The efficiency of the Carnot cycle
A ideal reversible cycle where heat is taken in at a constant upper temperature and rejected at a constant lower temperature was suggested by Sadi Carnot. The theoretically most efficient heat engine cycle, the Carnot cycle, consists of
- two isothermal processes and
- two adiabatic processes
Since the second law of thermodynamics states that not all supplied heat in a heat engine can be used to do work, the Carnot efficiency limits the fraction of heat that can be used.
The Carnot efficiency can be expressed as
μC = (Ti - To) / Ti (1)
μC = efficiency of the Carnot cycle
Ti = temperature at the engine inlet (K)
To = temperature at engine exhaust (K)
The wider the range of temperature, the more efficient becomes the cycle. The lowest temperature is limited by the temperature of the sink of heat - if it is the atmosphere or the ocean, river or whatever available. Normally the lowest temperature available is in the range 10 - 20 oC. The maximum temperature is limited by the metallurgical strength of available materials.