Fuel cell electric vehicles (FCEVs) are automobiles that use hydrogen fuel instead of carbon-based fuels. Because the byproduct of burning hydrogen fuel is water, it is a much cleaner form of energy at the vehicle level. Large companies such as General Motors plan to begin selling FCEVs by 2015, and over 100 FCEVs Chevrolets have collectively driven over one million miles. But the widespread adoption of FCEVs is hampered by the current cheap prices of carbon fuels and the general lack of hydrogen fuel infrastructure. Researchers believe, however, that solar energy hydrogen generation systems based in single homes are a viable system for fueling FCEVs. Kelly et al. (2011) at General Motors have already built a photovoltaic (PV) powered electrolyzing/storage/dispensing (ESD) system for use as a single FCEV home fueling system. However, the system has only been tested for 14 days, and the widespread effect of day to day operation of the system on its efficiency is unknown. The next step is to measure the efficiency and other characteristics of such systems.—Alan Hu
N.A., Kelly, T.L., Gibson, D.B., Ouwerkerk. 2011. Generation of high-pressure hydrogen for fuel cell electric vehicles using photovoltaic-powered water electrolysis. International Journal of Hydrogen Energy 36, 15803–15825.
Kelly et al. at General Motors previously built PV-ESD system consisting of a set of solar arrays and an ESD system. Four solar arrays were used, each having 10 Sanyo HIP-190BA3 modules. The modules were wired parallel in each array, and each array was wired parallel to the electrolyzer. As such, the PV system voltage output was equal to that of one module whereas the system current output was 40 times that. The solar array tilt angle could be significantly altered to maximize solar irradiance depending on the season. The second part of the PV-ESD system is the electrolyzer/storage/dispenser system. The Avalence electrolyzer used was cylindrical and could contain hydrogen and oxygen produced at high pressures up to 6500 psi. Due to problems in past experiments, however, the system was not run at 6500 psi but rather at 2000 psi. As such, the system could store only 2kg of hydrogen as opposed to 6kg.
In order to evaluate the performance of the PV-ESD system, Kelly et al. used the Sandia Photovoltaic Array Performance Model (SPAPM) to measure the voltage, current, and power values of the PV-ESD system. The SPAPM uses a set of outdoor performance measurements and output current at two other voltage values to determine the I-V curve of the PV module. Researchers analyzed the I-V curves of the PV and electrolyzer separately. A resistive load bank was used to measure the I-V curve of the PV-ESD on a sunny day while a Sorensen DC power supply was used to measure the I-V curve of the electrolyzer. These figures were used to calculate important PV figures through the SPAPM model including module efficiency and maximum power.
After constructing a theoretical model, researchers began actual testing of the PV-ESD system. The study was run from November 2008 to October 2009 with a period of inactivity between December 16, 2008 and March 24, 2009 due to the inability of the ESD to run in sub-freezing temperatures. The study was run for a total of 109 days between sunrise and sunset, though for about 10% of the days, less than one hour of data was collected due to problems in the electrolyzer system. Researchers found that on sunny days, system efficiency started low, peaked, and then dipped at around noon. In the afternoons, after the noon dip, efficiency rose until the sunset drop. On cloudy days, the PV efficiency showed sensitivity to short increases in solar irradiance caused by passing clouds and the associated increase in temperature. In general, researchers found that PV efficiency was tied to solar irradiance, the temperature of the PV system, and the impedance of the load that it is connected to. The ESD efficiency was dependent on the operating voltage and the electrolysis cell temperature. Electrolyzer operative voltage depended on its impedance and electrolysis cell temperature affected the electrolyte conductivity.
Based on the experience of the researchers various possibilities for improvement were considered. The experimenters recommended that the anode and cathode be reversed, that compression energy stored in high pressure hydrogen be captured and used to do work, that the membrane of the electrolyzer be made alkaline, that excess heat from the PV, which decreases its efficiency, be transported to the ESD, which increases its efficiency. In general, the experiment was a proof-of-concept for a single FCEV fueling system. The first phase of the experiment concerned the design and construction of the PV-ESD system while the second tested the built PV-ESD system. It was found that the electrolyzer responded well to the constantly changing solar irradiance caused by passing clouds and that day to day temperature variations did not decrease its efficiency. The coupling factor of the combined PV-ESD system was calculated by comparing maximum power and efficiency of the PV and the actual power and efficiency of the PV. The researchers concluded that 1) the system operated without any major failures of the high-pressure electrolysis system, which had previously been a problem, 2) solar energy to hydrogen efficiency averaged 8.2%, 3) coupling factor averaged 0.91, 4) the system produced 0.67 kg of hydrogen over a full day of operation, and 5) solar to hydrogen efficiency is less than a third as efficient on an energy utilization per mile basis as solar battery charging.