The word “cryogenic” refers to extremely cold temperatures ranging from -150°C to -273°C. An environment is considered to be cryogenic when gases begin to liquify.
Cryogenics is an interesting field of study because extremely low temperatures vary the chemical properties of materials. At -273°C (zero kelvins), the molecules would be at their lowest state of energy. This state is referred to as Absolute Zero.
In the field of cryogenics, researchers examine ultra-cold materials. These materials typically transition from a gaseous state to a liquid state at temperatures below -90°C (-130°F). Helium, for example, condenses into a liquid at -269°C. Some of the most common cryogenic substances include liquid nitrogen, oxygen, helium, and hydrogen.
Applications for cryogenics
Cryogenics are used in a wide variety of applications across industries. Medical applications include liquid nitrogen for thermal ablation treatments. Manufacturing applications include bulk and electronic specialty gases (ESGs) for semiconductor fabrication.
Hydrogen fuel cells
One exciting cryogenic application is hydrogen storage for fuel cell electric vehicles (FCEVs). Reliable, compact, safe, and cost-effective storage is necessary to commercialize FCEVs and other applications successfully. The main challenge is storing sufficient hydrogen in the vehicle without sacrificing cargo and passenger space.
Hydrogen can be stored as a liquid or a gas. Gas storage typically uses high-pressure tanks of 350-700 bar. This method of storing gas under pressure is also referred to as cryo-compression.
When stored as a liquid, hydrogen is kept at cryogenic temperatures to prevent it from boiling off (it vaporizes at −253°C). Cryogenic hydrogen is colorless, odorless, and the lightest available liquid.
The energy per mass of hydrogen is greater than that of other fuels. However, its energy by volume is less than that of other liquid fuels like gasoline. Cryogenic storage contains the same quantity at lower pressure or smaller quantities at similar pressures.
Hydrogen is a rapidly growing industry, and researchers across public and private sectors are working to improve storage technologies. More efficient systems will enable higher storage densities for commercial applications of fuel cell electric vehicles.
Why thermal insulation is essential for cryogenic systems
Thermal insulation is necessary to store cryogenics in liquid form while maintaining safe temperatures and pressures. To maintain ultra-cold temperatures, cryogenic systems use insulation to minimize incoming heat. Without insulation, thermal energy enters the tanks and piping systems via convection, conduction, and radiation.
If the temperature increases past the boiling point of the stored fluid, the liquid will begin to vaporize. As vapor collects, pressure increases inside the structure. Pressure release valves are important components that prevent internal pressure from reaching dangerous levels.
Vacuum Super Insulation
Vacuum insulation is considered the industry-standard thermal barrier for cryogenic storage, piping, and other systems. Vacuum insulation is often combined with superinsulation, also referred to as multi-layer insulation (MLI).
Multi-layer insulation (MLI)
MLI consists of a series of thin, reflective surfaces and helps to reduce radiation heat transfer. Some common multi-layer insulations are made of materials such as aluminized Mylar, separated by layers of nylon or silk netting. Other multi-layer insulations use proprietary materials and technologies.
The insulation performance of the MLI depends in part on the vacuum level of the surrounding environment. At a high vacuum level, there will be minimal air between the layers. Air contributes to conductive and convective heat transfer, so less air means less heat passing through the material. That’s why MLI is typically placed within the vacuum space of a double-walled, vacuum insulated container.