In order to achieve a larger solar contribution in a higher proportion of the housing stock, a new generation of thermal energy storage systems are needed. These storage systems must be compact, cost-effective, safe, clean and easy to handle. These challenges include the development of new materials and technologies, taking into account a set of boundary conditions for application on a very large scale, at acceptable cost.

Four main types of thermal energy storage technologies can be distinguished: sensible, latent, sorption and thermochemical heat storage. Sensible heat storage systems use the heat capacity of a material. When heat is stored, the temperature of the material increases. The vast majority of systems on the market are of this type and use water for heat storage. Water-based heat storage units cover a very broad range of capacities, from several hundred litres to tens of thousands m3.

For sensible heat storage at medium temperatures, the energy is transferred to a single-phase storage medium; the charging status corresponds to the temperature of the storage material. Present candidate storage materials are concrete, molten salt or pressurised liquid water. Storage systems using molten salt at temperatures between 300°C and 400°C have been integrated into solar thermal power plants and are expected to become operational in 2008.

In latent heat storage systems, thermal energy is stored during the phase change, either melting or evaporation, of a material. Depending on the temperature range, this type of storage is more compact than heat storage in water. Most of the currently used latent heat storage technologies for low temperatures are for storing heat in building structures to improve thermal performance, or in cold storage systems. For medium-temperature storage, nitrate salts are used as the storage materials. Pilot storage units in the 100 kW range are currently operated using solar steam

In sorption heat storage systems, heat is stored in materials using water evaporation. The material can be either a solid (adsorption) or a liquid (absorption). These technologies are still largely in the development phase, although there are some systems on the market. In principle, sorption heat storage densities can be more than four times higher than sensible heat storage in water.

In thermochemical heat storage systems, the heat is stored by splitting a chemical compound into its components. Taking the reaction enthalpy of the compound into consideration, a heat storage density 20 times higher than water could be reached. In practice, however, this would be lower, at around 8 to 10 times higher than water. This thermochemical type of heat storage is still at the research stage, with only a limited number of chemical storage principles having been demonstrated. The materials cur­rently under investigation are all compounds of a salt with water (hydrates). Thermochemical heat storage is likely to provide compact storage solutions for both low and medium temperature heat storage applications.

Fundamental research on new materials for thermal energy storage is essential in order to advance thermal storage with a high energy density. For more compact systems, new materials in the class of thermochemical thermal storage systems need to be developed. New and improved materials are also needed in order to improve existing thermal storage systems, such as systems based on phase-change materials and sorption.