Ranj Sirwan 1, Yusoff Ali 1, and K. Sopian 2 1 Department of Mechanical and Materials Engineering 2 Solar Energy Research Institute Faculty of Engineering and Built environment National University of Malaysia Malaysia
Introduction Absorption cooling system has broadly paid attention in present era, due to ozone depletion effect and the environmental hazards through using CFC and HCFC refrigerant. The increase in power consumption of air-conditioning applications pushes researchers toward the absorption cooling system that uses low energy input such as renewable energy. the performance of the absorption cooling system is still a challenging task since the coefficient of performance (COP) is generally poor when it compared with the conventional vapor compression cycle.
Intro In this study, a solar driving combined absorption-ejector cooling system has developed and studied theoretically by adding a flash tank between the condenser and the evaporator. A computer simulation program is developed to analyze the effect of adding flash tank on the COP of the system.
System Description the arrangement of the proposed modified combine cycle by adding an ejector, and flash tank to the absorption cooling cycle.
System Description At the generator the NH3-H2O solution is heated by solar energy to produce ammonia vapor solutions at (1). The ammonia vapor solution processed into the rectifier to remove water vapor as much as possible. Then water vapor returns to the generator through (16). A highly concentrates ammonia vapor preceded to the ejector (2) On the ejector, the vapor coming from the flash tank (secondary flow), with the primary ammonia vapor from the rectifier are mixed and flow to the condenser. Inside the condenser, the vapor condensates to liquid (3). At the flash tank The condensate ammonia expanded to the intermediate pressure, to get the saturation liquid at point (4), and the saturation vapor at point (18).
System Description The saturated liquid ammonia from the point (4) is expanded adiabatically through the expansion valve (5) which enters the evaporator and produces the necessary cooling effect. After the evaporation of liquid ammonia inside evaporator (6) is calculated. A quantity of the vapor sucked by the ejector using the booster, and the remaining entered the absorber. At the absorber, the strong solution absorbs the refrigerant vapor comes from (6). The strong liquid solution from the generator (11) passed through the solution heat exchanger (12) then expansion valve (13). Finally, the weak solution point (7) pump by the solution pump (8) to the rectifier (9) then get sensible heat via solution heat exchanger (10) and enter the generator. And by this the cycle is completed.
Mathematical Model The Mathematical models for different components of the system is essential before the simulation program of the system is consider. The mass and energy balance applied to yield the thermal load required in all components of the system. To evaluate the COP for the cycle, energy balance required through the generator and evaporator.
Results and Discussion A computer simulation program has been investigated to analyze the proposed combined absorption-ejector cycle. The operations condition of the cycle were selected as: Generator Temperature 60-120 o C Condenser temperature 20-50 o C Absorber Temperature 20-50 o C Evaporator Temperature -14-14 o C The circulation flow rate amount of the refrigerant 0.0166 kg/s The effectiveness of the heat exchanger assumed to be 0.5
Results and Discussion This figure shows the effects of variation in generator temperature on the COP. Three different cycles were studied and compared. The COP increased until it reached the maximum values, then decreased smooth with increasing in generator temperature. The highest value of COP is obtained for the modified combined cycle. The optimum COP is 0.861 at generator temperature 92 o C which give a high advice to use with the evacuated solar system. Variation of the COP via generator temperature
Results and Discussion Variation of the COP via Condenser temperature In this figure the effects of variation at condenser temperature on the COP are shown. The increase in the condenser temperature cause a decrease in the COP The combined cycle (ejector + flash tank) obtains the highest value comparing with the others two cycles
Results and Discussion Variation of the COP via evaporator temperature A comparison among the three cycles are obtained at different evaporator temperature. The COP values increases with increase in the evaporator temperature The highest value of COP occurs with combined cycle (ejector and flash tank) at lowest evaporating temperature, while at highest evaporating temperature, the combined cycle (ejector) gives optimum value.
Results and Discussion The comparison of COP values vs. absorber temperature for the three cycles is presented The effect of variation in absorber temperature is similar to the condenser, as the absorber temperature increases the COP for the cycles decrease At absorber temperature of 48 o C, the cycle stopped and the COP tend to zero. Because there is no longer refrigerant absorbed by the absorber. The results show that the COP for the combined cycle (ejector flash tank) is higher than the other two cycles Variation of the COP vs. absorber temperature
Conclusion A theoretical study of adding flash tank to the solar driving combined absorption-ejector cooling system was investigated The results show an improvement on the COP of the modified combined cycle at different operating conditions The modified cycle investigates the ability to work with highest condenser temperature and lowest evaporator temperatures. However, in some operating design, the cycle performance cannot achieve the optimum performance, due to usage a fix geometry ejector deign. To overcome with this problem, a variable ejector design parameter can be suggested and investigated for the future study.
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