Forschungsgruppe-NET - Hochschule Offenburg

Solar Heat

District Heating Network Holzgerlingen, Housing District Hülben

The circuit diagram below shows a simplified version of the solar thermal system and all its corresponding components. The whole system is subdivided into two main circuits: the collector circuit and the buffer storage circuit. From there, the heat is fed directly to the heating network.

The district heating network is divided into two subnets (north and south). The southern subnet is designed for 229 kW at a feed-in temperature of 70°C and an outgoing temperature of 40°C. 36 apartments in different complexes, 11 single family houses and the kindergarten are connected to this network. This corresponds to a total living space of 5.342 m². The northern subnet is designed for 340 kW at a feed-in temperature of 65°C and an outgoing temperature of 35°C. 33 single family houses with a total living space of 4.233 m² are connected to this second network.

The collectors are installed on the kindergarten roof. The collectors are fixed on metal trays, which were placed on the flat roof and additionally secured with the weight of the gravel on the roof. 

They cover an area of 248.8 m2; the azimuth of the entire field lies at 8° West at an inclination angle of 30°. The installation of the collectors was optimized for the summer so that the entire demand of heat for water heating could be covered by the solar thermal system.

In order to be able to store the heat, three buffers storage tanks were set up and connected in series. Each tanks has a capacity of 5 m3. The solar heat is transported via an underground pipe from the collector field to the central heating. There, it is fed into the buffer storage by a heat exchanger. When feeding the buffer storage, cold water is drawn either at the bottom of buffer storage 3 or at the reflow point of the district heating network and fed into the top part of buffer storage 1 after re-heating. A bypass was included to optimize the feed-in; if the temperature of buffer storage 1 is high, the bypass switches to buffer storage 3. A regulation valve at the storage circuit makes sure that the volume flow at the heat exchanger can be regulated such that ideally water at a temperature of 70° is fed into the buffer storage. The buffer storage, the heat exchanger and the wood pellet boiler are housed in the facility management building, 60 meters away from the collector field. 

Wood pellet boilers are not as easy to control as gas or oil boilers. In order to reduce the disturbance caused by unfavorable cycles of the wood pellet stove, it is also connected to the buffer storage. This way, buffer storage 1 is also the wood pellet stove buffer.

The facility management of the connected buildings was adapted to the demand of the solar heat supply. The reflow temperature, for instance, was reduced to a low level. This can be achieved when the heaters of the family homes are supplied directly rather than using a heat exchanger. The transfer stations were equipped with continuous flow systems for warm water. The apartment complex features underfloor heating. The warm water is generated with the help of a special transfer system with included storage.

Simplified hydraulic diagram of the solar thermal system of the dictrict heating network Holzgerlingen


This table shows the most important specifications of the solar thermal system for feeding into the district heating network of Holzgerlingen.

Project data
Surface249 m²
Collector typeflat plate collectors
Buffer storage3 x 5000 ltrs
Project costs153 275 € (618 €/m²)
Heat exchanger1 plate heat exchanger
Azimuth/ InclinationSouth +8° / 30°
Lowest energy costs0,14 €/kWh
Highest solar yield99.200 kWh/a (399 kWh/m²a)

Click on the pdf-file to receive detailed information on the specifications of the solar thermal system at Holzgerlingen. 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 and monitoring points of the solar thermal system at Holzgerlingen

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 solar thermal system for the supply of solar heat for the district heating network Holzgerlingen can also 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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