No abstract

This study analyses the technical and private economic aspects of integrating a large capacity of electric driven heat pumps (HP) in the Greater Copenhagen district heating (DH) system, which is an example of a state-of-the-art large district heating system with many consumers and suppliers. The analysis was based on using the energy model Balmorel to determine the optimum dispatch of HPs in the system. The potential heat sources in Copenhagen for use in HPs were determined based on data related to temperatures, flows, and hydrography at different locations, while respecting technical constraints. The Balmorel model was developed further in order to provide a better representation of HPs, for analysing the seasonal variations of COP, and to represent the difference in performance of HPs connected to either distribution or transmission networks. The optimization yields roughly 3,500 full load hours (FLH) for the HPs connected to the DH distribution networks when considering a current scenario. In a zero carbon-dioxide emission scenario expected in year 2025, approximately 4,000 FLH, are achieved. In the case where HPs are connected to the DH transmission network at elevated temperatures, their operation decreases by roughly 1,000 FLH. No significant impact was found when comparing fixed and varying operation characteristics of the HP.

Combined heat and power (CHP) production in connection with district heating (DH) systems has previously demonstrated a significant reduction in primary energy consumption. With extended installation of intermittent sustainable sources, such as eg. wind turbines rather than thermal units, the changed distribution of generation technologies may suggest a reconsideration of optimum for DH network temperatures, in order to achieve low cost and minimize carbon emissions. A mixed integer linear optimisation model was used to investigate the changed operation based on changed network characteristics. Utility plants and demand curves corresponded to the current and future scenarios for the DH system of Greater Copenhagen. Performance curves from typical CHP-plant technologies were used to represent the changed operation of power and heat production for changed DH temperatures. The results show that primary fuel consumption is reduced approximately 5-7 % at DH design temperatures of 60 -70 C. Further reduction in DH temperatures resulted in opposing tendencies, as hot tap water requires electricity to reach the required temperatures. The results are network-specific, as they represent the given network and production units, but similar trends can be expected for other large networks.

The ammonia-water hybrid absorption-compression heat pump (HACHP) has been proposed as a relevant technology for industrial heat supply, especially for high sink temperatures and high temperature glides in the sink and source. This is due to the reduced vapour pressure and the non-isothermal phase change of the zeotropic mixture, ammonia-water. To evaluate to which extent these advantages can be translated into feasible heat pump solutions, the working domain of the HACHP is investigated based on technical and economic constraints. The HACHP working domain is compared to that of the best available vapour compression heat pump with natural working fluids. This shows that the HACHP increases the temperature lifts and heat supply temperatures that are feasible to produce with a heat pump. The HACHP is shown to be capable of delivering heat supply temperatures as high as 150 ○ C and temperature lifts up to 60 K, all with economical benefits for the investor.

A large amount of operational and economic constraints limit the applicability of heat pumps operated with natural working fluids. The limitations are highly dependent on the integration of heat source and sink streams. An evaluation of feasible operating conditions is carried out considering the constraints of available refrigeration equipment and a requirement of a positive net present value of the investment. The considered sink outlet temperature range is from 40 C to 140 C, but for the six heat pump systems considered in this paper, the upper limit of their working domain is at 120 C. For each set of heat sink and source temperatures the optimal solution is determined. At low sink temperature glide, either R717 or R600a heat pumps are optimal depending on the sink outlet temperature. At higher sink temperature glide the transcritical R744 also becomes important in a limited domain.