In order to reduce the system dependency on the actual solar insolation availability, stor­age tanks have been applied in the collector / heat pump circuit by some researchers (sys­tems S2 and C3 of Figure 1). Table 1 shows the characteristics of some solar heat pump systems in experimental houses with different types of storage in combination with solar collectors and a heat pump.

Water stores are the easiest option and show a considerable performance gain. An inves­tigation of the Philips Experimental House showed that a solar fraction of 72% can be achieved by this collector-storage-configuration. Figure 4 illustrates that the fraction of heat dissipated of the complete insolation on the collector area is relatively high in comparison to customary solar systems. The highest monthly collector degree of utilisation was 50%; its average value was 40%. The curve of the storage temperature proves that solar energy is also stored seasonally in this system. As Table 1 illustrates, storage capacity is of major influence for the solar fraction that can be realised in the heating systems. However, the Philips Experimental House suffers from very high space demand for the water tank (42.000l).

The utilisation of phase change energy storage has been investigated by several re­searchers. For example the BBC AG, Walldorf (D), replaced the water store (8.000l) by a latent heat store (2.100l) in its solar house. As heat storage medium paraffin was investi­gated. It was found that the replacement of the storage tank (and the reduction of storage volume) was of little influence on the system’s performance [9]. Kaygusuz et al. [10][11] operated a solar assisted heat pump system at Karadeniz Technical University, Trabzon (TR). This experimental system incorporated a latent heat storage tank in combination with flat plate collectors (30m2, single-glazed, blackboard paint). As phase change material
(PCM), a salt hydrate (1500kg calcium chloride hexahydrate encapsulated in polyvinyl chloride containers) with a melt­ing point of 28…30°C was used. The sys­tem was designed for series, parallel and dual source (alternative source: ambient air) operation. Measurements over a day in the Black Sea region comparable weather conditions showed a consider­able improvement of the system effi­ciency by the use of the latent heat store in a series system in comparison to non­storage series and parallel configurations

In comparison to the typical PCMs (paraf­fin, salt hydrate), water as a PCM prom­ises to enhance the collector efficiency due to its lower melting point. Apart from that, water is a favourable storage me­dium because of its remarkable proper­ties (nontoxic, noncombustible, easy to handle, inexpensive,…).

The utilisation of the phase change of water in a solar heat pump system was investigated by Ltibeck University of Ap­plied Sciences for instance (Table 1). The insulated concrete storage tank (18.000…20.0001) was equipped with helical tube heat exchangers. It was heated up to approximately 80°C in summer. From June to November the thermal energy was transferred directly into a short-term storage tank (3000…4000l) for hot water preparation and space heating. During the heating season it was cooled down by the heat pump to 0°C utilising the phase transition. The store, however, was solidified to a degree of approx. only 60%(vol.) [12][13].

As a private initiative, a house with a solar heat pump is operated in Bad Reichenhall (D). W. Hesse planned and built the solar heating system. The speciality of this system is a cistern (40.000l) in the garden, which is used as a seasonal storage tank. The cistern is not insulated, thus makes use of thermal energy from the ground in winter times when the storage temperature is below ambient. The latent heat of water is also utilised in this sys­tem by a so-called dynamic heat exchanger. The heat pump evaporator is mounted above the cistern. Water is poured over the heat exchanger plates; the ice is automatically re­moved after a certain period. The system operates satisfactorily and provides a sufficient supply for the building’s heat demand [14][15].

A further similar solar heat pump system with water/ice storage in a cistern is commercially available from HitSolar21, Ramsau (A). The system incorporates solar absorbers without insulation, a heat pump and a patented heat exchanger in a cistern. Several systems in family houses have obviously been realised in Germany and Austria [16].

Unfortunately, a technical analysis of both “modern” systems, Hesse and HitSolar21, is not available.

SHAPE * MERGEFORMAT

In the 1970s a very interesting system for winter heating and summer cooling was developed and investigated by the Oak Ridge National Laboratory (USA). The so — called Annual Cycle Energy System (ACES) incorporates an insulated under­ground water tank being frozen during the heating season for summer cooling. The system is particularly advantageous in cli­matic zones with nearly equal heating and cooling loads and showed an annual elec­tricity saving of more than 50% in compari­son to a system based on full electric en­ergy supply. When the heating require­ments exceeded the cooling load, unglazed solar absorbers were used as an additional heat source [22][23].

Apart from these mainly experimental stud­ies, a simulation study of a solar heat pump system with unglazed solar absorbers and a water / ice storage tank was carried out Figure 5: Influence of Storage Capacity on by Posorski [24]. Figure 5 shows that the Additional Heating Energy Re­incorporation of such a storage tank in the quired (Climate: Hamburg/D, PCM:

collector / heat pump circuit reduces the Water) [24]

additional heating required to a minimum at an acceptable collector area for a Northern German climate. Furthermore, it is concluded, that the heat pump power required for monovalent operation is reduced considerably.

Collector-side storage also has a favour­able influence on the operation of the heat pump, as the source temperature is smoothed by the store. Kaygusuz et al.

[25][26] demonstrated that a storage tank leads to a more stable heat pump COP.

The Centre of Excellence for Solar En­gineering at Ingolstadt University of Ap­plied Sciences and Ratiotherm Hei — zung+Solartechnik GmbH&Co. KG, Dolln — stein (D) are investigating a solar heat pump heating system incorporating a low — temperature latent heat storage tank (phase change material: water) plus a stratification tank (Figure 6) [27]. The major advantages of the heating system are con­sidered to be its flexible application (suit­able for new and existing buildings be­cause of acceptable space demand) as well as the improvement of solar fraction, as described above. Although the desig­nated applications for the proposed heating…

system are typical new and redeveloped Figure 6: Solar Heating System investigated family houses in Central Europe, the heat — by Ingolstadt University of Applied

Sciences and Ratiotherm [27]

ing system is considered to be adaptable to two-family-houses and multifamily residential buildings as well. Furthermore, it promises to be applicable to a wider set of locations. A more favourable climate as for example in Southern Europe with higher solar radiation on the one hand and lower heat demand on the other hand reduces both the required storage capacity and the size of the collector area. The emphasis of this research project is placed on the development of a latent heat storage tank suitable for this application, the dimen­sioning of the components and the system, as well as on the development and optimisa­tion of system control strategies.