Types of Energy Storage
Storage options include batteries, thermal, or mechanical systems. All of these technologies can be paired with software that controls the charge and discharge of energy.
There are many types of energy storage; this list serves as an informational resource for anyone interested in getting to know some of the most common technologies available. You can learn more about these and other energy storage technologies in the U.S. Department of Energy’s Energy Storage Handbook.
There are various forms of batteries, including: lithium-ion, flow, lead acid, sodium, and others designed to meet specific power and duration requirements.
Initially used for consumer products, lithium-ion batteries now have a range of applications including smaller residential systems and larger systems that can store multiple megawatt hours (MWh) and can support the entire electric grid. These systems typically house a large number of batteries together on a rack, combined with monitoring and management units. These systems have a small footprint for the amount of energy they store. For example, a system the size of a small refrigerator could power an average home for several days. A utility-scale system of 100 MWh could fit on less than 0.5 acres.
Lithium-ion batteries have received a lot of press for their rapidly declining costs, due to the growing popularity of electric vehicles.
A different type of battery is a flow battery in which energy is stored and provided by two chemicals that are dissolved in liquids and stored in tanks. These are well suited for longer duration storage.
Thermal systems use heating and cooling methods to store and release energy. For example, molten salt stores solar-generated heat for use when there is no sunlight. Ice storage in buildings reduces the need to run compressors while still providing air conditioning over a period of several hours. Other systems use chilled water and dispatchable hot water heaters. In all cases, excess energy charges the storage system (heat the molten salts, freeze the water, etc.) and is later released as needed.
Flywheels store energy in a rapidly spinning mechanical rotor and are capable of absorbing and releasing high power for typically 15 minutes or less, although longer duration systems are being developed. These systems can balance fluctuations in electricity supply and demand where they respond to a control signal adjusted every few seconds. They also recapture braking energy from electric trains in some installations or provide short-term power until backup generation comes online during a grid outage, such as in a critical manufacturing process where product would be lost by a momentary electric interruption.
Pumped Hydro Power
Pumped hydroelectric facilities are the most common form of energy storage on the grid and account for over 95% of the storage in use today. During off-peak hours, turbines pump water to an elevated reservoir using excess electricity. When electricity demand is high, the reservoir opens to allow the retained water to flow through turbines and produce electricity. Siting these systems can be difficult because of the terrain needed (an upper and lower pool of water) and large footprint.
Compressed air, superconducting magnets, underground pumped storage, and hydrogen storage are all forms of emerging energy storage that are in different stages of development. Like NYSERDA, many storage vendors are technology agnostic—they can use their software to dispatch different storage technologies and will procure the storage technology from a manufacturing partner that best suits the requirements of the site.