Thermal energy storage is a means of storing energy for later use, by means of a thermal reservoir. There are a number of different types of thermal reservoirs, some being maintained at a higher temperature than the surrounding environment, and some being maintained at a lower temperature. The use of stored ice or cold water to cool buildings or other environments during the day is one application of thermal energy storage. Another application is the storage of heat from plants capturing solar thermal energy, for example, in an insulated storage facility for later heating of buildings and other environments, or the production of heated water for commercial or residential use.
The technology behind the storage of thermal energies is usually based on the ability of water to store large amounts of heat or cold. This characteristic of water is called the heat of fusion. This is a measure of how much heat thermal energy must be absorbed by a substance for it to change from a solid to a liquid state, or the reverse. A relatively small volume of water can store a large amount of energy. A typical measurement of heat of fusion might be the amount of energy that can be stored by, for example, one cubic meter of water, which is 93 kilowatts.
One of the most common industrial applications of thermal energy storage is that of air conditioning systems for large buildings. In commercial buildings, air conditioning systems are often the biggest contributors to electricity usage during the day, especially in summertime. As such, using thermal energy storage in ice form may enable air conditioning units to be run more economically by having chiller equipment running during the night, when electricity may be cheaper, and storing energy as ice for use in cooling air during the day.
Ocean thermal energy conversion is a technology that makes use of the natural thermal energy storage that exists in the sea. Using this technology, electricity may be generated by a heat engine which is driven by the temperature differential between shallow and deeper water. Generally, deep ocean water is colder than the water at shallower depths. This use of thermal energy storage works on similar principles to those used in turbines, by the transfer of thermal energy into work energy and then into electricity.