■ Several varieties of centralized systems were investigated; freshwater storage systems, "storage loaded” systems (that is, double storage tank combinations with combined buffer and freshwater storage tanks) and buffer storage systems with additional freshwater unit. The storage tank buffer system with a freshwater unit and the storage loaded system with the heating element in the freshwater tank showed the same energy efficiency if the design and the system concept was optimized. Both systems have a significant advantage over the "storage loaded” system with the heating element in the buffer storage tank, but only an insignificant advantage over the freshwater storage system (which requires a larger collection area).

■ Among the centralized systems, no one system showed itself to be clearly better than the others in terms of cost. When choosing a system, it is recommended that other aspects which are less quantifiable be considered. For example, factors such as bulky design requiring more space, costly installation or more difficult adjustment and operating instructions in the case of the double storage tank systems should also be taken into account.

■ It is possible to achieve an improvement in the solar contribution (depending on the system) of between 2 and 8 % by using a stratified charging in the centralized solar storage tanks.

Organizational Aspects

The most important criteria in making the decision to convert to a centralized heating system are the aspects of ownership and responsibility. Both when the Housing Committee organize the installation of the system or when an external company is employed, the house owners are the official owners of the heating system. If the heating system is leased, then the Contractor remains the owner of it. Many building companies view the idea of leasing a heating system as a psychological impediment for prospective
house buyers, who are in effect, purchasing a house ‘without a heating system’. On the other hand, factors such as the cost of the house being reduced significantly due to the many investors in the heating system, and a much better service in terms of maintenance could be seen as a definite incentive to install such a system. It is important that the decision in favor of a centralized system or otherwise is taken as early as possible so that the correct model can be selected and the appropriate choice of heating company can be made, as well as the arrangement of the contracts. Highest priority should be given to the signing of contracts as early as possible, whether the house owners or the leasing company own the heating system. It is very important for all parties involved to have a clear agreement on issues such as ownership, guarantees and the responsibility of each party, all which must be clearly defined.

Table 1 shows an extract from the technical and economic values of comparable heating systems concepts investigated during the project /2/.

4 Conclusion

The above results show that “mini-centralized” hot water systems using solar units between 20-60 m2 collector area offer clear advantages, especially from an economic point of view. Certainly, there are a variety of environmental factors which can influence results, so that no general recommendations (or rules of thumb) can in fact be given. An appropriate concept must be devised to accommodate each individual housing requirement. The above findings from the comparison project can serve as a reliable data source when planning solar hot water systems in the future.

5 References

/1/ K. Schwarzer, C. Wemhoner, B. Hafner:

Berechnung von Solaranlagen mit CARNOTunter MATLAB-Simulink®,

10. Symposium Thermische Solarenergie, Staffelstein, 2000

/2/ K. Schwarzer, C. Faber, T. Hartz, F. Spate, C. Petersdorff, J. Backes:

Planungshilfe solare Brauchwasserversorgung in Siedlungen — zentral oder dezentral?, Ecofys GmbH, Eupener Strafte 59, 50933 Koln