Thermal modeling of an alkaline electrolyser under variable load

Abstract

The unprecedented increase in mean ambient temperature due to immoderate CO2 emissions has shifted the energy policy towards the replacement of fossil fuels with renewable energy sources. Nevertheless, the intermittency of these technologies renders them unsuitable for reliable energy supply. A prominent solution is the utilization of renewable energy to produce green hydrogen that can be stored for later use.

The most environmental-friendly viable method for green hydrogen production is the electrolysis of water. Between the already existing types, alkaline electrolyser is the one with the highest technological maturity and lowest cost of hydrogen production. Despite that, the low energy efficiency, and the energy losses associated with the materials and the geometrical configuration make it difficult to produce hydrogen at competitive prices compared to fossil fuels. The connection with renewables exposes the electrolyser to variable loads that negatively affect the life of materials and the purity of hydrogen since the operating conditions like temperature and electrical current change constantly. Previous research work has shown that the energy losses owing to the electrode kinetics, diaphragm, and electrolyte resistivity are responsible for higher energy consumption than the thermodynamic minimum. These losses are a strong function of temperature and current density. This research project aims to investigate the performance of the electrolyser under fluctuating working conditions to get an insight into how energy consumption and efficiency are affected.

The research objectives are encountered by building a physical model that accurately predicts the electrochemical behavior of the cell under various circumstances and simulates the thermal response of the electrolyser. The numerous experiments performed at XINTC’s laboratory enabled the improvement of the model’s accuracy and the consideration of effects that are not easily confronted in a purely theoretical investigation. The results showed that the model accurately describes the experimental data of the cell with less than a 2% discrepancy. The model was also used for simulating the cell behavior under different pressures, electrodes, and diaphragm materials. The elevated temperatures and highly electroactive materials such as Nickel significantly reduce the voltage of the cell. Regarding the thermal model of the theoretical electrolysis system, sensitivity analysis of the current density, ambient conditions, and electrolyte flow was performed. The temperature across the system was uniform at an electrolyte volume flow of 5 Lt/min and the cooling load accounted for 20% of the electrolyser power. The thermal modeling of the real experimental setup was successful (<1.5°C deviation) at high current densities indicating the sound assumptions of the model. On the contrary, at low current densities, the discrepancy between the model and the experiments reached 10% due to the additional heat generation induced by the shunt currents. For that reason, a simplified electrical circuit analysis was formulated and included in the model. This led to reducing the deviation to less than 5% and at the same time calculation of the current efficiency in the absence of measuring devices was achieved.