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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.

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.

Document Version Peer reviewed versionLink back to DTU Orbit Citation (APA): Ommen, T. S., Thorsen, J. E., Markussen, W. B., & Elmegaard, B. (2017). Performance of ultra low temperature district heating systems with utility plant and booster heat pumps. Energy, 137, 544-555.Please cite this article as: Ommen T, Thorsen JE, Markussen WB, Elmegaard B, Performance of ultra low temperature district heating systems with utility plant and booster heat pumps, Energy (2017), AbstractThe optimal integration of booster heat pumps in ultra low temperature district heating (ULTDH) was investigated and compared to the performance of low temperature district heating. Two possible heat production technologies for the DH networks were analysed, namely extraction combined heat and power (CHP) and central heat pumps (HPs). The analysis focussed on the characteristic heat demands of newly build multi-story buildings and the results were based on the ratio of the individual demands compared to the total. It was found that the optimal return temperature was dependent on the forward temperature and the heat consumption profile. For reference conditions, the optimal return of ULTDH varies between 21°C and 27°C. When using a central HP to supply the DH system, the resulting coefficient of system performance (COSP) was in the range of 3.9 (-) to 4.7 (-) for equipment with realistic component efficiencies and effectiveness, when including the relevant parameters such as DH system pressure and heat losses. By using ULTDH with booster HPs, performance improvements of 12 % for the reference calculations case were found, if the system was supplied by central HPs. Opposite results were found for extraction CHP, were ULTDH with booster HPs resulted in decreasing COSP of 20 %. Keywords: 4gDH, Ultra low temperature district heating, Heat pumps, Combined heat and power 15 well as to allow more heat sources to be easily integrated in the heat network [6]. Lowering DH supply * Corresponding author Email addresses: tsom@mek.dtu.dk (Torben Ommen), jet@Danfoss.com (Jan Eric Thorsen), wb@mek.dtu.dk (Wiebke Brix Markussen), be@mek.dtu.dk (Brian Elmegaard)

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Ammonia-water hybrid absorption-compression heat pumps (HACHP) are a promising technology for development of efficient high temperature industrial heat pumps. Using 28 bar components HACHPs up to 100• C are commercially available. Components developed for 50 bar and 140 bar show that these pressure limits may be possible to exceed if needed for actual applications. Feasible heat supply temperatures using these component limits are investigated. A feasible solution is defined as one that satisfies constraints on the COP, low and high pressure, compressor discharge temperature, vapour water content and volumetric heat capacity. The ammonia mass fraction and the liquid circulation ratio both influence these constraining parameters. The paper investigates feasible combinations of these parameters through the use of a numerical model. 28 bar components allow temperatures up to 111