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Energies 2021, 14, 4103 5 of 28 2.1. Design and Operation Figure 1 illustrates the SC integrated CO2 unit. The evaporation level consists of four ambient air fin-and-tube evaporators and two glycol AC heat exchangers, which function to supply the heating and cooling load, respectively. Simultaneous operation with both air and glycol evaporators is limited, as SH and AC cooling loads are rarely concurrent. The cooling demand of the building is realized by employing the plate evaporators, EVAPAC,1 and EVAPAC,2 in Figure 1, which supply chilled glycol to the AC units in the building. Each evaporator’s thermostatic expansion valve controls the mass flow of CO2 according to the superheat at the exit of the respective unit. The mass flow of glycol is controlled to achieve the AC setpoint temperature. DHW SUPPLY SPACE HEAT GCSH MV DV1 P1 Tn ... T2 T1 DHW STORAGE CITY WATER GCDWH, 2 1 DV2 MV3 LIQUID RECEIVER AC COOLING FGV COMPRESSOR RACK 1 2 3 4a (VSD) EVAPAC, 1 EVAPAC,2 Figure 1. Simplified representation of the standard integrated CO2 concept, equipped with single- stage compression (SC). High-pressure lines are indicated in red, intermediate pressure lines in yellow, and low-pressure lines in dark blue. The fin-and-tube heat exchangers are exclusively connected to either the high-pressure side (red line) for gas coolers operation or the low-pressure line (dark blue) for evaporation at low temperature (LT). The distribution lines through the heat exchangers are switched between the different pressure levels, dependent on the mode of operation. Traditionally, a design selection with separate evaporators and gas coolers is preferred for simplicity and heat transfer considerations. However, due to advances within heat exchanger develop- ment and operation, several suppliers offer combined units to reduce the investment cost. The air heat exchangers are employed according to the heating demand of the building when operating in evaporator mode. Airflow through the evaporators is controlled by fans equipped with high-efficiency brushless DC motors. The CO2 evaporation pressure is controlled by the thermostatic expansion valves, which secure a superheat at the exit of each evaporation unit. The superheat is fixed to a minimum, such that enhanced heat transfer is achieved in the heat exchangers. The suction accumulator and the internal heat exchanger, FGT and IHX1 in Figure 1, respectively, ensure no liquid carryover to the compressors. EVAPORATORS, EVAP / AIR GAS COOLERS, GCAIR HPV IHX 1 FGT GC DWH, 1PDF Image | Evaluation of Integrated Concepts with CO2 for Heating
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