1.1 Literature Overview

There are many theoretical studies on 5GDHC systems, which mainly focus on the development of simulation^{[4, 8-11]} and mathematical optimization models.^{[1, 12]} Some studies present operational optimization models,^{[13, 14]} while other publications provide real-world experience from a practical planning perspective: Zeh et al.^[15] discuss the planning of the 5GDHC network in Wüstenrot (Germany) and their experience with installing the first large-scale agrothermal collector with a used land area of 4400 m² and a depth of 1.5 m. The Wüstenrot district is also discussed by Brennenstuhl et al.^[16] They also provide detailed information about two other geothermal collector systems in the cities Neustadt am Rübenberge and Bad Nauheim. Then et al.^[17] provide a technical description of the 5GDHC network in the district Largarde-Campus in Bamberg, in which geothermal probes and horizontal collectors as well as sewage water is used as the heat source. Vetterli et al.^[18] present monitoring data for the Suurstoffi district in Rotkreuz (Switzerland). They analyze how the water network temperature develops over 3 years of operation. The current status of the minewater project in Heerlen (Netherlands) is presented by Verhoeven et al.^[19] They elaborate on future network expansion plans and how the business case of this project is realized. In general, there is a lack of design guidelines for 5GDHC systems as discussed by Boesten et al.^[7] First approaches have been presented by Revesz et al.,^[20] who provide a holistic design approach for 5GDHC systems on a conceptual level. Pass et al.^[21] and Wirtz et al.^[22] introduce two different concepts to quantify the demand balancing in 5GDHC systems and to support design decisions in the early planning phase.

For conventional district heating and district cooling networks, comprehensive statistics exist as shown in refs. [23, 24] For 5GDHC systems, however, there is only very little data and only two publications are available, which provide an overview of the technology usage: Reiners^[25] shows a map with 22 5GDHC networks in Germany and indicates which heat source is used for every system. The data has been collected from publicly available sources. In his list, 7 systems use geothermal probes and 7 systems groundwater or water from old mines (especially in the Ruhrgebiet area in Germany). However, the data is from 2019 and except for the heat source, he does not provide any further details about the systems. In addition, Buffa et al.^[26] present a list of 40 5GDHC networks in Europe with 15 systems in Germany, 15 in Switzerland, and 5 in Italy. They find that the heat sources are very diverse, ranging from geothermal energy to surface water (river, sea, or lake), to excess heat or ambient air. They also provide data on the network temperatures, which range from 0 to 35 °C. Twelve systems are operated with fixed temperatures and 16 networks with seasonal shifting temperatures. In the article by Zeh et al.,^[15] the 5GDHC systems surveyed by Buffa et al. are visualized on a map.

The literature review shows that there is no overview of currently operated 5GDHC systems in Germany or Europe and that the literature mainly focuses on the investigation of individual case studies which makes it difficult to derive general conclusions or design guidelines.

1.2 Contributions

The main contribution of this article is an analysis of survey data of 53 5GDHC networks in Germany. The conduction of the survey has multiple objectives: Listing use cases of successful 5GDHC projects proves to decision-makers that 5GDHC systems have been successfully implemented in many cases in the past. This helps to build confidence in the technology and encourages decision-makers to implement innovative, emission-free district energy systems. Second, a comprehensive project list increases the visibility of 5GDHC networks and shows district design engineers how other 5GDHC districts have been planned in the past, including what key performance indicators have been achieved (e.g., primary energy factors) or which technologies and heat sources have been used. Third, the list of 5GDHC systems can support district design engineers in the planning process, since they can identify districts where systems with similar boundary conditions (e.g., specific combination of heat sources) have been implemented and can reach out to the respective planners. Lastly, the survey data is also a contribution to identify best practices and general design guidelines for 5GDHC networks, e.g., with respect to network temperatures or pricing models. In summary, the conducted survey helps to increase the global visibility and penetration of 5GDHC networks.

2.1 Survey and Questions

The goal of the survey is to cover the most relevant aspects of 5GDHC networks, ranging from technical aspects and key figures to political aspects. Basic information of the districts are the district name and city, the (planned) year of commissioning, and the operator of the 5GDHC network. To characterize the network and operation, the network temperatures and the pipe type were surveyed. The size of the system is quantified by the network length as well as the number of connected buildings. In many cases, 5GDHC networks were planned for newly built areas. Therefore, the share of new and existing buildings (floor area) is an interesting key figure for 5GDHC networks as well. One of the most important aspects of 5GDHC networks is the used heat source. The participants could select from a list or describe the heat source in a text field. The survey contains additional questions regarding the ability of the network to provide cooling energy, a district-wide obligation to connect to the network, the ownership of the decentral heat pumps, and the price model for heat and cold. All survey questions are listed in the Appendix.

2.2 Conduct of the Study

Districts with a 5GDHC network have been identified using a web search and a search in scientific literature as well as presentations at conferences and seminars. The goal was to identify a large number of 5GDHC networks, however, the total number of 5GDHC systems in Germany is unknown. The participants were contacted by email and asked to participate in the survey provided as a Google Form in German. The survey was conducted between September 2021 and May 2022. The participants were allowed to provide estimations (e.g., for network length) if exact numbers were not available or the system was not built yet. As a result, the authors cannot guarantee that all provided data is completely accurate. In the survey, only 5GDHC networks were included (no conventional district heating systems). For this purpose, the definition of 5GDHC systems by Buffa et al.^[26] has been applied. This means that district heating networks that provide heat directly to buildings throughout the year, i.e., without decentral heat pumps, are not considered a 5GDHC network. Also, district cooling networks (with the primary goal of providing cold) are not included, although their network temperature is in the same range as 5GDHC systems.

3.1 General Information

In Table 1, the key facts of all surveyed 5GDHC districts are listed. In most cases, the operators are city utilities (municipalities) or energy utility companies, such as E.ON,^[27] EnBW,^[28] or Energiedienst.^[29] Twenty networks are already in operation, 12 are commissioned or planned to be commissioned in 2022, and 8 networks are planned to start operation in 2023. Figure 1 provides an overview of the (planned) year of commissioning of the districts (the 5GDHC network from 1981 is not depicted). The increasing number of projects is an indication of the growing interest in 5GDHC networks in recent years. However, it should be noted that recently planned projects tend to be over-represented in this study due to the better availability of information and technical planners.

3.2 Technical Data

In Table 2, the technical parameters of the networks are listed. In some districts, the number of buildings and the network length has been estimated by the survey participants since no exact data is available. The network temperatures range from −5 to 75 °C (c.f. Figure 2). Usually, networks with 75 °C are not considered fifth but fourth-generation district heating (4GDH) networks. However, these networks are operated in summer as a 5GDHC network and in winter as a 4GDH network and are therefore a hybrid network type. In this study, the networks in Bedburg, Berlin-Tegel, Bodenmais, Dollnstein, Flehingen, Haßfurth and Kerpen are identified as hybrid 5GDHC/4GDH solutions. The other 5GDHC networks are operated below 40 °C during all seasons. Some networks use constant temperatures in the warm and cold pipe throughout the year, others have a temperature window that depends on the load situation of the network, the ground temperature, or the temperature of the heat source (e.g., in the case of river water). The network temperature strongly affects the pipe type: Networks with higher temperatures tend to use insulated pipes, since otherwise heat losses become dominant. As a result, the hybrid networks use insulated pipes. In contrast, if the network temperature is close to the ambient temperature level, insulation is usually omitted. Without insulation and with low network temperatures, heat gains from the surrounding soil to the pipe become relevant. Heat gains can make up a substantial share of the heat load of the district, especially for districts with low heat density. Since most of the networks are operated close to the ground temperature, plastic pipes are the most frequent pipe type (42 of 53 networks). In some cases, both types, plastic and steel, as well as both insulation types, with and without insulation, are used in a network, for example, in the district in Herne.

The network lengths vary within a wide range: The smallest network has a single length of 150 m (Achern). This network connects 4 buildings with a central heat source. The largest 5GDHC network has a length of 12.5 km (Berlin–Tegel). Other districts with large networks are Schleswig (Berender Redder, 13.9 km, 199 buildings), Soest (7.3 km, 417 buildings), Gelting (6.6 km, 118 buildings), Warendorf (5.5 km, 180 buildings), Bamberg (5.5 km, 60 buildings), and Nieby (4.9 km, 49 buildings). The network length of 32 districts (60%) is below 2 km.

The majority of projects are new build districts. Only in 10 districts, the share of newly built buildings (floor area) is smaller than 100%. One reason is the low-temperature requirements of underfloor heating systems in new residential buildings, which result in high heat pump efficiencies. Another reason is that for new built districts, natural-gas-based solutions are often not an accepted solution anymore.

The heat sources, the heating capacity, the provision of cold as well as the primary energy factors are listed in Table 3. 5GDHC networks are supplied by a wide range of heat sources (c.f. Figure 3): In 23 of the 53 districts, horizontal geothermal collectors are installed. For a detailed review of geothermal collectors, we refer to ref.15 If geothermal collector tubes are installed in multiple layers and in close proximity, the surrounding ground freezes in winter. This further increases the heat potential and is often described as earth ice storage. An earth ice storage is used, for example, in the district in Schleswig (Wichelkoppeln). The second most common heat source is (vertical) geothermal probes, which is used in 17 districts. They usually reach depths of 100–200 m. Another proven heat source, that is available in every district, is ambient air. Ambient air can be used in combination with an air-source heat pump or an air-to-water heat exchanger. Ambient air is used in 9 surveyed districts. In 5 districts, sewage water is used as the heat source. Especially for urban areas, sewage water is a promising heat source: Sewage water has a largely constant temperature throughout the year and can be used directly to heat a 5GDHC network. A further very promising heat source is waste heat: Waste heat can result from a variety of different processes and buildings: Industrial processes, data centers, process cooling in commercial buildings (e.g., supermarket), or from space cooling. Since space cooling of connected buildings is usually an integral function of the 5GDHC network, it is not considered an explicit heat source in this study. In 6 districts, waste heat is used as a heat source: In Shamrockpark in Herne, for example, waste heat from a nearby industrial site as well as a data center is used to cover the district's heating demands. Other examples are the two districts in Jülich, where waste heat from a data center and a supercomputer are used. In the district in Meitingen, industrial waste heat from a large industrial site is used to cover the heat demands of the 5GDHC district. Other heat sources are river water, wells and groundwater, or the return pipe of a conventional high-temperature district heating network. Using the return pipe of a district heating network to supply a 5GDHC network increases the temperature difference between the supply and return pipe, and thus reduces the pump work of the district heating network. Buffa et al. find in their study that 65% of the investigated 5GDHC networks are connected to only a single heat source and the rest uses multiple heat sources.^[26] This finding is close to the data of this study, where 57% (30 of 53) of the districts have a single heat source.

The cumulated heating capacity of all heat sources/generation units of a district varies largely from 50 kW (Achern) to 43 MW (Berlin–Tegel). While a crucial advantage of 5GDHC networks over 4GDH networks is the possibility to provide cold with the same infrastructure, the cooling supply is not realized in every district. In 39 of the 53 districts, the 5GDHC network also covers the buildings’ cooling demand. In contrast to that, most of the hybrid 5GDHC/4GDH networks do not provide cooling. Also, the districts in Meitingen and Jülich (FZ) are supplied with waste heat at around 30 °C, and therefore do not provide cold.

The surveyed primary energy factors (PEFs) span a large interval from 0.1 to 0.5. Since no formal definition of the PEF was given to the participants of the survey, it is not possible to comprehend the underlying calculation of the provided PEF values. However, in Germany, the calculation is usually conducted according to the German AGFW standard (FW 309)^[30] and it can be assumed that most of the surveyed values are based on this standard. Consequently, the surveyed data allows drawing the following conclusions: In most districts, for which a PEF value has been provided, it is between 0.4 and 0.5. If ambient heat (e.g., from geothermal sources with PEF = 0) is used as a heat source, the PEF of the network is predominantly affected by the electricity demand for the hydraulic pumps and decentral heat pumps. For example, a PEF of 0.45 is obtained when geothermal heat is used and a heat pump coefficient of performance of 4, and a PEF for electricity of 1.8 (according to the German standard) is assumed. A PEF of 0.45 is in line with typical 5GDHC networks that use a single geothermal source such as Hilter (0.46), Rottenburg, (0.42), Neustadt am Rübenberge (0.45), Salzgitter-Thiede (0.45), Soest (0.44), Warendorf (0.47), and Winsen/Luhe (0.45).

3.3 Political and Economic Data

Besides technical design parameters, political factors are also important for a successful implementation of a 5GDHC network. An overview of the surveyed data is given in Table 4. The municipality has to decide whether there is an obligation to connect to the 5GDHC network or not (German: Anschlusszwang). In 27 districts, an obligation to connect is imposed, in 22 this is not the case. In the districts, Achern and Flehingen, the entire district is developed by a single company. In Meitingen, a connection to the network is compulsory in some parts of the district. With the obligation to connect, the building owners cannot choose which heating system they want to install and must connect their building energy system to the thermal network. In some cases, building owners are hesitant to connect to a thermal network because the network operator has a de facto monopoly on the heat supply in the district. From an energy and economic perspective, a connection obligation makes sense, since a high connection rate reduces the heat supply costs for all buildings.

Another political design parameter is the ownership of the decentral heat pumps. Decentral heat pumps are either owned by the network operator or by the building owner. Due to the current subsidy policy in Germany, in most cases, it is beneficial if the network operator covers the investments of the heat pumps since then, the heat pumps are part of the publicly subsidized district energy system. In the surveyed districts, the heat pumps are owned by the network operator in 29 cases, in 18 systems they are owned by the building owner. In the district in Emmerthal, the operator owned the heat pumps for the first 20 years, now they are owned by the building owners. The transition of ownership from the network owner to the building owner after a certain time period is a typical agreement in 5GDHC networks in Germany.

In conventional district heating systems, the price policy is usually similar: The consumer pays a base and energy price (for the supplied heat at the substation) and, in some cases, a capacity price for the peak power. An energy price is charged in all systems, since a higher heat demand leads to higher fuel and operational costs for the network operator. 5GDHC networks offer more flexibility for designing price models: At first, it must be decided, where the billing boundary is located: The useful heat behind the heat pump, the low-temperature heat from the network, or just a flat rate service for supplying heat. Due to the ambient heat sources in 5GDHC networks, the operational costs of the network itself are very low and consist of maintenance costs as well as electricity costs for the hydraulic pumps. In most surveyed 5GDHC networks, the billing boundary for delivered heat is at the condenser of the heat pump (34 districts). The price models vary across the surveyed networks: The most widely used price model for heat supply is charging a base price (€ year⁻¹) as well as an energy price (€ kWh⁻¹) (30 districts). In 9 districts, a capacity charge (€ kW⁻¹) applies. Another 8 districts have a flat rate model and only charge an annual base price that depends on the expected annual heat demand. For example, in the districts of Fischerbach and Gutach-Bleibach, the annual base price depends on the building area. In the district in Dorsten-Wulfen, the supplied heat at the evaporator is charged based on a liter price for cold water. In Reichenbach a. d. Fils, cold water from the network is charged with a base price, and the electricity costs for the heat pumps are covered by building owners. Since the Wüstenrot district was part of a research project, heat is supplied for free and only a one-time fee was charged. In general, a one-time connection fee is charged in many districts, however, no data has been surveyed in this regard.

The provision of cold is often for free (11 districts). Keeping cooling prices low can be beneficial to incentive a high cooling demand in summer to regenerate geothermal sources. Other operators charge a base price (8 districts) or a base and energy price (8 districts).

In two districts (Bainergy Park in Jülich and Berlin–Tegel), prosumer buildings are connected to the network which receives a compensation for the feed-in of heat. Only a few values have been provided by the participants regarding the levelized costs of heat. Since the underlying cost calculation is not comprehensible for the authors, the values cannot be compared with each other and therefore, the data is not presented in this paper.

Several points have been mentioned by the participants regarding the regulations or legal provisions that are considered obstacles to the development of 5GDHC networks: One central problem is the use of an appropriate heat carrier fluid. Currently, a water–glycol mixture is widely used for 5GDHC networks. However, glycol but also alternative heat carrier fluids like bioethanol are classified as harmful to the environment in Germany (classified as Wassergefährdungsklasse 1). This classification leads to difficulties in the regulatory approval process. In some cases double-walled pipes have to be installed which has a substantial impact on the efficiency of 5GDHC networks: Heat gains from the ground, which can make up to 20% or more of the total heat source capacity (according to statements of survey participants), are substantially reduced. This means heat sources have to be sized larger which increases the investment cost and necessary installation area. In addition, the complex regulations and additional fees for self-used photovoltaic electricity is another regulatory obstacle for 5GDHC networks (decentral PV power is ideally used by decentral heat pumps in the same building^[31]). Other difficulties are long approval processes for subsidy applications or complex geological regulations for geothermal probes with a depth of more than 100 m (application of Bergrecht regulations in Germany).