Multi-Stage Electrolysis for Reduction of Electricity Consumption in Hydrogen Production

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Aulakh, Deepinder Jot Singh
Electrolysis , Stages , Electric work , CFD , Thermodynamics , Solid Oxide , Hydrogen
A novel approach to perform electrolysis in stages is developed for reducing the share of electric work in the process and replacing the same with thermal energy. The study is divided into two parts, the first part is a theoretical study of electrolysis in stages and the second part is a computational fluid dynamics study of the same. In the first part the thermodynamics of single stage electrolysis is studied in detail by performing parametric studies of varying steam utilization and inlet steam concentration. Subsequently, a formulation for electrolysis in stages is developed in order to calculate the utilization of each stage, keeping the inlet steam temperature and electrolyte temperature for each stage constant. The utilization of each stage is calculated such that overall utilization of the process is as desired. Electric and thermal energy requirements of the process are determined and are compared with the case of single stage electrolysis. The electric energy required for the process of 5 stages is 25.4 kWh (per kilogram of H2) which is substantially lower than the 50-65 kWh/kg used by commercial electrolysers today. Electric energy savings of up to 1.2 kWh/kg are predicted for a case with 5 stages as compared to single stage. These savings increase with an increase in the number of stages and the required inlet steam temperature is also reduced. The assumptions of this study are no Ohmic losses and constant temperature electrolysis. In the second part, a detailed CFD study of staged electrolysis is undertaken. The variation of temperature within the electrolyte is considered along with Ohmic losses. The electric energy requirement for a 5 stage process was found to be 25.5 kWh/kg. The electric energy savings in the CFD study are calculated to be 5.7 kWh/kg for a case with 5 stages as compared to a single stage. The higher energy savings are due to higher average electrolyte temperatures in the CFD study as compared to theoretical value. Similar to the first study the electrical energy savings increase as the number of stages increases. Finally, a hypothesis for the case with an infinite number of stages is developed in order to determine the absolute minimum work requirement for process in stages. By taking this as benchmark the effectiveness of the staging process is determined as a ratio of the electric energy requirement in infinite stages to that in a given number of stages. The study can be further enhanced by performing optimization to generate minimum work for any given number of stages.
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