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despite all good efforts in recent years, district heating in Germany is still very reliant on fossil fuels. The most recent statistics from 2020 show that natural gas accounts for around 44 % of the fuel input. Together with coal, lignite, and oil, the share of fossil fuels amounts to over 65 %. Of course, a large share of this fossil fuel input is used for co-generation and thus, district heating contributes to very efficient fuel utilization. Yet, against the backdrop of the current energy crisis in Europe and beyond, the transition towards renewable heat sources has gained more urgency in recent months. It will take some time until this urgency will be visible in the scientific literature. Nevertheless, we already wanted to take a slightly different approach for this issue 22 of our newsletter and instead of summarizing a single paper in detail give a broader overview and highlight some papers that have already investigated strategies for the energy transition in district heating networks for the example of Germany in the last years.
Already in 2019 Popovski et al. investigated The role and costs of large-scale heat pumps in decarbonising existing district heating networks – A case study for the city of Herten in Germany. In this study, the authors benchmarked 7 future scenarios against a reference scenario with the original setup of the district heating network in Herten. In that reference scenario, the two-part district heating network in Herten was largely supplied by a coal-fired CHP with a small additional share of heat from a waste incineration plant. For the future scenarios, the authors considered introducing new heat sources like solar thermal, biomass, and especially heat pumps. In addition, they considered reduced heat demands due to building retrofits.
One important take-away from the analysis was that it is not possible to reach the city's long-term decarbonization targets without changing the supply of the district heating system. And based on the cost assumptions from 2019, the paper showed that the transition to large-scale heat pumps was not cost-competitive to the reference scenario without several changes to the boundary conditions. These changes included lower network temperatures, lower electricity prices, lower CAPEX for the heat pumps, and higher CO2-prices. Of course, from today's perspective, the boundary conditions have changed dramatically, there has been much progress regarding large scale heat pumps development, and cost-competitiveness with the status-quo is not the only dimension to consider regarding the transition away from fossil fuels. Yet, this first case study for the transition of a German district heating network still contains many useful insights and sets the stage for other studies performed for the energy transition of other cities.
In a study from 2020, Gonzalez-Salazar et al. performed an Evaluation of Energy Transition Pathways to Phase out Coal for District Heating in Berlin. Similar to the Herten-study, this analysis used a heat demand model of the building stock and defined future scenarios to optimize the future heat sources for the district heating network. The case of Berlin is especially interesting as it involves the largest district heating system in Germany with around 2,000 km length and over 10 TWh of heat supply per year.
This study also considered several heat sources including biomass, waste and sewage, waste heat from industry, solar thermal, geothermal energy, and others. For the goal of the transition roadmap, the paper defined to phase out coal-fired CHP until 2030. As a solution the paper suggested to utilize a mix of renewable heat sources but found their potential limited to cover the entire heat demand of the network. To close the gap, the paper suggested a mix of natural gas, synthetic renewable gas and hydrogen to be used in CHPs together with power-to-heat plants. Thus, this is another good example for how the fast-changing energy landscape can show results from only 2 years ago in a new light. But similar to the Herten-study, this paper still describes a valuable approach to optimizing the energy transition for a large district heating network, even if with today's different boundary conditions the results may have been different.
In a paper from 2021, Leiss et al. evaluated the topic of Integrating deep, medium and shallow geothermal energy into district heating and cooling system as an energy transition approach for the Göttingen University Campus. Compared to Berlin, the investigated district heating and cooling system for the university campus in Göttingen is significantly smaller, with network length of around 13 km and around 250 buildings connected. This network is currently supplied by a gas-fired CHP and the paper investigates how a combination of geothermal sources can help the transition away from fossil fuels.
For this transition, the paper suggests to explore the potential of a deep geothermal well (3000 - 5000 m depth) in combination with a medium deep seasonal geothermal storage and shallow geothermal as well as additional biomass boilers. As especially deep geothermal resources come with large uncertainties, the paper does not yet include a final evaluation of the transition roadmap. But it argues to establish Göttingen as a demo site to learn about unlocking geothermal potential in wide areas of Europe. And it seems that the paper happened to address the topic of replacing natural gas as the main supply for a district heating and cooling system just before the topic became very urgent in recent months.
Even more up to date is a study by Kleinertz et al. with the title Heat Transformation Munich – Analysis and Strategy Definition for a Systemic Cost-Optimal Heat Supply Transformation which is still in pre-print, but already available. The starting point for this paper is the city of Munich's goal to be climate-neutral by 2035. To evaluate transition roadmaps, the paper compares 2 future scenarios with a reference scenario as the baseline. In the larger context of the entire city, the paper suggests building retrofits, heat pumps and climate-neutral district heating as the most important technical solutions. To achieve the climate-neutral district heating, the study identifies a wide mix of heat sources including a baseline of waste incineration, a large share of geothermal as well as smaller shares of biomass and hydrogen to replace today's large share of natural gas.
While the paper shows that many of the necessary measures included in the scenarios actually save costs, the main conclusion is that climate-neutrality can only be achieved by adding CO2-compensation measures outside of Munich into the balance at additional costs. Yet, even though this paper is very recent and still in pre-print, again the changed boundary conditions especially regarding the costs of natural gas in the reference scenario and the significant push towards decarbonization of recent months may make the suggested measures more attractive.
We hope that these 4 examples illustrate how different research teams envision the decarbonization of district heating networks for different cities in Germany. Furthermore, we think that all these examples show how dramatic changes in boundary conditions can completely change the fundamental outlook on this topic. Nevertheless, all these studies still present important methods and insights. It will be interesting to see how future publications will address the new challenges we face for decarbonization of district heating systems. And in addition to the examples above, the very first issue of our newsletter already featured a thorough overview of the potential for waste heat and solar thermal in Germany, which is also very relevant in the context of the 4 papers we showed in this newsletter.
The next issue of our newsletter will be published September 7.