Low-power cryocooling

Small-power cryorefrigeration typically covers mechanical refrigeration machines capable of producing cold at temperatures below around 120 K (boiling of liquefied natural gas LNG), with refrigeration powers typically below a few kW at 80 K (boiling of liquid nitrogen LN2), a few hundred watts at 20 K (boiling of liquid hydrogen LH2) or a few watts at 4 K (boiling of liquid helium LHe). This reference to boiling temperatures near atmospheric pressure of common cryogenic fluids reflects the fact that these cryorefrigerators have been developed to replace, in many laboratory and industrial applications, the use of the latent heat of boiling of these fluids as a source of cold. The main advantage of these machines is that they produce continuous cold power, eliminating the need for periodic refilling of cryostats using boiling fluids, which can be impractical and costly, especially for applications requiring continuous operation (superconductivity applications), or sensitive to orientation (detectors installed in astrophysics antennas, etc.), or located on sites that are difficult to access (space applications, etc.). They can also be used for in situ recondensation of cryogenic fluids. These machines usually feature closed cycles in which a gas (generally helium, the gas with the lowest liquefaction temperature) is compressed to ambient temperature (at the cost of mechanical power), pre-cooled (in a counter-current exchanger or regenerator), then cold-expanded (with or without recovery of expansion work, lowering of temperature and extraction of cooling power). Numerous machines are commercially available, operating with different types of thermodynamic cycles and in a wide range of temperatures and cooling capacities. The development of these machines is aimed at steadily improving their thermodynamic efficiency, reliability and, consequently, their acquisition and running costs, by using frictionless compression and expansion techniques (gas, magnetic or flexible bearings) and seeking to minimize or even eliminate mechanical parts that move when cold (pulsed gas tubes).