Electric energy storage involves amassing and saving electricity for use at a later time. Unlike natural gas, which is easy to store, electricity storage is more complex and until recently has mostly involved pumped hydroelectric storage. The primary focus of energy storage historically has been for utilities to serve peak electricity demand and to save money by avoiding usage of high-cost peaking power plants.
As the grid evolves and new storage technologies are implemented, storage has the potential to provide many grid and customer services including:
Customer energy management: tools for controlling bills through trimming peak demand, shifting usage to lower-price time periods, and shifting distributed generation output to higher-price time periods.
T&D infrastructure: capacity available on a specific distribution circuit or transmission path that allows the circuit or path to serve peak demand without the need to invest in new facilities such as substations or line capacity, also described as a type of non-wires alternative.
Ancillary services: provision of various services required to maintain grid reliability including frequency response, voltage support, fast ramping, spinning reserves, non-spinning reserves, and blackstart.
Capacity: supply available to grid operators to reliably meet demand during high-load periods or when other sources of supply are unavailable.
Renewables integration: ability to absorb excess supply during times of high renewables output, and to replace renewable supply when renewable availability drops due to weather conditions.
Time-shifting supply: ability to absorb supply when supply exceeds demand and return the supply to the grid when demand rises.
Price arbitrage: ability to absorb supply when prices are low and return the supply to the grid when prices rise.
There are various types of energy storage systems (ESS). Some discharge stored energy very quickly and some are considered long-duration.
The technical and cost characteristics of the various types of storage determine which service each is most suited to provide. These technologies are summarized below:
Some technologies are commercially available, and some are in the development phase:
Following are details on key ESS technologies that have been implemented as of 2020:
Pumped hydroelectric storage The most widely used form of energy storage is pumped hydroelectric power. Pumped hydro involves pumping water from one water reservoir to another reservoir, the second of which is located higher geographically from the first body of water. The pumping is done during off-peak hours when electricity is relatively cheap.
During the peak hours when electricity is expensive, water is sent from the upper reservoir to the lower reservoir and run through a turbine that generates electricity. Thus, the energy in the water that turns the turbine is in effect the storage. Pumped hydro is a mature technology but is typically expensive to build and has very specific geographical requirements.
Typical duration for pumped hydro: Minutes to hours Typical size for pumped hydro: 10-1500 MW
Thermal energy storage Thermal energy storage (TES) converts electric energy to thermal energy that can be stored for later use. Examples include ice storage for air conditioners or other cooling loads and heat storage in hot water heaters.
Typical duration for ice storage: Hours Typical size for ice storage: 10 kW-1 MW Typical duration for hot water storage: Minutes to hours Typical size for hot water storage: < 1 kW
Batteries Battery energy storage systems (BESS) convert electrical energy to chemical energy, which is then converted back to electricity when it is needed. Lithium-ion batteries have undergone rapid development in recent years and are becoming increasingly common on transmission and distribution systems as well as within customer facilities. Multiple other types of batteries are under development and may become commercial in the future.
Typical duration for lithium-ion battery: Seconds - hours Typical size for lithium-ion battery: 1 kW - 1 MW Typical duration for zinc air battery: Hours Typical size for zinc air battery: 100 kW – 1 MW Typical size of advanced lead acid battery: Minutes – hours Typical size for advanced lead acid battery: 1 kW – 10 MW
Flywheel A flywheel is a mechanical device that stores kinetic energy (the energy of motion). The flywheel is spun by an electric motor and contains a spinning mass at its center in a device designed for very low resistance. When electricity is needed, the kinetic energy is converted back into electricity.
Typical duration for a flywheel: Seconds Typical size for a flywheel: 10 kW -1 MW
Compressed air energy storage (CAES) CAES facilities use a concept similar to pumped hydroelectric power, however instead of using water to turn a turbine, CAES utilizes air. Ambient air is compressed and stored underground, much as natural gas is stored underground in pressurized caverns. The plant generates electricity by heating the pressurized air and expanding in an expansion turbine, which drives a generator.
Typical duration for CAES: Hours Typical size for CAES: 10 – 1000 MW
By 2020, numerous other storage technologies are being implemented into the grid in commercial scale projects. Technologies used include batteries, flywheels, gravity-based rail systems, and thermal storage. Installation of battery energy storage systems (BESS), consisting mostly of lithium-ion batteries, are increasing rapidly. To drive down the cost and improve the performance of storage technologies, significant research and development efforts are being expended on compressed air energy storage (CAES), hydrogen storage, and improvement of various battery technologies.