In the Twin-Storage-System (see figure 1) a combistore (nominal volume 600 l) is used for hot water preparation (by means of a stainless steel tube) and for space heating. It is charged by the collector via an immersed heat exchanger located in the lower part. The auxiliary part of the combistore is charged by the heat pump. Another pressureless buffer store (nominal volume 800 l) serves in addition to the borehole heat exchanger as a heat source for the heat pump. This store is charged by the solar collector only.

2.1. System with integrated heat pump (system B)

In this system (see figure 2) the heat pump exclusively uses a borehole heat exchanger as heat source. The heat pump is fitted at the combistore (nominal volume 750 l) and the condenser is located in the auxiliary part of the combistore. The return flow of the space heating loop is connected to the combistore using a special device for stratified discharging of the store. The heat gained by the collector is added to the combistore via an internal heat exchanger which is also equipped with a special stratification device. An external heat exchanger is used for hot water preparation.