Design of serially connected ammonia-water hybrid absorption-compression heat pumps for district heating with the utilisation of a geothermal heat source

District heating (DH) can reduce the primary energy consumption in urban areas with significant heat demands. The design of a serially connected ammonia-water hybrid absorption-compression heat pump system was investigated for operation in the Greater Copenhagen DH network in Denmark, in order to supply 7.2 MW heat at 85 °C utilizing a geothermal heat source at 73 °C. Both the heat source and heat sink experience a large temperature change over the heat transfer process, of which a significant part may be achieved by direct heat exchange. First a generic study with a simple representation of the heat pump was used 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 a base case using direct heat exchange and a single heat pump, under the assumption that the exergetic efficiencies of the heat pumps are similar. Next, ammonia-water hybrid absorption-compression heat pumps were selected as heat pump technology, since these may increase the performance due the non-isothermal phase change. Detailed thermodynamic models predict that an exergetic efficiency of the system of 0.5 to 0.65 is possible. The technical feasibility as well as the economic viability of this installation was investigated for a range of preferred solutions. The analysis recommends a heat pump configuration with an exergetic efficiency of 0.63 which was within 2 % of the theoretical economic optimum.