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Thermal performance analysis of an electrochromic vacuum glazing with low emittance coatings
Available online at www.sciencedirect.com
Solar Energy 84 (2010) 516–525 www.elsevier.com/locate/solener
Thermal performance analysis of an electrochromic vacuum glazing with 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, UK b 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 2009 Available 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. The EC 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 contiguously 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 diameter square 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 evacuated glass unit. The whole unit was rebated by 10 mm within a solid wood frame. The low-e coating absorbed 10% of solar energy incident on it. With the EC VG installed with the EC component facing the outdoor environment, for an incident solar radiation of 300 W m2, simulations demonstrated that when the EC layer is opaque for winter conditions, the temperature of the inside glass pane is higher than the 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 heat transferred from the glazing to the interior environment. When the emittance was lower to 0.02, the outdoor and indoor glass pane temperatures of the glazing with single and two low-e coatings are very close to each other. For an insolation of 1000 W m2, the outdoor glass pane temperature exceeds the indoor glass pane temperature, consequentially the outdoor glass pane transfers heat to the indoor glass pane. Ó 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 glazing (VG) reported by Robinson and Collins (1989) used a low melt solder glass gasket that formed a contiguous edge seal at temperatures of above 450 °C. At such temperatures, 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*
Corresponding author. Tel.: +44 2890368227; fax: +44 2890238239. E-mail addresses: [email protected], [email protected] (Y. Fang). 0038-092X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2009.02.007
comes this (Griffiths et al., 1998). The procedure for fabricating a VG at low temperature (Hyde et al., 2000) and its measured performance properties (Fang et al., 2000, 2006; Fang, 2002) have been reported. A measured heat transmittance of less than 1 W m2 K1 in the centre-of-glass area of the VG has been achieved. An electrochromic (EC) film can be used to control visual light transmittance when a 1– 2 V direct current (DC) switching power is applied. Simulation studies of window systems have considered the effect of 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 inclusion of an EC double glazed window can enable a building to
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Nomenclature A a E h I k L P Q T S
radiation intensity at the surface of a finite volume unit (W) radius of support pillar (m) extinction coefficient of glass (m1) surface heat transfer coefficient (W m2 K1) insolation intensity (W m2) heat conductivity of glass (W m1 K1) radiation flux path length through glazing (m) pillar separation (m) heat transfer (W) temperature (°C) area (m2)
consume less cooling energy by controlling visual light transmission and solar heat gain coefficient when compared to conventional double glazings. In a heating-dominated climate, as an EC window should remain in its bleached state during the heating season, it does not improve energy performance. An EC VG combines an EC smart window with a VG providing both very low heat transmittance and optimal visible light transmittance whilst allowing control of solar gain and thus thermal comfort for building occupants. It may reduce peak energy demands both for cooling during summer and heating during winter. In a study of the effect of insolation intensity on the thermal behaviour of EC VG (Fang and Eames, 2006a), it was found that to avoid intolerable overheating discomfort, the EC layer between the VG and the third glass pane must face the outdoor environment. Depending on its emittance value, employing a low-e coating on one or two glass surfaces within the vacuum gap of the VG reduces radiative heat transfer across the glazing significantly (Collins and Simko, 1998). A low-e coating also reduces solar and visual transmittance. Nevertheless, the use of a low-e coating increases thermal resistance much more than it reduces the solar gain (Hollands et al., 2001). For a glazing, to obtain net energy performance, both the total heat transmittance and the total solar energy transmittance should be calculated. The overall heat transfer coefficient of a VG has been determined experimentally by Fang et al. (2006). The solar heat gain coefficient can be determined in accordance with ISO 9050 (2000) employing calculations to determine how much radiation is absorbed and re-emitted inwardly.
t U
thickness of glass panes (m) heat transmittance (W m2 K1)
Greek letters hemispherical emittance of a surface r Stefan–Boltzmann constant (5.67 108 W m 2 K4) Subscripts 1, 2 internal and external sample surfaces in incident Z (1, 2, 3, . . ., n) position of radiation considered
Fang et al., 2006) was modified to analyse heat transfer through an EC VG under testing standard winter conditions (ASTM C1363, 2005). A simple analytic model (Collins and Simko, 1998) developed to predict heat flow through each individual support pillar of a VG was not used in this analysis since the pillar array is incorporated and modelled directly in the finite volume model. In the model, a square cross section pillar with the same area represents the circular cross section pillars of the fabricated system (Griffiths et al., 1998). The discretization of the mesh for numerical calculation of thermal conduction refined to provide a high density of nodes in and around each pillar to enable an accurate prediction of heat trans-
Glass panes Support pillars EC layer
Outdoor side
Vacuum edge seal
Two glass panes comprising the vacuum glazing
Indoor side Low-e coatings
Pillars
Edge seal
Glass panes
2. Methodology EC layer
The modelled EC VG consisted of a low temperature fabricated VG combined with an additional glass pane with an EC layer incorporated as shown in Fig. 1. A finite volume model (Eames and Norton, 1993) validated experimentally in extensive previous research (Fang, 2002;
Outdoor side Fig. 1. Schematic diagram of an EC VG. The third glass pane coated with EC layer is combined with the normal vacuum glazing. One or two low-e coatings are coated on the internal glass surface within the vacuum gap. The diagram is not to scale.
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fer. The simulated glazing system comprised two low-e film coated glass panes with an emittance of 0.18 separated by a vacuum gap with a third glass pane with an EC layer attached. Due to symmetry considerations, the one quarter pane of the EC VG modelled gives a representation of the behaviour of a complete EC VG window. In the finite volume model, the radiative heat transfer rate between each of the finite volume surfaces with areas S and mean surface temperatures T1 and T2, that form the two plane parallel glass surfaces containing the vacuum with hemispherical emittance e1 and e2 was determined (Collins and Simko, 1998) using: Qradiation ¼ eeffective rSðT 41 T 42 Þ
ð1Þ
where r is Stefan–Boltzmann constant and the effective emittance, eeffective, was determined by: 1 eeffective
¼
1 1 þ 1 e1 e2
ð2Þ
Radiation reflectance was assumed to be independent of the wavelength, angle of the incident radiation and surface temperature. The maximum error associated with ignoring these effects has been shown to be about 4% for two uncoated glass sheets (Zhang et al., 1997). Interactions between the levels of heat conduction through the pillars and radiation between the two internal glass surfaces within the evacuated gap for a well made vacuum glazing when compared to the total heat flow through the overall glazing system is small and can be ignored. The total thermal conductance can then be determined by (Collins and Simko, 1998): C glass–glass;centre-of-glazing ¼C glass–glass;gas þ C glass–glass;radiation þC glass–glass;pillars ¼0:8P
ð3Þ
þ 4effective rT 3average þ 2k glass a=P 2
ð4Þ
where P is the internal pressure, measured in Pascal. For a successful vacuum glazing sample P is less than 0.1 Pa, thus Cglass–glass,gas can be ignored. Taverage is the average of the temperatures T1 and T2, kglass is the glass thermal conductivity, a is the pillar radius, p is the pillar separation and eeffective is the effective emittance.
Iin dx
Glass
dy
Z1
dz
Z2
Z3 Zn
Coating Fig. 2. Schematic diagram of a glass pane with a low-e coating illustrating a finite volume unit area dx, dy and incident insolation Iin.
As can be seen from Fig. 2, solar radiation absorbed in each finite volume can be determined by: dI absorbed ¼ I in ðAZ AZþ1 Þdxdy
ð5Þ
where Az and Az+1 are the intensities of solar radiation at the two surfaces of the finite volume determined using Eq. (6). AL ¼ 1 eELZ
ð6Þ
where E, the extinction coefficient of glass is a measure of how the glass absorbs electromagnetic radiation (Duffie and Beckman, 1991). The extinction coefficient of glass varies from 32 m1 for ‘‘greenish cast of edge” glass to 4 m1 for ‘‘water white” glass (Duffie and Beckman, 1991). In this work E was assumed to be 30 m1. LZ is the path length through the glazing from the front glass surface. 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 was calculated and included in the energy balance for each finite volume. The thermal performance of a small central area (25 mm by 25 mm) of the vacuum glazing including a singular pillar was simulated using a mesh of 80 80 28 nodes. The mesh was graded to provide a denser number of nodes closer to the pillar. 28 nodes were distributed in a graded mesh through the glazing thickness of 12.12 mm. The thermal conductance of this simulated unit with a pillar in the centre was in good agreement with the analytic prediction with 1.5% variation, which is comparable to the result of Wilson et al. (1998). This level of agreement indicates that the density of nodes is sufficient to simulate the realistic level of heat flow with high accuracy. 3. Thermal performance of an EC VG The EC layer in the transparent state absorbs approximately 10% of the incident solar energy, in the opaque state, the energy absorption of an EC layer is 60%. These conditions were incorporated in the simulations. In the simulations, the indoor air set-point temperature and the outdoor ambient air temperatures were assumed to be constant at 21.1 and 17.8 °C, respectively, the convective heat transfer coefficients on the indoor and the outdoor glass surfaces were assumed to be 8.3 and 30 W m2 K1, respectively, corresponding to those in ASTM measurement standards for winter conditions (ASTM C1363, 2005). The simulated EC VG was 0.5 m by 0.5 m and comprised three 4 mm thick glass panes. The VG components comprised two glass panes coated with one or two low-e coatings, separated by 0.12 mm, supported by a 0.32 mm diameter pillar array spaced at 25 mm in a regular square pattern. The edge seal was a 6 mm wide band of indium. In the simulation, it was also assumed that the thermal conductivity of the EC layer was 1 W m1 K1 with thickness
Y. Fang et al. / Solar Energy 84 (2010) 516–525 Table 1 Parameters of modelled EC VG. Parameter Dimensions Glass pane thickness Emittance Edge seal width Pillar diameter Pillar height Pillar space Wood frame rebate depth Thickness of EC layer Energy absorption of low-e coating Energy absorption of EC layer Ambient temperature Glazing surface heat transfer coefficient Thermal conductivities
Value Thickness, width, length One or two surfaces
Transparent Opaque Outside Indoor External surface Internal surface Indium Glass and EC layer Pillar
12.22, 500, 500 mm 4 mm 0.02 or 0.18 6 mm 0.32 mm 0.12 mm 25 mm 10 mm 0.1 mm 10% 10% 60% 17.8 °C 21.1 °C 30 W m2 K1 8.3 W m2 K1 83.7 W m1 K1 1 W m1 K1 20 W m1 K1
of 0.1 mm. The frame insulation height was 10 mm. The parameters are summarised in Table 1. The thermal performance of an EC VG was investigated for glass with the EC layer facing towards the outdoor environment. If the EC layer were to face the indoor environment, overheating of the glazing would occur when the
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insolation is over 600 W m2 (Fang et al., 2006b). Temperatures for an EC VG with an EC layer absorption of 60% subject to 300 W m2 insolation incident normal to the glass surface were calculated using the finite volume model. Predicted isothermal plots with emittances of either 0.02 or 0.18 on the both glass panes surfaces within the vacuum gap are presented in Figs. 3 and 4. The emittance value of 0.02 or 0.18 means that 98% or 82% radiation within the long wavelength range (3–50 lm) will be reflected by one low-e coating. The combined effective emittance value of two low-e coated glass panes was determined by Eq. (2). Figs. 3 and 4 show the temperature difference between the two glass sheets of the vacuum glazing which is due to the high thermal resistance of the vacuum gap. The temperature difference between one of vacuum glazing located in the centre of EC VG and the external glass pane with EC layer is also discernable. This is due to solar energy absorption at the EC layer between the two glass sheets. The heat conduction through the indium based edge seal lead to the temperature of the edge area of the indoor glass pane being lower than the central area of the VG. Figs. 3 and 4 clearly show the isotherms of the support pillars in the glazing representing the heat conduction through the support pillars. The surface temperatures at the centre of glazing area in Figs. 3 and 4 are 23.1 and 22.2 °C, respectively. The heat conductance at the centre glazing area of the glazing with 0.02 emittance coatings is less than that with 0.18 emittance coatings.
Fig. 3. Under 300 W m2 insolation and at opaque state, predicted isotherms for an EC VG with the EC layer facing the outdoor environment and both glass pane surfaces within the vacuum gap of the VG coated with low-e coatings with emittance of 0.02. Other parameters of the EC VG are listed in Table 1.
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Fig. 4. Under 300 W m2 insolation and opaque state, predicted isotherms for an EC VG with the EC layer facing the outdoor environment with the two glass 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 VG are listed in Table 1.
Detailed analysis has shown the influence of low-e coatings on the heat transmission of a VG (Fang, 2002; Fang et al., 2007). For the EC VG, the effect of varying the emittance of the low-e coatings between 0.02 and 0.18 on the surface temperatures of the inside and outside glass panes subject to 300 and 1000 W m2 insolation were simulated and the predictions are presented in Figs. 5 and 6. In Fig. 5 when the insolation is 300 W m2, for the opaque state (energy absorption 60%), with 0.02 and 0.18 emittance of low-e coatings, the indoor glass pane mean temperatures are 22.1 and 21.5 °C, respectively, which are higher than the inside air temperature of 21.1 °C, thus the EC VG transfers heat to the indoor environment. The outdoor glass pane mean temperatures of the glass panes are 5.2 and 5.4 °C. The mean temperature difference between the indoor and outdoor glass panes with low-e coatings with emittance of 0.02 and 0.18 are 16.9 and 16.1 °C, respectively. Heat flow through the entire indoor glass pane to the outdoor glass pane of the glazing with 0.02 emittance of low-e coating is less than that with 0.18 emittance of low-e coating. For the transparent state, when the emittance is 0.02, the indoor and outdoor glass pane temperatures are 20.1 and 11.5 °C, the temperature difference between the outdoor and indoor glass panes is 31.6 °C; when emittance is 0.18, the indoor and outdoor glass pane temperatures are 18.6 and 11.1 °C, the temperature difference is 29.7 °C. In the transparent state, the tem-
perature difference is much greater than that of opaque state, thus the low-e coating performed better as a heat barrier to the radiative heat transfer than in opaque state. With increasing insolation, both outdoor and indoor glass pane mean temperatures increase, but the rate of increase in the mean temperature of outdoor glass pane is rapid than that of indoor glass pane (Fang and Eames, 2006a). When insolation increases to 675 W m2, the mean temperatures of the indoor and outdoor glass panes are equal (Fang and Eames, 2006a). In Fig. 6, when the insolation is 1000 W m2, for the opaque state, with 0.02 and 0.18 emittance, the indoor glass pane temperatures are 35.1 and 37.3 °C, respectively, which are higher than the indoor air temperature of 21.1 °C, consequentially the indoor glass pane transfers heat to the indoor environment. The outdoor glass pane temperatures are 55.9 and 55.3 °C, respectively, which are higher than those of the indoor glass pane due to absorbed solar energy by the EC layer and low-e coatings. Heat is transferred from the outdoor glass pane to the indoor glass pane. For transparent state, with 0.02 and 0.18 emittance, the indoor glass pane temperatures are 28.0 and 26.7 °C, and consequentially the indoor glass pane transfers heat to the indoor environment; the outdoor glass pane temperatures are 0.2 and 0.6 °C, respectively. When the emittance is 0.02 and 0.18, the mean temperature difference between the outdoor and indoor glass panes are 27.8 and 26.1 °C,
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25
20
20
15
15
10
10
5
5 Energy absoption 10%
0
0
-5
-5
-10
-10
-15
Indoor glass pane (60% energy absorption)
o
C)
Energy absorption 60%
Temperature (
Temperature ( o C)
Insolation 300W.m-2 25
Outdoor glass pane (10% energy absorption) Indoor glass pane (10% energy absorption) Outdoor glass pane (60% energy absorption)
-15 0
0.04
0.08
0.12
0.16
0.2
Emittance Fig. 5. Indoor and outdoor glass mean surface temperature variation with emittance when the EC layer was either transparent (energy absorption 10%) or opaque (energy absorption 60%) with 300 W m2 insolation. Other parameters of the EC VG are listed in Table 1 except the emittance of low-e coating.
Insolation 1000W.m -2
65
65
45
35
35
25
25
15
15
Energy absorption 10%
5
5
-5
Temperature ( o C)
Energy absorption 60%
45
o
Temperature ( C)
Outdoor glass pane (60% energy absorption)
55
55
Indoor glass pane (60% energy absorption) Outdoor glass pane (10% energy absorption) Indoor glass pane (10% energy absorption)
-5 0
0.04
0.08
0.12
0.16
0.2
Emittance Fig. 6. Indoor and outdoor glass mean surface temperature variation with emittance when the EC layer was either transparent (energy absorption 10%) or opaque (energy absorption 60%) with 1000 W m2 insolation. Other parameters of the EC CG are listed in Table 1 except emittance of the low-e coating.
respectively. So in the transparent state, the temperature difference between the indoor and outdoor glass panes at an insolation of 1000 W m2 is less than that at an insolation of 300 W m2, since the higher insolation increases the outdoor glass pane temperature leading to the temperature difference 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 variations of both the indoor and outdoor glass panes were less than those for the glazing with 10% solar energy absorption in the EC layer. This was because solar energy absorbed by the 60% absorption EC layer increased the
outdoor glass pane surface temperatures more significantly than that of the indoor glass pane, leading to a temperature difference across the glazing with 60% solar energy absorption in the EC layer being smaller than that with 10% solar energy absorption in the EC layer. From Eq. (1), it can be seen that for larger temperature differences between the two glass surfaces, emittance has the great effect on radiative heat transfer and thus on the mean surface temperatures of both the indoor and outdoor glass panes. The influence of frame rebate depth on the thermal performance of EC VG at opaque and transparent states when insolations are 300 and 1000 W m2 were simulated and
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and outdoor glass panes is less than that for an insolation of 300 W m2 for both transparent and opaque states, the influence of frame rebate depth on the glass mean temperatures in negligible. When the insolation is 1000 W m2, in the opaque state, the outdoor glass pane temperature is higher than the indoor glass pane temperature. With increasing the frame rebate depth, the indoor glass pane temperature decreases very gradually, since incident solar energy incident on the outdoor glass pane is shaded by the increased frame rebate depth. In the opaque state, the comparison of the indoor and outdoor glass pane temperatures with one and two low-e coatings at insolations of 300 and 1000 W m2 is presented in Fig. 9, which shows that when the emittance is close to
the results are presented in Figs. 7 and 8. Fig. 7 shows that when the insolation is 300 W m2, with increasing the frame rebate depth, the outdoor glass pane temperature increases, until the rate in increase of indoor glass pane temperature reaches an asymptote at frame rebate depths above 15 mm. Frame rebate depth has a greater influence on the indoor glass pane temperature for the transparent state than the opaque state. Because the solar energy absorbed by the EC layer increases the outdoor glass pane temperature and reduces the temperature difference between the indoor and outdoor glass panes, the influence of frame rebate depth on the glass mean temperature decreases. Fig. 8 shows that at an insolation of 1000 W m2, the temperature difference between the indoor
30
30
25
25
20
20
15
Energy absorption 60%
15
o
Temperature ( C)
Insolation: 300W.m -2
10
10
Energy absorption 10%
Indoor glass pane Outdoor glass pane
5
5
0
0
Indoor glass pane
-5
Outdoor glass pane
0
10
20
30
40
50
-5
Rebate depth (mm) -10
-10
-15
-15
Fig. 7. Mean temperatures of indoor and outdoor glass panes with various rebate depth at opaque and transparent states when the insolation is 300 W m2. Other parameters of the EC VG are listed in Table 1 except frame rebate depth.
70
70 Insolation: 1000W.m-2
60
60
50
o
Temperature ( C)
50 Energy absorption 60%
Outdoor glass pane
40
40
30
30
Outdoor glass pane
20
20
Indoor glass pane
Energy absorption 10% 10
10
0
0 0 -10
Indoor glass pane
10
20
30
Rebate depth (mm)
40
50 -10
Fig. 8. Mean temperatures of indoor and outdoor glass panes with various rebate depth at opaque and transparent states when the insolation is 1000 W m2. Other parameters of the EC VG are listed in Table 1 except frame rebate depth.
Y. Fang et al. / Solar Energy 84 (2010) 516–525
523
60
Outdoor glass (two coatings)
50 Outdoor glas s pane
Temperature ( o C)
Insolation 1000W.m-2
Indoor glass (two coatings)
40
Outdoor glass (one coating) 30
Indoor glass (one coating)
Indoor glas s pane
Outdoor glass (two coatings)
20
Indoor glass (two coatings)
Outdoor glas s pane
Insolation 300W.m-2
10
Outdoor glass (one coating) 0 0
0.04
0.08
0.12
0.16
Indoor glass (one coating)
0.2
Emittance 2
2
Fig. 9. With insolation of 300 W m and 1000 W m , the indoor and outdoor glass pane mean temperatures of EC VG at opaque state with low-e coatings on one and two internal glass surfaces with various emittance. Other parameters are listed in Table 1 except emittance of low-e coating.
0.02, the indoor and outdoor glass pane temperatures approach each other for both insolations of 300 and 1000 W m2. This is comparable with the result of Fang and Eames (2006b). When the insolation is zero and the emittance is close to 0.02, the two low-e coatings provide limited improvement in the thermal performance of VG
(Fang and Eames, 2006b). When the insolation is 300 W m2, heat is transferred from the indoor glass pane to the outdoor glass pane of the glazing with one or two low-e coatings. The indoor glass pane temperature of the glazing with two low-e coatings is greater by 0.6 °C than that with one low-e coating. This is because two low-e coat-
1.4 Energy absorption 60%, opaque states
1000 W.m-2
1
-2
-1
(W.m .K )
Radiative heat conductance
1.2
0.8
One low -e coating
0.6
0.4
Pillar conductance
0.2 Tw o low -e coatings 300 W.m-2 0 0
0.05
0.1 0.15 Emittance of low-e coating
0.2
Fig. 10. With insolation of 300 and 1000 W m2 and opaque states, the relative radiative heat conductance of EC VG at opaque state with one and two low-e coatings. The pillar conductance is included for comparison. Other parameters are listed in Table 1 except emittance.
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ings more efficiently reduce the radiative heat flowing from the indoor glass pane to the outdoor glass pane than the single low-e coating. In the finite volume model, each low-e coating absorbs 10% of solar energy passing through it. For EC VG with two low-e coatings on the two internal surfaces of glass panes within the vacuum gap, the temperatures of both glass panes of vacuum glazing are affected by the absorbed solar energy. Since the insolation passes through the coating on the central glass pane first, the temperature increase at the central glass pane caused by absorbed solar energy should be larger than that of the indoor glass pane. The reason that indoor glass pane temperature of a glazing system with two low-e coatings is higher than that of a glazing system with one low-e coating is that the effective emittance of two low-e coatings is lower than that with one low-e coating, thus it effectively reduces the radiative heat transfer from the indoor glass pane to the outdoor glass pane. When the insolation is 1000 W m2, the outdoor glass pane temperature is much greater than that of the indoor glass pane due to absorbed solar energy by the outdoor facing EC layer. The heat is transferred from the outdoor glass pane to the indoor glass pane. The two low-e coatings more efficiently reduces the heat transferred from the outdoor glass pane to the indoor glass pane than the single coating, so the indoor glass pane temperature of the glazing with two low-e coatings is lower by 1.6 °C than that with a single coating. This investigation considers the relative radiative heat conductance between the two glass surface within the vacuum gap and heat conductance of pillar array within the EC VG. The heat conductance of pillar array and radiative heat conductance are determined by Eq. (4) and eeffective for one and two coatings is calculated by Eq. (2). The emittance of uncoated glass was assumed to be 0.9 (Manz et al., 2006). Fig. 10 shows that at an insolation of 300 W m2, the pillar and radiative heat conductance are equal to 0.512 W m2 K1 for two low-e coatings having emittance of 0.174. The radiative conductance is 0.054 W m2 K1, nearly an order of magnitude smaller than the pillar conductance for two coatings of emittance of 0.02. Under 1000 W m2 insolation, two low-e coatings can more significantly reduce the radiative conductance compared to one low-e coating than under 300 W m2. When insoaltion close to 0.02, this reduction in radiative conductance by using two coatings is reduced. As solar energy is absorbed at the low-e coating and EC layer, when considering the total energy balance of EC VG, a full study of solar heat gain of the EC VG is needed, which beyond the range of this work. 4. Conclusions Under the ASTM winter boundary conditions, the thermal performance of a 0.5 m by 0.5 m EC VG rebated within a solid wood frame and with two 4 mm thick lowe coated glass panes and a 0.1 mm thick EC layer coated 4 mm thick glass pane was simulated using a finite volume
model. The indoor air set-point temperature and the outdoor ambient air temperatures were assumed to be constant at 21.1 and 17.8 °C, respectively, the convective heat transfer coefficients on the indoor and the outdoor glass surfaces were assumed to be 8.3 and 30 W m2 K1, respectively, in accordance with the ASTM winter conditions. Each low-e coating absorbed 10% of solar energy pass through it. Other parameters of the EC VG configuration are listed in Table 1. With 60% solar energy absorption at the EC layer and 300 W m2 insolation incident perpendicular to the outdoor glass pane surface, for the EC layer facing the outdoor environment, when the emittance value of both the low-e coatings decreased from 0.18 to 0.02, the temperature of the indoor glass pane increased from 21.5 to 22.1 °C; the outdoor glass pane temperature decreased from 5.4 to 5.2 °C, and the temperature difference increased from 16.1 to 17.0 °C. The temperature of the indoor glass pane of the EC VG was higher than the indoor air temperature of 21.1 °C, transferring heat into the indoor environment. With 1000 W m2 insolation incident perpendicular to the outdoor glass pane surface, when the emittance value of both the low-e coatings decreased from 0.18 to 0.02, the temperature of the indoor glass pane decreased from 37.3 to 35.1 °C; the outdoor glass pane temperature increased from 55.3 to 55.9 °C, the temperature difference increased from 18.0 to 20.8 °C. The heat is transferred from the outdoor glass pane to the indoor glass pane and thus transferred into the indoor environment. The influence of frame rebate depth on the glass pane temperatures at an insolation of 300 W m2 is greater than for an insolation of 1000 W m2, since at the higher insolation of 1000 W m2 there is a significant decrease in the mean temperature difference between the outdoor and indoor glass panes, which reduces the influence of frame rebate depth. Under both 300 and 1000 W m2 insolations, when using one and two low-e coatings, with an emittance of 0.02, the temperatures of both the indoor and outdoor glass panes approach each other. At 1000 W m2 insolation, the outdoor glass pane temperature is much higher than that of the indoor glass pane, the two low-e coatings reduce the indoor glass pane temperature by 1.6 °C more than one low-e coating. At 300 W m2 insolation, the two low-e coatings increase the indoor glass pane temperature by 0.6 °C more than one low-e coating. Acknowledgements The authors acknowledge the support provided by the Charles Parson Energy Research Award, from the Department of Communications and Energy, the Republic of Ireland. References ASTM C1363, 2005. Standard test method for thermal performance of building materials and envelop assemblies by means of a hot box
Y. Fang et al. / Solar Energy 84 (2010) 516–525 apparatus, ASTM International, West Conshohocken, PA, 2005, doi:10.1520/C1363-05. Available from:. Collins, R.E., Simko, T.M., 1998. Current status of the science and technology of VG. Solar Energy 62, 189–213. Duffie, Beckman, 1991. Solar Engineering of Thermal Processes. Wiley, New York (Chapter 5). Eames, P.C., Norton, B., 1993. A validated unified model for optics and heat transfer in line-axis concentrating solar energy collectors. Solar Energy 50, 339–355. Fang, Y., 2002. An experimental and theoretical investigation into the design, development and performance of evacuated glazing, Ph.D. Thesis, University of Ulster, UK. Fang, Y., Eames, P.C., 2006a. Thermal analysis of electrochromic vacuum glazing. Energy Conversion and Management 47, 3602–3610. Fang, Y., Eames, P.C., 2006b. The effect of glass coating emittance and frame rebate on heat transfer through VG and EC vacuum glazed windows. Solar Energy Materials & Solar Cells 90, 2683–2695. Fang, Y, Eames, P.C., Norton, B., 2000. Influence of insolation level and glass thickness on the thermal performance of evacuated glazing. In: Proceedings of the World Renewable Energy Congress VI, Brighton, UK, pp. 1878–1881. Fang, Y., Eames, P.C., Norton, B., Hyde, T.J., 2006. Experimental validation of a numerical model for heat transfer in evacuated glazing. Solar Energy 80, 564–577. Fang, Y., Eames, P.C., Norton, B., Hyde, T., Zhao, J., Wang, J., Huang, Y., 2007. Low emittance coatings and the thermal performance of vacuum glazing. Solar Energy 81, 8–12. Griffiths, P.W., Leo Di, M., Cartwright, P., Eames, P.C., Yianoulis, P., Leftheriotis, G., Norton, B., 1998. Fabrication of evacuated glazing at low temperature. Solar Energy 63, 243–249.
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Hollands, K.G.T., Wright, J.L., Granguist, C.G., 2001. In: Gordon, J. (Ed.), Solar Energy – The State of the Art, James & James Ltd., London, UK (Chapter 2). Hyde, T.J., Griffiths, P.W., Eames, P.C., Norton, B., 2000. Development of a novel low temperature edge seal for evacuated glazing. In: Proceedings of the World Renewable Energy Congress VI, Brighton, UK, pp. 271–274. ISO 9050, 2000. Glass in building – determination of light transmittance, solar direct transmittance, total solar energy transmittance and ultraviolet transmittance, and related glazing factors, Geneva, Switzerland. Kailsson, J., Kailsson, B., Ross, A., 2000. Control strategies and energy savings. In: Proceedings of the EuroSun 2000, Copenhagen, Denmark, June 2000. Manz, H., Brunner, S., Wullschleger, L., 2006. Triple vacuum glazing: heat transfer and basic mechanical design constraints. Solar Energy 80, 1632–1642. Robinson, S.J., Collins, R.E., 1989. Evacuated windows-theory and practice. In: ISES Solar World Congress, International Solar Energy Society, Kobe, Japan. Selkowitz, S., Rubin, M., Lee, E.S., Sullivan, R., 1994. A review of EC window performance factors. SPIE 2255, 226–248. Sullivan, R., Lee, E.S., Rubin, M., Selkowitz, S., 1996. The energy performance of EC windows in heating-dominated geographic location, LBNL-Report NR, LBNL-38252. Wilson, C.F., Simko, T.M., Collins, R.E., 1998. Heat conduction through the support pillars in vacuum glazing. Solar Energy 63, 393–406. Zhang, Q.-C., Simko, T.M., Dey, C.J., Collins, R.E., Turner, G.M., Brunotte, M., Gombert, A., 1997. The measurement and calculation of radiative heat transfer between uncoated and doped tin oxide coated glass surface. International Journal of Heat and Mass Transfer 40, 61–71.
Solar Energy 84 (2010) 516–525 www.elsevier.com/locate/solener
Thermal performance analysis of an electrochromic vacuum glazing with 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, UK b 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 2009 Available 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. The EC 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 contiguously 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 diameter square 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 evacuated glass unit. The whole unit was rebated by 10 mm within a solid wood frame. The low-e coating absorbed 10% of solar energy incident on it. With the EC VG installed with the EC component facing the outdoor environment, for an incident solar radiation of 300 W m2, simulations demonstrated that when the EC layer is opaque for winter conditions, the temperature of the inside glass pane is higher than the 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 heat transferred from the glazing to the interior environment. When the emittance was lower to 0.02, the outdoor and indoor glass pane temperatures of the glazing with single and two low-e coatings are very close to each other. For an insolation of 1000 W m2, the outdoor glass pane temperature exceeds the indoor glass pane temperature, consequentially the outdoor glass pane transfers heat to the indoor glass pane. Ó 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 glazing (VG) reported by Robinson and Collins (1989) used a low melt solder glass gasket that formed a contiguous edge seal at temperatures of above 450 °C. At such temperatures, 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*
Corresponding author. Tel.: +44 2890368227; fax: +44 2890238239. E-mail addresses: [email protected], [email protected] (Y. Fang). 0038-092X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2009.02.007
comes this (Griffiths et al., 1998). The procedure for fabricating a VG at low temperature (Hyde et al., 2000) and its measured performance properties (Fang et al., 2000, 2006; Fang, 2002) have been reported. A measured heat transmittance of less than 1 W m2 K1 in the centre-of-glass area of the VG has been achieved. An electrochromic (EC) film can be used to control visual light transmittance when a 1– 2 V direct current (DC) switching power is applied. Simulation studies of window systems have considered the effect of 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 inclusion of an EC double glazed window can enable a building to
Y. Fang et al. / Solar Energy 84 (2010) 516–525
517
Nomenclature A a E h I k L P Q T S
radiation intensity at the surface of a finite volume unit (W) radius of support pillar (m) extinction coefficient of glass (m1) surface heat transfer coefficient (W m2 K1) insolation intensity (W m2) heat conductivity of glass (W m1 K1) radiation flux path length through glazing (m) pillar separation (m) heat transfer (W) temperature (°C) area (m2)
consume less cooling energy by controlling visual light transmission and solar heat gain coefficient when compared to conventional double glazings. In a heating-dominated climate, as an EC window should remain in its bleached state during the heating season, it does not improve energy performance. An EC VG combines an EC smart window with a VG providing both very low heat transmittance and optimal visible light transmittance whilst allowing control of solar gain and thus thermal comfort for building occupants. It may reduce peak energy demands both for cooling during summer and heating during winter. In a study of the effect of insolation intensity on the thermal behaviour of EC VG (Fang and Eames, 2006a), it was found that to avoid intolerable overheating discomfort, the EC layer between the VG and the third glass pane must face the outdoor environment. Depending on its emittance value, employing a low-e coating on one or two glass surfaces within the vacuum gap of the VG reduces radiative heat transfer across the glazing significantly (Collins and Simko, 1998). A low-e coating also reduces solar and visual transmittance. Nevertheless, the use of a low-e coating increases thermal resistance much more than it reduces the solar gain (Hollands et al., 2001). For a glazing, to obtain net energy performance, both the total heat transmittance and the total solar energy transmittance should be calculated. The overall heat transfer coefficient of a VG has been determined experimentally by Fang et al. (2006). The solar heat gain coefficient can be determined in accordance with ISO 9050 (2000) employing calculations to determine how much radiation is absorbed and re-emitted inwardly.
t U
thickness of glass panes (m) heat transmittance (W m2 K1)
Greek letters hemispherical emittance of a surface r Stefan–Boltzmann constant (5.67 108 W m 2 K4) Subscripts 1, 2 internal and external sample surfaces in incident Z (1, 2, 3, . . ., n) position of radiation considered
Fang et al., 2006) was modified to analyse heat transfer through an EC VG under testing standard winter conditions (ASTM C1363, 2005). A simple analytic model (Collins and Simko, 1998) developed to predict heat flow through each individual support pillar of a VG was not used in this analysis since the pillar array is incorporated and modelled directly in the finite volume model. In the model, a square cross section pillar with the same area represents the circular cross section pillars of the fabricated system (Griffiths et al., 1998). The discretization of the mesh for numerical calculation of thermal conduction refined to provide a high density of nodes in and around each pillar to enable an accurate prediction of heat trans-
Glass panes Support pillars EC layer
Outdoor side
Vacuum edge seal
Two glass panes comprising the vacuum glazing
Indoor side Low-e coatings
Pillars
Edge seal
Glass panes
2. Methodology EC layer
The modelled EC VG consisted of a low temperature fabricated VG combined with an additional glass pane with an EC layer incorporated as shown in Fig. 1. A finite volume model (Eames and Norton, 1993) validated experimentally in extensive previous research (Fang, 2002;
Outdoor side Fig. 1. Schematic diagram of an EC VG. The third glass pane coated with EC layer is combined with the normal vacuum glazing. One or two low-e coatings are coated on the internal glass surface within the vacuum gap. The diagram is not to scale.
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fer. The simulated glazing system comprised two low-e film coated glass panes with an emittance of 0.18 separated by a vacuum gap with a third glass pane with an EC layer attached. Due to symmetry considerations, the one quarter pane of the EC VG modelled gives a representation of the behaviour of a complete EC VG window. In the finite volume model, the radiative heat transfer rate between each of the finite volume surfaces with areas S and mean surface temperatures T1 and T2, that form the two plane parallel glass surfaces containing the vacuum with hemispherical emittance e1 and e2 was determined (Collins and Simko, 1998) using: Qradiation ¼ eeffective rSðT 41 T 42 Þ
ð1Þ
where r is Stefan–Boltzmann constant and the effective emittance, eeffective, was determined by: 1 eeffective
¼
1 1 þ 1 e1 e2
ð2Þ
Radiation reflectance was assumed to be independent of the wavelength, angle of the incident radiation and surface temperature. The maximum error associated with ignoring these effects has been shown to be about 4% for two uncoated glass sheets (Zhang et al., 1997). Interactions between the levels of heat conduction through the pillars and radiation between the two internal glass surfaces within the evacuated gap for a well made vacuum glazing when compared to the total heat flow through the overall glazing system is small and can be ignored. The total thermal conductance can then be determined by (Collins and Simko, 1998): C glass–glass;centre-of-glazing ¼C glass–glass;gas þ C glass–glass;radiation þC glass–glass;pillars ¼0:8P
ð3Þ
þ 4effective rT 3average þ 2k glass a=P 2
ð4Þ
where P is the internal pressure, measured in Pascal. For a successful vacuum glazing sample P is less than 0.1 Pa, thus Cglass–glass,gas can be ignored. Taverage is the average of the temperatures T1 and T2, kglass is the glass thermal conductivity, a is the pillar radius, p is the pillar separation and eeffective is the effective emittance.
Iin dx
Glass
dy
Z1
dz
Z2
Z3 Zn
Coating Fig. 2. Schematic diagram of a glass pane with a low-e coating illustrating a finite volume unit area dx, dy and incident insolation Iin.
As can be seen from Fig. 2, solar radiation absorbed in each finite volume can be determined by: dI absorbed ¼ I in ðAZ AZþ1 Þdxdy
ð5Þ
where Az and Az+1 are the intensities of solar radiation at the two surfaces of the finite volume determined using Eq. (6). AL ¼ 1 eELZ
ð6Þ
where E, the extinction coefficient of glass is a measure of how the glass absorbs electromagnetic radiation (Duffie and Beckman, 1991). The extinction coefficient of glass varies from 32 m1 for ‘‘greenish cast of edge” glass to 4 m1 for ‘‘water white” glass (Duffie and Beckman, 1991). In this work E was assumed to be 30 m1. LZ is the path length through the glazing from the front glass surface. 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 was calculated and included in the energy balance for each finite volume. The thermal performance of a small central area (25 mm by 25 mm) of the vacuum glazing including a singular pillar was simulated using a mesh of 80 80 28 nodes. The mesh was graded to provide a denser number of nodes closer to the pillar. 28 nodes were distributed in a graded mesh through the glazing thickness of 12.12 mm. The thermal conductance of this simulated unit with a pillar in the centre was in good agreement with the analytic prediction with 1.5% variation, which is comparable to the result of Wilson et al. (1998). This level of agreement indicates that the density of nodes is sufficient to simulate the realistic level of heat flow with high accuracy. 3. Thermal performance of an EC VG The EC layer in the transparent state absorbs approximately 10% of the incident solar energy, in the opaque state, the energy absorption of an EC layer is 60%. These conditions were incorporated in the simulations. In the simulations, the indoor air set-point temperature and the outdoor ambient air temperatures were assumed to be constant at 21.1 and 17.8 °C, respectively, the convective heat transfer coefficients on the indoor and the outdoor glass surfaces were assumed to be 8.3 and 30 W m2 K1, respectively, corresponding to those in ASTM measurement standards for winter conditions (ASTM C1363, 2005). The simulated EC VG was 0.5 m by 0.5 m and comprised three 4 mm thick glass panes. The VG components comprised two glass panes coated with one or two low-e coatings, separated by 0.12 mm, supported by a 0.32 mm diameter pillar array spaced at 25 mm in a regular square pattern. The edge seal was a 6 mm wide band of indium. In the simulation, it was also assumed that the thermal conductivity of the EC layer was 1 W m1 K1 with thickness
Y. Fang et al. / Solar Energy 84 (2010) 516–525 Table 1 Parameters of modelled EC VG. Parameter Dimensions Glass pane thickness Emittance Edge seal width Pillar diameter Pillar height Pillar space Wood frame rebate depth Thickness of EC layer Energy absorption of low-e coating Energy absorption of EC layer Ambient temperature Glazing surface heat transfer coefficient Thermal conductivities
Value Thickness, width, length One or two surfaces
Transparent Opaque Outside Indoor External surface Internal surface Indium Glass and EC layer Pillar
12.22, 500, 500 mm 4 mm 0.02 or 0.18 6 mm 0.32 mm 0.12 mm 25 mm 10 mm 0.1 mm 10% 10% 60% 17.8 °C 21.1 °C 30 W m2 K1 8.3 W m2 K1 83.7 W m1 K1 1 W m1 K1 20 W m1 K1
of 0.1 mm. The frame insulation height was 10 mm. The parameters are summarised in Table 1. The thermal performance of an EC VG was investigated for glass with the EC layer facing towards the outdoor environment. If the EC layer were to face the indoor environment, overheating of the glazing would occur when the
519
insolation is over 600 W m2 (Fang et al., 2006b). Temperatures for an EC VG with an EC layer absorption of 60% subject to 300 W m2 insolation incident normal to the glass surface were calculated using the finite volume model. Predicted isothermal plots with emittances of either 0.02 or 0.18 on the both glass panes surfaces within the vacuum gap are presented in Figs. 3 and 4. The emittance value of 0.02 or 0.18 means that 98% or 82% radiation within the long wavelength range (3–50 lm) will be reflected by one low-e coating. The combined effective emittance value of two low-e coated glass panes was determined by Eq. (2). Figs. 3 and 4 show the temperature difference between the two glass sheets of the vacuum glazing which is due to the high thermal resistance of the vacuum gap. The temperature difference between one of vacuum glazing located in the centre of EC VG and the external glass pane with EC layer is also discernable. This is due to solar energy absorption at the EC layer between the two glass sheets. The heat conduction through the indium based edge seal lead to the temperature of the edge area of the indoor glass pane being lower than the central area of the VG. Figs. 3 and 4 clearly show the isotherms of the support pillars in the glazing representing the heat conduction through the support pillars. The surface temperatures at the centre of glazing area in Figs. 3 and 4 are 23.1 and 22.2 °C, respectively. The heat conductance at the centre glazing area of the glazing with 0.02 emittance coatings is less than that with 0.18 emittance coatings.
Fig. 3. Under 300 W m2 insolation and at opaque state, predicted isotherms for an EC VG with the EC layer facing the outdoor environment and both glass pane surfaces within the vacuum gap of the VG coated with low-e coatings with emittance of 0.02. Other parameters of the EC VG are listed in Table 1.
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Fig. 4. Under 300 W m2 insolation and opaque state, predicted isotherms for an EC VG with the EC layer facing the outdoor environment with the two glass 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 VG are listed in Table 1.
Detailed analysis has shown the influence of low-e coatings on the heat transmission of a VG (Fang, 2002; Fang et al., 2007). For the EC VG, the effect of varying the emittance of the low-e coatings between 0.02 and 0.18 on the surface temperatures of the inside and outside glass panes subject to 300 and 1000 W m2 insolation were simulated and the predictions are presented in Figs. 5 and 6. In Fig. 5 when the insolation is 300 W m2, for the opaque state (energy absorption 60%), with 0.02 and 0.18 emittance of low-e coatings, the indoor glass pane mean temperatures are 22.1 and 21.5 °C, respectively, which are higher than the inside air temperature of 21.1 °C, thus the EC VG transfers heat to the indoor environment. The outdoor glass pane mean temperatures of the glass panes are 5.2 and 5.4 °C. The mean temperature difference between the indoor and outdoor glass panes with low-e coatings with emittance of 0.02 and 0.18 are 16.9 and 16.1 °C, respectively. Heat flow through the entire indoor glass pane to the outdoor glass pane of the glazing with 0.02 emittance of low-e coating is less than that with 0.18 emittance of low-e coating. For the transparent state, when the emittance is 0.02, the indoor and outdoor glass pane temperatures are 20.1 and 11.5 °C, the temperature difference between the outdoor and indoor glass panes is 31.6 °C; when emittance is 0.18, the indoor and outdoor glass pane temperatures are 18.6 and 11.1 °C, the temperature difference is 29.7 °C. In the transparent state, the tem-
perature difference is much greater than that of opaque state, thus the low-e coating performed better as a heat barrier to the radiative heat transfer than in opaque state. With increasing insolation, both outdoor and indoor glass pane mean temperatures increase, but the rate of increase in the mean temperature of outdoor glass pane is rapid than that of indoor glass pane (Fang and Eames, 2006a). When insolation increases to 675 W m2, the mean temperatures of the indoor and outdoor glass panes are equal (Fang and Eames, 2006a). In Fig. 6, when the insolation is 1000 W m2, for the opaque state, with 0.02 and 0.18 emittance, the indoor glass pane temperatures are 35.1 and 37.3 °C, respectively, which are higher than the indoor air temperature of 21.1 °C, consequentially the indoor glass pane transfers heat to the indoor environment. The outdoor glass pane temperatures are 55.9 and 55.3 °C, respectively, which are higher than those of the indoor glass pane due to absorbed solar energy by the EC layer and low-e coatings. Heat is transferred from the outdoor glass pane to the indoor glass pane. For transparent state, with 0.02 and 0.18 emittance, the indoor glass pane temperatures are 28.0 and 26.7 °C, and consequentially the indoor glass pane transfers heat to the indoor environment; the outdoor glass pane temperatures are 0.2 and 0.6 °C, respectively. When the emittance is 0.02 and 0.18, the mean temperature difference between the outdoor and indoor glass panes are 27.8 and 26.1 °C,
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25
20
20
15
15
10
10
5
5 Energy absoption 10%
0
0
-5
-5
-10
-10
-15
Indoor glass pane (60% energy absorption)
o
C)
Energy absorption 60%
Temperature (
Temperature ( o C)
Insolation 300W.m-2 25
Outdoor glass pane (10% energy absorption) Indoor glass pane (10% energy absorption) Outdoor glass pane (60% energy absorption)
-15 0
0.04
0.08
0.12
0.16
0.2
Emittance Fig. 5. Indoor and outdoor glass mean surface temperature variation with emittance when the EC layer was either transparent (energy absorption 10%) or opaque (energy absorption 60%) with 300 W m2 insolation. Other parameters of the EC VG are listed in Table 1 except the emittance of low-e coating.
Insolation 1000W.m -2
65
65
45
35
35
25
25
15
15
Energy absorption 10%
5
5
-5
Temperature ( o C)
Energy absorption 60%
45
o
Temperature ( C)
Outdoor glass pane (60% energy absorption)
55
55
Indoor glass pane (60% energy absorption) Outdoor glass pane (10% energy absorption) Indoor glass pane (10% energy absorption)
-5 0
0.04
0.08
0.12
0.16
0.2
Emittance Fig. 6. Indoor and outdoor glass mean surface temperature variation with emittance when the EC layer was either transparent (energy absorption 10%) or opaque (energy absorption 60%) with 1000 W m2 insolation. Other parameters of the EC CG are listed in Table 1 except emittance of the low-e coating.
respectively. So in the transparent state, the temperature difference between the indoor and outdoor glass panes at an insolation of 1000 W m2 is less than that at an insolation of 300 W m2, since the higher insolation increases the outdoor glass pane temperature leading to the temperature difference 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 variations of both the indoor and outdoor glass panes were less than those for the glazing with 10% solar energy absorption in the EC layer. This was because solar energy absorbed by the 60% absorption EC layer increased the
outdoor glass pane surface temperatures more significantly than that of the indoor glass pane, leading to a temperature difference across the glazing with 60% solar energy absorption in the EC layer being smaller than that with 10% solar energy absorption in the EC layer. From Eq. (1), it can be seen that for larger temperature differences between the two glass surfaces, emittance has the great effect on radiative heat transfer and thus on the mean surface temperatures of both the indoor and outdoor glass panes. The influence of frame rebate depth on the thermal performance of EC VG at opaque and transparent states when insolations are 300 and 1000 W m2 were simulated and
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and outdoor glass panes is less than that for an insolation of 300 W m2 for both transparent and opaque states, the influence of frame rebate depth on the glass mean temperatures in negligible. When the insolation is 1000 W m2, in the opaque state, the outdoor glass pane temperature is higher than the indoor glass pane temperature. With increasing the frame rebate depth, the indoor glass pane temperature decreases very gradually, since incident solar energy incident on the outdoor glass pane is shaded by the increased frame rebate depth. In the opaque state, the comparison of the indoor and outdoor glass pane temperatures with one and two low-e coatings at insolations of 300 and 1000 W m2 is presented in Fig. 9, which shows that when the emittance is close to
the results are presented in Figs. 7 and 8. Fig. 7 shows that when the insolation is 300 W m2, with increasing the frame rebate depth, the outdoor glass pane temperature increases, until the rate in increase of indoor glass pane temperature reaches an asymptote at frame rebate depths above 15 mm. Frame rebate depth has a greater influence on the indoor glass pane temperature for the transparent state than the opaque state. Because the solar energy absorbed by the EC layer increases the outdoor glass pane temperature and reduces the temperature difference between the indoor and outdoor glass panes, the influence of frame rebate depth on the glass mean temperature decreases. Fig. 8 shows that at an insolation of 1000 W m2, the temperature difference between the indoor
30
30
25
25
20
20
15
Energy absorption 60%
15
o
Temperature ( C)
Insolation: 300W.m -2
10
10
Energy absorption 10%
Indoor glass pane Outdoor glass pane
5
5
0
0
Indoor glass pane
-5
Outdoor glass pane
0
10
20
30
40
50
-5
Rebate depth (mm) -10
-10
-15
-15
Fig. 7. Mean temperatures of indoor and outdoor glass panes with various rebate depth at opaque and transparent states when the insolation is 300 W m2. Other parameters of the EC VG are listed in Table 1 except frame rebate depth.
70
70 Insolation: 1000W.m-2
60
60
50
o
Temperature ( C)
50 Energy absorption 60%
Outdoor glass pane
40
40
30
30
Outdoor glass pane
20
20
Indoor glass pane
Energy absorption 10% 10
10
0
0 0 -10
Indoor glass pane
10
20
30
Rebate depth (mm)
40
50 -10
Fig. 8. Mean temperatures of indoor and outdoor glass panes with various rebate depth at opaque and transparent states when the insolation is 1000 W m2. Other parameters of the EC VG are listed in Table 1 except frame rebate depth.
Y. Fang et al. / Solar Energy 84 (2010) 516–525
523
60
Outdoor glass (two coatings)
50 Outdoor glas s pane
Temperature ( o C)
Insolation 1000W.m-2
Indoor glass (two coatings)
40
Outdoor glass (one coating) 30
Indoor glass (one coating)
Indoor glas s pane
Outdoor glass (two coatings)
20
Indoor glass (two coatings)
Outdoor glas s pane
Insolation 300W.m-2
10
Outdoor glass (one coating) 0 0
0.04
0.08
0.12
0.16
Indoor glass (one coating)
0.2
Emittance 2
2
Fig. 9. With insolation of 300 W m and 1000 W m , the indoor and outdoor glass pane mean temperatures of EC VG at opaque state with low-e coatings on one and two internal glass surfaces with various emittance. Other parameters are listed in Table 1 except emittance of low-e coating.
0.02, the indoor and outdoor glass pane temperatures approach each other for both insolations of 300 and 1000 W m2. This is comparable with the result of Fang and Eames (2006b). When the insolation is zero and the emittance is close to 0.02, the two low-e coatings provide limited improvement in the thermal performance of VG
(Fang and Eames, 2006b). When the insolation is 300 W m2, heat is transferred from the indoor glass pane to the outdoor glass pane of the glazing with one or two low-e coatings. The indoor glass pane temperature of the glazing with two low-e coatings is greater by 0.6 °C than that with one low-e coating. This is because two low-e coat-
1.4 Energy absorption 60%, opaque states
1000 W.m-2
1
-2
-1
(W.m .K )
Radiative heat conductance
1.2
0.8
One low -e coating
0.6
0.4
Pillar conductance
0.2 Tw o low -e coatings 300 W.m-2 0 0
0.05
0.1 0.15 Emittance of low-e coating
0.2
Fig. 10. With insolation of 300 and 1000 W m2 and opaque states, the relative radiative heat conductance of EC VG at opaque state with one and two low-e coatings. The pillar conductance is included for comparison. Other parameters are listed in Table 1 except emittance.
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Y. Fang et al. / Solar Energy 84 (2010) 516–525
ings more efficiently reduce the radiative heat flowing from the indoor glass pane to the outdoor glass pane than the single low-e coating. In the finite volume model, each low-e coating absorbs 10% of solar energy passing through it. For EC VG with two low-e coatings on the two internal surfaces of glass panes within the vacuum gap, the temperatures of both glass panes of vacuum glazing are affected by the absorbed solar energy. Since the insolation passes through the coating on the central glass pane first, the temperature increase at the central glass pane caused by absorbed solar energy should be larger than that of the indoor glass pane. The reason that indoor glass pane temperature of a glazing system with two low-e coatings is higher than that of a glazing system with one low-e coating is that the effective emittance of two low-e coatings is lower than that with one low-e coating, thus it effectively reduces the radiative heat transfer from the indoor glass pane to the outdoor glass pane. When the insolation is 1000 W m2, the outdoor glass pane temperature is much greater than that of the indoor glass pane due to absorbed solar energy by the outdoor facing EC layer. The heat is transferred from the outdoor glass pane to the indoor glass pane. The two low-e coatings more efficiently reduces the heat transferred from the outdoor glass pane to the indoor glass pane than the single coating, so the indoor glass pane temperature of the glazing with two low-e coatings is lower by 1.6 °C than that with a single coating. This investigation considers the relative radiative heat conductance between the two glass surface within the vacuum gap and heat conductance of pillar array within the EC VG. The heat conductance of pillar array and radiative heat conductance are determined by Eq. (4) and eeffective for one and two coatings is calculated by Eq. (2). The emittance of uncoated glass was assumed to be 0.9 (Manz et al., 2006). Fig. 10 shows that at an insolation of 300 W m2, the pillar and radiative heat conductance are equal to 0.512 W m2 K1 for two low-e coatings having emittance of 0.174. The radiative conductance is 0.054 W m2 K1, nearly an order of magnitude smaller than the pillar conductance for two coatings of emittance of 0.02. Under 1000 W m2 insolation, two low-e coatings can more significantly reduce the radiative conductance compared to one low-e coating than under 300 W m2. When insoaltion close to 0.02, this reduction in radiative conductance by using two coatings is reduced. As solar energy is absorbed at the low-e coating and EC layer, when considering the total energy balance of EC VG, a full study of solar heat gain of the EC VG is needed, which beyond the range of this work. 4. Conclusions Under the ASTM winter boundary conditions, the thermal performance of a 0.5 m by 0.5 m EC VG rebated within a solid wood frame and with two 4 mm thick lowe coated glass panes and a 0.1 mm thick EC layer coated 4 mm thick glass pane was simulated using a finite volume
model. The indoor air set-point temperature and the outdoor ambient air temperatures were assumed to be constant at 21.1 and 17.8 °C, respectively, the convective heat transfer coefficients on the indoor and the outdoor glass surfaces were assumed to be 8.3 and 30 W m2 K1, respectively, in accordance with the ASTM winter conditions. Each low-e coating absorbed 10% of solar energy pass through it. Other parameters of the EC VG configuration are listed in Table 1. With 60% solar energy absorption at the EC layer and 300 W m2 insolation incident perpendicular to the outdoor glass pane surface, for the EC layer facing the outdoor environment, when the emittance value of both the low-e coatings decreased from 0.18 to 0.02, the temperature of the indoor glass pane increased from 21.5 to 22.1 °C; the outdoor glass pane temperature decreased from 5.4 to 5.2 °C, and the temperature difference increased from 16.1 to 17.0 °C. The temperature of the indoor glass pane of the EC VG was higher than the indoor air temperature of 21.1 °C, transferring heat into the indoor environment. With 1000 W m2 insolation incident perpendicular to the outdoor glass pane surface, when the emittance value of both the low-e coatings decreased from 0.18 to 0.02, the temperature of the indoor glass pane decreased from 37.3 to 35.1 °C; the outdoor glass pane temperature increased from 55.3 to 55.9 °C, the temperature difference increased from 18.0 to 20.8 °C. The heat is transferred from the outdoor glass pane to the indoor glass pane and thus transferred into the indoor environment. The influence of frame rebate depth on the glass pane temperatures at an insolation of 300 W m2 is greater than for an insolation of 1000 W m2, since at the higher insolation of 1000 W m2 there is a significant decrease in the mean temperature difference between the outdoor and indoor glass panes, which reduces the influence of frame rebate depth. Under both 300 and 1000 W m2 insolations, when using one and two low-e coatings, with an emittance of 0.02, the temperatures of both the indoor and outdoor glass panes approach each other. At 1000 W m2 insolation, the outdoor glass pane temperature is much higher than that of the indoor glass pane, the two low-e coatings reduce the indoor glass pane temperature by 1.6 °C more than one low-e coating. At 300 W m2 insolation, the two low-e coatings increase the indoor glass pane temperature by 0.6 °C more than one low-e coating. Acknowledgements The authors acknowledge the support provided by the Charles Parson Energy Research Award, from the Department of Communications and Energy, the Republic of Ireland. References ASTM C1363, 2005. Standard test method for thermal performance of building materials and envelop assemblies by means of a hot box
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