Energetically-optimal PEM electrolyzer pressure in power-to-gas plants

Hydrogen production from renewable electricity in power-to-gas concepts is promising for future energy storage systems since hydrogen offers high energy density and can be used emission free. Economically viable power-to-gas applications require high efficiency and thus low specific energy demand of the hydrogen production. Energy is required for hydrogen production via water electrolysis, but also for gas conditioning. Gas conditioning includes mechanical gas compression to a defined storage pressure and gas drying to purify the raw hydrogen. The energy demand of gas conditioning can be reduced by operating pressurized electrolyzers. However, pressurized operation increases the energy demand of the electrolyzer. To determine the optimal operating pressure of the electrolyzer, the overall power-to-gas process has to be considered. In this paper, the energy demand of the overall power-to-gas plants is optimized considering compression and temperature swing adsorption (TSA)-drying of hydrogen. It is shown that an optimum pressure for each operating condition in the electrolyzer in relation to the efficiency exists. This optimal operating pressure depends on the current density in the stack and the hydrogen storage pressure. When operating the system with load adapted operating pressure efficiencies between 55% and 73% for the whole power-to-gas plant can be achieved.