Device or physical media that store some form of energy to perform some useful operation at a later time.
There are so many options for storing energy, let’s concentrate on electrical energy. Of course this can and often does entail a transfer between electrical and another form:
- Hydroelectric: electricity is used to drive electric motors that mechanically drive pumps to move water from a lower to a higher height above ground level. Then when the energy is required the water drives the pumps in reverse and the electric motors generate electricity.
- Compressed Air: similar to hydroelectricity, but the working medium is air and it is compressed and re-expanded.
- Batteries: electrical energy is stored electrochemically. There are many options in terms of chemistry for battery energy storage, some better suited to stationary applications – such as flow batteries.
- Supercapacitor: often used for high power/low energy applications as capacitors are able to deliver very high discharge rates, but energy storage density is lower than batteries.
- Thermal Storage: electricity is use to heat a thermal mass. The thermal mass is insulated to reduce heat loss. The thermal mass is then used directly to heat air/water for building heating or to drive steam turbines to turn the energy back into electricity.
- Hydrogen: electricity is used to electrolyse water, the resultant hydrogen is compressed or liquidised and stored. A fuel cell can then be used to generate electricity from the hydrogen gas.
These technologies are not mutually exclusive: combining two or more of them into a single system, to take advantage of their complementary strengths, is the basis of hybrid energy storage systems.