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Temperature heat transfer (T_Q) diagram shown in figure 12 shows the difference in heat exchange process across the heat sink in conventional and CO2 operated water cooled system, as the graph shows the cooling profile of CO2 closely matches with the heating profile of water and therefore involves less thermal losses. 2.4 CO2 heat pump water heater case studies As stated earlier CO2 is an old known refrigerant and was used in the early applications and usage declined after the discovery of synthetic refrigerants. Recent revival of the CO2 started after a proposal from Prof. Gustav Lorentzen [1], In his publication Professor Lorentzen proposed CO2 as an alternative refrigerant for automotive air conditioning (highly dominant sector for refrigerant leakage losses) and claimed that testing results with their prototype showed comparable performance with other alternatives. In the same publication Lorentzen also discussed the application of CO2 in heat pumps for water and space heating application. Petter Nekså [2] has experimentally investigated the performance of CO2 heat pump water heater, the prototype was having heating capacity of 50 kW and utilized tube in tube type heat exchanger for the gas cooler, however, evaporator was plate in shell type heat exchanger. He heated up the water from 8-60 oC at evaporation temperature of 0 oC and quoted the COP value of 4.3 (shaft power instead of power input to the motor was measured and motor efficiency was considered to be 90% ). Variation of COP with variation of evaporation temperature and set point temperature for hot water was also shown in the study. In his detailed publication, Man-Hoe Kim [13] discussed the process details and system design issues when dealing with CO2 vapor compression systems. In another study Jørn stene [8] described the experimental results for a residential CO2 heat pump system for combined space heating and hot water heating applications. The employed prototype was having heating capacity of 6.5 kW and utilized single pass tube in tube heat exchanger for the purpose of gas cooler. The gas cooler was divided/splitted into three parts for preheating of hot water, low temperature space heating and reheating of hot water. The prototype was tested in three different modes, for space heating, District heating water (DHW) heating, and for combined space and DHW heating. Different temperature sets were used for space heating and hot water set temperature of 60, 70 and 80 oC were used. The evaporation temperature was - 5 oC and average city water temperature was about 6.5 oC. Figure 13 shows the results with various combinations for hot water heating and space water heating applications with different heating capacity ratios. 11PDF Image | heat recovery from R744 based refrigeration system
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