The solar heat pump systems without storage in the collector/heat pump circuit (systems S1 and C1 of Figure 1) are a straightforward option. The collectors are directly connected to the heat pump evaporator. Thus, the heat pump COP strongly depends on the present solar insolation. The collector temperature restricts heat pump operation.

For these systems usually plane energy absorbers or solar absorbers are used. These absorbers are simple unglazed collectors that are connected to a hydraulic circuit. Solar absorbers were developed for example by Shinobu et al. [3] or by Rheinzink GmbH&Co. KG, Datteln (D) for roof integration (Figure 2 and Figure 3). The Rheinzink Quick Step — Solar Thermie Module has recently been made commercially available.

Dietrich et al. [5] showed that a direct combination of plane energy absorbers with a heat pump is feasible. The temperature of the evaporator can be increased by solar absorbers, but an additional heat source (oil or gas) is still required, as heat pump operation is limited to source temperatures above -10°C.

A laboratory-scale solar heat pump system with two different types of unglazed flat plate collectors (convective / radiative type) was investigated by Ito et al. [6]. It was shown that using the two collectors in parallel raised the heat pump COP at low solar insolation. Fur­thermore, Ito et al. [7] analysed the performance of such a system with photovoltaic cells bonded on the surface of the thermal collectors by conductive silicone, which did not influ­ence the performance of the heat pump appreciably.