Design of serially connected district heating heat pumps utilising a geothermal heat source

The design of two heat pumps (HP), connected in series, was investigated for operation in the district heating (DH) network of the Greater Copenhagen area, Denmark. The installation was dimensioned to supply 7.2 MW of heat at a temperature of 85 °C. The heat pumps utilise a geothermal heat source at 73 °C. Both heat source and sink experience a large temperature change, which may lead to decreased performance for single vapour compression HP. The performance may be increased by using HPs connected in series and by applying HPs with zeotropic mixtures. First a generic study with a simple representation of the HP was applied to investigate optimal system configurations. It was shown that using two heat pumps in series with direct heat exchange in parallel with the first heat pump could increase the performance compared to the HP performance. Detailed thermodynamic models of a zeotropic mixture HP predicted that an exergetic efficiency of the units between 50% and 65% is possible. The technical feasibility as well as the economic viability of this installation was investigated for a range of optimal configurations. The analysis recommends a heat pump configuration with a system exergetic efficiency of 63%.