Bjarne Bach: Integration of Heat Pumps in Greater Copenhagen, MSc Thesis, DTU, 2014
The municipality of Copenhagen has the ambitious goal of becoming the first CO2 neutral capital before 2025. Using large heat pumps for district heat production in Copenhagen can contribute to a more effective way of producing heat in the future, and thereby help to fulfil the goal of CO2 neutrality. At the same time heat pumps can be an important player in the integration of the fluctuating electricity production from renewable energy sources, and relieve the pressure on the limited biomass resources. This study analyses the technical and economical aspects of integrating heat pumps in the Greater Copenhagen district heating system. Three main aspects of heat pump integration have been investigated: (I) the potential sources, (II) the feasible heat pump technologies, and (III) the economical competitiveness of heat pumps in the district heating system.
The analyses of the heat sources were based upon gathered data on temperatures, flows, hydrography, and locations, as well as considering the technical and economical challenges. The thermodynamic modelling of heat pumps were done in EES. And in this study an auxiliary program, COPcalc, for calculations of seasonal variations in COP and capacities of heat pumps was developed. The output from COPcalc is a COP and capacity profile, which can be implemented in the energy system optimization model Balmorel. The Balmorel model was developed further to make a better representation of heat pumps, for analysing the seasonal variations of COP, and to be enable to represent heat pumps connected on the distribution grid.
The natural heat sources investigated were; sewage water, sea water, drinking water, ground water, air, and ground. The most promising sources for the Copenhagen system were found to be sewage, drinking, and sea water. It is found that total of around 87 MWth could be connected to sewage water facilities, and around 13 MWth could be connected to drinking water facilities. The sea water potential was found to be practically infinite, but there was found to be some challenges with depths, pumping, piping, and freezing of water. From analyses of the demand in the near coast distribution areas it was estimated that a total capacity of 160 MWth of sea water heat pump could be implemented in the system. The potential of sea water as heat source was found to be highly dependent on the hydrography of the system analysed.
The current most suitable heat pump technology for the district heating system, was found to be an ammonia two-stage compression heat pump, heating the district heating water in a tandem system. The operational calculations of such a heat pump connected to the distribution grid yields a COP of around 2.8 to 3.1 in winter time, and a COP of 3.2 to 3.6 in the summer time, depending on the source. Connecting the heat pumps to the transmission grid gives a much lower COP of around 2.5 in winter time and 2.9 in summer time, for all sources.
Using Balmorel to analyse the competitiveness of a total 260 MWth heat pump capacity distributed onto the district heating system, yields around 3500 full load hours for the heat pumps connected to the distribution grids in 2013, and around 4000 full load hours in 2025.
When connected to the transmission grid the number decreases by around 1000 full load hours for both years. The poor competitiveness when connected to the transmission grid was found to be caused by the lower COP. The heat pumps mainly displace production at peak load in winter time, and heat production by combined heat and power plants in summer time. Based on the 3500 full load hours the integration of especially sewage water and drinking water heat pumps was found to be economical feasible already today, if connected to the distribution grid. The study concludes that implementation of heat pumps directly to the distribution grids in larger cities in Denmark is private economical feasible using the Danish taxation legislation from 2013.