Organic Rankine Cycle

The Rankine cycle is a thermodynamic cycle which converts heat into work. The heat is supplied externally to a closed loop, which usually uses water as the working fluid. This cycle generates about 80% of all electric power used throughout the world, including virtually all solar thermal, biomass, coal and nuclear power plants.

The organic Rankine cycle (ORC) uses a high molecular organic low boiling fluid 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.

The Rankine cycle does not restrict the working fluid in its definition, so the inclusion of an “organic” cycle is simply a marketing concept that should not be regarded as a separated thermodynamic cycle.

T-s diagram of a typical Rankine cycle


There are four processes in the Rankine cycle:
  • 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 waste heat source to become a dry saturated vapor
  • Process 3-4: The dry saturated vapor expands through a turbine. This decrease the temperature and pressure of the vapor, and some condensation may occur. 
  • Process 4-1: The wet vapor then enters a condenser where it is condensed at constant pressure and temperature to become a saturated 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(temperature-entropy) diagram and more closely resemble that of the Carnot cycle. The Rankine cycle prevents the vapor ending in the superheat region after the expansion in turbine, which reduces the energy removed by the condensers.





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