Hydrogen Energy Storage
Electricity can be converted into hydrogen by electrolysis. The hydrogen can be then stored and eventually re-electrified. The round trip efficiency today is as low as 30 to 40% but could increase up to 50% if more efficient technologies are developed. 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 at 100~300 bar or liquefied at 20.3K (-423 deg F). Alternatively, solid metal hydrides or nanotubes can store hydrogen with a very high density. Very large amounts of hydrogen can be stored in man made underground salt caverns of up to 500,000 m3 at 200 bar (2,900 psi), corresponding to a storage capacity of 167 GWh hydrogen (100 GWh 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.
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 future fuel cell cars or blending with natural gas of up to 5 to 15% in natural gas pipelines. Electrolytic hydrogen can also be used for the production of synthetic liquid fuels from biomass, thereby increasing significantly the efficiency of the biomass utilization.
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. To date there are two full size hydrogen caverns in operation in Texas, USA, a third one is under construction, three older caverns are operating at Teesside, UK.