Temperature control strategies for fifth generation district heating and cooling systems: A review and case study

Temperature control exerts a significant influence on the performance of Fifth Generation District Heating and Cooling (5GDHC) systems. Ineffective temperature control could lead to not only occupant discomfort and higher operation costs due to increased energy consumption and reduced equipment lifespan, but also a negative environmental impact through higher carbon emissions. Despite its importance, this area remains underexplored, with limited studies conducting in-depth analyses of temperature control strategies applied in 5GDHC systems. To bridge this knowledge gap, this study conducts a comprehensive literature review on temperature control strategies for 5GDHC systems. Three prevalent temperature control strategies have been identified: constant temperature control, multi-stage temperature control, and free-floating temperature control. To facilitate the implementation and relatively fair comparison of these strategies and their various settings, detailed 5GDHC system models have been developed using Modelica. The case study encompasses two network configurations, spans across four distinct climate zones, and evaluates the system performance using five key performance indicators, including total energy consumption, system coefficient of performance (COP), carbon emissions, peak electricity demand and plant capacity. The simulation results reveal that the constant operating temperature control strategy can outperform the free-floating temperature control strategy when appropriate network operating temperature settings are adopted but also exhibit greater sensitivity to changes in these settings. Simple multi-stage temperature control strategies are found to be insufficient to achieve the best system performance. In addition, substation direct cooling could be an effective means to enhance the system performance but necessitates coordination with suitable temperature control strategies to maximize energy savings. In general, the performance of temperature control strategies in 5GDHC systems is strongly influenced by multiple factors such as load profiles, system configurations and specific strategy settings. This study underscores the importance of selecting and designing temperature control strategies based on comprehensive system analyses to achieve high performance 5GDHC systems during operations.