The sun provides us with a quantity of energy that far exceeds demand, is permanently renewable, does not cause pollution and does not produce waste. Solar thermal energy is produced by converting the sun's rays into heat. It can be used for numerous purposes, ranging from room heating and electricity production through to firing ceramics and sewage treatment. The SFOE Solar Heat and Heat Storage research programme focuses on applications in the buildings sector, and thus on uses at low to medium temperatures of between 0° and 150° C. Installations for converting solar energy (solar collectors) are particularly efficient at these temperatures. Modern collectors have annual efficiency levels of between 40 and 50 percent. In order to utilise even larger quantities of solar energy, it needs to be stored: during the day for use at night, and during the summer for use in winter. For this reason, heat storage forms an integral part of the solar heat programme. But it is also a component of the control mechanisms of other energy systems, for example in "Minergy" buildings or production systems that are fed with wood or by a heat pump.