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7 – Discussion and Analysis It is important to emphasize that the SPF calculations for heat pump systems only should be regarded as rough estimates due to the simplified calculation method and the fixed boundary conditions. At low seasonal DHW heating capacity ratios, the state-of-the-art heat pump system was considerably more energy efficiency than that of the CO2 systems due to the poor COP of the CO2 system during operation in the SH mode. At increasing seasonal DHW heating capacity ratios, the seasonal performance of the CO2 systems was gradually improved, since an increasing part of the heating demand was covered by operation in the combined mode and the DHW mode. On the other hand, the SPF of the state-of-the-art heat pump systems dropped off quite rapidly at increasing seasonal DHW heating capacity ratios, since the COP during operation in the DHW mode was about 30% lower than that of the SH mode. At the actual operating conditions, the break-even for the prototype CO2 system occurred at a seasonal DHW heating capacity ratio around 35%, whereas the break-even for the improved and more energy efficient CO2 system was about 10 percentage points lower. Consequently, in existing houses where the DHW ratio typically ranges from 10 to 15% (ref. Section 2.2, Heating Demands in Houses), a state-of-the-art brine-to- water heat pump system will be considerably more energy efficient than a brine-to-water CO2 heat pump system for combined space heating and hot water heating. However, in new houses or in so-called low-energy houses, where the DHW ratio typically ranges from 20 to 45% (ref. Section 2.2), an optimised CO2 heat pump system may achieve the same or higher SPF than that of the most energy efficient state-of-the-art brine-to-water heat pump systems. However, the latter presupposes that: ♦ The CO2 heat pump unit covers the entire DHW heating demand, and the annual heat delivered for DHW production is minimum 25 to 30% of the total annual heat delivered from the heat pump unit. ♦ The CO2 heat pump is operated in the combined heating mode when there is a simultaneous space heating and DHW heating demand. ♦ The CO2 heat pump delivers heat to a space heating system with a relatively low return temperature (about 30oC or lower). ♦ The city water temperature is relatively low (about 10oC or lower), and the mixing of hot and cold water as well as the conductive heat transfer inside the DHW tank is at a low level. 228PDF Image | Residential CO2 Heat Pump System for Combined
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