News – What`s happening in energy research

read short description

District heat station for the new housing estate „Badener Hof“, Heilbronn
© ZfS Energietechnik GmbH
Long-term experience with large solar energy systems

High yields for the last twelve years

A solar energy system with around 130 square metres of collector surface area has been providing heat for a youth hostel for twelve years already, and a large solar energy system in Heilbronn has been supporting the local heating network in a housing estate for the last eight years. Both of these systems were installed as part of the Solarthermie-2000 research programme and provided the planning basis for the construction of large solar energy systems. These two projects have been subject to ongoing monitoring and measurement. ZfS-Energietechnik GmbH has now presented the results of this long-term experience in two short reports.

The solar energy system at the youth hostel in Saarbrücken was designed in 1994 and went into operation three years later – this was back at a time when not much experience with large solar energy systems was available. Since it was not possible to run the system as a drinking water preheating system as originally planned because of the insufficient hot water consumption, the building circulation was subsequently connected to the system in order to increase utilisation. In their report, the ZfS researchers provide tips on what to take into account when carrying out this type of retrofitting measure. With additional optimisation steps also implemented, the system can now serve as an example for new projects. Reiner Croy, the head of the accompanying scientific research, specifies certain restrictions, for example regarding the storage concept: "Balancing storage tanks that are connected in parallel is complex. As long as there is no solution which has a reliable design, is easy to use and inexpensive, we recommend that the buffer storage tanks be connected in series."

Leaks in the collector circuit meant that the solar energy system had to be shut down for several months. "Ever since the repair work, the system in Saarbrücken has been delivering very good results", explains Mr. Croy. "No signs of performance-reducing degradations have been detected. We assume that the system will be in operation for at least another ten years." This would fit in with the research results from Solarthermie-2000 which provide evidence of an average service life of at least 20 to 25 years for solar energy systems.

Local heating in Heilbronn

The solar energy system with 376 square metres of collector surface area is designed to increase the return temperature for a local heating network and has been running reliably for almost eight years. Major alterations to the system were not necessary. The only change was the installation of a bypass around the solar buffer storage tank, so that the return pipe of the local heating network would not run through the storage tank. This reduces heat losses of conventional energy. Similar bypasses were also retrofitted to other network systems.


The short reports on the long-term performance of the solar energy systems in Saarbrücken and Heilbronn can be downloaded as PDF files. Short reports on four further systems are published on ZfS’s website. Additional reports are to be published in spring of 2010.

BINE publications on this topic

The solar energy system in Heilbronn has already been presented in the BINE Projektinfo brochure ‘Solare Nahwärme in neuen Wohnsiedlungen’ in 2005 (in German). The Themeninfo brochure ‘Large-scale solar thermal systems for buildings’ covers further projects and research results from Solarthermie-2000.


BINE subscription

Subscribe to newsletter