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Energies 2020, 13, 5654 6 of 18 are radiant systems. However, in these applications, the temperature drops do not exceed 3–4 ◦C, as suggested by the standards, in order to guarantee a uniform temperature of the radiant surface and good thermal comfort for the users. The layout of the heat pump prototype allows this last problem that limits the use of the transcritical cycle in the field of air conditioning of buildings to be overcome. In this case study, the heat pump exploits the advantages of the medium/high-temperature cycle, using the CO2 refrigerant in the transcritical cycle and the heat exchanged at high temperature by the subcritical cycle, which uses the R1234ze refrigerant. The total heat flux exchanged with the heat carrier fluid is the sum of the heat flux exchanged at the gas cooler of the CO2 transcritical cycle and the heat flux exchanged at the condenser of the high-temperature cycle working with the HFO refrigerant. As is well known, the CO2 is not suitable for every application in refrigeration, air-conditioning, and heat pump systems, and it presents some limitations in order to guarantee good energy performance. The CO2 has a vapor pressure much higher than other traditional refrigerant fluids and its critical temperature is around 31 ◦C. This last characteristic leads to the impossibility of discharging heat to the external air through condensation in a subcritical cycle, when the air temperature, or in general, the heat carrier fluid, is above the critical temperature. The main consequence is that CO2 can be used efficiently for cooling applications when heat is discharged below the critical temperature, while at higher temperatures, when the cycle works at supercritical pressures, only gas cooling is possible. Therefore, in cooling applications, the use of the ground as the heat sink is a possible and efficient solution. On the other hand, when the heat pump operates in heating mode, the HVAC (Heating Ventilation Air Conditioning) applications could result more complex because, generally, the supply temperature for the terminal units is above 30 ◦C, except for radiant systems. However, CO2 is a natural fluid, its ODP (ozone depletion potential) is null, it is not flammable and not toxic (A1), and its GWP is equal to 1. For this reason, CO2 was also used in the past when environmental advantages and safety restrictions reasons overcame the energy drawbacks. The R1234ze refrigerant is an alternative fluid to the R134a and its environmental impact is lower if compared to the latter. The main properties of the two refrigerants are summarized in Table 1. Table 1. Main properties of the refrigerants used in the heat pump [28]. CO2 (R744) R1234ze ODP GWP 0 1 0 6 Boiling Point @ 1 atm [◦C] −57 −19 Critical Temperature [◦ C] 31 109 Critical Pressure [bara ] 74 36 ASHRAE Safety Group A1 A2 The thermodynamic cycles of the heat pump are represented in Figure 3 in a temperature–entropy chart. The diagram shows, from a qualitative point of view, the operating points of the thermodynamic cycles during the heat pump operations. The letters and the numbers in the chart refer to the points reported in Figure 2, which shows the scheme of the heat pump’s high and low-temperature cycles. The properties of the main components of the heat pump are summarized in Table 2. The heat pump is a prototype because it is not available in the market yet, even though all the devices constituting the machine are employed in other applications. For example, the ejector used in the CO2 cycle is not commonly used in HVAC application, but rather in commercial refrigeration. One of the main objectives during the development of this prototype was to investigate the energy performance potential of this technology at the time of the project. In the last two years, the HVAC market started to introduce new devices and components properly developed for HVAC applications that use the CO2 as the refrigerant. These new products can certainly guarantee better results than those obtained with the technology available about 3–4 years ago and used in the prototype; therefore, the results of this study can be considered the starting point of this emerging technology. The investigated heat pump was thought to be used in plug-and-play applications, as the generation module and the hydraulic module have been included in a common box. The control of the heat pump worksPDF Image | Novel Ground-Source Heat Pump with R744
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