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M.-H. Kim et al. / Progress in Energy and Combustion Science 30 (2004) 119–174 155 mode, but that is not a feasible option for automobiles because the small volume of air in the passenger compart- ment would cool immediately. Connecting to another heat source for defrosting may also be problematic, especially after a series of short trips during which the engine never reaches normal operating temperature. On the other hand, during normal operation, the heat pump needs only to provide supplemental capacity for a short period after startup, while the engine is warming up. After that, sufficient heat should be available for defrosting; the only problem is how to transfer it. Other options for avoiding frosting, such as obtaining heat from the engine coolant, have their own unique difficulties: for example, causing the engine to operate at suboptimal temperatures and exceed air pollutant emission standards. These difficulties might be avoided through careful design in the future, by integrating and improving thermal management within the engine. Hammer and Wertenbach [147] showed test data for an Audi A4 car with 1.6l gasoline engine, comparing a standard heater and a CO2 heat pump system based on engine coolant as heat source. Fig. 47 shows measured air temperatures at foot outlet nozzles and passenger compartment temperatures using standard heater core (‘production’), and a heat pump system (without heater core). The more rapid heating up with heat pump is clear, with almost 50% reduction in the heating-up time from 2 20 to þ 20 8C. Since the heat pump used engine coolant as heat source, the possible risk of extended heating-up time for the engine was of some concern. Measurements showed that owing to the added load on the engine by the heat pump compressor, the heating-up time was in fact slightly reduced even when heat was absorbed from the coolant circuit. Despite the technical challenges, the ability of a heat pump to provide ‘instant heat,’ and the capability of a CO2 heat pump to deliver that heat at higher temperatures while moving less air, create value that may warrant development of more complex automotive climate control systems. This feature, combined with the long-term need for increasing vehicle efficiency, is driving ongoing research in both universities and industry. Moreover, the emergence of electric and hybrid vehicles is causing the scope of the investigations to be expanded to reconsider electrically driven hermetically sealed heat pumps. 8.3. Residential cooling The first assessment of transcritical CO2 systems for residential air conditioning was done by simulating operation of an Asian-style ductless minisplit system, comparing CO2 to a baseline R-22 system [148]. Evaporator temperatures were higher in the CO2 system, and very small approach temperatures were estimated for the CO2 gas cooler. The mechanically expanded round-tube heat exchangers were designed within the same core dimensions and air-side pressure drop in both systems. The effects of pressure drop, particularly in the evaporator and suction line of the R-22 system and the superheat characteristics of the expansion valve, gave cooling COPs (summer operation) that were similar in both systems, even at high ambient temperatures. An extensive set of experiments was conducted on a prototype North American-style ducted split air condition- ing system. The baseline R-410A system selected was the most efficient commercially available, and the CO2 Fig. 47. Measured air temperatures in during start-up of an Audi A4 test vehicle (production) and same car with CO2 heat pump (‘heat pump’) [147].PDF Image | CO2 Vapor Compression Systems
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