Ground-mounted solar thermal collectors in front of the central heating plant at the Hallerndorf district heating (Source: Guido Bröer)

Emission-Free, Carbon-Neutral, and Efficient: Solar District Heating Support Local Climate Protection

Steinbeis experts coordinate climate protection project to support the transition to environmentally friendly heating

Solar district heating combines two established and perfected forms of technology: solar thermal energy (“solarthermics”) and district heating. The general expansion of district heating and, in particular, solar district heating represents a significant opportunity for local authorities to play their part in climate protection. Heating currently accounts for more than 50 % of the annual final energy required by the housing sector in Germany. Funded by the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU), the SolnetPlus climate protection project supports the development of district heating based on large-scale solar thermal systems – thus also supporting the transition to environmentally friendly heating. The project is being coordinated by Solites, the Steinbeis Research Center for Solar and Sustainable Thermal Energy Systems.

The German federal government has been promoting the development and construction of large-scale solar thermal systems on open spaces and roofs since the 1990s. More than 44 such large-scale plants are in operation now, supplying thermal energy for heating and hot water to a large number of residential and non-residential buildings. The fastest growth is currently being witnessed in urban district heating networks and in the district heating of smaller cities and communities in rural areas.

Basically, there are two different types of collectors used to generate heat: high-temperature flat plate collectors and evacuated tube collectors. When the sun shines, absorbers heat up a liquid heat transport medium inside the collectors. Depending on the specific design and construction, systems deliver different temperature levels of output, achieve different efficiencies and result in different costs for each type of collector. By connecting multiple panels to form large-scale collector farms (tens of thousands of square meters are possible), heat can be transported to a heating plant. From the heating plant, the thermal energy is shared with connected buildings via insulated pipelines as part of so-called district heating.

The advantages of solar energy

Using solar energy as the primary power source makes it possible to generate 100 % emission-free heat. It is also carbon-neutral. In addition, these systems achieve stable heating costs, typically for many years because they are unaffected by fuel prices and offer low maintenance and servicing costs. These costs are highly competitive compared to other heat generation systems. Aside from the energy saved when cutting carbon emissions, ground-mounted solar thermal systems can also have a positive impact on the ecology and biodiversity of the surroundings. Allowing meadows to fill with flowers and designing the technology to blend in with its environment offers new habitats to native plant life and insects, enhancing the overall area.

Key factors affecting solar thermal systems

In principle, solar thermal collectors can be used on any open space. A number of factors need to be taken into account, however, such as proximity to the heating plant, the orientation of collectors and local topography. A key challenge with large-scale solar thermal systems is increasing competition over land use with farming, businesses and other renewable energy sources. In part, competition can be mitigated by, for instance, using old landfills or occupying land converted from other uses. As solar thermal collectors are the most efficient way to use open spaces – even compared to other renewables, including solar energy used for photovoltaics – the priority with suitable land should be to earmark it for solar thermal energy.

A key prerequisite for using large-scale solar thermal collectors is an existing or recently constructed district heating system. (Solar) district heating makes an important contribution to municipal climate protection. Within three years, a large number of buildings can be supplied with climate-neutral solar heat, either through completely new district heating systems in rural areas, for example in combination with biomass, or by feeding thermal energy into existing urban district heating networks. Integrating solar thermal systems into existing district heating works well if the flow and return temperatures of the district heating system are kept as low as possible. The lower the temperatures are, the more efficiently solar-generated heat can be used. This enables the solar thermal collectors to cover the summer heating requirements of an entire network. During this time, alternative heat generators can be left to idle. Not only does this save expensive fuel, it also makes it possible to carry out maintenance and servicing.

It also becomes possible to cover more demand through solar energy. Examples of systems in Denmark and Germany show that using seasonal heat storage systems makes it possible to achieve “solar fractions” of 50 %. In the process, solar surpluses generated in the summer can be stored over extended periods and used in the winter months. Making multifunctional use of thermal storage systems also improves the cost-effectiveness of seasonal heat storage.

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SolnetPlus contributes to climate protection and the green energy transition

More and more heating suppliers are now constructing large-scale solar thermal systems in Germany – primarily because highly appealing subsidies are now available, but also for commercial reasons. As a result, a number of solar thermal collector farms have been constructed in recent years, ranging in size from 2,000 to 14,800 square meters. Measuring 18,700 sqm, Germany’s largest solar thermal plant to date is currently under construction in Greifswald.

To keep up the momentum of this trend, the Steinbeis Research Center for Solar and Sustainable Thermal Energy Systems (Solites) is coordinating a project called SolnetPlus – Solar district heating as a Solution for Municipal Climate Protection. The initiative is receiving just under € 1 million of funding from the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety. Working alongside project partners AGFW (a rationalization, information, and standardization specialist), Hamburg Institut Research (HIR), the German Institute for Urban Affairs (Difu), and freelance journalist Guido Bröer, the aim is to improve awareness of solar heating system solutions and to accelerate municipal climate protection through the implementation of concrete projects.

The SolnetPlus project team at the official handover of the funding check with Parliamentary State Secretary Rita Schwarzelühr-Sutter, Steinbeis Board Member Prof. Dr. Michael Auer, and representatives of the Solar Heating Networks Industrial Initiative (picture taken July 8, 2021 at the Steinbeis House for Management and Technology (SHMT) in Stuttgart. Source: Guido Bröer)


For the project, local authorities are being targeted with the aim of offering them information and advisory services that could motivate them to contribute to local climate protection and promote the transition to environmentally friendly heating. The other important target group in transforming the sector is heating suppliers, which are also being approached and offered training.

In addition, assessments are being carried out not only to understand the underlying factors that shape planning and approval processes, but also to establish potential areas that concrete improvements could be made in – and to propose recommended actions among governmental institutions. The project is being supported by both professional PR experts, with the aim of increasing impact and a project advisory board comprising manufacturers and supplier companies.

The innovative climate protection project is funded by the German Federal Ministry for the Environment through the National Climate Initiative (NKI). Further information on the National Climate Initiative of the German Federal Ministry for the Environment:


Patrick Geiger (author)
Steinbeis Research Center Solar and Sustainable Thermal Energy Systems (Stuttgart)