Publisher Summary Heat capacity, or sensible heat storage, is accomplished by changing the temperature of a material without changing its phase or chemical composition. The actual heat storage devices may be of modest size, aiming at delivering heat during the night after a sunny day, or they may be somewhat larger, capable of meeting the demand during a number of consecutive overcast days. The storage system may provide seasonal storage of heat, as required at high latitudes where seasonal variations in solar radiation are large, and, furthermore, heat loads are to some extent inversely correlated with the length of the day. Another aspect of low-temperature heat storage is the amount of decentralization. Many solar absorption systems are conveniently placed on existing rooftops, that is, in a highly decentralized fashion. A sensible-energy heat store, however, typically loses heat from its container, insulated or not, in proportion to the surface area. Heat energy intended for later use at temperatures below 100 ˚C may conveniently be stored as hot water, owing to the high heat capacity of water combined with the fairly low thermal conductivity of water. Truly underground storage of heat takes place in geological formations capable of accepting and storing water, such as rock caverns and aquifers. In the aquifer case, it is important that water transport be modest such that hot water injected at a given location stay approximately there and exchange heat with the surroundings only by conduction and diffusion processes. In such cases, it is estimated that high cycle efficiencies can be attained after breaking the system in, that is, after having established stable temperature gradients in the surroundings of the main storage region.