Process of Hydrogen Liquefaction

The Joule-Thomson Effect

Hydrogen exists naturally in a gaseous form. Gases change temperature and pressure by the application and removal of both heat and pressure. According to Astronautix, hydrogen liquefies at 20.24 K, or -252.87 C. Achieving a temperature this low consumes a great deal of energy, but the Joule-Thomson effect mitigates this somewhat. The Joule-Thomson effect dictates the behavior of gases when they undergo a change in pressure. For most gases, a drop in pressure causes a drop in temperature in ambient room temperature, but behavior reverses when temperatures drop to a certain point. For hydrogen and helium, it's the opposite--at exceptionally low temperatures, an increase in pressure further lowers the temperature of the gas.

Regenerative Cooling

According to NASA, regenerative cooling works by allowing a compressed gas to expand. Manufacturers of liquid hydrogen commonly use regenerative cooling by first introducing the cooled hydrogen to a concentration of liquid nitrogen, which lowers its temperature more. When the gas expands, it takes heat from its surroundings, and passes through a heat exchanger. In the case of liquid hydrogen, it expands through a valve that contacts the liquid nitrogen. The hydrogen can then be re-pressurized and the process can repeat until liquefaction.

Storing Liquid Hydrogen

HILTech explains that hydrogen cannot be in its natural state for effective storage because of its extremely low density and volatility. Liquefaction, chemical bonding or compression work as means of storage but have their drawbacks. Liquefaction takes an enormous amount of energy to maintain the low temperature, and compression requires high-quality seals because of the small molecules in hydrogen. Chemical bonding creates an electromagnetic bond between the hydrogen molecules and another element. According to HILTech, the compounds for attracting hydrogen should be liquid or metal. These materials carry electric charge most easily, especially at lower temperatures; therefore, they lend themselves well to chemical and electromagnetic bonding.

References

• Astronautix - Liquid Air/LH2
• NASA - History of Hydrogen Through the 19th century
• HILTech - Hydrogen and Storing Hydrogen