UNIVERSITY OF SOUTHAMPTON The Efficiency of Photovoltaic Systems by Athanasios Gousiopoulos A thesis for the degree of Doctor of Philosophy in Electrical and Electronic Engineering Faculty of Physical Sciences and Engineering Department of Electronics and Computer Science November 2016 UNIVERSITY OF SOUTHAMPTON ABSTRACT FACULTY OF PHYSICAL SCIENCES AND ENGINEERING DEPARTMENT OF ELECTRONICS & COMPUTER SCIENCE Doctor of Philosophy The Efficiency of Photovoltaic Systems by Athanasios Gousiopoulos At this work the principle aim is to create new low cost hardware test circuitry that can emulate the behavior of solar cells under different insolation and temperature conditions. Another goal is to propose a new MPPT algorithm that could accurate and reliable estimate the maximum power operating point. The transcendental equation describing a solar cell IV characteristic contains current on both sides in a function of the form I = f (V, I). In this work, a current- independent voltage expression is derived for the maximum power point as a function of a new variable which is mathematically well defined. Validation is performed on four different photovoltaic modules. The best case scenario has shown a divergence in Pmpp of 0.08% while the worst 0.84%. The new method is examined for sensitivity, up to ± 5% on values of five fitting parameters (Rs, Rsh, n, Io, Iph), with Iph to have the higher impact effect (up to 5% error). Two topologies, of VBE multiplier, have been proposed that can reliably emulate solar cell operation. Analysis shows that both circuits have the potential to deliver good quality characteristic IV curves with small RMSE (10-3). The second improved VBE multiplier has the ability to operate over wider range of illumination and temperature conditions. Through this model it was possible to build a low cost (£230) fully functional prototype digital controlled emulator solar system “DiceSol”. A MPPT device has implemented based on a powerful mathematical modeling tool. The proposed method “Goose Waddle (GW)” was employed on a boost DC/DC converter configuration. Excellent static and dynamic performances were exhibited. At all cases considered convergence is higher than 98.53% while convergence time is on average 220 msec. Table of Contents Table of Contents ................................................................................................... i List of Tables ....................................................................................................... vii List of Figures ....................................................................................................... xi Declaration of Authorship ................................................................................. xxi Acknowledgements ........................................................................................... xxiii 1 Introduction ........................................................................................... 1 Background and Motivation ...................................................................... 1 Aims of this Work ...................................................................................... 7 Structure of Report ..................................................................................... 8 2 Literature Review ................................................................................ 11 Device Physics ......................................................................................... 11 P-N Junction............................................................................................. 14 Solar Cell Characteristics ......................................................................... 17 Fundamentals of Silicon Solar Cell Design ............................................. 19 Efficiency Limits ..................................................................................... 20 Equivalent Circuits................................................................................... 22 2.6.1 Comparison of single diode model and two diode model of photovoltaic module. ................................................................. 25 Dependencies on insolation and temperature .......................................... 28 2.7.1 Insolation ................................................................................... 28 2.7.2 Temperature ............................................................................... 29 Software Simulator Program - PC1D ...................................................... 31 2.8.1 PC1D modelling ........................................................................ 32 DC to DC Converters ............................................................................... 34 2.9.1 Buck, Boost, Buck-Boost converter .......................................... 34 2.9.2 Buck Converter .......................................................................... 35 2.9.2.2 Continuous – Conduction Mode ................................................ 37 2.9.3 Boost Converter ......................................................................... 39 2.9.3.4 Inductor Current Ripple – Inductor Choice ............................... 48 2.9.3.5 Output Capacitor Choice ........................................................... 50 i 2.9.4 Buck-Boost Converter ............................................................... 53 2.9.5 Measuring Regulator Efficiency of DC-DC Converters ........... 57 MPPT Methodologies -Techniques ......................................................... 58 2.10.1 Introduction ............................................................................... 58 2.10.2 Perturb and Observe .................................................................. 58 2.10.3 Hill-Climbing ............................................................................ 60 2.10.4 The incremental conductance method ....................................... 61 2.10.5 Constant Voltage (CV) .............................................................. 63 2.10.6 Fractional open-circuit voltage control ..................................... 64 2.10.7 Fractional short-circuit current control ..................................... 65 2.10.8 Other “hill climbing” maximum power point tracking methods66 2.10.9 Temperature Gradient Method .................................................. 69 2.10.10 Current and Temperature Method (I&T) .................................. 70 2.10.11 Look Up Table (LUT) ............................................................... 70 2.10.12 Simulation Results Evaluating MPPT Methods ........................ 71 2.10.13 Discussion ................................................................................. 76 Review on current solar simulators ......................................................... 79 2.11.1 Solar simulator based on PV Sandia Database ......................... 79 2.11.2 Solar simulator light sources ..................................................... 80 2.11.3 SunaPlayer: Emulation Platform of Solar Cells ........................ 83 Conclusions ............................................................................................. 85 3 Derivation of maximum power point ................................................ 89 Introduction ............................................................................................. 89 The current - voltage relationship for a photovoltaic device ................... 89 Theoretical Analysis - Equations for Determining Impp, Vmpp, Pmpp and A90 Experimental Validation.......................................................................... 96 Resistor Sensitivity to MPP ..................................................................... 97 Sensitivity of parameters Io, Iph, n to MPP ............................................ 102 Conclusion ............................................................................................. 110 4 Modeling of Photovoltaic Cell Operation and Behavior Using and Improved VBE Multiplier .................................................................. 111 Introduction ........................................................................................... 111 ii Schematic overview of the experimental setup. .................................... 112 4.2.1 Electronic load - Principle of operation ................................... 113 Analytical Model of a PV Cell............................................................... 115 Basic VBE Multiplier ............................................................................... 118 4.4.1 Parameter extraction from PC1D ............................................. 120 4.4.2 Parameter extraction for the Transistor ................................... 123 4.4.3 Error Calculation ..................................................................... 126 4.4.4 Series Resistance Compensation ............................................. 126 4.4.5 Experimental Results – Discussion ......................................... 129 Improved VBE Multiplier ........................................................................ 136 4.5.1 Extracting PV Cell Parameters from PC1D ............................. 138 4.5.2 Improved VBE Multiplier – Application to Modelling Photovoltaic Cell Behavior ...................................................... 139 4.5.3 Parameter Extraction for the Transistor ................................... 143 4.5.4 Series Resistance Compensation ............................................. 145 4.5.5 Temperature Compensation ....................................................