The Carnot cycle is a cycle    composed of the totally reversible processes of isentropic compression and expansion and isothermal heat addition and rejection. The  thermal efficiency of a Carnot cycle depends only on the absolute temperatures of the two reservoirs in which heat transfer takes place, and for a power cycle is: 
where  is the lowest cycle temperature and  the highest. For Carnot power cycles the coefficient of performance for a heat pump is: 
and for a refrigerator the coefficient of performance is: 
The second law of thermodynamics limits the efficiency and COP for all cyclic devices to levels at or below the Carnot efficiency. The Stirling cycle and Ericsson cycle are two other  reversible cycles that use regeneration to obtain isothermal heat transfer.