CHARACTERISATION AND PERFORMANCE ANALYSIS OF BIFACIAL SOLAR CELLS AND MODULES JAI PRAKASH (M.Tech., IIT Bombay, Mumbai, India) (B.Tech., Jamia Millia Islamia, New Delhi, India) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2014 DECLARATION I hereby declare that the thesis is my original work and it has been written by me in its entirety. I have duly acknowledged all the sources of information which have been used in the thesis. The thesis has also not been submitted for any degree in any university previously. Jai Prakash 18th December 2014 ACKNOWLEDGEMENTS Firstly, I would like to express my sincere gratitude and appreciation to my supervisors Prof. Armin G. Aberle and Dr. Timothy M. Walsh for their continuous support, encouragement and guidance throughout the course of this research. I thank Prof. Aberle for giving me the opportunity to work at the Solar Energy Research Institute of Singapore (SERIS), NUS and for his invaluable feedback on my research progress and journal publication. I personally thank Dr. Timothy Walsh for his daily supervision and valuable feedback on my research work and publication. Tim has been a great mentor and friend. I would also like to thank Dr. Marius Peters and Dr. Johnson Wong for their scientific advice on my research work. I would like to thank Yong Sheng and Chai Jing for scientific discussion and helping me with experiment. I would also like to thank my colleagues Siyu Guo, Ye Jiaying, Ankit Khanna, Avishek Kumar, Sandipan Chakraborty and Mridul Sakhuja for fruitful discussion and the exchange of ideas. The PhD journey would be incomplete without the friends at SERIS. I would like to thank Chai Jing, Yong Sheng, Avishek, Sandipan, Kishan, Shubham, Vinodh, Deb, Basu and Samuel for giving nice company during my PhD. The journey has also been coloured by the following people: the late Jenny Oh and Natalie Mueller for organizing the fun bowling sessions. I would also like to thank Ann Roberts and Maggie Keng for their administration support. I would like to give special thanks to all my fellow peers and staff at SERIS who have helped me in one way or another during this journey. I Last but not least, I would like to thank my parents, my wife Richa, and my in-laws for their endless love, encouragement and support during my PhD journey. Finally, I would like to thank Almighty God, who always showers his kindness on me at every moment of my life. A big heartfelt thanks to everyone! Jai Prakash II TABLE OF CONTENTS ACKNOWLEDGEMENTS I TABLE OF CONTENTS . III ABSTRACT . VII LIST OF FIGURES X LIST OF TABLES XIV LIST OF SYMBOLS AND ABBREVIATIONS XV CHAPTER - Introduction 1.1 Solar photovoltaics: A promising renewable energy source . 1.2 Cost of PV electricity and benefits of bifacial PV modules 1.3 Thesis motivation and objectives 1.4 Thesis Structure . CHAPTER 2.1 Background, applications and challenges with bifacial solar cells and modules . 10 Background . 10 2.1.1 Bifacial solar cells and module structures . 10 2.1.2 History of bifacial solar cells and modules 12 2.2 Applications and potential benefits . 13 2.2.1 Terrestrial albedo collection configuration 14 2.2.2 Vertically mounted bifacial PV modules . 15 2.2.3 Bifacial modules for space applications 17 2.2.4 Static concentrators 17 2.2.5 Building integrated PV applications 19 2.3 Challenges with bifacial PV devices . 21 2.3.1 Installation-based performance dependence 21 2.3.2 Cell processing steps and associated cost 22 2.3.3 Characterisation and standardisation of bifacial devices . 24 2.3.4 Rating and cost estimation of bifacial solar cells and modules . 26 CHAPTER 3.1 Fabrication and measurement techniques . 28 Introduction . 28 III 3.2 PV module fabrication 28 3.2.1 Cell interconnection and making electrical contacts . 29 3.2.2 Lamination . 30 3.3 Measurements of bifacial solar cells and modules 32 3.3.1 Current-voltage (I-V) measurements 32 3.3.2 Spectral response measurement and quantum efficiency 36 3.3.3 Suns-Voc measurements 38 3.3.4 UV-VIS spectrophotometer measurements . 39 CHAPTER - A new method to characterise bifacial solar cells 40 4.1 Introduction . 40 4.2 The method: Bifacial 1.x efficiency and gain-efficiency product for bifacial solar cells 42 4.2.1 Definitions 42 4.2.2 Calculation of effective Voc (Voc-bi) 44 4.2.3 Calculation of effective FF (FFbi) . 45 4.2.4 Bifacial 1.x efficiency and gain-efficiency product 47 4.3 Examples: Analysis of bifacial solar cells . 49 4.3.1 Comparison of two bifacial cells with different front and rear side electrical parameters . 49 4.3.2 Effect of various electrical parameters on bifacial 1.x efficiency and gain-efficiency product 51 4.4 Conclusions . 56 CHAPTER - A new method to characterise bifacial PV modules 57 5.1 Introduction . 57 5.2 I-V characterisation of bifacial modules: The method 59 5.2.1 Monofacial indoor measurements of bifacial modules 60 5.2.2 Calculation of Isc-bi . 61 5.2.3 Calculation of Voc-bi 62 5.2.4 Calculation of FFbi . 63 5.3 Indoor bifacial module measurements and charac-terisation 67 5.3.1 Comparison of simulated and measured I-V parameters . 68 5.3.2 Bifacial module characterisation for bifacial illumination 70 5.4 Conclusions . 74 IV CHAPTER - Investigation of PV module structures with bifacial solar cells and performance analysis under STC . 75 6.1 Introduction . 75 6.2 Quantifying the effects of different module structures on module current 77 6.2.1 Effect of bifacial cell transmittance on module current . 78 6.2.2 Effect of cell-gap region on module current 83 6.2.3 Mini-module fabrication and experimental analysis 89 6.3 Comparison of glass/glass and glass/backsheet module structures . 92 6.3.1 Maximum possible benefit from glass/backsheet module: Module optimisation 92 6.3.2 Benefits of the glass/glass bifacial module structure . 94 6.4 Glass/glass bifacial module measurement under STC 95 6.5 Conclusions . 97 CHAPTER - Cell-to-module losses in silicon wafer-based bifacial (and monofacial) PV modules 98 7.1 Introduction . 98 7.2 Quantifying CTM loss: The methodology 101 7.2.1 CTM loss for single-sided (monofacial) illumination . 102 7.2.1.1 Optical loss/gain . 102 7.2.1.2 Mismatch loss 105 7.2.1.3 Resistive loss 106 7.2.2 CTM loss under bifacial illumination 108 7.2.2.1 Optical loss/gain under bifacial illumination . 108 7.2.2.2 Mismatch loss under bifacial illumination . 109 7.2.2.3 Resistive loss under bifacial illumination 110 7.3 CTM loss analysis: Experimental . 111 7.3.1 CTM loss for single-side illumination (front side) 112 7.3.1.1 Optical loss . 112 7.3.1.2 Mismatch loss 115 7.3.1.3 Resistive loss 115 7.3.2 7.4 CTM loss under bifacial illumination 118 Conclusions . 120 V CHAPTER - Investigation of outdoor performance and cost potential of bifacial PV modules 122 8.1 Introduction . 122 8.2 Outdoor installation set-up and measurement . 123 8.3 Analysis of experimental data and results . 125 8.3.1 Comparison of bifacial and monofacial modules 125 8.3.1.1 Performance ratio of the module at different tilt angles 125 8.3.1.2 Isc gain for different time of the day . 128 8.3.1.3 Variation of Isc gain with diffuse/global irradiance ratio . 129 8.3.2 Comparison between vertical and 10° South facing installation of bifacial modules . 130 8.4 LCOE analysis of PV systems based on energy gain from bifacial PV modules . 132 8.5 Conclusions . 135 CHAPTER - Conclusions and proposed future work 136 9.1 Thesis conclusions . 136 9.2 Original contributions . 140 9.3 Proposed future work 142 Publications arising from this work . 144 Bibliography . 146 VI ABSTRACT Bifacial solar cells can convert incident sunlight to electrical energy from both sides of the cells. Thus, bifacial photovoltaic (PV) modules can effectively increase the energy yield as compared to conventional monofacial modules by utilizing the albedo (light scattered from the ground and the surroundings) when operating in real-world outdoor conditions. However, there are a number of technical challenges in the development of bifacial devices and deploying them into the mainstream PV systems. One of the main challenges is the lack of an established indoor measurement standard to characterise bifacial solar cells and modules. This thesis focuses on characterisation and standardisation of bifacial solar cells and modules, and on performance evaluation of these devices in indoor and outdoor environments. Various new methodologies are developed to investigate the performance of bifacial devices, by employing in-depth analysis of various electrical and optical loss mechanisms. Initially, to characterise bifacial solar cells and modules for simultaneous bifacial illumination, new methods were introduced. The proposed methods require only standard monofacial indoor measurement setups to measure the front and rear side of the device separately under standard test conditions (STC). Two new parameters, bifacial 1.x efficiency and gainefficiency product (GEP) are introduced for a complete characterisation of bifacial solar cells. The new methods provide 1) a means for fundamental study and optimisation of bifacial solar cells and modules under bifacial illumination conditions, and 2) information related to energy yield and the VII end-use benefits in real-world operating conditions. The validity of these methods is examined using measurements on a silicon wafer based bifacial module. The module’s output power calculated using the method agrees to within 1% with the measured power for a number of illumination conditions on front and rear sides of the bifacial modules. This thesis also investigates the performance of bifacial silicon solar cells encapsulated in two different module structures: glass/glass and glass/ backsheet. It is found that, under STC measurements, a glass/glass module construction causes a net cell-to-module current loss due to the rear-side encapsulation. In contrast, a glass/backsheet module with a standard cell gap offers 2-3% higher power output under STC as compared to a glass/glass module. The results show that, under STC, the maximum possible cost reduction benefits of glass/backsheet modules over glass/glass modules are limited to approximately 3.3%. Considering this result and the outdoor potential of bifacial PV modules, a methodology to measure and rate bifacial glass/glass modules under STC is presented. The new rating methodology, if accepted by the PV community, would allow module manufacturers to get some of the benefits, by being able to sell bifacial modules at a premium price compared to glass/backsheet modules while retaining substantial benefits for the end-users. Further, a method to quantify the losses in the cell-to-module (CTM) process is developed for silicon wafer based bifacial PV modules. The CTM losses are quantified in terms of the individual loss components, i.e. optical, mismatch and resistive losses, for single-sided illumination (monofacial) and then extended to bifacial illumination conditions. The method is useful in VIII [3] J. P. Singh, Y. S. Khoo, A. G. Aberle, and T. M. Walsh, “Standardization of bifacial PV devices”, presented in PV Asia Scientific conference, Singapore, 2014. [4] Y. S. Khoo, J. P. Singh, T. M. Walsh and A. G. 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Reise, "High Performing PV Systems for Tropical Regions-Optimization of Systems Performance," in 27th European Photovoltaic Solar Energy Conference and Exhibition, Messe Frankfurt, Germany, 2012, pp. 3763-3769. 160 [...]... bifacial solar cell and modules on mass scale [31] 1.3 Thesis motivation and objectives Despite the obvious advantages of bifacial solar cells and modules, and their promising potential for cost reductions of PV power, the share of bifacial modules in the market as of today is almost negligible There are a number of challenges and problems associated with the deployment of the bifacial modules in solar. .. challenges and problems in detail and suggest solutions to increase the market share of bifacial solar cells and modules The development of robust characterisation techniques and standard tests for bifacial solar cells and modules is important for two reasons: 1) They enable researchers to properly measure and characterise the devices in order to understand their behaviour and improve their performance, ... advantage of cost reduction via energy gain from a bifacial PV module, it is necessary to address the challenges mentioned above Each of the topics mentioned above is vast and can be studied in separation This work focuses on the characterisation and standardisation of bifacial solar cells and modules and the performance of bifacial PV modules in indoor and outdoor conditions The main objectives of this... standards to characterise bifacial solar cells and modules [32-34] 2 Rating and standardisation: No standard method is available to rate the bifacial cells and modules using indoor measurements [35] 3 Bifacial solar cell (and module) fabrication: Additional complex steps in the cell fabrication process and associated costs [36-38] 6 4 Outdoor energy yield: Installation dependent module performance and. .. Thus, one of the major focuses of this work will be on development of characterisation methods for bifacial solar cells and modules and standardising these devices under indoor testing conditions In addition, the indoor and outdoor performance of bifacial PV modules will be analysed and compared with that of monofacial modules 7 1.4 Thesis Structure This PhD thesis consists of 9 chapters Chapter 1 highlights... APPLICATIONS AND CHALLENGES WITH BIFACIAL SOLAR CELLS AND MODULES 2.1 Background As discussed in the previous chapter, the use of bifacial PV devices can significantly reduce the cost of PV electricity Bifacial devices offer several advantages over standard monofacial solar cells and modules in terms of additional energy yield in outdoor conditions as well as cell efficiency improvements under standard test... fabrication, such as hetero-junction bifacial cells [40], c-Si bifacial cells (n-type/p-type, mono/multi) [41-43], dyesensitized bifacial solar cells [44], rear contact bifacial cells [45], etc With bifacial solar cells, two different module structures are possible, i.e bifacial (glass/glass) and monofacial (glass/backsheet) structures The most important application of bifacial solar cells is the glass/glass PV... Figure 2.1 Schematic of a standard monofacial (left) and bifacial (right) silicon wafer solar cell [36] The rear side of the bifacial cell structure shown above is without texture However, almost all commercial bifacial cells are textured on both sides to enhance light trapping and hence current response 11 2.1.2 History of bifacial solar cells and modules Bifacial solar cells have been investigated...understanding the loss mechanisms and identifying the root causes of CTM losses in wafer-based bifacial (and monofacial) PV modules The calculations of individual loss components are explained with the fabrication and experimental analysis of single-cell mini -modules and 4-cell modules using bifacial solar cells The measurements show that the resistive loss in the CTM process is important for bifacial. .. both the cell and module level by reducing the wafer and cell breakage, and also allows the use of thinner wafers Thus, one of the motivations behind the development of bifacial solar cells was to improve the cell performance by minimizing the above mentioned loss and problems Due to the potential benefits of bifacial cells and modules, many researchers are exploring bifacial PV technologies and the PV . solar cells and modules. This thesis focuses on characteri- sation and standardisation of bifacial solar cells and modules, and on performance evaluation of these devices in indoor and outdoor. and associated cost 22 2.3.3 Characterisation and standardisation of bifacial devices 24 2.3.4 Rating and cost estimation of bifacial solar cells and modules . 26 CHAPTER 3 - Fabrication and. (FF bi ) 45 4.2.4 Bifacial 1.x efficiency and gain-efficiency product 47 4.3 Examples: Analysis of bifacial solar cells 49 4.3.1 Comparison of two bifacial cells with different front and rear side