Investigation of an Evaporative Cooler for Buildings in Hot and Dry Climates

Transcription

1 Investigation of an Evaporative Cooler for Builings in Hot an Dry Climates R. Boukhanouf, H. G. Ibrahim, A. Alharbi, an M. Kanzari architecture often integrate into the incatchers. Evaporative cooling is a lo carbon an economically feasible metho for cooling builings in hot an ry climates. Abstract The paper presents a computer moel an experimental results of a sub-et bulb temperature evaporative cooling system for space cooling in builings in hot an ry climates. The cooler uses porous ceramic materials as the et meia for ater evaporation. Uner selecte test conitions of airflo ry bulb temperature of up to 45o C an relative humiity of up to 50%, it as foun that the supply air coul be coole to belo the et bulb temperature ith a maximum cooling capacity of 280 W/m2 of the et ceramic surface area. It as also shon that the overall et bulb effectiveness is greater than unity. This performance oul make the system a potential alternative to conventional mechanical air conitioning systems in hot an ry regions. Inex Terms Builings air conitioning, cooling, heat an mass transfer, porous ceramics. II. EVAPORATIVE COOLING TECHNOLOGY Evaporative cooling technologies coul be classifie into irect an inirect system. A. Direct Evaporative Cooling Systems Direct evaporative cooling is the process of evaporating liqui ater to the surrouning air an causing its temperature to ecease. A typical irect evaporative cooler, as shon in Fig. 1, uses a fan to ra in outsie air through a pa etting meia an circulates the cool air through the builing. evaporative I. INTRODUCTION Energy consumption in builings stans at beteen 30-40% of the total primary energy use globally [1]. A major part of this is use to provie comfortable inoor climatic conitions for occupants. For example, in regions ith hot climates energy for space cooling accounts for over 60% of the total energy use in builings [1]. The groth in air conitioning systems in the orl is mainly riven by an increase in living stanars, afforability, population increase an cheap electrical energy in some regions such the Mile East. This has le many countries to buil ne poer generation plants an exten gri infrastructure to meet peak electricity loas, hich in turn impacts negatively on the environment through increase greenhouse gas emissions. Current air conitioning market is ominate by mechanical vapour compression systems, hich are energy intensive systems an suffer from lo thermal performance in hot climate conitions. Hence, there is a renee interest in the use of evaporative cooling for thermal comfort in builings. The earliest use of evaporative cooling as by ancient Egypt an the Roman Empire using for example et mats (cooling pas) over oors an inos to cool the inoor air hen in ble through the mats [2]. Evaporative cooling is iely foun in Mile East an Persian Fig. 1. Schematic of a irect evaporative cooling system The energy require for evaporation of ater is provie by the air, though at the expense of increasing its moisture content an ecreasing its temperature. Since the process is aiabatic, the sensible heat loss by the air is balance out by latent heat gain, hich appears as moisture content increase. The heat an mass transfer beteen the arm ry air an ater can be expresse as follos [3]. & ah1 + m & v1hv1) m & vhfg = (m & ah2 + m & v2hv2 ) (m here ma, h1 an h2 are air mass flo rate, inlet an outlet enthalpy respectively. mv1, hv1, mv2 an hv2 are the ater vapour inlet mass flo rate, enthalpy, outlet mass flo rate an enthalpy respectively. mv an hfg are the ater evaporation rate an latent heat of evaporation respectively. The amount of ater require can be compute as: Manuscript receive June 11, 2013; revise July 16, This publication as mae possible by NPRP grant No from the Qatar National Research Fun (a member of Qatar Founation). The statements mae herein are solely the responsibility of the authors. R. Boukhanouf an A. Alharbi are ith The University of Nottingham, Department of Architecture an Built Environment, Nottingham, NG7 2RD, UK ( rabah.boukhanouf@nottingham.ac.uk, laxaa17@nottingham.ac.uk ). H. G. Ibrahim an M. Kenzari are ith Qatar University, Department of Architecture an Urban Planning, Doha, Qatar ( hatem_ibrahim@qu.eu.qa, meryem_kanzari@qu.eu.qa) DOI: /JOCET.2014.V2.127 (1) & v = m & a (g2 g1 ) m (2) here ma is ry air mass flo rate, g1 an g2 are the inlet an outlet air moisture content respectively. The effectiveness of irect evaporative coolers is primarily influence by the air et bulb temperature an in a 221

2 C. Wet Meia Materials The et meia use in evaporative coolers is an essential component of an evaporative cooler. It is usually mae of a porous material ith large surface area an capacity to hol liqui ater. Accoring to Wanphen an Nagano [9], the selection of et meia materials is base on their effectiveness, availability, cost, safety, an environment factors. Zhao et al [10] investigate various types of porous materials such as metal an plastic foams, zeolite an carbon fibres to be use as et meia for heat an mass transfer in evaporative cooling systems. Musa [11] also investigate the use of more common aspen pas materials for inirect evaporative cooling system. Riffat an Zhu [12] employe ceramic materials for inirect evaporative cooling systems. Fig. 3 shos some common et meia materials that can be foun in evaporative cooling systems. ell-esigne system the air coul be coole to ithin 2 to 3oC of the et bulb temperature hich presents a severe thermoynamic limitation. B. Inirect Evaporative Cooling an Sub Wet Bulb Temperature This has le several researchers to evelop an moify the thermal process of irect evaporative cooling system to achieving sub-et bulb temperature, referre to as De point or Sub-et bulb temperature evaporative cooling. In this coolers arrangement, the air streams are separate into ry channel for supply air an et channel for rejecting spent orking air. The supply air in the ry channel is coole inirectly by transferring its heat to the orking air in the et channel through a thin non-permeable channel all. To achieve sub-et bulb temperature, part of the cool air in the ry channel is iverte to accomplish the evaporation process in the et channel, as shon in Fig. 2. a) b) c) ) Fig. 3. Wet meia materials a) metal foams b) organic impregnate materials (Aspen) c) Celek paper ) fire- clay. Fig. 2. A simple schematic of a sub et bulb temperature inirect evaporative cooler. The avantage of this arrangement is that the moisture content of the coole air remains unchange. Hsu et al. [4] carrie out a theoretical an experimental stuy on to configurations of close-loop et surface heat exchangers to achieve sub-et bulb temperature cooling through counter flo an cross flo air stream arrangements. Boxem et al. [5] presente a moel for a compact counter flo Inirect Evaporative Cooler ith finne exchanger. The performance of a 400 m3/h air flo rate cooler as analyse an shoe that for inlet air temperatures higher than 24 C the moel results accuracy ere ithin 10%. Zhao et al. [6] presente a numerical stuy of a counter flo Inirect Evaporative Cooler for sub-et bulb temperature cooling. The authors suggeste a range of esign conitions to maximize the cooler performance incluing air velocity range, height of air passage, an length to height ratio of air flo ucts an foun that the cooler can yiel et bulb effectiveness of up to 1.3. Riangvilaikul et al. [7] presente experimental results for a sensible evaporative cooling system at ifferent inlet air conitions (temperature, humiity an velocity) covering ry, temperate an humi climates. The results sho that et bulb effectiveness range beteen 92 an 114%. A continuous operation of the system uring a typical ay of summer season in a hot an humi climate shoe that et bulb effectiveness as almost constant at about 102%. Hasan [8] also presente a theoretical moel of four ifferent configurations of inirect an sub et bulb temperature coolers: to-stage counter flo cooler, to-stage parallel flo cooler, single-stage counter flo regenerative cooler an combine parallel-regenerative cooler. The author conclue that ith higher number of stage coolers, the ultimate temperature to be reache is the e point of ambient air. 222 III. DESCRIPTION OF THE POROUS CERAMIC EVAPORATIVE COOLER In this project, porous ceramic materials in the form of hollo flat shells ere use as et meia in a sub-et bulb temperature evaporative cooler. Porous ceramic materials ere selecte for their stable structural, non-corrosion properties an easily moule into esire shape. Fig. 4 shos the configuration of the sub-et bulb temperature evaporative cooler using the porous ceramic material for ater evaporation. Fig. 4. A schematic of a builing integrate sub-et bulb temperature porous ceramic evaporative cooler The porous ceramic panels ere place beteen the ry an et air ucts to form small an narro ucts ith air floing at lo velocity. The ry channel sie of the porous ceramic panel is seale ith a thin non-permeable membrane hile the et channel sie allos ater to sip through its micro-pores onto its surface forming a thin ater film. This

3 allos irect contact ith the airflo an hence causing ater evaporation. The air streams in the ry an et channel flo in counter flo arrangement an the supply air exchanges sensible heat ith the ater in the porous ceramic panels that in turn are coole through ater evaporation on the et channel sie. This results in a rop in temperature of the air in the ry channel ithout changing its moisture content hile the air in the et channel is rejecte at saturation state. IV. MATHEMATICAL MODEL The sub-et bulb temperature evaporative cooler as moelle using common energy an mass conservation las. In the moel the ry an et channel ere ivie into small elements (finite volumes) to hich the energy an mass transfer equations ere applie. A. Energy Conservation in the Dry Channel Air is coole in the ry channel by transferring its sensible heat to the et channel through the non-permeable layer an the porous ceramic panels. This can be expresse as follos: h = U ( T T ) here m, h,t, A an U are the air flo mass rate, enthalpy, temperature, heat transfer coefficient an area of the ry channel. T is the temperature of the ater film on the et channel sie. B. Energy Conservation in the Wet Channel The heat transfer mechanism in the et channel is more complicate than in the ry channels, as sensible an latent heat is exchange beteen the airflo an the ater film on the surface of the porous ceramics. This is expresse as [13]: h = κ ( T T ) + σ ( g g ) h here m, h,t, g, an κ are the air mass flo rate, enthalpy, temperature, moisture content an convective heat transfer coefficient in the et channel. T, g, an σ are the ater film temperature, saturate air moisture content, an mass transfer coefficient. It is assume that air flo regime in both ry an et channel are laminar an the mass transfer coefficient, σ obeys the folloing Leis number correlation [8], [13]: Le p fg (3) (4) κ = (5) σ c here Leis number, Le, value ranges from to 0.9 to 1.15 an to simplify the analysis it is often taken to be 1, c p is specific heat of humi air. C. Mass Conservation in the Wet Channel Water evaporation from the ceramic panel surface appears as an increase of the air moisture content along the length of the et channel. The mass balance for the ater vapour in the et channel can be ritten as: g = σ ( g g ) D. Overall Energy Balance The overall energy an mass balance at the ater film interface beteen the air flo in the ry channel, the ater film on the ceramic surface an the air flo in the et channel can be expresse as: C p T (6) = U ( T T ) σ ( g g ) h fg α ( T T ) here C p is specific heat of ater. The governing ifferential equations ere iscretise an applie to each finite volume element along the ry an et channel length. In the computer moel, it as assume that the air properties, heat an mass transfer coefficients are constant in each finite control volume, the ater film an the non-permeable membrane thermal resistances ere assume to be negligible. The initial conitions use in this moel inclue knon air properties (temperature an moisture content) for the ry channel an air moisture content for the et channel. The main esign parameters of the system are given in Table I. TABLE I: DESIGN AND MODELLING PARAMETERS Air channel Length Air channel With Air channel Height Mass lo rate in the ry channel Mass flo rate in the et channel Air flo regime Fan poer rating Porous Ceramic materials composition Porosity Density Thermal conuctivity 0.64 m 0.93 m m 0.03 (kg/s) (kg/s) Laminar 16-24V, 14 W Al 2 O 3, SiO 2, Si 3 N 4 17% 2300 kg/m W/mK Computation of the operating parameters of airflo along the air ucts length as performe iteratively until converging conitions ere satisfie giving a temperature ifference beteen to consecutive iterations of less than 0.01 o C. V. EXPERIMENTAL TEST RIG AND RESULTS A laboratory test rig as built to test the porous ceramic sub-et bulb temperature evaporative cooler is shon in Fig. 5. The porous ceramic panels ere fille ith ater through an overhea tank hile a fan as use to ra air at controlle temperature an relative humiity from an environmental chamber an circulate it through the evaporative cooler ry an et channels. The rig as fully instrumente to measure the air temperature, moisture content an flo rates along the ry an et channels. The thermal performance of the laboratory prototype as measure uner controlle conitions of inlet air temperature an humiity. The results of hich ere also compare to that of the computer moel. Fig. 6 shos the computer moel (7) 223

4 an experimental results of the airflo temperatures in the ry an et channel. For an initial inlet air flo (ambient conitions) ry bulb temperature of 35oC an relative humiity of 35% (i.e., et bulb temperature of 23.1oC an e point of 18oC), the computer moel preicts that the supply air coul be coole to 22.3o C, achieving sub-eb bulb temperature conitions. The airflo along the et channel, on the other han, increase from 22.3o C to 26.2o C, as the balance beteen sensible heat loss to evaporate ater on the surface of the ceramics an latent heat gain from the aition of ater vapour to the air is positive. The experimental measurements hoever sho the supply air is coole to about 22.9oC an that of the rejecte air from the et channel is 23.2oC. The iscrepancy beteen the experimental an computer moel results coul be explaine by the ifficulties in obtaining a uniform air istribution in the ie section of the ry an et channel. Fig. 7. Temperature evolution in the evaporative cooler (m= 0.03kg/s, RH=35%). Further evaluation of the performance of the evaporative cooler as carrie out by calculating its cooling potential at various ry bulb temperature an relative humiity conitions, as shon in Fig. 8. It can be seen that the cooling capacity is strongly influence by the ry bulb temperature an relative humiity of the ambient air. The cooling capacity ecrease nearly proportionally to increasing relative humiity. This cause by the iminishing rate of ater evaporation as the air in the et channel nears the saturation conitions. The cooling capacity at 40oC an 35% relative humiity as measure to be about 240 W/m2 of the et porous ceramic surface area. Fig. 5. A laboratory experiment rig of the ceramic evaporative cooler Fig. 8. Cooling capacity (m= 0.03kg/s). Finally, the effectiveness of the evaporative cooler as etermine using the et bulb an e point effectiveness. The et bulb effectiveness is the ratio of the ifference beteen inlet an outlet airflo ry bulb temperature to the ifference beteen inlet airflo ry bulb temperature an its corresponing et bulb temperature [14]. The mathematic expression of the et bulb effectiveness is given by: ε b = Fig. 6. Temperature profile along the ry an et channel (air inlet from left to right) air mass flo rate m=0.03 kg/s The experimental steay state temperature of the supply airflo, rejecte air an ater film ere recore as shon in Fig. 7. It can be seen that the supply airflo temperature (airflo in the ry channel) as reuce from the inlet conitions of 35oC to about 25oC (a rop of 10oC) at a constant relative humiity of 35% an mass flo rate of 0.03kg/s. Tb,in Tb,out Tb,in Tb,in (8) The e point effectiveness, on the other han, is the ratio of the ifference beteen inlet airflo ry bulb temperature to the ifference beteen inlet airflo ry bulb temperature an its e point temperature [15]. The mathematic expression of the e point effectiveness is expresse as follos: 224

5 ε p = Tb,in Tb,out Tb,in Tp [10] X. Zhao, Z. Duan, C. Zhan, an S. B. Riffat, Dynamic performance of a novel e point air conitioning for the UK builings, International Journal of Lo Carbon Technologies, vol. 4, pp , [11] M. Musa, Novel Evaporative Cooling Systems for Builing application, in Architectur an Built EnvironmentMay, PhD thesis, The University of Nottingham: Nottingham, pp , 234, [12] S. B. Riffat an J. Zhu, Mathematical moel of inirect evaporative cooler using porous ceramic an heat pipe, Applie Thermal Engineering, vol. 24, no. 4, pp , [13] B. Halasz, A general mathematical moel of evaporative cooling evices, Elsevier, Paris, pp , [14] B. Riangvilaikul an S. Kumar, Numerical stuy of a novel e point evaporative cooling system, Energy an Builings, vol. 42, no. 11, pp , [15] B. Frank, On-site experimental testing of a novel e point evaporative cooler, Energy an Builings, vol. 43, no. 12, pp , (9) For a typical inlet airflo conition of 35oC an 35% relative humiity, the eb bulb effectiveness of experimental test as 1.02 an e point effectiveness as This shos that the evaporative cooler achieve et bulb effectiveness greater than unity, a thermal performance that coul compare favourably ith mechanical vapour compression systems use in hot climates ith the aitional benefit of improving comfort level in builings. VI. CONCLUSION R. Boukhanouf is a lecturer in sustainable energy technologies at the Department of Built Environment, University of Nottingham. His experience in research an teaching in the area of energy efficient an lo carbon technologies extens for over 15 years. He obtaine his PhD in 1996 from the University of Manchester, UK. Dr. Boukhanouf orke on numerous research projects fune by inustry an government agencies in the area of small scale combine heat an poer, active an passive heating an cooling systems for builings, an avance heat transfer enabling evices. He publishe a number of journal an conference papers an is name as the inventor in six international patents. A computer moel an experimental results of a sub-et bulb temperature evaporative cooler using porous ceramic materials ere presente. It as shon that the evaporative cooler can achieve high thermal performance in terms of lo air supply temperatures an effectiveness. The structural stability an manufacturing controllability of ceramic materials len them ell to integration into builings an performing the function of air conitioning in regions ith hot an ry climatic conitions. A. Alharbi is a PhD egree caniate in the epartment of the Built Environment, University of Nottingham. His main research topic is evaporative cooling technology in hot an ry climates. Mr. Alharbi has a MSc an BEng egree in mechanical engineering. He has a long inustrial experience in air conitioning systems. REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] A. Dooo, L. Gustavsson, an R. Sathre, Builing energy-efficiency stanars in a life cycle primary energy perspective, Energy an Builings., vol. 43, no. 7, pp , D. R.Vissers, Stuy on Builing integrate evaporative cooling of glass-covere spaces, in Builing Physics an Systems, Einhoven University of Technology, W. P. Jones, Air Conitioning Engineering, 5th Eition, Butterorth Heinemann, S. T. Hsu, Z. Lavan, an W. M. Worek, Optimization of et-surface heat exchangers, Energy, vol. 14, pp , G. Boxem, S. Boink, an W. Zeiler, Performance moel for small scale inirect evaporative cooler, in Proceeings of Clima WellBeing Inoors, REHVA Worl Congress, Helsinki, Finlan, 2007, No X. Zhao, J. M. Li, an S. B. Riffat, Numerical stuy of a novel counter-flo heat an mass exchanger for e point evaporative cooling, Applie Thermal Engineering, vol. 28, no , pp , B. Riangvilaikul an S. Kumar, An experimental stuy of a novel e point evaporative cooling system, Energy an Builings, vol. 42, pp , A. Hasan, Inirect evaporative cooling of air to a sub-et bulb temperature, Applie Thermal Engineering., vol. 30, no. 16, pp , S. Wanphen an K. Nagano, Experimental stuy of the performance of porous materials to moerate the roof surface temperature by its evaporative cooling effect, Builing an Environment, vol. 44, no. 2, pp , H. G. Ibrahim is an associate professor at Qatar University. Dr. Ibrahim has a long an establishe research experience incluing managing green construction, carbon abatement in construction inustry using knolege base programming, an preservation of traitional architectural an urban heritage of Qatar. The latter being particular an ass-on avantage for reconciling the integration of ne lo carbon technologies ith the traitional architectural concepts. Meryem Kanzari is currently a research assistant at Qatar University an orking on a novel esign of evaporative cooling system for application in hot an ry climates. Previously, she orke on various engineering projects incluing time frequency analysis of vibration an acoustic systems, fault etection an inustrial maintenance an instrumentation. Ms Kanzari obtaine Engineer egree from Institut National es Sciences Appliquées et e Technologie (INSAT), TUNISIA, in 2008 an a MSc egree from Institut National es Sciences Appliquées e Lyon (INSA Lyon), FRANCE in