A thermal power plant is a facility in which heat from combustion produces steam that drives a steam turbine which in turn is connected to an electrical generator. The heat source comes from the combustion of fossil fuels, biomass, or waste; the fissioning of enriched uranium in nuclear power plants; the sun in solar thermal power plants; or heat from the Earth’s core in geothermal power plants. In 2022, more than 70% of electricity generated in the world came from a thermal power plant.1
Efficiency is a key engineering, economic, and environmental characteristic of a thermal power plant. The shorthand description of efficiency is how much electricity you get out compared to the energy input to the plant. The technical definition of efficiency is the heat content of a kilowatt-hour (kWh) of electricity divided by the heat rate of the plant, which is the amount of energy used to generate one kWh of electricity, expressed as a percentage. Efficiency is always less than 100% because, as dictated by the second law of thermodynamics, some of the energy input is converted into low quality “waste” heat.
The average efficiency of a thermal power plant using fossil fuels in the United States increased from about 4% in 1900 to about 39% in 2023. The tremendous gains in efficiency were achieved by reducing heat loss in the three main energy conversions in a thermal power plant: 2
- The boiler, where fuel heat is converted to steam energy.
- The turbine, where steam heat is converted to mechanical rotational energy.
- The generator, where rotational energy is converted to electric power.
The efficiency of coal, oil, and nuclear power plants plateaued beginning in the 1970s due to diminishing returns to technological and operational advances. The efficiency of natural gas plants has increased over the past few decades due to the deployment of so-called combined cycle plants in which some of the waste heat from natural gas combustion in gas turbines is captured and used to generate additional electricity in a conventional steam turbine cycle. The best combined cycle natural gas power plants have efficiencies that exceed 60%.
Efficiency gains have widespread benefits. As the efficiency of a power plant increases, the fuel and operating cost of the facility declines, which helps reduce the cost of electricity to consumers. Improved efficiency also reduces the amount of coal, oil, natural gas, and uranium that must be extracted and processed to generate one kilowatt hour of electricity. Less cooling water is needed per kWh hour in a more efficient plant. The combustion of fossil fuels and biomass fuels release a wide range of air pollutants that harm the health of people and natural systems. Thermal power plants also account for about 25% of greenhouse gas emission in the United States.3 The dramatic improvements in the efficiency of thermal power plants have dramatically lowered the per kWh severity of such impacts compared to a world with no efficiency gains.
1 Ember, Electricity Data Explorer, Accessed June 15, 2023,https://ember-climate.org/data/data-tools/data-explorer/
2 Understanding Coal Power Plant Heat Rate and Efficiency, Power, November 1, 2014, https://www.powermag.com/understanding-coal-power-plant-heat-rate-and-efficiency/
3 U.S. Environmental Protection Agency,Sources of Greenhouse Gas Emissions, Accessed June 15, 2023, https://www.epa.gov/ghgemissions/sources-greenhouse-gas-emissions