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Figure 6: Control Optimization of the R134a/R744 blend at Condition M10a The above results show the favorable performance of R445A compared to R134a in heat pump operation. Significant study has been performed on R445A as a drop-in for air conditioning as part of a SAE cooperative research program: MAC Refrigerant Blend Cooperative Research Program (MRB CRP). The results have been presented in the white paper, Development and Evaluation of AC5 and AC6 Refrigerants for MAC Applications (SAE, 2013). The study has shown that R445A as a drop-in has similar performance to R134a in A/C operation at high load conditions while there is a loss in COP of 5% to 10% at part load conditions. With the selection of an optimized internal heat exchanger and separated receiver it is possible to improve the A/C performance of R445A relative to R134a at part load and achieve equivalent or better performance relative to R1234yf. Thus with R445A significant performance gains are achievable in heat pump operation while minor shortcomings in A/C operation have been observed. 5. R744 EXPERIMENTATION AND RESULTS Carbon Dioxide was first explored for use in automotive air conditioning in the 1990s and early 2000s as a replacement for R134a due to the greatly reduced global warming potential (1 compared to 1430). Despite comparable cooling capacities and COP values, R744 was eventually surpassed by new drop-in refrigerants such as R1234yf which require very few changes on the component level compared to R744. Heat pump operation was explored as part of the initial A/C research for R744 at the University of Illinois Air Conditioning and Refrigeration Center (Musser 2005) and those results have been revisited in this paper for comparison to R134a and the R134a/R744 blend. R744 has recently gained traction again in automotive air conditioning, specifically in Europe, due to flammability concerns with R1234yf. If R744 is implemented for vehicle air conditioning, usage in vehicle heat pumps is expected to soon follow. R744 has been used in heat pumps for hot water heating beginning in the mid 1990s and is prevalent in Japan. Use in commercial hot water heating is also being explored (Petersen et al, 2012). R744 is also used in many other cooling applications on the market today ranging from light commercial to supermarket refrigeration. The experimental facility used for the automotive R744 heat pump exploration consisted of 2 calorimeters and is similar in function to the facilities described above. The vehicle components used were designed for a mid-sized sedan as with the other refrigerants discussed but in this case a belt-driven compressor was used. The system consisted of components primarily designed for A/C operation and not optimized for use in heat pump operation. The system used only 2 heat exchangers and is presented in heat pump configuration in Figure 7 below. 15th International Refrigeration and Air Conditioning Conference at Purdue, July 14-17, 2014 2509, Page 6PDF Image | Mobile Heat Pump Exploration Using R445A and R744
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