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Energies 2021, 14, 4103 17 of 28 charging strategy, explained by the elevated gas cooler outlet temperature during this mode of operations. 100 80 60 40 20 0 SC PC EJ Compressor 1 Compressor 2 Parallel Base Compressor 3 Compressor 4 -15 -10 -5 0 5 10 15 20 25 30 35 Ambient temperature [°C] Figure 9. Electric power consumption and arrangement of the compressors when applying the leveled charging strategy. The three different bars at each temperature interval represents the designs of SC, PC, and EJ. 0.4 0.3 0.2 0.1 essive charging 0 -15 -10 -5 0 5 10 15 20 25 30 35 Ambient temperature [°C] - Leveled charging - Aggr - Leveled charging - Aggressive charging Figure 10. Ejector efficiency (η) and entrainment ratio (ω) for the EJ design when applying the leveled and aggressive charging strategy. As illustrated in Figure 7, a constant DHW demand pattern was selected for this investigation. However, fluctuations will occur in real-life systems due to variation in hotel guest load, which will highly influence the magnitude of DHW demand peaks. Thus, it is likely that the CO2 system is occasionally forced to operate with aggressive charging to fulfill excessive DHW demands. An alternative scenario is heat pump operations during periods when the guest load is low. In these instances, a surplus of stored DHW will result in periods of operation without DHW production, and as a consequence, elevated gas cooler outlet temperatures. Therefore, the advantages of the ejector are likely more prominent than what was achieved in this investigation. This may speak for applying ejector technology in CO2 heat pump and chiller systems for hotels, despite the fact that the benefits and influence on COP are limited when applying the leveled charging strategy. 4.2. Annual Energy Consumption and Environmental Impact The annual COP and energy consumption of the investigated solutions for different locations are listed in Table 5. The EJ design demonstrated superior performance, followed by PC and SC for all locations independent of charging strategy. The COP at the locations were improved by 3.1% (Copenhagen) to 4.1% (Athens) when comparing EJ to SC (leveled charging strategy). Similar results were obtained with the aggressive charging strategy, in which the COP at each location increased by 4.7% (Stockholm) to 6.9% (Athens) with the EJ design. [-] / [-] Power [kW]PDF Image | Evaluation of Integrated Concepts with CO2 for Heating
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