Energy storage as a natural process is billions of years old - the energy produced in the initial creation of the Universe has been stored in stars such as our Sun, and is now being used by humans directly (e.g. through solar cells) or indirectly (e.g. by growing crops). As a purposeful activity, energy storage has certainly existed since pre-history, though it was often not recognized as such. An example would be the use of logs or boulders as defensive measures in ancient forts - the logs or boulders would be collected at the top of a hill, and the energy thus stored would be released as a defense against invaders.

A more recent application was the control of waterways to power water mills for processing grain or powering machinery. Often complex systems of reservoirs and dams were constructed to store and release water (and the potential energy it contained) when required.

Energy storage only became a major concern, however, with the introduction of electricity. Unlike the other common power sources at the time, such as natural gas, electricity had to be used as it was generated. This meant that changes in demand were difficult to cater for without either cutting supplies at times, or having expensive excess capacity.

An early solution was the battery, but this is of limited use both due to its small capacity and relatively high cost. A similar solution with the same type of problems is the capacitor. Some areas of the world (Washington and Oregon in the USA, and Wales in the United Kingdom are examples) have used geographic features to store large quantities of water in reservoirs at the top of hills, using excess electricity at times of low demand to pump water into the reservoirs, then letting the water fall through generators to retrieve the energy when demand peaks.

In the past, energy storage on a large scale had been limited to storage of fuels. For example, large amounts of natural gas and petroleum are routinely stored. On a smaller scale, electric energy is stored in batteries that power automobile starters and a great variety of portable appliances. In the future, energy storage in many forms is expected to play an increasingly important role in shifting patterns of energy consumption away from scarce to more abundant and renewable primary resources.

An example of growing importance is the storage of electric energy generated at night by power plants to meet peak electric loads during daytime periods. This is achieved by pumped hydroelectric storage, that is, pumping water from a lower to a higher reservoir at night and reversing this process during the day, with the pump then being used as a turbine and the motor as a generator.

Off-peak electric energy can also be converted into mechanical energy by pumping air into a suitable cavern where it is stored at pressures up to 80 atm (8 megapascals). Turbines and generators can then be driven by the air when it is heated and expanded.

The development of advanced batteries (such as nickel-zinc, nickel-iron, zinc-chloride, and sodium-sulfur) with characteristics superior to those of the familiar lead-acid battery could result in use of battery energy storage on a large scale. For example, batteries lasting 2000 or more cycles could be used in installations of several-hundred-thousand-kilowatt-hour capacity in various locations on the electric power grid, as an almost universally applicable method of utility energy storage. Batteries combining these characteristics with energy densities (storage capacity per unit weight and volume) well above those of lead-acid batteries could provide electric vehicles with greater range.

Ceramic brick “storage heaters” that store off-peak electricity in the form of heat have gained wide acceptance for heating buildings in Europe, and the barriers to their increased use in the United States are more institutional and economic than technological.

Solar hot-water storage is technically simple and commercially available. However, the use of solar energy for space heating requires relatively large storage systems, with water or rock beds as storage media, and difficulties can arise in integrating this storage with existing buildings while keeping costs within acceptable limits.

Heat or electricity may be stored by using these energy forms to force certain chemical reactions to occur. Such reactions are chosen so that they can be reversed readily with release of energy; in some cases the products can be transported from the point of generation to that of consumption. For example, reactions which produce hydrogen could become attractive since hydrogen could be stored for extensive periods of time and then conveniently used in either combustion devices or in fuel cells.

Electrical energy can be stored directly in the form of large direct currents used to create fields surrounding the superconducting windings of electromagnets. In principle such devices appear attractive because their storage efficiency is high. However, the need for maintaining the system at temperatures approaching absolute zero and, particularly, the need to physically restrain the coils of the magnet when energized require expensive auxiliary equipment (insulation, vacuum vessels, and structural supports).

Storage of kinetic energy in rotating mechanical systems such as flywheels is attractive where very rapid absorption and release of the stored energy is critical. However, research indicates that even advanced designs and materials are likely to be too expensive for utility energy storage on a significant scale, and applications will probably remain limited to systems where high power capacity and short charging cycles are the prime consideration.

Portability is the area of greatest success for current energy storage technologies. Single-use and rechargeable batteries are ubiquitous, and provide power for devices with demands as varied as digital watches and cars. Advances in battery technology have generally been slow, however, with much of the advance in battery life that consumers see being attributable to efficient power management rather than increased storage capacity. This has become an issue as pressure grows for alternatives to the internal combustion engine in cars and other means of transport. These uses require far more energy density (the amount of energy stored in a given volume or weight) than current battery technology can deliver. Liquid hydrocarbon fuel (such as gasoline, ethanol/petrol and diesel) have much higher energy densities.

Virtually all devices that operate on electricity are adversely affected by the sudden removal of their power supply. Solutions such as UPS (uninterruptible power supplies) or backup generators are available, but these are expensive. Efficient methods of power storage would allow for devices to have a built-in backup for power cuts, and also reduce the impact of a failure in a generating station. Examples of this are currently available using fuel cells and flywheels. 1