Sustainable Operation of Large Scale Heat Pumps in Cogeneration dominated District heating Networks.

Abstract

Large scale heat pumps (LSHP) have seen a rapid proliferation in recent years, with projects and initiatives aimed at integrating electrically driven LSHP into existing district heating networks (DHN). The financial advantages have driven most LSHP installations to be powered by electricity generated by cogeneration (CHP), given CHP's dominant role in DHN. This LSHP + CHP combination poses the question, to what extent ecological improvements, measured as a reduction in specific CO₂-emissions through LSHP operation can be achieved, contingent on the source of electricity and heat source employed. In this paper, the LSHP + CHP combination was generally analysed under usage of exergy analysis and a universal method is presented that allows to evaluate the magnitude of ecological benefit through LSHP operation across all common heat sources in 1st to 4th (current) generation DHNs in conjunction with various types of CHP. Results show, that the heat from a LSHP using either of the 2 heat-sources surfaced-water or ambient air, inevitably results in higher specific CO₂-emissions from LSHP + CHP combination over the CHP-heat. Only when using a sufficiently hot and emissions-free heat source, the LSHP-heat from a LSHP + CHP combination can yield to an annual best case of half the CO₂-emissions of the CHP. Importantly, the choice of power source has shown to have a far more significant impact on emission reduction, as electricity sourced from renewable energy results in over an eight-fold reduction in emissions compared to the LSHP + CHP combination. Therefore, power consumption with sufficiently low specific CO₂-emissions out of external source is imperative to fully realize the ecological benefit of LSHP in DHN, underlining that the successful heat transition through LSHP relies heavily on the transformation of the electricity supply.