Thermodynamic cycle

The circle of the image represents a system that evolves through thermodynamic cycles.

The thermodynamic cycle is any series of thermodynamic processes such that, after all of them, the system returns to its initial state; that is to say, that the variation of the thermodynamic magnitudes proper to the system cancels out.

However, the aforementioned does not apply to variables such as heat or work, since these are not state functions of the system, but energy transfers between it and its surroundings. A characteristic fact of thermodynamic cycles is that the first law of thermodynamics dictates that: the sum of heat and work received by the system must be equal to the sum of heat and work done by the system.

Representation of a thermodynamic system in a P-V diagram

Carnot cycle pressure-temperature diagram

Represented in a P-V diagram (pressure/specific volume), a thermodynamic cycle adopts the shape of a closed curve. In this diagram the volume of a system is represented in abscises and the pressure in order so that as the work by volume change (or in general, if a pallet wheel is not used or similar procedure) is equal to the area described between the line that represents the process and the axis of abscises. W=− − ∫ ∫ PdV{displaystyle qquad W=-int P dV}

The direction of advance, indicated by the arrowheads, tells us if the volume increase is positive (to the right) or negative (to the left) and, as a consequence, if the work is positive or negative, respectively.

Therefore, it can be concluded that the area enclosed by the curve representing a thermodynamic cycle in this diagram, indicates the total work done (in one complete cycle) by the system, if this advances in a clockwise direction or, conversely, the total work exerted on the system if it does so in an anticlockwise direction.

Practical uses of thermodynamic systems

Getting a Job

Obtaining work from two thermal sources at different temperatures is used to produce movement, for example in motors or alternators used in the generation of electricity. Efficiency is the main parameter that characterizes a thermodynamic cycle, and is defined as the work obtained divided by the heat expended in the process, in the same complete cycle time if the process is continuous.

This parameter is different according to the multiple types of thermodynamic cycles that exist, but it is limited by the factor or performance of the Carnot Cycle.

Generally, the fluid used is water, although other substances can be used. For example, the Ericsson engine uses air. When the temperature reached by the hot source is not too high (approximately less than 300 °C), such as in low-power solar thermal power plants or geothermal power plants, the use of organic fluids, making use of the organic Rankine cycle, provides higher performance.. For large solar thermal power plants, on the other hand, supercritical carbon dioxide mixtures are being developed.

Work contribution

General Carnot Cycle based on temperature and entropy.

A reverse thermodynamic cycle seeks the opposite of the thermodynamic cycle of obtaining work. External work is contributed to the cycle to ensure that heat transfer occurs from the colder to the hotter source, the reverse of how it would happen naturally. This arrangement is used in air conditioning and refrigeration machines.