The effect of a ripple wave power supply on PEM electrolysis efficiency

Abstract

The two matured water electrolysis technologies commercially available today are Alkaline water electrolysis and Proton exchange membrane water electrolysis (PEMWE). Although alkaline water electrolysis is in a more mature stage of development, PEMWE holds a few advantages over this technology. These advantages include minimum mass transport limitations, the compact cell design which results in rapid response times, the simplicity of the cell design, the high current density capabilities, the ability to discharge hydrogen gas at high pressures and the high purity hydrogen gas produced. However, PEMWE is still a relatively new technology compared to Alkaline water electrolysis and consequently has the highest need for research and development to reduce the cost and improve the efficiency of this technology. Past research and development in this field focused mainly on material science of the electrolyser components and the operating conditions such as temperature and pressure to improve the efficiency and reduce the cost of this technology. Water electrolysers are usually supplied with a steady DC current, and it is common to use the electrical characteristics (i.e. voltage, current density, and impedance) as a measure of the electrolyser's performance. Unfortunately, very little research is available on the effect that specific electrical characteristics of the power supplied to the PEM electrolyser (i.e. ripple current) have on the performance and efficiency of the cell. In the little literature that is available, the main focus was on the efficiency of the converters used and the effect that different converter topologies have on the efficiency of a PEM electrolyser. Much more literature is however available on the effect of electrical characteristics, such as a fluctuating current or voltage, on the cell efficiency of alkaline water electrolysers. These studies indicated that electrical characteristics such as the ripple factor, frequency, and waveform of the applied electrical power affect the hydrogen production rate as well as the power consumption rate of the electrolyser. Although some respectable information was obtained from the literature, all the studies made use of alkaline water electrolysis systems and the effects of a ripple current on PEMWE are still unknown. This study aims to determine how the efficiency of a PEM electrolyser is affected by a ripple current. The primary objective was determining how the ripple current frequency, ripple factor, and waveform affects the hydrogen production, power consumption, efficiency and durability of a PEM electrolyser. Electrochemical impedance spectroscopy (EIS) was used to characterise the PEM electrolyser after which the impedance data were used to develop an equivalent electrical circuit (EEC) model. The EEC model was then used to simulate the effects of different ripple currents on the voltage response and power consumption of the cell. An active laboratory sized PEM electrolysis system was used to further investigate the impact of varying ripple currents on the efficiency of the system and to verify and validate the simulation results.