AbstractClosed sorption heat storage opens up a way to achieve seasonal thermal storage without loss during storage. Thermal energy is not stored as sensible heat or latent heat, but by the separation of substances. In this manner it is possible to store heat, or better said, the potential to regain heat.Such a system functioning as an absorption heat pump with intermediate storage is well suited for long term thermal energy storage. Nevertheless, the conversion losses in both the regenerating and heating process make it inferior to sensible thermal storage for short storage cycles. For this reason a hybrid system including a water tank as sensible thermal storage to cover the thermal demands of several days, and a closed sorption heat storage to cover longer periods of insufficient solar thermal input is proposed.The storage system energy density is directly proportional to the sorbate mass difference between regenerated and diluted sorbent. In order to reach high utilization, a large dilution in the sorbent must be reached during heating mode. Thus, the operating temperature parameters must be adjusted and regulated accordingly.In the scope of the EU funded project COMTES, a prototype system based on the working pair sodium hydroxide and water is under construction. The system is dimensioned to cover space heating as well as domestic hot water in a single family house in Zurich, built to passive energy standards. The prototype is starting operation in spring 2014 whereby the system is regenerated to be ready to cover the heating demands the following winter.