Stirling coolers transfer heat in or out of the working fluid during all four stages of their
operation, and their coefficient of performance depends on whether the non-isothermal heat exchanges
are performed reversibly or irreversibly. Both of these possibilities can in principle be arranged.
Notably, if the working fluid is an ideal gas, the input of energy in the form of heat during one
isochoric step is equal in magnitude to the output during the other isochoric step in the cycle. The
theoretical performance of the fridge can then attain the reversible Carnot limit if a regenerator
is used, which is a high heat capacity material through which the gas flows. Various Stirling
refrigerator configurations are analysed in this article at a level of presentation suitable for an
introductory undergraduate thermodynamics course.