Forschungsgruppe-NET - Hochschule Offenburg

Solar Heat

Solar Heating of Potable Water at Wilmersdorferstraβe 3 and 5

The collector field of 228m² is oriented South, at an inclination angle of 30°. The roof space of one building did not provide enough space for the amount of collectors required for potable water heating. Therefore, the collector field was put on the roofs of both buildings. The two parts are connected with a transfer pipe.

The collector field on number 3 covers 154.6m²; it is divided into four subfields with parallel flows. The collector field on number 5 covers 73.6 m² and is divided in two parallel subfields. The interconnection of both subfields is shown below.

Roof interconnection of the solar thermal system Wilmersdorferstraβe 3 and 5

The individual collector modules are mounted on standard aluminum profiles, which are mounted on a specially-built construction of galvanized steel beams. The steel beams are bolted to support pillars anchored in concrete of the flat roof, which is sealed with a cooper sheet.

The collector circuit pump switches once the solar irradiation reaches the set limit of 150 W/m². Then, the water-glycol mixture of the collector circuit is circulated. When flowing through the absorber pipes of the collector, the mixture absorbs the heat provided by irradiation which is then transported to the heat exchanger of the collector circuit in the basement. If the temperature is high enough, the buffer storage pump switches on and pumps the heating water out of the buffer storage tanks (each with capacity of 4500 ltrs) into the heat exchanger of the collector circuit. There, the water from the buffer storage absorbs the heat from the collector circuit and feeds it into the buffer storage.

A second heat exchanger, the so-called discharge heat exchanger, is responsible for the discharge of the buffer storage, i.e. the discharge of the stored energy to the potable water. If the temperature is high enough in the buffer storage, the discharge pump and the charging pump of the potable water circuit switch on and feed the heat to the preheating tanks of the potable water circuit.

When the warm water is extracted, the heated potable water flows from the preheating tank to the reheating tank; if need be, it will be heated up to 60°C by two oil-fired boilers. The reheating tanks are connected to the preheating tank. They have a capacity of 2 000 ltrs.

For reasons of disinfecting the preheating reservoir, the legionella control makes sure that it is flooded once a day with hot water such that all the water in the tank reaches a temperature of 65 °C.

The diagram below shows a simplified scheme of the solar thermal system and its components. The entire system is subdivided in three different circuits: the collector circuit, the buffer storage circuit and the potable water circuit.

Simplified diagram of the solar thermal system at Wilmersdorferstraβe 3/5


This table shows the most important specifications of the solar thermal system of the solar system on Wilmersdorferstraβe 3 and 5.

Surface228 m²
Collector typeflat plate collectors
Buffer storage2 x 4.500 ltrs
Project costs215 638,- € (945,- €/m²)
Heat exchanger2 plate heat exchanger
Preheating tank2 000 ltrs
Azimuth/ InclinationSouth / 30°
Lowest energy cost0.13 €/kWh
Highest solar yield137 680 kWh/a (604 kWh/m²a)


Click on the pdf-file to receive detailed information on the specifications of the solar thermal system of Wilmersdorferstraße 3 and 5. The file also includes details of the collectors, the pipes, the carrier medium in the collector circuit and the heat exchanger between the different circuits.

Measurement Technique

The following diagram shows the system’s measurement and monitoring sensors. The monitoring sensors are essential for the functioning of the whole system; the measurement sensors are used for the extended functionality control of the solar thermal system.

Diagram of the measurement points of the solar thermal system on Wilmersdorferstraβe 3 and 5

DATAPOOL provides access to our data server

 Via DATAPOOL  you can access selected data of this solar thermal system. You can choose from various illustrations like line charts, carpet plots, or scatter plots.

The apartment complex of Freiburg Wilmersdorferstraße can be found on the Datapool pages under Solar Thermal Systems.

The individual measurement points of the data pool can be found in the diagram above or here.

For a list of the measurement points, click here.

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