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Energies 2021, 14, 8238 8 of 25 These tests were also used to verify the polynomial curves describing the overall compressor efficiency, thus, the compressor power consumption. When the fixed speed compressor was considered, the experimental reading could be directly compared to the manufacturer’s data. On the other side, in the case of inverter controlled compressor, we have to account for the inverter efficiency. This was assumed to be 95%, according to the inverter data sheet. Figure 5b presents the same operating points of Figure 5a in terms of nominal against experimental electrical power consumption. For both the compressors, the power was sys- tematically underestimated by the nominal data. For each frequency lower than 50 Hz, the average error was 5.8%, which is consistent with the tolerance allowed for data declaration (5% according to EN12900:2013) and the measurement accuracy. The 60 Hz actual power was 10.5% higher than the expected. This can be ascribed to a drop in the compressor efficiency at higher speeds and/or a decrease in the inverter performances. This aspect will need further investigation. The heat flow rate at the internal heat exchanger (Equation (4)), was evaluated as the average value of the heat flow rate in the high (Equation (2)) and low pressure (Equation (3)) side of the heat exchanger. Q.HP =m. (h−h) (2) IHX CO2 6 7 Q.LP=m. (h−h) (3) IHX CO2 12 13 . HP . LP = QIHX +QIHX (4) Q. IHX Qgc(1) COPheat−pump = P (5) el,comp When the system was operating in chiller configuration, the heat flow rate at the 2 The COP of the heat pump was calculated according to Equation (5). . gas-cooler(2) was evaluated according to (Equation (6)): Q.CO2 =m. (h−h) (6) gc(2) CO2 3 4 The heat flow rate at the finned coil gas-cooler was evaluated according to Equation (7): Q. =m. (h −h) (7) gc(3) CO2 4 5 Lastly, the heat flow rate at the brazed-plate evaporator(1) was evaluated according to (Equation (8)), when the refrigerant at the evaporator(1) outlet was in a superheated state. Q.CO2 =m. (h −h) (8) eva(1) sn SL 9 When the refrigerant was in a two-phase state, the heat flow rate at the evaporator was evaluated with (Equation (9)), as the refrigerant enthalpy at evaporator outlet was not defined using pressure and temperature at point 9: Q.w =m. (h −h ) (9) eva(1) w,3 w,6 w,7 As for the evaporator(2), the heat flow rate at the evaporator was given only on the water side (Equation (10)), as the refrigerant mass flow was not available. Q.w =m. (h −h ) (10) eva(2) w,3 w,5 w,6PDF Image | Dynamic Modelling and Validation of an Air-to-Water Reversible R744
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