Solar Energy Conversion and PEM Technology for a Residential Back-up Power System by Aaron Biddings A thesis submitted to the Graduate Faculty of Auburn University in partial fulfillment of the requirements for the Degree of Master of Science Auburn, Alabama May 4, 2013 Keywords: photovoltaic, electrolysis, proton exchange membrane Approved by Steven M. Halpin, Chair, Professor, Electrical and Computer Engineering Department Charles A. Gross, Professor Emeritus, Electrical and Computer Engineering Department Mark Nelms, Professor and Chair, Electrical and Computer Engineering Department ABSTRACT Shown in this work are the characterization, simulation, and implementation of a small- scale back-up power system powered through solar energy conversion. The technical approach utilizes solar arrays with a proton exchange membrane (PEM) electrolyzer and fuel cell. This facilitates the production and storage of electrical energy using hydrogen as a carrier. Proton exchange membrane (PEM) technology was chosen due to its relatively high power density (in comparison with other fuel cell types), its low operating temperature range, and lack of noxious emissions. The setup employs a solar array designed to provide sufficient power to the electrolyzer under varying temperature and lighting conditions. The electrical power is derived directly from the electrochemical energy stored in the hydrogen supply. In this way, the inherent instability and fluctuations in power due to the intermittent nature of solar energy are diminished. The focus of this study is strictly technical. Monetary cost considerations are beyond the scope of this document. ii TABLE OF CONTENTS Abstract…………………………………………………………………………………..….…....ii List of Tables………………………………………………………………………………..……iv List of Illustrations…………………………………………………………………………..…….v List of Abbreviations………………………………………………………………………..……vi Chapter 1: Introduction……………………..………………………………………………...…..1 Chapter 2: Proton Exchange Membrane Fuel Cell….………………………………………….....3 Chapter 3: Proton Exchange Membrane Electrolyzer..………………………………………….10 Chapter 4: Solar Energy Conversion…………….………………………………………………16 Chapter 5: Conclusion…………………………………………………………………………...33 Chapter 6: Further Research……………………………………………………………………..35 References……………………………………………………………………………...………..36 Appendix A: Determination of Irradiance……………………………………………………….38 Appendix B: Matlab Code For Simulations……………………………………………………...43 iii LIST OF TABLES Table A-1. Light Energy, Power, and Irradiance………………………………………………...40 Table A-2. Error Calculations……………………………………………………………………41 Table A-3. Adjusted Light Energy, Power, Irradiance, and Percent Error………………………42 iv LIST OF ILLUSTRATIONS Figure 2-1. A) Membrane Electrode Assembly, B) Gas Diffusion Layers, C) Bipolar Plates D) Current Collectors, E) Compression Plates……………….………………….…...3 Figure 2-2. Activation Loss: 0A-0.2A; Resistive Loss: 0.2A-0.6A; Mass Transport Loss: 0.6A-0.99A……………………………………………………6 Figure 3-1. A) Membrane Electrode Assembly, B) Gas Diffusion Layers, C) Bipolar Plates D) Current Collectors, E) Compression Plates……………………………………...10 Figure 3-2. Input Voltage vs. Input Current…..…….…………………………………………...12 Figure 3-3. Hydrogen Production Rate vs. Input Power.………………………………………...13 Figure 4-1. Silicon Solar Cell Cross-section……………………………………………………..17 Figure 4-2. Simple Model of Silicon Solar Cell….……………………………………………...18 Figure 4-3. Typical Solar Cell I-V Curve………………………………...……………………...19 Figure 4-4. Effect of Decreasing Shunt Resistance…………………………………….………..21 Figure 4-5. Effect of Increasing Series Resistance…………….………………………………...22 Figure 4-6. Effect of Increasing Current…………………………………..……………………..24 Figure 4-7. Measured Output Current/Voltage Characteristic…………………………………...25 Figure 4-8. Impedance Plot (0.001 Hz ≤ f ≤ 1MHz)…………………………………………….27 Figure 4-9. Nyquist Plot Representative Circuit…………………………………………………27 Figure 4-10. Effect of Light Intensity on Solar Cell Performance………………….………...…29 Figure 4-11. V and I as Functions of Temperature (°C)…………………………………….31 OC SC v LIST OF ABBREVIATIONS EIA Energy Information Administration H O Water (Dihydrogen Monoxide) 2 M Molar Solution (mol/L) MEA Membrane Electrode Assembly PEM Proton Exchange Membrane vi CHAPTER 1: INTRODUCTION Since the late nineteenth century, when Thomas A. Edison premiered his Pearl Street Station in New York City on September 4, 1882, the generation, transmission, and utilization of electric power has become an indispensible part of everyday living. Much advancement in both equipment and operating practices has been put into effect since those early days. In the United States, the decade of the 1960’s was a time of high load growth due to both industrial and commercial expansion. Initially, regulated utilities generated and delivered electric power within localized service areas. Today, the transmission system delivers across states or even regions [1]. In the United States, due to the ubiquity and reliability of the grid, the availability of electric power is generally taken for granted by the public at large. That is, until problems arise and the system fails to deliver. One extreme example of such an outage occurred in 2003 when a wide-area power failure affected millions of Americans across several northeastern states. While outages of this magnitude aren’t the norm in the United States, smaller failures do happen occasionally due to various causes ranging from bad weather to aging equipment. In the absence of the main power source, alternate sources may be desired for residential application. Solar arrays used in conjunction with PEM technology may make this a possibility. Proton exchange membrane (PEM) fuel cells run on hydrogen, which is reported to be the most abundant element on earth. However, it is not usually found in its pure form and must be extracted from a compound. One such well-known compound is none other than H₂O 1 (water). In the United States, water is supplied directly to most homes. Furthermore, it is unlikely that both the electric power and water supply would fail simultaneously. Therefore, a back-up power system utilizing solar arrays, a PEM electrolyzer, and a PEM fuel cell may be a viable source of intermittent power. The U.S. Energy Information Administration (EIA) reported the average annual electricity consumption for a U.S residential utility customer to be 11,496 kWh in 2010, an average of 958 kWh per month [2]. Thus, the back-up system would need to generate and store enough hydrogen to supply about 32 kWh for daily use. 2 CHAPTER 2: PROTON EXCHANGE MEMBRANE FUEL CELL The proton exchange membrane (PEM) fuel cell is an electrochemical cell which functions to combine gaseous hydrogen with oxygen. In the process, only electricity, water, and heat are generated. The basic structure of the PEM fuel cell consists of a membrane electrode assembly (MEA), two gas diffusion layers, two bipolar plates, two current collectors, and two compression plates. See Figure 2-1 for the diagram. Figure 2-1. A) Membrane Electrode Assembly, B) Gas Diffusion Layers, C) Bipolar Plates D) Current Collectors, E) Compression Plates The electrochemical reaction that occurs within the fuel cell is described by the equations 2-1 through 2-3 [3]. Anode (oxidation): (2-1) Cathode (reduction): (2-2) Overall reaction: (2-3) The relationship between the fuel cell potential and the concentrations of the products and reactants involved in the electrochemical reaction is described by equations 2-4 and 2-5 [4]. 3 ∏ (2-4) ∏ (2-5) Equation 2-5 is known as the Nernst equation, where the following definitions apply: ( ) ( ) Note that in these definitions, standard conditions are P = 1 atm, T = 298.15 K (77 °F), and 1.0 M concentrations. In order to calculate the fuel cell potential from the redox reaction, it is necessary to obtain the standard potential values for each half-cell reaction within the cell. These may be obtained from the list of standard reduction potentials and are exemplified by equations 2-6 and 2-7 [5]. (0.0 V) (2-6) (1.23 V) (2-7) 4