Rankine cycle

The Rankine cycle is a thermodynamic cycle. It is named after the German physicist Rudolf Julius Emanuel Clausius and the Scottish engineer William Rankine John Macquorn. It serves as a comparison process for the steam power plant in its simplest constellation with turbine T, capacitor Co, Sp feed pump and boiler with superheater Ke. As with all thermodynamic cycle processes he can not surpass the efficiency of the corresponding Carnot cycle. In steam power plants is first "created" mechanical energy by and vaporizes a working fluid (usually water, but for example, ammonia) is condensed alternately at low pressure at high pressure. The pressure is applied by the feed pump by expenditure of work and reduced in the turbine with the release of work. The working medium is guided in a closed circuit.

The four changes of state


From the Ts diagram can be seen that most of the heat for evaporation is applied. Advantage of the steam power process over the processes with inert gases is the large specific cycle work ( in the diagrams of the yellow colored area) because of the small work of the feed pump ( small specific volume of the liquid ). The ratio of the specific volumes between saturated steam and liquid water is not directly readable from the pv diagram, since the abscissa is divided logarithmically. At 50 bar, it is about 31, at 0.03 bar approx 46000 The efficiency of the comparison process is calculated with. :

For example, a steam condition of 50 bar at 400 ° C and a condenser pressure of 0.03 bar results in:

The numerical values ​​in the equation are the enthalpies in kJ / kg. The units cut out. The difference in parenthesis is the work of the boiler feed pump, only about 0.5 percent of the turbine work.


The process can be improved by:

  • Increasing the steam pressure and steam temperature. Thus, the steam wetness in the last turbine stage is not too high, a reheat is also required, which in turn contributes to improve efficiency (see turbine power plant ).
  • Feed water by extraction steam from the turbine. Wherein the mean temperature of the heat supply is increased and the efficiency approaches the thermodynamic maximum of the Carnot efficiency of. Why they are called such and similar efficiency improvements Carnotisierung.

Actual process

The usual maximum temperature in fossil-fired steam power plant is now bar at 570 ° C, the pressure at 250. A reheat is required. The pressure in the condenser is - depending on the cooling - from about 0.03 bar (ie negative pressure ) corresponding to a temperature of about 25 ° C. In nuclear power stations, only saturated steam is produced at a temperature below 300 ° C. Overheating of the live steam is not possible there, only a reheating by live steam.

When real power plant process, the turbine is indeed largely adiabatic (see adiabatic machine ), but by throttle, shock and friction processes (dissipation ) is not made the work completely to the shaft, the entropy increases. For large turbines of the grade is about 0.9. Next are efficiency reducing the flow pressure losses in the system, especially in the boiler ( no preheating and no isobaric isobaric or isothermal evaporation, especially no isobaric overheating). The feed pump is not working isentropic.

Opposites process for cooling

A corresponding process in the opposite direction ( " counter-clockwise " ) can be used for chillers and heat pumps. In this case, the steps in the circuit diagram and the diagram on the right are:

  • 4-1: Evaporation at a low temperature and pressure level ( heat absorption in cooling coils, instead of Qin Qout )
  • 1 - 2: compression (eg compressor in a refrigerator)
  • 2-3: cooling, condensing and subcooling at a high temperature and pressure level ( heat Qab instead of Qin )
  • 3-4: Relaxing of the liquid phase, wherein a partial evaporation is performed.

The last step could theoretically be carried out adiabatically with a turbine or piston engine, then this would correspond to an ideal process a left-handed Rankine process. In practice, however, is dispensed with compression chillers on the energy yield of this step ( 1% of total sales) to facilitate the construction. A turbine for evaporating liquid would also be very difficult to realize. Therefore relaxes irreversibly through a choke, the enthalpy remains constant. In the TS diagram item # 4 above then diagonally right below point 3, the energy is not delivered hence must not be resumed as Qin and the coefficient of performance is reduced slightly.