The options of solar collector and heat pump connection for domestic heating systems are shown in Figure 1. Several examples of realised or theoretically investigated solar heat pump systems from literature are described above. It is evident, that the system configura­tions have diverse advantages and disadvantages and are thus often applicable only in particular areas or buildings. Table 2 gives an overview of the system approaches pre­sented, evaluates their characteristics (especially concerning their applicability in Central European climate and buildings) and describes the problems and challenges linked with each concept.

The parallel solar heat pump concepts with ground collector and air source are in principle good solutions for domestic heating, especially in comparison to fossil fuel fired systems. However, one problem is the high space demand for the collector and the effort for its in­stallation in the ground. The air source heat pump clearly suffers from the long winter peri­ods in Central Europe and the problems linked with it (e. g. icing of the evaporator).

The same problem occurs in non-storage solar heat pump systems with a direct heat pump-collector connection. Monovalent operation seems not to be feasible in Central European climates with long frost periods and for example snow lying on the absorber area, which means an additional heat source (e. g. furnace) is required.

The water storage systems on the one hand have the disadvantage of large storage vol­umes and therewith high space demand. On the other hand, they use an easy to handle and low-cost storage medium in contrary to the latent heat storage tanks working with par­affin or salt hydrate. These tanks also have the problem that the melting point of many PCMs is relatively high and that the heat exchanger design is complex.

The heat exchanger design is also a challenge for stores that use the water/ice transition. Nevertheless, a water/ice-latent heat storage tank has the most positive influence on the collector degree of utilisation due to the low melting point of water. While collector effi­ciency and degree of utilisation are increasing, the heat pump’s COP is decreasing, hence heat pump development to achieve an acceptable seasonal performance factor (SPF: ratio of heat delivered and total energy supplied over the season) is necessary. Furthermore, the storage volume can be reduced considerably in comparison to a sensible water store by the utilisation of the phase change.