Electricity in the U.S. Basics
Most of the electricity in the United States is produced using steam turbines
A turbine converts the kinetic energy of a moving fluid (liquid or gas) to mechanical energy. In a steam turbine, steam is forced against a series of blades mounted on a shaft. The steam rotates the shaft connected to a generator. The generator, in turn, converts its mechanical energy to electrical energy based on the relationship between magnetism and electricity.
In steam turbines powered by fossil fuels (coal, natural gas, and petroleum), the fuel is burned in a furnace to heat water in a boiler to produce steam.
Most of U.S. electricity is generated using fossil fuels
In 2016, natural gas was the largest energy source for the 4 trillion kilowatthours of electricity generated in the United States.
Natural gas was the source of about 34% of U.S. electricity generation in 2016. In addition to burning natural gas to heat water for steam, it is also burned to produce hot combustion gases that pass through a gas turbine, spinning the turbine's blades to generate electricity.
Coal was the second-largest energy source for U.S. electricity generation in 2016—about 30%. Nearly all coal-fired power plants use steam turbines. A few coal-fired power plants convert coal to a gas for use in a gas turbine to generate electricity.
Petroleum can be burned to produce hot combustion gases to turn a turbine or to make steam that turns a turbine. Residual fuel oil and petroleum coke, products from refining crude oil, are the main petroleum fuels used in steam turbines. Distillate (or diesel) fuel oil is used in diesel-engine generators. Petroleum was the source of less than 1% of U.S. electricity generation in 2016.
Nuclear power provides about one-fifth of U.S. electricity
Nuclear power plants produce electricity with nuclear fission to create steam that spins a turbine to generate electricity. Nuclear power was the source of about 20% of U.S. electricity generation in 2016.
Renewable energy sources provide 15% of U.S. electricity
Hydropower, the source of about 7% of total U.S. electricity generation (about 44% of electricity generation from renewable energy) in 2016, is a process in which flowing water spins a turbine connected to a generator. Most hydropower production is at large facilities built by the federal government, such as the Grand Coulee Dam. Many of the largest hydroelectric dams are in the western United States, but many hydropower facilities operate around the country.
Wind power is produced by converting wind energy into electricity with wind turbines. Electricity generation from wind has increased significantly in the United States since 1970. In 2016, wind power provided almost 6% of U.S. electricity generation (about 37% of electricity generation from renewable energy).
Biomass is material derived from plants or animals and includes lumber and paper mill wastes and the food scraps, grass, leaves, paper, and wood in municipal solid waste (garbage). Biomass also includes forestry and agricultural residues such as wood chips, corn cobs, and wheat straw. These materials can be burned directly in steam-electric power plants, or they can be converted to a gas that can be burned in steam generators, gas turbines, or internal combustion engine-generators. Biomass accounted for about 2% of total U.S. electricity generation in 2016.
Solar power is produced with energy from the sun. Photovoltaic (PV) and solar-thermal power are the two main types of technologies used to convert solar energy to electricity. PV conversion produces electricity directly from sunlight in a photovoltaic (solar) cell. Solar-thermal power generators concentrate solar energy to heat a fluid and produce steam to drive turbines. In 2016, about 1% of U.S. electricity generation was from solar energy.
Geothermal power comes from heat energy beneath the surface of the earth. In some areas of the United States, geothermal energy is close enough to the earth's surface to heat underground water into steam, which is tapped for use at steam-turbine plants. Geothermal electricity generation was less than 1% of total U.S. electricity generation in 2016.
How Electricity Gets to Your Home
Electricity is delivered to consumers through a complex network
Electricity is generated at power plants and moves through a complex system, sometimes called the grid, of electricity substations, transformers, and power lines that connect electricity producers and consumers. Most local grids are interconnected for reliability and commercial purposes, forming larger, more dependable networks that enhance the coordination and planning of electricity supply.
In the United States, the entire electricity grid consists of hundreds of thousands of miles of high-voltage power lines and millions of miles of low-voltage power lines with distribution transformers that connect thousands of power plants to hundreds of millions of electricity customers all across the country.
The stability of the electricity grid requires the electricity supply to constantly meet electricity demand, which in turn requires coordination of numerous entities that operate different components of the grid. The U.S. electricity grid consists of three large interconnected systems that operate to ensure the stability and reliability of the grid. To ensure coordination of electric system operations, the North American Electric Reliability Corporation developed and enforces mandatory grid reliability standards approved by the Federal Energy Regulatory Commission (FERC).
The smart grid
The smart grid incorporates digital technology and advanced instrumentation into the traditional electrical system, which enables utilities and customers to receive information from and communicate with the grid. A smarter grid makes the electrical system more reliable and efficient by helping utilities reduce electricity losses and to detect and fix problems more quickly. The smart grid can help consumers intelligently manage energy use, especially at times when demand reaches significantly high levels or when a reduced energy demand is needed to support system reliability.
Smart devices in homes, offices, and factories can inform consumers and their energy management systems of times when an appliance is using relatively higher-priced electricity. This helps consumers, or their intelligent systems, to optimally adjust settings that, when supported by demand reduction incentives or time-of use electricity rates, can lower their energy bills. Smart devices on transmission and distribution lines and at substations allow a utility to more efficiently manage voltage levels and more easily find out where an outage or other problem is on the system. Smart grids can sometimes even remotely correct problems in the electrical distribution system by digitally sending instructions to equipment that can adjust the conditions of the system.
Electricity comes from various sources and types of providers
The origin of the electricity that consumers purchase varies. Some electric utilities generate all the electricity they sell using just the power plants they own. Other utilities purchase electricity directly from other utilities, power marketers, and independent power producers or from a wholesale market organized by a regional transmission reliability organization.
The retail structure of the electricity industry varies from region to region. The company selling you power may be a not-for-profit municipal electric utility; an electric cooperative owned by its members; a private, for-profit electric utility owned by stockholders (often called an investor-owned utility); or in some states, you may purchase electricity through a power marketer. A few federally-owned power authorities—including the Bonneville Power Administration and the Tennessee Valley Authority, among others—also generate, buy, sell, and distribute power. Local electric utilities operate the distribution system that connects consumers with the grid regardless of the source of the electricity.
The process of delivering electricity
The electricity power plants generate is delivered to customers over transmission and distribution power lines. High-voltage transmission lines, like those that hang between tall metal towers, carry electricity over long distances to where it is needed. Higher voltage electricity is more efficient and less expensive for long distance electricity transmission. Lower voltage electricity is safer for use in homes and businesses. Transformers at substations increase (step up) or reduce (step down) voltages to adjust to the different stages of the journey from the power plant on long distance transmission lines to distribution lines that carry electricity to homes and businesses.
Electricity & the Environment
Although electricity is a clean and relatively safe form of energy, the generation and transmission of electricity has environmental impacts. Nearly all types of electric power plants have an effect on the environment. Some power plants have a bigger effect than others.
The two coal-fired power plants of the Crystal River North Steam Complex in Crystal River, Florida
Hunter Power Plant, a coal-fired power plant south of Castle Dale, Utah
The United States has laws that govern the environmental impacts of electricity generation and transmission. The Clean Air Act regulates air pollutant emissions from most power plants. The U.S. Environmental Protection Agency (EPA) administers the Clean Air Act and sets emissions standards for power plants through various programs like the Acid Rain Program. The Clean Air Act has helped to substantially reduce emissions of some major types of air pollutants in the United States.
The impact of power plants on the landscape
All power plants have a physical footprint (the location of the power plant). Some power plants are located inside, on, or next to an existing building, so the impact of the footprint is fairly small. Most large power plants require land clearing to build the power plant. Some power plants may also require access roads, railroads, and pipelines for fuel delivery, electricity transmission lines, and cooling water supplies. Power plants that burn solid fuels may have areas to store the combustion ash.
Many power plants are large physical structures that alter the visual landscape. In general, the larger the structure, the more likely it is that the power plant will affect the visual landscape.
Fossil fuel, biomass, and waste burning power plants
In the United States, the energy sources for about 68% of total electricity generation in 2015 were: fossil fuels (mainly coal, oil, and natural gas), materials that come from plants (biomass), and municipal and industrial wastes. Emissions that result from combustion of these fuels include:
- Carbon dioxide (CO2)
- Carbon monoxide (CO)
- Sulfur dioxide (SO2)
- Nitrogen oxides (NOx)
- Particulate matter (PM)
- Heavy metals such as mercury
Nearly all combustion byproducts have negative impacts on the environment and human health:
- CO2 is a greenhouse gas, and it contributes to the greenhouse effect.1
- SO2 causes acid rain, which is harmful to plants and to animals that live in water. SO2 also worsens respiratory illnesses and heart diseases, particularly in children and the elderly.
- NOx contribute to ground level ozone, which irritates and damages the lungs.
- PM results in hazy conditions in cites and scenic areas, and coupled with ozone, contributes to asthma and chronic bronchitis, especially in children and the elderly. Very small, or fine PM, is also believed to cause emphysema and lung cancer.
- Heavy metals such as mercury are hazardous to human and animal health.
Power plants use air emission controls to limit their environment impact
Air pollution emission standards limit the amounts of some of the substances that power plants can release into the air. Power plants meet these standards in several ways:
- Burning low sulfur content coal reduces SO2 emissions. Pretreating and processing coal can also reduce the level of undesirable compounds in combustion gases.
- PM emission control devices that treat combustion gases before they exit the power plant include:
- Bag-houses are large filters that trap particulates.
- Electrostatic precipitators use electrically charged plates that attract and pull particulates out of the combustion gas.
- Wet scrubbers use a liquid solution to remove PM from combustion gas.
- Wet and dry scrubbers mix lime in the fuel (coal) or spray a lime solution into combustion gases to reduce SO2 emissions. Fluidized bed combustion also results in lower SO2 emissions.
- NOx emissions controls include low NOx burners during the combustion phase or selective catalytic and non-catalytic converters during the post combustion phase.
Some power plants also produce liquid and solid wastes
Ash is the solid residue that results from burning solid fuels such as coal, biomass, and municipal solid waste. Bottom ash includes the largest particles that collect at the bottom of the combustion chamber. Fly ash is the smaller and lighter particulates that collect in air emission control devices. Fly ash is usually mixed with bottom ash. The resulting sludge, which contains all the hazardous materials that pollution control devices capture, may be put in retention ponds, sent to landfills, or sold for use in making concrete blocks or asphalt. Many coal-fired power plants have large sludge ponds. Several of these ponds have burst and caused extensive damage and pollution downstream.
Most power plants produce greenhouse gases
Electricity generation is one of the leading sources of greenhouse gas emissions in the United States. Power plants that burn fossil fuels or materials made from fossil fuels, and some geothermal power plants, are the sources of nearly 40% of total U.S. energy-related carbon dioxide emissions.
Nuclear power plants produce different kinds of waste
Nuclear power plants do not not produce greenhouse gases or PM, SO2, or NOx, but they do produce two kinds of radioactive waste:
- Low-level radioactive waste is stored at nuclear power plants until the radioactivity in the waste decays to a level where reactor operators can dispose of it as ordinary trash or to a level where they can send it to a low-level radioactive waste disposal site.
- Spent (used) nuclear fuel assemblies are highly radioactive and reactor operators must initially store it in specially designed pools of water resembling large swimming pools. These pools cool the fuel and act as a radiation shield. Operators can also store spent nuclear fuel in specially designed dry storage containers. An increasing number of reactor operators now store older spent nuclear fuel in dry storage facilities using special outdoor concrete or steel containers with air cooling. All commercial nuclear power plants store spent nuclear fuel assemblies at the plant because, at this time, the federal government has not approved any other places (repositories) for storing the waste.
Electric power lines and other distribution infrastructure also have a footprint
Electricity transmission lines and the distribution infrastructure that carries electricity from power plants to customers also have environmental impacts. Most transmission lines are above ground on large towers. The towers and lines alter the visual landscape, especially when they pass through natural areas. Trees near the wires may be disturbed and may have to be continually managed to keep limbs from touching the wires. These activities can affect native plant populations and wildlife. Power lines can be placed underground, but it is more expensive and may result in a greater landscape disturbance than overhead lines.
1. U.S. Environmental Protection Agency, Climate Change Science