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18. Jun 2015 - Feeding solar thermal energy into district heating networks

Functional model of a combined house connection and grid feed-in station (HANEST)

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Photo: Functional model of a combined house connection and grid feed-in station (HANEST)
© TU Dresden, GEWV


The first large-scale solar thermal systems connected to local and district heating networks date back to the 1970s. To date, there are over 170 such systems across Europe with a rated thermal output of more than 350 kW, whereby about 60 plants produce over 1 MW. The solar thermal systems are situated either centrally at heating plants – often in combination with large seasonal heat storage systems – or decentrally at suitable locations and are integrated into the district heating network. In this case, the solar thermal systems use the heating network for storage.

Since 2011, the EnEff:Wärme research alliance "Decentralised feed into local and district heating systems using the example of solar thermal energy" has been investigating the technical and environmental impacts of decentralised infeeds using coupled simulations for three network types for providing district heating. Solar thermal energy was selected from the range of renewable energy sources because it presents particularly high demands in terms of its feed-in into heating networks. This is because it is an energy source that cannot be controlled, or only to a limited extent, with load change situations from 0 to 100% during the day. The main task was to secure a disruption-free grid operation, whereby it was intended to determine the bandwidth to facilitate a decentralised infeed.

New requirements for network and system components

Using actual network data and structures, various switching and control systems for the solar thermal/district heating transfer technology were designed, simulated and partially tested as functional models. These include house connection and grid feed-in stations (HAST, NEST) as well as the combined version developed for the project by the Technical University of Dresden as a single system (HANEST). Possible integration points and capacity limits of the heat storage systems were investigated as well as the behaviour of the solar thermal and heating networks in terms of the network hydraulics, infeed points and load changes.

The project results open up new development opportunities for already existing central heating networks. They also provide the basis for developing future-proof, multifunctional heating supply systems. Nevertheless, several questions still remain, such as how much decentralisation can be expected in existing district heating networks. Technical pricing models still need to be conceived and the overall expenditure for district heating systems with and without decentralised infeeds still needs to be balanced. One issue is whether the solar thermal yields can really compensate for the additional expenditure required, such as for powering pumps.

Practical test in Dusseldorf’s district heating network

Since May 2015, different possibilities for integrating solar thermal heat into the district heating system belonging to the Stadtwerke Düsseldorf municipal utility company have been tested as part of EnEff:Wärme’s new joint SWD.SOL project. For this purpose, a measurement rig is being installed in a building belonging to the Rheinwohnungsbau Gesellschaft Düsseldorf (RWB) housing association. On this basis, a locally adapted concept for integrating solar thermal heat shall be developed by 2018 for the Stadtwerke Düsseldorf’s district heating system.

Further information is provided in the project description "Decentralised feed into local and district heating networks – using the example of solar thermal energy".

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