Thermal Behavior of Photovoltaic Devices

Transcription

1 Thermal Behavior of Photovoltaic Devices

2 Olivier Dupré Rodolphe Vaillon Martin A. Green Thermal Behavior of Photovoltaic Devices Physics and Engineering 123

3 Olivier Dupré Centre for Energy and Thermal Sciences of Lyon, CNRS, Univ Lyon, INSA-Lyon Université Claude Bernard Lyon 1 Villeurbanne France Rodolphe Vaillon Centre for Energy and Thermal Sciences of Lyon, CNRS, Univ Lyon, INSA-Lyon Université Claude Bernard Lyon 1 Villeurbanne France Martin A. Green Australian Centre for Advanced Photovoltaics, School of Photovoltaic and Renewable Energy Engineering University of New South Wales Sydney Australia and Radiative Energy Transfer Laboratory, Department of Mechanical Engineering University of Utah Salt Lake City USA ISBN ISBN (ebook) DOI / Library of Congress Control Number: Springer International Publishing AG 2017 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. Printed on acid-free paper This Springer imprint is published by Springer Nature The registered company is Springer International Publishing AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

4 Preface Although long known that temperature negatively affects the performances of PV devices, research and engineering in photovoltaics has been mainly focused on reducing the optical and electrical losses. This book is the result of several years of research dedicated to understand better the thermal behavior of photovoltaic devices. The work on this topic was triggered by simple questions such as: Why does temperature negatively affect the efficiency of most photovoltaic systems? Is it possible to engineer the temperature sensitivities of solar cells? If it is not possible to make their performance completely independent of temperature, what can be done to minimize their operating temperatures? In this book, we discuss the different temperature-driven phenomena that are important in the field of photovoltaics. We provide detailed physics-based explanations of the mechanisms involved and review different solutions and strategies to mitigate the temperature-induced losses. The original perspective we have taken in this book to describe the fundamental principles of photovoltaic conversion has its origins in the diversity of our backgrounds. Throughout the book, we introduce a novel approach to optimize the design of photovoltaic devices where thermal criteria are integrated. Because the thermal behavior of a photovoltaic system is deeply intertwined with its electrical and optical/radiative characteristics, the optimum set of design parameters is always a function of the operating conditions, which depend on the location and the mounting configuration of the system. This means that photovoltaic devices can be designed to maximize their energy yield in specific conditions (climates, installations, etc.). The present book provides models and concepts to guide researchers and engineers towards the development of this kind of optimization that goes beyond the standard test conditions. We are very grateful to all the persons that have motivated, through direct contacts or publications, our research and those who helped during the preparation of the book. Yannick Riesen and Ricky Dunbar deserve special mention for stimulating discussions about the specific thermal behavior of silicon heterojunction and perovskite-based solar cells. We also thank particularly Anne Gerd Imenes, Sarah Kurtz and David Moser for kindly providing data of photovoltaic modules installed all around the world. v

5 vi Preface Rodolphe Vaillon was hosted by the Department of Mechanical Engineering at the University of Utah when the present book was prepared. The financial support from the College of Engineering (W.W. Clyde Visiting Chair award) is acknowledged. Villeurbanne, France Salt Lake City, USA Sydney, Australia Olivier Dupré Rodolphe Vaillon Martin A. Green

6 Contents 1 Thermal Issues in Photovoltaics and Existing Solutions The Effects of Temperature on the Performances of Photovoltaic Devices Reversible Decrease of PV Performances with Temperature Thermal Annealing and Staebler-Wronsky Effect in Amorphous Silicon Solar Cells Temperature and Module Degradation Predicting the Operating Temperature and Energy Yield of PV Installations Reducing the Operating Temperature of PV Devices Common Cooling Strategies Hybrid Photovoltaic-Thermal Solutions Building Integrated PV and Floating PV Radiative Cooling Thermal Design of PV Cells and Modules References Temperature Coefficients of Photovoltaic Devices Definition Fundamental Conversion Losses and Temperature Coefficients of Solar Cells The Detailed Balance Principle and the Thermodynamic Argument Influence of Bandgap Temperature Dependence and Incident Spectrum Loss Mechanisms and Temperature Coefficients of Actual Solar Cells Open-Circuit Voltage Temperature Sensitivity Short-Circuit Current Temperature Sensitivity Fill Factor Temperature Sensitivity vii

7 viii Contents 2.4 Tuning the Temperature Coefficients Conclusion Appendix Appendix References A Thermal Model for the Design of Photovoltaic Devices Why a Thermal Model for Photovoltaic Devices? Model for the Heat Source Model for the Equilibrium Temperature Dependence on Voltage of the Heat Source and the Cell Temperature Temperature Dependent Power Output as a Function of Voltage Revisiting the Definition of Nominal Operating Cell Temperature Beyond Standard Test Conditions: Taking into Account Field Operating Conditions in the Design of Photovoltaic Devices Sub-bandgap Energy Photon Filtering High-Energy Photon Filtering Bandgap Optimization Optimization of Other Parameters Conclusion References Specificities of the Thermal Behavior of Current and Emerging Photovoltaic Technologies Standard Silicon Solar Cells Silicon Hetero-Junction Solar Cells Compensated Silicon Solar Cells Amorphous Silicon Solar Cells Perovskite Solar Cells Multi-junction Solar Cells Concentrator Photovoltaics Thermophovoltaic Devices References Index

8 Nomenclature Fundamental Constants c Speed of light in vacuum ( ms 1 ) h Planck constant ( Js) k Boltzmann constant ( JK 1 ) q Elementary charge ( C) r Stefan Boltzmann constant r ( Wm 2 K 4 ) Latin Symbols D Diffusion coefficient (m 2 s 1 ) E Photon energy (ev) E c/v Conduction/valence band edge (ev) E Fn/p Quasi-Fermi level of the electrons/holes (ev) E g Energy bandgap (ev) EQE External quantum efficiency ( ) ERE External radiative efficiency ( ) f Fraction of photons ( ) G Parameter of interest h Heat transfer coefficient (W m 2 K 1 ) I Current (A) J Current density (A m 2 ) L Diffusion length (cm) n Photon flux density (m 2 ) or diode ideality factor ( ) or electron concentration (cm -3 ) N Doping concentration or density of states (cm -3 ) p Hole concentration (cm -3 ) P Power loss per unit area not being a heat source (W m 2 ) ix

9 x Nomenclature PFD Photon flux density (m 2 ) PFD bb(t) Photon flux density emitted by a blackbody at temperature T (m 2 ) Q Component of the heat generated in the device per unit area, heat flux (W m 2 ) R(E) Spectral reflectance of the PV device ( ) R Resistance (X) S Entropy (J/K) T(E) Spectral transmittance of the PV device ( ) T Temperature (K or C) V Voltage (V) V max Maximum voltage (V) W Cell thickness (m) or useful work X Concentration factor ( ) Greek Symbols b Temperature coefficient (ppm K 1 or % K 1 ) η Conversion efficiency ( ) D For specifying a current (DJ) or a voltage (DV) loss with respect to the maximum e(e) Spectral emittance of the PV device ( ) X (projected) solid angle P Peltier coefficient (V) l Chemical potential (ev) s Carrier lifetime (s) n Parameter closely related to the product of carriers np introduced in (Green 2003) c Parameter that includes the temperature dependences of several parameters determining J 0 Subscripts 0 In the dark (applies to the current density) 1sun Under one sun illumination abs Applies to the (projected) solid angle within which the cell absorbs radiation from the source am Refers to the angle mismatch loss amb Ambient

10 Nomenclature bb below E g c Carnot cond, conv, rad current DJ, DV e emit ex G i ib inc MPP oc par rad s sc sh xi Refers to the case where the illumination (or the cell) radiation is modelled by a blackbody (Planck function) Refers to the sub-bandgap loss Refers to the cell Carnot fundamental loss Apply to the heat fluxes exchanged by the cell with its surrounding by conduction, convection, radiation For the fraction of photons that participate to the current Apply to the heat source components associated to a current loss or a voltage loss Environment with which the device exchanges thermal radiation Applies to the (projected) solid angle within which the cell emits radiation For the fraction of emitted photons that exit the cell Applies to the parameter G of interest Intrinsic (applies to the carrier concentration without doping) For the fraction of photons absorbed through the interband process Incident upon the cell Refers to the maximum power point Refers to the open circuit configuration For the fraction of photons absorbed through parasitic processes In the radiative limit Refers to the Sun (equivalent blackbody temperature only) or series (for resistances) Refers to the short circuit configuration Shunt (for resistances)

11 Abbreviations AM ARC BC BSF c-si CdS CdTe CIGS CPV EG-Si EQE ERE EVA FCA FF FZ GaAs Ge HIT InP IR LED MPP NF-TPV NOCT NRR PERC PERL PFD PR Air mass Anti reflection coating Boundary condition Back surface field Crystalline Silicon Cadmium Sulfide Cadmium Telluride Copper Indium Gallium Selenide Concentrated photovoltaics Electronic grade silicon External quantum efficiency External radiative efficiency Ethylene-vinyl acetate Free carrier absorption Fill factor Float zone Gallium Arsenide Germanium Heterojunction intrinsic thin layer Indium Phosphide Infrared Light-emitting diode Maximum power point Near-field thermophotovoltaic Nominal operating cell temperature Nonradiative recombination Passivated emitter and rear cell Passivated emitter with rear locally diffused Photon flux density Photon recycling xiii

12 xiv Abbreviations PV RR SHJ Si SiN SQ SRH STC STPV TC TPV UV ZnS Photovoltaic Radiative recombination Silicon heterojunction Silicon Silicon Nitride Shockley Queisser Shockley Read Hall Standard test conditions Solar thermophotovoltaic Temperature coefficient Thermophotovoltaic Ultraviolet Zinc Sulfide