Energy storage systems play an important role in the efficient use of natural energy resources. Especially thermal energy storage becomes an issue of crucial importance when replacing conventional heating technologies (direct fossil fuel heating) by e.g. solar systems or innovative energy conserving concepts for hot water production and room heating (heat pumps, ..).

Since a large fraction of fossil fuels is used up in low temperature heat production there is a tremendous potential for saving natural resources by using these more efficient technologies coupled with good storage systems. In particular when using the sun as energy source, supply and demand often do not coincide. Thus by appropriate storage techniques the gap can be bridged and the regenerative cover rate can be raised noticeably.

Thermal energy storage technologies can be classified into four categories:

Depending on which time span must be covered by the storage and on which temperature level the heat (or cold) should be available for use, there are several different storage concepts and techniques at hand.

For room climatisation, cold can be stored in winter and used in summer (5°C-20°C) and summer heat can be stored for wintertime heating (30°-60°C). Besides, heat can be stored to produce hot water for domestic use. The necessary temperature lies then between 60°C and 90°C. In addition, there is need for heat storage at higher temperature levels for industrial processes.

The most important criteria for the choice of a heat storage technology (besides cost) are the storage density, defined as energy stored per unit volume, and the storage efficiency, defined as the fraction of energy recoverable after the storage period.  

For seasonal heat storage large water tanks, gravel/water storages, aquifers or ground storage are up to now the way to go, since during the lifetime of the storage only a limited amount of cycles will be achieved and thus systems with low building costs have to be chosen.

For shorter cycle times also other concepts are possible which are not based on storing the sensible heat. Phase change materials (PCM), which use the latent heat stored in a liquid-solid phase transition, may as well be considered. Especially in the strongly growing market field of cold storage there are very interesting possibilities to connect PCM heat or cold storage with traditional building concepts.

The technology of  ad- and absorption heat storages, which use the additional heat of ad- or absorption, though not yet as widely spread, are a further way to make heating or cooling systems more efficient. Adsorption heat storage offers the technological advantage that its storage efficiency does not depend (in principle) on storage time: The adsorption of a gas on the surface of a solid is an exothermal process that here provides the heat released or "discharged" from the storage. Preventing this discharge simply requires to keep the dry solid adsorbent and the adsorbing fluid, called adsorptive, in separate closed vessels. Besides, the achievable storage densities are comparably higher which is an advantage when small systems are needed.