How does a Rankine cycle work

Clausius-Rankine cycle

Lexicon> Letter C> Rankine cycle

Definition: a thermodynamic cycle that describes the basic principle of steam turbines and steam engines

English: Clausius Rankine cycle

Categories: Engines and Power Plants, Physical Basics

Author: Dr. Rüdiger Paschotta

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Original creation: December 20, 2014; last change: 07/20/2020

URL: https://www.energie-lexikon.info/clausius_rankine_kreisprozess.html

The Clausius-Rankine cycle is a simplified thermodynamic model for closed cycle processes as they are technically implemented in steam turbines and steam engines. It was named after the German physicist Rudolf Julius Emanuel Clausius and the Scottish engineer William John Macquorn Rankine and played an essential role in the effort to understand the thermodynamic fundamentals and the principal limitations of the efficiency of steam turbines.

What is a thermodynamic cycle?

In general, one speaks of a thermodynamic cycle when a working medium such as water in a machine or system undergoes a series of changes in state (e.g. changes in temperature, pressure and state of aggregation) that repeat themselves periodically (i.e. always the same). One is looking at a machine or system that works in a steady state, i. H. has adjusted to this in a number of cycles under constant external conditions. A closed cycle process is present when there is no material exchange of the working medium with the environment during operation. This is usually the case with steam turbine systems, sometimes also with steam engines, but generally not with gas turbines.

Basically, the Rankine cycle is about the realization of a heat engine, which converts heat in part into mechanical energy. Part of the fundamental principle is that the working medium is alternately compressed and expanded, whereby the conditions are set in such a way that less mechanical work has to be done for compression than the medium does during expansion. This means that mechanical energy remains in the machine's entire cycle and can be extracted as useful energy. In the Rankine cycle process, a working medium such as water is used, which is alternately present in the process in a liquid and a gaseous phase.

The four steps of the Rankine cycle

The Rankine cycle can be described as follows:

  • The initially liquid working medium is strongly heated at constant high pressure and evaporated in the process, with the entropy of the medium increasing sharply. Three processes actually occur here: first heating to the boiling point, then evaporation (which requires most of the heat input) and finally further heating (overheating) of the steam. (Here it is assumed that the pressure remains below the critical pressure, so that there is a clear distinction between the liquid and gaseous phase.) In practice, this can be done in a steam boiler, for example, to which heat is supplied from the combustion of a fuel or is obtained in a nuclear reactor.
  • The working medium is now expanded without being able to exchange further heat with its surroundings (adiabatic expansion). In doing so, it does work, i. H. it gives off mechanical energy and its temperature drops sharply in the process, with the entropy ideally remaining unchanged. The steam can be partially condensed during the expansion. This step can take place, for example, in one or more turbines or reciprocating piston engines.
  • The remaining steam is then condensed by extracting heat from it at a low temperature. In the model used, it is assumed that the pressure remains constant, namely at the level that is reached at the end of the expansion phase. The temperature also remains constant in the wet steam area and the entropy of the medium drops sharply. In practice, a so-called condenser is used for this step, which can be, for example, a heat exchanger cooled with cold water.
  • In order to return to the original state, the liquid medium must now be brought back to the original high pressure with a pump (again without heat exchange with the environment), with which it can be fed into the steam generator. The pump has to do work here - but much less than the medium did during the expansion, because the volume to be pumped in liquid form is much smaller. The pump therefore only consumes a small part of the mechanical energy released.

Analysis of the cycle; achievable efficiency

The process can now be analyzed quantitatively, taking into account the physical properties of the working medium (equations of state and enthalpy of evaporation or condensation). The efficiency of the machine working with this cyclic process can be estimated from the results, although deviations occur due to various simplifications (idealizations); a real machine is somewhat less efficient as a result of additional flow pressure losses (friction losses), leaks, undesired heat conduction, etc., which are not taken into account in the theoretical model.

High energy efficiency requires a high steam temperature and a low temperature of the condenser.

The calculated efficiency is higher, the higher the temperature of the generated steam and the more the medium can be expanded. The possible strength of the expansion is greater, the lower the temperature at which the waste heat can be dissipated. The efficiency cannot in principle exceed the so-called Carnot efficiency and in fact remains well below it; For example, for a steam temperature of 400 ° C and condensation at 25 ° C, an efficiency of 39.5% is calculated, while the Carnot efficiency would be 55.7%.

The lower efficiency is due to the fact that the Rankine cycle is not thermodynamically optimal. In particular, it is not optimal to use only the heat supplied from the outside for heating the working medium. It would be more efficient to first use extraction steam from the turbine for heating at a low temperature level, which is also practiced in today's power plants. So-called reheating of the steam between the turbine stages is also common today, which also improves efficiency and also protects the last turbine stage. Such measures are called Carnotization because they bring the efficiency closer to the ideal Carnot efficiency.

In the vast majority of cases, water is used as the working medium - for example in almost all steam turbine power plants. However, if only a heat source with a relatively low temperature is available, the thermodynamic properties of water are not optimal or even make it completely unsuitable. In such cases, organic working media with a significantly lower boiling point are sometimes used and this modified process is then referred to as Organic Rankine Cycle.

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See also: thermodynamics, steam turbine, steam engine, efficiency
as well as other articles in the categories of engines and power plants, physical principles