In recent years, and especially in the last few months, the demand for new, innovative district heating network solutions has risen strongly. One concept that is being investigated and implemented are ultra-low temperature district heating networks. These networks capture sources at a low temperature level (e.g. waste heat from data centers or geothermal energy) and distribute it at the low temperature level. Decentralized heat pumps in the buildings use the network as a source and raise the temperature to the individually required level in the buildings. More information on ultra-low temperature district heating can be found in our references and demos: https://heatbeat.de/de/referenzen/shamrockpark/, https://heatbeat.de/de/simulation/kalte-nahwaerme/
In our July issue of the heatbeat Research Newsletter, we would like to present a research article that summarizes the hydraulic and thermal challenges of these networks and offers possible solutions. The article was written by T. Sommer, A. Sotnikov, M. Sulzer, V. Scholz, S. Mischler, B. Rismanchi, K. Gjoka and S. Mennel at the Institute of Building Technology and Energy, Lucerne University of Applied Sciences. In the article "Hydrothermal challenges in low-temperature networks with distributed heat pumps" the authors show challenges for the operation of distributed heat pumps in ultra-low temperature district heating networks. Low or highly fluctuating volume flow rates are challenging compared to operation in conventional networks. The authors investigate the influence of the volume flow through the evaporator on the heat pump efficiency (coefficient of performance) and introduce a new evaluation variable "agent authority", which is like the concept of valve authority. In addition to the pure physical evaluation variables, the study also introduces soft factors that evaluate, for example, the expandability of the network or the resilience of the network. The developed evaluation variables are applied to two case studies in Switzerland and Australia. The article is available at https://doi.org/10.1016/j.energy.2022.124527
Heat pumps cannot be operated with arbitrary temperatures, temperature differences and volume flows in the evaporator. These operational limits of heat pumps mark important boundary conditions of ultra-low temperature district heating networks. To guarantee good heat transfer in the evaporator, turbulent flow through the heat exchanger at the evaporator must be ensured. This results in a minimum volume flow and prevents freezing of the heat transfer medium (water) in particularly ultra-low temperature district heating networks. The inlet temperature to the evaporator must also be kept within limits. This ensures that the working pressures and temperatures of the refrigerant are not exceeded or undercut in the refrigeration circuit of the heat pump.
While the influence of the temperature on the efficiency of the heat pump is often considered (the higher the temperature difference, the lower the efficiency), Sommer et al. show that the volume flow also has an influence on the Coefficient of Performance of the heat pump. The volume flow influences the effectiveness of heat transfer in the evaporator and thus also determines the evaporation of the refrigerant. At very low volume flows, there is a risk that the refrigerant will not evaporate completely. In this case, the internal control of the heat pump intervenes and lowers the pressure and thus also the temperature of the evaporation. This results in a higher temperature difference within the heat pump and efficiency decreases. Large volume flows generally result in better heat pump efficiencies, but also increase the energy required to circulate in the heat network. In addition to the temperatures, the volume flows in ultra-low temperature district heating networks must therefore be monitored.
The agent authority describes the ratio of the pressure loss of an agent in the network to the total pressure loss of the network. An agent can be a consumer or a supply. Thus, this concept is very similar to the valve authority in the classical definition, but also includes the pressure losses of additional appliances (heat exchangers, heat pumps, etc.). The agent authority ranges from 0 to 1. If the agent authority is 1, all participants in the network are hydraulically decoupled. The authors deduce that the flow rate in a network remains well controllable and can meet the requirements set above if the flow rate changes are about 20% of the design flow rate, for which the agent authority must be about 0.7.
To keep the minimum volume flows and their fluctuations low, the authors describe several possible measures:
The authors' findings are applied to two use cases. The first use case is an ultra-low temperature district heating network in Switzerland (13 buildings and 1200 meters of pipe length). An analysis of the network shows that a generous design of the pipes already achieves a high agent authority of about 0.93 for the last consumer. Volume flow fluctuations in individual agents therefore cause only minor fluctuations in the entire network. In addition, the share of circulators in the total electricity demand (heat pumps is low). These two properties of the network mean that optimization of the volume flow by intelligent control in a building has only a very small influence (about 0.3 %).
The second use case is a new network to be built for the University of Melbourne campus. Three different system configurations are compared. A conventional ultra-low temperature district heating network, a bi-directional network, and a novel network design with only one pipe (reservoir network). The three concepts are evaluated using a total of 13 indicators. In addition to physical and energy aspects (e.g., agent authority, low electricity demand of heat pumps and circulators), softer factors such as controllability, resilience, and expandability were evaluated. Overall, the conventional ultra-low temperature district heating network was rated most suitable for this use case, ahead of the reservoir and bi-directional network. However, the evaluation also depends on the available heat source and the individual agents in the network.
For a detailed evaluation of the use cases and the derivation of agent authority, we recommend the full-length article by Sommer et al. The article is available at https://doi.org/10.1016/j.energy.2022.124527 The next issue of our newsletter will be published on August 1, 2022. Until then, feel free to follow us on LinkedIn where we share smaller use cases and information.
The next issue of our newsletter will be published August 3.
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