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Polish bus manufacturer started selling battery-driven electric buses with reversible R-744 heat pump systems for heating and cooling (Solaris, 2012). In 2018, Daimler launched an electric city bus with a R-744 charged MAC system; the first units will be handed over by the end of 2018 (Daimler, 2018) Environmental and fuel price concerns lead bus and coach manufacturers to pursue development of hybrid and battery driven electric vehicles. The concept makes traditional engine-driven compressors obsolete and favours self-contained hermetic systems, where the refrigerant charge and leakage rate are lower. The electric power is either supplied from on-board sources, or from engine-driven generators. These systems are not limited to electric vehicles, but can also be applied in traditional buses and coaches. Installations can be found in trolleybuses. 10.5 Energy efficiency in MAC applications Based on average annual values, HFO-1234yf as well as R-774 systems exhibit efficiencies comparable to HFC-134a for cooling and heating (Koehler, 2018; Westerloh, 2019). To achieve these performance levels in case of the use of HFO-1234yf, manufacturers either apply an IHX or larger degree of sub-cooling, while for R-744 appropriate system design (e.g. IHX) and controls are required (Zilio, 2009, Strupp, 2011, Koehler, 2018; Westerloh, 2019). 10.6 Concluding remarks Currently, more than one refrigerant is used in the car and light truck mobile air conditioning: HFC-134a remains largely adopted worldwide, while HFO-1234yf is currently the main refrigerant option in Europe and North America. The deployment of highly electrified vehicles (PHEV and BEV) in Europe, China and North America will lead to the implementation of a heat pump function. Manufacturers are working on the improvement of this feature by using cycle variations such as by using an economizer coupled with vapour injected compressors. Two German OEMs have started to apply R-744 in some of their premium models. If the cost of the R-744 system will become competitive, it may be possible that R-744 will be taken into consideration especially for highly electrified vehicles, due to its good performance when operated as a heat pump. For the Indian market, one car manufacturer has expressed interest in the adoption of HFC- 152a. Other low GWP synthetic refrigerants and refrigerant blends (e.g. R-444A, R-445A) have been investigated but it is not likely that they will be used in the near future. In existing systems, primarily designed for CFC-12 or HFC-134a, hydrocarbons (e.g. R-290) are used in some regions (e.g., Australia). At the moment, it cannot be foreseen whether or not all these refrigerants will be used in parallel in the market for a long period of time. It is also unclear whether the bus sector (where currently HCFC-22, HFC-134a, R-407C or R-410A are used) and the heavy-duty truck sector will follow these trends. 10.7 References Andersen, 2013, S.O.A., Baker J., Craig T., Kapoor S.H. 2013. The New Business Case For Secondary Loop Mobile AC Systems In Climates with High Fuel Prices and Long, Hot and Humid Cooling Seasons. ATA Mobile Air Conditioning and Thermal System Workshop, Turin 2013. 196 2018 TOC Refrigeration, A/C and Heat Pumps Assessment ReportPDF Image | Heat Pumps Technical Options
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