180J. Guo, H.G. Shen / Energy and Buildings 41 (2009) 175–181Fig. 8. Hourly ambient temperature variations in Shanghai (July).Fig. 11. Variation of COP of the SERS with time.Fig. 9. Hourly <strong>solar</strong> radiation in Shanghai (July).4.2. Performance of the SERSThe climate conditions of Shanghai were used for theperformance prediction of the SERS. Figs. 8 and 9 show the hourlyoutdoor ambient temperature and total <strong>solar</strong> radiation on a typicalday in July calculated from the model proposed by Liu and Jordan[19]. Given the performance of the <strong>ejector</strong> as mentioned above andthe climate conditions as shown in Figs. 8 and 9, the hourlyperformance of the ERS and SERS can be obtained (Figs. 10 and 11).Fig. 10 shows the hourly COP of the ERS with the evaporatortemperature at 8 8C and the condenser temperature varying withthe ambient temperature. Under fixed inlet pressures of motivefluid and entrained fluid, the mixed fluid is easier to flow throughwith higher condenser temperature, therefore, more refrigerantfluid can be entrained and the entrainment ratio of the <strong>ejector</strong>increases, consequently, the cooling capacity and the COP of theERS also increase.Comparing Figs. 8–10, although the <strong>solar</strong> radiation reachesmaximum at 12:00, the ambient temperature and the entrainmentratio of the <strong>ejector</strong> reach maximum at 14:00. It indicates that thecondenser temperature has greater effect on the performance ofthe ERS than the generator temperature. As the condensertemperature not only determines the condenser pressure whichin turn influences the entrainment ratio and COP of the ERS asmentioned above, but it also influences the heat required by thegenerator. Under a higher condenser temperature, the <strong>ejector</strong>entrains more refrigerant and supplies more cooling capacity.Furthermore, a higher condenser temperature causes a decrease inthe heat required by the generator when it generates the samequality and quantity of motive fluid.Fig. 10. Variation of COP of ERS with time.Fig. 12. Hourly <strong>solar</strong> fraction.
J. Guo, H.G. Shen / Energy and Buildings 41 (2009) 175–181 181Considering the efficiency of the <strong>solar</strong> collector, heat loss of thestorage tank and pipes as well as the heat transfer efficiency in thegenerator, the hourly overall performance of the SERS is as shownin Fig. 11. From 8:00. to 16:00, the <strong>system</strong> worked under steadyperformance between 0.43 and 0.53 with cooling capacity of 6 kW.During other times, the <strong>solar</strong> radiation intensity weakens andthe ambient temperature drops, therefore, the overall COP of the<strong>system</strong> decreases sharply. From the view of this character, the<strong>system</strong> exerts its best performance when being used in daytime.Therefore, it’s appropriate for the <strong>system</strong> to supply <strong>air</strong> conditioningfor office buildings.The hourly <strong>solar</strong> fraction of the <strong>system</strong> is shown in Fig. 12. Withmore <strong>solar</strong> energy gain from 10:00 to 13:00, the <strong>solar</strong> fractionduring this period is more than 1.0, which means no additionalelectric energy is needed (except that used for the instrument andcirculation pumps) for the <strong>system</strong> to supply <strong>air</strong> conditioning.During other hours at daytime, the <strong>solar</strong> fraction of the <strong>system</strong> isbetween 0.45–0.94 except at 17:00, the <strong>solar</strong> fraction drops to aslow as 0.15. When the <strong>system</strong> is equipped in office buildings, andthe office time is from 9:00 to 17:00, the average <strong>solar</strong> fraction ofthe <strong>system</strong> is 0.82. That is to say, only 18% electric energy is neededto provide the same cooling capacity. Compared with traditionalcompressed <strong>air</strong> conditioning <strong>system</strong>, the SERS can conserve morethan 75% of electric energy.5. ConclusionsIn this study, the lumped method combined with dynamicmodel for performance prediction of <strong>solar</strong>-<strong>driven</strong> <strong>ejector</strong> <strong>refrigeration</strong><strong>system</strong> for providing <strong>air</strong> conditioning to office buildingswas investigated. The results of the mathematical simulation havedemonstrated that the <strong>solar</strong>-<strong>driven</strong> <strong>ejector</strong> <strong>refrigeration</strong> <strong>system</strong>can be designed to meet the cooling requirements of <strong>air</strong>conditioning for office buildings. The following conclusions wereobtained:(1) For the studied case, the condenser temperature influencesmore on the performance of the SERS than the generatortemperature.(2) From 9:00 to 17:00, on typical clear sky days, the average COPof the <strong>system</strong> is 0.48 with most of the daytime remainingsteady between 0.43–0.53, except at 17:00, when it drops aslow as 0.29. 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