What is Hydroelectric Power Plant?
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 until 2022, hydropower was the largest source of total annual U.S. renewable electricity generation.
In 2023, hydroelectricity accounted for about 6.7% of total U.S. utility-scale1 electricity generation and 32% of total utility-scale renewable electricity generation. Hydroelectricity’s share of total U.S. electricity generation has decreased over time, mainly because of increased electricity generation from other sources.
Hydropower relies on the water cycle
Understanding the water cycle is important to understanding hydropower. The water cycle has three steps:
- Solar energy heats water on the surface of rivers, lakes, and oceans, which causes the water to evaporate.
- Water vapor condenses into clouds and falls as precipitation rain, and snow.
- 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 variations in precipitation and long-term changes in precipitation patterns, such as droughts, can have large effects on the availability of hydropower production.
How does a Hydroelectric Power plant Work?
Actually, hydroelectric and coal-fired power plants produce electricity in a similar way. In both cases, a power source is used to turn a propeller-like piece called a turbine, which then turns a metal shaft into an electric generator, which is the motor that produces electricity.
A coal-fired power plant uses steam to turn the turbine blades; whereas a hydroelectric plant uses falling water to turn the turbine. The results are the same.
Take a look at this diagram (courtesy of the Tennessee Valley Authority) of a hydroelectric power plant to see the details:
The theory is to build a dam on a large river that has a large drop in elevation (there are not many hydroelectric plants in Kansas or Florida). The dam stores lots of water behind it in the reservoir. Near the bottom of the dam wall, there is a water intake.
Gravity causes it to fall through the penstock inside the dam. At the end of the penstock, there is a turbine propellor, which is turned by the moving water. The shaft from the turbine goes up into the generator, which produces the power.
Power lines are connected to the generator that carries electricity to your home and mine. The water continues past the propellor through the tailrace into the river past the dam. By the way, it is not a good idea to be playing in the water right below a dam when water is released!
A turbine and generator produce the electricity
A hydraulic turbine converts the energy of flowing water into mechanical energy. A hydroelectric generator converts this mechanical energy into electricity. The operation of a generator is based on the principles discovered by Faraday.
He found that when a magnet is moved past a conductor, it causes electricity to flow. In a large generator, electromagnets are made by circulating direct current through loops of wire wound around stacks of magnetic steel laminations.
These are called field poles and are mounted on the perimeter of the rotor. The rotor is attached to the turbine shaft and rotates at a fixed speed. When the rotor turns, it causes the field poles (the electromagnets) to move past the conductors mounted in the stator. This, in turn, causes electricity to flow and a voltage to develop at the generator output terminals.
Pumped storage: Reusing water for peak electricity demand
Demand for electricity is not “flat” and constant. Demand goes up and down during the day, and overnight there is less need for electricity in homes, businesses, and other facilities.
Hydroelectric plants are more efficient at providing for peak power demands during short periods than are fossil-fuel and nuclear power plants, and one way of doing that is by using “pumped storage”, which reuses the same water more than once.
Pumped storage is a method of keeping water in reserve for peak period power demands by pumping water that has already flowed through the turbines back up a storage pool above the power plant at a time when customer demand for energy is low, such as during the middle of the night. The water is then allowed to flow back through the turbine generators at times when demand is high and a heavy load is placed on the system.
Types of Hydropower Plant
There are three types of hydropower facilities: impoundment, diversion, and pumped storage. Some hydropower plants use dams and some do not.
Although not all dams were built for hydropower, they have proven useful for pumping tons of renewable energy to the grid. In the United States, there are more than 90,000 dams, of which less than 2,300 produce power as of 2020. The other dams are used for recreation, stock/farm ponds, flood control, water supply, and irrigation.
Hydropower plants range in size from small systems suitable for a single home or village to large projects producing electricity for utilities. Learn more about the sizes of hydropower plants.
The most common type of hydroelectric power plant is an impoundment facility. An impoundment facility, typically a large hydropower system, uses a dam to store river water in a reservoir.
Water released from the reservoir flows through a turbine, spinning it, which in turn activates a generator to produce electricity.
The water may be released to meet changing electricity needs or other needs, such as flood control, recreation, fish passage, and other environmental and water quality needs.
A diversion, sometimes called a “run-of-river” facility, channels a portion of a river through a canal and/or a penstock to utilize the natural decline of the river bed elevation to produce energy.
A penstock is a closed conduit that channels the flow of water to turbines with water flow regulated by gates, valves, and turbines. A diversion may not require the use of a dam.
3. Pumped Storage
Another type of hydropower called pumped storage hydropower, or PSH, works like a giant battery. A PSH facility is able to store the electricity generated by other power sources, like solar, wind, and nuclear, for later use. These facilities store energy by pumping water from a reservoir at a lower elevation to a reservoir at a higher elevation.
When the demand for electricity is low, a PSH facility stores energy by pumping water from the lower reservoir to an upper reservoir. During periods of high electrical demand, the water is released back to the lower reservoir and turns into a turbine, generating electricity.
Sizes Of Hydroelectric Power Plants
Hydropower facilities range in size from large power plants, which supply many consumers with electricity, to small and even ‘micro’ plants, which are operated by individuals for their own energy needs or to sell power to utilities.
1. Large Hydropower
Although definitions vary, DOE defines large hydropower plants as facilities that have a capacity of more than 30 megawatts (MW).
2. Small Hydropower
Although definitions vary, DOE defines small hydropower plants as projects that generate between 100 kilowatts and 10 MW.
3. Micro Hydropower
A micro-hydropower plant has a capacity of up to 100 kilowatts. A small or micro-hydroelectric power system can produce enough electricity for a single home, farm, ranch, or village.
Advantages Of Hydropower
- Hydropower is a renewable source of energy. The energy generated through hydropower relies on the water cycle, which is driven by the sun, making it renewable.
- Hydropower is fueled by water, making it a clean source of energy. Hydroelectric power won’t pollute the air like power plants that burn fossil fuels, such as coal or natural gas.
- Hydroelectric power is a domestic source of energy, allowing each state to produce its own energy without being reliant on international fuel sources.
- Impoundment hydropower creates reservoirs that offer recreational opportunities such as fishing, swimming, and boating. Most hydropower installations are required to provide some public access to the reservoir to allow the public to take advantage of these opportunities.
- Hydroelectric power is flexible. Some hydropower facilities can quickly go from zero power to maximum output. Because hydropower plants can generate power to the grid immediately, they provide essential backup power during major electricity outages or disruptions.
- Hydropower provides benefits beyond electricity generation by providing flood control, irrigation support, and clean drinking water.
- Hydropower is affordable. Hydropower provides low-cost electricity and durability over time compared to other sources of energy. Construction costs can even be mitigated by using preexisting structures such as bridges, tunnels, and dams.
- Hydropower compliments other renewable energy sources. Technologies like pumped storage hydropower (PSH) store energy to use in tandem with renewables such as wind and solar power when demand is high.
Disadvantages Of Hydropower
1. Hydropower plants can adversely affect surrounding environments
While hydropower is a renewable energy source, there are some important environmental impacts that come along with building hydroelectric plants to be aware of. Most importantly, storage hydropower or pumped storage hydropower systems interrupt the natural flow of a river system.
This leads to disrupted animal migration paths, issues with water quality, and human or wildlife displacement.
These negative environmental impacts of hydropower are typically lower with run-of-river, wave energy, or tidal power setups, but the vast majority of current hydropower systems are storage or pumped storage systems that block river flow.
2. Building hydropower facilities is expensive up-front
Many hydropower plants are large infrastructure projects that involve building a dam, a reservoir, and power-generating turbines. requiring a significant monetary investment. While a large hydropower facility can often provide low-cost electricity for 50 to 100 years after being built, the upfront construction costs can be large.
This, combined with the fact that suitable places for reservoirs are becoming rarer over time means that large-scale hydropower plant construction costs may continue to rise.
3. Hydropower facilities rely on local hydrology
Hydropower is a reliable energy source, but it is still ultimately controlled by weather and precipitation trends. Because most hydropower generation relies on river water, droughts that cause lower water flow impact hydroelectric generation capacity.
Month to month and year to year, the amount of water available for hydropower systems can vary, thus electricity production at a hydroelectric facility can also vary.