A HOLLOW FIBER MEMBRANE-BASED LIQUID DESICCANT AIR DEHUMIDIFICATION SYSTEM FOR SOLAR ENERGY RECLAMATION

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1 SASEC01 Third Southern African Solar Energy Conference May 01 Kruger National Park, South Africa A HOLLOW FIBER MEMBRANE-BASED LIQUID DESICCANT AIR DEHUMIDIFICATION SYSTEM FOR SOLAR ENERGY RECLAMATION Zhang L.Z.*, Zhang N., Cai R.R. *Author for correpondence Key Laboratory of Enhanced Heat Tranfer and Energy Conervation of Education Minitry, School of Chemitry and Chemical Engineering, South China Univerity of Technology, Guangzhou 106, China, Lzzhang@cut.edu.cn ABSTRACT A mathematical model i propoed to tudy the heat and ma tranfer performance of a hot water driven hollow fiber membrane-baed liquid deiccant air dehumidification ytem. The ytem can be extended to olar energy cooling in ummer, epecially in hot and humid region. The benefit with thi technology i that the liquid deiccant i not in direct contact with the proce air, therefore the problem of liquid droplet cro over i prevented. With the model, the effect of variou operating parameter like freh air temperature and humidity on ytem performance, in term of SDP (pecific dehumidification power) and COP (coefficient of performance), are dicued. INTRODUCTION Air conditioning in building ha accounted for 1/3 of the overall energy ue in the world. The heavy burden on economic growth and environmental protection ha forced people to reduce energy ue and to ue renewable energy ource. Solar energy driven liquid deiccant air cooling ytem i jut uch a promiing alternative to the traditional air conditioning ytem. It ue low-grade hot water (which can be produced by olar energy) rather than high-grade electricity. It ha the ability to torage energy by liquid deiccant, which olve the intermittent nature of olar energy. However traditional liquid deiccant ytem [1,] ha the eriou problem of liquid droplet cro over, which i rather corroive. In recent year, membrane have been ued to olve thi problem [3]. In thi ytem, membrane are ued to eparate the liquid deiccant from air tream. Moiture in air can be aborbed by the liquid deiccant inide membrane fiber, but liquid droplet croover i prevented. The ytem i promiing. To analye the ytem performance, in thi tudy, a mathematical model i propoed to imulate a hot water driven hollow fiber membrane-baed liquid deiccant air dehumidification ytem. With the model, the effect of variou operating parameter like freh air temperature and humidity on ytem performance, in term of SDP (pecific dehumidification power), and COP (Coefficient of Performance), are dicued. SYSTEM DESCRIPTION Figure 1 i the ytem modelled. A een, it i imilar to a traditional liquid deiccant air dehumidification ytem, but two hollow fiber membrane module, one for air dehumidification and the other for olution regeneration, are employed. The former one i called the dehumidifier and the latter one i called the regenerator. The tructure for the two are the ame. One air tream i freh air that i dehumidified, and the other one i regenerating air that i in fact another freh air. The cycle for the olution i It comprie of four procee. (1) Proce 1-, dehumidification. In the olution container there i cool and concentrated liquid deiccant at tate 1. LiBr olution i ued a the deiccant olution. It i pumped to the tube ide fiber in the dehumidifier. In the dehumidifier, the liquid deiccant aborb water vapor from the air flowing in the hell ide. The air i dehumidified. The olution become diluted with a temperature rie to tate, after aborbing aborption heat. () Proce -3, olution heating. The olution at tate i then heated by a hot water bath which i et to 7C. The temperature rie and the olute ma fraction remain unchanged. Thi i o called pure heating proce. (3) Proce 3-4, regeneration. The hot olution at tate 3 flow through the hollow fiber in the regenerator. It water content i driven to the regeneration air flowing in the hell ide. Solution become concentrated with a temperature decreae to tate 4, after releaing aborption heat. The Regeneration air i exhauted to the ambient. (4) Proce 4-1, olution cooling. The warm and concentrated olution at tate 4 flow to a cooler by city water. It i further cooled and tranported to the olution container at tate 1. The olute ma fraction remain contant in thi tep. Thi i called pure cooling proce. After thi tep, the cycle i completed. Beide olution, the thermodynamic path for the air tream are alo depicted. The proce for the dehumidified air i expreed a -6. The proce for the regeneration air i expreed a

2 1 Concentrated olution Pump Solution Container Cold water bath 0C Flow meter T Senor Dehumidified air 6 Dehumidifier Freh air diluted olution 3 Exhaut air 7 Regenerator 4 Regeneration air Hot water bath 80C Figure 1 A hot water driven hollow fiber membrane liquid deiccant air dehumidification ytem Air out Shell Hollow fiber membrane Solution in Solution out Tube plate Air in Tube plate Figure Structure of a hollow fiber membrane module The tructure of a ingle membrane module i hown in Fig.. A een, it i imilar to a hell-and-tube heat exchanger. However, membrane tube are ued rather than metal tube. Both heat and moiture can be tranferred from one fluid to another through the membrane tube wall. Heat and ma tranfer in the membrane module are chematically illutrated in Figure 3. Heat and moiture can be permelectively tranported from air to liquid olution in dehumidification, or from liquid olution to air tream in regeneration. Liquid cannot travel through membrane. Figure 3 Heat and ma tranfer through membrane Heat and moiture conervation in the air tream are dt h A dx L dωa ρaka dx L a acpa T Ta (1) m m a ω ωa () where in the equation, A i the al area of the outer urface of fiber (m ), x i axial coordinate (m) and L i fiber length (m). In the equation, i the equilibrium humidity of air with olution at T and water ma fraction (1-X ). In air 37

3 dehumidification operation, the change in the liquid ma flow rate in the module can be neglected. Heat conervation in olution m c dt dx ha L m h dωa dx T Ta a ab (3) where in the equation, the firt term on right ide i convective heat tranferred from olution to air, and the econd term i aborption heat extracted by water deorption. Variable the al ma flow rate of olution (kg/). Equilibrium humidity of olution i a function of temperature and olute fraction, or dω ω Solution ide preure drop ω m dt dx (4) T X p u fl () in which, f, u and d i are friction factor for olution ide, ma flux and inner diameter of fiber. For laminar flow in round tube, friction factor and Reynold number atifie [4] f Re 64 (6) Air ide preure drop aua,max pa NLm fa (7) in which N L i the number of fiber along the air flow direction, m i a correction factor, f a i friction factor acro tube bank lited in, u a,max i maximum ma flux in air flow direction. Specific dehumidification power (SDP), and COP of the whole ytem are defined to evaluate the performance of thi ytem. SDP and COP expre the dehumidification ability of the device and they can be calculated by the following equation: ava ω a,deh,iωa,deh, o SDP (8) A w m a e COP m c pw d i a,deh, i T e het, i a,deh, o Thet, o where ρ a i air denity, V a i air volumetric flow rate and ω i humidity ratio, e i enthalpy (kj/kg). Subcript a,, deh, i, w and o, het refer to air, liquid deiccant, dehumidifier, inlet, water and outlet, heater, repectively. The detailed calculation of other parameter can be found in [4]. Eq. (1)-(4) are the governing partial differential equation for heat and moiture tranfer in the module. A heat and ma tranfer are coupled, they are olved by control volume baed finite difference method. A grid independence tet i found that grid are enough for numerical accuracy (le than % difference compared to grid). After the olution of the module, the tate point of the ytem are determined, and then the ytem performance i evaluated. Temperature, humidity, and olution concentration of liquid deiccant are governed by equilibrium thermodynamic graph a hown in Fig.3. The model i (9) for the module ha been experimentally validated in [4]. Following are the performance analyi. Vapor preure (kpa) % LiBr ma fraction (%) Figure 3 Thermodynamic tate graph for LiBr olution RESULTS AND DISCUSSION The dehumidification ytem i imulated by the model and the operating condition for the module are hown in Table 1. Temperature and relatively humidity of outdoor air are typical weather condition in hot and humid Southern China. Volumetric air flow rate of 1m 3 h -1 i elected for freh air requirement of a 100m office building with 4- occupant. The ambient freh air i alo ued for olution regeneration. The volumetric flow rate i equal to that of outdoor freh air for preure balance in room. Inlet olution concentration i et to 0.4 and inlet temperature i et to 0 for the dehumidifier. Volumetric flow rate of heating and cooling water i 4 Lh -1 and the inlet olution temperature of dehumidifier i kept to 0. Table 1 Geometrical and phyical propertie of the module Parameter Decription or value Dehumidifier/regenerator Module length (y 0) 380 mm Module width (x 0) 00 mm Fiber inner diameter (d i) 1.1 mm Fiber outer diameter (d 0) 1. mm Total area (A ) 7m Bundle arrangement Staggered Membrane thickne () 1m Moiture diffuivity (D vm) 1.6e-9m / Air flow rate are varied to tudy the effect of air flow rate on performance while other parameter are fixed. The regeneration air flow rate are equal to freh air. The pecific dehumidification power and COP under varying inlet air 7% RH 38

4 temperature and humidity are plotted in Fig. and 6 repectively. SDP (g/h/m ) SDP (g/h/m ) COP SDP COP COP p a (Pa) Figure 7 Effect of fluid velocity on preure drop through the membrane module Air Fluid velocity (m/) Solution p (Pa) Freh air temperature (C) Figure Effect of freh air temperature on performance SDP COP Freh air relative humidity (RH) Figure 6 Effect of freh air humidity on performance A outdoor air temperature change from time to time, the influence hould be conidered. A een, generally, the higher the air temperature i, the lower the SDP and COP are. The reaon i that the working zone move to a higher temperature one. The fluid cycle at higher temperature. The ability to dehumidify air decreae. At the ame time, the higher air temperature and the much more latent heat releaed during the dehumidification proce all contribute to the rie in olution temperature. Therefore, the temperature of the liquid deiccant i higher and the coefficient of performance i lower. On the other hand, the higher olution temperature would make the regeneration temperature rie and conequently a lower performance. The SDP i about 1 to kg/h/m, which indicate that the new ytem i compact and high efficient. For humidity, the higher the freh air humidity i, the higher the SDP and the COP are. Thi i becaue, the higher the freh air humidity i, more moiture i dehumidified and the better the performance. Generally, the performance of the membrane ytem i comparable to a packed column air dehumidification ytem. However the liquid droplet croover problem can be prevented. The preure drop through the module with varying air and olution flow rate are hown in Fig.7. Generally, the preure drop are low. The preure drop in air ide i from 30 to 60Pa, while the preure drop in liquid ide i from 3 to 600 Pa. A 0W blower and a W platic pump are ued repectively to drive the air tream and the liquid tream. The preure drop induced energy ue i trivial compared to the energy aving by the low-grade hot water ytem. A tet proype i being built in our laboratory to help to commercialize the ytem. Detailed heat and ma tranfer analyi for component deign i conducted [,6]. CONCLUSION A mathematical model i developed to imulate a hot water driven and membrane-baed liquid deiccant air dehumidification ytem. The membrane module i modeled in 39

5 detail to decribe the performance of the whole ytem. The ytem performance are invetigated under different operating condition. Following concluion can be made: The temperature and humidity of the freh air have a tremendou impact on ytem performance. The higher the temperature i, the le the dehumidification capability and the COP. For humidity, the higher the freh air humidity i, the higher the SDP and the COP are. Generally, the performance of the membrane ytem i comparable to a packed column air dehumidification ytem. The preure drop are acceptable. However the liquid droplet croover problem can be prevented. ACKNOWLEDGMENTS The project i upported by National Natural Science Foundation of China, ; and by the National Science Fund for Ditinguihed Young Scholar of China, No It i alo upported by Educational Commiion of Guangdong Province, China, No. 013KJCX0017, Science and Technology Planning Project of Guangdong Province, China, No. 013B01006, and the State Key Laboratory of Sub-tropical Building Science. REFERENCES [1] Fumo N., Gowami D.Y., Study of an aqueou lithium chloride deiccant ytem: air dehumidification and deiccant regeneration, Solar Energy, vol. 7, 00, pp [] Babakhani D., Soleymani M., Moheb A., Heat and ma tranfer between air and liquid deiccant in cro-flow contact ytem, Chemical Engineering and Technology, vol. 33, 010, pp [3] Zhang L.Z., Progre on heat and moiture recovery with membrane: From fundamental to engineering application, Energy Converion and Management, vol. 63, 01, pp [4] Zhang L.Z., Zhang N., A heat pump driven and hollow fiber membrane-baed liquid deiccant air dehumidification ytem, Energy, vol. 6, 014, pp [] Zhang L.Z., Conjugate Heat and Ma Tranfer in Heat Ma Exchanger Duct, 1t edition, Academic Pre, Elevier, New York, 013. [6] Zhang L.Z., Total Heat Recovery: Heat and Moiture Recovery from Ventilation Air, Nova Sci. Pub. Co., New York,