Like a classic Rankine cycle plant, Organic Rankine Cycle (ORC) plants generate power using a working fluid to rotate a generator. The term “organic” indicates that the cycle relies on an organic fluid instead of water as the working fluid. This blog discusses the difference between these two cycles and the advantages of the Organic Rankine Cycle.
In the classic Rankine Cycle, the working fluid is water. The circulating water is heated until it changes phases to steam. The steam’s energy drives a spinning turbine connected to a generator which generates electricity. Used steam exhausts from the turbine and enters a condenser where the steam condenses back to water. A feed pump pumps the water (also called condensate) back to the boiler to repeat the cycle.
The Organic Rankine Cycle uses an organic fluid instead of water. This fluid may be one of several organic fluids such as a hydrocarbon (toluene, isobutene, isopentane, etc.) or a refrigerant. Otherwise, the Organic Rankine Cycle fluid uses the same steps as the Rankine Cycle (i.e., boiler, turbine, condenser, feed pump).
So why use the Organic Rankine Cycle? The conversion of water to steam requires 940 Btu/lb at atmospheric pressure. This is significant heat input, requiring substantial fuel. Organic fluids require significantly less specific heat (Btu/lb) to affect the phase change. Because the organic fluid evaporates with a lower specific heat, the ORC can utilize a lower temperature and pressure to drive the cycle. Therefore, heat sources such as solar, geothermal, waste heat, and biomass may be used. Additionally, because the organic fluid remains superheated through the entire turbine, cycle efficiency increases further.
So why not always use the ORC for electricity generation? An ORC power turbine does not produce the high power of a conventional Rankine power turbine. This is because of the low enthalpy of the fluid. A typical ORC generator might have an output of 400kW to a few megawatts maximum. Compare this Rankine cycle generators, which may have outputs of over one thousand megawatts.
If a plant has a low power requirement, ORC generators may be best. Similarly, a plant with relatively low heat generation may choose an ORC where a steam Rankine cycle is not possible. With advancements in fluid technologies, outputs of organic Rankine cycle plants should continue to increase output.
Ambient Factors Conditions and Combustion Turbine Performance
