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7 – Discussion and Analysis 7.1.2 Energy Efficiency 7.1.2.1 The COP vs. the Temperature Levels in the Space Heating and DHW Systems The coefficient of performance (COP) of an integrated brine-to-water CO2 heat pump unit for combined space heating and hot water heating is strongly affected by the temperature levels in the space heating (SH) system and the domestic hot water (DHW) system. The experimental and simulation results regarding the impact of the temperature levels on the COP of the prototype CO2 heat pump unit, can be summarized as follows: Simultaneous Space Heating and Hot Water Heating (Combined Mode) ♦ The highest COP, approx. 4.0, was measured at 33/28°C supply/- return temperatures for the SH system, 60°C DHW temperature, 7oC city water temperature and 8 MPa high-side pressure. ♦ At 60, 70 and 80oC DHW temperature, the COP dropped off by approximately 3, 6 and 9%, respectively, when the supply/return temperatures in the SH system was increased from 33/28 to 40/35oC. ♦ At 33/28, 35/30 and 40/35oC supply/return temperatures in the SH system, the COP dropped off by approximately 10, 12 and 16% when the DHW temperature was altered from 60 to 80oC. ♦ At 60°C DHW temperature and 35/30 to 40/35oC supply/return temperatures for SH system, the COP dropped off by approximately 0.8% per oC rise in the inlet water temperature for the DHW pre- heating gas cooler. At 80oC DHW temperature, the corresponding figures were about 0.5% and 0.6%, respectively. Hot Water Heating Only (DHW Mode) ♦ The highest COP, approx. 3.8, was measured at 60°C DHW tempe- rature, 5.5oC city water temperature and 9 MPa high-side pressure. ♦ The COP dropped off by approximately 6% when the set-point was altered from 60 to 70oC (i.e. ~0.6% per °C). The maximum COP at 80oC was not found due to operational limitations of the test rig. ♦ At 60oC and 80oC DHW temperature, the COP dropped off in ave- rage by approximately 1.1% and 0.8% per oC rise for the inlet water temperature to the DHW preheating gas cooler. 206PDF Image | Residential CO2 Heat Pump System for Combined
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