Analyzing the Efficiency of Solar-to-Hydrogen Production for Renewable Energy Storage

Abstract

Although the advent of renewable energy technology (solar panel, windmills, etc..) has been around for decades, it is still plagued with issues of being intermittent and requiring additional technology for storing excess energy. One such technology being investigated is converting excess electrical power, from solar panels, into hydrogen gas. The hydrogen gas can then be stored and used, at a later time, by either burning it and driving a steam turbine or with fuel cells. To produce the hydrogen gas, a DC current is often utilized to split water into its two components: oxygen and hydrogen. This phenomena is called electrolysis and is realized using an electrolyzer. As with any energy conversion, efficiency is an important consideration, and energy is lost when converting from one form to another. In the case of converting sunlight to hydrogen, a major source of energy loss is the connection between the solar panels and the electrolyzer. This is a result of the mismatch between the ideal impedance of the electrolyzer and the maximum power operating point (MPOP) of the solar panel. The purpose of this thesis is to study this mismatch and to improve the efficiency of the energy conversion process using silicon photovoltaic (PV) cells, a proton exchange membrane (PEM) electrolyzer and a DC-DC converter. The experiment is set up to use a solar panel assembled from individual PV cells, combined in parallel and series connections, to match the MPOP of the solar panel to the ideal power operating point of the PEM electrolyzer. This matching was done at the sun's zenith (when the greatest amount of irradiance hits Earth's surface) with respect to Earth's horizon, during a day's rotation. At first, the panel was connected directly to the electrolyzer and the efficiency was measured. For the second part of the experiment, the panel was connected to a DC-DC converter which regulated the solar panel's output voltage to the electrolyzer and adjusted the impedance to match the MPOP of the panel throughout the day. Results from outdoor testing show that regulating the impedance improved the solar panel's efficiency, but decreased the overall efficiency of the system. This is mainly due to the fact that, in order to regulate the impedance, the current is reduced to achieve the desired MPOP; this in turn decreases the production of hydrogen gas because there are less electrons for the electrolysis reaction to take place with. At an essential level, this study's aim is to provide an understanding of the conversion process for utility companies to build and optimize solar-to-hydrogen systems for energy storage.