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## Can you give a quick review of the concept of exergy?

### Can you give a quick review of the concept of exergy?

Can you provide a definition of exergy efficiency?

Answer: Exergy: H - T0.S situates a quantity of energy with respect to a temperature level T0. To illustrate, 100 kW at 20°C is not the same as 100 kW at 800 °C. You can't do the same thing with it. For an open system, the first two laws of thermodynamics taken together lead to the expression of the difference in exergy between the input and the output. This is: + useful (recoverable) work, + the quantity of heat multiplied by (1 - T0/Tc), + the generation of entropy. The multiplication factor (1 - T0/Tc) is the Carnot factor (efficiency of a Carnot cycle functioning between T0 and Tc). The exergy balance leads to adding actual work and an ideal work that you could obtain from a given quantity of heat. For a machine with a dual heat source, the energy efficiency is W/Q, and the exergy efficiency is the ratio of actual work to ideal work. Exergy efficiency is therefore deduced from the energy efficiency using the Carnot factor. The choice of the reference temperature T0 is difficult. The exergy efficiency is an intrinsic characteristic of a machine if we choose the cold source carefully (example discharge temperature for a gas turbine).