How Hydrogen Energy Storage Works
Electricity can be converted into hydrogen by electrolysis. The hydrogen can be then stored and eventually re-electrified. The round trip efficiency today is lower than other storage technologies. Despite this low efficiency the interest in hydrogen energy storage is growing due to the much higher storage capacity compared to batteries (small scale) or pumped hydro and CAES (large scale).
Alkaline electrolysis is a mature technology for large systems, whereas PEM (Proton Exchange Membrane) electrolyzers are more flexible and can be used for small decentralized solutions. The conversion efficiency for both technologies is about 65%~70% (lower heating value). High temperature electrolyzers are currently under development and could represent a very efficient alternative to PEM and alkaline systems, with efficiencies up to 90%.
Small amounts of hydrogen (up to a few MWh) can be stored in pressurized vessels, or solid metal hydrides or nanotubes can store hydrogen with a very high density. Very large amounts of hydrogen can be stored in constructed underground salt caverns of up to 500,000 cubic meters at 2,900 psi, which would mean about 100 GWh of stored electricity electricity. In this way, longer periods of flaws or of excess wind / PV energy production can be leveled. Even balancing seasonal variations might be possible.
Hydrogen can be re-electrified in fuel cells with efficiencies up to 50%, or alternatively burned in combined cycle gas power plants (efficiencies as high as 60%).
Other Uses of Hydrogen
Because of the limited round trip efficiency, direct uses of green hydrogen are under development, e.g. as feedstock for the chemical and the petrochemical industry, as fuel for fuel cell cars or blending with natural gas of up to 5 to 15% in natural gas pipelines.
Several European and American companies offer integrated hydrogen solutions for the supply of electric power to small isolated sites or islands. Demonstration projects have been performed since 2000 in Europe and the USA and commercial products are available. Large scale hydrogen storage in salt cavern is standard technology.