Impact of variable heating loads on the performance of a return-line integrated heat pump in district heating systems

Abstract

Heat pumps (HP) are a suitable solution for utilizing low-grade heat sources to meet the heating demands of modern buildings, with the return pipeline of district heating systems (DHS) representing a significant low-temperature energy source. Therefore, this study presents multivariate simulation calculations of a HP system integrated into the DHS return line under variable operating conditions and heating loads, using return flow as a low-grade heat source. The system performance is evaluated under varying return water temperatures (45–60 °C) to the DHS and heating loads (20–100 kW), as well as under three operating temperature regimes of the building heating system (BHS): 60/40 °C, 70/50 °C, and 80/60 °C. The analysis focuses on the coefficient of performance (COP). The results show that the highest efficiency is achieved at the lowest temperature regime (60/40 °C), with COP values ranging from 5.27 to 6.94. Increasing the temperature level of the BHS leads to a reduction in performance, with COP decreasing by 17–26% for the 70/50 °C regime and 36–43% for the 80/60 °C regime. The findings highlight the strong influence of heating system temperature levels on heat pump efficiency and demonstrate the advantages of low-temperature operation in district heating applications.