Thermal Transmittance around Edge of Vacuum Glazing
Ivan ChmurnyIOP Conference Series: Materials Science and Engineering
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The main objective of the calculations thermal performance properties of the edge of vacuum glazing was the quantification of the heat flow around edge of glazing. Additional heat flow around edge of glazing is expressed by means linear thermal transmittance. Linear thermal transmittance of the junction between edge vacuum glazing and wooden frame or wooden-aluminium frame was calculated by using a two dimensional (2D) numerical calculations. Edge sealing of vacuum glazing is assumed by means solder glass with Indium metal strip, so it is metal-based edge seal. In vacuum glazing the edge seal acts as a thermal bridge between the glass sheets. Heat flows from the glass sheet on the warm side through the edge seal to the sheet on the cold side, not only does it increase the overall thermal performance of the glazing, but also it causes temperature variation (decrease surface temperatures) and stress in the edge region.
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Thermal Transmittance around Edge of Vacuum Glazing
To cite this article: Ivan Chmurny 2019 IOP Conf. Ser.: Mater. Sci. Eng. 603 022059
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IOP Publishing
IOP Conf. Series: Materials Science and Engineering 603 (2019) 022059 doi:10.1088/1757-899X/603/2/022059
Thermal Transmittance around Edge of Vacuum Glazing
Ivan Chmurny 1
1
Slovak Technical University, Radlinskeho 11, 810 05 Bratislava, Slovakia
[email protected]
Abstract. The main objective of the calculations thermal performance properties of the edge of
vacuum glazing was the quantification of the heat flow around edge of glazing. Additional heat
flow around edge of glazing is expressed by means linear thermal transmittance. Linear
thermal transmittance of the junction between edge vacuum glazing and wooden frame or
wooden-aluminium frame was calculated by using a two dimensional (2D) numerical
calculations. Edge sealing of vacuum glazing is assumed by means solder glass with Indium
metal strip, so it is metal-based edge seal. In vacuum glazing the edge seal acts as a thermal
bridge between the glass sheets. Heat flows from the glass sheet on the warm side through the
edge seal to the sheet on the cold side, not only does it increase the overall thermal
performance of the glazing, but also it causes temperature variation (decrease surface
temperatures) and stress in the edge region.
1. Introduction
The main objective of the calculations thermal performance properties of the edge of vacuum glazing
was the quantification of the heat flow around edge of glazing. Additional heat flow around edge of
glazing is expressed by means linear thermal transmittance. Linear thermal transmittance of the
junction between glazing and wooden frame was calculated by using a two dimensional (2D)
numerical calculations.
At present, vacuum glass is used in different kinds of buildings such as office building, building
group, exhibition center and showrooms, library, greenhouse, house and apartment, many of which
were “world first” or “world greatest”.
A different fabrication method of vacuum glazing using a indium alloy edge seal technique was
developed at the University of Ulster (UU) 1, and in China 2. Vacuum glazing samples
incorporating two tempered or heat-strengthened glass of 5 mm thick were produced with the center of
glazing U-value of 0,58 W.m-2.K-1.
2. Description of vacuum glazing
The vacuum glazing consists of two glass panes separated by a vacuum layer which is 0.2 mm wide.
The glass sheets are separated by small circular metal pillars with diameter of about 0.25 mm and
spaced 40 mm apart. There is a low-emission layer on the inner part of the glazing, which reduces heat
loss by radiation.
Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution
of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Published under licence by IOP Publishing Ltd
1
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IOP Conf. Series: Materials Science and Engineering 603 (2019) 022059 doi:10.1088/1757-899X/603/2/022059
According to the description from Beijing Synergy Vacuum Glazing Technology Co., Ltd., vacuum
glazing has a dimension of 978 mm x 578 mm and structure (Figure 1):
T5 + V + TL5
where T is tempered or heat-strengthened glass of 5 mm thick;
V is vacuum layer of 0.15 to 0.2 mm thick;
TL is tempered or heat-strengthened low-E glass of 5 mm thick.
Solar factor (g-value) of glazing = 0.638;
Light-transmission factor V = 0.79;
The edge of the vacuum glazing is 10 to 12 mm wide;
Amount of pillars (supporting profiles) 13 x 24 = 312 for vacuum glazing area of 0.565 m2 (Figure 2);
Declared value of glass thermal transmittance Ug = 0,58 W.m-2.K-1.
These properties were stated by the manufacturer based on the calculation in Window 7 by Canadian
Standard NFRC 100-2010.
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978
Figure 1. Vacuum glazing – section (not to scale) 3
578
Figure 2. Vacuum glazing – support pillar field – view
Synergy Vacuum Glazing Technology uses infrared edge sealing technology 2. The technology
of making tempered vacuum glass in rapid step infrared heating furnace and edge sealing by infrared
glass solder was first proposed in Chinese and American patents [5]. In recent years, a Chinese
company not only developed low-melting-point and lead-free glass solder which can heat up rapidly
by absorbing infrared, but also designed and manufactured step infrared continuous heating furnace
2
WMCAUS 2019
IOP Publishing
IOP Conf. Series: Materials Science and Engineering 603 (2019) 022059 doi:10.1088/1757-899X/603/2/022059
5 0.2
5
with 6 minutes a beat. The edge of glass is sealed at a relatively fast speed where the temperature of
glass center is 30-50 ºC lower than the edge. The technology not only decreased the anneal extent of
tempered glass, but also reduced electricity consumption and production. Model of the edge of
vacuum glazing (Figure 2) is based on thermal conductivity of applied materials. Solder glass 1
W/(m.K) and Indium 83.7 W/(m.K).
= 1 W/(m.K)
ekv´= 0,0063 W/(m.K)
11
= 83.7 W/(m.K)
Figure 3. Model of edge of vacuum glazing (not to scale)
3. Method for testing heat flow around edge of glazing
In general, the thermal transmittance or U-value of the window or door product or assembly is
calculated as a function of the thermal transmittance of the components and their geometrical
characteristics, plus the thermal interactions between the components.
EN ISO 10077-1 6 provides a calculation method to obtain the thermal transmittance of windows
and pedestrian doors consisting of glazed and/or or opaque panels fitted in a frame. EN ISO 10077-2
7 specifies the method for numerical calculation of the thermal transmittance of frames Uf and the
linear thermal transmittance . The value describes the heat loss caused by fitting the glass unit in
the frame (or mullion/transom). The amount of this value varies depending on the spacer used in the
insulating glass unit. A distinction is made between conventional spacers and thermally improved
spacers. The calculation of the thermal transmittance of frame profiles, and the linear thermal
transmittance according to EN ISO 10077-2 is carried out using a two dimensional numerical method.
4. Calculations for vacuum glazing
The thermal transmittance of the frame section, Uf, is defined as follows. In the calculation model the
glazing is replaced by an insulation panel with thermal conductivity λ = 0,035 W/(m·K) inserted into
the frame, with clearance b1 not less than 5 mm. The overlap b2 is equal to that of the glazing which
the insulation panel replaces. The length of the panel shall be at least 190 mm measured from the most
protruding part of the frame, ignoring any protruding gasket(s). The opposite end of the panel is
considered as an adiabatic boundary. The frame model shall contain all materials used in
manufacturing the window except the glazing or opaque panel, which is replaced by the insulation
panel. The thickness d of the insulation panel shall be of the glazing or opaque panel being replaced;
To calculate the two-dimensional thermal conductance of the section consisting of the frame and
the glazing including the spacer effect, the frame section with a projected frame width, bf, and thermal
transmittance Uf is completed by glazing with thermal transmittance Ug and length bg (Figure.4). The
value of the linear thermal transmittance, Ψ, is defined by Equation (1).
L2D U f bf U g bg
(1)
where
Ψ
2D
L
Uf
Ug
is the linear thermal transmittance, expressed in W/(m·K);
is thermal conductance of the section shown in Figure.4, expressed in W/(m·K);
is the thermal transmittance of the frame section, expressed in W/(m2·K);
is the thermal transmittance of the central area of the glazing, expressed in W/(m2·K);
3
WMCAUS 2019
IOP Publishing
IOP Conf. Series: Materials Science and Engineering 603 (2019) 022059 doi:10.1088/1757-899X/603/2/022059
bf
bg
is the projected width of the frame section, expressed in m;
is the visible width of the glazing, expressed in m.
Figure 4. Schematic of profile section with glazing installed 7
5. Calculations vacuum glazing in wooden frame
Calculation of linear thermal transmittance was by means Therm program 8. Results of calculations
are in Table 1. In the first column are results for wooden frame I.4 taken from EN ISO 10077-2. In this
frame was installed calibration panel with thermal conductivity λ = 0,035 W/(m·K) inserted into the
frame, with clearance b1 in order to obtain Uf value for glazing with dg = 10.2 mm (for vacuum
glazing). This is in second column in Table 1. In the third column are results for calculation of linear
thermal transmittance of vacuum glazing. Here we have from Table 1:
Uf = 1.56 W/(m2.K) and bf = 110 mm
Ug = 0.58 W/(m2.K) and bg = 190 mm
L2D= 0.3042 W/(m.K).
For linear thermal transmittance according equation (1) is
= 0.3046 – 1.56 x 0.110 – 0.58 x 0.19 = 0.023 W/(m.K)
(2)
6. Calculations vacuum glazing in wooden-aluminium frame
The wood-aluminium window has a dimension of 800 x 1,200 mm, the surface of the window Asp =
0,96 m2 (Figure. 5). The frame construction of the wood-aluminium window is based on the wooden
profile IV88.
Figure 5. Window with vacuum glazing - section
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IOP Publishing
IOP Conf. Series: Materials Science and Engineering 603 (2019) 022059 doi:10.1088/1757-899X/603/2/022059
Table 1. Results of calculations of thermal transmittance VG in wooden frame
Calibration and comparison
results from EN ISO 10077-2
Figure I.4:
dg = 28 mm
Uf = 1.36 W/(m2.K)
L2D= 0.346 W/(m.K)
Results for frame with
calibration panel with
dg = 10.2 mm
Uf = 1.56 W/(m2.K)
L2D= 0.586 W/(m.K)
Results for vacuum glazing
dg = 10.2 mm
Uf = 1.56 W/(m2.K)
Ug = 0.58 W/(m2.K)
L2D= 0.3046 W/(m.K)
Table 2. Results of calculations of thermal transmittance VG in wooden-aluminium frame
Results for frame with calibration Results for vacuum glazing
panel
dg = 10.2 mm
dg = 10.2 mm
Uf = 1.286 W/(m2.K)
Uf = 1.286 W/(m2.K)
Ug = 0.58 W/(m2.K)
L2D= 0.5615 W/(m.K)
L2D= 0.2942 W/(m.K)
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WMCAUS 2019
IOP Publishing
IOP Conf. Series: Materials Science and Engineering 603 (2019) 022059 doi:10.1088/1757-899X/603/2/022059
Here we have from Table 2:
Uf = 1.286 W/(m2.K) and bf = 115 mm
Ug = 0.58 W/(m2.K) and bg = 190 mm
L2D= 0.2942 W/(m.K).
For linear thermal transmittance according equation (1) is
= 0.2942 – 1.286 x 0.115 – 0.58 x 0.19 = 0.036 W/(m.K)
(3)
7. Conclusions
The edge area o vacuum glazing using solder glass and indium alloy edge seal has linear thermal
transmittance
= 0,023 W/(m.K) for wooden frame,
= 0,036 W/(m.K) for wooden-aluminium frame.
These values should be used if thermal performance of window with vacuum glazing is expressed
according system of European standards EN ISO 10077-1 and EN ISO 10077-2.
Acknowledgement
The authors express thanks to Ministry of Education SR for financial support of the projects VEGA
1/0113/19 and APVV-16-0126.
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Y. Fang, T. J. Hyde, F. Arya, N. Hewitt, P. C. Eames, B. Norton, S. Miller Indium alloy-sealed
vacuum glazing development and context, Renewable and Sustainable Energy Reviews 37
(2014) 480-501
Y. Zhao Method for Making Tempered Vacuum Glass and Production Line. China,
CN201510940006.1, filed Dec. 15, 2015, and issued May 4, 2016
H. Hohenstein, The dawn of a new era – New option for windows, facades and walls with
vacuum glass. Synergy VG, 2014
I. Chmúrny, D. Szabó, Thermal Irregularities in Vacuum Glazing. IOP Conf. Series: Materials
Science and Engineering 471 (2019) 062012, doi:10.1088/1757-899X/471/6/062012
J. Tang, X. Tang and W. Xu: Development of Vacuum Glass in China – Both Opportunity and
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EN ISO 10077-1 Thermal performance of windows, doors and shutters. Calculation of thermal
transmittance. Part 1: General (ISO 10077-1: 2017)
EN ISO 10077-2 Thermal performance of windows, doors and shutters. Calculation of thermal
transmittance. Part 2: Numerical methods for frames (ISO 10077-2: 2017)
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6
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