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With reference to Section 5.1.3.2, Simultaneous Space Heating and Hot Water Heating, the DHW heating capacity ratio at optimum high-side pressure during operation in the combined mode increased when the temperature level in space heating system was increased or the set-point temperature for the hot water was reduced. At 60°C hot water temperature and 33/28oC supply temperature in the space heating system (Figure 6.8), the temperature curves for the CO2 and the water converged at the hot end of the DHW preheating gas cooler. However, by changing the supply/return temperature to 40/35oC (Figure 6.12), the water flow rate increased and the temperature profiles became divergent. By increasing the hot water temperature to 80oC, the water flow rate dropped off by about 50%, and only 30 to 60% of the heat transfer surface for the DHW preheating gas cooler was actually used for heat transfer (Figure 6.9, 6.11 and 6.13). The temperature profiles for the CO2 and the water in the DHW reheating gas cooler were also greatly influenced by the operating conditions. At 60°C hot water temperature (Figures 6.8, 6.10 and 6.12), the temperature curves diverged at the hot end of the gas cooler, and the higher the tempe- rature level in the space heating system, the more pronounced the diver- gence. At 80°C hot water temperature, the situation was quite different due to the 50% reduction in the water flow rate. Whereas the curves were convergent at 33/28 and 35/30oC supply/return temperatures in the space heating system (Figures 6.9 and 6.11), the curves were virtually parallel at 40/35oC (Figure 6.13). Regarding the space heating gas cooler, the average temperature diffe- rence during heat rejection at optimum high-side pressure dropped off quite rapidly when the temperature level in the space heating system was increased. This was the main reason for the considerable reduction in the heating capacity at high supply/return temperatures in the space heating system. Due to the relatively large heat transfer surface and the fact that the CP-value of the water flow was larger than that of the CO2 flow during all operating conditions, the temperature curves were convergent at the cold side of the gas cooler. It should also be noted that the temperature approach for the space heating gas cooler unit was also affected by the outlet water temperature from the DHW preheating gas cooler. As a general rule, the larger the water flow rate in the hot water circuit, the lower the temperature approach. 6 – Modelling 187PDF Image | Residential CO2 Heat Pump System for Combined
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