A Comparative Analysis of Energy Performance of Ultra-Low Temperature District Heating and Cooling Networks with Different Substation Configurations

This study aims to explore the differences in the energy performance among ultra-low temperature district heating and cooling (ULTDHC) networks with different substation configurations. Three substation configurations, including free cooling (using heat exchangers only for substation cooling), active cooling (using chillers only for substation cooling), and hybrid cooling (utilizing both chillers and heat exchangers for substation cooling), are investigated through a simulation-based case study in Atlanta, GA. A detailed ULTDHC system model is developed using Modelica. The simulation results indicate that without integrating the sewage water, compared to the system with active cooling configuration, the system with free/hybrid cooling configuration exhibits annual electric energy savings of 5.5% for radiant systems and 2.6% for all-air systems, respectively. However, when the excess waste heat from the sewage water is recycled, the free cooling configuration performs the poorest among the three. In comparison to the system with free cooling, the system with hybrid cooling achieves energy savings of 4.0% for radiant systems and 4.7% for all air systems. These findings reveal that although substation free cooling makes it possible to reduce the energy consumed by the substation cooling process, the overall system performance varies depending on multiple factors such as the availability of thermal energy sources and building-side HVAC system requirements. In addition, the simulation results underscore that network operating temperature control plays a crucial role in the system's performance. Therefore, detailed system analyses are recommended to guide the selection of the most suitable configuration and temperature control settings during ULTDHC system design.