The Rankine cycle is a term describing a process that uses heat to perform work. It is named after the Scottish engineer and physicist William John Macquorn Rankine. Generally, the heat produces steam that drives a turbine. It is primarily used to generate electricity, and as of 2011, accounts for approximately 80% of the electricity used throughout the world.
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 vapor that drives the turbine and is then condensed into a liquid, which is pumped back into the boiler. The vapor 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, high-pressure gas that drives the turbine shaft, producing a rotation that can generate electricity or drive an engine. Rankine cycle efficiency can reach around 40%.
This process is the basis for most power plants where fuel is used as a source of heat 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. Rankine cycle technology on a smaller scale can also be used to exploit waste heat from traditional power plants. For example, gas-burning power stations use combustion turbines that generate hot exhaust gas as, unlike Rankine cycle turbines, they do not operate in a closed loop. It is possible to combine these with a Rankine cycle generator that uses the hot gas to produce the steam, thus greatly improving the energy efficiency.
Fluids other than water can be used in a Rankine cycle where they offer advantages in particular applications. For example, power plants that operate in very cold conditions may require a fluid with a lower freezing point. A number of organic fluids have been used and pentane has been employed successfully in a Rankine cycle generator that exploits hot exhaust gas from a gas-fired power station 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 stations, and this system has been adapted to be used in some solar power plants, where the sunlight is focused to provide the heat to drive the cycle. Small-scale generators that use this technology have become available to exploit waste heat from a variety of industrial processes.