- Liquefaction plays an important part in the storage and transport of hydrogen as an energy source.
- The supply of liquid hydrogen to filling stations provides full flexibility.
- It may easily be pumped to high pressure and gasified for storage in compressed form in a vehicle's tanks, stored as a liquid, in the case of larger vehicles, or in a partly-cooled and compressed form for future applications.
- The supply of hydrogen in liquid form also reduces the volume that has to be transported. This means fewer tank trucks on the road, and no need for expensive and energy-demanding compression at the filling stations.
Owing to its special physical properties, hydrogen is an almost permanent gas, since it only liquefies at very low temperatures (below –253°C). Gases can be liquefied either by decreasing the temperature and/or raising the pressure. However, there is a critical temperature above which a gas can no longer be liquefied, no matter how high the pressure. In the case of hydrogen the critical temperature is –239.96°C (33.19 K). If hydrogen is to be liquefied, its temperature must be below this point. Therefore the liquefaction of hydrogen requires mostly cooling and an additional compression, and the method is also called as cryogenic hydrogen storage.
The liquefication of hydrogen is a rather energy demanding process. Firstly pure hydrogen gas is compressed to a high pressure and obviously its temperature also rises. It is important to use pure hydrogen as impurities could solidify at the later stages causing problems in the overall system. The hot pressurized gas is then passed through two heat exchangers and again combined to be passed through a tank containing liquid nitrogen, finally to pass through another heat exchanger.
The FCH JU project IDEALHY brought together world experts to develop a generic process design and plan for a prospective large scale demonstration of efficient hydrogen liquefaction in the range of up to 200 tonnes per day. It investigated the different steps in the liquefaction process in detail, using innovations and greater integration in an effort to reduce specific energy consumption by 50% compared to the state of the art, and simultaneously to reduce investment cost.