Simple tool to evaluate the impact of daylight on building energy consumption

Transcription

1 Downloaded from orbt.dtu.dk on: Oct 15, 2018 Smple tool to evaluate the mpact of daylght on buldng energy consumpton Hvd, Chrstan Anker; Nelsen, Toke Rammer; Svendsen, Svend Publshed n: Proceedngs of the 8th Symposum on Buldng Physcs n the Nordc Countres Publcaton date: 2008 Document Verson Early verson, also known as pre-prnt Lnk back to DTU Orbt Ctaton (APA): Hvd, C. A., Nelsen, T. R., & Svendsen, S. (2008). Smple tool to evaluate the mpact of daylght on buldng energy consumpton. In Proceedngs of the 8th Symposum on Buldng Physcs n the Nordc Countres (Vol. 1, pp ). Copenhagen: Dansh Socety of Engneers, IDA. General rghts Copyrght and moral rghts for the publcatons made accessble n the publc portal are retaned by the authors and/or other copyrght owners and t s a condton of accessng publcatons that users recognse and abde by the legal requrements assocated wth these rghts. Users may download and prnt one copy of any publcaton from the publc portal for the purpose of prvate study or research. You may not further dstrbute the materal or use t for any proft-makng actvty or commercal gan You may freely dstrbute the URL dentfyng the publcaton n the publc portal If you beleve that ths document breaches copyrght please contact us provdng detals, and we wll remove access to the work mmedately and nvestgate your clam.

2 Smple tool to evaluate the mpact of daylght on buldng energy consumpton Chrstan Anker Hvd, Indutral Ph.D. student, Alecta A/S; Toke Rammer Nelsen, Assoc. prof., Depatment of Cvl Engneerng, Techncal Unversty of Denmark; Svend Svendsen, Professor, Department of Cvl Engneerng, Techncal Unversty of Denmark; KEYWORDS: Smulaton, daylght, valdaton, radosty, ntegraton, buldng desgn SUMMARY: Ths paper presents a smple buldng smulaton tool for ntegrated daylght and thermal analyss. The tool s capable of mportng the thermal and vsual propertes for dfferent glazngs and shadng postons from the Wndow Informaton System (WIS) program. Radosty methodology s used to derve the daylght levels for dfferent sky condtons on an hourly bass. The daylght levels are fed nto an exstng smple thermal smulaton program capable of calculatng energy demand and the ndoor envronment. Straghtforward control systems for general and task lghtng systems have been mplemented together wth a shadng control strategy that adjusts the shadng accordng to the ndoor operatve temperature and the profle angle of the sun. The mplemented daylght calculaton method allows for shades from the wndow recess and overhang. Comparsons wth the raytracng program Radance show that the accuracy of ths approach s adequate for predctng the energy mplcatons of photoresponsve lghtng control. 1. Introducton For ntegrated daylght and thermal smulatons several approaches and programs have been developed. One approach whch s mplemented n the program Adelne (Fraunhofer-Insttut für Bauphysk 2006) generates an annual output fle for lghtng whch may be used as an nternal load fle n a thermal smulaton program. However, ths method lacks nteractvty between daylght, lghtng, solar shadng and the thermal performance of the buldng. Another approach s to use Radance (Ward, G.L. and Shakespeare, R.A. 1998) n combnaton wth a thermal smulaton program. Ths approach has been mplemented n ESP-r (Clarke, J. and Janak, M. 1998). Generally lghtng smulaton packages nvolve a lengthy learnng effort, whch restrcts ther use to expert desgners, and they are computatonally costly for general archtectural and engneerng purposes, especally durng the ntal desgn stage. In order to reduce the computatonal burden the daylght coeffcent method has been suggested as a thrd approach (Tregenza,P.R. and Waters, I.M. 1983; Renhart, C.F. and Herkel, S. 2000). The tool descrbed n ths artcle encompasses an ntegrated thermal and lghtng smulaton approach for evaluatng the mpact of daylght and dynamc shadng devce desgn on energy demand. The amount of nput s small yet t provdes detaled hourly output of the daylght level, the electrcal energy consumpton for lghtng, heatng load, coolng load and ndoor operatve temperature. An exstng smplfed thermal smulaton tool BuldngCalc (Nelsen, T.R. 2005) and a daylght smulaton tool LghtCalc (Nelsen, T. 2005) formed the startng pont for the work. 2. Calculaton procedures There are several dfferent methods and tools for determnng daylght dstrbuton n rooms. Methods vary from smple factor calculatons through radosty methods to complex computer algorthms, such as ray-tracng. In ths tool, the radosty method s employed for nternal daylght reflectons, whle the ncdent ntal lght s calculated usng a ray-tracng approach. Ths gves a reasonable balance between accuracy and calculaton tme.

3 2.1 External lght dstrbuton External daylght may be dvded nto drect lght from the solar dsc, dffuse lght due to the scatterng propertes of the atmosphere, and dffuse lght reflected from the ground and surroundngs. The dffuse lght s modelled usng the approach n Robnson and Stone (2006) and summarzed here. An upper sky dome for atmospherc lght and a lower (nverted) sky dome for ground reflectons (one above and one below the horzontal plane) are used to model dffuse lght. Each sky vault s dvded nto 145 patches usng a dscretzaton scheme proposed by Tregenza (1987). Each patch subtends a smlar sold angle Φ (Sr), whch enables every patch to be treated as a pont source wth nsgnfcant error. The sky vault s dvded nto seven azmuthal bands of equal angular heght (sn γ,max -sn γ,mn ), n whch the azmuthal range Δα ncreases towards zenth (12, 12, 15, 15, 20, 30, 60 ). ( sn γ γ ) Φ = Δα (1), max sn,mn Let L denote the lumnance (Lm m -2 Sr -1 ) of the th patch, ξ the mean angle of ncdence (rad), and σ (0 σ 1) the vsble proporton of the patch, then the llumnance E sky on an external plane due to dffuse lght from the sky vault s expressed as: E sky = 145 ( LΦ cosξ ) = 1 σ (2) Let E n denote the drect normal llumnance and ξ the ncdence angle (solar zenth angle), then the llumnance on an external nclned plane due to drect lght E sun s expressed (Scharmer, K. and Gref, J. (2000): E sun = E n cosξ (3) Havng determned the lght sources, the reflectng ground can be represented as a lumnous up-sde down sky wth constant brghtness. Gven the ground patch lumnance L *, the llumnance due to reflected lght E ground s wrtten as: E ground = 145 * ( L Φ cosξ ) j= σ (4) j where L * s expressed as a functon of the total horzontal dffuse llumnance E sky, the drect llumnance E sun on a horzontal plane and the mean ground reflectance ρ (albedo): ( ) * ρ L = E sun + E sky π The Perez ansotropc sky model (Perez et al. 1993) s amenable to mplementaton n a computer program whle mantanng good overall performance. The lumnance of a sky pont L s gven here: (5) L = 145 j = 1 lv dh ( lvφ cosξ ) j (6) where the relatve lumnance lv s normalzed to dffuse horzontal llumnance dh as recommended by Perez et al. (1993). Dffuse horzontal and drect normal llumnances are obtaned from measured horzontal and drect normal rradances respectvely by a lumnous effcacy η gven n Perez et al. (1990). The vsble proporton σ s calculated by establshng a 10x10 grd of each patch and evaluatng the vsblty of each grd pont for all nternal surfaces. Thus σ s a functon of both dstant objects (other buldngs, the landscape) and near shades lke the wndow recess and overhang. Reflected lght from opposng buldng façades s not yet treated. 2.2 Internal daylght dstrbuton The calculaton of the nternal dstrbuton of lght s based on the lumnous extance method. Ths method s analogous to the radosty method, n that all the restrctons and assumptons are the same. Internal subsurfaces ht by transmtted drect and dffuse lght act as lght sources, wth the ntal extance Mo, f we assume these

4 surfaces have Lambertan optcal characterstcs and reflect ncdent lght perfectly dffusvely and gnore any specular propertes. The methodology and mplementaton of the daylght dstrbuton algorthms are descrbed n detal by Park (2003). 2.3 Couplng of external and nternal lght dstrbuton To establsh the ntal lght extance Mo of a subsurface the amount and the drecton of the lght and the reflectance of the surface has to be known. Therefore the external and nternal lght dstrbutons s coupled n a smple ray-tracng approach that assumes the lumnance of the sky hemsphere and ground hemsphere patches can be consdered as pont sources Dffuse lght For dffuse sky and ground lght penetratng nto the room, the extance for each nternal subsurface s calculated usng eq. (2) and eq. (4) multpled by the profle-angle dependent lght transmttance τ θ and the surface reflectance ρ: * Mo = ( Φ ) + ( Φ ) ρ L σ cosξτ θ L σ cosξτ θ j (7) = 1 j= 1 The profle angle θ s defned as the lne of elevaton projected unto the vertcal normal plane of a surface. We may also name t the perpendcular ncdence angle on a vertcal surface. The profle-angle dependent lght transmttance s used because t s calculated drectly by the WIS program (van Djk, D. and Oversloot, H. 2003), see secton 2.4. However for clear glazngs wth sotropc optcal propertes we use the profle-angle dependent data drectly as dependent on ncdence angle Drect lght For drect lght a dfferent approach s appled. It s evdent that all drect lght transmtted through the glazng hts a subsurface. Subdvdng the nternal surfaces, however, may result n false predcton of the amount of ncomng drect lght. Let E dr denote the ncdent sun lght on the wndow plane obtaned by eq. (3), A g the glazng area, A j the area of the j th nternal subsurface and m the total number of nternal subsurfaces. If we defne a m normalzaton factor χ = Edr Agτ θ E j = n A 1 jτθ cosξ j then the ntal extance Mo of the th subsurface s wrtten: cosξ Mo = E ρ τ cos ξ χ = E A ρ τ (8) n θ dr g θ m j= A j cosξ When the drect lght s transmtted through the glazng, some of the drect lght may be transformed nto dffuse lght n a dffusng devce, e.g. blnds placed n conjuncton wth the glazng. Ths effect s taken nto consderaton by calculatng the lght contrbuton from sun, sky, and ground on the wndow plane by usng eq. (2), (3), and (4). The extance of the nner glazng surface Mo g s determned by multplyng the total lght contrbuton wth the lght transmttance for drect lght that s dffused when t passes the glazng/shadng system: τ dr df. Ths lght transmttance s calculated by WIS, see secton 2.4. ( Esun + Esky + E ground ) dr df Mo g > j = τ (9) Devces that redrect the ncomng lght, e.g. a specular lght shelve are modelled usng a smple mplementaton. It s acheved by settng a specal redrectng lght transmttance τ redr to a value between 0 and 1 where 0 means that no lght s redrected and 1 that all ncomng lght s redrected. Ths means that for an ncomng ray of lght wth a gven profle angle θ the followng apples: τθ + τ dr df, θ + τ redr, θ = 1 (FIG. 1). The nclnaton angle β of the slat or lght shelve determnes the reflecton angle. Only fully specular devces are consdered and any specular nterreflectons between slats and between slats and glazng are gnored.

5 τ redr,θ β+θ β θ τ dr df,θ τ θ FIG. 1. Illustraton of how an ncomng ray of lght from the sky, sun or ground s transmtted drectly, dffused n the combned glazng and shadng, or redrected specularly wth equal nbound and outbound angle. 2.4 Lght transmttances A crtcal element n the daylght calculaton routne s the lght transmttance of the combned glazng/shadng system. For ths purpose the European software tool called WIS (van Djk, D. and Oversloot, H. 2003) s used. Ths tool mplements algorthms from the ISO standard (ISO ) capable of calculatng the lght transmttance of a transparent system for both drect and dffuse lght. WIS calculates the thermal and solar performance of multlayered wndow systems, allowng the user unlmted combnatons of glazng and solar shadng devces. Ths makes WIS a very powerful tool for evaluatng varous ntegrated daylght desgns. Currently the mprovement and verfcaton of WIS, and ts database format and database populaton are the responsblty of the EU Thematc Network WnDat, whch conssts of major European research nsttutons and manufacturers of wndow components. The output from a WIS calculaton may be n the format of a text fle. The fle ncludes the lght transmttances and solar energy transmttances for dfferent solar profle angles (-90 to 90 at 10 ncrements), and may be loaded seamlessly nto the tool descrbed n ths artcle. If the shadng devce has multple shadng postons, e.g. Venetan blnds, the user may generate and load fles for every poston requred. The tool wll lnearly nterpolate between the transmttance data loaded, thus makng the number of loaded postons a queston of desred accuracy. Because the employed method of calculatng ncdent lght on nternal subsurfaces s equvalent to a ray-tracng technque, the WIS transmttance for drect lght s employed for both dffuse and drect lght. WIS cannot yet handle specular shadng devces, e.g. lght shelves or lght redrectng devces. 3. CONTROL STRATEGIES 3.1 Thermal smulaton The smplfed thermal model s descrbed n detal n Nelsen (2005). It s capable of evaluatng the thermal ndoor envronment and heatng and coolng loads n a buldng wth very few nput parameters whle provdng the opton of sophstcated controls. 3.2 Electrcal lghtng The electrcal lghtng system can be dvded nto general and task lghtng. Both systems are defned by the power consumpton of the lghtng fxtures n W/m 2 when provdng an llumnance of 100 lux, and the mnmum (standby) power consumpton. The lnear relatonshp s shown on FIG. 2. The values for power densty and correspondng llumnance are often suppled by the producers of lghtng fxtures, and the maxmum llumnance s calculated usng the maxmum power densty.

6 Power densty [W/m2] Max densty W/m2/100 lux Mn. densty 100 Illumnance [Lux] FIG. 2. Defnton of llumnance and power densty relatonshp for lghtng systems n the tool. Both systems can be defned and controlled separately wth respect to daylght n two arbtrary ponts wthn the smulated enclosure. The possble control strateges are always max, always mn, on-off, and contnuous. The hourly ncomng daylght n the pont s evaluated and the electrcal lghtng s swtched on/off or dmmed accordng to the chosen control strategy. The on-off control swtches between the maxmum and mnmum power consumpton when the daylght level s below or above the llumnance setpont. The contnuous control nterpolates lnearly between the maxmum and mnmum power consumpton n order to meet the specfed setpont. Electrcal losses n the ballasts must be ncluded n the power densty. 3.3 Shadng The ndoor ar temperature controls the systems ncludng the shadng. Ths means that shadng s actvated only when excess heat gans occur, thus gnorng the rsk of glare for the tme beng. In the case of screens or smlar, the controls are lmted to screen up or screen down. Sun w Outsde Insde d θ β FIG. 3. Illustraton of the cut-off shadng control strategy for adjustable slats. In the case of adjustable slats or smlar the shadng s lowered and adjusted to cut-off angle when the ndoor ar temperature exceeds the specfed coolng setpont. Let d denote the dstance between two slats (m), θ the profle angle of the sun (degrees), and w the wdth of the slats (m), then the cut-off angle s calculated: β cut off d cosθ = arcsn θ (10) w 3.4 Thermal smulaton couplng It requres a sophstcated couplng to calculate the ncomng daylght, the effect of shadng on daylght levels, and thus electrcal lghtng consumpton and ndoor ar temperature. Ths s acheved by pre-calculatng the hourly daylght levels n the room wthout shadng, ntate the thermal smulaton, evaluate the hourly ndoor operatve temperature wth respect to the coolng setpont, possbly lower the shadng and adjust the slat angle (for blnds) to cut off drect sunlght, and calculate the daylght levels agan. If the operatve temperature stll exceeds the coolng setpont, ventng, ncreased ventlaton, and mechancal coolng are employed n that order. The daylght levels are evaluated at two arbtrary ponts specfed by the user.

7 4. Valdaton Of the numerous lghtng smulaton programs avalable, Radance has been extensvely valdated and repeatedly surpassed competng programs n terms of both functonalty and accuracy. For these reasons, we chose Radance as our reference model. The valdaton of the daylght calculaton algorthm s carred out for a sngle offce room as depcted on FIG. 4. Three dfferent setups are carred out for a clear glazng, external blnds adjusted to cut-off angle and an external screen. The property data s lsted n TABLE. 1 TABLE. 1: Input data for valdaton. Propertes WIS Code Property Value [-] Remarks Glazng 4-15Ar-SN4 Lght transmttance Double glazng w/ lowe coatng Blnds Wndat #01 Dffuse reflectance Slat wdth: 0.08m, slat dstance: 0.072m Screen Verosol Slverscreen Lght transmttance Total transmttance for screen+glazng FIG. 4. Room dmensons for valdaton. The chosen valdaton date s the 21st of September at 3 p.m. because t nvolves complex calculaton of solar poston, ncdence angles and cut-off angle. The radaton data s from the Dansh Desgn Reference Year and the measurements are performed at desktop heght 0.85m n nterval ponts along the centre lne of room. 18,0 16,0 14,0 12,0 10,0 8,0 6,0 4,0 2,0 0, Illumnance [lux] ,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5 5,0 5,5 Dstance from wndow [m] Radance Tool Rel. err. FIG. 5. Clear glazng. Absolute llumnance levels and relatve error. Relatve error [%] FIG. 5 to FIG. 7 compares the llumnance levels computed by Radance and by the smple tool. The relatve error s wthn ±20% for the clear glazng and for the blnds whch s consdered satsfactory n daylght research. However the largest dscrepancy s for the screen, but ths s due to two facts: the lght transmttances from WIS are undrectonal but screens have bdrectonal propertes and the screen s defned n Radance as a glazng wth reduced transmttance gnorng the dffusng propertes of the real screen.

8 Illumnance [lux] ,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5 5,0 5,5 20,0 15,0 10,0 5,0 0,0-5,0-10,0-15,0-20,0-25,0 Relatve error [%] Dstance from wndow [m] Radance Tool Rel. err. FIG. 6. External blnds at cut-off angle. Absolute llumnance levels and relatve error. Illumnance [lux] ,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5 5,0 5,5 Dstance from wndow [m] Radance Tool Rel. err. 80,0 70,0 60,0 50,0 40,0 30,0 20,0 10,0 0,0 Relatve error [%] FIG. 7. External screen. Absolute llumnance levels and relatve error. 5. Combned smulaton FIG. 8 depcts the mplcatons of daylght responsve lghtng systems. The artfcal lghtng system adjusts contnuously wth respect to the ncomng daylght complementng the llumnance defct. If overheatng occurs the shadng devce s actvated and the effect on daylght, casual gan and the thermal balance s quantfed. Consequently the tool ads n producng daylght frendly desgn and quantfes the effect of smart solar shadngs that allows the maxmum amount of drect sunlght to be tranferred dffusvely n order to save electrcal energy. FIG. 8. Daylght and artfcal lght n a pont. Contnuous dmmng control of the lghtng system. 6. Concluson The tool descrbed here s developed to evaluate the mpact of ncomng daylght on the energy consumpton for lghtng. The tool calculates the daylght dstrbuton on the bass of a ray-tracng approach and the radosty

9 method to enhance accuracy whle mantanng calculaton speed. The sky s dvded nto patches wth ndvdual lumnances n order to mtate the energy dstrbuton of the real sky. Rays are traced from each ndvdual sky patch to the surfaces of the room through glazng and shadng gvng the ntal nput to the radosty algorthm whch s employed for nternal lght dstrbuton. Wndow overhang and recess as well as dstant objects lke other buldngs reduce the vsble proporton of the sky. The daylght dstrbuton s calculated every hour, thus provdng the nformaton necessary for the thermal program to control the photoresponsve lghtng and to calculate the heat load of the electrcal lghtng system. The daylght and thermal smulatons are ntegrated meanng that the ndoor temperature s recalculated f overheatng has caused the shadng to be actvated. The daylght algorthms were valdated by comparson wth the state-of-the-art ray-tracng program, Radance, usng the Perez ansotropc sky. The results show agreement wthn 20 % relatve error, thus the smplfed tool s adequate for predctng the electrcal energy consumpton of photoresponsve lghtng systems, ncludng the mpact of complex shadng systems such as external Venetan blnds. 7. References Clarke, J. and Janak, M. (1998). Smulatng the thermal effects of daylght-controlled lghtng. Proceedngs of Buldng Performance (BEPAC UK), Issue 1. Fraunhofer-Insttut für Bauphysk, ADELINE 3.0, Abtelung Wärmetechnk, Stuttgart, Germany. Avalable from: ISO (2003). ISO 15099:2003 Thermal performance of wndows, doors and shadng devces Detaled calculatons, Internatonal Organzaton for Standardzaton, Geneva, Swtzerland. Nelsen T.R. (2005a). Smple tool to evaluate energy demand and ndoor envronment n the early stages of buldng desgn, Solar Energy, Vol. 78, No. 1, Nelsen, T., Nelsen, T. R. and Svendsen, S. (2005b). Calculaton of daylght dstrbuton and utlzaton n rooms wth solar shadngs and lght redrectng devces. Proceedngs of 7th Symposum on Buldng Physcs n the Nordc Countres, Park K.-W. and Athents A.K. (2003). Workplane llumnance predcton method for daylghtng control systems, Solar Energy, Vol. 75, No. 4, Perez R., Inechen P., Seals R., Mchalsky J. and Stewart R. (1990). Modelng daylght avalablty and rradance components from drect and global rradance, Solar Energy, Vol. 44, No. 5, Perez R., Seals R. and Mchalsky J. (1993). All-weather model for sky lumnance dstrbuton - prelmnary confguraton and valdaton, Solar Energy, Vol. 50, No. 3, Renhart C.F. and Herkel S. (2000). The smulaton of annual daylght llumnance dstrbutons - a state-of-theart comparson of sx RADIANCE-based methods, Energy and Buldngs, Vol. 32, No. 2, Robnson D. and Stone A. (2006). Internal llumnaton predcton based on a smplfed radosty algorthm, Solar Energy, Vol. 80, No. 3, Scharmer, K. and Gref, J. (2000). The European Solar Radaton Atlas. École des Mnes de Pars, France. Tregenza P.R. (1987). Subdvson of the sky hemsphere for lumnance measurements, Lghtng Research & Technology, Vol. 19, No. 1, Tregenza P.R. and Waters I.M. (1983). Daylght coeffcents, Lghtng Research & Technology, Vol. 15, No. 2, van Djk, D. and Oversloot, H. (2003). WIS, the European tool to calculate thermal and solar propertes of wndows and wndow components. Proceedngs of IBPSA, Buldng Smulaton, Vol. 1, Ward, G. L. and Shakespeare, R. A. (1998). Renderng wth Radance - The art and scence of lghtng vsualzaton. Morgan Kaufmann, San Francsco.