Hydraulic Configurations for Implemented Low-Temperature District Heating Networks
The decarbonization of the heat supply using district heating networks requires a consistent transformation. Fossil energy sources, which were planned and implemented earlier, must be replaced by renewable energies in district heating systems. This transformation of district heating networks is often understood as a transformation to the 4th and 5th generation, and it applies to both new and existing district heating systems. For the exact differences in the definition of 4th and 5th generation, we would like to refer to our issue 9 of our newsletter. In this issue we summarize both generations as networks with low temperatures both in the distribution network and in the buildings. In addition to low temperatures and the associated low distribution losses, the focus of the 4th and 5th generation is on the integration of renewable heat and waste heat.
In the 23rd issue of the heatbeat Research Newsletter we present an article by Sven Werner (Halmstadt University, Department of Energy and Construction Engineering). The article Network configurations for implemented low-temperature district heating examines the hydraulic design of low-temperature networks that have already been implemented or are currently being planned. The author not only derives the typical configurations of early implementation of low-temperature networks, but also describes advantages and disadvantages of the different variants and derives the suitability of the different network configurations for different cases. We find it particularly interesting that the focus of the article is not to collect all new innovations, but to concentrate on actual implementations. Sven Werner identifies six different network configurations for low-temperature networks. These differ in temperature, number of pipes, and how buildings and power supplies are integrated. For some configurations, the author gives additional variants. The six main configurations were identified using 153 networks investigated. The biggest difference between the variants is the distinction in temperatures. Low-temperature networks distribute heat at the level of the consumers (in this article the author calls them warm networks), and cold heat networks can be an option when heat sources with this temperature level are not available. This leads to the most important insight from the article: which configuration is appropriate for a network depends largely on the heat source and its temperature level. The article is freely available at https://doi.org/10.1016/j.energy.2022.124091.
As mentioned above, this article examines several implemented or actually planned district heating networks. For this purpose, a total of 153 heat network projects were evaluated. The data was collected from various data sources. A lot of data could be obtained from past and current projects of the International Energy Agency (https://www.iea-dhc.org/home) and EU research projects. National funding programs (such as the German Wärmenetzsysteme 4.0) were also considered. This data from projects was supplemented by a literature and internet research. The author limited his research to district heating networks that use water as a heat carrier.
From these 153 district heating networks, six different configurations are identified, which will be presented below. We will also discuss the advantages and disadvantages and the applicability of the networks.
Classic Configurations with Warm Networks
In district heating networks with low temperatures, the classic structure of district heating networks can still be used. For this purpose, a flow and a return pipe are implemented. Normally, each building has its own transfer station. Typical temperatures in this configuration are between 60 °C and 65 °C. Especially the prevention of legionella in the domestic hot water supply limits further temperature reduction. In the networks under investigation, the return temperature can be between 30 °C and 35 °C. A major advantage of this configuration is that all planning steps and components are widely available and can thus be easily implemented. A major disadvantage, on the other hand, is that the customer with the highest temperature requirement also specifies the temperature in the network. Variants of the classic network are, on the one hand, that sub-networks can be formed, in which individual parts of the network are separated from the main network. Another variant includes a so-called return integration. Buildings with particularly low temperature requirements are connected to the return flow of the network, so that a higher cooling of the return flow can be achieved.
Modified Classic Configuration with Warm Networks
To reduce the flow temperature to the lowest possible level, the second configuration includes a key change compared to the classic network. A third pipe is introduced in addition to the supply and return pipes. This pipe provides the necessary circulation in the network, which lowers the return temperatures of the network, especially during low loads. In addition, in apartment buildings, transfer stations are rigorously used in the apartments themselves. In this way, circulation in the building can be avoided, buffer storage tanks are not required and there is therefore no risk of legionella formation. This measure allows lower supply temperatures to be used in the apartments. According to the author, flow and return temperatures can be lowered by up to 10 K compared to the classic district heating network. Higher investments in a third pipe and building transfer stations are disadvantages.
Multi-Level Configurations with Warm Networks
The third configuration includes several different pipes with different supply temperatures. Thus, different temperatures and temperature spreads can be handled with one network. This corresponds to a cascading of heat sources as well as heat sinks. The advantages are that the temperature can be tailored to the customer, especially in small networks, without the need for heat generators in the buildings. On the other hand, there is an increased control effort to provide the correct volume flow in each pipe. This configuration can also be divided into two sub-variants. The first variant has one pipe for heating purposes and one pipe for providing domestic hot water. The second variant uses a second pipe for lower temperatures only in parts of the network.
Ultra-Low Configuration with Cold Networks
The fourth configuration describes cold district heating networks (or ultra low temperature district heating networks). These networks distribute the energy at the level of the source or with a small central preheating. Decentralized heat pumps in the buildings (or electric heaters) raise the temperature to a usable level. Thus, a wide range of possible supply temperatures can be exploited; this offers the potential to incorporate many different, otherwise unused, heat sources. Another advantage is that different temperature levels can be realized in the building, so even old existing buildings can be integrated into the district heating network. These systems are particularly economical if the source can be accessed at low cost. High investments in the transfer stations with heat pumps are an obstacle to this configuration.
CHC Configurations with Cold Networks
The second last configuration uses synergies between heating and cooling demand in the same network. For this purpose, heat pumps are used, which provide thermal energy in the condenser and cooling energy in the evaporator simultaneously. The district heating network has a cold and a warm pipe. This principle is already used by large commercial buildings with simultaneous heating and cooling requirements. A key feature of this configuration is that each building must have its own circulation pump in the district heating network. Heating and cooling demands can balance each other, but if one of the two demands dominates, energy must be added to (heating case) or extracted from (cooling case) the district heating network at a central location. The temperatures in these networks are between 10 °C and 45 °C. The synergies exploited by the heat pumps represent the greatest advantage of this configuration. Challenges concern in particular the control and the fact that, for efficient operation, heating and cooling demand must be as simultaneous as possible. Two variants of this configuration are particularly relevant. One is a configuration where the cold pipe can be used directly for cooling. The second variant is the so-called reservoir network, in this configuration only one pipe with one temperature is used.
CHC Configuration with Warm Networks
In this configuration, separate heating and cooling networks are installed. This results in a four-pipe system. The transfer in the buildings is realized by classical transfer stations. A large heat pump is built at a central location. The evaporator is connected to the cooling network and cools the flow of this network. The condenser serves as a heat source for the district heating network. An advantage over cold district heating networks with combined heating and cooling is that it can be applied to existing networks. A disadvantage is that the heat pumps must cover the complete temperature difference between the cooling and district heating networks and thus operate at poorer operating points.
Most district heating networks with low temperatures are built according to the classic configuration. A total of 45 % use these configurations. A total of 41 % of the investigated networks use cold district heating networks with decentralized heat pumps or combined heating and cooling and are thus an important component for future district heating networks. Configurations 2, 3 and 6 are rarely used.
Further Information
As always, we recommend the full-length article, which is freely available at https://doi.org/10.1016/j.energy.2022.124091. The next issue of our newsletter (2nd anniversary) will be published on October 5, 2022. Until then, feel free to follow us on LinkedIn where we share smaller use cases and information.
