Integrating Industrial Waste Heat and Solar Thermal Energy into District Heating

Dear reader,

thank you for your interest in our first heatbeat Research Newsletter. We are very happy to start this exciting journey together. As part of our internal research and development, we closely follow current developments and publications in the research areas of district heating, district cooling and district energy systems. With the monthly publication of our newsletter, we want to present to you one selected research article from these subject areas in each issue.

The short version

For this issue, we have selected the paper *Potential of integrating industrial waste heat and solar thermal energy into district heating networks in Germany* by Johannes Pelda, Friederike Stelter, and Stefan Holler from HAWK University of Applied Science and Art Hildesheim / Holzminden / Göttingen. It examines the potential for integrating waste heat and solar thermal energy into existing district heating networks for 42 major cities in Germany.

The results for the waste heat potential show significant differences between cities with potentials ranging from the possibility to completely cover the network's annual heat demand (e.g. Karlsruhe and Trier) to very small estimated shares of less than 1% of the annual heat demand (e.g. Berlin and Munich). Further investigating the potential of renewable heat sources, the paper estimates the spatial requirements for solar thermal heat collectors needed to achieve given target values ​​for the solar fraction of the network feed-in.

We found it particularly interesting that by making intelligent use of open data, the researchers succeeded in estimating the waste heat potential not on a regional level or by example of a single network, but on the city level across a large number of actual German cities. From our point of view, this creates a very interesting foundation for further work facilitating an increase in the share of renewable heat sources in the heat supply in Germany and beyond.

Our full summary

The research article *Potential of integrating industrial waste heat and solar thermal energy into district heating networks in Germany* addresses an important aspect of the German Federal Government's Climate Action Plan 2050: The integration of waste heat and solar thermal energy into district heating networks as an important approach to reduce the CO2 emissions of the heat supply in Germany. The paper pursues an interesting approach to assess the potential of these two renewable heat sources. What is special here is that the assessment is not carried out on a regional level or for a single sample network, but across as many large cities in Germany as possible, each at city level. To accomplish this, the authors use two approximations:

  • As a starting point for determining the waste heat potential, they assume the officially reported CO2 emissions of large industrial sites. Based on knowledge of the CO2 emissions, they estimate the original primary energy input for each location. This leads to the definition of 3 scenarios in which the authors assume that 10%, 20%, or 30% of this primary energy input can be used for the integration of waste heat into a district heating network.

  • For the potential of solar thermal energy, the researchers follow a calculation approach the other way round. They start with the annual heat supply of the district heating networks in the investigated cities. For this annual heat supply, they assume a target value for the solar fraction of 1%, 5%, or 10% in 3 further scenarios. From this they determine the heat that would have to be fed in from the solar thermal system and estimate the space required for this. For a plausibility check, these areas are compared to the total area of ​​the considered cities.

In order to apply these methods to as many large cities in Germany as possible, the paper starts the investigation with 80 cities with more than 100.000 inhabitants each. For 42 of these cities, data for the annual heat supply for the district heating networks as well as the necessary CO2 emission data of industrial sites in these cities were found.

As a result of the investigations, the paper finds large differences for the waste heat potential and their possible share in the total input of the existing district heating networks. Waste heat potentials are determined for the cities of Bremen, Karlsruhe, Saarbrücken, and Trier which could cover approximately 100% of the required heat input. In contrast, the waste heat potential for Munich and Berlin is given as less than 1%. The authors reason that these low shares are caused by relatively large district heating networks in these cities with high heat demands but only relatively low reported industrial CO2 emissions.

For the solar thermal potentials, the paper concludes that even a solar fraction which at 15% exceeds the originally assumed scenarios has the space requirements of less than 1% of the total area of ​​the cities concerned. While this number appears small at first, the absolute required space for Munich for example is given at around 4 km².

Of course, both a single large-scale solar thermal plant and a large number of distributed solar thermal collectors would each face significant challenges for actual implementation. In order to show the general feasibility of high solar fractions nevertheless, the paper cites the examples Senftenberg and Crailsheim, where solar fractions of 4% in Senftenberg and even 50% in Crailsheim have already been achieved in actual implementations. The solar fractions in the paper were also chosen in such a way that investments in additional heat storage are not absolutely necessary in order to achieve the target values.

For this assessment of potentials, the paper deliberately does not investigate the concrete feasibility for the individual cities. Instead, after estimating the potentials, the authors emphasize that a specific assessment of the local conditions should be carried out in each individual case. Nevertheless, we think this research work shows an interesting approach to supplement the usual perspectives of the regional analysis and individual case studies with an analysis across a larger number of large cities.

As a result, the identified potentials for the integration of industrial waste heat and solar thermal energy can be a good starting point for examining the interesting questions about actual implementations. The range of possible questions covers the spatial conditions with the specific locations of the waste heat potential and the areas that can be used by solar as well as the optimal operation of existing networks in order to make the best possible use of the discovered renewable heat sources.

To further validate the potentials for specific cases, detailed evaluations need to go beyond the annual heat input or averaged temperatures. Instead, the time series of the feed-in and the demands must be evaluated over the course of different operation conditions. Of course, the hydraulic behavior of the network must also be adjusted to best accommodate additional and possibly intermittent heat sources. Lower network temperatures favor the integration of renewable energies, while lower supply temperatures can also lead to higher mass flow rates and thus to higher pumping costs and possible bottlenecks in the network.

For these evaluations, it is important to weigh the advantages and disadvantages for each individual case and to find the best solutions for the local conditions. For these and similar questions, we at heatbeat work with dynamic simulation models in order to test and optimize the integration of renewable heat sources in a virtual prototype.

Further information

If this summary has sparked your interest, please refer to the original article that appeared in *Energy* in May 2020 and is freely available under open access: . The blog of HAWK also provides further information on the research project "DEKADE-F-Waerme" under which the featured research was carried out. And the researchers continue to work on the subject of waste heat utilization in the research project "MEMPHIS", for which you can find more information at and under

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Best regards,
Your heatbeat team