Clustering Energy Systems for District Heating in Germany

One key to increase the share of renewable energies in the heating sector is district heating. The focus is on both new efficient district heating networks and the transformation of existing ones. District heating networks are individually grown systems, which often differ significantly in their detailed design. However, for a comprehensive transformation of district heating networks it is crucial to develop transferable concepts and methods. For this purpose, district heating networks must be categorized and characterized. Often, categorization is based on the predominant temperatures in the district heating network and possible generation capacities (see also our 9th newsletter). In the paper we present this month, the authors have made a categorization of existing district heating systems in Germany based on the actual installed capacities of the energy supply systems. The authors establish a total of eight different clusters of district heating systems. This classification not only helps to develop national transformation strategies for the energy and heat transition but can also be used to scale technical solutions to other systems.

The paper Landscape of district heating systems in Germany - Status quo and categorization was written by Triebs, Papadis, Cramer and Tsatsaronis of the Chair of Energy Engineering and Environmental Protection, Technical University of Berlin. The authors collected data from a total of 141 district heating network systems from German cities with at least 100,000 inhabitants and a larger share of heat supply by district heating networks. A major challenge was the availability of data. Unlike the electric sector, district heating networks are not liberalized. District heating networks are independent systems operated by individual companies (e.g., municipal utilities). To generate a complete data set, the authors used different sources (e.g., Federal Environmental Agency, Federal Network Agency, studies, and statistics of the AGFW, individual utilities/companies, and other sources). From the 141 district heating systems, 82 systems, each with a net thermal capacity of more than 50 MW (thermal), were selected and classified.

The authors use four KPIs for classification, which group the systems based on the fuel share (for cogeneration), the storage size, the power-to-heat ratio (for cogeneration), and the installed capacity. It is noticeable that fossil fuels are dominant for large district heating systems. Natural gas, coal and lignite dominate (about 80% of fuel use in the systems studied, year 2018). Storage systems are used especially if the system relies on gas-fired CHP solutions, the share of power-to-heat (electric boiler as well as heat pump) also increases in this category. The power-to-heat ratio is higher for coal-fired than for primarily gas-fired cogeneration. These are mostly large power plants (e.g., lignite). Nearly all systems are supplemented by large capacities of (mostly gas-fired) peak-load boilers. The study shows the great need for transformation of district heating networks to dispense fossil fuels for heat generation by 2045 and is therefore an important contribution. In the following, we would like to go into more detail about the methodology and the authors' results.

Technologies and indicators of heat supply of large district heating networks

To derive categories and classify the district heating systems later, the technologies used must first be defined and described in more detail. For this purpose, the authors describe seven different technologies (CHP, boilers, power-to-heat, solar thermal, waste heat and waste incineration, and storage).

An important technology for providing heat in district heating networks is cogeneration. Due to the selected size of the installed capacity (at least 50 MW), the authors only consider large power plants with steam cycle. For this purpose, the authors emphasize that it is important to distinguish between back pressure turbines and extraction-condensing turbines. Boilers are used as peak load systems in almost all district heating networks. As power-to-heat, the authors include both direct electric heaters and heat pumps. While direct electric heaters can be used primarily for peak loads and as control reserve for the electric grid, heat pumps are intended to be operated for longer periods due to their efficiency. To consider solar thermal systems, the authors point out that a distinction should be made between two technologies (flat plate collector and evacuated tube collector), as they can provide different temperatures. Triebs et al. describe waste heat and waste incineration as further important technologies. Waste heat can be provided from industrial processes as well as from coupled district heating networks. Thermal storage systems are characterized by their design (atmospheric, pressurized, 2-zone) and the resulting storage density. After describing the technologies, the authors define four metrics they would like to use to classify district heating networks.

The first key figure puts the installed capacity of the individual generators in relation to the total installed capacity. Even without classification, some trends stand out. The capacity of boilers is the largest on average. Heating boilers are used to cover peak loads. As a result, the capacity of these systems is often very high, whereas their share of heat supply is low on average (only 10.5% 2015-2020). After that, CHP plants (of both types) and waste incineration dominate the generation portfolio of district heating networks. Power-to-heat technology is considered with 5% of the total capacity on average. Solar thermal takes only a negligible share of the total generation capacity.

The second key figure describes the maximum possible time period for which the storage units can replace CHP plants running at full load. In the considered district heating systems, hardly any long-term storage is used. A total of 47 systems have a storage system, 44 of which have a maximum storage time of less than 12 hours.

To describe the capacity of the CHP plants in more detail, the power-to-heat ratio (ratio between installed electrical and thermal capacity) was included as a key figure. This indicator describes the potential for sector coupling with the power grid.

The fourth and last key figure describes the fuel input of the CHP plants. The fuel input of the boilers is not examined further. However, the authors state that boilers mostly use natural gas, oil more rarely hard coal and biomass.

Eight categories of district heating networks in Germany

The key figures are calculated for all district heating systems examined and sorted using a special clustering algorithm. The result is a set of eight different categories for district heating systems in Germany. It should be noted that only systems with an installed capacity of 50 MW (thermal) or more were considered.

• The first category is constituted by gas-fired power plants with extraction-condensation turbines (combined-cycle plant). Often, waste incineration plants and smaller storage facilities are also included in this category. The average installed capacity is about 450MW.
• The second category is formed by smaller gas-fired CHP (about 250 MW on average). However, only back pressure turbines are used in this category. The capacity of the power-to-heat technologies used is higher than in the first category.
• The third category is similar to the second category. However, the significantly larger share of storage capacities is striking, which is also accompanied by an extended increase in Power-To-Heat technologies. In 5 out of 8 grids in this category, PtH plants are used with up to 11% of the installed capacity.
• The fourth category consists of large-scale power plants that run on hard coal. The thermal capacity is on average about 850 MW and in one system even reaches more than 2000 MW. Only few storage facilities are used.
• The fifth category also includes large power plants (1050 MW on average), which are, however, in the transition phase between coal and gas-fired CHP plants. Approximately 50 % of the fuel used is hard coal and natural gas. In this category, biomass is increasingly used in CHP systems.
• The sixth category includes systems that are operated by waste incineration or waste heat utilization. The installed capacity is significantly lower than in the previous categories (approx. 150 MW on average) and no CHP systems are used. Waste heat recovery and waste incineration make the use of storage uneconomical, so that only one out of eight systems examined has a storage system.
• The seventh category includes biomass CHP plants. The installed CHP capacities are lower in relation to the previous categories, also the total installed capacity is lower with approx. 200 MW on average.
• The last category is formed by lignite-fired power plants. The systems have an average size of approx. 350 MW. The high power-to-heat ratios are particularly striking, which means that the heat extracted only accounts for a small proportion of the installed electrical output.

With their work, the authors have improved the data quality and data availability of large district heating systems in Germany. In this way, they improve predictions on fundamental trends and national transformation strategies for large district heating systems. This also promotes the transferability of specific technical solutions that we are dealing with in the transformation of district heating networks.

Further information on the classification of district heating networks

The article by Triebs et al. is freely available at https://doi.org/10.1016/j.ecmx.2020.100068 and, in addition to the results presented here, mainly includes the graphical representations of the cluster analyses. We recommend reading the full paper.

The next issue of our newsletter will be published on 1st December 2021.