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8 – Conclusions and Suggestions for Further Work 230 ♦ In contrary to conventional heat pump systems for combined space heating and DHW heating, the integrated CO2 heat pump system achieves the highest COP in the combined heating mode and the DHW heating mode, and the lowest COP in the space heating mode. Hence, the larger the annual DHW heating demand, the higher the SPF of the integrated CO2 heat pump system. ♦ The lower the return temperature in the space heating system and the lower the DHW storage temperature, the higher the COP of the inte- grated CO2 heat pump. A low return temperature in the space heating system also results in a moderate DHW heating capacity ratio, which means that a relatively large part of the annual space heating demand can be covered by operation in the combined heating mode, where the COP is considerably higher than in the space heating mode. ♦ During operation in the combined heating mode and the DHW heating mode, the COP of the integrated CO2 heat pump is heavily influenced by the inlet water temperature for the DHW preheating gas cooler unit. The lower the inlet temperature, the higher the COP. The CO2 system will therefore achieve the highest COP at low city water temperatures, and when there is negligible mixing and minimum conductive heat transfer between the hot and cold water in the DHW tank during the tapping and charging periods. ♦ The COP for the integrated CO2 heat pump is generally more sensitive to variations in the compressor efficiency than that of con- ventional brine/water-to-water heat pump systems. It is therefore of particular importance to apply a high-efficiency compressor. ♦ At each operating mode and temperature programme, there will be an optimum gas cooler (high-side) pressure that leads to a maximum COP for the integrated CO2 heat pump. However, at moderate DHW temperatures, the heat pump can be operated at constant high-side pressure in all heating modes with only a minor reduction in the COP. This is favourable, since it simplifies the operation of the system and reduces the first cost. ♦ During operation in the combined heating mode, the COP for the integrated CO2 heat pump may be higher than in the DHW heating mode due to similar temperature approaches at the cold outlet of the gas coolers and lower optimum high-side pressure. The higher the DHW temperature, the larger the COP difference for the operating modes.PDF Image | Residential CO2 Heat Pump System for Combined
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