ENERGY MODELLING OF ETFE MEMBRANES IN BUILDING APPLICATIONS

Eleventh Internatinal IBPSA Cnference Glasgw, Sctland July 27-30, 2009 ENERGY MODELLING OF ETFE MEMBRANES IN BUILDING APPLICATIONS Harris Pirazis, Mikkel Kragh, and Charlie Hgg Arup, 13 Fitzry Street, Lndn W1T 4BQ, United Kingdm ABSTRACT ETFE (ethylene tetraflurethylene) is a lightweight material increasingly used in building applicatins. It has gained ppularity mainly due t its daylight transmittance and the ptential fr energy savings. When used as cladding ETFE sheets are usually assembled int cushins, which are inflated fr structural reasns. ETFE cushins can prvide thermal insulatin with reduced initial csts and less structural supprts as cmpared with a cnventinal glazed rf. Limited research regarding the mdelling f ETFE in building applicatins and limited availability f infrmatin n material prperties led t the present study. Designers are currently facing difficulties when carrying ut energy ptimisatin studies as part f the design prcess. Fr example, since ETFE is nt entirely paque t lngwave radiatin, merely treating the material as a standard glass layer can lead t errrs when evaluating its thermal perfrmance. In rder t enable building designers t assess the perfrmance f these systems, maximising perfrmance and managing risk, it is essential t gain knwledge and develp methds t mdel this nvel material. This study takes int accunt the lngwave transmissin prperties f the ETFE material and discusses the need fr a methdlgy fr estimating surface temperatures, heat lsses, and slar gains. Guidelines fr integratin are needed t define its prperties and t evaluate perfrmance during the building design prcess. INTRODUCTION ETFE is a relatively new, lightweight material increasingly used in buildings, mainly due t its lightweight prperties, its high daylight transmittance and the ptentials fr energy savings. When used fr cladding, sheets f ETFE are usually assembled int cushins which are inflated (fr structural reasns) by means f cmpressrs. The system cnsists f tw r mre sheets f fil laid n tp f each ther and jined at the edges t frm the cladding equivalent f an inflatable cushin. As stated, ETFE cushins can prvide thermal insulatin, with reduced initial cst investments and fewer supprts cmpared with a glazed rf (Rbinsn, 2005). Hwever, due t the lack f infrmatin n the material prperties it becmes difficult fr designers t deliver energy perfrmance ptimised designs. Additinally, since ETFE is nt paque t lngwave radiatin, treating it as a glass layer can lead t errrs, when evaluating its perfrmance. Therefre, it becmes essential t gain knwledge and develp methds t mdel this material in rder t maximise perfrmance (and minimise risk). AIM OF THE STUDY This paper deals with energy transmissin aspects f ETFE in building design. The study fcuses n thermal and ptical perfrmance f ETFE. Initially, a brief descriptin f the ETFE material is given in rder t reach a fundamental understanding f its perfrmance. Calculatin methds and appraches fr existing prjects are mentined and a first evaluatin f these methds is utlined. The paper presents recmmendatins fr further studies f ETFE energy transfer mdelling fr building perfrmance simulatin. BACKGROUND ETFE in building design ETFE has apprximately 95% light transmittance, but des nt ffer the clear visibility/transparency f glass (Rbinsn, 2005). As a result, ETFE slutins therefre initially fund use n prjects such as btanical gardens, zlgical gardens, swimming pls, and exhibitins spaces. Hwever, ETFE is increasingly finding its place in mre traditinal buildings as rfing fr curtyards, shpping malls, atria and stres. The ETFE material has been used n prminent architectural prjects such as the Eden Centre and the Water Cube and it is currently cnsidered fr a number f high prfile internatinal sprts venues. Previus ETFE studies have fcused mainly n structural prperties and related issues, while little research has been carried ut in rder t determine energy transmissin prperties and characteristics in terms f envirnmental building design. - 696 -

transmittance thrugh the ETFE layers, in practice ETFE fils are usually mdelled as glazing units. Depending n the building use, the building design, the site, and gegraphical lcatin f the building, this simplificatin may impact n the accuracy f the simulated building perfrmance, as discussed in the fllwing. Figure 1 ETFE in building design Mdelling f ETFE in building simulatin tls Implementing ETFE cushins in building design is a cmplicated task due t the unusual transmissin characteristics f the material. Since currently available cmmercial sftware tls are nt develped t take int accunt the lngwave Shrtwave and lngwave radiatin This sectin presents a brief theretical backgrund, in rder t gain an understanding f the particular prperties f ETFE and the resulting ptential shrtcmings f current energy mdelling tls and methds. Electrmagnetic radiatin is an energy frm, which cmprises what we refer t as heat and light. The electrmagnetic spectrum is utlined in Figure 2. The term thermal radiatin (relating t heat transfer) ranges frm a wavelength f apprximately 0.1μm t 100μm and includes part f the ultravilet (UV) and all f the visible light and infrared (IR) radiatin. Shrtwave Lngwave Thermal radiatin Visible Gamma rays X rays Ultravilet Infrared Micrwave 10-5 10-4 10-3 10-2 10-1 1 10 10 2 10 3 10 4 Figure 2 Bands f the electrmagnetic radiatin spectrum All bdies emit and absrb energy in the frm f electrmagnetic radiatin. At a given temperature, the thermal radiatin emitted frm a surface varies fr different wavelengths. The term spectral is used t indicate this dependence. The spectral distributin depends n the characteristics and temperature f the emitting surface. In rder t accurately quantify radiative heat transfer, the spectral and directinal effects shuld be taken int accunt (Incrpera et al., 2002). ETFE THERMAL AND OPTICAL PROPERTIES General One f the main reasns fr using ETFE is the lw thermal transmittance achieved fr large span mdules (Rbinsn-Gayle et al., 2001). In Table 1 a cmparisn f thermal transmittance (U-value) and ttal slar energy transmittance (g-value) f insulating glazing units and ETFE cushins is presented. - 697 -

Table 1 Thermal and slar transmittance fr glazing units and ETFE cushins (Salz et al., 2006) U-value g-value (W/m 2 K) 6mm mnlithic glass 5.9 0.95 6-12-6 Duble Glazing 2.8 0.83 Unit (DGU) 6-12-6 High 2.0 0.35 Perfrmance Duble Glazing Unit (DGU) 2 Layer ETFE Cushin 2.9 0.71-0.22 (with frit) 3 Layer ETFE Cushin 1.9 0.71-0.22 (with frit) 4 Layer ETFE Cushin 1.4 0.71-0.22 (with frit) The thermal and ptical prperties f the ETFE cushins can be altered significantly by applicatin f catings, print, gemetry and the build-up in which they are applied. The fllwing tw examples illustrate hw energy transmissin thrugh an ETFE cushin (transmissin, reflectin and absrptin) can be mdified (Salz et al., 2006). Applicatin f a reflective frit t an inflatable intermediate cushin; the intermediate fils can be in an pen r clsed psitin allwing heat and daylight int the inner space as shwn in Figure 3. summer case reflective frit at the intermediate layer winter case utdrs lw E cating indrs slar cntrl cating Figure 4 Catings in ETFE cushin Usually ETFE cushins incrprate tw r three air chambers. Cnvective heat transfer within these air chambers will influence the thermal perfrmance f the cushin and estimatin f U-values is generally cmplex. Mdelling f ETFE cushins as part f building perfrmance simulatin is therefre nt straightfrward. The perfrmance f the systems can be assessed by means f cmputatinal fluid dynamics (CFD) and/r by empirical (ht bx) testing. Transmissin prperties f ETFE and glass A ptentially imprtant difference between glass and ETFE is the way in which lngwave radiatin is blcked r transmitted. Glazing is virtually paque t lngwave radiatin, while ETFE transmits part f the lngwave radiatin as indicated in Figure 6 (Salz et al., 2006). It is difficult t btain infrmatin n the physical prperties f ETFE fr the lngwave spectrum. Precise knwledge f the ETFE spectral behaviur is essential fr increasing the cnfidence in predictins fr the impact f lngwave radiatin n the building perfrmance. It shuld be clearly stated that the infrmatin in Figure 5 is nt cnfirmed fr its accuracy and has been included fr illustrative purpses nly. Figure 3 Frit in ETFE cushin Applicatin f catings (lw emissivity cating in rder t reduce the lngwave transmissin lsses i.e. during a cld winter night prviding lwer thermal transmittance values and/r slar cntrl cating in rder t reduce the slar transmittance) as shwn in Figure 4. - 698 -

Figure 5 Spectral transmissin thrugh Glass and ETFE The visual light transmittance f ETFE is 94-97% with ultravilet transmittance being in the 83-88% range. Within the visible part f the slar spectrum the frequencies are fairly evenly transmitted thrugh the material, which means that the clurs viewed thrugh the ETFE are nt disturbed (Rbinsn, 2005). As mentined abve, glass is practically paque t lngwave radiatin. Fr a given glass surface, the lngwave radiatin emitted depends n the surface emissivity and temperature. A temperature difference between the pane and its surrundings will result in exchange f lngwave radiatin. In Figure 6 a cmparisn f the mechanism between a triple glazed unit and a triple-layer ETFE cushin is presented. - Triple Glazed Unit, expsed t slar radiatin: Incident slar radiatin reaches the 1 st pane; part shrtwave and part lngwave radiatin. The shrtwave radiatin is transmitted, absrbed, and reflected. The absrbed energy will lead t an increase in the temperature f the 1 st pane. The same applies fr the 2 nd and 3 rd pane. The glazing is practically paque t lngwave radiatin. In terms f lngwave radiatin exchange, the relatively very high temperature f the sun means that the exchange is cmpletely dminated by the incident lngwave radiatin frm the sun. Hwever, depending n the temperature difference between the panes and the emissivities f their surfaces, lngwave radiatin is exchanged between the 1 st and 2 nd pane similar is the mechanism between the 1st pane and the utdr envirnment. Depending n the radiative temperatures f the panes and the surrundings, the net (r resulting) lngwave radiatin flux can be inwards r utwards. The resulting temperature f the panes depends n the lngwave radiatin exchange, cnvectin between the panes and absrptin (including the effect f multiple reflectins). The ttal slar transmittance (gvalue) f the triple glazed system is the sum f the shrtwave transmitted part, the net lngwave radiatin emitted frm the 3rd pane t the indr side, and the energy transfer by cnvectin frm the 3 rd pane t the indr side. By reducing the emissivity f ne f the panes (lw emissivity catings), we reduce the lngwave radiative exchange and therefre reduce the thermal transmittance f the system. By filling the cavities with Argn r Kryptn, we reduce the heat exchange due t cnvectin between the panes, achieving similar results (lwer U- values). - 699 -

Slar radiatin Shrtwave radiatin Lngwave radiatin 1 2 3 1 2 3 Glazing ETFE Figure 6 Mechanism f slar radiatin fr triple glazing and ETFE fils - ETFE layers, expsed t slar radiatin: Incident slar radiatin reaches the 1 st layer; part shrtwave and part lngwave radiatin As fr the glazing system, the shrtwave radiatin is transmitted absrbed and reflected by the layer. The absrbed shrtwave energy leads t an increase in the temperature f the 1 st layer. The mechanism is similar fr the 2 nd and 3 rd layer. ETFE is nt paque t lngwave radiatin. Therefre, when slar radiatin reaches the 1 st layer, part f the lngwave radiatin is transmitted. The mechanism f ETFE is similar t the ne f glass, but in this case, a reduced part will be absrbed and re-emitted due t the transmissin. In the case f ETFE the lngwave transmittance impacts n the transmissin f energy absrbed in layers (fr instance a fritted layer) which is emitted and transmitted thrugh ther layers, as well situatins where lngwave exchange ccur between inside and utside acrss the ETFE. The significance f these effects will vary with the envirnmental cnditins and the prperties f the ETFE build-up. ETFE PERFORMANCE This study has identified a prnunced need fr detailed infrmatin and dcumentatin f ETFE material prperties. In particular, infrmatin n energy transfer characteristics ETFE is scarce. Cnsequently, a level f uncertainty still surrunds the thermal perfrmance f ETFE. Effect f lng wave transmissin n building perfrmance In rder t better understand the ptential effect f lngwave transmissin n the resulting building perfrmance, a space with a glazed rf is cmpared with a space with an ETFE rf fr tw scenaris: a cld (winter) night and a warm sunny (summer) day. During a cld winter night with clear sky the temperature f the flr within the space will be higher than the radiant temperature f the sky. Since the indr space will be able t transmit lngwave radiatin thrugh the ETFE rf directly t the cld sky, a lngwave exchange between the indr space and the utdr envirnment will take place. The resultant heat transfer depends n the temperature difference, the temperature f the ETFE layers and their lngwave transmissin prperties. Similarly and depending n view factrs, lngwave radiatin exchange may ccur between the ccupied space and the surrunding buildings. When expsed t slar radiatin, during a warm summer day, the shrtwave energy transmissin will typically dminate, but lngwave radiatin exchange will ptentially affect the resulting heat transfer, depending again n the temperature differences between the different ETFE layers (which may include fritted and thus absrbing layers) the flr f the ccupied space, the sky and any surrunding buildings. - 700 -

Impact f frit n lngwave transmissin A key parameter influencing the perfrmance f ETFE cushins is the pssible presence f a fritted intermediate layer (Figure 7). In general, the main purpse f the frit is t intrduce shading and reduce the transmitted slar energy int the ccupied space. The slar transmissin may be variable by means f multiple fritted layers, which can be regulated t vary the cmbined shading effect. During a cld night, such a frit wuld increase the thermal insulatin, since its pacity t lngwave wuld reduce the lngwave heat transfer between the flr and the sky. During a warm sunny day, the frit will reduce the amunt f shrtwave radiatin entering the space; the higher the frit density, the lwer the direct shrtwave penetratin. The frit will reflect a part f the shrtwave and absrb anther part increasing the temperature f the layer. A highly absrbing frit wuld increase the temperature f the middle layer mre than a highly reflecting ne, increasing the emitted lngwave radiance (twards indrs and utdrs depending n the temperature differences and emissivities). On the ther hand, since the frit is paque t lngwave radiatin, a fritted intermediate layer wuld shield frm transmissin f lngwave radiatin frm utdrs. lngwave cntributin due t frit Figure 7 Impact f fritting n ETFE perfrmance The impact f fritting reduces the risk f verheating f a space, but the energy transfer mechanism is cmplicated. Quantifying the impact f an lngwave transmissin thrugh an ETFE rf; a case study Accurate assessment f ETFE systems and their impact n building energy perfrmance (including thermal cmfrt) is currently nt pssible due t (a) uncertainty regarding lngwave physical prperties f ETFE and (b) dynamic building perfrmance simulatin sftware tls d nt take int accunt the spectral prperties f ETFE in the underlying calculatin methds. Hwever, in rder t assess the effect f lngwave transmissin and investigate whether the current practice f mdelling the ETFE as glass significantly impacts n the simulated building perfrmance, a preliminary study was carried ut fr a summer scenari. The study was divided in three parts: Cmparisn f the relative effect f heat transfer by cnvectin and lngwave radiatin Estimatin f expected maximum ETFE layer temperatures Develpment f a simplified mathematical mdel based n heat transfer balance fr each layer The simplified mathematical mdel was develped in rder t estimate the effect f lngwave radiative transmissin thrugh an ETFE cushin. The study was carried during the early design stage f a stadium prject. The main bjective was t evaluate whether the lngwave cntributin affects substantially the simulated building perfrmance and therefre, whether, cnsequently, it shuld be included in the dynamic thermal mdel. The ETFE cushins were mdelled as a rf and the calculatins were carried ut fr the prject design summer cnditins. The heat flw mdel fr the ETFE cushin was develped by taking int cnsideratin the heat balance fr each f the cmpnent ETFE layers. Fr each f the layers the radiative and cnvective heat transfer were included in the heat balance (including the transmitted radiatin). The main utput f the develped mdel were the ETFE layer temperatures and the rf element heat fluxes. This utput is purely indicative and serves t assess the impact f lngwave radiative transmissin prperties n the resulting thermal perfrmance f the element. The assumptins and cnstraints incrprated int the simplified mdel are described belw: The mdel deals with a single rf element; therefre, wall effects are nt included. Linear radiatin heat transfer cefficients were assumed. The same cefficients were used fr all layers f the cushins irrespectively f their temperature. This ptentially leads t underestimatin f emissin frm ht surfaces and verestimatin f radiatin frm cld surfaces. The air temperature within the cushin is taken as the mean temperature f the tw adjacent layers. The sky radiatin temperature and the utdr dry bulb temperature are assumed the same (humid cnditins). The indr mean radiant temperature is assumed identical t the indr dry bulb temperature. The layers have a lngwave reflectivity f 0 and a lngwave emissivity f 1. - 701 -

The amunt f incming slar radiatin absrbed by the ETFE layers is assigned explicitly t each layer (see Figure 8). The ETFE layers in the build-up all have the same transmittance. The ETFE lngwave transmittance was arbitrarily set t 0.2 (while the crrespnding transmittance f glass is 0.0) This apprach des nt accunt fr any lngwave absrptin f interlayer shading. The effect f the intermediate layer frit n the lngwave transmissin was nt accunted fr. Table 2 Heat fluxes and temperatures fr a simplified glass and ETFE mdel (fr bth cases direct slar transmittance (T dir ) f 6% was assumed) Lngwave radiative gain [W/m 2 ] Cnvective gain [W/m 2 ] GLASS ETFE 195 218 73 56 Ttal gain [W/m 2 ] 328 334 External layer temperature [ C] 48 45 Interlayer temperature [ C] 81 74 Internal layer temperature [ C] 58 53 Figure 8 Bundary cnditins fr mathematical mdel f the rf (lngwave radiative cnvective heat transfer evaluated by the mdel are nt shwn) Fr cmparisn the perfrmance f glass and ETFE systems have been calculated fr particular envirnmental cnditins, chsen t represent a rf slutin with a fritted interlayer in a ht humid climate. Table 2 presents the heat fluxes t the internal envirnment and the temperatures f each layer within the glazing and ETFE system given by the simplified mdel. The main cnclusins drawn frm this study are: The ETFE cnstructin has a 12% increase in the lngwave heat flux frm the element when cmpared t a glass cnstructin. The ETFE cnstructin has a 2% increase in the ttal heat gain thrugh the element. In a scenari with slar radiatin, the effects f lngwave transmissin thrugh the ETFE are nt cnsidered significant (especially when cnsidering the errrs that may ccur by inaccurate input in the thermal mdels). Fr ther cnfiguratins and different envirnmental cnditins, the lngwave transmissin is likely t have a mre significant impact n the simulated thermal perfrmance f ETFE building elements. A particular cnditin where the simulated perfrmance f ETFE may be significantly different t that f glazed systems is that f radiatin frm the indr side t a cld sky at night. The magnitude f this impact has nt yet been specifically investigated but will be the subject f further research and simulatin activities. CONCLUSIONS AND SUGGESTIONS FOR FURTHER RESEARCH The main aim f the present study was t describe the state-f-the-art regarding the understanding f ETFE energy transfer characteristics and perfrmance assessment methds. Further research is suggested with a view t evaluating in mre detail the perfrmance f ETFE and increase cnfidence in its implementatin during the design prcess. ETFE prperties Available infrmatin n ETFE spectral prperties is scarce. Previus studies in this field are limited and manufacturers appear t treat the infrmatin as - 702 -

cnfidential. As a first step, accurate and validated data n ETFE spectral prperties (transmittance, absrptance, reflectance) have t be gathered. The infrmatin shwn indicatively in Figure 6 abve shuld be verified. In the absence f verified manufacturers data, it may be necessary t cllect representative samples and carry ut certified measurements in cllabratin with relevant research centres. Assessing the impact f lngwave transmissin Previusly, effrts have been made t evaluate the impact f lngwave transmissin n ttal heat transfer. The mathematical mdel develped as part f the present study was highly simplified and fcusing n the maximum ptential effect during a warm summer day. Accrding t the results btained frm this tl, the impact f lngwave transmissin is insignificant fr a warm sunny day. The impact in a cld night scenari shuld be investigated, but the present mdel is nt suited due t the specific simplifying assumptins intrduced. Tentative mdels did nt prvide cnfidence in the applicability f the tl fr the different bundary cnditins and further wrk is required. The assumptins made were entirely reasnable fr the specific study, but befre the tl is applied t ther cases cnsideratin shuld be given t the fllwing: Errr due t linearised radiative heat transfer cefficients. Inclusin f lngwave pacity f the frit. Individual layers mdelled separately in terms f transmittance, reflectance, and emissivity. The mdel shuld be further validated. A tl that can assess the lngwave transmissin effects fr steady state bundary cnditins wuld be essential in rder t assess its impact and evaluate whether necessary t implement the spectral transmissin mdel cncept in a dynamic building perfrmance simulatin tl. Is it reasnable t assume an average transmittance acrss the whle spectrum fr ETFE materials? Since the magnitude f radiatin at any wavelength varies with the characteristics and temperature f the emitting surface, the amunt f estimated transmitted energy frm/thrugh an ETFE layer may depend n the assumed spectral prperties. Similarly, is it reasnable t assume an average emissivity fr all wavelengths fr an ETFE material? T sum up, it wuld be relevant t investigate hw varius available building perfrmance simulatin tls treat lngwave radiatin and (depending n the results f the research prpsed abve) cnsider further develpment f tls either fr generic use r specifically fr the purpse f assessment f lngwave transmissin effects. REFERENCES Incrpera P.F., De Witt P.D., 2002. Fundamentals f heat and mass transfer (5 th editin), Jhn Wiley and Sns, ISBN 0-471-38650-2 Rbinsn A.L. 2005. Structural Oprtunities f ETFE (ethylene tetra flur ethylene), Masters thesis at the Massachusetts Institute f Technlgy, USA Rbinsn-Gayle S., Klktrni M., Cripps A., Tannb S., 2001. ETFE fil cushins in rfs and atria, Jurnal f Cnstructin and Building Materials 15, p 323-327 Salz C., Schepers H., 2006. Arup s ETFE Material nte Implementatin f lngwave transmittance mdel in building perfrmance simulatin tls Currently, there is n dynamic building perfrmance simulatin tl that incrprates spectral prperties f layers in the windw mdel. In mst f the cases an average transmittance, reflectance and absrptance are defined fr the shrtwave regin and reflectance and absrptance are defined fr the lngwave part f the spectrum. The emissivity f the different layers is als defined as cnstant values. Fr cnventinal glazing these assumptins are acceptable, since the transmittance f panes is relatively cnstant acrss the shrtwave range. Hwever, the fllwing questins arise: Hw is reflectance and absrptance treated acrss the lngwave part f the spectrum? - 703 -