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Energies 2019, 12, x FOR PEER REVIEW Energies 2019, 12, 457 14 of 33 Energies 2019, 12, x FOR PEER REVIEW 16 of 35 14 of 33 Figure 4. The ejector efficiencies for the unit with different diameters (mm) and rotational speeds (RPM) at the inlet duct of the motive nozzle. Reproduced with permission from [96]. Elsevier, 2016. Figure 4. The ejector efficiencies for the unit with different diameters (mm) and rotational speeds Figure 4. The ejector efficiencies for the unit with different diameters (mm) and rotational speeds (RPM) at the inlet duct of the motive nozzle. Reproduced with permission from [96]. Elsevier, 2016. In addition, the adjustable ejector configuration was found to enhance the system COP up to (RPM) at the inlet duct of the motive nozzle. Reproduced with permission from [96]. Elsevier, 2016. 30% compared to the fix-geometry ejector [97]. This was attributed to the fact that the adjustable In addition, the adjustable ejector configuration was found to enhance the system COP up to 30% ejector can provide a flexible control over the mass flow rate, which is the key to performance compared to the fix-geometry ejector [97]. This was attributed to the fact that the adjustable ejector In addition, the adjustable ejector configuration was found to enhance the system COP up to improvement. Lie et al. [98] performed a study to analyze the performance of an adjustable ejector can provide a flexible control over the mass flow rate, which is the key to performance improvement. 30% compared to the fix-geometry ejector [97]. This was attributed to the fact that the adjustable for aLsiemeut altla. n[9e8o]upserhfoeramtiendgasntdudcyootolianngalaypzpeltihceatpieornfo.rTmhaenycefoufnandathdjautsatanbolepetjiemctuormfohreaastimngulatanndeocuosoling ejector can provide a flexible control over the mass flow rate, which is the key to performance heating and cooling application. They found that an optimum heating and cooling performance can performance can be achieved by regulating the ejectors’ internal geometries. Xu et al. [99] made an improvement. Lie et al. [98] performed a study to analyze the performance of an adjustable ejector be achieved by regulating the ejectors’ internal geometries. Xu et al. [99] made an effort to optimize effort to optimize the high-side pressure through an adjustable ejector (Figure 5), which uses a stepper forasimtuhletahnigeho-suidsehperaetsisnugreatnhrdoucgoholaingadajupsptalbicleateijoecnto.rT(hFeigyufroeu5)n,dwhthicahtuasnesoapsttiemppuemrmhoetaotrimngovaingdcooling motor moving a needle forward and backward to adjust the nozzle throat area. The study showed a needle forward and backward to adjust the nozzle throat area. The study showed that such an performance can be achieved by regulating the ejectors’ internal geometries. Xu et al. [99] made an that such an ejector could regulate the throttling area for an optimal high-side pressure. The ejector could regulate the throttling area for an optimal high-side pressure. The optimized pressure effort to optimize the high-side pressure through an adjustable ejector (Figure 5), which uses a stepper optimized pressure has a positive effect on the system performance and outweighs the low ejector has a positive effect on the system performance and outweighs the low ejector efficiency. Additionally, motoefrfimcieonvciyn.gAadndeiteiodnlealflyo,rwXaurdetaanld. [b9a9c]kdweavredloptoedadajucsotntthroelnsotrzaztlegythrtoamt axreima.izTehethsetuCdOyPshboywed Xu et al. [99] developed a control strategy to maximize the COP by correlating the CO2 pressure and thatcosurrcehlataingetjheectCorO2copurelsdsuregaunladtetemthpeertahturorettlaint gtheargeas fcorolearnexoipt.tiAmamluhltig-vha-rsiiadbelepardejsusutarbel.eThe temperature at the gas cooler exit. A multi-variable adjustable ejector controller [100] and an on-line ejector controller [100] and an on-line quasi-cascade controller [101] are among the other control optimizeqduapsir-ecasscuarde choanstraollpero[s1i0t1iv] earefafmecotngonthethoethesrycsotnemtrolpseyrstfeomrms faornocpetiamnidzinogutthwe peeigrfhosrmtahnecelowf ejector systems for optimizing the performance of adjustable ejectors. For the later one, the nozzle throat adjustable ejectors. For the later one, the nozzle throat area varies with the compressor speed and CO efficiency. Additionally, Xu et al. [99] developed a control strategy to maximize the2COP by areamvarsiseflsowitrhatteh, esoctohmatparceosrsroerlastipoenefdoratnhde oCpOti2mmumassnoflzozwle trhartoea,tsaorethaactanabceordrerliavteidonfofroargtahsecopolteimr um correlating the CO2 pressure and temperature at the gas cooler exit. A multi-variable adjustable nozzpleretshsruoreatbasreda ocannthbeeddyenraimveicdsfyosrteamgraespconosle.r pressure based on the dynamic system response. ejector controller [100] and an on-line quasi-cascade controller [101] are among the other control systems for optimizing the performance of adjustable ejectors. For the later one, the nozzle throat area varies with the compressor speed and CO2 mass flow rate, so that a correlation for the optimum nozzle throat area can be derived for a gas cooler pressure based on the dynamic system response. Figure 5. 3-D model of the adjustable ejector. Reproduced with permission from [99]. Elsevier, 2012. Figure 5. 3-D model of the adjustable ejector. Reproduced with permission from [99]. Elsevier, 2012. Among many types of expansion devices, the electronic expansion valve (EEV) and capillary tubes have been studied for CO2 HPs. Zhang et al. [102] experimentally studied the effect of the refrigerant charge amount and EEV opening on the performance of a CO2 HP water heater. The EEV Figure 5. 3-D model of the adjustable ejector. Reproduced with permission from [99]. Elsevier, 2012. opening of 40% was found to be optimal for their system. Increasing the EEV opening from its optimal value to 60% decreased the heating capacity up to 30% due to an increase in the refrigerant chargePDF Image | Recent Advances in Transcritical CO2 (R744) Heat Pump System
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