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7 – Discussion and Analysis Hot water heating has been regarded as one of the most energy efficient applications for the transcritical CO2 heat pump process due to the virtually perfect temperature fit between the supercritical CO2 and the water, and the consequent low exergy loss during heat rejection. When heat is given off at three different temperature levels in a tripartite gas cooler, the same temperature approach can be achieved but at a lower high-side pressure. Consequently, at low return temperatures in the SH system, the COP in the combined mode can be even higher than that of the DHW mode. The COP of the CO2 heat pump unit during operation in the SH mode was considerably less than that of the other modes. This was mainly the result of the bad temperature fit between the CO2 and the water, and the relatively high CO2 temperature before the throttling valve. In the SH mode, an integrated CO2 heat pump unit will generally benefit less from a low temperature level in the SH system than that of conventional heat pump units, where heat is given off by means of condensation of the working fluid. 7.1.2.2 The COP vs. the Evaporation Temperature The evaporation temperature affects the heating capacity of the tripartite gas cooler, the input power to the compressor and consequently the COP of the heat pump unit. The experimental results regarding the impact of the evaporation temperature on the COP of the prototype CO2 heat pump unit can be summarized as follows: ♦ In the combined mode and the DHW mode, the COP dropped off by approximately 6 and 10%, respectively, when the evaporation tempe- rature was reduced from -5 to -10oC. ♦ In the SH mode, the COP dropped off and rose by approximately 7.5 and 5.5%, respectively, when the evaporation temperature was altered from -5 to -10oC and from -5 to 0oC. The experiments were carried out at 35/30oC supply/return temperatures for the SH system and 60oC DHW temperature. Assuming a linear relationship between the evaporation temperature and the COP, the proto- type CO2 heat pump unit would have achieved a COP of approximately 4.2 and 4.1 in the combined mode and the DHW mode, respectively, if the evaporation temperature had been increased from -5 to 0oC. 208PDF Image | Residential CO2 Heat Pump System for Combined
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