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for the space heating system, and the lower the set-point temperature for the DHW system, the larger the DHW heating capacity ratio. At 40/35 8C supply/return tempera- ture for the space heating system and 608C DHW temperature, the DHW heating capacity ratio was almost 70%, which means that the heat pump was practically operating as a heat pump water heater. On the other hand, at 33/28 8C supply/return temperatures in the space heating system and 80 8C DHW temperature, the DHW heating capacity ratio was about 30%, and more than 2/3 of the heat was given off to the space heating system. Fig. 5 shows the measured maximum COP at 60 8C DHW temperature and various supply/return temperatures for the space heating system when operating in the combined mode, DHW mode and SH mode. The numbers at the bottom of the combined mode bars display the DHW heating capacity ratio. While the COPs during testing in the combined mode and DHW mode were almost identical, the COPs in the SH mode were roughly 20–30% lower than that of the combined mode. The largest COP difference was measured at the highest temperature level in the space heating system. By increasing the DHW temperature to 80 8C, the COP in the combined mode and the DHW mode dropped by roughly 15%, and the average COP difference between the combined mode and the SH mode was reduced from about 25 to 15%. The reduction in the COP was a result of the lower water flow rate in the DHW circuit. The measured overall isentropic efficiency for the prototype rolling piston compressor ranged from about 0.52 to 0.55 at 6000 rpm. An exergy analysis of the prototype CO2 heat pump system showed that the relative exergy loss for the compressor constituted about 40% in the SH mode and about 55% in the combined mode and DHW mode [7]. This is consider- ably higher than that of conventional brine-to-water heat pump systems. Consequently, for integrated CO2 heat pumps it is of particular importance to use a high- efficiency compressor. 5. Simulation results An in-depth analysis of the DHW storage tank proved that conductive heat transfer between the DHW and the cold city water in the tank during the tapping and charging periods may result in a considerable increase in the average inlet water temperature for the DHW preheating gas cooler unit (Fig. 1, gas cooler unit A) [7]. Inevitable mixing of hot and cold water in the tank will lead to a further increase in average inlet water temperature. Fig. 6 shows the simulated relative COP for the prototype heat pump in the combined mode at varying inlet water temperature to the DHW preheating gas cooler unit. The supply/return temperatures for the space heating system were 35/30 or 40/35 8C, the DHW temperature was 60 or 80 8C, and the high-side pressure for each temperature program was kept constant at the measured optimum value (Section 4). The reference temperature for the inlet water was 5 8C, which corresponds to a relative COP of 100% at the different operating conditions. The inlet water temperature had a significant impact on the COP for the CO2 heat pump unit during operation in the combined mode, since it governed the maximum possible cool-down of the CO2 in the DHW preheating gas cooler unit. As an example—by increasing the inlet water temperature from 5 to 208C, the COP at 35/30–608C dropped by approximately 15%. In the DHW mode, the COP was even more sensitive to variations in the inlet water temperature, since the entire heating capacity for the heat pump was used for DHW production. The relative drop in COP was smaller at 80 8C hot water temperature than at 60 8C due to the lower water flow rate in the hot water circuit. The impact of the conductive heat transfer inside the DHW storage tank on the COP of the prototype CO2 heat pump was simulated at different operating conditions [7]. As an example—at initial city water and DHW temperatures of J. Stene / International Journal of Refrigeration 28 (2005) 1259–1265 1263 Fig. 5. The measured maximum COP at 60 8C DHW temperature and various supply/return temperatures in the space heating system during operation in the combined mode, DHW mode and SH mode. Fig. 6. The simulated relative COP in the combined mode as a function of the inlet water temperature and varying set-point temperatures for the space heating and DHW systems.PDF Image | Residential CO2 heat pump system space and hot water heating
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