Liquid hydrogen (LH2) is the liquid state of the element hydrogen (H2), a colorless, odorless gas. Converting hydrogen gas into its liquid form, a process called liquefaction, requires substantial cooling. This transformation is necessary because liquid hydrogen is a highly concentrated form of energy storage. It occupies a volume approximately 800 times smaller than the equivalent amount of gaseous hydrogen at standard conditions.
The Specific Temperature of Liquid Hydrogen
To achieve the liquid state, hydrogen gas must be cooled to its precise boiling point of 20.28 Kelvin (K), or -252.87 degrees Celsius (°C). This temperature is the point at which liquid hydrogen transitions back into a gas under normal atmospheric pressure. For context, this temperature is significantly colder than other common cryogenic fluids. Liquid nitrogen, for example, boils at 77 K, making liquid hydrogen nearly four times colder on the Kelvin scale. The freezing point of liquid hydrogen, where it becomes a solid, is only slightly lower at 14.01 K, or -259.14 °C.
The Physics of Liquefaction
The process of cooling hydrogen gas to 20.28 K is challenging due to specific thermodynamic properties. All gases must be cooled below their maximum inversion temperature for the Joule-Thomson effect to produce a cooling effect upon expansion. For hydrogen, this inversion temperature is relatively low at approximately 202 K. If hydrogen gas at room temperature is expanded, it exhibits a negative Joule-Thomson effect and heats up instead of cooling down.
Therefore, the gas must first be pre-cooled below 202 K before the expansion cooling cycle can begin. This precooling step often employs other cryogens, such as liquid nitrogen or a helium refrigeration cycle. Once pre-cooled, the gas is compressed and then allowed to expand through a valve, causing a dramatic temperature drop that leads to liquefaction.
A complication in liquefaction is the conversion of hydrogen’s spin isomers, known as ortho-hydrogen and para-hydrogen. Normal hydrogen gas is composed of 75% ortho-hydrogen and 25% para-hydrogen. However, at the boiling point of 20 K, the equilibrium mixture shifts to nearly 100% para-hydrogen. The conversion from the ortho to the para form is an exothermic reaction, releasing a significant amount of heat.
This released heat is greater than the latent heat of vaporization of the liquid. If this heat is not removed during the liquefaction process, it causes the liquid to boil off rapidly. Therefore, catalysts are used to force the conversion to occur efficiently before the liquid is placed into storage.
Handling and Storing Cryogenic Fluids
Storing liquid hydrogen requires specialized engineering solutions to combat continuous heat gain from the surrounding environment. Primary storage vessels are double-walled, vacuum-jacketed containers, similar to a large Dewar flask. The vacuum space between the inner and outer walls is maintained at a high level to prevent heat transfer via conduction and convection.
To further reduce heat ingress, the vacuum space is typically filled with multi-layer insulation (MLI). MLI consists of numerous reflective layers that minimize heat transferred to the extremely cold liquid by reflecting thermal radiation. Despite these measures, some heat still penetrates the container, causing a small amount of liquid hydrogen to vaporize, a phenomenon known as “boil-off.”
Handling liquid hydrogen also introduces specific safety concerns. The ultra-low temperature can cause immediate and severe cold burns or frostbite upon contact with skin or tissue. Furthermore, standard materials like carbon steel become brittle and fracture when exposed to such extreme cold. This necessitates the use of specialized materials, such as certain aluminum alloys or austenitic stainless steels, for tank construction.
The extremely high expansion ratio of hydrogen gas—approximately 700 to 1 when converting from liquid to gas—is another hazard. Any liquid spill can quickly displace oxygen in the air, creating a risk of asphyxiation in enclosed spaces.
Primary Applications Requiring Extreme Cold
The decision to store hydrogen as a liquid, despite the complex cooling process, is driven by the advantages of high-density storage. This volumetric efficiency is a primary driver for its use as a rocket propellant. In aerospace, liquid hydrogen is combined with liquid oxygen to create a highly efficient rocket fuel, producing only water vapor as a byproduct.
Its extremely low molecular weight and high energy release per unit mass translate to a superior specific impulse, which is a measure of engine efficiency. Beyond rocketry, liquid hydrogen is gaining importance in the emerging hydrogen economy. It serves as a means for large-scale energy storage and distribution. Its ability to be transported in bulk via cryogenic tankers makes it a viable candidate for moving vast amounts of energy to be used in fuel cells or to store surplus renewable energy.