Energy storage via electrolysis/fuel cells

Abstract The application of renewable energy sources requires buffer technology between energy generation and consumption. Energy must be converted into a storable energy carrier such as hydrogen to ensure long‐term supply to the energy consumer. The components under consideration for the conversion processes and for storage are water electrolyzers, fuel cells and gas storage systems. There are currently two competing methods in water electrolysis: alkaline and polymer membrane electrolysis. Alkaline electrolysis is an established technique operated on an industrial scale. Polymer membrane electrolysis makes use of a solid electrolyte with good conductivity for hydrogen ions and sufficient resistance to oxygen as the separator between the anode and cathode compartment. An interesting alternative from the systems engineering aspect is the bifunctional cell. Equipped with a polymer electrolyte membrane (PEM), the cell can be operated in either mode: as an electrolyzer or a fuel cell. In fuel cells, the stored hydrogen can be directly converted into electrical energy with oxygen or atmospheric oxygen. Due to the process engineering requirements such as frequent startup and shutdown, as well as the use of hydrogen as the fuel gas, fuel cell types with low operating temperatures are especially suitable for this application, e.g., the alkaline fuel cell (AFC) and the polymer electrolyte fuel cell (PEFC). The most economical form of gas storage is in tanks at high pressure. Suitable process components are mechanical, electrochemical and thermal compressors. The energetically most favorable process is the compression of liquid water with subsequent decomposition in a high‐pressure electrolyzer. The only hydrogen compression technique that does not need electrical energy is thermal hydrogen compression with metal hydrides.