Editing Cryocooler (section)

== Ideal heat exchangers and regenerators ==
[[Heat exchanger]]s 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 ''T''<sub>X</sub> of the heat exchanger. Note that even a perfect heat exchanger will not affect the entrance temperature ''T''<sub>i</sub> 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 thermal 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 complicated, so one usually makes simplifying models. In its most extreme form an ''ideal'' regenerator has the following properties:
* large volumetric heat capacity of the material;
* perfect heat contact between gas and matrix;
* zero flow resistance of the matrix;
* zero porosity (this is the volume fraction of the gas);
* zero thermal conductivity in the flow direction;
* the gas is ideal.
Progress in the cryocooler field in recent decades is in large part due to development of new materials having high heat capacity below 10&nbsp;K.<ref>T. Kuriyama, R. Hakamada, H. Nakagome, Y. Tokai, M. Sahashi, R. Li, O. Yoshida, K. Matsumoto, and T. Hashimoto, Advances in Cryogenic Engineering 35B, 1261 (1990)</ref>