The Rankine cycle uses heat to produce steam which drives a turbine, generating electricity. It involves a closed loop with four main components: a pump, a boiler, a turbine, and a condenser. The efficiency can reach about 40%. Other fluids besides water can be used, such as organic fluids in an organic Rankine cycle (ORC). ORC technology is used in geothermal and some solar power plants and can harness waste heat from industrial processes.
Rankine cycle is a term that describes a process that uses heat to do work. It is named after the Scottish engineer and physicist William John Macquorn Rankine. Generally, the heat produces steam which drives a turbine. It is mainly used to generate electricity and, as of 2011, accounts for approximately 80% of the electricity used worldwide.
The basic principle involves a fluid – usually water, but referred to as the “working fluid” – flowing in a closed loop with four main components: a pump, a boiler, a turbine and a condenser. In the boiler, an external heat source converts the liquid working fluid into a steam that drives the turbine and is then condensed into a liquid, which is pumped back into the boiler. The steam leaves the boiler at a high temperature – up to 1,000°F (538°C) – and pressure – up to 4,500 psig (31,026.4 kPag) – and it is the expansion of this hot gas at high pressure that drives the turbine shaft, producing a rotation that can generate electricity or drive a motor. The efficiency of the Rankine cycle can reach about 40%.
This process underlies most power plants where fuel is used as a heat source to generate electricity. The fuel can be oil, coal, gas, biomass or nuclear; since the heat source is external, anything that can generate sufficient heat can be used. Small-scale Rankine cycle technology can also be used to exploit waste heat from traditional power plants. For example, gas-fired power plants use combustion turbines that generate hot exhaust gas because, unlike Rankine cycle turbines, they do not operate in a closed loop. It is possible to combine them with a Rankine cycle generator which uses hot gas to produce steam, thus significantly improving energy efficiency.
Fluids other than water can be used in a Rankine cycle where they offer advantages in particular applications. For example, power plants operating in very cold conditions may require a fluid with a lower freezing point. A variety of organic fluids have been used, and pentane has been successfully employed in a Rankine cycle generator using the hot exhaust gas from a gas-fired power plant as a heat source, as described above. A process that uses organic fluid in this way is known as an organic Rankine cycle (ORC).
Many renewable energy projects use ORC technology to generate power. It is often used in geothermal power plants, and this system has been adapted for use in some solar power plants, where sunlight is concentrated to provide the heat needed to drive the cycle. Small-scale generators have become available that use this technology to harness waste heat from a variety of industrial processes.
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