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October 31, 2019

Most pumped storage electricity generators in the U.S. were built in the 1970s

U.S. operating hydroelectric pumped storage capacity
Source: U.S. Energy Information Administration, Annual Electric Generator Report

Pumped storage plants for hydroelectric power in the Unites States were built primarily between 1960 and 1990; nearly half of the pumped storage capacity still in operation was built in the 1970s. Pumped storage power plants are the largest source of electricity storage technology used in the United States, both in terms of capacity and number of plants. (Virtually all remaining commercial-sized electrical storage use thermal energy or batteries.)

Pumped storage involves pumping water into a storage reservoir during times of relatively low electricity demand and low electricity prices, such as during the night. When electricity demand is high, water flows downhill from the reservoir through hydroelectric generators at a dam. Pumped storage plants cost more to construct than conventional hydro plants because of the storage reservoirs, additional pipes, and pumps that carry water from the reservoir to the dam. They also incur a sizable additional operating expense from pumping water uphill.

In 2018, the United States had 22.9 gigawatts (GW) of pumped storage hydroelectric generating capacity, compared with 79.9 GW of conventional hydroelectric capacity, according to the U.S. Energy Information Administration’s (EIA) most recent inventory of power plants. California has the most pumped storage capacity, with 3.9 GW, or 17% of the national total. Other states such as Virginia, South Carolina, and Michigan each have at least 2 GW of hydroelectric pumped storage capacity.

Pumped storage hydropower systems are generally one of two types. An open-loop system has a continuous source of downstream water that is pumped uphill to an upper storage reservoir. Typically, an open-loop plant pumps water that has already passed through the dam up to the storage reservoir above the dam. By contrast, closed-loop systems pump water from a lower storage reservoir, which is not continuously filled with water and is generally not connected to a flowing source.

open- and closed-loop pumped-storage hydropower
Source: U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy

Historically, most U.S. pumped storage plants were installed on rivers as open-loop systems. However, more of the recent applications filed with the Federal Energy Regulatory Commission (FERC) for pumped storage hydro project licenses have been for closed-loop plants. One reason for the increase in closed-loop plants is that they capture streams of runoff water from agricultural irrigation systems that would otherwise go unused.

Even though FERC has issued licenses for three proposed pumped storage projects since 2014, no new pumped storage projects have come online in the United States since 2012. EIA’s most recent survey of planned power plant additions shows no pumped storage projects under development.

Because these plants require energy to pump water uphill, pumped storage hydro plants have net negative electricity generation balances, meaning that they consume more energy than they store. Most pumped storage systems require 15% to 30% more electricity to pump water uphill than what the water generates when it flows back downhill.

To provide a better understanding of how electricity storage systems are used, EIA recently added tables to its Electric Power Annual that show adjusted capacities and usage factors for the two main energy storage technologies: pumped storage and batteries. Each month, pumped storage systems operate at between 8% and 17% of their capacity. Pumped storage’s usage factor generally follows the pattern of total electricity demand: a large peak in the summer months, a smaller peak in the winter months, and the lowest use in the spring and fall.

U.S. monthly hydroelectric pumped storage usage factor
Source: U.S. Energy Information Administration, Electric Power Annual

Principal contributor: Fred Mayes