Heat pumps supplying district heating and ancillary services for the power system

Abstract

Transforming the energy system to a renewable energy system requires rethinking the structure of the system. One way to allocate large shares of renewable power production in the system is the coupling of different energy sectors in an integrated energy system. Large-scale heat pumps are expected to play a key role within these integrated energy systems, as they provide a link between the district heating sector and power sector. Thereby, they may unlock the thermal storage capacity of the district heating system to provide flexibility, while enabling the supply of heat from renewable heat sources. To analyse, whether and to what extend heat pumps may serve this purpose, the technical and economical feasibility of heat pumps supplying district heating and ancillary services for the power grid was assessed in this study, which was divided into three main parts. The first part analysed, how the district heating system should be optimally designed to allow for the most feasible heat supply from heat pumps through a techno-economic approach. This allowed analysing the influence of the boundary conditions of the supplied area on the economic feasibility of the assessed heat pump based supply solutions. In the second part, the technical ability of large-scale ammonia heat pumps to supply frequency regulation was assessed. This was done using a dynamic model of a heat pump that was validated against measurements of an existing heat pump in Nordhavn, Copenhagen. It was assessed how the design of the heat pump system and the corresponding control structure influence the ability of the heat pump to operate flexibly to provide primary frequency regulation. Finally, the feasibility of providing frequency regulation from large-scale heat pumps was assessed and compared to a combined frequency regulation service delivered by the heat pump together with fast regulating electric vehicles. For this study, a simplified dynamic model of the overall heat pump and district heating system was used. This model further included the calculation of exergy and cost of all streams in the system. Based on these exergoeconomic data, a method to allocate cost to the two products of the heat pump, i.e. heat and ancillary services, was proposed. The following findings may be summarised based on the work presented in this thesis. Firstly, for future suburban and urban areas in Denmark supplied by heat pumps, the supply of low-temperature district heating from central large-scale heat pumps is most feasible. For areas with low linear heat demand densities, individual heat pumps become more feasible, while the feasibility of ultra-low temperature district heating suffers from the requirement of large space heating shares and additional cost for decentral heating units, despite the higher thermodynamic efficiency. Secondly, the dynamic behaviour of largescale heat pumps and especially the ability to change load quickly is influenced strongly by the control structure of the system, controller parametrisation and the refrigerant mass in the system. A limiting factor of fast regulation of ammonia heat pumps is the risk of droplet formation in the suction line during fast ramp down. Within this work, it was shown that future fast regulating heat pumps should be designed, such that the refrigerant mass is minimised, especially in the vessels. Further, it is advantageous if the power uptake is controlled directly. Thirdly, it was shown that the assessed largescale ammonia heat pump was able to follow a given frequency deviation and thereby may in principle provide frequency controlled reserve. If the allowed regulation time of the frequency regulation service is long enough, i.e. 150 s like FCR-N service in Eastern Denmark, the heat pump may provide frequency regulation on its own, which would maximise the obtainable income for the heat pump owner. If the heat pump is not fast enough to provide the service alone, e.g. for the required regulation times of 30 seconds in Western Denmark, it may be operated in combination with fast regulating bidirectional units, such as electric vehicles. This allows for a combined service provision and the exploitation of synergies between the different units, as the heat pump may generate an additional income from supplying ancillary services, while ensuring that the electric vehicles can bid a larger power capacity into the market. Finally, the proposed allocation method allows quantifying the influence of different operation strategies on the cost of the two different products of the system, i.e. heat and ancillary services. Overall, this work showed that large-scale heat pumps may be equipped to be able to take over the intended role as flexible units between the heating and power system and that this operation mode can be economically feasible within the Danish market framework.