It is already well known that thermal solar systems provide benefits for the environment. Nevertheless in the present investigation this was confirmed once more by the short energy payback times. The minimal values for the solar domestic hot water systems were

1.3 years (system H13) and 2.0 years for the solar combisystems (system C2).

In addition the development of the system costs is quite positive. Figure 3 shows average values of the system costs (including VAT and installation) for solar domestic hot water systems (SDHW) and solar combisystems (COMBI).

It is obvious that the price degradation observed in the past for SDHW systems nowadays also appears for solar combisystems.

Although thermal solar systems, especially with regard to single and double family houses, are usually not sold in order to save money it is important to know the price of one kilowatt hour of solar energy. Therefore figure 4 shows the energy savings and the heat prices of the systems investigated.

The cost accounting was performed on the basis of the annuity method (interest rate 4 %, lifetime 20 years) without taking into account subsides. Due to the fact that the systems C9, C10 and C11 were combisystems with an integrated gas burner the heat prices of these systems are not included since they are not direct comparable with the other systems.

On the basis of theoretical considerations it can be expected that the heat prices will increase with increasing (fractional) energy savings, and therefore decreasing system efficiency. This consideration is valid for solar domestic hot water systems as well as for solar combisystems. However figure 4 shows that this effect can (up to now) not be observed. This indicates that heat prices and system costs are primarily determined by other parameters such as the system technology or the individual cost structure of the manufacturers or traders. On the basis of this fact it can in general be concluded that most of the systems are still far away from a cost minimum.

Figure 4 shows also that the minimum heat price is in the range of 0,12 to 0,13 €/kWh. Taking into account the present German subsides of 110 €lm collector area leads to heat prices of approximately 0,10 €/kWh. This value is very close to the current price of heat generated with individual oil or gas boilers.

3 Conclusions

Compared to the previous comparison test carried out in 1998 solar technology made one more step towards professionalism. Most of the investigated products convinced due to good quality and performance. This was indicated e. g. by the fact that during reliability and durability testing only one collector failed with a major failure. Furthermore no significant lacks of safety such as underdimensioned expansion vessels, have been noticed. This comparison test showed that by now thermal solar systems are well introduced to the market and are a serious technology for the generation of heat for domestic hot water and space heating.

/1/ Stiftung Warentest, Berlin, Test solar domestic hot water systems: "Eine Technik zum Erwarmen", Consumers’ magazine "test", 4/2002, pages 56 — 61, April 2002

/2/ Stiftung Warentest, Berlin, Test solar combisystems: " Sonne tanken",

Consumers’ magazine "test", 4/2003, pages 69 — 73, April 2003

/3/ DFS hot water comfort test. Described in: Stryi-Hipp, G., Kerskes, H., DrQck, H., Bachmann, S.: Abschlussbericht des Projekts "Testverfahren fQr Solaranlagen zur kombinierten Brauchwassererwarmung und Raumheizung (Kombianlagen)", Institut fQr Thermodynamik und Warmetechnik (ITW), Universitat Stuttgart, 2001

Acknowledgement:

The results presented in this paper have been obtained by the engineers of the Test and Research Centre for Thermal Solar Systems (TZS) located at ITW: S. Bachmann, S. Fischer, M. Hampel, H. Kerskes, M. Peter und E. Streicher. This work was partly financed by the Deutsche Bundesstiftung Umwelt. The authors gratefully acknowledge this support.