The following HPS system concept has been selected for the investigations. It may be described by the characteristics given below:

• Unglazed Collectors: Inexpensive, uncritical operation mode (no stagnation, no steam generation), uncomplicated opportunities for building integration

• In order to realize an uncomplex system: Solar heat only directed to the evaporator side of the heat pump and: No option for direct use of solar heat for domestic hot water or space heating foreseen.

• Limitation of the investigations: Heating systems for small buildings (< 30 MWh/a heating demand)

• Thermal soil regeneration as the main target: A permanent increase of the yearly mean HPS source temperature if compared to a non-solar BHE system is the aim, but a significant seasonal storage effect is not planned. Thus, even with SC the ground is mainly serving as a heat source.

All these characteristics and restrictions form a reasonable unit. As for small buildings the seasonal storage effect may not be achieved due to the high thermal losses, the solar heat is mainly used for the regeneration of the soil. This leads to lower source temperatures in winter. Under these condi­tions unglazed SC find an ideal application temperature level, due to the fact that they are operated nearby or even below ambient air temperature.

1. BH discharging ►

2. BH charging

…………………………. ►

3. Serial-operation

The system (Fig. 1) is operated in three modes: Charging, discharging and serial operation. While operating in the discharging mode the BHE delivers with its own ground heat to the heat pump, whereas in the serial-operation mode the BHE is supported by the SC as additional heat source. In the operation mode “charging”, the collector is heating up and thus regenerating the soil around the BHE, while the HP is turned off. This concept realizes a simple hydraulic connection of the com­ponents and therefore allows different volume flow rates in the collector and BHE circuit.