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Mobile Heat Pump Exploration Using R445A and R744 Andrew MUSSER1*, Pega HRNJAK1 1Creative Thermal Solutions, Inc. Urbana, IL, USA (217) 344-7663, andrew.musser@creativethermalsolutions.com * Corresponding Author ABSTRACT The increased usage of hybrid and electric vehicles where waste heat availability is limited has spurred research and development of mobile heat pump systems. Many options exist for heat pump system architectures and refrigerants to be used. Currently R134a use is prevalent in vehicle air conditioning systems but offers poor heat pump performance at low ambient temperatures. Two refrigerants will be explored in this paper, R744 and R445A. Both of these refrigerants are getting attention in vehicle A/C systems because of their relatively low GWP but each offers benefits over R134a in heat pump systems as well. Both refrigerants operate at higher pressures which improves the performance at low ambient temperatures in part due to higher compressor inlet refrigerant densities. R134a (and R445A to a lesser extent) also suffer from the drawback of going into sub-atmospheric pressure operation at temperatures commonly seen in vehicles. Data will be shown for multiple system architectures comparing these refrigerants to R134a. Advantages and disadvantages of each refrigerant will be shown. System control and optimization is important to get the maximum performance from each refrigerant and system. Control exploration will be presented for each alternative refrigerant. 1. INTRODUCTION Electric vehicle range is greatly limited in cold ambient conditions by the heating requirements for the cabin and batteries of the vehicle. The most prevalent form of heating in electric vehicles is electric resistance heat. The maximum coefficient of performance (COP), defined by the heating capacity divided by the power input, for resistance heating is 1. Implementing a heat pump system can more than double or triple the COP thus greatly increasing the driving range of an electric vehicle. Some electric vehicles on the road today have heat pumps or heat pump options such as the BMW i3, Renault Zoe, and Nissan Leaf. These heat pumps use low pressure refrigerants commonly used in vehicle air conditioning such as R134a and R1234yf. Both refrigerants have the limitation of low performance and sub-atmospheric pressure operation at low ambient temperatures at or below -20°C. R1234yf has been introduced as a drop-in alternative to R134a due to its low GWP (4 compared to 1430). Another low GWP drop-in alternative, R445A (GWP of 135) has been explored and has advantages in heat pump operation due to higher pressures. R744 (GWP of 1) has been shown to be the most promising fluid for heat pump operation among the refrigerants listed above but is not a drop-in due to the requirement for components able to withstand significantly higher pressures. Both R445A and R744 are under the European Union regulatory limit for GWP of 150 which is generally accepted worldwide as the threshold for classifying as low-GWP refrigerants. R445A is a ternary refrigerant mixture of R1234ze(E), R134a, and R744 in the nominal proportions of 85%/9%/6% respectively. For the purposes of this study, it was not feasible to test with R445A, thus an alternative was used to simulate the properties of R445A. This alternative was chosen to consist of the same percentage of R744 at 6% with the balance being R134a (94%). The proportion of R1234ze(E) in R445A is present to reduce the GWP and has similar performance to R134a thus replacing the R1234ze(E) with R134a was not expected to have a large impact on the overall performance. The 94% R134a, 6% R744 blend will be referred to as the ‘R134a/R744 blend’ for the purposes of simplicity in the remainder of this paper. 15th International Refrigeration and Air Conditioning Conference at Purdue, July 14-17, 2014 2509, Page 1PDF Image | Mobile Heat Pump Exploration Using R445A and R744
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