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

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##### Date

##### Authors

Aulakh, Deepinder Jot Singh

##### Keyword

Electrolysis , Stages , Electric work , CFD , Thermodynamics , Solid Oxide , Hydrogen

##### Abstract

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.