H. Druck, W. Heidemann, H. Mtiller-Steinhagen

Universitat Stuttgart, Institut fur Thermodynamik und Warmetechnik (ITW) Pfaffenwaldring 6, D-70550 Stuttgart Tel.: 0711/685-3536, Fax: 0711/685-3503

email: drueck@itw. uni-stuttgart. de, Internet: http://www. itw. uni-stuttgart. de

This paper presents the results on the latest comparison tests of thermal solar systems for domestic hot water preparation and space heating carried out for the German consumers’ magazine “test”. The systems were tested with regard to thermal performance, durability and reliability, environmental aspects as well as safety aspects. The test procedures as well as the results obtained are described and discussed. Possible future trends e. g. with regard to the development of the technology and the system costs will be shown.

1 Introduction

Following the last comparison test of solar thermal systems performed in 1998 a new series of test results was published by "Stiftung Warentest" in the German consumers’ magazine "test" in 2002 and 2003. The results of the test of 16 solar domestic hot water systems were published in /1/. A report on the test of 11 thermal solar systems for combined domestic hot water production and space heating, so-called combisystems, can be found in /2/.

In addition to the customer-oriented and product related results already published in the consumers’ magazine "test", this paper provides further background information. The test procedure is described in detail. Furthermore, the ranking of the results, especially with regard to the assessment of the thermal performance, is presented. Finally the long-term development of thermal solar system technology, e. g. with regard to the cost development is discussed.

2 Systems tested

The systems had to be dimensioned by the supplier or manufacturer respectively for a single-family house located at WUrzburg, Germany. The house is equipped with a 45 ° inclined, south facing roof. The daily hot water load is 200 litres (at 45 °C) and the heat insulation standard of the building with a heated living area of approx. 130 m2 fulfils the requirements according to the German Energy Saving Directive (Energieeinspar — verordung: EnEV) which is the present German directive concerning the primary energy demand of buildings. Based on this the yearly heat demand for space heating reaches 9090 kWh or 71 kWh/m2 respectively.

With regard to the 16 thermal solar systems for domestic hot water preparation (DHW) the effective collector area varied between 3.2 m2 (system H13, H15,H16) and 5.7 m2 (system H12). 12 systems are equipped with flat plate collectors and four with vacuum tube collectors (systems H13, H14, H15, H16). The effective usable storage volume of the domestic hot water stores is in the range of 268 litres (system H10) up to 419 litres (system H14). The amount of hot water that is available if only the auxiliary heated part of the store is in operation varies between 100 litres for system H6 and a maximum of 200 litres for system H11.

Concerning the system and storage concepts most of the systems are designed as the typical German „standard systems" shown in fig 1.

For all 16 systems the solar energy is transferred to the domestic hot water via a plain tube heat exchanger. The store of system H1 is the only one that is additionally equipped with a device for stratified charging. Additionally the store of system H11 is charged in a stratified way by using two solar loop heat exchangers: One located in the upper and one located in the lower part of the store. In the case of high temperatures delivered by the solar collector the flow is directed additionally via the upper heat exchanger following a special control strategy.

With regard to the solar combisystems, systems for combined domestic hot water preparation and space heating, the spectrum of investigated system concepts is much broader. Concerning space heating, for 7 of the 11 systems tested the space-heating loop is operated in the pre-heating mode. This means, that the return line of the space heating loop is only directed through the store if the temperature at the store’s space heating outlet connection is above the return temperature of the space heating loop. The energy delivered by the external auxiliary heater is only transferred to the combistore in order to heat the auxiliary part required for domestic hot water preparation. Auxiliary energy required for space heating is fed directly into the space heating loop (see figure 2a).

The other 4 systems (system C4, C5, C10, C11) use the combistore as a buffer store.

This means that the auxiliary energy is always transferred to the store and that the space heating loop is continuously supplied from the combistore (see figure 2b).

In comparison to a similar investigation of solar combisystems carried out five years ago it is remarkable that the solar combisystem technology has moved towards a higher level of integration. Five years ago when ordering a combisystem, one was supplied with a collection of separate, individual components. Today in many cases the appearance of the components already shows obviously that they belong to the combisystem of a certain manufacturer.

The positive trend towards compact systems was also documented by the fact that the manufacturers did not anymore select so-called two-store-systems for the test. The state of the art concerning maximum compactness are systems were the gas burner is already integrated into the combistore (system C9 and C11) or directly connected at the combistore (system C10).

The effective collector area of the combisystems investigated is in the range of 5.7 m2 (system C2) to 14.2 m2 (system C7). 6 systems use flat plate collectors. The other 5 systems are using vacuum tube collectors (system C2, C3, C4, C5, C10). The volume of the combistores varies from 450 litres for system C9 up to nearly 1000 litres for system C7 and C11. The usable hot water volume is in the range from 100 litres (system C2 and C3) up to a maximum of 300 litres for system C4. Concerning the systems using the combistore as a buffer for the auxiliary heater, the buffer volume for the boiler or gas burner, respectively, varies between 80 litres for system C4 and 225 litres for system C10.