Hydrogen is the most abundant element in the universe. Composed of one proton and one electron, it is the lightest element known to mankind – and due to its ability to carry energy along with its abundance on Earth, hydrogen may be the key to a cleaner, more efficient power supply. However, when it comes to the task of storing hydrogen for use, there is a hurdle to clear: Hydrogen exists as a gas by default but is most useful when stored as a liquid. Unfortunately, liquefying hydrogen isn't as easy as turning steam into liquid water. It takes a lot more work to create liquid hydrogen – but methods to do so have existed for nearly 150 years, and scientists are making it easier all the time.
TL;DR (Too Long; Didn't Read)
While hydrogen is liquefied primarily to store large quantities of the element at once, liquid hydrogen is used as cryogenic coolant, as a component of advanced fuel cells and as a critical component of the fuel used to power the engines of space shuttles. To liquefy hydrogen, it must be brought to its critical pressure and then cooled down to temperatures below 33 degrees Kelvin.
Liquid Hydrogen Uses
While scientists are still researching ways to turn hydrogen into a useful, large-scale power source, liquid hydrogen is used for a variety of applications. Most famously, NASA and other space agencies use a combination of liquid hydrogen and other gases like oxygen and fluorine to power large rockets – and outside of Earth's atmosphere, hydrogen stored in liquid form is used as a propellant to move space vehicles. On Earth, liquid hydrogen has also found widespread use as a cryogenic coolant and as a component of advanced fuel cells that may one day power cars, homes and factories.
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Turning Gas to Liquid
Not all elements behave the same under the natural temperature range, atmospheric pressure and gravity of Earth. Water is unique in that it can shift between its solid, liquid and gaseous states under these conditions, but iron is solid by default – whereas hydrogen is normally gas. Solids can be turned to liquids and finally gases by applying heat until the element reaches its melting and then boiling point, and gases work in reverse: Regardless of elemental composition, a gas can be liquefied by cooling it, turning to liquid at the point of condensation and solid at the point of freezing. To effectively store and transport hydrogen for use, the gaseous element must first be turned into a liquid, but elements like hydrogen that exist on Earth as gases by default cannot just be cooled to turn them into liquids. These gases must be pressurized first, to create conditions where the liquid element can exist.
Coming to Critical Pressure
Hydrogen's boiling point is incredibly low – at just under 21 degrees Kelvin (roughly -421 degrees Fahrenheit), liquid hydrogen will turn into a gas. And because pure hydrogen is incredibly flammable, for safety's sake the first step to liquefying hydrogen is to bring it to its critical pressure – the point at which, even if hydrogen is at its critical temperature (the temperature at which pressure alone cannot turn a gas into a liquid), it will be forced to liquefy. Hydrogen is pumped through a series of condensers, throttle valves and compressors to bring it to its pressure of 13 bar, or roughly 13 times the standard atmospheric pressure of Earth. While this occurs, the hydrogen is being cooled to keep it in its liquid form.
Keeping Things Cool
While hydrogen must always be pressurized to maintain a liquid state, the process of cooling it down to keep it a liquid can differ. Small, specialized cooling units can be used, as can powerful heat exchangers that work alongside the pressurization process. Regardless, the hydrogen gas must be brought under at least 33 degrees Kelvin (hydrogen's critical temperature) to become a liquid. These temperatures must be maintained at all times in order to ensure that the liquid hydrogen stays in that form; at temperatures just under 21 degrees Kelvin, you reach the hydrogen boiling point, and the liquid element will begin to return to its gaseous state. This temperature and pressure maintenance is what makes storing, transporting and using liquid hydrogen so expensive at the moment.