ENERGY SAVING SOLAR FACADE FOR NON-RESIDENTIAL BUILDINGS FOR CLIMATIC CONDITION IN THE CZECH REPUBLIC Jr Sedlak and Mlos Kalousek Department of Buldng Desgn Engneerng, Faculty of Cvl Engneerng, Techncal Unversty of Brno, Vever 95, Brno, 662 37, Czech Republc, Telephone No. +420 5 41147404, +420 5 41147440, Fax No. +420 5 41240996, E-mal address: sedlak@rochla.fce.vutbr.cz, pskal@fce.vutbr.cz Abstract - The energy savng double solar facades of the Moravan Regonal Lbrary Buldng n Brno has been expermentally and numercally nvestgated to predct heat gans for heatng n the ar ventlaton system n the lbrary. The double-sknned facade conssts from an outer sngle glazng system wth adjustable louvers, a shadng system and the nner wall conssts of double glazed wndows and the sll wall from porous concrete. For the evaluaton of the solar facade the thermal and solar measurements where carred out on the fragment of the solar facade. The nfluence of nner ar space (ventlated cavty) and the constructon of the solar facade on both recovery of the solar energy durng sunshne perods and on heat losses durng nght hours has been assessed for the heatng perod. Results from the expermental and numercal nvestgatons wll be used n the regulaton system and for the operaton of the solar facade. At the present tme the lbrary buldng s under constructon. Complete montorng ncludng further expermental measurements on the solar facade wll be carred out later ths and n the next year. 1. INTRODUCTION The wder use of double-sknned solar walls wth external transparent glazng systems n modern or ntellgent offce buldngs have been known for the last two decades n Europe. A transparent external skn desgned as a part of an nteractve double solar facade s very senstve to clmate changes and must be carefully desgned for requred functons of a buldng and evaluated for regonal and local purposes all the year round accordng clmate condtons. The well desgned double skn has envronmental benefts to protect the nteror of a buldng aganst the weather and wth the applcaton of passve or actve solar systems gves hgher energy effcency and energy savngs. For the concept of the solar double-sknned facades on the south-facng walls of the Moravan Regonal Lbrary n Brno wthn total area 1740 m 2 there has been used low energy desgn whch ncludes passve solar heatng and natural ventlaton. The outer glazng skn appled on two southerly orented facades of the buldng provdes the buffer from wnd and ran, reducng unwanted nfltraton and heat losses and offerng possbltes for natural ventlaton for much longer. The shadng system of the facade s created by adjustable shutters on the wndows n the nner wall and by openng grate walkways placed n each floor on the facade (Fg. 1). The structure of the double facade provdes an external and nternal walkways for wndows cleanng and mantenance. The natural cross ventlaton of the lbrary buldng s enabled by openng wndows placed n the nner wall and adjustable glass louvers of the outer glazng skn controlled by an automatc measurng and regulaton system. The natural ventlaton s ncreased by the stack effect n the space between the outer skn and the nner wall and s regulated by means of motorzed dampers at the top of the solar facades. The nner space of the double facades nduces stack effect, whch s benefcal n provdng ventlaton and free coolng approprate to the season. One of two double-sknned solar walls n the area of 1170 m 2 placed on a south wng of the buldng provdes on cooler but sunny days the solar-warmed ar n the vod between the outer glazng skn and the nner wall whch s used to the mechancal ventlaton system to reduce heatng load n the heatng perod between September and May. The Am of ths contrbuton s to present some expermental measurements (carred out durng constructon of the buldng) and numercal nvestgatons of the breathng double-sknned ventlated glazed solar wall projected for the Moravan Regonal Lbrary n Brno. These results should be used for the mprovements of the project before fnshng of the buldng constructon for the operaton and energy management of the buldng. 2. PROBLEM DESCRIPTION Solar double walls used as ar heaters n heatng perod can provde heat supply durng recovery perods,.e. when ncdent solar energy suffcently rases the temperature of the channel borderng walls above the ambent temperature. In the heatng perod an addtonal heat s delvered from the adjacent rooms by conducton through the nternal wall of the double facade when temperatures n the channel are lower n cooler days and n non-recovery perods. Suffcent thermal capactance of the nternal wall due to thermal mass of the structure can prolong heat supply also after recovery perods. As far as nght hours and cloudy perods (non-recovery perods) n cooler days whch are concerned the double facade has heat losses lke a normal facade. In order to lmt the extent of these losses, nlet and outlet sectons have to be closed, thus preventng the nflow of could ar nto the
channel due to stack effect of preheated ar from the nternal wall or from effect of wnd. The Am of the expermental and numercal nvestgaton of the solar facade on the south wng of the buldng was to predct an nfluence of constructon materals desgned for the facade and the nfluence of convecton channel spacng on both recovery of solar energy (of the solar-warmed ar n the vod between the outer glazng skn and the nner wall) for the ventlaton and heatng system durng sunshne perods and on heat losses durng nght hours. Fg. 1 Secton of solar double-sknned facade wth openng louvers and nternal and external open grate walkways on the Moravan Regonal Lbrary n Brno The measurements of the solar double-sknned wall were carred out on the facade element (Fg. 1) before the facade erecton to evaluate the archtectural desgn and to predct energy and thermal functons of the facade for regonal clmate condtons. The element of the facade used for thermal measurements conssts from two vertcal modules-floors nstalled n the south wng of the buldng. The facade elements 1.5 m wde had an nlet and outlet sectons provded measurements under dfferent condtons for ar flow n the convecton channel of the double facade. For the convecton of ar n the facade element due to stack effect n the space between outer glazng skn and the nner wall the nlet and outlet sectons were opened and for non-ventlated cavty of the facade element the nlet and outlet sectons were closed. The measurements were carred out n the three perods wthn January March ths year. Clmatc data of global solar radaton, ar bulb temperature, relatve humdty, wnd drecton and prevalng wnd speed for the evaluaton of expermental measurements were used from the meteorologcal staton located on the unversty buldng about 200 m dstant from the ste of the Moravan Regonal Lbrary. For numercal nvestgatons of the solar double facade there was used ANSYS/FLOTRAN software. The smulaton model for energy and thermal analyss of the solar facade (n full sze conssts from 8 vertcal modules) was carred out for two veloctes of ar flow n the double facade. For the smulaton of thermal and energy balance of the solar facade durng wnter perod (to use solarwarmed ar n the vod between the outer and nner wall for mechancal ventlaton and heatng system n the buldng) there were used nput clmatc hourly data for the South Morava regon. The nput data are n the format TRY created from the meteorologcal measurements of the staton Kucharovce. The nfluence of convecton channel spacng on both recovery of solar energy durng sunshne perods and on heat losses durng nght hours was evaluated by smulaton for two thckness 0.55 m and 0.8 m of the channel.
3. MATHEMATICAL FORMULATION The double-sknned solar ventlated facade s a sold (external glazng, nternal sll wall and double glazed wndow) and flud regon (ventlated ar cavty). The mathematcal formulaton of ths conjugate the heattransfer problem whch s manly based on the followng assumptons: - Conducton and convecton are non-steady state. - Flow and temperature felds are two-dmensonal. - Ar flow s lamnar. Calculatons of the dfferent models show that n the range of our smulaton, transtons to turbulent flow n the ventlated cavty does not arse. - The deal gas equaton of state wth constant pressure s vald n order to evaluate densty varatons n the flud flows. - Vscous dsspaton and compressblty effects are neglected n the energy equaton. - Physcal propertes of ar and all materals nvolved are constant. Wth regard to the general assumptons the governng and basc equatons for the convecton heat and flud flow nsde of convecton ar gap n the facade cavty can be wrtten as: Heat flow The frst law of thermodynamcs states that thermal energy s conserved and specalzng ths to a dfferental control volume whch s gven by the equaton: T c + t T T { v} { L} T + { L} { q} q = &&& ρ (1) where:{ L } = { } v = v v x y X Y = vector operator = velocty vector for mass transport of heat { q} & q& - heat flux vector - heat generaton rate per unt volume Expandng the Stefan-Boltzmann Law to a two-surface radaton equaton, the heat transfer rate between two surfaces and j s: Q where j 4 4 ( T T ) = σε F A (2) Q σ ε F Flud flow j A T T, j j - heat transfer rate from surface - Stefan-Boltzmann constant - effectve emssvty - vew factor from surface to surface j - area of surface - absolute temperature at surface and surface j For the calculaton was used the FLOTRAN (ANSYS) soluton method wth elements FLUID 141 for the calculaton of 2-D velocty and pressure dstrbutons n a sngle phase, Newtonan flud ncludng thermal effects. The flud flow problem s defned by the laws of conservaton of mass, momentum, and energy. Assumptons for the flud flow calculatons of the facade are: the flud s Newtonan, there s only one phase, the problem doman does not change, the problem s turbulent and ncompressble. From the law of conservaton of mass law there comes out the contnuty equaton: where ñ (V ) ( ρ(vy )) x = + = 0 t X Y V, V x x (3) - components of the velocty vector n the x and y drectons σ - densty x, y - global Cartesan coordnates - tme t In the Newtonan flud, the relatonshp between the stress and rate of deformaton of the flud s: = P + u u + + u λ j τ j δ j µ δ j (4) x j x x where τ j u - stress tensor - orthogonal veloctes ( u V u = ) 1 = x, 2 V y
µ - dynamc vscosty λ - second coeffcent of vscosty The fnal term, the product of the second coeffcent of vscosty and the dvergence of the velocty, s zero for a constant densty flud. The equaton (4) transforms the momentum equatons to the Naver-Stokes equatons. 4. MEASUREMENTS OF SOLAR FACADE The solar double facade was measured and expermentally evaluated on the solar double wall element n three dfferent perods wthn January March ths year. The solar wall element conssts from two vertcal modules wth the hgh of 7.2 m and 1.5 m wde and nstalled on the level of two last floors of the buldng (Fg. 2). Results of measurements of the solar double facade on the Moravan Regonal Lbrary for the perod of 28 January tll 3 February (Fg. 3) are for the non-ventlated channel (cavty of double facade) wth the closed nlet and outlet secton. The ncreased temperatures n the cavty of the facade element are n good relaton to drect solar radaton for the measured perod. The ncreased temperatures n the solar cavty for non-recovered perods and cloudy days were nfluenced by heat transfer through the nternal wall. Resduals of temperatures between external temperature and temperature n the cavty was elevated up to 15 C and more on sunny days and recovery perods and less than 5 C n nght hours and on cloudy days. The nternal temperature n the cavty of the facade and drect solar radaton (measured on the meteorologcal staton located at the unversty campus 200m from the ste) s n good correlaton (see Fg. 3). Lower ar temperatures from the measurement n the cavty of solar element were nfluenced and lmted by the vertcal sze of 7.2 m of the facade element n comparson of the facade hgh 24.7 m. Measured data n the wnter perod were used for an assessment of thermal propertes of the double facade constructons and effect of buffer zone (ar vod between nner and outer skn of the solar facade) reducng heat losses of the buldng. For the further evaluaton of low energy desgn and proposed energy balance expected durng operaton of the buldng ncludng ventlaton and heatng system wll be carred out after the completon of the facade structure n the next wnter perod. channel (forced ar flow n the cavty) for the perod 18 24 February (Fg. 4). Thermophyscal propertes of constructon materals used for physcal model are specfed n Tab. 1. Other propertes of facade constructons used for the physcal model are determned for glazng of external envelope (thckness of glass 10 mm, transmttance τ = 0.80). ROOF CONSTRUCTION 8. FLOOR 5. COMPUTER SIMULATION AND RESULTS For smulaton of thermal behavor and energy balance of the solar facade where used clmate hourly data from the test reference year from meteorologcal staton Kucharovce valdated for the South Moravan regon. The calculaton of energy balance was carred out by the smulaton of ar temperatures n the convecton Fg. 2 Two segments used for prelmnary thermal measurements of the solar double facade
For the absorpton of solar radaton for wndows n the nternal wall where was used α = 0.20 and for wood-chps board t was α = 0.90. Geometry of 2D model (8 sectons) was created n ANSYS/FLOTRAN envronment of conssts from 1000 CFD elements FLUID 141. Example of grd sze and dstrbuton of elements n one segment s on Fg. 6. Model facade s created from 8 segments. Other propertes of facade constructon for the physcal model were determned for glazng of external envelope (thckness of glass 10 mm, transmttance τ = 0.80). For the absorpton of solar radaton for wndows n nternal wall there was used α = 0.20 and for woodchps board t was α = 0.90. Materal Thckness Densty Specfc heat Conductvty (mm) (kg/m3) (J/kg.K) (W/m.K) Ytong 300 600 840 0.21 Wood-chps boards 10 800 1500 0.22 Ar cavty 500 Glass 10 2600 840 0.76 Table 1 Thermophyscal propertes of materals used for smulaton of solar double-facade (specfcaton n secton of sll wall) For the smulaton t was determned ncompressble and turbulent ar flow. For the begnnng of smulaton there where appled 50 teraton steps for calculaton statonary for ntroductory state. Next calculaton was carred out under non-statonary state. In each step (per hour) there were solved 15 teratons wth applcaton of pressure solver CFD (TDMA method). Boundary condtons: for outer glazng of the facade there were appled external temperature (Fg. 5) wth convecton flm 15 W.m -2.K -1. For the surface on the nternal wall there was appled nternal temperature 20 C (Fg. 5) for heatng perod from 6.00 am to 8.00 pm and 15 C for nght tme wth convecton flm 8 W.m - 2.K -1. In convecton channel on both surfaces there was appled zero velocty. On the top of the channel n the outlet secton there were determned two varants of of ar flow velocty 0.1 and 0.2 m.s -1. Ventlaton of the channel was regulated accordng to dfference of external and output cavty temperature. For the ventlaton there was requred mnmum dfference of 5 C. Ventlaton (forced ar) was actve only n the heatng perod (Fg. 6) and when outdoor temperature s less 15 C. For ncdent solar radaton there were used hourly sums of drect radaton data from the TRY (Fg. 5) calculated for the vertcal wall. Fg. 3 Example of the grd sze dstrbuton of the frst segment of the facade model (8 segments) CONCLUSIONS Based on the results from the measurements and numercal nvestgatons the followng conclusons can be drawn: (1) Smulaton of heat gans from solar heated ar n the convecton channel (cavty) of the solar facade for heatng system n evaluated perod gves hgh energy effcency. Heat gans calculated from resduals of external and cavty output temperature (Fg. 6) and volume of solar heated ar are on average 24.5 W.m -2 of facade and 28.7 kw (for 1170 m 2 of facade) for velocty 0.1.s -1 and on average 42.2 W.m -2 of facade and 49.4 kw (for 1170 m 2 of facade) for velocty 0.2 m.s -1. Dstrbuton of heat gans smulated for the perod 18 24 February s expressed n graphs on Fg. 7. (2) The energy effcency of the facade s suffcently hgh for the maxmum velocty of ar flow 0.2 m.s -1 n convecton channel for the ventlaton and heatng system of the buldng. (3) From complementary smulaton studes and results the large channel spacng (cavty) more than 0.6 m wde s not suffcent for both recovery and nonrecovery perods.
30,00 Date 28.01.2000 29.01.2000 30.01.2000 31.01.2000 01.02.2000 02.02.2000 03.02.2000 1600 25,00 1400 20,00 1200 EXTERNAL Temperature ( o C) 15,00 10,00 1000 800 600 KJ/m 2 INTERNAL OUTPUT AIR IN CAVITY DIRECT SOLAR RADIATION ON HORIZONTAL 5,00 400 0,00 200-5,00 Hours 0 Fg. 4 Measurements of the solar double facade Moravan Regonal Lbrary n the perod 28.1-3.2.2000 Date 25 18.02 19.02 20.02 21.02 22.02 23.02 24.02 450 400 20 350 EXTERNAL 15 300 INTERNAL Temperature ( o C) 10 5 250 200 150 100 W/m 2 OUTPUT AIR IN CAVITY FOR AIR FLOW 0.1M/S OUTPUT AIR IN CAVITY FOR AIR FLOW 0.2M/S DIRECT SOLAR RADIATION ON THE HORIZONTAL 0 50-5 Hours 0 Fg. 5 Smulaton of ar temperatures n the cavty of double solar facade preheated from drect radaton for two steady forced ar flows 0.1 and 0.2 m.s-1. Clmate output data are used from TRY (staton Kucharovce, CZ)
20,00 15,00 INTERNAL Temperature ( o C) RESIDUAL EXTERNAL AND CAVITY FOR AIR FLOW 0.1M/S 10,00 RESIDUAL EXTERNAL AND CAVITY FOR AIR FLOW 0.2M/S 5,00 18.02 18.02 19.02 19.02 20.02 20.02 21.02 21.02 22.02 Hours and Dates Fg. 6 Resduals of external and cavty output temperature smulated for ar flow velocty 0,1 m/s and 0,2 m/s n the convecton channel of the solar facade n the recovery perod 22.02 23.02 23.02 24.02 24.02 80,0 70,0 60,0 50,0 Heat Gan (W/m 2 ) 40,0 30,0 HEAT GAINS FOR AIR FLOW 0.1M/S HEAT GAINS FOR AIR FLOW 0.2M/S 20,0 10,0 0,0 Hours Fg. 7 Smulaton of heat gans from solar preheated ar n the facade used for heatng n ventlaton system REFERENCES Journal artcles: [1] Mootz F. and Bezan J. J., (1996). Numercal study of ventlated facade panel. Solar Energy. Vol. 57, No. 1, pp 29-36. [2 ] Webler M. and Gessler G., (1996) Bürogebäude Götz, GmbH Würzburg. Deutsche Bauzetung, Sonderheft Jun, pp 14-37 Books: [3] Compagno A., (1995). Intellgent Glass Facades Materal Practce Desgn, Frst edton, Brkhäser Verlag, Basel Boston Berln. [4] Duffe A. J. and Beckman A. W. (1991), Solar Engneerng of Thermal Processes, 2 nd edn., pp.733-767. John Wley & Sons, Inc., New York. [5] Wggnton M. (1996). Glass n archtecture, Phadon Press Ltd, London.