Liquid hydrogen

Liquid hydrogen (H₂(l)) is the liquid state of the element hydrogen. Hydrogen is found naturally in the molecular H₂ form. To exist as a liquid, H₂ must be cooled below its critical point of 33 K. For it to be in a fully liquid state at atmospheric pressure, H₂ needs to be cooled to 20.28 K (−252.87 °C; −423.17 °F). A common method of obtaining liquid hydrogen involves a compressor resembling a jet engine in both appearance and principle.

Liquid hydrogen is typically used as a concentrated form of hydrogen storage. 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 kept as a liquid for some time in thermally insulated containers.

There are two spin isomers of hydrogen. Room temperature hydrogen is mostly one type, and at very cold temperatures it changes to another type. Special methods are used during the liquefaction process to prevent problems with heat release during storage.

Hydrogen needs a lot of energy to be turned into a liquid. Current facilities use more energy than the theoretical minimum, but future facilities are expected to use less energy.

History

Further information: Timeline of low-temperature technology

In 1885, Zygmunt Florenty Wróblewski found important temperatures and pressures for hydrogen. Hydrogen became a liquid thanks to James Dewar in 1898. In 1929, Paul Harteck and Karl Friedrich Bonhoeffer created a special form of liquid hydrogen.

Spin isomers of hydrogen

Main article: Spin isomers of hydrogen

Hydrogen molecules can be in two different forms because of the spin of their atoms. One form is called parahydrogen and is more stable. At normal temperatures, hydrogen is mostly in the other form. But when it becomes a liquid and is cooled, it slowly changes to the stable form. This change gives off heat, which can make some of the liquid turn back into gas. To stop the liquid hydrogen from disappearing, it is changed into the stable form when it is made, using special materials like iron oxide or activated carbon.

Uses

Liquid hydrogen is used as a special fuel for rockets. It helps cool parts of the rocket engine before mixing with liquid oxygen to create water vapor. Some cars, submarines, and a ferry called the MF Hydra also use liquid hydrogen as fuel.

Liquid hydrogen can cool tiny particles called neutrons for science experiments. It was also used in the first big nuclear explosion called Ivy Mike.

Properties

When liquid hydrogen burns with pure oxygen, it makes only water vapor. But if it burns in normal air, it can make harmful gases because of the high heat and nitrogen in the air. Even though water vapor seems safe, it can still affect the climate, especially in airplanes. Liquid hydrogen gives more energy than gasoline or natural gas, but it is very light, weighing only about 70.85 kg for every cubic meter.

Keeping liquid hydrogen safe is hard because it must stay very cold. Special containers are needed, and some hydrogen can still escape into the air. Like other forms of hydrogen, it can be dangerous because of its very low temperature and its ability to explode under certain conditions.

Safety

Liquid hydrogen can be very dangerous because it is extremely cold and can cause serious burns. It is also very flammable and can catch fire or explode if it mixes with air and there is a spark or heat nearby.

When liquid hydrogen turns back into a gas, it becomes very cold and heavier than air. This can create dangerous mixtures with air that are easy to ignite. If containers holding liquid hydrogen are not properly insulated, the cold temperature can cause oxygen from the air to gather on the outside, which can also lead to sudden fires. Therefore, liquid hydrogen must be kept away from any sources of heat or flame unless ignition is planned.