Control Strategy of Rectifier for a PV-Grid-Powered Electrolyser

Abstract

Green hydrogen can be produced from various renewable energy sources like PV, wind, geothermal and biomass. Especially PV or wind sourced hydrogen production systems are becoming more prevalent as a solution to produce green hydrogen, owing to their regional abundance. In this master thesis project, a PV-grid powered electrolyser shall be taken into consideration. The electrolyser is an equipment that produces hydrogen through electrolysis. Here, an alkaline electrolyser is taken into consideration, which is a DC load and hence, it requires a rectifier to convert the AC power from the grid and from the PV plant (DC PV power will be converted to AC through an inverter) into DC power. The amount of hydrogen produced is proportional to the amount of current flowing into the electrolyser. As the electrolyser operates, the process of electrolysis will, more often than not, produce heat. The heat produced during electrolysis, along with the ambient conditions, will result in variation of the temperature of the electrolyser. Some systems make use of a thermostat to control the temperature of the electrolyser whereas some employ a cooling system to extract heat and utilise it to perform useful work. If there is no system to control or maintain its temperature, the electrolyser is bound to have varying temperature while it is operational. This research aims to develop a control strategy for a PV-grid powered electrolyser to ensure the optimal operation of the electrolyser despite its varying temperature. The need for such a control strategy for the electrolyser rises due to the dependence of the I-V characteristics of the electrolyser on the temperature. As the temperature of the electrolyser increases, the I-V curve shifts in such a way that a given current value will be produced at a lower value of voltage. Since the amount of hydrogen produced depends on the amount of current flowing into the electrolyser, the control strategy aims to maintain the input current to the electrolyser at the rated value irrespective of the change in the temperature of the electrolyser. The control strategy is tested on Simulink with the help of an electrolyser emulator into which data is fed through a look-up table. Real-time data for irradiance and ambient temperature has been used to perform day-long simulations. Based on the results obtained, the average efficiency of the converter was found to be 93.88%.