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𝐙𝐙 Optimized variables 𝜂𝜂comp Isentropic efficiency compressor [%] 𝜂𝜂ejec Ejector efficiency [%] 1 Introduction Heat pumps used for combined space and domestic hot water (DHW) heating, also known as integrated heat pumps, represent a technology under development, as discussed in the field study report by Stene and Alonso (2016). Integrated heat pumps can be used simultaneously for heating and cooling (Byrne et al., 2009), or designed to operate in reversible mode for space cooling during summer (Minetto et al., 2016), or in large district heating networks (Elmegaard et al., 2016). The history and current status is also discussed in the review articles by Hepbasli and Kalinci (2009) and Arpagaus et al. (2016). Heat pumps are typically compared on energy efficiency and cost, but the refrigerant used is also an important factor. Replacing hydrofluorocarbons (HFCs) with more environmentally friendly alternatives has become compulsory worldwide. This is due to the entry into force of strict legislative acts aimed at environment preservation, such as the EU F-Gas regulation (EU, 2014), which are pushing towards the adoption of refrigerants with low global warming potential (GWP), like CO2 with a GWP=1. The focus of this study is CO2, which is also inexpensive, non-flammable, non-toxic, and has other advantages since it operates transcritical and therefore can match the temperature profile of the DHW heating. Energy efficient CO2-based processes typically have three gas coolers to match the refrigerant temperature profile in the exchangers with the space and hot water heating (Stene, 2005). Process performance depends on system design and operating conditions, e.g. inlet and outlet water temperatures, exchanger sizes (gas coolers, internal heat exchanger and evaporator), compressor efficiency and the profile of the heating demand. The ratio between space and DHW heating is particularly important for the heating efficiency (Stene, 2007a). Houses constructed according to the Norwegian building code from 1997 are between 2.2 and 3.4 (Stene, 2007a). Buildings with a large showering consumption such as sport centers and hotels often have much lower space heating demand relative to DHW (Stene and Alonso, 2016). Low-energy buildings and passive houses in Norway have annual ratios ranging from 0.40 to 0.85 (Stene, 2007b), and are estimated to increase rapidly from around 3 % in 2014 to 40 % in 2050 (Nord et al., 2016). Another important performance factor is the feedwater temperature, which is related to the city water temperature, but also affected by thermodynamic losses in the DHW tank, caused by mixing of hot and cold water which increases the feedwater temperature (Stene, 2005; Yokoyama et al., 2007). For a large variety of heat pumps, the energy efficiency can be improved if a two-phase ejector for expansion work recovery is included (Stene, 2007a; Banasiak et al., 2012; Banasiak et al., 2015; Pottker and Hrnjak, 2015; Boccardi et al., 2017). Two-phase ejectors in systems with two gas coolers have previously been modelled by Minetto et al. (2016). However, to the best of the authors' knowledge, the benefits of using ejectors in energy efficient integrated heat pumps with three gas coolers have not been studied in detail. Conventional one-stage integrated heat pump systems are typically based on HFCs, like R410A (GWP = 1924), and operate with one condenser and one desuperheater in subcritical processes (Blanco et al., 2012). The first transcritical integrated heat pump prototypes were introduced by Nekså (2002) and Stene (2005), and where CO2 (R744) processes with two and three gas coolers, respectively. Heat pumps based on CO2 have been known for more than two decades to be very efficient for tap water heating (Nekså et al., 1998), but for space heating processes R410A is reported as more efficient than CO2 Richter et al. (2003). That is, integrated CO2 heat pumps outperform conventional HFC-based systems if the space heating demand is small compared to the DHW heating demand (Stene, 2007b). However, there is little study work on where the trade-off point is. To the best of the authors' knowledge, most of the relevant studies have only discussed pure DHW heating or pure space heating, i.e. systems with only one gas cooler or condenser. A few 2PDF Image | CO2 Heat Pump Performance
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