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Solar Energy 84 (2010) 516–525

Thermal performance analysis of an electrochromic vacuum glazingwith low emittance coatings

Yueping Fang a,*, Trevor Hyde a, Neil Hewitt a, Philip C. Eames b, Brian Norton c

a Centre for Sustainable Technologies, School of the Built Environment, University of Ulster, Newtownabbey, BT37 0QB N. Ireland, UKb Centre for Research in Renewable Energy Science and Technology, University of Loughborough, UK

c Dublin Energy Lab, Dublin Institute of Technology, Aungier Street, Dublin 2, Ireland

Received 30 September 2008; received in revised form 16 February 2009; accepted 20 February 2009Available online 28 March 2009

Communicated by: Associate Editor: J.-L. Scartezzini

Abstract

Thermal performance of an electrochromic (EC) vacuum glazing (VG) was modelled under ASTM standard winter conditions. TheEC VG comprised three 0.5 m by 0.5 m glass panes with a 0.12 mm wide evacuated space between two 4 mm thick panes sealed contig-uously by a 6 mm wide indium based edge seal with either one or two low-emittance (low-e) coatings supported by a 0.32 mm diametersquare pillar grid spaced at 25 mm. The third glass pane on which the 0.1 mm thick EC layer was deposited was sealed to the evacuatedglass unit. The whole unit was rebated by 10 mm within a solid wood frame. The low-e coating absorbed 10% of solar energy incident onit. With the EC VG installed with the EC component facing the outdoor environment, for an incident solar radiation of 300 W m�2,simulations demonstrated that when the EC layer is opaque for winter conditions, the temperature of the inside glass pane is higher thanthe indoor air temperature, due to solar radiation absorbed by the low-e coatings and the EC layer, the EC VG is a heat source with heattransferred from the glazing to the interior environment. When the emittance was lower to 0.02, the outdoor and indoor glass pane tem-peratures of the glazing with single and two low-e coatings are very close to each other. For an insolation of 1000 W m�2, the outdoorglass pane temperature exceeds the indoor glass pane temperature, consequentially the outdoor glass pane transfers heat to the indoorglass pane.� 2009 Elsevier Ltd. All rights reserved.

Keywords: Electrochromic vacuum glazing; Low-e coating; Thermal performance; Emittance; Insolation

1. Introduction

The first successful method for fabricating vacuum glaz-ing (VG) reported by Robinson and Collins (1989) used alow melt solder glass gasket that formed a contiguous edgeseal at temperatures of above 450 �C. At such tempera-tures, tempered glass and many types of soft low-emittance(low-e) coatings degrade. A low temperature method (i.e.less than 200 �C) for producing an edge seal for a VG over-

0038-092X/$ - see front matter � 2009 Elsevier Ltd. All rights reserved.

doi:10.1016/j.solener.2009.02.007

* Corresponding author. Tel.: +44 2890368227; fax: +44 2890238239.E-mail addresses: y.fang@ulster.ac.uk, fangyueping@yahoo.com (Y.

Fang).

comes this (Griffiths et al., 1998). The procedure for fabri-cating a VG at low temperature (Hyde et al., 2000) and itsmeasured performance properties (Fang et al., 2000, 2006;Fang, 2002) have been reported. A measured heat transmit-tance of less than 1 W m�2 K�1 in the centre-of-glass areaof the VG has been achieved. An electrochromic (EC) filmcan be used to control visual light transmittance when a 1–2 V direct current (DC) switching power is applied. Simu-lation studies of window systems have considered the effectof control strategies on energy saving (Kailsson et al.,2000). Selkowitz et al. (1994) and Sullivan et al. (1996)showed that in a cooling-dominated climate, the inclusionof an EC double glazed window can enable a building to

EC layer

Vacuum edge seal

Two glass panes comprising the vacuum glazing

Glass panesSupport pillars

PillarsLow-e coatings Edge sealPillarsLow-e coatings Edge seal

Indoor side

Outdoor side

Nomenclature

A radiation intensity at the surface of a finite vol-ume unit (W)

a radius of support pillar (m)E extinction coefficient of glass (m�1)h surface heat transfer coefficient (W m�2 K�1)I insolation intensity (W m�2)k heat conductivity of glass (W m�1 K�1)L radiation flux path length through glazing (m)P pillar separation (m)Q heat transfer (W)T temperature (�C)S area (m2)

t thickness of glass panes (m)U heat transmittance (W m�2 K�1)

Greek letters

� hemispherical emittance of a surfacer Stefan–Boltzmann constant (5.67 � 10�8 W m

�2 K�4)

Subscripts

1,2 internal and external sample surfacesin incidentZ (1, 2, 3, . . .,n) position of radiation considered

Y. Fang et al. / Solar Energy 84 (2010) 516–525 517

consume less cooling energy by controlling visual lighttransmission and solar heat gain coefficient when com-pared to conventional double glazings. In a heating-domi-nated climate, as an EC window should remain in itsbleached state during the heating season, it does notimprove energy performance.

An EC VG combines an EC smart window with a VGproviding both very low heat transmittance and optimalvisible light transmittance whilst allowing control of solargain and thus thermal comfort for building occupants. Itmay reduce peak energy demands both for cooling duringsummer and heating during winter. In a study of the effectof insolation intensity on the thermal behaviour of EC VG(Fang and Eames, 2006a), it was found that to avoid intol-erable overheating discomfort, the EC layer between theVG and the third glass pane must face the outdoorenvironment.

Depending on its emittance value, employing a low-ecoating on one or two glass surfaces within the vacuumgap of the VG reduces radiative heat transfer across theglazing significantly (Collins and Simko, 1998). A low-ecoating also reduces solar and visual transmittance. Never-theless, the use of a low-e coating increases thermal resis-tance much more than it reduces the solar gain (Hollandset al., 2001). For a glazing, to obtain net energy perfor-mance, both the total heat transmittance and the total solarenergy transmittance should be calculated. The overall heattransfer coefficient of a VG has been determined experi-mentally by Fang et al. (2006). The solar heat gain coeffi-cient can be determined in accordance with ISO 9050(2000) employing calculations to determine how muchradiation is absorbed and re-emitted inwardly.

Glass panes

EC layer

Glass panes

EC layer

Outdoor side

Fig. 1. Schematic diagram of an EC VG. The third glass pane coated withEC layer is combined with the normal vacuum glazing. One or two low-ecoatings are coated on the internal glass surface within the vacuum gap.The diagram is not to scale.

2. Methodology

The modelled EC VG consisted of a low temperaturefabricated VG combined with an additional glass pane withan EC layer incorporated as shown in Fig. 1. A finite vol-ume model (Eames and Norton, 1993) validated experi-mentally in extensive previous research (Fang, 2002;

Fang et al., 2006) was modified to analyse heat transferthrough an EC VG under testing standard winter condi-tions (ASTM C1363, 2005). A simple analytic model (Col-lins and Simko, 1998) developed to predict heat flowthrough each individual support pillar of a VG was notused in this analysis since the pillar array is incorporatedand modelled directly in the finite volume model. In themodel, a square cross section pillar with the same area rep-resents the circular cross section pillars of the fabricatedsystem (Griffiths et al., 1998). The discretization of themesh for numerical calculation of thermal conductionrefined to provide a high density of nodes in and aroundeach pillar to enable an accurate prediction of heat trans-

518 Y. Fang et al. / Solar Energy 84 (2010) 516–525

fer. The simulated glazing system comprised two low-e filmcoated glass panes with an emittance of 0.18 separated by avacuum gap with a third glass pane with an EC layerattached. Due to symmetry considerations, the one quarterpane of the EC VG modelled gives a representation of thebehaviour of a complete EC VG window.

In the finite volume model, the radiative heat transferrate between each of the finite volume surfaces with areasS and mean surface temperatures T1 and T2, that formthe two plane parallel glass surfaces containing the vacuumwith hemispherical emittance e1 and e2 was determined(Collins and Simko, 1998) using:

Qradiation ¼ eeffectiverSðT 41 � T 4

2Þ ð1Þwhere r is Stefan–Boltzmann constant and the effectiveemittance, eeffective, was determined by:

1

eeffective

¼ 1

e1

þ 1

e2

� 1 ð2Þ

Radiation reflectance was assumed to be independent ofthe wavelength, angle of the incident radiation and surfacetemperature. The maximum error associated with ignoringthese effects has been shown to be about 4% for twouncoated glass sheets (Zhang et al., 1997).

Interactions between the levels of heat conductionthrough the pillars and radiation between the two internalglass surfaces within the evacuated gap for a well madevacuum glazing when compared to the total heat flowthrough the overall glazing system is small and can beignored. The total thermal conductance can then be deter-mined by (Collins and Simko, 1998):

Cglass–glass;centre-of-glazing¼Cglass–glass;gasþCglass–glass;radiation ð3ÞþCglass–glass;pillars¼0:8P þ4�effectiverT 3

averageþ2kglassa=P 2 ð4Þ

where P is the internal pressure, measured in Pascal. For asuccessful vacuum glazing sample P is less than 0.1 Pa, thusCglass–glass,gas can be ignored. Taverage is the average of thetemperatures T1 and T2, kglass is the glass thermal conduc-tivity, a is the pillar radius, p is the pillar separation andeeffective is the effective emittance.

Coating

Z1

Z2

Z3

dy

dx

Iin

dz

Zn

Glass

Fig. 2. Schematic diagram of a glass pane with a low-e coating illustratinga finite volume unit area dx, dy and incident insolation Iin.

As can be seen from Fig. 2, solar radiation absorbed ineach finite volume can be determined by:

dIabsorbed ¼ I inðAZ � AZþ1Þdxdy ð5Þ

where Az and Az+1 are the intensities of solar radiation atthe two surfaces of the finite volume determined usingEq. (6).

AL ¼ 1� e�ELZ ð6Þ

where E, the extinction coefficient of glass is a measure ofhow the glass absorbs electromagnetic radiation (Duffieand Beckman, 1991). The extinction coefficient of glassvaries from 32 m�1 for ‘‘greenish cast of edge” glass to4 m�1 for ‘‘water white” glass (Duffie and Beckman,1991). In this work E was assumed to be 30 m�1. LZ isthe path length through the glazing from the front glasssurface.

It was assumed that each low-e coating absorbs 10%of the incident solar radiation (Hollands et al., 2001).The radiation absorbed within each finite volume wascalculated and included in the energy balance for eachfinite volume. The thermal performance of a small cen-tral area (25 mm by 25 mm) of the vacuum glazingincluding a singular pillar was simulated using a meshof 80 � 80 � 28 nodes. The mesh was graded to providea denser number of nodes closer to the pillar. 28 nodeswere distributed in a graded mesh through the glazingthickness of 12.12 mm. The thermal conductance of thissimulated unit with a pillar in the centre was in goodagreement with the analytic prediction with 1.5% varia-tion, which is comparable to the result of Wilson et al.(1998). This level of agreement indicates that the densityof nodes is sufficient to simulate the realistic level of heatflow with high accuracy.

3. Thermal performance of an EC VG

The EC layer in the transparent state absorbs approxi-mately 10% of the incident solar energy, in the opaquestate, the energy absorption of an EC layer is 60%. Theseconditions were incorporated in the simulations. In thesimulations, the indoor air set-point temperature and theoutdoor ambient air temperatures were assumed to be con-stant at 21.1 and �17.8 �C, respectively, the convectiveheat transfer coefficients on the indoor and the outdoorglass surfaces were assumed to be 8.3 and 30 W m�2 K�1,respectively, corresponding to those in ASTM measure-ment standards for winter conditions (ASTM C1363,2005). The simulated EC VG was 0.5 m by 0.5 m and com-prised three 4 mm thick glass panes. The VG componentscomprised two glass panes coated with one or two low-ecoatings, separated by 0.12 mm, supported by a 0.32 mmdiameter pillar array spaced at 25 mm in a regular squarepattern. The edge seal was a 6 mm wide band of indium.In the simulation, it was also assumed that the thermal con-ductivity of the EC layer was 1 W m�1 K�1 with thickness

Table 1Parameters of modelled EC VG.

Parameter Value

Dimensions Thickness, width,length

12.22, 500,500 mm

Glass pane thickness 4 mmEmittance One or two surfaces 0.02 or 0.18Edge seal width 6 mmPillar diameter 0.32 mmPillar height 0.12 mmPillar space 25 mmWood frame rebate depth 10 mmThickness of EC layer 0.1 mmEnergy absorption of low-e

coating10%

Energy absorption of EC layer Transparent 10%Opaque 60%

Ambient temperature Outside �17.8 �CIndoor 21.1 �C

Glazing surface heat transfercoefficient

External surface 30 W m�2 K�1

Internal surface 8.3 W m�2 K�1

Thermal conductivities Indium 83.7 W m�1 K�1

Glass and EC layer 1 W m�1 K�1

Pillar 20 W m�1 K�1

Y. Fang et al. / Solar Energy 84 (2010) 516–525 519

of 0.1 mm. The frame insulation height was 10 mm. Theparameters are summarised in Table 1.

The thermal performance of an EC VG was investigatedfor glass with the EC layer facing towards the outdoorenvironment. If the EC layer were to face the indoor envi-ronment, overheating of the glazing would occur when the

Fig. 3. Under 300 W m�2 insolation and at opaque state, predicted isothermboth glass pane surfaces within the vacuum gap of the VG coated with lowlisted in Table 1.

insolation is over 600 W m�2 (Fang et al., 2006b). Temper-atures for an EC VG with an EC layer absorption of 60%subject to 300 W m�2 insolation incident normal to theglass surface were calculated using the finite volume model.Predicted isothermal plots with emittances of either 0.02 or0.18 on the both glass panes surfaces within the vacuumgap are presented in Figs. 3 and 4. The emittance valueof 0.02 or 0.18 means that 98% or 82% radiation withinthe long wavelength range (3–50 lm) will be reflected byone low-e coating. The combined effective emittance valueof two low-e coated glass panes was determined by Eq. (2).

Figs. 3 and 4 show the temperature difference betweenthe two glass sheets of the vacuum glazing which is dueto the high thermal resistance of the vacuum gap. The tem-perature difference between one of vacuum glazing locatedin the centre of EC VG and the external glass pane with EClayer is also discernable. This is due to solar energy absorp-tion at the EC layer between the two glass sheets. The heatconduction through the indium based edge seal lead to thetemperature of the edge area of the indoor glass pane beinglower than the central area of the VG. Figs. 3 and 4 clearlyshow the isotherms of the support pillars in the glazing rep-resenting the heat conduction through the support pillars.The surface temperatures at the centre of glazing area inFigs. 3 and 4 are 23.1 and 22.2 �C, respectively. The heatconductance at the centre glazing area of the glazing with0.02 emittance coatings is less than that with 0.18 emittancecoatings.

s for an EC VG with the EC layer facing the outdoor environment and-e coatings with emittance of 0.02. Other parameters of the EC VG are

Fig. 4. Under 300 W m�2 insolation and opaque state, predicted isotherms for an EC VG with the EC layer facing the outdoor environment with the twoglass pane surfaces within the vacuum gap coated with two low-e coatings with emittance of 0.18 within the vacuum gap. Other parameters of the EC VGare listed in Table 1.

520 Y. Fang et al. / Solar Energy 84 (2010) 516–525

Detailed analysis has shown the influence of low-e coat-ings on the heat transmission of a VG (Fang, 2002; Fanget al., 2007). For the EC VG, the effect of varying the emit-tance of the low-e coatings between 0.02 and 0.18 on thesurface temperatures of the inside and outside glass panessubject to 300 and 1000 W m�2 insolation were simulatedand the predictions are presented in Figs. 5 and 6.

In Fig. 5 when the insolation is 300 W m�2, for the opa-que state (energy absorption 60%), with 0.02 and 0.18 emit-tance of low-e coatings, the indoor glass pane meantemperatures are 22.1 and 21.5 �C, respectively, which arehigher than the inside air temperature of 21.1 �C, thusthe EC VG transfers heat to the indoor environment.The outdoor glass pane mean temperatures of the glasspanes are 5.2 and 5.4 �C. The mean temperature differencebetween the indoor and outdoor glass panes with low-ecoatings with emittance of 0.02 and 0.18 are 16.9 and16.1 �C, respectively. Heat flow through the entire indoorglass pane to the outdoor glass pane of the glazing with0.02 emittance of low-e coating is less than that with 0.18emittance of low-e coating. For the transparent state, whenthe emittance is 0.02, the indoor and outdoor glass panetemperatures are 20.1 and �11.5 �C, the temperature differ-ence between the outdoor and indoor glass panes is31.6 �C; when emittance is 0.18, the indoor and outdoorglass pane temperatures are 18.6 and �11.1 �C, the temper-ature difference is 29.7 �C. In the transparent state, the tem-

perature difference is much greater than that of opaquestate, thus the low-e coating performed better as a heat bar-rier to the radiative heat transfer than in opaque state.With increasing insolation, both outdoor and indoor glasspane mean temperatures increase, but the rate of increasein the mean temperature of outdoor glass pane is rapidthan that of indoor glass pane (Fang and Eames, 2006a).When insolation increases to 675 W m�2, the mean temper-atures of the indoor and outdoor glass panes are equal(Fang and Eames, 2006a).

In Fig. 6, when the insolation is 1000 W m�2, for theopaque state, with 0.02 and 0.18 emittance, the indoor glasspane temperatures are 35.1 and 37.3 �C, respectively, whichare higher than the indoor air temperature of 21.1 �C, con-sequentially the indoor glass pane transfers heat to theindoor environment. The outdoor glass pane temperaturesare 55.9 and 55.3 �C, respectively, which are higher thanthose of the indoor glass pane due to absorbed solar energyby the EC layer and low-e coatings. Heat is transferredfrom the outdoor glass pane to the indoor glass pane.For transparent state, with 0.02 and 0.18 emittance, theindoor glass pane temperatures are 28.0 and 26.7 �C, andconsequentially the indoor glass pane transfers heat tothe indoor environment; the outdoor glass pane tempera-tures are 0.2 and 0.6 �C, respectively. When the emittanceis 0.02 and 0.18, the mean temperature difference betweenthe outdoor and indoor glass panes are 27.8 and 26.1 �C,

-15

-10

-5

0

5

10

15

20

25

0 0.04 0.08 0.12 0.16 0.2

Emittance

Tem

pera

ture

(o C

)

-15

-10

-5

0

5

10

15

20

25

Tem

pera

ture

(o C

)

Indoor glass pane(60% energyabsorption)

Outdoor glasspane (10% energyabsorption)

Indoor glass pane(10% energyabsorption)

Outdoor glasspane (60% energyabsorption)

Energy absorption 60%

Energy absoption 10%

Insolation 300W.m-2

Fig. 5. Indoor and outdoor glass mean surface temperature variation with emittance when the EC layer was either transparent (energy absorption 10%) oropaque (energy absorption 60%) with 300 W m�2 insolation. Other parameters of the EC VG are listed in Table 1 except the emittance of low-e coating.

-5

5

15

25

35

45

55

65

0 0.04 0.08 0.12 0.16 0.2Emittance

Tem

pera

ture

(o C

)

-5

5

15

25

35

45

55

65Te

mpe

ratu

re (

oC)

Outdoor glass pane(60% energyabsorption)

Indoor glass pane(60% energyabsorption)

Outdoor glass pane(10% energyabsorption)

Indoor glass pane(10% energyabsorption)

Insolation 1000W.m -2

Energy absorption 60%

Energy absorption 10%

Fig. 6. Indoor and outdoor glass mean surface temperature variation with emittance when the EC layer was either transparent (energy absorption 10%) oropaque (energy absorption 60%) with 1000 W m�2 insolation. Other parameters of the EC CG are listed in Table 1 except emittance of the low-e coating.

Y. Fang et al. / Solar Energy 84 (2010) 516–525 521

respectively. So in the transparent state, the temperaturedifference between the indoor and outdoor glass panes atan insolation of 1000 W m�2 is less than that at an insola-tion of 300 W m�2, since the higher insolation increases theoutdoor glass pane temperature leading to the temperaturedifference between two glass panes being reduced.

Figs. 5 and 6 show that for the glazing system with 60%absorption in the EC layer, the rates in temperature varia-tions of both the indoor and outdoor glass panes were lessthan those for the glazing with 10% solar energy absorp-tion in the EC layer. This was because solar energyabsorbed by the 60% absorption EC layer increased the

outdoor glass pane surface temperatures more significantlythan that of the indoor glass pane, leading to a temperaturedifference across the glazing with 60% solar energy absorp-tion in the EC layer being smaller than that with 10% solarenergy absorption in the EC layer. From Eq. (1), it can beseen that for larger temperature differences between thetwo glass surfaces, emittance has the great effect on radia-tive heat transfer and thus on the mean surface tempera-tures of both the indoor and outdoor glass panes.

The influence of frame rebate depth on the thermal per-formance of EC VG at opaque and transparent states wheninsolations are 300 and 1000 W m�2 were simulated and

522 Y. Fang et al. / Solar Energy 84 (2010) 516–525

the results are presented in Figs. 7 and 8. Fig. 7 shows thatwhen the insolation is 300 W m�2, with increasing theframe rebate depth, the outdoor glass pane temperatureincreases, until the rate in increase of indoor glass panetemperature reaches an asymptote at frame rebate depthsabove 15 mm. Frame rebate depth has a greater influenceon the indoor glass pane temperature for the transparentstate than the opaque state. Because the solar energyabsorbed by the EC layer increases the outdoor glass panetemperature and reduces the temperature differencebetween the indoor and outdoor glass panes, the influenceof frame rebate depth on the glass mean temperaturedecreases. Fig. 8 shows that at an insolation of1000 W m�2, the temperature difference between the indoor

-15

-10

-5

0

5

10

15

20

25

30

0 10 20 30 40

Rebate depth (mm)

Tem

pera

ture

(o C

) Energy absorpt

Energy absorption 10%

Insolation: 300W.m -2

Fig. 7. Mean temperatures of indoor and outdoor glass panes with variou300 W m�2. Other parameters of the EC VG are listed in Table 1 except fram

-10

0

10

20

30

40

50

60

70

0 10 20 30 40

Rebate depth (mm)

Tem

per

atu

re (

oC

)

Insolation: 1000W.m-2

Energy absorption 60%

Energy absorption 10%

Fig. 8. Mean temperatures of indoor and outdoor glass panes with variou1000 W m�2. Other parameters of the EC VG are listed in Table 1 except fram

and outdoor glass panes is less than that for an insolationof 300 W m�2 for both transparent and opaque states, theinfluence of frame rebate depth on the glass mean temper-atures in negligible. When the insolation is 1000 W m�2, inthe opaque state, the outdoor glass pane temperature ishigher than the indoor glass pane temperature. Withincreasing the frame rebate depth, the indoor glass panetemperature decreases very gradually, since incident solarenergy incident on the outdoor glass pane is shaded bythe increased frame rebate depth.

In the opaque state, the comparison of the indoor andoutdoor glass pane temperatures with one and two low-ecoatings at insolations of 300 and 1000 W m�2 is presentedin Fig. 9, which shows that when the emittance is close to

50

-15

-10

-5

0

5

10

15

20

25

30

Indoor glasspane

Outdoorglass pane

Indoor glasspane

Outdoorglass pane

ion 60%

s rebate depth at opaque and transparent states when the insolation ise rebate depth.

50-10

0

10

20

30

40

50

60

70

Outdoorglass pane

Outdoorglass pane

Indoor glasspane

Indoor glasspane

s rebate depth at opaque and transparent states when the insolation ise rebate depth.

0

10

20

30

40

50

60

0 0.04 0.08 0.12 0.16 0.2Emittance

Tem

pera

ture

(o

C)

Outdoor glass (two coatings)

Indoor glass (two coatings)

Outdoor glass (one coating)

Indoor glass (one coating)

Outdoor glass (two coatings)

Indoor glass (two coatings)

Outdoor glass (one coating)

Indoor glass (one coating)

Insolation 1000W.m-2

Insolation 300W.m-2

Indoor glass pane

Outdoor glass pane

Outdoor glass pane

Fig. 9. With insolation of 300 W m�2 and 1000 W m�2, the indoor and outdoor glass pane mean temperatures of EC VG at opaque state with low-ecoatings on one and two internal glass surfaces with various emittance. Other parameters are listed in Table 1 except emittance of low-e coating.

Y. Fang et al. / Solar Energy 84 (2010) 516–525 523

0.02, the indoor and outdoor glass pane temperaturesapproach each other for both insolations of 300 and1000 W m�2. This is comparable with the result of Fangand Eames (2006b). When the insolation is zero and theemittance is close to 0.02, the two low-e coatings providelimited improvement in the thermal performance of VG

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 0.05 0Emittance o

Rad

iativ

e he

at c

ondu

ctan

ce

(W.m

-2.K

-1)

1000 W

300 W.m-2

One low -e coating

Energy absorption 60%, o

Fig. 10. With insolation of 300 and 1000 W m�2 and opaque states, the relativlow-e coatings. The pillar conductance is included for comparison. Other para

(Fang and Eames, 2006b). When the insolation is300 W m�2, heat is transferred from the indoor glass paneto the outdoor glass pane of the glazing with one or twolow-e coatings. The indoor glass pane temperature of theglazing with two low-e coatings is greater by 0.6 �C thanthat with one low-e coating. This is because two low-e coat-

.1 0.15 0.2f low-e coating

.m-2

Tw o low -e coatings

paque states

Pillar conductance

e radiative heat conductance of EC VG at opaque state with one and twometers are listed in Table 1 except emittance.

524 Y. Fang et al. / Solar Energy 84 (2010) 516–525

ings more efficiently reduce the radiative heat flowing fromthe indoor glass pane to the outdoor glass pane than thesingle low-e coating. In the finite volume model, eachlow-e coating absorbs 10% of solar energy passing throughit. For EC VG with two low-e coatings on the two internalsurfaces of glass panes within the vacuum gap, the temper-atures of both glass panes of vacuum glazing are affectedby the absorbed solar energy. Since the insolation passesthrough the coating on the central glass pane first, the tem-perature increase at the central glass pane caused byabsorbed solar energy should be larger than that of theindoor glass pane. The reason that indoor glass pane tem-perature of a glazing system with two low-e coatings ishigher than that of a glazing system with one low-e coatingis that the effective emittance of two low-e coatings is lowerthan that with one low-e coating, thus it effectively reducesthe radiative heat transfer from the indoor glass pane to theoutdoor glass pane. When the insolation is 1000 W m�2,the outdoor glass pane temperature is much greater thanthat of the indoor glass pane due to absorbed solar energyby the outdoor facing EC layer. The heat is transferredfrom the outdoor glass pane to the indoor glass pane.The two low-e coatings more efficiently reduces the heattransferred from the outdoor glass pane to the indoor glasspane than the single coating, so the indoor glass pane tem-perature of the glazing with two low-e coatings is lower by1.6 �C than that with a single coating.

This investigation considers the relative radiative heatconductance between the two glass surface within the vac-uum gap and heat conductance of pillar array within theEC VG. The heat conductance of pillar array and radiativeheat conductance are determined by Eq. (4) and eeffective forone and two coatings is calculated by Eq. (2). The emit-tance of uncoated glass was assumed to be 0.9 (Manzet al., 2006). Fig. 10 shows that at an insolation of300 W m�2, the pillar and radiative heat conductance areequal to 0.512 W m�2 K�1 for two low-e coatings havingemittance of 0.174. The radiative conductance is0.054 W m�2 K�1, nearly an order of magnitude smallerthan the pillar conductance for two coatings of emittanceof 0.02. Under 1000 W m�2 insolation, two low-e coatingscan more significantly reduce the radiative conductancecompared to one low-e coating than under 300 W m�2.When insoaltion close to 0.02, this reduction in radiativeconductance by using two coatings is reduced. As solarenergy is absorbed at the low-e coating and EC layer, whenconsidering the total energy balance of EC VG, a full studyof solar heat gain of the EC VG is needed, which beyondthe range of this work.

4. Conclusions

Under the ASTM winter boundary conditions, the ther-mal performance of a 0.5 m by 0.5 m EC VG rebatedwithin a solid wood frame and with two 4 mm thick low-e coated glass panes and a 0.1 mm thick EC layer coated4 mm thick glass pane was simulated using a finite volume

model. The indoor air set-point temperature and the out-door ambient air temperatures were assumed to be con-stant at 21.1 and �17.8 �C, respectively, the convectiveheat transfer coefficients on the indoor and the outdoorglass surfaces were assumed to be 8.3 and 30 W m�2 K�1,respectively, in accordance with the ASTM winter condi-tions. Each low-e coating absorbed 10% of solar energypass through it. Other parameters of the EC VG configura-tion are listed in Table 1. With 60% solar energy absorptionat the EC layer and 300 W m�2 insolation incident perpen-dicular to the outdoor glass pane surface, for the EC layerfacing the outdoor environment, when the emittance valueof both the low-e coatings decreased from 0.18 to 0.02, thetemperature of the indoor glass pane increased from 21.5 to22.1 �C; the outdoor glass pane temperature decreasedfrom 5.4 to 5.2 �C, and the temperature difference increasedfrom 16.1 to 17.0 �C. The temperature of the indoor glasspane of the EC VG was higher than the indoor air temper-ature of 21.1 �C, transferring heat into the indoor environ-ment. With 1000 W m�2 insolation incident perpendicularto the outdoor glass pane surface, when the emittance valueof both the low-e coatings decreased from 0.18 to 0.02, thetemperature of the indoor glass pane decreased from 37.3to 35.1 �C; the outdoor glass pane temperature increasedfrom 55.3 to 55.9 �C, the temperature difference increasedfrom 18.0 to 20.8 �C. The heat is transferred from the out-door glass pane to the indoor glass pane and thus trans-ferred into the indoor environment.

The influence of frame rebate depth on the glass panetemperatures at an insolation of 300 W m�2 is greaterthan for an insolation of 1000 W m�2, since at the higherinsolation of 1000 W m�2 there is a significant decrease inthe mean temperature difference between the outdoor andindoor glass panes, which reduces the influence of framerebate depth. Under both 300 and 1000 W m�2 insola-tions, when using one and two low-e coatings, with anemittance of 0.02, the temperatures of both the indoorand outdoor glass panes approach each other. At1000 W m�2 insolation, the outdoor glass pane tempera-ture is much higher than that of the indoor glass pane,the two low-e coatings reduce the indoor glass pane tem-perature by 1.6 �C more than one low-e coating. At300 W m�2 insolation, the two low-e coatings increasethe indoor glass pane temperature by 0.6 �C more thanone low-e coating.

Acknowledgements

The authors acknowledge the support provided by theCharles Parson Energy Research Award, from the Depart-ment of Communications and Energy, the Republic ofIreland.

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