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Energies 2021, 14, 4103 8 of 28 The parallel compressor section controls the latter in order to meet the total cooling demand from the building. EVAPAC,2 is installed at the LT evaporation level and ensures cooling of the chilled water to setpoint conditions. The HPV in Figure 2a is replaced by a ejector block in Figure 2b. The ejector is connected to the high-pressure side and recovers the expansion work in the high-pressure stream to lift the pressure of refrigerant from the LT pressure level. The vapor ejector operates in parallel to the LT compressors and is installed downstream of IHX1, and so provides a dual benefit. First, it ensures a high value of superheat at the suction port of both the ejector and the compressor, which allows the evaporators to operate with a low superheat. Second, additional cooling downstream of the gas cooler is provided to reduce expansion losses. Typically, applications of ejectors demonstrate the largest benefits when the gas cooler outlet temperature is elevated. Such a scenario transpires in both winter and summer when either SH or AC cooling loads are high and DHW demands are low. 2.3. Operational Modes The system designs have been established to provide flexibility and a high degree of operational freedom independent of the specific mode of operation, e.g., summer and winter. 2.3.1. Winter Mode The air evaporators shown in Figure 1, EVAPair, are employed based on SH demand. Vapor is sucked from EVAPair through IHX1 by the LT base compressors. The compressor capacity is mainly controlled by the SH demand, as additional heat for DHW is continu- ously recovered as a byproduct if needed. Neither of the AC cooling evaporators is active during winter mode. For the the system solutions shown in Figure 2, parallel compressors are employed to remove flash gas from the system and control the pressure of the liquid receiver. The number of compressors employed at LT and INT pressure levels is determined based on heating demand. Vapor from the liquid receiver is superheated in IHX2 before compression. The ejector in Figure 2b introduces expansion work recovery from the high pressure level. Thus, a portion of the required compressor capacity is moved from the LT section to the INT section, reducing the total work of the system. 2.3.2. Summer Mode The base compressor capacity is controlled by the cooling load, as the LT pressure is regulated to meet the requested AC cooling demand of the building. The mass flow rate of chilled water is controlled to meet the setpoint at the outlet of the AC evaporators. For the AC-chiller arrangements presented in Figure 2, two separate pressure levels are used to chill down the liquid. After the first stage of cooling, chilled water from EVAPAC,1 is directed to the second AC evaporator, EVAPAC,2, for further cooling until the setpoint is reached. EVAPAC,2 is installed in parallel to the air evaporators, which are generally not employed during summer mode. The pressure of EVAPAC,2 is controlled by the LT base compression block, which during cooling mode operates 4 to 6 bar below the INT level. The parallel compressor(s) will increase capacity and thus reduce INT pressure if additional cooling is needed. Simultaneously, the AC chilled water pumps will increase the mass flow rate through the heat exchangers. In the case of the ejector-supported system displayed in Figure 2b, a large portion of the CO2 is lifted from LT to the INT pressure level. The DHW storage functions as the only useful heat sink to the system during summer mode, as SH demand is lacking during high ambient temperature operations. If removal of excess heat is needed, DV1 directs the flow towards GCair for rejection towards the ambient air. 3. Methodology 3.1. Numerical Model Detailed models of the thermal systems were created in the object-oriented program- ming language Modelica. The programming environment Dymola 2018 was applied toPDF Image | Evaluation of Integrated Concepts with CO2 for Heating
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