Potential and limitations of using low-temperature district heating and cooling networks for direct cooling of buildings

Abstract Low-temperature district heating and cooling (LTDHC) networks use heat from cooling loads of a district to serve the heating demand of nearby residential areas. These networks consist of a dual-pipe system connected to decentralized heat pumps that serve the heating or cooling needs, and to borehole storages that even out seasonal mismatch between the demands for heating and cooling. The software Polysun® has been modified for the simulation of LTDHC networks, and models have been validated with measured data from an existing network in Zurich. Simplified simulation cases were used to analyze the potential for direct or “free” cooling by LTDHC networks. Our results show that “free” cooling of residential buildings can only contribute to a minor extent to the regeneration of borehole field storages connected to LTDHC. Therefore, other sources such as waste heat or solar thermal energy are needed to fully regenerate the borehole fields in order to avoid long-term freezing of the heat carrier fluid. These sources induce elevated temperatures in the network/borehole storage in summer and thereby further limit possible contributions of direct cooling. Deep lake water has an elevated potential for direct cooling of buildings. The potential of superficial water may be largely overestimated when using standard (artificial) weather data for the determination of the thermal building loads in combination with measured water temperatures. For realistic results, the water temperatures must correlate with the weather data.