Thermodynamic Cycles

Classification of Thermodynamic Cycles

In general, thermodynamics is the science that deals with energy production, storage, transfer and conversion. Our goal here will be to introduce thermodynamics as the energy conversion science. At present, fossil fuel is still the world’s predominant energy source. But the burning of fossil fuels generates only thermal energy, therefore these energy sources are so called “primary energy sources”, that must be converted to secondary energy source, so called energy carriers (electrical energy etc.). To convert thermal energy into another form of energy a heat engine must be used.

Many heat engines operate in a cyclic manner, adding energy in the form of heat in one part of the cycle and using that energy to do useful work in another part of the cycle.

A process that eventually returns a system to its initial state is called a cyclic process. At the conclusion of a cycle, all the properties have the same value they had at the beginning. Typical thermodynamic cycle consists of a series of thermodynamic processes transferring heat and work, while varying pressure, temperature, and other state variables, eventually returning a system to its initial state.

The first law of thermodynamics dictates that the net heat input is equal to the net work output over any cycle.

The increase in internal energy of a closed system is equal to the heat supplied to the system minus work done by it.

∆Eint = Q – W

This is the First Law of Thermodynamics and it is the principle of conservation of energy, meaning that energy can neither be created nor destroyed, but rather transformed into various forms as the fluid within the control volume is being studied.

It is the most important law for analysis of most systems and the one that quantifies how thermal energy is transformed to other forms of energy.

The thermodynamic cycles can be divided into two primary classes:

  • Power cycles. Power cycles are cycles which convert some heat input into a mechanical work output. Thermodynamic power cycles are the basis for the operation of heat engines, which run the vast majority of motor vehicles and generate  most of the world’s electric power.
  • Heat pump cycles.  Heat pump cycles transfer heat from low to high temperatures using mechanical work input. There is no difference between thermodynamics of refrigerators and heat pumps. Both work by moving heat from a cold space to a warm space.

The following classification of thermodynamic cycles is made according to their constituent thermodynamic processes. In practice, simple idealized thermodynamic cycles are usually made out of four thermodynamic processes. In general, the following processes usually constitute thermodynamic cycles:

Classification of Thermodynamic Cycles: