DEVELOPMENT OF APPROACH TO OPTIMIZATION OF BUILDING ENVELOPE DESIGN IN ASPECT OF THERMAL COMFORT AND ENERGY USE. Ruey Lung Hwang 1, Fu Shun Tsai 2

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1 Proceedngs: Buldng Smulaton 27 DEVELOPMENT OF APPROACH TO OPTIMIZATION OF BUILDING ENVELOPE DESIGN IN ASPECT OF THERMAL COMFORT AND ENERGY USE Ruey Lung Hwang 1, Fu Shun Tsa 2 1 Department of Occupatonal Safety and Health, Chna Medcal Unversty Tachung, Tawan, Chna 2 Department of Mechancal Engneerng, CECI Engneerng Consultants, Inc Tape, Tawan, Chna ABSTRACT Ths study examned the effect of buldng envelope on thermal comfort. The effects of key energy conservaton measures, such as wndow/wall rato, transmttance of fenestraton glass and shadng devces, were studed. The output from EnergyPlus was use to predct ther nfluence on thermal comfort. Standard energy conservng measures proposed by ENVLOAD to reduce ndoor thermal dscomfort and coolng energy consumpton were examned. Ths study proposes an approach whch smultaneously optmzes both energy savngs and thermal comfort. Employng ths approach to optmze coolng setpont can optmze energy savngs. KEYWORDS Buldng envelope, thermal comfort, energy savng. INTRODUCTION Lghtweght constructon desgns have ganed popularty n Tawan, partcularly the use of glass curtan walls or large wndows. In summer, the nteror surface of wndows may ncrease dramatcally due to absorpton of ncdental solar energy even though room ar temperature may reman at a comfortable temperature. The ndoor thermal envronment, normally a major concern of occupants, s degraded by the large, hot surfaces of wndows. Therefore, lowerng ndoor ar temperature set-pont, whch gves the penalty of ncreasng energy consumpton, s guaranteed to the comfort of occupants.. Conversely, the green buldng ntatves am to acheve thermal comfort by mnmzng energy consumpton. A well-desgned buldng envelope s consdered the best approach to achevng ths goal. As n most other ndustralzed countres, the buldng envelope s defned n Tawan by ENVLOAD [CPA 23], an energy conservaton ndex whch s calculated by the followng expresson: ENVLOAD = a k =1 ( A η K IH ) + b Aen (1) where η s the transmttance of glass; K s the correcton factor for a shadng devce; IH s the nsolaton hours, n WH/m 2 -yr; A s the area of wndow, n m 2 ; Aen s the area of buldng envelope, n m 2 ; a, b are constant. As equaton (1) shows, the three key factors n ENVLOAD are wndow-wall-rato, transmttance of fenestraton glass and effect of shadng devce. Unfortunately, most studes nvolvng ENVLOAD focus on desgnng a buldng envelope to save energy rather than mprove the ndoor thermal envronment. Ths study therefore examnes the effect of buldng envelope on thermal comfort. The role of ENVLOAD n promotng thermal comfort s also examned. SIMULATION Methodology A typcal space was examned as a reference. Features of envelope desgn affectng coolng energy requrements and thermal comfort were dentfed. The EnergyPlus [Crawley et al. 21] computer program was then run for parametrc smulatons by varyng the values of chosen parameters over ther specfed range. The ndoor clmate data produced by each buldng envelope desgn were used as nput to the Fanger comfort equaton [ISO 199] to measure thermal comfort. The Fanger comfort equaton was programmed nto an EXCEL spreadsheet. Hourly EnergyPlus output was mported nto the same spreadsheet to calculate hourly comfort ndces. In order of better analyssng ths aspect of the problem, two ndex are here ntroduced: and seasonal mean predcted percentage of dssatsfed(ppd). The s smply the total hours of predcted mean votes (PMV) values over.5. The total and seasonal mean PPD are evaluated as follows: N Overheated hours = Δt for PMV.5 (2) =

2 Proceedngs: Buldng Smulaton 27 mean PPD = PPD N = 1 Δt (3) where PMV and PPD are the PMV and PPD values n the th tme perod Δt. The examned tme perod was the hours of occupaton durng weekdays between Aprl 1 and October 31. Reference Space Descrpton The reference space s 5. m by 5. m square wth walls 3. m hgh. One wall s exteror and facng west, and three walls are nteror. A wndow s nstalled on the west façade. The constructon characterstcs of the floor, celng, exteror wall and nteror walls are dentcal to those for a medumweght constructon shown n the ASHRAE Handbook of Fundamentals [ASHRAE 21]. Internal heat gans from occupants, lghtng and equpment were ncluded n all calculatons. The coolng capacty of the HVAC system was automatcally determned by the program EnergyPlus. Coolng capacty was the peak load of the space under ASHRAE summer desgn condtons for Tape [ASHRAE 21]. Typcal Tape weather was assumed for annual smulaton. Parameters Analyzed Buldng envelope factors nfluencng coolng energy requrements and thermal comfort, ncludng wndow-wall-rato, transmttance and absorbance of fenestraton glass, shadng devces, glass conductance, thermal nsulaton and thermal mass of walls, floor and roof. For smplcty, the three ENVLOAD parameters, wndow/wall-rato, transmttance of glass and shadng devce, were dentfed n the followng parametrc smulatons. Three curtans were selected to analyze the effect of shadng devces on ndoor thermal envronment: lght color curtan wth η=.7, medum color curtan wth η=.5 and dark color curtan wth η=.1. The two selected wndow szes were.mx3.m and.mx1.5m wth correspondng wndow/wall ratos of.9 and.5, respectvely. Table 1 shows thermal characterstc of seven generc wndows. Calculatng PMV and PPD As formulated by ISO 773[ISO 1991], the Fanger equaton s expressed n two comfort varables (.e., PMV and PPD). The PPD s gven n terms of the PMV as The followng summarzes the treatment or extracton of each parameter from the EnergyPlus hourly output. Table 1 Thermal characterstc of seven generc wndows No Descrpton Code η + α + 1 6mm clear S-Clr mm green S-Tn mm reflectve S-Ref mm Low-E S-LoE mm ar gap, D-Clr mm clear 6 6mm ar gap, D-Tn mm green 7 6mm ar gap, 6mm Low-E D-LoE.28.5 Ar temperature, Ta: Ths varable s determned by EnergyPlus output data. Relatve humdty,φ: The zone relatve humdty, also extracted from the EnergyPlus data.. Relatve ar Speed, v: Snce EnergyPlus cannot calculate zone ar speed, v s treated as a constant.25 m/s; that s, the ASHRAE acceptable summer ar speed. Metabolc rate, Mw: Specfed as a constant 1.2 met, the metabolc rate for general offce actvty. Clothng ndex, Icl: Held constant at.6 clo, whch s the nsulaton value of typcal summer clothng. Mean radant temperature, Tmrt: The mean radant temperature s derved from the absolute surface temperature of the surroundng surfaces, T, and the angle factors between the person and the surroundng surfaces, F p- T mrt = T 1 F p 1 + T2 Fp L T F (6) n p n In equaton (6) the temperature of surroundng surfaces are extracted from the EnergyPlus output data. The angle factor can be computed as a functon of wdth a, and heght b, of the surface and as a functon of the dstance c, between the person and the surface, by the followng equaton: [Rzzo 1991] PPD = 1 95exp[ (.3353PMV PMV () The PMV s then related to sx parameters by PMV = f ( Ta, φ, v, Tmrt, Mw, Icl) (5) 2 )] F P = F max [1 e ( a / c) / τ where τ = A + B( a / c) γ = C + D( b / c) + E( a / c) ][1 e ( b / c) / r ] (7) (8)

3 Proceedngs: Buldng Smulaton 27 The coeffcents Fmax, A, B, C, D and E are calculated for an occupant sttng along the centerlne of the wndow. The occupant s seated 1.5 m away from the wndow. DISCUSSION AND RESULTS Envronmental dscomfort s usually due to solar radaton and outdoor temperature. Both factors affect the surface temperature of wndows and surroundng, whch sgnfcantly affect mean radant temperature and thus comfort. Fgure 1 shows the transmtted sun radaton and surface temperatures of seven generc wndows wth dfferent curtans under ASHRAE s 1% coolng desgn condton of Tape. The surface temperature of wndows s relatvely hgher than the outsde ar temperature. Hgh absorbance glass such as sngle pane tnted glass and sngle pane reflectve glass have nsde surface temperatures of 2ºC or more whle clear glass has an nsde surface temperature of 35ºC due to low absorptance. The curtan also dramatcally affects the nsde surface temperature of the glass. However, the ncreased transmttance of the curtan lmts temperature reducton. surface temperture, C curtan opened medum color curtan lght color curtan dark color curtan S-Clr S-Tn S-Ref S-LoE D-Clr D-Tn D-LoE types of glass Fgure 1 Varaton n nsde surface temperature for seven generc glass wndows nsolaton, W/m no curtan medum color curtan S-Clr S-Tn S-Ref S-LoE D-Clr D-Tn D-LoE Type of Glass lght color curtan dark color curtan Fgure 2 Varaton n radaton transmsson for seven generc glass wndows The sgnfcant varaton n transmtted sun radaton n seven generc glass wndows s due mostly to the effect of transmttance. Fgue 2 shows that the ncreased transmttance of the curtan sgnfcantly dlutes solar radaton. Fgures 3-5 show the mpact of varyng values for dentfed parameters on thermal comfort. As Fg. 3 shows, wndow/wall rato clearly reduces. As no curtan s used, the elmnated n the seven generc wndows are smlar. The large wndow (WWR=.9) has an average of 25 more than the small wndow (WWR=.5). The hgher average s due to dfferences n solar radaton passng through the wndows and somewhat to dfference of the angle factors between large wndow and small wndow. Conversely, the patterns of overheated hours for both wndow szes s dfferent as medum color curtan s used. Wndow/wall rato produces a dfference of 25 n clear glass cases, 15 hours n tnted glass cases and as few as 2 hours n low-e glass and reflectve glass cases. Reducton n s less dramatc n small wndows wth glass transmttance lower than.3 because the dfference n sun radaton s less sgnfcant WWR=.9 WWR=.5 no curtan D-LoE S-Ref S-LoE D-Tn S-Tn S-Clr D-Clr type of glass medum color curtan Fgure 3 Effect of wndow/wall rato on thermal comfort As Fg. shows, glass type affects oveheated hours. Fgure lsts glass types n order of transmttance. The double loe-e glass had mnmum transmttance, but t dd not exhbt mnmum oveheated hours due to the absorbtance affectng the nsde surface temperature of the glass. As Fg. 1 shows, the nsde surface temperature of double low-e glass s 37ºC, whch s 6ºC less than that of sngle reflectve glass. The decreased nsde surface temperture tends to dmnsh the dscomfort effect of radaton through the wndow. Fgure also shows that overheated hours ncrease wth ncreasng transmttance of glass. Excludng double low-e glass, the change n

4 Proceedngs: Buldng Smulaton WWR=.9 WWR=.5 S-Ref D-LoE S-LoE D-Tn S-Tn D-Clr S-Clr type of glass Fgure Effect of glass transmttance on comfort s approxmately 18 hours for both large and small wndow areas. Fgure 5 shows that the s very senstve to specfc combnatons of glass type and curtan type. At WWR=.9, the number of n sngle reflectve glass or double low-e glass was below 5 hours when curtans are used, even wth transmttance as hgh as.7 (lght color). Where s zero, such as n the combnaton of double low-e glass and lght colored curtan, further energy-savng s stll possble by allowng ncreased ar temperature wthout sacrfcng thermal comfort. However, n sngle clear glass, the are stll approxmately 1 hours even though a dark colored curtan (transmttance =.1) s used. Ths ndcates that the only way to reduce dscomfort s by lowerng ar temperature n the space, whch thus ncreases energy consumpton of the ar condtonng system. Therefore, choosng the approprate combnaton of glass and curtan s mportant not only for thermal comfort but also for energy conservaton. Comfort trends versus energy savngs measures were also examned, as Fgs. 6-7 llustrate. Fgure 6 estmated aganst seasonal mean PPD for entre cases consstng of ffty-sx runs of the EnergyPlus programs. Fgure 5 demonstrates that no are observed when the seasonal mean PPD s lower than 13%. Ths means that occupant dscomfort does not happen n the range of seasonal mean PPD<13%. When seasonal mean PPD>13%, the value of ncreases lnearly aganst seasonal mean PPD untlt reaches 2%. In the range of seasonal mean PPD>2%, the value of also ncreases lnearly but the ncrease s gradual over the perod. Fgure 7 dsplays the lnear relatonshp establshed between mean PPD and ENVLOAD. The R 2 value for the ftted lne s.8889 and sgnfcant. The ENVLOAD benchmark s 8 KWH/m 2 -yr. As Fg. 6 llustrates, the seasonal mean PPD at benchmark value s approxmates13%, ndcatng no dscomfort. Ths mples that a buldng envelope met the energy conservaton benchmarks of ENVLOAD also ensure no dscomfort occurrs n the coolng season. Overheated hours Mean PPD, % Fgure 6 Overheated hours aganst seasonal mean PPD D-LoE, large wndow D-LoE, small wndow S-Ref, large wndow S-Ref, small wndow D-Tn,large wndow D-Tn, small wndow S-Clr, large wndow S-Clr, small wndow Mean PPD, % lght color curtan medum color curtan dark color blnd R 2 = no lght medum dark types of curtan ENVLOAD, KWH/m 2 -yr Fgure 5 Effect of shadng devce on comfort

5 Proceedngs: Buldng Smulaton 27 Fgure 7 Relatonshp between energy conservaton measures and seasonal mean PPD The above dscusson and smulaton results assume a 2ºC coolng set-pont.however, a more practcal soluton s settng a coolng set-pont lmt above whch ar temperature adjustment for energy savngs s nadvsable. If the actual coolng set-pont for a bult envronment s below the specfed lmt, then addtonal energy conservaton measures may be mplemented, but ndoor thermal comfort attaned would reman at the same level,.e., no overheated hours are observed. Table 2 dsplays the addtonal energy savng models examned n RUN #1 and RUN #2. Table 2 Energy and comfort comparson of two cases tem RUN #1 RUN #2 ENVLOAD 7 9 Coolng set-pont temperature=2ºc Coolng energy Mean PPD 5.9% 7.% Coolng set-pont temperature=25ºc Coolng energy Mean PPD 9.9% 12.8% Coolng set-pont temperature=26ºc Coolng energy Mean PPD 12.9% 21.1% Unt: ENVLOAD n KWH/m 2 -yr; Coolng energy n KWH/m 2 -yr The RUN#1 model had an ENVLOAD value of 7 KWH/m 2 -yr whle the RUN#2 had a value of 9 KWH/m 2 -yr. The coolng set-pont temperatures were ncreased from a base of 2ºC to 26ºC. Although seasonal mean PPD ncreased progressvely over the perod, Run #1 exhbted an opportnuty of energy savngs untl 26ºC. Addtonal savngs of 11.3 KWH/m 2 -yr (about 17%) coolng energy were observed. In RUN#2, the only energy savngs,. KWH/m 2 -yr (about %) coolng energy, occurred 25ºC. However, wth a well-desgned buldng envelope, total savngs of 12.8 KWH/m 2 -yr coolng energy was achevable. CONCLUSION In ths study, the concept of PMV-PPD was employed to assess the performance of buldng envelopes. Among the envelope parameters, those affectng solar transmsson were also found to sgnfcantly affect ndoor thermal comfort. These parameters, wndow wall rato, transmttance of fenestraton glass and shadng devce, are also the key features of the ENVLOAD standard energy conservng measures n Tawan. Ths study also examned whether ENVLOAD energy conservng measures can be appled for optmzng thermal comfort. To explore ths nference, hourly EnergyPlus output were correlated wth thermal comfort ndcatons.a sgnfcant lnear correlaton between ENVLOAD and mean PPD was observed. Thus, ENVLOAD can be employed for both energy conservaton as well as thermal comfort. The two example runs verfed that addtonal energy conservaton can be acheved f buldng envelope and coolng set-pont of ar temperature are consdered n buldng desgn. ACKNOWLEDGMENT The authors would lke to thank the Archtecture and Buldng Reaserch Insttute, Mnstry of Interor of the Republc of Chan, Tawan for fnancally supportng ths research under Contract No. MOIS REFERENCES ASHRAE, 21, ASHRAE Handbook of Fundamentals, Atlanta: Amercan Socety of Heatng, Refrgeratng, and Ar-Condtonng Engneers, Inc. CPA. 23. Techncal code and calculatng example foe energy conservaton desgn of buldng for offce buldngs, 23 Edton, Constructon and Plannng Agency, Mnstry of the Interor, Tawan Crawley, Drury B., Lnda K. Lawre, Curts O. Pedersen, Rchard K. Strand, Rchard J. Lesen, Frederck C. Wnkelmann, W. F. Buhl, Y. Joe Huang, A. Ender Erdem, Danel E. Fsher, Mchael J Wtte, and Jason Glazer. 21. "EnergyPlus: Creatng a New-Generaton Buldng Energy Smulaton Program," n Energy & Buldngs, Volume 33, Issue, pp ISO, ISO 773:199 Moderate thermal envronments - Determnaton of the PMV and PPD ndces and specfcaton of the condtons for thermal comfort. Rzzo G, Franztta G, Cannstraro G. 1991, Algorthms for the calculaton of the mean projected area factors of seated and standng persons. Energy and Buldngs; vol. 17, pp