Cryocooler
A Cryocooler is a standalone cooler, usually of table-top size. It is used to cool some particular application to cryogenic temperatures. A recent review is given by Radebaugh. The present article deals with various types of cryocoolers and is partly based on a paper by De Waele.
Heat exchangers are important components of all cryocoolers. Ideal heat exchangers have no flow resistance and the exit gas temperature is the same as the (fixed) body temperature TX of the heat exchanger. Note that even a perfect heat exchanger will not affect the entrance temperature Ti of the gas. This leads to losses.
An important component of refrigerators, operating with oscillatory flows, is the regenerator. A regenerator consists of a matrix of a solid porous material, such as granular particles or metal sieves, through which gas flows back and forth. Periodically heat is stored and released by the material. The heat contact with the gas must be good and the flow resistance of the matrix must be low. These are conflicting requirements. The thermodynamic and hydrodynamic properties of regenerators are very complicated, so one usually makes simplifying models. In its most extreme form an ideal regenerator has the following properties:
The recent progress in the cryocooler field is for a great deal due to the development of new materials with a high heat capacity below 10K.
The basic type of Stirling-type cooler is depicted in fig. 1. From left to right it consists of a piston, a compression space, and heat exchanger (all at ambient temperature Ta), a regenerator, and a heat exchanger, expansion space, and a piston (all at the low temperature TL). Left and right the thermal contact with the surroundings at the temperatures Ta and TL is supposed to be perfect so that the compression and expansion are isothermal. The work, performed during the expansion, is used to reduce the total input power. Usually helium is the working fluid.
The cooling cycle is split in 4 steps as depicted in fig. 2. The cycle starts when the two pistons are in their most left positions:
In the pV diagram (fig. 3) the corresponding cycle consists of two isotherms and two isochores. The volume V is the volume between the two pistons. In practice the cycle is not divided in discrete steps as described above. Usually the motions of both pistons are driven by a common rotary axes which makes the motions harmonic. The phase difference between the motions of the two pistons is about 90°. In the ideal case the cycle is reversible so the COP (the ratio of the cooling power and the input power) is equal to the Carnot COP given by TL/(Ta-TL).
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