Hydroelectric Energy

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 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.

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, and often referred to as head—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 head, the more electricity a hydropower plant can produce.

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 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.

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 supplied by hydro turbines or by other types of power plants including fossil fuel or nuclear power plants. They usually pump water to storage when electricity demand and generation costs, and/or when wholesale electricity prices are relatively low and release the stored water to generate electricity during peak electricity demand periods when wholesale electricity prices are relatively high. Pumped-storage hydroelectric systems generally use more electricity to pump water to the upper water storage reservoirs than they produce with the stored water. Therefore, pumped-storage facilities have net negative electricity generation balances.