Efficiency of Carnot

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

μ_C = (T_i - T_o) / T_i         (1)
where
μ_C = efficiency of the Carnot cycle
T_i = temperature at the engine inlet (K)
T_o = 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.