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5 – Experimental Results According to Figure 5.38, the comparison of the measured COPs for the different heating modes can be summarized as follows: ♦ At 33/28°C supply/return temperatures in the space heating system and 60 to 80oC DHW temperature, the COP in the combined mode was about 5 to 10% higher than that of the DHW mode, and the higher the hot water temperature, the larger the difference. This was mainly a result of the 1 to 2.5 MPa lower optimum high-side pressure and with that 10 to 20% less power input to the compressor. ♦ At 35/30oC supply/return temperatures and 60 to 70oC hot water temperature, the COPs in the combined mode and the DHW mode were virtually the same. However, at 80oC hot water temperature the COP was about 7% higher in the combined mode. ♦ At 40/35oC supply/return temperatures, the COP in the combined mode and the DHW mode were practically identical at all hot water temperatures. This was owing to similar optimum high-side pressures at 60oC hot water temperature, and the fact that the nega- tive impact of the higher high-side pressure in the DHW mode at 70 to 80oC hot water temperature was outweighed by a lower tempera- ture approach. ♦ The COP in the SH mode was considerably lower than that of the COP in the combined mode, due to a very bad temperature fit in the gas cooler and a relatively high CO2 outlet temperature (ref. Figure 5.34). The higher the temperature level in the space heating system, the larger the relative difference in COP. At 60oC hot water tempera- ture and 33 to 40oC supply temperature, the COP in the space heating mode was 21 to 28% lower than that of the combined mode. The corresponding figures at 70 and 80oC hot water temperature were 16 to 21% and 12 to 14%, respectively. Optimum High-Side Pressure As demonstrated in the previous sections, the temperature levels in the space heating and hot water systems had a major impact on the optimum high-side pressure for the different heating modes. Figure 5.39 presents the COP as a function of the high-side pressure at 60, 70 and 80°C hot water temperature and 33/28, 35/30 and 40/35oC supply/return tempera- tures for the space heating system. 144PDF Image | Residential CO2 Heat Pump System for Combined
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