The maximum efficiency that a heat engine can have is from the Carnot efficiency. It is defined in the Carnot’s theorem, which states that no engine operating between two heat reservoirs can be more efficient than a Carnot engine operating between the reservoirs.

The efficiency of a Carnot heat engine can be obtained from the following equation:

E=W/QH=1-TC/TH

  • W: Work done by the system
  • QH: Heat put into the system
  • TC: Absolute temperature of the cold reservoir
  • TH: Absolute temperature of the hot reservoir

 

From the above equation, the higher TH is or the lower TC is, the more work will be available for the heat engine, and the more efficient will be the engine.

The energy for work comes from a decrease in the total energy of the fluid used in the system. Therefore, the greater the temperature change is, the greater this decrease in the fluid and thus the greater energy available to do work is.

An efficiency of 100% only could be accomplished if TC = 0K, because the only way to transform all the heat transferred from the hot reservoir into work would be removing all thermal energy, what means that the cold reservoir would be at absolute zero, what is not possible.

The best efficiency that can be obtained from this cycle is about 0.7