Heat Engines
Posted under Power Source
Over 90% of U.S. electricity is generated in power plants that convert heat into mechanical work. The heat may be the result of nuclear reactions, fossil-fuel combustion, or even concentrated sunlight focused onto a boiler.
Almost all of this 90% is based on a heat source boiling water to make steam that spins a turbine and generator, but there is a rapidly growing fraction that is generated using gas turbines. The best new fossil-fuel power plants use a combination of both steam turbines and gas turbines to generate electricity with very high efficiency.
Steam engines, gas turbines, and internal-combustion engines are examples of machines that convert heat into useful work. What we are interested in here is, How efficiently can they do so? This same question will be asked when we describe fuel cells, photovoltaics, and wind turbines in future chapters, and in each case we will encounter quite interesting, fundamental limits to their maximum possible energy-conversion efficiencies.
Heat Engines
Very simply, a heat engine extracts heat QH from a high-temperature source, such as a boiler, converts part of that heat into work W, usually in the form of a rotating shaft, and rejects the remaining heat QC into a low-temperature sink such as the atmosphere or a local body of water.
The thermal efficiency of a heat engine is the ratio of work done to input energy provided by the high-temperature source:
Since energy is conserved,
QH = W + QC
which leads to another expression for efficiency:
