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7 – Discussion and Analysis system and 60oC DHW temperature, the DHW heating capacity ratio was almost 70%, which means that the prototype CO2 heat pump unit practically operated as a heat pump water heater. At 33/28oC supply/return temperature and 80oC DHW temperature, the DHW heating capacity ratio was about 30%, and most of the heat was rejected to the SH system. At constant temperature level in the SH system, both the DHW heating capacity ratio and the COP for an integrated CO2 heat pump unit will diminish at elevated DHW temperatures due to the reduced water flow rate in the DHW circuit and the consequent less cool-down of the CO2 in the DHW preheating gas cooler unit. The compressor power input will also affect the COP, since the optimum high-side pressure increases slightly when the DHW temperature is increased. The higher the DHW tempe- rature, the larger the relative drop-off in the DHW heating capacity ratio. At constant DHW temperature, the COP for an integrated CO2 heat pump will diminish whereas the DHW heating capacity ratio will increase when the temperature level in the SH system is increased. The latter can be explained by the lowered mean temperature difference between the CO2 and the water in the SH gas cooler, and the consequent reduction of the heating capacity. Although the total heating capacity of the tripartite gas cooler will increase due to a larger water flow rate in the DHW circuit, the rise in the optimum high-side pressure will lead to a drop in the COP. Figure 7.2 shows the measured relative heating capacities for the three gas cooler units at optimum high-side pressure and varying temperature levels in the SH system and the DHW system. The COP and the relative heating capacities at 35/30oC supply/return temperatures for the SH system and 70oC DHW temperature did not follow the general trends in Figures 7.1 and 7.2. This was because the pressure measurements were carried out at 0.5 MPa intervals, and that the optimum high-side pressure was between 8.5 and 9.0 MPa. Hence, at the real optimum high-side pressure, the DHW heating capacity ratio and the COP would most likely have been around 0.48 and 3.65, respectively. The heat pump unit was designed for 35/30oC supply/return temperatures for the SH system and 60oC DHW temperature. When the heat pump unit was tested at the other temperature programs, the measured COP at the optimum high-side pressure was the highest possible COP that could be obtained with the actual gas cooler design. Thus, if the tripartite gas cooler had been redesigned and optimized for each temperature program, both the COP and the DHW heating capacity ratio would have increased. 210PDF Image | Residential CO2 Heat Pump System for Combined
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