Second-law efficiency

By Jai Schrem

The Second Law of Thermodynamics states that no real process can be completely isentropic. Or, restated in terms of exergy, no real process can use 100% of the exergy within a system.

Engineering aims to minimise the generation of entropy. Second-law efficiencies measure how successfully entropy generation has been minimised in a device by comparing real processes to an equivalent idealised isentropic process via their thermal efficiencies or COPs are appropriate:

Think of the amount of exergy not used as the energy lost due to the generation of entropy. Engineering aims to maximise the use of exergy. Hence these formulas can be rewritten in terms of exergy: