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Energies 2021, 14, 4103 22 of 28 Table 8. Net present value (NPV) and discounted payback period (DPP) for selected cities with the EJ design at different electricity prices when applying the leveled and aggressive charging strategy. Strategy Leveled charging Aggressive charging Variable/Location Stockholm Copenhagen Tromsø Helsinki Munich Rome Athens Madrid Stockholm Copenhagen Tromsø Helsinki Munich Rome Athens Madrid NPV at Selected Electricity Prices [k€] 0.06 0.14 0.20 0.10 0.77 1.27 0.07 0.71 1.18 0.15 0.90 1.46 0.12 0.82 1.35 0.07 0.71 1.19 0.00 0.55 0.96 <0 0.50 0.89 0.02 0.59 1.02 0.07 0.71 1.19 0.05 0.65 1.11 0.13 0.84 1.38 0.10 0.77 1.27 0.05 0.66 1.12 <0 0.50 0.89 <0 0.45 0.82 0.00 0.55 0.96 DPP at Selected Electricity Prices [years] 0.06 0.14 0.20 10.8 3.7 2.5 11.7 4.0 2.7 9.2 3.3 2.2 10.1 3.5 2.4 11.6 3.9 2.7 14.9 4.7 3.2 15.0 5.0 3.4 13.8 4.5 3.0 11.5 3.9 2.6 12.5 4.1 2.7 9.7 3.4 2.3 10.7 3.6 2.5 12.4 4.1 2.7 15.0 4.9 3.3 15.0 5.3 3.5 14.9 4.6 3.1 5. Conclusions In this paper, three different designs of integrated CO2 systems with thermal storage for hotels were evaluated through an energetic and economic analysis. The investigated designs were standard single-stage compression (SC), parallel compression (PC), and ejector-supported parallel compression (EJ). The performance of the systems was numer- ically investigated by implementing two separate DHW charging strategies: aggressive and leveled charging. In addition, variations in loads, ambient temperatures, and setpoints were applied to each numerical model. Evaluations of the annual efficiency, emissions, energy performance, net present value, and discounted payback period were carried out at eight different locations, ranging from Scandinavia to the Mediterranean. The main conclusions drawn from this investigation are as follows. • The EJ design demonstrated enhanced annual performance, followed by PC and SC, at all locations independent of charging strategy. Annual COPs of 4.27 to 5.03 were achieved at the location in central Europe and Scandinavia. In the Mediterranean loca- tions, Annual COPs in the range of 5.40 to 5.70 were obtained. Considerable reductions in both related emissions and energy consumption were achieved at all locations. • The highest annual COPs were achieved when applying the leveled charging strategy, independent of design. An increase in COPs of up to 7.3% was attained compared to the aggressive charging strategy. Thus, control of the DHW charging is a larger influencing factor on performance than system designs. • DPP and NPV for the Scandinavian locations were found to be between 6.3 to 7.7 years and 0.23 to 0.34 M€ at typical Scandinavian electricity prices. Hotels in temperate and Mediterranean climates obtained DPPs of approximately 3 and 4.5 to 7.5 years, respectively, and NPVs in the range of 0.25 to 0.95 M€. Thus, integrated heating and cooling systems with CO2 can be an efficient, cost-effective and environmentally friendly solution for hotels located in temperate and Mediterranean locations. Integrated CO2 systems have proven to be efficient and sustainable alternatives for hotel applications. However, high investment cost decelerates the rate of installation for these type of solutions. History has shown that potential harmful refrigerants are eventually heavily regulated and even banned. As a result, HFO alternatives may pose an economic risk for hotel owners. Thus, the somewhat high investment costs of integrated CO2 systems can be justified. Further research is necessary to establish integrated CO2PDF Image | Evaluation of Integrated Concepts with CO2 for Heating
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