Electrical Energy Storage (EES) refers to the process of converting electrical energy into a stored form that can later be converted back into electrical energy when needed.1 Batteries are one of the most common forms of electrical energy storage, ubiquitous in most peoples' lives. The first battery—called Volta’s cell—was developed in 1800. The first U.S. large-scale energy storage facility was the Rocky River Pumped Storage plant in 1929, on the Housatonic River in Connecticut.2,3 Research in energy storage has increased dramatically, especially after the first U.S. oil crisis in the 1970s, and resulted in advancements in the cost and performance of rechargeable batteries.2,4,5 The impact energy storage can have on the current and future sustainable energy grid is substantial.6
- EES systems are characterized by rated power in megawatts (MW) and energy storage capacity in megawatt-hours (MWh).7
- In 2020, the U.S. had over 23.2 GW of capacity in energy storage compared to 1,100 GW of total installed generation capacity.8,9 Globally, installed energy storage capacity totaled 173.6 GW.10
- 1,355 energy storage projects were operational globally in 2020, with 11 projects under construction. 40% of operational projects are located in the U.S.10
- California leads the U.S. in energy storage with 215 operational projects (4.2 GW), followed by Hawaii, New York, and Texas.10
Number of Grid-Connected Energy Storage Projects by State10
Key EES technologies include: Pumped Hydroelectric Storage (PHS), Compressed Air Energy Storage (CAES), Advanced Battery Energy Storage (ABES), Flywheel Energy Storage (FES), Thermal Energy Storage (TES), and Hydrogen Energy Storage (HES).13 PHS and CAES are large-scale technologies capable of discharge times of tens of hours and power capacities up to 1 GW, but are geographically limited. ABES and FES have lower power and shorter discharge times (from seconds to 6 hours), but are often not limited by geography.14
Maturity of Energy Storage Technologies11
Pumped Hydroelectric Storage (PHS)
- PHS systems pump water from a low to high reservoir and, when electricity is needed, water is released through a hydroelectric turbine, generating electrical energy from kinetic energy.14,15
- 96% of global energy storage is from PHS.15
- PHS plants have long lifetimes (50-60 years) and have operational efficiencies of between 70 and 85%.14,15
Compressed Air Energy Storage (CAES)
- CAES systems store compressed air in an underground cavern. To create electricity, the pressurized air is heated and expanded in a natural gas combustion turbine, driving a generator.16,17
- Existing CAES plants are based on the diabatic method, where the compression of the combustion air is separate from the gas turbine. The diabatic method can generate 3 times the output for every natural gas input, reduce CO2 emissions by 40-60%, and enable plant efficiencies of 42-55%.17
- As of August 2019, there were 2 CAES plants operating in the U.S. and Germany. The U.S. facility is a 110 MW plant in Alabama.18
Advanced Battery Energy Storage (ABES)
- ABES stores electrical energy in the form of chemical energy, which is then converted back into electricity when needed.19
- Batteries contain two electrodes (anode and cathode) composed of different materials and an electrolyte that separates the electrodes. The electrolyte enables the flow of ions between the two electrodes and external wires to allow for electrical charge to flow.2,19
- The U.S. has several operational battery-related energy storage projects based on lead-acid, lithium-ion, nickel-based, sodium-based, and flow batteries.10 These batteries account for 0.74 GW of rated power in 2020 and have round-trip efficiencies (the ratio of net energy discharged to the grid to the net energy used to charge the battery) between 60-95%.10,20
Flywheel Energy Storage (FES)
- FES is mainly used for power management rather than longer-term energy storage. FES systems store kinetic energy by spinning a rotor in a frictionless enclosure.17 The rotor is sped up or down to shift energy to or from the grid, which steadies the power supply.14
- There are two categories of FES: low-speed and high-speed. These systems rotate at rates up to 10,000 and 100,000 RPM (rotations per minute), respectively, and are best used for high power/low energy applications.17
- In 2020, flywheels accounted for 0.058 GW of rated power in the U.S. and have efficiencies between 85-87%.10,20
Characteristics of Energy Storage Technologies12
U.S. Energy Storage Projects by Technology Type in 202010
(including announced projects)
- EES has many applications, including energy arbitrage, generation capacity deferral, ancillary services, ramping, transmission and distribution capacity deferral, and end-user applications (e.g., managing energy costs, power quality and service reliability, and renewable curtailment).22
- EES can operate at partial output levels with fewer losses and can respond quickly to adjustments in electricity demand.23 Much of the current energy infrastructure is approaching—or beyond—its intended lifetime.24 Storing energy in off-peak hours and using that energy during peak hours saves money and prolongs the lifetime of energy infrastructure.21
- Round-trip efficiency, annual degradation, and generator heat rate have a moderate to strong influence on the environmental performance of grid connected energy storage.25
- Energy storage will help with the adoption of renewable energy by storing excess energy for times when intermittent renewable energy sources are unavailable.26
Daily Energy Storage and Load Leveling21
Five Categories of Energy Storage Applications23
Research & Development
- The U.S. Department of Energy (DOE) administered $185 million of the American Recovery and Reinvestment Act (ARRA) funding to support 16 large-scale energy storage projects with a combined power capacity of over 0.53 GW.27
- Storage technologies are becoming more efficient and economically viable. One study found that the economic value of energy storage in the U.S. is $228.4 billion over a 10 year period.23
- Lithium-ion batteries are one of the fastest-growing energy storage technologies due to their high energy densities, high power, near 100% efficiency, and low self-discharge.28,29 The U.S. has 630,000 tonnes of lithium in reserves; globally, there are 17 million tonnes of reserves.30
- Long-term (10-100 hours) and seasonal (100+ hours) energy storage are also important areas of research. Hydrogen, compressed air, and hydropower are the most viable technologies for this type of storage.31
Policy & Standardization
- The Energy Independence and Security Act of 2007 enabled an Energy Storage Technologies Subcommittee to form through the Electricity Advisory Committee (EAC), whose members assess and advise the U.S. DOE every two years on progress of domestic energy storage goals.32
- In 2010, California approved Assembly Bill 2514, requiring the California Public Utilities Commission (CPUC) to set and meet energy storage procurement targets for investor-owned utilities, totaling 1.33 GW of storage capacity completed by 2020 and implemented by 2024.33 Massachusetts, Oregon, Nevada, New Jersy, and New York all have similar mandates.34
- In 2018, the U.S. Federal Energy Regulatory Commission (FERC) issued Order No. 841, which requires wholesale electricity markets to establish participation models that recognize energy storage's physical and operational characteristics. The order builds on past FERC Orders No. 755 and No. 784.35
- Chen, H., et al. (2009) “Progress in Electrical Energy Storage System: A Critical Review.” Progress in Natural Science, 19:291–312.
- Whittingham, S. (2012) History, Evolution, and Future Status of Energy Storage. Proceedings of the Institute of Electrical and Electronics Engineers (IEEE).
- National Hydropower Association (NHA) (2012) Challenges and Opportunities For New Pumped Storage Development.
- Sandia National Laboratory (SNL) (2014) “DOE OE Energy Storage Systems (ESS) - Overview.”
- National Renewable Energy Laboratory (NREL) (2018) 2018 U.S. Utility-Scale Photovoltaics-Plus-Energy Storage System Costs Benchmark.
- Verma, H., et al. (2013) “Energy storage: A review.” International Journal of Innovative Technology and Exploring Engineering, 3(1): 63-69.
- Pacific Northwest National Laboratory (PNNL) & U.S. Department of Energy (DOE) (2014) Protocol for Uniformly Measuring and Expressing the Performance of Energy Storage Systems.
- U.S. Energy Information Administration (EIA) (2020) Form EIA-860.
- U.S. EIA (2020) Electric Power Monthly April 2020.
- U.S. DOE (2020) “Global Energy Storage Database Projects.”
- World Energy Council (2020) Five Steps To Energy Storage.
- SNL (2015) DOE/EPRI Electricity Storage Handbook in Collaboration with NRECA.
- U.S. DOE (2019) Solving Challenges in Energy Storage.
- U.S. DOE (2013) Grid Energy Storage.
- Gür, T. M. (2018). “Review of electrical energy storage technologies, materials and systems: challenges and prospects for large-scale grid storage.” Energy & Environmental Science, 11(10), 2696–2767.
- U.S. Environmental Protection Agency (2018) Energy and the Environment - Electricity Storage.
- Energy Storage Association (ESA) (2020) “Mechanical Energy Storage.”
- PNNL (2019) Compressed Air Energy Storage.
- Massachusetts Institute of Technology School of Engineering (2015) “Ask an Engineer: How Does a Battery Work?”
- State Utility Forecasting Group (2013) Utility Scale Energy Storage Systems.
- Sabihuddin, S., et al. (2015) A Numerical and Graphical Review of Energy Storage Technologies.
- Sioshansi, R., et al. (2012) Market and Policy Barriers to Deployment of Energy Storage.
- SNL (2010) Energy Storage for the Electricity Grid.
- U.S. DOE (2014) Large Power Transformers and the U.S. Electric Grid April 2014 Update.
- Arbabzadeh, M., et al. (2017) “Parameters driving environmental performance of energy storage systems across grid applications.” Journal of Energy Storage 12: 11–28.
- National Renewable Energy Laboratory (NREL) (2010) The Role of Energy Storage with Renewable Electricity Generation.
- U.S. DOE (2014) Storage Plan Assessment Recommendations for the U.S. DOE.
- U.S. DOE (2011) Energy Storage Activities in the United States Electricity Grid.
- U.S. DOE (2012) Lithium-Ion Batteries for Stationary Energy Storage.
- U.S. Geological Survey (2020) Mineral Commodity Summaries.
- NREL (2020) "Declining Renewable Costs Drive Focus on Energy Storage."
- U.S. DOE (2014) Storage Plan Assessment Recommendations for the U.S. DOE.
- California Independent System Operator, California Public Utilities Commission, and the California Energy Commission (2014) Advancing and Maximizing the Value of Energy Storage Technology: A California Roadmap.
- DSIRE (2020) Summary Maps: Energy Storage Target.
- U.S. Federal Energy Regulatory Commission (2018) Order No. 841. Electric Storage Participation in Markets Operated by Regional Transmission Organizations and Independent System Operators.