Yueping Fang Final Version

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E n e r g y P e r f o r m a n c e i n B u i l d i n g s

The Influence of Isolation on the ThermalPerformance of Triple Vacuum GlazingYueping Fang, Trevor J Hyde, Neil J HewittSchool of the Built Environment, University of Ulster, BT37 0QB, Newtownabbey, Northern Ireland, UK

Abstract

The thermal performance of triplevacuum glazing subjected to varioussolar insolation levels was simulatedusing a finite volume model. Simulationresults show that with increasinginsolation, the temperatures of the threeglass panes increase; however the rateof temperature increase of the middleglass pane is the largest. This is due tothe high thermal insulation providedby the vacuum gaps either side of themiddle glass pane; consequently theheat absorbed from the sun by themiddle glass pane cannot be easilytransferred to the indoor and outdoorenvironments. For 0.5 m by 0.5 mand 1 m by 1 m triple vacuum glazingexposed, to isolation levels greater than200 W.m-2 and 180 W.m-2 respectivelywith four 0.03 emittance coatings, themiddle glass sheet temperature is larger

than that of the indoor and outdoorglass sheets. Thus the heat absorbedfrom solar radiation by the middleglass sheet flows to both the indoorand outdoor glass sheets. When theinsolation is less than 200 W.m-2 and180 W.m-2 for 0.5 m by 0.5 m glazingand 1 m by 1 m glazing respectively, theheat flows from the indoor glass sheetto the middle glass sheet and then tothe outdoor glass sheet. For a 0.5 mby 0.5 m triple vacuum glazing withfour 0.18 emittance coatings, wheninsolation is greater than 400 W.m-2, themiddle glass sheet temperature is higher

than that of indoor glass sheet.

Introduction

Double vacuum glazing was firstpatented by Zoller (1913). Since then,many patents have been reported,however only recently was the firstsuccessful fabricated vacuum glazingreported by a team from the Universityof Sydney (Collins and Robinson, 1991).This vacuum glazing used solder glasswith a melting point in the region of450 °C as a vacuum edge seal. Sincethen a team at the University of Ulster

(Hyde et al., 2000, Griffiths et al., 1998)employed an indium based alloy witha melting point of less than 200°C toseal the evacuated space between theglass sheets, making possible the use

of a large range of soft low-emittance(low-e) coatings and tempered glasswhich would otherwise degrade at highsealing temperatures. Using the lowtemperature indium based edge seal,samples have been fabricated with aheat transmission U-value of 0.90 W.m-

2.K-1 in the centre-of-glazing area for a

0.5 m by 0.5 m sample. More recentlyresearch into new sealing techniques(Wittwer V., 2005), outgassing (Minaaiet al., 2005; Ng and Collins, 2005),thermal performance modeling andexperimental validation (Collins andSimko, 1998; Fang et al., 2005, 2006,2009a, 2009b, and Zhao et al., 2007)and vacuum glazing application(Trushevskii and Mitina, 2008) has beenpublished in peer reviewed scientific journals.

In an attempt to further reduce theheat transmission through the glazing,the concept of triple vacuum glazing

as shown in Fig. 1 was presented by ateam of Swiss Federal Laboratories for

Material Testing and Research with basicmechanical design constraints discussed(Manz et al., 2006). A heat transmissionof 0.2 W.m-2.K-1 in the centre-of-glazingarea was predicted when using astainless steel pillar array with a pillardiameter of 0.3 mm and the four glasssurfaces within the two vacuum gaps

coated with low-e coatings. The thermalperformance of the entire triple vacuumglazing was simulated by Fang et al.,(2010) varying parameters such as edgeseal width, edge seal material and framerebate depth. With such a low thermaltransmission and consequently apotentially high temperature differentialfrom one side of the glazing to theother, the level of insolation has animportant bearing on the thermalperformance of the triple vacuumglazing. In the work presented in thispaper, a three-dimensional finite volumemodel was developed to simulate the

thermal performance of the entiretriple vacuum glazing with the support

Fig. 1 (a)

Schematic diagram of triplevacuum glazing within abuilding; (b) enlarged view of heat flow through triplevacuum glazing under solar radiation.



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pillar arrays within the two vacuumgaps integrated and modeled directly.The numerical simulation results werecompared with those calculated usingthe analytical model. Using this finitevolume model, this paper investigatedthe effects of isolation on the glasssurface temperatures of triple vacuumglazing.

Methodology

The simulated triple vacuum glazingcomprised three glass sheets with thefour internal glass surfaces coatedwith low-emittance (low-e) coatingsas shown in Fig. 1. A finite volumemodel (Eames and Norton, 1993)extensively validated experimentally inprevious research (Fang et al., 2006)was modified to analyse heat transferthrough a triple vacuum glazing understandard winter conditions (EN ISO,2000) and various insolations. Thesimple analytic model (Collins andSimko, 1998) developed to predict heatflow through each individual supportpillar of a vacuum glazing was not usedin this analysis since the pillar array wasincorporated and modeled directly inthe finite volume model. In the model,a square cross section pillar with thesame cross sectional area representsthe circular cross section pillars of thefabricated system since the pillars withthe same cross section conducted similaramounts of heat under same conditions(Holman, 1989). A mesh refined toprovide a high density of nodes in andaround the pillar was used to enable anaccurate prediction of heat transfer in

this area. The simulated glazing systemcomprised four low-e coated glasspanes with an emittance of 0.03 and0.18 separated by two vacuum gaps.Due to symmetry considerations, onlyone quarter pane of the triple vacuumglazing was modeled to represent thethermal performance of the entireglazing.

In the finite volume model, the rateof radiative heat transfer between eachof the finite volume surfaces with areasS and mean surface temperatures T

1

and T 2

, that form the two plane parallelglass surfaces containing the vacuum

with hemispherical emittance e1 ande2

was determined (Collins and Simko,1998) using:

(1)

Where the effective emittance,, was determined by:

(2)

Radiation reflectance was assumedto be independent of the wavelength,angle of the incident radiation andsurface temperature. The errorassociated with calculating emittanceusing equation 2 in this researchis about 4% (Zhang et al., 1997).This ignores dependence on surfacetemperature, the wavelength and angleof incidence of the radiation.

As can be seen from Fig. 2, solarradiation absorbed in each finite volumecan be determined by:

(3)

where A z

and A z+1

are the intensitiesof solar radiation at the two surfacesof the finite volume determined usingequation 4.

(4)

where Ce is the extinction coefficientof glass which is a measure of how wellit absorbs electromagnetic radiation(Duffie and Beckman, 1991). Duffieand Beckman (1991) reported that theextinction coefficient of glass varies

from 32 m-1

for “greenish edge” glassto 4 m-1 for “water white” glass. In thiswork Ce was assumed to be 30 m-1. Z

L

is the path length through the glazingfrom the front glass surface.

It was assumed that each low-ecoating absorbs 10% of the incidentsolar radiation (Hollands et al., 2001).The radiation absorbed within eachfinite volume was calculated andincluded in the energy balance for eachfinite volume.

Thermal performance of triplevacuum glazing under solarradiation

The thermal performance of a triplevacuum glazing with 0 Wm-2 and

500 Wm-2 insolation was simulatedby using the finite volume modeldescribed in section 2. The isothermsof the simulated triple vacuum glazingare presented in Fig. 3. The boundaryconditions of EN ISO 10077-1 (2000)

standard are listed in table 1. The otherparameters of the glazing are listed intable 2.

Fig. 3a shows the temperaturegradient from the indoor to the outdoorglass sheets. The large temperaturedifference between each of the glasssheets was caused by the high thermalinsulation provided by each vacuumgap. The heat conduction through thesupport pillars can be clearly seen. Themean temperature at the centre-of-glazing area is clearly higher than at theedge area due to conduction throughthe edge seal. Fig. 3b shows that the

middle glass sheet temperature is higherthan both the indoor and outdoor glasssheets, since the heat absorbed bythe middle glass sheet cannot transfer

dxdy A A I dI Z Z inabsorbed

)(1+

=

LCeZ

L e A−

−= 1

)( 4

2

4

1 T T S eQ effectiveradiation −= σ

1111

21

−+=

eeeeffective

eeffective

Fig 2

Schematic diagram of a glass sheet with low-e coating illus-trating a finite volume unit areadx, dy and incident insolationIin

perpendicular to the glazing surface.

Table 1

Boundary conditions of the triple vacuum glaz-ing

Table 2

Parameters of modeled triple vacuum glazing.



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E n e r g y P e r f o r m a n c e i n B u i l d i n g s

to the indoor or outdoor glass sheetsdue to the high thermal insulation ofthe two vacuum gaps at either side ofmiddle glass sheet.

With various insolations incidentperpendicular to the outdoor glasssurface, the thermal performance of0.5 m by 0.5 m and 1 m by 1 m triplevacuum glazings with a 6 mm wideindium edge seal were simulated usingthe boundary conditions of EN ISO

10077-1 (2000) listed in Table 1 and theresults are presented in Fig. 4. All otherconfiguration parameters are shown inTable 2.

Fig. 4 shows that for the 0.5 m by0.5 m triple vacuum glazing, with 0W.m-2 insolation, the temperatures ofthe outdoor, middle and indoor glasspanes are 0.9 °C, 6.2 °C and 15.2 °Crespectively. When the insolation is 900W.m-2, the temperatures of the outdoor,middle and indoor glass panes are 15.5°C, 72.4 °C and 45.8 °C respectively. For1 m by 1 m triple vacuum glazing, with0 W.m-2 insolation, the temperatures of

the outdoor, middle and indoor glasspanes are 0.6 °C, 6.3 °C and 16.3 °Crespectively. When insolation is 900W.m-2, the temperatures of the outdoor,middle and indoor glass panes are 14.6°C, 81.4 °C and 47.9 °C respectively.With increasing insolation, thetemperatures of the three glass panesincrease, however the rate in increaseof the middle glass pane temperatureis the greatest. Due to the high thermalinsulation of the vacuum gaps eitherside of the middle glass pane, the heatabsorbed from solar radiation by themiddle glass pane cannot easily flow

to the indoor or outdoor glass pane.For 0.5 m by 0.5 m triple vacuumglazing, when the isolation is greaterthan 200 W.m-2, the middle glass panetemperature is greater than that of the

indoor glass pane; for the 1 m by 1 mtriple vacuum glazing, when insolationis greater than 180 W.m-2, the middleglass pane temperature is greater thanthat of the indoor glass pane, thus theheat absorbed from solar radiationby the middle glass pane flows to theindoor and outdoor glass panes.

The outdoor glass pane temperatureof the 0.5 m by 0.5 m triple vacuumglazing is greater that of the 1 m by

1 m triple vacuum glazing; the indoorglass pane temperature of the 0.5 mby 0.5 m triple vacuum glazing is lowerthan that of the 1 m by 1 m triplevacuum glazing. The middle glass panetemperature of the 0.5 m by 0.5 mtriple vacuum glazing is lower than thatof 1 m by 1 m triple vacuum glazing.This is because the heat flow from the

middle glass pane to the indoor andoutdoor glass panes of the 0.5 m by0.5 m glazing is greater than that ofthe 1 m by 1 m glazing system as thethermal conductance of the 0.5 m by0.5 m glazing is greater than that of the1 m by 1 m glazing system. Due to theedge conduction influence, the largerthe glazing size, the lower the heattransmission of the total glazings areawill be (Fang et al., 2010).

The temperature variations of thethree glass panes of triple vacuumglazing with low-e coatings ofemittance 0.03 and 0.18 subject tovarious levels of insolation are presentedin Fig. 5. This figure shows thatsubjected to the same insolation level,the indoor glass sheet temperatureof the glazing with 0.03 emittance is

Fig. 3 (a)

Isotherms of triple vacuum glazing with insolation of 0 Wm-2 , (b) of 500 Wm-2.

3 a 3 b

Fig. 4

The mean glass panetemperatures of triplevacuum glazing under various insolations inci-dent perpendicular to theoutdoor glass surface.



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higher than that of the glazing with0.18 emittance coatings, (a maximumdifference of 1.9 °C occurred at aninsolation of 900 W.m-2), while theoutdoor glass sheet temperature ofthe glazing with 0.03 emittance islower than that with 0.18 emittancecoatings (a maximum difference of 1.7°C occurred at an insolation of 900W.m-2); i.e. the temperature differencebetween the indoor and outdoor glasssheets of the triple vacuum glazing with0.03 emittance coatings is larger thanthat with 0.18 emittance coatings. Themiddle glass sheet temperature of triplevacuum glazing with 0.03 emittance islarger than that of glazing with 0.18emittance by 22.6 °C at an insolation of900 W.m-2, i.e. when emittance valueof four low-e coatings within the triplevacuum glazing are 0.03 and 0.18, themiddle glass sheet temperature of thetriple vacuum glazing with emittanceof 0.03 is significantly larger than thatwith emittance of 0.18. This is becausethe radiative heat flow across thevacuum gap of triple vacuum glazingwith 0.03 emittance is much lowerthan that with 0.18 emittance. For atriple vacuum glazing with four 0.03emittance coatings, when the insolationincreases to 200 W.m-2, the middleglass sheet temperature is higher thanthat of indoor glass sheet; for thetriple vacuum glazing with four 0.18emittance coatings, when insolationincreases to 400 W.m-2, the middle glasssheet temperature is higher than that ofindoor glass sheet.

Conclusions

Simulation results showed that withincreasing insolation, the temperaturesof the three glass panes increase;however the rate of temperatureincrease of the middle glass pane isthe largest. Due to the high thermalinsulation of the vacuum gaps eitherside of the middle glass pane, the heatabsorbed from the sun by the middleglass pane cannot easily transfer tothe indoor and outdoor environments.For 0.5 m by 0.5 m triple vacuumglazing, when the insolation is largerthan 200 W.m-2, the middle glass pane

temperature is greater than that ofthe indoor glass pane; for the 1 m by1 m triple vacuum glazing, the wheninsolation is larger than 180 W.m-2, themiddle glass pane temperature is largerthan that of the indoor glass pane. Thusthe heat absorbed by the middle glasspane from the solar radiation flowsto both the indoor and outdoor glasspanes.

For triple vacuum glazing with fourlow-e coatings of emittance of 0.03and 0.18, the temperatures both ofthe indoor and outdoor glass sheets ofthe two triple vacuum glazing exhibits

approximately 2 °C of a difference whenthe insolation is 900 W.m-2, howeverthere is a significant temperaturedifference between the middle glasssheets of the two triple vacuum glazing.

Fig. 5

Temperature variationsof triple vacuum glazing glass sheets with emit-tance of 0.03 and 0.18.

When subjected to an insolation of900 W.m-2, the middle glass sheettemperature of the glazing with 0.03emittance is 22.6 °C larger than thatwith 0.18 emittance coatings. Whenthe insolation is less than 400 W.m-2 for0.5 m by 0.5 m triple vacuum glazingwith four 0.18 emittance coatings, theheat flows from the indoor glass paneto the middle glass pane, then to theoutdoor glass pane. When the insolationis higher than 400 W.m-2, the middleglass sheet temperature is higher thanthat of indoor glass sheet. The heatflows from the middle glass sheet to theindoor glass sheet, then to the indoorenvironment.

Acknowledgements

The authors acknowledge the supportfrom the Charles Parson EnergyResearch Awards through the National

Development 2007-2013 of theDepartment of Communications, Marineand Natural Resources, Dublin, Ireland.

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Yueping Fang Final Version