This study demonstrated an increase in the thermodynamic performance of a booster heat pump, which was achieved by choosing the working fluid among pure and mixed fluids. The booster heat pump was integrated in an ultra-low-temperature district heating network with a forward temperature of 40 °C to produce domestic hot water, by heating part of the forward stream to 60 °C, while cooling the remaining part to the return temperature of 25 °C. The screening of working fluids considered 18 pure working fluids and all possible binary mixtures of these fluids. The most promising solutions were analysed with respect to their performance under off-design conditions and their economic potential. The best-performing mixture showed a coefficient of performance (COP) of 9.0 and thereby outperformed R134a by 47 %. Although the mixed working fluids resulted in higher investment cost, the economic performance was comparable to the pure fluids. The mixtures showed similar performance as the pure fluids at off-design conditions. It was concluded that the mixtures 50 % Propylene / 50 % Butane and 50 % R1234yf / 50 % R1233zd(E) could considerably improve the thermodynamic performance of the overall heat supply system while being economically competitive to pure fluids.

The integration of different energy sectors, such as the electricity and heating sector, is an effective way to integrate large shares of renewable energy into the energy system. Heat pumps allow efficient heat production based on electricity. As such, they may be used to provide two different services-the generation of heat and the provision of demand flexibility as ancillary services for the power system. The paper presents a method to assess the impact of providing demand flexibility on the performance of the conversion system based on a dynamic exergoeconomic analysis. A way to allocate the cost of heat and flexibility products based on the difference in exergy destruction was proposed. The method was applied to a case of a groundwater-source heat pump system supplying a district heating island system. It was found that providing demand flexibility causes higher exergy destruction, mainly due to heat losses during storage and the need to reheat the fluid using an electric heater. The major part of the additional exergy destruction was not related to heat pump regulation. When providing flexibility the overall cost of the system increased and according to the proposed allocation, demand flexibility accounted for 12 % of the overall cost.

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