INFLUENCE OF INSOLATION LEVEL AND GLASS THICKNESS ON THE
THERMAL PERFORMANCE OF EVACUATED GLAZING
Yueping Fang, P. C. Eames and B. Norton
Centre for Sustainable Technologies,
University of Ulster,
Newtownabbey,
BT37 0QB,
Northern Ireland
Abstract -Flat evacuated glazing consists of two plane glass sheets separated by a narrow internal evacuated
space. The space is maintained by an array of small support pillars. Evacuated glazings of up to 0.5m by
0.5m have been fabricated, using glass sheets with thickness of 4 or 6mm. The 0.32mm diameter pillars
0.12mm high are arranged on a regular square grid with a separation of up to 40mm. A detailed 3dimensional finite volume model has been employed to determined the thermal performance variation of
evacuated glazing as a function of insolation intensity and the glass pane thickness.
Keywords: Evacuated glazing; U-value; insolation; thermal performance; glass thickness.
1. Introduction
Evacuated glazing comprises two contiguously sealed glass sheets between which a vacuum (<0.1Pa)
effectively eliminates gaseous conduction and convection. Transparent low-emittance coatings on the
interior surfaces of the glass sheets reduce radiative heat transfer to a low level. Conduction heat transfer
occurs through both the support pillars and the vacuum edge seal.
The successful fabrication of a vacuum glazing with low gas conduction was first reported from the
University of Sydney in 1989 (Robinson and Collins, 1989). The edge seal of the glazing was made of a
solder glass, with a welding temperature of about 400 0 C . The drawback of solder glass is that its melting
temperature is too high to be used in conjunction with many soft low-e coatings and tempered glass.
However the University of Ulster has fabricated successfully an evacuated glazing with a metal edge seal
with a melting point below 200 0 C (Griffiths et al., 1998). Many low emittance coated films can tolerate this
temperature, so the choice of possible low-E coatings is increased.
The structure of the evacuated glazing modelled consists of two 4mm or 6mm thick glass sheets with a
0.12mm narrow internal evacuated space. The separation of the sheets under the influence of atmospheric
pressure is maintained by an array of small support pillars. The diameter of each pillar is 0.32mm and pillar
separation is 40mm on a regular square grid. A finite volume model was used to simulate the thermal
performance of this evacuated glazing for different levels of insolation and with different glass thicknesses.
The temperatures of warm indoor air and ambient air were set at 21.1 0 C and -17.8 0 C respectively. The
emittances of the low-e coatings on both interior surfaces within the vacuum gap were set to be 0.2. The
convective heat transfer coefficients from the cold and warm side external glazing surfaces were set to be
30 Wm 2 K 1 and 8.3 Wm 2 K 1 respectively.
2. The effect of insolation on the thermal performance of evacuated glazing
Insolation incident on the external surface of an evacuated glazing affects greatly the thermal performance of
the evacuated glazing, the predicted effect is shown in Fig. 1. In Fig.1, the average external and internal
surface temperature and centre glass C-value refer to the exposed glass surface region.
35
1.3
30
0.8
20
15
0.3
10
5
-0.2
0
-5 0
200
400
600
800
1000
-0.7
-10
Average ambient external
surface Temperature
C,U-value (W/Km²)
Temperature (°C)
25
Average ambient internal
surface Temperature
Room air temperature
Center glass C-value
Total system C-value
-15
-20
-1.2
Insolation (W/m²)
Fig. 1. Variation of average surface temperature and heat transfer
coefficient with increasing insolation intensity. The edge seal height
is 2mm, the frame insulation height for both the internal and external
surface is 0.0497m. The coating emittances on both internal and
external glass surfaces are 0.2. The pillar diameter is 0.32mm and the
pillar separation is 40mm. The thickness of two glass panes are
6mm.
It can be seen from Fig. 1 that
with increasing insolation both
the internal and external average
surface temperature increase as
the glazing absorbs solar energy.
The internal average glass
surface temperature increases at
a greater rate that the external
glass surface temperature as the
heat transfer from the inner glass
sheet to the outer glass sheet
decreases. When the insolation
increases to 300W/m 2 , the
average predicted internal glass
surface
temperature
was
0
21.61 C , which is higher than
the indoor air temperature of
21.1 0 C , as shown as in Fig.1.
The evacuated glazing is now an
heat source, transferring heat into
the internal room air.
18
1.4
17.5
1.3
1.2
17
1.1
16.5
1
16
0.9
15.5
3.1 Finite volume model analysis
C,U-value (W/Km²)
Temperature (°C)
3. Thermal performance analysis with different thickness of glass sheet
Internal
surface Temp.
Center glass
C-value
Total C-value
0.8
Total U-value
15
0.003
0.004
0.005
0.7
0.006
Thickness of glass panes (m)
Fig. 2. Thermal performance variation due to changing the
thickness of glass sheet. The emittance of coatings on
interior vacuum gap surfaces are 0.2. The dimensions of the
evacuated glazing was 0.5m by 0.5m with pillars of 0.32mm
diameter separated at 40mm. The vacuum space was
0.12mm wide.
The thickness of glass sheet is one of the
most important parameters in determining
the selection of pillar separation for an
evacuated glazing system (Simko, 1996).
In simulations undertaken, the glass sheet
thickness is varied but the pillar
separation is maintained at a constant
distance, i.e. the tensile stress within the
glazing is not considered in this analysis.
The width of edge seal for these
simulations was 3mm, the height of frame
insulation was 2cm. The predicted
thermal performance variation with
changing thickness of the glass sheet is
shown in Fig. 2.
It can be seen from Fig. 2 that increasing the thickness of glass sheet with constant pillar separation, the Uvalue of the evacuated glazing increases. The ambient internal average surface temperature decreases. The
heat transfer rate through the full glazing system increases.
2.2 Analytic model analysis
The heat flow per unit length of edge due to the edge conduction is (Simko, 1996):
Qedge 
kt (Ti  T0 )
w1  w2  kt / h1  kt / h2
(1)
Heat transfer resistance and the U-value through one pillar (Wilson et al., 1998):
1
 2t

1 1
1
Rair toair 
 (

)  2 kg a  
hwarm A  kg A hrad A
hcold A

Uone, pillar  1 / ( Rair toair A)
1
(2)
(3)
2.4
1.1
2.2
1
2
0.9
1.8
0.8
1.6
1.4
0.003
U-value (W/K m²)
Heat transfer
Corresponding to different glass sheet thickness t, the rate of heat transfer per unit length of edge calculated
by equation (1) is presented in Figure 3. The heat transfer rate through a single pillar calculated using
equation (2) and (3) are also shown in Figure 3. The heat transfer through the glazing system and the Uvalue of the glazing system calculated by the finite volume (FV) model are also included.
Heat transfer per
unit length of edge
(W/m)
Heat transfer
through glazing
system (W)
U-value through
one pillar
0.7
0.004
0.005
0.6
0.006
U-value of glazing
system
Thickness of glass panes (m)
Fig. 3. The effect of glass sheet thickness on the thermal
performance of an evacuated glazing calculated by analytic
model and finite volume models. The emittance of both
internal and external glass surface coatings were 0.2. The
diameter of the pillar was 0.32mm at a separation of 40mm.
The height of frame insulation was 2cm. The size of the
evacuated glazing was 0.5m by 0.5m.
It can be seen from Fig. 3 that with
increasing glass sheet thickness, the air
to air U-value through a single pillar
decreases, this is because the greater
glass thickness increases the thermal
resistance above the pillar ends.
With increasing glass sheet thickness,
the heat transfer per unit length of the
edge due to edge conduction increases,
its rate of increase is larger than the rate
of decrease of heat transfer through the
pillar array, this leads to the heat transfer
rate through the whole glazing
increasing, so the U-value of the whole
glazing system increases. A schematic
diagram for heat transfer in an evacuated
glazing is shown in Fig. 4.
Consideration of the tensile stress for
thinner glass sheets would indicate the
pillar separation should decrease. The analysis of the glazing U-value requires further detail consideration.
It can be concluded that if the pillar separation is kept constant for the system modelled, the thicker the
glass sheet, the larger the U-value of the glazing system.
4. Conclusions
The thermal performance of an evacuated glazing was analyzed with a detailed finite volume model. It was
found that when incident insolation intensity increased to a value of 300 W / m2 , the inside glass surface
temperature was 21.61 0 C . This is larger than the indoor air temperature of 21.1 0 C . The evacuated glazing
became a heat source transferring heat into the inside room air.
0.12mm
In general, for a standard
Frame insulation
glazing system, the thicker the
glass sheet is, the smaller the
w
U-value of the system would
Heat transfer through
be,
i.e.
the
thermal
pillars
performance of the glazing
will be better. But for a small
Cold air
Warm air
area evacuated glazing, the
opposite was observed. When
hc ,out
hc ,in
increasing the glass sheet
thickness, the air to air U-value
through a single pillar
0.5m
decreases, this is because the
glass sheet is thicker and the
thermal resistance of the glass
Heat transfer per unit
above the two pillar ends is
length of edge
larger.
However
with
increasing
glass
sheet
Metal edge
thickness, the heat transfer per
Fig. 4. Schematic diagram of heat transfer in an evacuated glazing unit length of the edge due to
edge conduction increases, the
rate of its increase is larger than the rate of decrease of the heat transfer through the pillar array. This leads
to the heat transfer through the whole glazing system increasing, so that the U-value of the whole glazing
system increases.
Nomenclature
a
A
h
U
k
Q
R
t
T
w
Pillar radius, m
Area of model, or of unit cell, over which heat transfer process occurs, m 2
Heat transfer coefficient at the external surface of the glass sheets, Wm2 K 1
Thermal transfer coefficient, Wm2 K 1
Thermal conductivity, Wm1 K 1
Heat flow, W
Thermal resistance, K W 1
Thickness of glass sheet, m
Temperature, K
Height of frame insulation, m
Subscripts
1,2
Refer to external surfaces of glass sheet
g
Glass
rad
Radiation
References
Griffiths P.W., Leo M.Di, Cartwright P., Eames P.C. , Yianoulis P., Leftheriotis G and Norton B. (1998)
Fabrication of Evacuated Glazing at Low Temperature, Solar Energy Vol. 63, No. 4, 243-249.
Robinson S.J. and Collins R.E. (1989) Evacuated window-- theory and practice. In ISES Solar World
Congress, Internal Solar Energy Society, Kobe, Japan.
Simko T.M., (1996) Heat transfer process and stresses in vacuum glazing. Ph.D. thesis, The University of
Sydney.
Wilson C.F., Simko T.M., and Collins R.E., (1998) Heat Conduction Through The Support Pillars in
Vacuum Glazing, Solar Energy Vol. 63, No. 6, 393-406.