Drinking water supply as low-temperature source in the district heating system: A case study for the city of Copenhagen

This paper explores the potential for using large-scale heat pumps (HPs) to extract energy from Copenhagen’s drinking water network and deliver it to its district heating system. The system involves certain losses in terms of additional heat and power consumption for end-use water heating. The net potential for energy extraction was analysed by means of an EPANET model to simulate system-wide temperatures in a piped distribution network. The model was validated against measured data from the network. Heat transfer in service lines was computed analytically and included in the net potential for energy extraction, which was determined to be 21 MW in Copenhagen. Around 38% of the HP source demand was harnessed from the ground. With HP COPs between 2.8 and 3.2, the System COP was only 1.7, thus suggesting that the choice of drinking water as a low-temperature heat source should depend on the available alternatives. Drinking water HPs have the side-benefit of preventing high drinking water temperatures; if operated in the summer they increased the share of supplied water complying with a recommended upper temperature limit of 12 °C from 42% to 81%.