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2509, Page 8 Control optimization was also performed on the R744 heat pump system. Unlike the hybrid system with a variable speed electric compressor used with the low pressure fluids, the belt driven compressor used for the R744 experiments had a fixed speed dependent on the simulated vehicle speed. The most influential control variable explored was the expansion valve. Park et al. (1999) have shown using similar systems that maximum COP and capacity can be obtained for a transcritical R744 mobile A/C system by regulating high-side pressure. A relationship between maximum COP and high-side pressure was determined based on Park’s study. Giannavola (2001) further showed capacity and COP/HPF maximums were obtainable by regulating high side pressure in both A/C and H/P transcritical operation. The study presented here and originally shown by Musser (2005) extended this work to low temperatures where subcritical operation is expected for the heat pump system. Figure 9 shows high side pressure control exploration for -20°C ambient temperatures and multiple indoor air temperatures. As expected, the compressor power increases as the pressure increases. However, due to the loss of compressor efficiency at higher pressure ratios, the refrigerant mass flow rate and evaporator capacity reaches a maximum and then begins to drop. The initial evaporator capacity increase is due to increased subcooling caused by better condenser performance from higher temperature difference between refrigerant and air side but the loss in mass flow eventually outweighs the gains. The condenser or heating capacity is the sum of the compressor power and evaporator capacity thus it increases to the point at which the evaporator capacity loss becomes greater than the compressor power addition. This heating capacity maximum occurs at subcritical pressures for indoor temperatures of -10°C and -20°C for the -20°C ambient condition. The COP/HPF maximums occur at lower pressures than the capacity maximums due to relatively low heating capacity gains compared to the elevated compressor power input as the pressure increases. Thus, there is a trade-off between maximizing heating capacity and efficiency in subcritical operation and at these low temperatures capacity would likely be chosen over efficiency during the initial warm-up period. 15th International Refrigeration and Air Conditioning Conference at Purdue, July 14-17, 2014PDF Image | Mobile Heat Pump Exploration Using R445A and R744
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