Many renewable energy sources (most notably solar and wind) produce intermittent power. Wherever intermittent power sources reach high levels of grid penetration, energy storage becomes one option to provide reliable energy supplies. Individual energy storage projects augment electrical grids by capturing excess electrical energy during periods of low demand and storing it in other forms until needed on an electrical grid. The energy is later converted back to its electrical form and returned to the grid as needed.
Common forms of renewable energy storage include pumped-storage hydroelectricity, which has long maintained the largest total capacity of stored energy worldwide, as well as rechargeable battery systems, thermal energy storage including molten salts which can efficiently store and release very large quantities of heat energy, and compressed air energy storage. Less common, specialized forms of storage include flywheel energy storage systems, the use of cryogenic stored energy, and even superconducting magnetic coils.
Other options include recourse to peaking power plants that utilize a power-to-gas methane creation and storage process (where excess electricity is converted to hydrogen via electrolysis, combined with CO
2 (low to neutral CO
2 system) to produce methane (synthetic natural gas via the sabatier process) with stockage in the natural gas network) and smart grids with advanced energy demand management. The latter involves bringing "prices to devices", i.e. making electrical equipment and appliances able to adjust their operation to seek the lowest spot price of electricity. On a grid with a high penetration of renewables, low spot prices would correspond to times of high availability of wind and/or sunshine.
Another energy storage method is the consumption of surplus or low-cost energy (typically during night time) for conversion into resources such as hot water, cool water or ice, which is then used for heating or cooling at other times when electricity is in higher demand and at greater cost per kilowatt hour (KWh). Such thermal energy storage is often employed at end-user sites such as large buildings, and also as part of district heating, thus 'shifting' energy consumption to other times for better balancing of supply and demand.
Seasonal thermal energy storage (STES) stores heat deep in the ground via a cluster of boreholes. The Drake Landing Solar Community in Alberta, Canada has achieved a 97% solar fraction for year-round heating, with solar collectors on the garage roofs as the heat source. In Braestrup, Denmark, the community's solar district heating system also utilizes STES, at a storage temperature of 65°C (149°F). A heat pump, which is run only when there is surplus wind power available on the national grid, is used when extracting heat from the storage to raise the temperature to 80°C (176°F) for distribution. This helps stabilize the national grid, as well as contributing to maximal use of wind power. When surplus wind generated electricity is not available, a gas-fired boiler is used. Presently, 20% of Braestrup's heat is solar, but expansion of the facility is planned to raise the fraction to 50%.
In 2011, the Bonneville Power Administration in Northwestern United States created an experimental program to absorb excess wind and hydro power generated at night or during stormy periods that are accompanied by high winds. Under computerized central control, home appliances in the region are commanded to absorb surplus energy at such times by heating ceramic bricks in special space heaters to hundreds of degrees, and by also boosting the temperature of modified hot water heater tanks. After being fully charged the highly insulated home appliances then provide home heating and hot water at later times as needed. The experimental system was created as a result of a severe 2010 storm that overproduced renewable energy in the U.S. Northwest to the extent that all conventional power sources were completely shut down, or in the case of a nuclear powerplant, reduced to its lowest possible operating level, leaving a large swath of the region running almost completely on renewable energy.
