Energy storage for electricity generation

An energy storage system (ESS) for electricity generation uses electricity (or some other energy source, such as solar-thermal energy) to charge an energy storage system or device, which is discharged to supply (generate) electricity when needed at desired levels and quality. ESSs provide a variety of services to support electric power grids. In some cases, ESSs may be paired or co-located with other generation resources to improve the economic efficiency of one or both systems.

Types of energy storage systems for electricity generation

The five types of ESSs in commercial use in the United States, in order of total power generation capacity as of the end of 2022 are:

• Pumped-storage hydroelectric
• Batteries (electro-chemical)
• Solar electric with thermal energy storage
• Compressed-air storage
• Flywheels

Other types of ESSs that are in various stages of research, development, and commercialization include capacitors and super-conducting magnetic storage.

Hydrogen, when produced by electrolysis and used to generate electricity, could be considered a form of energy storage for electricity generation. Thermal ice-storage systems use electricity during the night to make ice in a large vessel, which is used for cooling buildings during the day to avoid or reduce purchasing electricity when electricity is usually more expensive.

Electricity generation capacity of energy storage systems

Two basic ratings for ESS electricity generation capacity¹ are:

• Power capacity—the maximum instantaneous amount of electric power that can be generated on a continuous basis and is measured in units of watts (kilowatts [kW], megawatts [MW], or gigawatts [GW])
• Energy capacity—the total amount of energy that can be stored in or discharged from the storage system and is measured in units of watthours (kilowatthours [kWh], megawatthours [MWh], or gigawatthours [GWh])

The U.S. Energy Information Administration (EIA) collects and publishes data on two general categories of ESSs based on the size of power generation capacity:

• Utility scale or large scale have at least 1 MW of net generation capacity and are mostly owned by electric utilities or independent power producers to provide grid support services.
• Small scale have less than 1 MW of net generation capacity, and many are owned by electricity end users that use solar photovoltaic systems to charge a battery. EIA publishes data only for small-scale battery ESS.

ESSs are not primary electricity generation sources. They must use electricity supplied by separate electricity generators or from an electric power grid to charge the storage system, which makes ESSs secondary generation sources. ESSs use more electricity for charging than they can provide when discharging and supplying electricity. Because of this difference, EIA publishes data on both gross generation and net generation by ESSs. Gross generation reflects the actual amount of electricity supplied by the storage system. Net generation is gross generation minus electricity used to recharge the storage system and the electricity consumed to operate the energy storage system itself. Net generation from ESSs is reported as negative in EIA data reports to avoid double counting the generation from charging sources for ESSs and the generation from ESSs. The difference between gross and net generation varies widely by type of ESS.

Most of the largest ESSs in the United States use the electric power grid as their charging source. An increasing number of battery ESSs are paired or co-located with a renewable energy facility, which in some cases may be used directly as a charging source. As of December 2022, about 3,612 MW of battery power capacity were located next to or close to solar photovoltaic and wind energy projects.

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Uses and benefits of energy storage systems for electricity generation

ESSs are used for many purposes and provide a number of benefits to the electric power industry and electricity consumers. The major uses and benefits of ESSs are:

• Balancing grid supply and demand and improving quality and reliability—Energy storage can help balance electricity supply and demand on many time scales (by the second, minute, or hour). Fast response (ramping) ESSs are well suited to provide ancillary services for electric power grids to help maintain electric grid frequency on a second-to-second basis. Power quality is an important attribute of grid electricity because momentary spikes, surges, sags, or outages can harm electric equipment, appliances, and other devices powered by electricity.
• Peak electricity demand shaving and price arbitrage opportunities—Charging an ESS during periods of lower electricity demand and discharging an ESS and using or selling the electricity during higher demand periods can help to flatten daily load or net load shapes. Shifting some or all of electricity use from peak demand periods to other times of a day can reduce the amount of higher-cost or seldom-used reserve generation capacity, which can result in overall lower wholesale electricity prices. The stored and discharged electricity may be sold at a premium (arbitrage) above the price or cost of the charging electricity or it can be used to avoid using or purchasing higher-cost electricity.
• Storing and smoothing renewable electricity generation—Energy storage can provide greater and more effective use of intermittent solar and wind energy resources. Pairing or co-locating an on-grid ESS with wind and solar energy power plants can allow those power plants to respond to supply requests (dispatch calls) from electric grid operators when direct generation from solar and wind resources is not available or limited. Alternatively, an ESS can help solar and wind power plants avoid reducing or curtailing generation when the availability of those resources exceeds electricity demand or power transmission line capacity or as required by grid operators. ESSs also allow for storing and using renewable energy where there is no access to an electric grid (an off-grid system).
• Deferring electricity infrastructure investments—Localized pockets of increasing electricity demand sometimes require electric utilities to upgrade existing or build new, expensive substations, and power transmission and distribution lines. ESSs at strategic locations on the grid can help utilities to manage growing electricity demand at lower cost than upgrading or expanding electric grid infrastructure.
• Back-up power—An ESS owned by on-grid electricity consumers can provide emergency back-up electricity during grid outages.
• Reducing end-user demand and demand charges—Commercial and industrial electricity consumers can deploy on-site energy storage to reduce their electricity demand and associated demand charges, which are generally based on their highest observed levels of electricity consumption during peak demand periods. An ESS can also be used by participants in utility demand-side management (DSM) programs.
• Integration with microgrids—ESSs are being integrated into microgrids that supply a relatively small geographic area or customer base to provide some or all of the uses and benefits of electricity storage listed above. A microgrid ESS may be isolated from a larger grid, or it may be connected to a larger grid with automatic isolation (disconnect) from the larger grid during grid supply interruptions.

ESSs are designed to supply electricity on varying timescales, which is reflected in the duration of their discharge-generation cycle length, and they can be grouped into two general categories according to their usual duration and main use:

• Short duration—on the scale of minutes and power oriented
• Diurnal or daily duration—on the scale of hours and energy oriented

Simple examples of duration cycles are two systems each with 2 MWh energy capacity, where one (usually) produces 2 MW for short periods of time (seconds to minutes, a short duration system) and the other (usually) produces less than 1 MW consistently for 4 hours (a diurnal duration system). In general, pumped-hydro, compressed-air, and large energy-capacity battery ESSs can supply a consistent level of electricity over extended periods of time (several hours or more) and are used primarily for moderating the extremes of daily and seasonal variations in electricity demand. Many battery storage systems, and flywheels and super capacitors, provide rapid response to electricity demand fluctuations on sub-hourly timescales—from a few minutes down to fractions of a second—to keep grid voltage and frequency characteristics within a narrow range and provide an expected level of power quality.

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Energy storage systems for electricity generation operating in the United States

Pumped-storage hydroelectric systems

Pumped-storage hydroelectric (PSH) systems are the oldest and some of the largest (in power and energy capacity) utility-scale ESSs in the United States and most were built in the 1970’s. PSH systems in the United States use electricity from electric power grids to operate hydroelectric turbines that run in reverse to pump water to a storage reservoir. When needed, the water is sent back down through the turbines to generate electricity. PSH systems are generally operated most often during summer months to help meet daily peaks in electricity demand that are often the result of increases in cooling demand by utility customers.

In 2022, the United States had 40 PSH systems operating in 18 states with a combined total nameplate power capacity of about 22,008 MW. (Energy capacity data are not available for these facilities.) The largest PSH is the Bath County facility in Virginia, which has six separate generators, each with 477 MW nameplate power capacity for a combined total of about 2,860 MW of nameplate power capacity that can discharge at full capacity for up to six hours or longer. The smallest and oldest PSH facility is the Rocky River plant in Connecticut, which began operation in 1928 and has two generators each with 3.5 MW of nameplate power capacity and one generator with 24 MW nameplate power capacity. The newest PSH system is the Lake Hodges Hydroelectric Facility in California, which became operational in 2012 and has 42 MW of nameplate power capacity.

Five states—California, Georgia, Michigan, South Carolina, and Virginia—combined, had 61% of the total U.S. PSH nameplate power generation capacity in 2022, and they accounted for about 67% of total gross electricity generation from PSH facilities in 2022.