Liquid hydrogen

Liquid hydrogen (H2(l)) is the <a href="/facts/Liquid_state/t9GkLWzt">liquid state</a> of the element <a href="/facts/Hydrogen/BoYHjl0J">hydrogen</a>. Hydrogen is found naturally in the <a href="/facts/Molecule/N9ljSfFq">molecular</a> H2 form.
To exist as a liquid, H2 must be cooled below its <a href="/facts/Critical_point_(thermodynamics)/pkM7RLJK">critical point</a> of 33 <a href="/facts/Kelvins/E3trzC4C">K</a>. However, for it to be in a fully liquid state at <a href="/facts/Atmospheric_pressure/wJLMSaVG">atmospheric pressure</a>, H2 needs to be cooled to 20.28 K (−252.87 °C; −423.17 °F). A common method of obtaining liquid hydrogen involves a <a href="/facts/Compressor/I0vV0Tx2">compressor</a> resembling a jet engine in both appearance and principle. Liquid hydrogen is typically used as a concentrated form of <a href="/facts/Hydrogen_storage/KPqx6qNG">hydrogen storage</a>. Storing it as liquid takes less space than storing it as a gas at normal temperature and pressure. However, the liquid density is very low compared to other common fuels. Once liquefied, it can be maintained as a liquid for some time in thermally insulated containers.
There are two <a href="/facts/Spin_isomers_of_hydrogen/zhN4qBMQ">spin isomers of hydrogen</a>; whereas room temperature hydrogen is mostly orthohydrogen, liquid hydrogen consists of 99.79% parahydrogen and 0.21% orthohydrogen.
Hydrogen requires a theoretical minimum of 3.3 kWh/kg (12 MJ/kg) to liquefy, and 3.9 kWh/kg (14 MJ/kg) including converting the hydrogen to the para isomer, but practically generally takes 10–13 kWh/kg (36–47 MJ/kg) compared to a 33 kWh/kg (119 MJ/kg) heating value of hydrogen.

Liquid hydrogen open-in-new

Liquid hydrogen