Hydropower basics

Hydropower is energy in moving water

People have a long history of using the force of water flowing in streams and rivers to produce mechanical energy. Hydropower was one of the first sources of energy used for electricity generation and is usually the largest single renewable energy source of annual electricity generation in the United States.

In 2022, hydroelectricity accounted for about 6.2% of total U.S. utility-scale electricity generation and 28.7% of total utility-scale renewable electricity generation.¹ Hydroelectricity generation varies annually, and it's share of total U.S. electricity generation generally decreased from the 1950's through 2020, mainly because of increases in electricity generation from other sources. Hydroelectricity's percentage share of total annual U.S. electricity generation in 2001 through 2022 averaged about 6.7%.

Hydropower relies on the water cycle

Understanding the water cycle is important to understanding hydropower. The water cycle has three steps:

1. Solar energy heats water on the surface of rivers, lakes, and oceans, which causes the water to evaporate.
2. Water vapor condenses into clouds and falls as precipitation—rain and snow.
3. Precipitation collects in streams and rivers, which empty into oceans and lakes, where it evaporates and begins the cycle again.

The amount of precipitation that drains into rivers and streams in a geographic area determines the amount of water available for producing hydropower. Seasonal and long-term changes in rain and snowfall, such as droughts, have a big effect on hydropower production.

Hydroelectric power is produced with moving water

Because the source of hydroelectric power is water, hydroelectric power plants are usually located on or near a water source. The volume of the water flow and the change in elevation (or fall) from one point to another determine the amount of available energy in moving water. In general, the greater the water flow and the higher the fall, the more electricity a hydropower plant can produce. A big river, such as the Columbia River that forms the border between Oregon and Washington, carries a great deal of energy in its flow. Water descending rapidly from a high point, such as Niagara Falls in New York, also has substantial energy in its flow.

At hydropower plants, water flows through a pipe, or penstock, then pushes against and turns blades in a turbine to spin a generator to produce electricity.

Conventional hydroelectric facilities include

• Run-of-the-river systems, where the force of the river's current applies pressure on a turbine. The facilities may have a weir (a short or low dam) in the water course to divert water flow to hydro turbines.
• Storage systems, where water accumulates in reservoirs created by dams on streams and rivers and is released through hydro turbines as needed to generate electricity. Most U.S. hydropower facilities have dams and storage reservoirs.

Pumped-storage hydropower facilities are a type of hydroelectric storage system where water is pumped from a water source up to a storage reservoir at a higher elevation and is released from the upper reservoir to power hydro turbines located below the upper reservoir. The electricity for pumping may be from hydroturbines or from other types of power plants including fossil fuel or nuclear power plants. They usually pump water to storage when electricity demand (or use) and prices are low and release the stored water to generate electricity when electricity demand and prices are higher. Pumped-storage hydroelectric systems generally use more electricity to pump water to the upper water storage reservoirs than they produce with the stored water.

History of hydropower

Hydropower is one of the oldest sources of energy for producing mechanical and electrical energy. Hydropower was used thousands of years ago to turn paddle wheels to help grind grain. Before steam power and electricity were available in the United States, grain and lumber mills were powered directly with hydropower. The first industrial use of hydropower to generate electricity in the United States was in 1880, when 16 brush-arc lamps were powered using a water turbine at the Wolverine Chair Factory in Grand Rapids, Michigan. The first U.S. hydroelectric power plant opened on the Fox River near Appleton, Wisconsin, on September 30, 1882. Most U.S. hydroelectricity is now produced at large dams on major rivers, and most of these hydroelectric dams were built before the mid-1970s.

Where hydropower is generated

Most U.S. hydroelectricity generation capacity is in the West

There are hydropower facilities in nearly every state. Most hydroelectricity is produced at large dams built by the federal government, and many of the largest hydropower dams are in the western United States.

About half of U.S. hydroelectricity generation capacity² is in Washington, California, and Oregon. Washington has the most hydroelectric generating capacity of any state and is the site of the Grand Coulee Dam, the largest hydropower facility in the United States. New York has the largest hydroelectricity generation capacity of all states east of the Mississippi River, followed by Alabama.

In 2022, total U.S. conventional hydroelectricity net summer electricity generation capacity 79,980 megawatts (MW)—or about 80 million kilowatts.

• The top five states and their percentage shares of U.S. total conventional hydroelectricity net summer electricity generation capacity in 2022 were:
• Washington27%
• California13%
• Oregon10%
• New York6%
• Alabama4%

Hydroelectricity generation varies with precipitation levels

Because hydroelectricity generation depends on precipitation, and precipitation levels vary seasonally and annually, the ranking of each state in annual hydroelectricity generation may be different from its ranking in generation capacity.

In 2022, total U.S. conventional hydroelectricity generation was equal to about 6.2% (about 262 billion kilowatthours [kWh]) of total U.S. utility-scale electricity generation

• The top five states and their percentage shares of U.S. total conventional hydroelectricity generation in 2022 were:
• Washington31%
• Oregon12%
• New York10%
• California7%
• Montana4%

Most dams were not built for electricity generation

Only a small percentage of the dams in the United States produce electricity. Most dams were constructed for irrigation and flood control and do not have hydroelectricity generators. The U.S. Department of Energy estimated that in 2012, non-powered dams in the United States had a total of 12,000 megawatts (MW) of potential hydropower capacity.

Hydropower and the environment

Dams are not just for generating electricity

Most dams in the United States were built mainly for flood control, municipal water supply, and irrigation. Although many dams have hydroelectric generators, most dams were not built specifically for hydropower generation. Hydropower generators produce clean energy because they do not directly emit air pollutants. However, dams, reservoirs, and hydroelectric generators can affect the environment.

Dams have some environmental impact

A dam that creates a reservoir—diverts water to a run-of-river hydropower plant—may obstruct fish migration. A dam and reservoir can change natural water temperatures, water chemistry, river flow characteristics, and silt loads. These changes can affect the ecology and the physical characteristics of the river, which may negatively affect native plants and animals in and around the river.

Reservoirs may also cover important natural areas, agricultural land, or archeological sites. A reservoir or a dam’s operation may also result in relocating people. The physical location of a dam or reservoir, the dam operations, or using the river’s water can change the environment over a much larger area than the reservoir.

Dams have associated emissions

Manufacturing the concrete and steel in hydropower dams requires equipment that may produce emissions. If fossil fuels are an energy source for making these materials, then the emissions associated with those fuels could be associated with the electricity that hydropower facilities generate. However, given the long operating lifetime of a hydropower plant (50 years to 100 years), those construction emissions are offset by the emissions-free hydroelectricity.

Greenhouse gases (GHG) such as carbon dioxide and methane form in natural aquatic systems and in human-made water storage reservoirs as a result of the aerobic and anaerobic decomposition of biomass in the water. The exact amounts of GHG that form in and are emitted from hydropower reservoirs is uncertain and depend on many site-specific and regional factors.

Fish ladders help fish reach their spawning grounds

Hydropower turbines kill and injure some of the fish that pass through the turbine. The U.S. Department of Energy has sponsored research and development of turbines that could reduce fish deaths to lower than 2%, compared with fish kills of 5% to 10% for the best existing turbines.

Many species of fish, such as salmon and shad, swim up rivers and streams from the sea to reproduce in their spawning grounds. Dams can block their way. Different approaches to fixing this problem include constructing fish ladders and elevators that help fish move around or over dams to the spawning grounds upstream.

Tidal power

Tidal power in the United States

The gravitational pull of the moon and sun along with the rotation of the earth create tides in the oceans. In some places, tides cause water levels near the shore to rise and fall up to 40 feet. People in Europe harnessed this movement of water to operate grain mills more than 1,000 years ago. Today, tidal energy systems generate electricity. Producing tidal energy economically requires a tidal range of at least 10 feet.

The United States does not have any commercially operating tidal power plants. However, a few places in the United States have potential for tidal power, including Cook Inlet of Alaska, which has the second-highest tidal range in North America; several places in Maine; and in New York. Demonstration projects that have been implemented or are in various stages of development include:

• Roosevelt Island Tidal Energy (RITE) Project Pilot in the East River of New York
• Western Passage Tidal Energy Project in Maine
• Cobscook Bay Tidal Energy Project in Maine

Types of tidal energy systems

Three types of tidal energy systems are used around the world:

• Tidal barrages
• Tidal turbines
• Tidal fences

Tidal barrages

A barrage, which is similar to a dam, is installed across a tidal basin. A tidal basin is an inlet of an ocean bay or lagoon that fills with water during high tide and empties during low tide. Sluice gates on the barrage control water levels and flow rates to allow the tidal basin to fill on the incoming high tides and to empty through an electricity turbine system on the outgoing ebb during low tide. A two-way tidal power system generates electricity from both the incoming and outgoing tides.

A potential disadvantage of tidal power is the effect a tidal station can have on plants and animals in estuaries of the tidal basin. Tidal barrages can change the tidal level in the basin and increase turbidity (the amount of matter in suspension in the water). They can also affect navigation and recreation.

Several tidal power barrages operate around the world. The largest is the Sihwa Lake Tidal Power Station in South Korea, at 254 megawatts (MW) of electricity-generation capacity. The oldest and second-largest operating tidal power plant is in La Rance, France, with 240 MW of electricity-generation capacity. Canada has the third-largest tidal power plant, at 20 MW of electricity-generation capacity, which is in Annapolis Royal in Nova Scotia. China, Russia, and South Korea also have tidal power plants.

Tidal turbines

Tidal turbines are similar to wind turbines because they both have blades that turn a rotor to power a generator. They can be placed on the sea floor where the tidal flow is strong. Because water is about 800 times denser than air, tidal turbines must be much sturdier and heavier than wind turbines. Tidal turbines are more expensive to build than wind turbines but capture more energy with the same size blades.

Tidal fences

A tidal fence has vertical-axis turbines mounted on the seabed in a fence or row, similar to tidal turbines. Water passing through the turbines generates electricity.

Wave power

Waves have a lot of energy

Waves form as wind blows over the surface of open water in oceans and lakes. Ocean waves contain tremendous energy. The theoretical annual energy potential of waves off the coasts of the United States is estimated to be as much as 2.64 trillion kilowatthours, which is equal to about 64% of total U.S. utility-scale electricity generation in 2021. (Utility-scale generation is from electric generators/power plants with at least one megawatt of generation capacity.) The west coasts of the United States and Europe, and the coasts of Japan and New Zealand, have potential sites for harnessing wave energy. Many different methods and technologies for capturing and converting wave energy to electricity are under development. These methods include placing devices on or just below the surface of the water and anchoring devices to the ocean floor.

Different ways to channel the power of waves

One way to harness wave energy is to bend or focus waves into a narrow channel to increase their size and power and to spin the turbines that generate electricity. Waves can also be channeled into a catch basin or reservoir where the water flows to a turbine at a lower elevation, similar to the way a hydropower dam operates.

The U.S. Department of Energy's Marine and Hydrokinetic Technology Database provides information on marine and hydrokinetic renewable energy, in the United States and around the world.

Ocean thermal

Temperature differences in ocean waters can produce electric power

Ocean thermal energy conversion (OTEC) is a process or technology for producing energy by harnessing the temperature differences (thermal gradients) between ocean surface waters and deep ocean waters.

Energy from the sun heats the surface water of the ocean. In tropical regions, surface water can be much warmer than deep water. This temperature difference can be used to produce electricity and to desalinate ocean water. Ocean Thermal Energy Conversion (OTEC) systems use a temperature difference (of at least 36° Fahrenheit or 20° Celsius) to power a turbine to produce electricity. Warm surface water is pumped through an evaporator containing a working fluid. The vaporized fluid drives a turbine or generator. The vaporized fluid is turned back to a liquid in a condenser cooled with cold ocean water pumped from deeper in the ocean. OTEC systems using seawater as the working fluid produce desalinated water through the condensation process.

The United States became involved in OTEC research in 1974 when the Natural Energy Laboratory of Hawaii Authority was established. The laboratory is one of the world's leading test facilities for OTEC technology. The laboratory operated a 250 kilowatt (kW) demonstration OTEC plant for six years in the 1990s. The United States Navy supported the development of a 105 kW demonstration OTEC plant at the laboratory site. This facility became operational in 2015 and supplies electricity to the local electricity grid.

Other larger OTEC systems are in development or planned in several countries, mostly to supply electricity and desalinated water for island communities.