Power and power plants

Summary: Modern power plants typically convert energy inputs into rotational energy to spin an electric generator and create electricity. Leading fuel inputs include coal, natural gas, nuclear fission, and hydropower.

Electricity is a versatile energy source used to power much of modern life. However, this form of energy is difficult and expensive to store in large quantities. As a result, most electricity must be generated as it is needed. This puts a premium on understanding the power plants used to generate the electricity we use.

Modern power plants tend to produce electricity using the same components: a fuel source to generate heat, a boiler or similar component to generate steam or to direct combusted gas, a turbine that converts that steam or gas into rotational energy, and an electrical generator that converts the rotational energy into electricity. The major sources of heat include coal, natural gas, and nuclear fission. Supplementing these are renewable sources such as hydropower, which generates electricity by harnessing falling or running water, and wind power, which harnesses the power of the wind to generate electricity.

Large-scale electric power facilities and related infrastructure tend to present certain environmental challenges regardless of the fuels they consume. Massive centralized power generation facilities require substantial amounts of land, and environmentally irresponsible operations can leave the sites badly polluted and the neighboring land significantly devalued. Power transmission lines have a negative aesthetic impact, particularly in natural areas. These large structures can also disrupt wildlife habitats, and where they cross agricultural land they have the potential to impede farming operations. If a power plant diverts water from a river to serve as a coolant and then returns the heated water to the river, the resulting thermal can cause a localized reduction in the river’s dissolved oxygen levels and otherwise negatively affect aquatic organisms and ecosystems.

When fossil fuels are burned, carbon combines with oxygen from the air to form gaseous carbon dioxide. In the last decades of the twentieth century, concerns arose that increased carbon dioxide levels would cause Earth’s atmosphere to act like the glass panels of a greenhouse, allowing solar radiation to enter the atmosphere but preventing infrared heat from escaping. Experts warned that global warming from this greenhouse effect could melt polar ice caps and snow cover, which in turn could raise the levels of the oceans and cause disastrous flooding of coastal communities. Also, a hotter and drier climate could create worldwide problems for agriculture. The primary way to reduce the production of carbon dioxide is to burn less coal, oil, and natural gas, and many world powers took actions to limit global warming in the late twentieth and early twenty-first centuries.

Electric Generators

The basic form of an electric generator is a loop of wire between two stationary magnets. When the wire rotates, the electric field generates voltage. Since the movement of the wire coil is in different directions relative to each of the two magnets, the voltage generated is an alternating current (AC), meaning that the direction of the flow of electrons in the system changes. The frequency of AC is measured in hertz, or cycles per second. In the United States, electricity from the electric grid has a frequency of 60 hertz, meaning that it alternates 60 times every second. In Europe, for example, electricity from the grid alternates at 50 hertz. In addition to AC, there is direct current (DC), which comes from batteries. The main difference between power plants tends to be the way in which the electric generator is made to spin: Thermal power plants boil water to make steam to spin a turbine, which is connected to the generator; gas turbines burn gas to accomplish the same result; other plants use a different mechanical force, such as moving water or wind, to turn the generator directly.

Measuring Output and Efficiency

The electric output of power plants is measured in units of energy called watts, equal to 1 joule per second. One kilowatt is equal to 1,000 watts and is very roughly equal to the amount of energy used by a typical household in the United States at any given moment. That means that over the course of one hour, such a household would have used 1 kilowatt-hour of electricity.

The maximum amount of electricity that a power plant can produce at one time is referred to as the plant’s capacity. The potential capacity of power plants connected to the electric grid is typically measured in megawatts; 1 megawatt is equal to 1 million watts. A typical large coal, nuclear, natural gas, or hydroelectric power plant may have a capacity of anywhere from a few hundred megawatts to more than 1,000 megawatts.

One important measure of a power plant is its operating efficiency. The efficiency of a modern thermal electric generator is measured as a percentage of the energy content of the fuel that is converted into electricity or other usable outputs. Efficiency generally increases as plant temperature and pressure increase. Although typical thermal power plants have had efficiencies of between 20 and 45 percent, plants that effectively utilize waste heat have achieved operating efficiencies of 60 percent or greater.

The availability factor of a power plant is how often the plant is available to produce electricity, as opposed to being off line for a mechanical failure or maintenance. Availability factors are rarely below 70 percent and can be above 98 percent for some nuclear or gas plants or wind turbines. A related measure is a power plant’s capacity factor, or how much electricity the power plant produces over a period of time compared to how much it could have produced if it had been running at full capacity for that entire period. Typical capacity factors can be very high, above 90 percent, for nuclear or coal plants that run nearly constantly. Capacity factors can also be much lower, in the range of 20 to 45 percent, for wind farms, and even lower for solar plants.

Coal-Fired Power Plants

Along with wood-fired and hydroelectric power, coal-fired power plants are one of the original sources of energy used in power plants to generate electricity. Coal-fired power plants remain a leading source of electricity in the world. In 2022, 36 percent of the world’s electricity was produced from coal. Originally, coal-fired boilers were fed directly with coal. Over time, a number of ways to improve the efficiency of the system were devised. By pulverizing coal before feeding it into a boiler, its surface area is increased and the boiler can be operated at a hotter temperature and higher efficiency. Other technological improvements allowed development to move from so-called subcritical coal plants, in which steam temperature may be about 1,000 degrees Fahrenheit and steam pressure of 2,400 pounds per square inch, to “critical,” “supercritical,” and “ultrasupercritical” coal power plants, in which steam temperatures may be above 1,100 degrees Fahrenheit with pressures of 4,500 pounds per square inch. The efficiency of coal plants increases from about 35 percent to 42 percent or more (an increase in efficiency of 20 percent or more) along this continuum from subcritical to ultrasupercritical.

Coal-fired power plants tend to be moderately expensive to build, traditionally costing between $3,000 and $5,000 per kilowatt of capacity. However, they tend to be cheap to operate because of the relatively low price of coal per unit of energy. Coal is a common choice for new power plants in developing countries, whose economies require large increases in electricity generation to be developed as cheaply as possible. In developed countries, where electricity demand tends to grow more slowly and more of a premium is placed on the environment, new coal power plants are built less frequently. Note that in coal-fired power plants, as with most technologies, expertise is highest and costs lowest where the most new development is occurring.

Burning coal can produce emissions of sulfur dioxide, nitrogen dioxide, particulate matter, mercury, and carbon dioxide. Many developed countries place environmental restrictions on coal generators to reduce the impact of some or all of these emissions. For example, regulations to limit the emissions of sulfur dioxide in the United States, including portions of the Clean Air Act, have led many coal plant operators to install “scrubbers” to capture the sulfur dioxide before it is released into the air. Scrubbers force the exhaust from the plant through a mixture including lime or limestone, which binds with the sulfur.

Due to rising concerns in the early twenty-first century regarding global warming, international efforts were undertaken to move away from a reliance on coal. Following the implementation of the Paris Agreement in 2015, the United Nations reported in 2021 that there was a 76 percent reduction in the planned construction of new global coal power plants (worth an estimated 1000 gigawatts (GW) of coal-powered electricity). However, the demand for coal power plants spiked in China in 2023, with the country responsible for 95 percent of coal-plant construction. The number of plants that China was building was four times what it built in 2019. However, experts believed that the spike in the number of coal plants would soon be offset by the retirement of coal plants in the United States and Europe.

Natural Gas–Fired Power Plants

There are three basic types of natural gas power plants: steam turbines, simple-cycle combustion turbines, and combined-cycle gas combustion turbines. Combustion turbines are more commonly used with natural gas. These power plants are known for being cheap to build, but they can have high and variable operating costs due to the fluctuating price of natural gas.

Simple-cycle gas turbines create a mixture of air, fed through a compressor at hundreds of miles an hour, and natural gas. The air-gas mixture is burned at temperatures above 2,000 degrees Fahrenheit. This stream of hot gas hits the turbine, which causes it to spin and run an electric generator and drive the compressor. Because of this simple design, basically that of a jet engine, a gas turbine is quick to start up. However, efficiencies range from only 20 to 35 percent. These units are typically used only for peak periods of demand, when an additional amount of electricity must be generated quickly and the cost of that generation is less important.

A combined-cycle gas turbine begins with the simple cycle, then passes the waste gas turbine exhaust through a heat-recovery steam generator to create steam to run a steam turbine. This process can increase efficiency to between 50 and 60 percent. With higher efficiencies, such units can be run regularly.

In 2023, approximately 23 percent of the world’s electricity was produced from natural gas. During the same year, natural gas accounted for nearly 43 percent of electricity generation in the United States. Natural gas–fired electricity generation produces almost no sulfur dioxide or mercury emissions, about one-quarter the amount of nitrogen oxides, and about half as much carbon dioxide as coal-fired generation.

Hydroelectric Power Plants

Using falling or running water for power is an ancient technique. Because hydropower was understood prior to the development of electric power in the late nineteenth century, hydroelectric power plants were quick to develop. As a result, the best places for large dams in most developed countries were quickly used. Since the best locations for hydroelectric power plants tended to be chosen first, and because environmental concerns around hydropower have increased, few new hydropower plants have been built over time, and hydropower has contributed a smaller portion of total electricity generation in the United States and most developed countries. However, some countries with an abundance of optimal hydroelectric sites, such as China and Brazil, have emphasized hydroelectricity production.

Typical hydroelectric power plants with dams may cost about $3,000 per kilowatt of capacity, but they have essentially zero operating costs. These units can provide base-load electricity generation, because operators control gates that hold water back and choose when and how much electricity to produce. This is not the case with pure run-of-the-river hydroelectric power plants, which do not have those options. Also, uniquely among large sources of power, hydroelectric power has the ability to store energy at a reasonable cost, through the use of pumped storage. Storage is a little more expensive, between $5,000 and $6,000 per kilowatt of capacity, but because water is pumped into a reservoir when electricity prices are low, the energy in that water can be stored until needed, when prices are higher.

Hydroelectric power plants generated 15.8 percent of global electricity in 2020, but with significant variability across countries. For example, in the United States that percentage was just 6.3 percent in 2022.

Nuclear Power Plants

Nuclear power plants differ from other thermal power plants in that the heat source is not a fossil fuel but refined uranium, U-235. Rods enriched to contain about 3.5 percent U-235 (compared to just 0.7 percent natural enrichment) are placed near each other in a pool of water. When a U-235 atom is hit by a neutron, it splits and throws off two or three additional neutrons. If those neutrons hit other U-235 atoms, a chain reaction of fission can occur, creating a large amount of heat. This heat is used to heat the water, making steam to turn a turbine.

The fission process, once started, cannot be easily ratcheted up or down. As a result, nuclear power plants are typically run at a high capacity, as often as possible. If needed, the fission reaction can be slowed or stopped under normal conditions by inserting control rods made of a neutron-absorbing substance such as boron, often encased in stainless steel, or by moving the fuel rods farther apart to slow the interaction between them.

Opponents of nuclear power point out the risks of radiation in case of accidents and the difficulties of storing nuclear waste. Nuclear supporters tout low operating costs and the fact that the operation of a nuclear power plant creates zero emissions. Nuclear power accounted for 9 percent of global electricity in 2023 and 18 percent of electricity in the United States in 2022.

Oil-Fired Power Plants

Oil can be used to make electricity in the same ways that natural gas is used: via steam turbines, simple-cycle combustion turbines, and combined-cycle combustion turbines. In 2020, oil-fired power plants were responsible for generating 3.1 percent of global electricity. Much of this generation is focused in oil-producing countries, where the cost of oil is low. In the United States, electricity from oil-fueled power plants fell from 3.0 percent in 2005 to only 1.1 percent and 1.0 percent in 2008 and 2009, respectively. This number had fallen to just slightly more than 0.6 percent by 2024. The quantity of oil-fueled electricity generation can be volatile, because many plants that are powered by oil can switch fuels to take advantage of fuel price fluctuations. Emissions from fueling a power plant with oil are slightly less than those from coal-fired plants.

Nonhydro Renewable Power Generation

Generation from fuel sources in the category of nonhydro renewable generation include wind, biomass, waste, geothermal, solar photovoltaic, solar thermal, and tidal energy. Of these resources, wind is the largest and fastest-growing source, both in the world and in the United States. Wind generation increased from 0.3 percent of electricity generation in the United States in 2003 to 10.2 percent in 2022. Onshore wind farms have experienced growth because the cost of construction, about $2,500 per kilowatt, is competitive with other resources and wind farms have almost no operating costs.

Wind turbines generate electricity at the hub, where the rotors connect to the tower. The spinning blades turn an electrical generator. This hub can sit 328 feet (100 meters) or more aboveground, and blades for larger units can measure between 196 and 328 feet (60 and 100 meters) in diameter. Most utility-scale onshore wind turbines have the capability to generate about 1.5 to 3 megawatts of electricity each. Offshore wind farms often are made of slightly larger turbines, and proposed turbines would be capable of generating as much as 7 megawatts. Either onshore or offshore, wind turbines can be placed individually or in a large array of dozens or hundreds.

However, wind power—as well as solar and tidal power—suffers from an inability to choose when power will be generated. This situation is referred to as intermittency. Intermittency limits the usability of these renewable energies because they cannot be increased or decreased to follow changes in electricity demand; rather, they are subject to the availability of the resource, whether wind, sunshine, or appropriately strong tides. Intermittency also reduces these resources’ capacity factor, or average amount of electricity generated compared to how much can be generated. The capacity factor of wind farms may be 20 to 40 percent, while solar photovoltaic (PV) cells, which produce electricity when photons of light hit a surface of silicon or other material and create a flow of electrons, have capacity factors of about 10 to 20 percent.

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