ABSTRACT
The term cycle refers to a thermodynamic process that takes a working substance, typically a gas, from an initial equilibrium state through a succession of other states (equilibrium and/or non-equilibrium), ending back in the initial equilibrium state. The most well known cycle is the clockwise reversible Carnot cycle. This serves as a model for real heat engines that operate between high and low temperatures. In all Carnot cycles, the energy sources and sinks at these temperatures are assumed to be constant-temperature reservoirs. The reversible counterclockwise cycle has the characteristics of a refrigerator, whose working substance (refrigerant) takes in energy from the refrigerator compartment at low temperature and rejects it to the kitchen at a higher temperature. Combined-cycles have become increasingly popular for electricity generation because, with larger high to low temperature ratios, they offer higher efficiencies than are possible otherwise.
