The most common type of hydroelectric power plant is an impoundment facility. An impoundment facility uses a dam to store river water in a reservoir. Water released from the reservoir flows through and rotates a turbine, which in turn spins a generator to produce electricity. The water may be released to meet changing electricity demand or other needs, such as flood control, recreation, fish passage, and other environmental and water quality needs.
A run-of-river, sometimes called a “diversion” facility, channels a portion of a river through a canal and/or a penstock to utilize the natural decline of the riverbed elevation to generate electricity. A penstock is a closed conduit that channels the flow of water to turbines with water flow regulated by gates, valves, and turbines.1 Most run-of-river projects have little or no storage capacity.
As the name implies, pumped storage hydropower is a form of energy storage that stores energy and generates electricity by moving water between two reservoirs at different elevations.1 The reservoir at a higher elevation stores energy in the form of gravitational potential energy just like water in the reservoir in an impoundment facility. At times of low electricity demand, like at night or on weekends, excess electricity is used to pump water to the upper reservoir. During periods of high electricity demand, the stored water is released through turbines in the same manner as a conventional hydro station, flowing downhill from the upper reservoir into the lower and generating electricity.2
Many of the earliest hydropower projects used run-of-the-river technology due to their simplicity and cost-effectiveness. Run-of-the-river systems require less infrastructure and engineering work because they do require extensive impoundment structures. This also leads to faster implementation.
Impoundment technology became a priority in the first half of the 20th century. Dam and reservoir construction are expensive and time-consuming, but they provide several advantages compared to run-of-the-river. Impoundment projects can store large volumes of water in reservoirs behind the dam. This stored water can be released as needed to generate electricity, allowing for more consistent and reliable power generation, especially during periods of high demand or when natural flow rates are low.
Pumped storage facilities have surged in the past few decades because they provide enhanced grid reliability in the face of the rapid increase in intermittent sources of electricity from solar and wind.
1 U.S. Department of Energy, “Types of Hydropower Plants,” accessed February 23, 2024, Link
2 National Hydropower Association, “Pumped Storage,” accessed February 23, 2024, https://www.hydro.org/policy/technology/pumped-storage/