- Process 1-2: The working fluid is pumped from low to high pressure. As the fluid is a liquid at this stage the pump requires little input energy.
- Process 2-3: The high pressure liquid enters a boiler where it is heated at constant pressure by an external heat source to become a dry saturated vapour. The input energy required can be easily calculated using mollier diagram or h-s chartor enthalpy-entropy chart also known as steam tables.
- Process 3-4: The dry saturated vapor expands through a turbine, generating power. This decreases the temperature and pressure of the vapour, and some condensation may occur. The output in this process can be easily calculated using the Enthalpy-entropy chart or the steam tables.
- Process 4-1: The wet vapour then enters a condenser where it is condensed at a constant pressure to become asaturated liquid.
In an ideal Rankine cycle the pump and turbine would be
isentropic, i.e., the pump and turbine would generate no entropy and hence maximize the net work output. Processes 1-2 and 3-4 would be represented by vertical lines on the
T-S diagramand more closely resemble that of the Carnot cycle. The Rankine cycle shown here prevents the vapor ending up in the superheat region after the expansion in the turbine, which reduces the energy removed by the condensers.
Variables
 | Heat flow rate to or from the system (energy per unit time) |
 | Mass flow rate (mass per unit time) |
 | Mechanical power consumed by or provided to the system (energy per unit time) |
 | Thermodynamic efficiency of the process (net power output per heat input, dimensionless) |
 | Isentropic efficiency of the compression (feed pump) and expansion (turbine) processes, dimensionless |
 | The "specific enthalpies" at indicated points on the T-S diagram |
 | The final "specific enthalpy" of the fluid if the turbine were isentropic |
 | The pressures before and after the compression process |
Equation
In general, the efficiency of a simple Rankine cycle can be defined as:
Each of the next four equations
[1] is easily derived from the
energy and
mass balance for a control volume.
defines the
thermodynamic efficiency of the cycle as the ratio of net power output to heat input. As the work required by the pump is often around 1% of the turbine work output, it can be simplified.
When dealing with the efficiencies of the turbines and pumps, an adjustment to the work terms must be made.
Real Rankine cycle
In a real power plant cycle (the name 'Rankine' cycle is used only for the ideal cycle), the compression
by the
pump and the expansion in the
turbine are not isentropic. In other words, these processes are non-reversible and
entropy is increased during the two processes. This somewhat increases the
powerrequired by the pump and decreases the power generated by the turbine.
In particular the efficiency of the steam turbine will be limited by water droplet formation. As the water condenses, water droplets hit the turbine blades at high speed causing pitting and erosion, gradually decreasing the life of turbine blades and efficiency of the turbine. The easiest way to overcome this problem is by superheating the steam. On the
Ts diagram above, state 3 is above a two phase region of steam and water so after expansion the steam will be very wet. By superheating, state 3 will move to the right of the diagram and hence produce a drier steam after expansion.
Variation of the basic Rankine Cylce
The overall
thermodynamic efficiency (of almost any cycle) can be increased by raising the average
heat input
temperature 
of that cycle. Increasing the temperature of the steam into the superheat region is a simple way of doing this. There are also variations of the basic Rankine cycle which are designed to raise the thermal efficiency of the cycle in this way; two of these are described below.
Rankine cycle with reheat[edit]
Rankine cycle with reheat
The purpose of a reheating cycle is to remove the moisture carried by the steam at the final stages of the expansion process. In this variation, two
turbines work in series. The first accepts
vapor from the
boiler at high pressure. After the vapor has passed through the first turbine, it re-enters the boiler and is reheated before passing through a second, lower-pressure, turbine. The reheat temperatures are very close or equal to the inlet temperatures, whereas the optimum reheat pressure needed is only one fourth of the original boiler pressure. Among other advantages, this prevents the vapor from
condensingduring its expansion and thereby damaging the turbine blades, and improves the efficiency of the cycle, given that more of the heat flow into the cycle occurs at higher temperature. The reheat cycle was first introduced in the 1920s, but was not operational for long due to technical difficulties. In the 1940s, it was reintroduced with the increasing manufacture of high-pressure
boilers, and eventually double reheating was introduced in the 1950s. The idea behind double reheating is to increase the average temperature. It was observed that more than two stages of reheating are unnecessary, since the next stage increases the cycle efficiency only half as much as the preceding stage. Today, double reheating is commonly used in power plants that operate under supercritical pressure.
Regenerative Rankine cycle[edit]
Regenerative Rankine cycle
The regenerative Rankine cycle is so named because after emerging from the condenser (possibly as a
subcooled liquid) the working fluid is heated by
steamtapped from the hot portion of the cycle. On the diagram shown, the fluid at 2 is mixed with the fluid at 4 (both at the same pressure) to end up with the saturated liquid at 7. This is called "direct contact heating". The Regenerative Rankine cycle (with minor variants) is commonly used in real power stations.
Another variation is where
bleed steam from between turbine stages is sent to
feedwater heaters to preheat the water on its way from the condenser to the boiler. These heaters do not mix the input steam and condensate, function as an ordinary tubular heat exchanger, and are named "closed feedwater heaters".
The regenerative features here effectively raise the nominal cycle heat input temperature, by reducing the addition of heat from the boiler/fuel source at the relatively low feedwater temperatures that would exist without regenerative feedwater heating. This improves the efficiency of the cycle, as more of the heat flow into the cycle occurs at higher temperature. This process ensures cycle economy.
Organic Rankine cycle
The organic Rankine cycle (ORC) uses an organic fluid such as n-pentane or toluene in place of water and steam. This allows use of lower-temperature heat sources, such as solar ponds, which typically operate at around 70–90 °C The efficiency of the cycle is much lower as a result of the lower temperature range, but this can be worthwhile because of the lower cost involved in gathering heat at this lower temperature. Alternatively, fluids can be used that have boiling points above water, and this may have thermodynamic benefits. See, for example, mercury vapour turbine.
The Rankine cycle does not restrict the working fluid in its definition, so the name “organic cycle” is simply a marketing concept and the cycle should not be regarded as a separate thermodynamic cycle.
Supercritical Rankine cycle
The Rankine cycle applied using a supercritical fluid combines the concepts of heat regeneration and supercritical Rankine cycle into a unified process called the Regenerative Supercritical Cycle (RGSC) cycle. It is optimised for temperature sources 125 - 450°C.
