There are five primary forms of commercial energy storage today: gravity, kinetic, chemical, thermal and thermodynamic.
gravity energy storage
The most common form of energy storage, and the most cost effective, is hydro. By using one reservoir and and source of water (open loop), or two reservoirs (close loop), it is possible to pump water up at times of low energy use, then let it drop through a turbine at peak time to regenerate stored energy. Cycle efficiency is high, in the neighborhood of 70%.
Beyond hydro, there are other possible uses of gravity. An example under investigation is the use of heavily laden trains, going uphill at nadir time, then downhill at peak time.
kinetic energy storage
Kinetic energy today is primarily used with flywheel storage, where heavy flywheels are accelerated in times of low use, then slowed down (typically electrically, through the use of windings and magnets) at peak time.
It is the most common form of energy storage in many customer applications: batteries are ubiquitous. Yet their use is still prohibitively expensive for most grid-scale uses. The battery industry is undergoing a tremendous amount of innovation today, with many new forms of chemical storage being investigated every year.
Thermal storage is inherently difficult to exploit with high efficiency. Specific applications, however, can make use of it favorably. Two primary scenarios exist today:
- Concentrated solar plants can heat a large mass of multiple hundreds of tons of molten salt with converging mirrors, then use its thermal inertia to drive a steam turbine.
- In commercial AC applications, it is possible to cool at times of low use a cooler with large thermal inertia, then use the cooler to cool an AC heat exchanger at peak time. This distributed storage technology is seeing significant adoption today.
Thermodynamic storage uses the thermodynamic properties of gases and liquids. A common application is air compression: air is compressed at times of low use, then expands when energy restitution is needed. Efficiency is difficult to obtain, but new technologies are now deploying that seem to significantly improve the cycle efficiency of this technology.
By far the most common application of energy storage is hydro. The DOE energy storage database, while very incomplete, is a good way to get a sense of the commercial distribution of these diverse technologies.