National Solar Thermal Test Facility (NSTTF), 90, 91 Near-term future work, 51, 78 Non-vacuum inkjet-printing route, 36–38 N2-plasma SnO2 electron-transport layer (ETL), 46, 48 O Off-grid applications, 135, 142 Off-grid RTPV systems, 141 On-sun test, 92 Optoelectronic parameters, 33 Organic photovoltaics (OPVs), 18 assessment, 43, 44 bandgap engineering, 39

Index charge-carrier dynamics, 41 device optimization, 39, 40 efficiencies, 39 energy-level positioning, 40 face-to-face meeting, 40 high-performance fluorinated OPV polymers, 42 IACS, 40 morphologies, 41 NREL’s, 39, 40 PCEs, 40 perfluoroalkylfullerenes, 42 photostability, 43 roll-to-roll technology, 39 SERIIUS, 39 synthesis, 39 transparency, 39 unique and diverse skillsets, 39 Organic Rankine cycles (ORCs), 20, 87 Oxidative stability, 40 P Parabolic trough (PT) collector, 113, 144 Partnership to Advance Clean Energy (PACE), 1 Passivated contact SHJ, 74, 75 Perfluoroalkylfullerenes, 42 Perovskite-based solar cells assessment engagement, 51 high-efficiency, 50 inorganic transport layers, 50 international collaboration, 50 long-term anticipated developments, 51 near-term future work, 51 SERIIUS, 50 software, 50 CRDC, 49 deposition processes and scale-up, 46 electrodeposited spray-coating technique, 46 electrospray deposition, MAI precursor, 46, 48 ETL, 46, 48 HTL, 46 hydrophobic TEOS, 46, 48 motivation, 44, 45 physics/reliability, 74–76 project goals, 49 R2R fabrication process, 46 research accomplishments, 49 science approach, 45 technological challenges, 46, 47 WUStL, 46 Phase-change material (PCM), 125, 126

163 Photobleaching, 41, 42 Photostability OPV active-layer, 43 Photovoltaics (PVs), 2, 5, 154, 155, 157 OPVs (see Organic photovoltaics (OPVs)) R2R processing (see Roll-to-roll (R2R) technology) reliability (see Reliability, PVs) solution-processing approaches (see Solution-processing approaches) Policy roadmap, 134, 142 Power conversion efficiencies (PCEs), 28, 30–32, 34–38, 40, 41 Power degradation rates, 60 Power-plant model, 136 Precursor films, 28 Pressurized volumetric receivers, 102 Program Management Committee (PMC), 7, 14, 16 Project Monitoring Committee (PMC), 11 PV solar pumps, 64 Q Quantum efficiency (QE), 32, 67 R Raman mapping, 32 Rankine cycle, 88, 89, 108 Rankine solar thermal systems, 155 Rapid-thermal-processing (RTP), 30 Raw materials, 136, 138 Ray-tracing analyses, 90 Ray-tracing tool, 94 Reaction-controlled grain growth, 31 Receiver design, 91 Receiver geometry, 94 Recuperative heat transfer, 89 Reduced-order model, 122, 124 Reliability, PVs awards, 69, 70 BLDC fans, 65 climatic dependence, 56–63 and durability, 56 encapsulants, 68, 69 hot climates, 61, 63, 64 LED lights, 65 soiling, 66–69 solar-powered GOA system, 65 solar-powered portable culture incubator, 65 solar pumps, 64 transparent conductors, 68, 69 water-pumping systems, 64

164 Renewable energy, 1 Research and development (R&D), 5 Research-scale processing equipment, 55 Revolutionary solar technologies, 157 “Risk Priority Number (RPN)” approach, 58, 61 Roll-to-roll (R2R) technology, 39 flexible glass substrates (see Flexible glass substrates) Rooftop photovoltaic (RTPV), 141, 143, 144 Rooftop solar, 135, 141 S Scanning electron microscope (SEM), 28 Scanning transmission electron microscope (STEM), 28 Scanning tunneling microscopy (STM), 41 s-CO2-based power plant, 19 Selenization amine-thiol molecular precursors, 35 annealing, 28 atmospheric-pressure, 36 CZTSe, 30 CZTSSe thin-film solar cells fabrication, 31 inert gas, 31 inkjet-printed CIG precursor, 37 nanocrystal-ink-based thin films, 31 precursor film, 30 in RTP, 34 sodium dopant, 30 thick films, 36 SERIIUS CIGS, 17 SERIIUS-MAGEEP Fellows and Scholars program, 15 Silicon-based solar cells, 26 Silicon heterojunction (SHJ) solar cells, 74, 75 Sine-wave pump controllers (SPCM), 64 Single-tank thermal storage, 118 Soda-lime glass (SLG), 30, 31 Sodium dopant, 30 Soiling, 18, 66–69 Solar deployment bankability, 155 policy analysis, 141 SAM, 143 site options, 135, 136 storage integration, 155 Solar energy, 2 Solar Energy Integration (SEI), 20–22 battery technologies, 147 consortium, 156 geographic smoothing, interconnected solar PV, 147 grid intermittency and variability, 147

Index modeling, 155 off-grid solar technology, 146 solar deployment (see Solar deployment) solar PV applications analysis, 149 autonomy, 148 battery utilization, 148, 149 solar microgrids, 149 techno-economic analysis, 147 storage-backed solar technologies, 147 techno-economic analysis (see Techno-­ economic analysis) techno-economic assessments, 135 tech-to-market analysis (see Tech-to-­ market analysis) Solar Energy Research Institute for India and the United Sates (SERIIUS) achievements, 11 active dialogue, 7 actual partners, 8 bankability, 3 collaborations, 156–158 communications, 7, 14–16 consortium, 154 crosscutting workshops, 157 CSP, 155 development, solar electric technologies, 5 effectiveness, 7 evolution, structure, 3, 4 external coupling, 15, 16 face-to-face contact, 153 functional integration, 10 global hot-climate region, 154 goals, 6 intellectual property, 10, 12 internal communication, 13 MAGEEP Fellowships, 154 management structure, 7, 9, 10, 13, 153 Multiscale CSP, 19, 20 outreach programs, 158 partner institutions, 154 partnership, 156 10-Point Plan, 10, 156 project control, 13, 14 project structure, 6 R&D, 5, 157 real-time communication, 7 revolutionary, 5 SEI (see Solar Energy Integration (SEI)) synergism and programmatic, 11 technology transfer, 157 thrust areas (see Thrust-based research) U.S.–India consortia, 3, 6, 7, 154, 155 vision, 5 workforce development program, 158

Index Solar energy targets, 133, 135 Solar integration, 133 Solar microgrids, 149 SolarPACES, 10, 14 Solar photoconversion, 2 Solar photovoltaics, 17 Solar plants, 135, 147, 148 Solar-powered GOA system, 65 Solar-powered portable culture incubator, 65 Solar PV solutions, 64 Solar resource assessment, 134 Solar tower (ST), 144, 145 SolTrace, 90, 91, 94 Solution-processing approaches OPVs (see Organic photovoltaics (OPVs)) perovskite-based solar cells (see Perovskite-based solar cells) thin-film solar cells (see Thin-film solar cells) Spectrally selective absorber computer modeling, 101 fabricating multilayer stacks, 100 magnetron sputtering, 100 photothermal conversion efficiency, 101 photothermal efficiency, 98 semiempirical model, 101 TiB2/TiB(N)/Si3N4 film, 98, 99 W/WAlN/WAlON/Al2O3, 99, 100 Spiral-wound tubular solar receiver, s-CO2 Brayton cycle air vs. CO2, 98 conical back reflector, 98 data exchange, coupled modeling, 94, 95 effect of change, 97 Fresnel lens-based concentrator, 95 geometry, 94 helical receiver and placement, 96 high-temperature power cycles, 93 interchanging inlet–outlet, 97 irradiated helical receiver, 96 mass flow rate, 94 optical-thermal-fluid assessment, 93, 94 S-CO2 operating conditions effect, 98 spectrally selective absorber, 98–100 surface heat flux, 97 temperature distribution, 97 thermal efficiency, 96 thermal storage, 101–107 thermophysical properties, 93 volumetric ceramic receiver, 101–107 Steady-state operation, 111 Storage and hybridization, CSP Brayton cycles

165 heat-storage system, 121 lab-scale thermal storage loop, 117, 118 temporal degradation, 120 TES, 116 thermocline degradation, 120 thermocline storage system, 117 thermocline TES system, 119, 120 low-temperature ORC storage, 125–128 Molten-salt thermocline tank performance economic optimization, 123 energy-collection data, 123 instantaneous temperature, 125 LCOE, 123 low-cost filler material, 122 phase-change energy storage, 124 reduced-order model, 122 SERIIUS, 122 simulation, 123 solid filler material, 125 system-level model, 123 TES, 122 thermocline energy storage, 124 SunShot Initiative, 135 Supercritical carbon dioxide (s-CO2), 19, 87 closed-loop Brayton cycles, 88 commercial scale, 109 components, 109 compressor, 107, 108 heat exchanger, 110, 111 isenthalpic and isobaric processes, 110 Rankine-based steam/gas-turbine Brayton systems, 107 SERIIUS, 109 smaller footprint, 107 startup operation, 111 steady-state operation, 111, 112 temperature, 108 thermodynamic cycle, 107, 108 System advisor model (SAM), 142, 143, 155 T Tandem cells, 73, 77 Technical Advisory Board, 9, 14 Techno-economic analysis computational tools, 142 CSTEM, 142, 143 CSTEM-CSP tool, 144–146 CSTEM-PV, 143, 144 RTPV tool, 143, 144 SAM, 143 Techno-economic assessments, 135

Index

166 Techno-economic models, 21 Technology roadmap SEI (see Solar energy integration (SEI)) Tech-to-market analysis cost share, various components, 136, 138 c-Si, 136 deployable technologies, 155 domestic modules, 136 feasibility analysis, c-Si PV manufacturing, 139, 140 financial model, 136, 137 manufacturing costs, 136 module manufacturing, 136–139 optimal requirements, 138 policies, 136 sensitivity analysis, 138 solar potential, 136 Temperature-dependent molten-salt, 119 Temporal degradation, 120 Thermal energy storage (TES), 104 Thermal-fluid evaluation, 93 Thermal performance index (TPI), 127 Thermal storage, 89, 90 Thermocline, 117 degradation, 120 TES system, 119, 120 Thermodynamics individual and tandem cells PV, 73 Thin-film solar cells Cu(In,Ga)Se2 (CIGSe) (see CIGSe solar cells) Cu2ZnSn(S,Se)4 (CZTSSe) (see CZTSSe solar cells) CZTS, 31, 32 PV applications, 26 Thrust-based research cutting-edge technologies, 17 earth-abundant photovoltaics (see Earth-abundant photovoltaics) India’s National Solar Mission, 17 SEI, 20–22 TiB2/TiB(N)/Si3N4, 98, 100 Time-of-use (TOU), 148 Time-resolved photoluminescence (TRPL), 34 Transmission electron microscopy (TEM), 99 Transparent conducting oxides (TCOs), 73 Transparent conductors, 68, 69

U Ultrahigh-temperature ceramics, 98 Ultrathin CIGSe solar cell, 36 Ultraviolet (UV)-visible-near-infrared (IR) spectrum, 54 Upstream supply chain, 136 Utilization factor, 138 V Vacuum-based processing, 52 Vacuum-based techniques, 28 Vacuum deposition, 55 Variable-frequency drive (VFD) controllers, 64 Voltage-dependent AS, 33 Voltage-dependent QE data, 33 Volumetric ceramic receiver and thermal storage absorption, radiation, 103 ceramic-based thermal storage, 104–107 direct heating, s-CO2, 102 element, 102 heat energy storage, 107 heat transfer, 103 metallurgical and mechanical failure, 102 regeneration cycle, 107 small quartz windows, 102 temperature, 103, 104 transparent window, 102 tubular receivers, 101 W Water-based systems, 67 Water-pumping systems, 64 Water vapor transmission rate (WVTR), 68, 69 Wavelength-dependent QE data, 33 Weather data, 135, 136 Working capital, 136 W/WAlN/WAlON/Al2O3, 99, 100 X X-ray diffraction (XRD) pattern, 35 X-ray photoelectron spectroscopy (XPS), 113