Modelling and experimental analysis for the influence of hard and
soft coatings on the performance of vacuum glazing
Yueping Fang, Philip C. Eames, Trevor J. Hyde, Junfu Zhao and Jinlei Wang
,
Centre for Sustainable Technologies, School of the Built Environment,
University of Ulster, Newtownabbey, BT37 0QB, N. Ireland UK
Abstract
The thermal performance of a vacuum glazing employing coatings with emittance values between 0.02 and 0.16 were simulated using a three-dimensional finite volume model. The thermal performance of vacuum glazings with hard and soft coatings with emittance values of 0.04, 0.12 and 0.16 were fabricated and characterised using a guarded hot box calorimeter. Both simulated and experimentally characterised vacuum glazings consisted of two 400mm by 400mm glass panes which were separated by a 0.15mm wide evacuated space sealed contiguously by a 6mm wide metal edge seal. The vacuum space formed between the glass sheets was supported by a 0.4mm diameter array of pillars spaced 25mm apart on a square grid pattern. Each glazing had either one or two low-emittance (low-e) coatings. Details of the effect of different low-e coatings on the thermal performance of the simulated glazing are presented. Experimentally determined thermal performance for vacuum glazing with soft (emittance 0.04) and hard coatings (emittance 0.12 and 0.16) were in good agreement with theoretical predictions. The simulations show that for a low value of emittance (e.g. 0.02), the use of two low-e coatings gives limited improvement in thermal performance of the glazing system. The use of a single high performance low-e coating in the vacuum glazing has been shown to provide excellent performance.
Keywoods Vacuum glazing, low-e coating, thermal performance, insolation
1. Introduction
In the first successful method for fabricating vacuum glazing (Robinson and Collins, 1989), a solder glass formed a contiguous edge seal at temperatures of above 450oC. At such temperatures, tempered glass and many types of soft low-e coatings degrade. Consequently only hard low-e coatings have been employed successfully in vacuum glazings fabricated using a solder glass edge seal. A new method for producing an edge seal at a low temperature (less than 200oC) for a vacuum glazing has been developed to overcome this obstacle. Significant work has been presented on the vacuum glazing fabrication (Hyde et al., 2000) and its predicted and experimentally determined thermal performance (Fang et al., 2000, 2005). A measured thermal transmittance of less than 1Wm-2K-1 for the centre-of-glass area of the vacuum glazing has been achieved.
Depending on its emittance value, the use of a low-e coating on one or two glass surfaces within the evacuated gap of the vacuum glazing reduces the 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 decreases the solar gain (Hollands et al., 2001). To obtain the net energy performance of a glazing, both the total heat transmittance and the total solar energy transmittance should be calculated. The overall heat transfer coefficient of a vacuum glazing has been determined using the developed guarded hot box calorimeter (Fang et al., 2005). The solar heat gain coefficient can be determined in accordance with ISO, (2000) employing calculations to determine how much radiation is absorbed and re-emitted inwardly. In this work the effect of different low-e coatings on the thermal performance of the vacuum glazing was simulated using the finite volume model and compared to experimental measurements. Experimental results for the thermal performance of vacuum glazings with soft (emittance 0.04) and hard coatings (emittance 0.12 and 0.16) were in good agreement with predictions to within calculated experimental error. The simulations show that if the emittance value is very low (0.02), using two low-e coatings rather than one gives limited improvement in thermal performance for the glazing system. The use of only one currently-expensive high performance low-e coating in the vacuum glazing can provide acceptable performance.
2. Simulation methodology
A schematic diagram of the vacuum glazing modelled theoretically and characterised experimentally is shown in Fig. 1. A finite volume model (Eames and Norton, 1993) was modified to analyse the heat transfer through a vacuum glazing under ASTM standard winter conditions (ASTM, 1995). The glazing comprised two low-e coated glass panes separated by a 0.12mm vacuum gap. Due to symmetry considerations, only a quarter pane of the vacuum glazing was modelled. An example of a finite volume mesh employed when analysing the thermal performance of the vacuum glazing is shown in Fig. 2.
In the finite volume model, the net rate of radiative heat flow between two plane parallel surfaces with area A, mean surface temperatures T1 and T2, hemispherical emittances ε1 and ε 2 is calculated by (Collins and Simko, 1998):
(1)
i.e. (2)
Where Taverage is the average of the temperatures T1 and T2 and the effective emittance, εeffective, is determined by:
(3)
Radiation reflectance was assumed to be independent of the wavelength and the angle of incident radiation. Theoretically, the emittance value depends on the surface temperature, the wavelength, and the angle of incidence of the radiation to the normal. The error that resulted from using equation 3 is approximately 4% (Zhang et al., 1997).
3. Thermal performance of a vacuum glazing with various low-e coatings
In the simulations, the indoor air set-point temperature and the outdoor ambient air temperatures were assumed to be constant at 21.1ºC and -17.8ºC respectively, the convective heat transfer coefficients on the indoor and the outdoor surfaces were assumed to be 8.3Wm-2K-1 and 30Wm-2K-1 respectively that correspond to those in ASTM measurement standards for winter conditions (ASTM, 1995). The simulated vacuum glazing was assumed to be of 400mm by 400mm in dimension comprising two low-e coated 4mm thick glass panes, separated by 0.15mm, supported by a 0.4mm diameter pillar array spaced at 25mm in a regular square pattern. The edge seal was assumed to be a 6mm wide band of indium metal. The glazing was rebated 15mm into a solid woof frame.
Using the finite volume model, temperatures across the glazing were calculated for a vacuum glazing under ASTM winter conditions. Predicted isothermal plots for emittances of 0.12 and 0.16 on either glass pane surfaces within the vacuum gap are presented in Fig. 3. The temperature gradient across the two glass panes due to the high thermal resistance of the vacuum gap can be clearly seen. Emittance values of 0.12 and 0.16 mean that 88% and 84% of radiation within the long wavelength range (3 - 50μm) are reflected by the low-e coating. The combined effectiveemittanceof two low-e coatings was determined by equation 3. The metal edge seal of the vacuum glazing significantly reduced the indoor glass surface temperature in the region near to the edge of the glazing. The predicted overall heat transfer coefficient U-value of the total window and the centre-of-glass area with boundaries 63.5mm from each sightline (Anon, 1992), in which edge effects have little influence, were calculatedand are presented in Fig. 4. It can be seen that with increasing low-e coating emittance, the U-value of both the total window and the centre-of-glass area increase. The U-value of a vacuum glazing with two low-e coatings is lower than that with one low-e coating and one non-coated float glass pane (emittance value: 0.87), however when the emittance of the low-e coating is close to 0.02, the use of two low-e coatings rather than one gives limited reduction in the U-values of vacuum glazing. The use of a single currently-expensive low-e coating in a vacuum glazing will provide acceptable performance.
4. Experimental analysis of the performance of vacuum glazing with various low-e coatings
4.1 Measurement method
The thermal performance of vacuum glazings with low-e coatings with emitance of 0.04, 0.12 and 0.16 were characterised using a guarded hot box calorimeter, the schematic diagram is shown in Fig. 5. The heat transfer through the test sample can be determined by:
(4)
Here heat flow through the metering box, Qb was controlled to be close to zero. Heat transfer through the mask wall, Qm was determined by experimental measurement using the mask wall mean surface temperatures in the hot and cold chambers. The flanking loss Qe through the edge of the test specimen was measured using a polystyrene standard specimen whose thermal properties were known. Qe was also determined using the finite volume model. The thermal resistance C-value of a specimen with area A can be determined by:
(5)
Ts is the specimen surface temperature. ISO (1994) combines radiant and air temperatures into a single index, the environmental temperature Tn, which represents the proper weighting of air and radiant temperatures for the purpose of determining the heat flow to the surface.
(6)
Tr and Ta are the baffle surface and air temperatures respectively, ε is the emittance and hr is the radiation coefficient. Using equation 6, in the hot and cold chambers, Tn1 and Tn2 can be calculated, subsequently the U-value can be determined by equation 7:
(7)
A detailed description of the guarded hot box calorimeter calibration and error analysis are presented in Fang et al., (2005). The uncertainty of the U-value measured by the guarded hot box calorimeter was determined to be less than 8.5%.
4.2 Description of samples
The three vacuum glazing samples characterised in this work were fabricated in the Centre for Sustainable Technologies, University of Ulster. The specifications of the three samples are detailed in section 3. The edge of the vacuum gap was sealed by a 6mm band of indium metal with a thermal conductivity of 83.7 Wm-1K-1 (Griffiths et al., 1998). For sample 1, the two glass panes were Pilkington K-glass (Pilkington, 1989) with emittance of 0.16; for sample 2, one pane was K-glass and the other was Guardian 1.4D glass with emittance of 0.12; for sample 3, one glass pane was K-glass and the other was Pilkington Optitherm with an emittance of 0.04. The three samples were rebated 15mm into a solid wood frame for characterisation using the guarded hot box calorimeter.
4.3 Experimental determination of heat transfer through vacuum glazing with different low-e coatings
The three vacuum glazings contained in the solid wood frames were installed into the mask wall of the guarded hot box calorimeter as shown in Fig. 5 to determine their U-values. A summary of the heat transfer through the mask wall, flanking loss and vacuum glazing windows are listed in table 1. The predicted and experimentally determined U-values for the total window and centre-of-glass areas are compared in table 2.
Table1 Experimentally measured heat transfer through three vacuum glazing samples within a guarded hot box calorimeter.
Sample number /(C) /
(W) /
(W) /
(W) /
(W) /
(W) /
() /
()
Vacuum glazing 1 / 25.1 / 16.05 / 6.62 / 2.27 / 3.26 / 3.90 / 2.87 / 1.30±0.10
Vacuum glazing 2 / 22.1 / 13.86 / 5.83 / 2.00 / 2.93 / 3.09 / 2.16 / 1.21±0.10
Vacuum glazing 3 / 20.3 / 12.80 / 5.48 / 1.89 / 2.90 / 2.54 / 1.76 / 1.15±0.09
Table 2 Comparison between predicted and experimentally determined U-values for vacuum glazing with different low-e coatings.
Sample No. / Emittance values / Predicted U-values(Wm-2K-1) / Experimentally determined U-values (Wm-2K-1)
ε1 / ε2 / Ucentre / UW / Ucentre / UW
Vacuum glazing 1 / 0.16 / 0.16 / 1.06 / 1.29 / 1.07±0.09 / 1.30±0.11
Vacuum glazing 2 / 0.12 / 0.16 / 0.97 / 1.22 / 1.00±0.09 / 1.21±0.10
Vacuum glazing 3 / 0.04 / 0.16 / 0.85 / 1.13 / 0.87±0.07 / 1.15±0.09
The results in tables 1 and 2 show that better coatings lead to lower U-values of the vacuum glazing systems. The experimentally determined U-values are in good agreement with the predictions and are within experimental error. Predictions indicate that when the emittance of both low-e coatings is changed from 0.16 to 0.04, the U-value of the centre-of--glass area reduces by 20.8% and that of the total window system reduces by 12.4%. The influence of emittance on the U-value of the centre glass area is significantly larger than that of the total window system for the 400mm by 400mm glazing simulated due to the frame area and edge effects.
5. Conclusions
The thermal performance of a 400mm by 400mm vacuum glazing with two 4mm thick low-e coated glass panes was simulated using a finite volume model. The simulations show that when the surface emittance approaches 0.02, the use of two low-e coatings gives limited improvement in thermal performance of a glazing system when compared to one low-e coated glass pane. The use of only one currently-expensive low-e coating in a vacuum glazing has been shown to provide good thermal performance. The thermal performance of vacuum glazings with soft (emittance 0.04) and hard coatings (emittance 0.12 and 0.16) were determined experimentally using a guarded hot box calorimeter and shown to be in good agreement with theoretical predictions. The effect of emittance on the U-value of the centre glazing area is significantly larger than that of the total window system due to the influence of frame and edge conduction.
Nomenclature
AArea (m2)CHeat transfer coefficient (Wm-2K-1)
Q Heat transfer (W)T Temperature (ºC)
U Heat transmittance (Wm-2K-1)
Greek letters
ε Emittance of surfaceσStefan-Boltzmann constant
(5.6710-8 Wm–2K–4)
Subscripts
1, 2Internal and external surfacesaAir temperature
bMetering boxeFlanking loss
fFrameg Glass
mMask wallnEnvironment temperature
rRadiationWWindow
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