Passive Cooling of Glazing Surfaces Using Solar Chimneys

Transcription

1 Intenational Confeence on Renewable Enegies and Powe Quality (ICREPQ 16 Madid (Spain, 4 t to 6 t May, 2016 Renewable Enegy and Powe Quality Jounal (RE&PQJ ISSN X, No.14 May 2016 Passive Cooling of Glazing Sufaces Using Sola Cimneys A. Al Touma 1, K. Gali 1 and N. Gadda 1 1 Depatment of Meanical Engineeing Ameican Univesity of Beiut, P.O. Box , Beiut , Lebanon Pone numbe: , Ext: 3438, aga32@mail.aub.edu, ka04@aub.edu.lb, faa@aub.edu.lb Abstact Te aim of tis poject is to educe te cooling loads in spaces by deceasing te dow tempeatue and sola adiation entance toug te use of a passive evapoative coole combined wit a sola imney and wit no additional enegy consumption. Te coole povides eat and mass exange tat ende te capable of cooling te space in font of te dow. On te ote and, te sola imney woks based on pinciples of buoyancydiven natual flows. Matematical models of te evapoative coole glazing section, sola imney ae developed and integated wit a space model in an algoitm wee te entie system is tested unde diffeent doo tempeatue and sola adiation conditions. As te e glass tempeatue typically epesents te tempeatue of te dow wit te installation of tis system, eductions of 8% to 12% in te dow tempeatues wee seen fo an doo tempeatue of 30 o C and a sola adiation of 300Wm 2 and fo small imney sizes. Key wods Key wods: Sola Cimney, Evapoative Coole Integated System, Window Tempeatue 1. Intoduction All spaces ae now equipped wit Heating, Ventilation and Ai-Conditioning (HVAC systems tat allow pope contol of te eating and cooling set-point tempeatues. Te building envelope is wat suounds te space fom te e envionment. It consists of walls, oof, ceiling, dows, mateials, insulation and so on. Exteme cae as to be taken wen designing te building envelope due to its impotance on te lifecycle of te building. Windows ae known to be te weakest components of te building envelope due to tei temo-pysical popeties tat ae vulneable to doo climatic conditions [1]. Significant amount of sola adiation and eat ente te building toug dows duing te summe and a consideable amount of eat leaks duing te te wi causes an incease in te cooling and eating loads, espectively. Fo instance, te dows contibution in te building enegy pefomance was aound 39% of te space total cooling load in te ot climate of Texas fo a space tat ad a Window-to-Wall Ratio (WWR of 18% only [2]. Te space load tat is caused by dows could be divided into two: diect sola adiation tansmitted into te space and adiation absobed by te envelope mateials. Extensive eseaes ave been conducted in te pupose of impoving dow pysical and optical popeties. Tsikaloudaki et al studied te effect of dow temal eat tansfe coefficient, sola tansmittance, fame faction and pane gap widt on te eduction in enegy consumption [3]. Vantegem et al consideed te possibility of anging te building oientation and te facades on wi dows ae installed in a manne tat educes te amount of sola adiation enteing te space [4]. Aici et al consideed te eat passage toug multiple pane dows and tei effect duing te summe in Tukey [5]. It was found tat inceasing te numbe of dow panes slows down te flow of eat in te cavities and acts as a adiation sield yielding enegy savings of 50% and 67% wen double pane dows ae eplaced by tiple-pane and quaduple pane dows. Altoug designing ig-pefomance dows tuned to be successful, yet tey wee unable to deteioate te dow tempeatue to educe its effect on space loads. One mitigation was te design of single and dual flow dows tat wok based on fee and foced convection between layes of glass tat captue sola eat. Single flow dows ave been designed to wok on fou modes: supply, exaust, indoo cutain and doo cutain [6]. In bot te supply and te indoo cutain modes, te sola eat is used to peeat te passing toug te dow to educe te eating load in cold seasons. In contay, te sola eat is used to extact te excess of eat aound dows in te exaust and doo cutain modes duing te ot seasons. Similaly, dual flow dows consisting of tipleglazed units wee subject to many studies [6, 7]. Altoug all of te afoementioned metods ave caused some deceases in te buildings enegy consumption, tey ae often eite unpactical in existing buildings o expensive to adopt. Hence, tis stu consides means of educing te dow tempeatue passively toug te natual flows wit any additional enegy consumption and wit sacificing te temal comfot of te occupants of te indoo quality inside te space. 2. Design of Poposed System Te combined system of passive evapoative coole and sola imney is sown in Fig. 1. Wat dives te flow of te entie system is te sola imney; as sola 625 RE&PQJ, No.14, May 2016

2 adiation its its e suface, it induces a diffeence in tempeatue between te doo and te inside te imney, wi ises due to buoyancy foces. As it moves upwads, passes toug te evapoative coole wee it exanges eat and mass wit te wate absobing seets installed along its eigt. Ten, eaes te glazing section, wi is composed of a dow and an e glass laye and extacts te eat accumulated witin tis small space until it eaes te sola imney wee it is once again dagged of te system. Assuming tat te gap is mu smalle tan te widt of te annel, te enegy balance equations descibing te tempeatue and umidity anges ae: dt c 2 (, u t p gap c T (1 wat T d w * u t 2 ( w w (2 gap m As fo te wate absobing seets, tei enegy balance equation is: ( w * w 0 c wat m fg (3 In Equations 1, 2 and 3, is te density, c p is te specific eat, u is te velocity, t is te annel gap, c and m ae te eat and mass convection coefficient, fg is te latent eat of wate and w* is te umidity of satuated. B. Glazing Section A sematic of te glazing section is sown in Figue 3. Figue 1: Passive evapoative coole and sola imney of te designed system. Note tat te system includes insulation layes on bot sides of te evapoative coole and on te space side of te sola imney so tat any eat tansfe in tese diections is neglected. 3. Matematical Fomulation Tis section descibes te matematical models adopted fo te suggested system. A. Evapoative Coole A sematic of te evapoative coole is sown in Fig. 2. Figue 3: Sematic of te glazing section. Accoding te stated assumptions, te glazing section is divided into tee main sub-components: e glass, and dow. Te e glass is subjected to diect sola adiation itting its e suface, convection wit doo, adiation exange wit te and gound fom te e envionment, convection wit te induced and adiation exange wit te dow. On te ote and, te induced is subjected to convection eat exange wit te e glass and te dow simultaneously. Lastly, te dow is subjected to sola adiation tat as been tansmitted toug te e glass, convection wit te induced, adiation exange wit te e glass, convection wit te oom and adiation exange wit te space sufaces. Tei tee enegy balance equations ae: Q 0 gd gd (4 Figue 2: Passive evapoative coole of te designed system. 626 RE&PQJ, No.14, May 2016

3 T d c ( (, u t T p gap c c T (5 T Q T q ad T 0 T In Equations 4, 5 and 6, is te absoptivity, is te tansmissivity, -, -gd and - ae te adiation eat tansfe coefficients of te e glass laye wit te, gound and te dow espectively, and q ad is te adiation exange between te dow and diffeent sufaces in te space including walls, floo ceiling, funitue, and so on. Te tempeatue model of Duffie and Beckman [8] as been adopted in tis stu and te gound tempeatue is assumed to be equal to te doo tempeatue. Nusselt numbe as been calculated using coelations of fee convective flow and te eat and mass convection coefficients ave been found accodingly assuming a Lewis numbe of unity [9]. Lastly, te adiation eat tansfe coefficients wee calculated in tei simplified foms using te view factos between diffeent sufaces [9]. C. Sola Cimney Simila to te glazing section, enegy equations wee caied fo te sola imney mateial and te passing toug its annel. Note tat a eduction of one enegy equation is noticed due to te installation of te insulation laye in te oiginal model. Q 0 T gd gd (6 (7 d c (, u t p gap c T (8 T C. Flow Model Te sola imneys flow model of Cen et al [10, 11] is adopted. Te model assumes unidiectional upwad flow of a vetical imney wit small gap-to-eigt atio (less tan 0.2. Te volume flow ate is calculated by assuming a balance between stack pessue and pessue loss toug te imney. Te model fo te aveage flow ate depends of te buoyancy flux B, wi is a function of te sola adiation intensity itting te imney suface aea and te doo conditions, and a coelation fo te pessue loss coefficients assuming a constant cosssectional aea. gq HW c T (10 p, amb A H 1 [ f ( cin c] (11 H 2D 2 In Equations 9, 10 and 11, A is te imney cosssectional aea, g is te gavity, D is te ydaulic diamete of te annel and c in, c and f ae te pessue loss coefficient at te inlet, let and toug te imney. C. Models Implementation A MATLAB-based algoitm as been implemented to pefom te enegy calculations and find te tempeatue distibutions ove te system components. Te algoitm stats wit te equied doo conditions to estimate te flow ate using te descibed model. It ten uses te enegy equations in te evapoative coole to solve fo te tempeatue and umidity distibution along its eigt. At te end, it uses tese values as bounday conditions fo te glazing section tat as e glass, and dow tempeatues initialized. Tis section undegoes an iteative pocess until te eo between two iteations at any node is less tan 0.1 o C. Upon convegence, te sola imney section similaly initializes te tempeatues and undegoes a simila iteative pocess until te same tempeatue citeia is met. Upon te implementation of te algoitm, it as been tested fo te conditions descibed in Table 1. Table 1: Simulation Paamete. Paamete Value Outdoo Tempeatue 30 o C Outdoo Relative Humidity 33% Outdoo Global Sola Radiation 400 W/m 2 Evapoative Coole Dimensions 0.6m x 0.8m Glazing Section Dimensions 0.8m x 0.8m Sola Cimney Dimensions 0.5m x 0.8m Entie System Ai Gap 7cm 4. Results and Discussion Te esult of te simulation is sown in Fig. 4. 1/3 V (9 A( 2 Figue 4: Tempeatue ange in te entie system. 627 RE&PQJ, No.14, May 2016

4 In te 0.6m ig evapoative coole gets cooled fom a tempeatue of 30 o C to 26.8 o C by exanging eat wit te wate seets until it eaes te glazing section. In tis 0.8m ig section, extacts te eat in between te e glass and te dow and its tempeatue inceases back. In te sola imney, extacts mu moe eat due to te installation of te insulation laye and its tempeatue eaes aound 31 o C at te let. Note tat te system as managed to ende te dow at a tempeatue elatively lowe tan tat of te e glass by aound 2 o C. pecentage eduction deceased as te sola adiation intensity inceased, wi educed te system efficiency in seving its pupose. Lastly, te pecentage eduction inceased at a ige doo -bulb tempeatue. Tis is attibuted to te enancement of te evapoative coole effect at ige doo tempeatues tat ende te moe capable of extacting eat and educing te dow tempeatue. Paametic Stu Based on te conducted analysis, te effect of te annel gap as been studied fo doo tempeatues of 35 o C and 40 o C and fo diffeent sola adiation intensities anging between 300W/m 2 and 700W/m 2. Results of te dow tempeatue at a eigt of 0.4m (in te middle ae sown in Fig. 5. Note tat te same system dimensions as in Table 1 wee used. Figue 6: Pecentage diffeent in dow and e glass tempeatues fo diffeent tempeatues and sola adiations. Figue 5: Window tempeatue Vs Sola adiation fo diffeent annel gaps. As seen in Fig. 5, te dow tempeatue deceases as te gap of te annel is smalle. Tis could be attibuted to te fact tat as te gap is smalle te flow in te entie annel is faste and so becomes te eat extaction pocess. Howeve to bette undestand te effect of gap, te ange in te dow tempeatue wit and wit te installation of te suggested system sould be analyzed. Since te e glass tempeatue is elatively an estimation of ow mu te tempeatue of te dow would be if te system as not been installed, te pecentage eductions in tempeatues between te dow and te e glass ave been found and esults ae sown in Fig. 6. As it could be seen, te pecentage eduction deceases as te annel gap deceases In fact, aving a smalle gap as a counte-effect in deceasing te flow of and te effect of te sola imney, wi endes te two layes at tempeatues elatively close to ea ote. Regading te doo conditions, it was noted tat te 5. Conclusion Tis stu consideed new means of educing dow tempeatues passively and wit no additional enegy consumption. Te system consisted of a passive evapoative coole combined wit a sola imney and applied above and below glazing sufaces. Numeical models fo ea component of te system ave been developed and integated tete. Simulations ave poved te capability of te system to educe te dow tempeatue wen compaed to te e glass laye. A paametic stu as been conducted on te annel gap; it was sown tat smalle gaps tend to incease te flow velocity in te annel and educe moe te dow tempeatue. Wen compaed to te e glass, smalle eductions in tempeatues wee seen as te gap is deceased. Hige eductions could be aieved by enlaging te evapoative coole and sola imney sizes. 6. Refeences [1] E. M. Alawadi, Effect of an incompletely closed dow sutte on indoo illuminance level and eat gain, in Enegy and Buildings, 2015, 110, [2] D. Casa N. Moye A. Rudd, D. Pake S. Canda, Measued and simulated cooling pefomance compaison; insulated concete fom vesus fam constuction, in: Poceeding in ACEEE 2002 Summe Stu, Ameican Council fo an Enegy Efficiency Economy, Wasington, DC, August, [3] Tsikaloudaki, K., Laskos, K., Teodosiou, T., & Bikas, D, Te Enegy Pefomance of Windows in te Mediteanean Regions. Enegy and Buildings, 2015, 92, RE&PQJ, No.14, May 2016

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