Dear Reader,

For the 54rd issue of our heatbeat Research Newsletter, we take a look at the return temperature reduction methods in district heating networks again.

We covered this already in newsletter 49 where we summarized the paper Return-Temperature Reduction at District Heating Systems: Focus on End-User Sites" from Tol et al. The authors now published a second review paper with the title “Enhancing District Heating System Efficiency: A Review of Return Temperature Reduction Strategies” from Tol et al. reviewing not only the possibilities at the end user site, but also the possibilities on the overall network system level.The paper reviews strategies for reducing return temperatures in district heating (DH) systems to enhance energy efficiency. DH systems distribute centrally generated heat to buildings, and the EU Heating and Cooling Strategy aims to increase renewable sources and recover industrial waste energy. Reducing return temperatures improves cooling of heating water and increases temperature differences in DH systems, leading to better efficiency and lower operational costs. Quantitative measurements show significant benefits from return temperature reductions, such as economic savings and improved system performance. Previous studies have addressed various aspects of DH systems, but none have specifically focused on return temperature reduction at a system level. This review synthesizes existing research on system-level components and operational strategies.The paper covers five areas:

1. DH Networks: Hydraulic balancing ensures even heat distribution and prevents return temperature spikes. Bypass applications, such as thermostatic bypasses, minimum flow bypasses, and flow control bypasses, are crucial for managing return temperatures. Cascading applications utilize residual heat effectively before rejection, reducing return temperature.
2. Heat Storage Systems: Medium-scale heat storage systems at the network level can stabilize temperature variations and improve efficiency. The capacity of thermal storage increases with decreasing DH return temperature.
3. Control Strategies: Various control mechanisms, including flow control (constant-flow and variable-flow strategies) and supply temperature adjustments (weather compensation curves and adaptive control methods), are essential for maintaining low return temperatures. Low-flow operation strategies, such as the Kiruna Method, help manage reduced return temperatures and increased temperature differentials.
4. Heat Sources: The impact of return temperatures on different heat sources, such as boilers, heat pumps, solar collectors, and industrial excess heat recovery systems, is examined. Lower return temperatures generally benefit performance near the heat source.
5. System-Level Optimization: A holistic approach integrating technical, operational, and economic factors is necessary for optimizing DH system efficiency. This includes site-specific temperature adjustments, annual analyses, and balancing energy efficiency with economic considerations.

Effective implementation requires addressing both technical and non-technical aspects, with tariff structures and consumer awareness playing critical roles. Pressure-independent thermostatic radiator valves and decentralized pumps are innovative solutions. Advanced thermo-hydraulic fluids, heat pumps sourced by return medium, and continuous monitoring and control are promising areas for future research. The review highlights the importance of a holistic approach to optimizing DH efficiency. Future research should focus on integrated design, smart adaptive control systems, and site-specific optimization to achieve low return temperatures and enhance overall system performance.By addressing these areas collectively, the paper provides a comprehensive framework for advancing DH system performance through optimized return temperature management.

Further Information

As always, we recommend reading the article in full. In addition to this research newsletter and various blog posts, we have added a monthly feature update to our blog, summarizing important developments and new features in our heatbeat Digital Twin. You can find the first entry at "https://heatbeat.de/en/blog/64/"

The next issue of our newsletter will be published on May 7, 2025.