A system of devices for the conversion of fossil energy to mechanical work or electric energy.

The main systems are the Steam (Rankine) Cycle and the Gas Turbine (Brayton Cycle).

1. Steam (Rankine) Cycle is an ideal thermodynamic cycle that consists of four processes:
   • Heat transfer to the system at constant pressure.
   • An expansion at constant entropy.
   • A constant-pressure heat transfer from the system.
   • A compression at constant entropy; used as a standard of efficiency.
2. Gas Turbine (Brayton) Cycle, an ideal gas cycle used as a standard for the actual performance of a simple gas turbine, consisting of four processes:
   • A reversible adiabatic (no heat transfer) compression at constant entropy.
   • A heat transfer at constant pressure up to the maximum temperature.
   • An adiabatic expansion at constant entropy back to the original pressure.
   • A heat transfer at constant pressure back to the original volume and entropy.
3. Entropy in thermodynamics is a measure of the disorder or randomness of a closed system; more entropy means less energy is available for doing work.

The total entropy of an isolated system cannot decrease when the system undergoes a change; it can remain constant for reversible processes, and will increase for irreversible ones.