The second law of thermodynamics identifies the direction of spontaneous heat transfer: heat always flows from hotter objects towards colder objects, and not vice versa.

But the direction of the heat flow can be reversed by applying external work. Such machines are called heat pumps or refrigerators. In the case of cryogenic temperatures, these machines are called cryocoolers.
In a closed cycle Stirling cryocooler, the gaseous working agent is cyclically compressed and expanded in the compression chamber of a piston compressor.

The piston is in direct pneumatic communication with the warm chamber of an expander unit.

The expansion chamber of the expander unit is separated from the warm chamber by a pneumatically-actuated expansion piston (displacer) containing a porous regenerative heat exchanger.
During the expansion stage of the thermodynamic cycle, the expanding working agent perform work on the motion of the displacer.

This operation results in a cooling effect, followed by a heat absorption on the cold side of the cryocooler, and in the recovery of the expansion work.

During the compression stage, the absorbed heat and the compression heat are rejected to the environment from the expander base (the warm side of the cryocooler).

This expansion work recovery is one of the outstanding features of Stirling cryocoolers and the key to their superior performance.

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