General considerations on thermal energy storage with closed adsorption systems

Heat supply for buildings accounts for about 40% of primary energy onsumption in Europe. The main obstacle to a sustainable energy supply for buildings is the time gap between energy availability (solar heat in summer) and heating demand in winter. Storage of thermal energy may allow to bridge this gap and to reach high solar coverage rates for the heating supply of buildings. Within a study of a reference house in three climatic regions, with three insulation standards, two collector types, two storage designs (T-proportional as for hot water tanks and isothermal storage as for PCM storages) and for two heating temperatures (30°C and 50°C, no DHW) a variation of collector area and storage capacity has been carried out. From the results, typical storage capacities and energy savings per storage capacity over the lifetime of the storage can be extracted. Sorption storages theoretically provide a promising possibility of compact thermal energy storage without losses over long times. In practice it proves to be very difficult to reach the aimed goals, especially for closed systems. To show some general reasons for this, the specific thermodynamic and technical limitations of closed adsorption storage systems are evaluated with the given storage system boundaries. The influence of losses both on an adsorption as well as on a reference hot water storage are discussed.