|
Today, many energy-saving shading
options exist with a uniform means to determine their solar optical
properties and their affects on the thermal properties of windows.
Thus, with no recommended standards or procedures when making spectroradiometric
measurements (NFRC 2002), this study uses industry accepted fenestration
software to determine the thermal and optical effects of solar screen
material on windows.
In this study, the thermal calculations
for the window are made using the recommended Window 4.1 program (Finlayson
et al. 1994). The thermal properties of the solar screen material
were measured at the Yellott Solar Energy Laboratory under the direction
of John Elliot Engineering Associates, Inc. (1982). See Appendix.
In addition, the glass optical properties were determined using the
spectral database for Varicon glass products. It is important
to note that field measurements were not within the scope of this
study.
The solar screen material is a three-layer
material consisting of a transparent polyester film, a perforated
vinyl screen, and a perforated aluminum film. It is designed
to reflect heat outward during the summer months and to conduct heat
inward during the winter months. However, this study makes no
recommendations or assertions to the configuration of the solar screen
material. Accordingly, the solar screen thermal properties are
shown in Table 1 .
Table
1 . Solar Screen Thermal Properties
| |
Transmittance1
|
Reflectance1
|
Absorbtance1
|
Emittance
|
|
Aluminized Surface
|
0.28
|
0.65
|
0.07
|
0.69
|
|
Black Surface
|
0.30
|
0.19
|
0.51
|
0.86
|
1
The value was
determined with the indicated surface facing the sun.
Source:
John Elliot Engineering Associates, Inc. (1982)
In the case of the transparent polyester
film, no values were determined. In this study, the polyester
film is presumed to have no significant effect on the solar screen
values. However, additional testing of the transparent polyester
film is required.
This study developed a prototype,
single glazed window using Window 4.1. The window was
then modified by adding a second layer for the solar screen material.
Overall the study varied the winter and summer environmental conditions
and the orientation of the solar screen. Four window conditions
were studied using the optical data for 1/8 inch single pane glass
manufactured by Varicon and compared to the base case for winter and
summer conditions defined within the environmental conditions library.
In this study ASHRAE winter conditions (for both U-VALUE and SOLAR
subsets) and ASHRAE summer conditions (for both U-VALUE and SOLAR
subsets) were used.
| |
A1
|
A2
|
|
B1
|
y11
|
y12
|
|
B2
|
Y21
|
Y22
|
Figure
1
. Window Assessment Model
Figure 1 is the model used to compare
the window conditions. “A” represents the orientation
of the solar screen (aluminized facing outward and black facing outward).
“B” represents the environmental conditions used to develop
the thermal properties of the window, which were summer and winter.
To analyze the data, the U-value, the shading coefficient, the solar
heat gain coefficient, and the visible transmittance of the four conditions
were compared.
Figure 2
. Solar Screen Cutting Method.
After review of the laboratory result
discussed earlier in this report, transmittance of the solar screen
material was reexamined. As a result, the transmittance values
for the solar screen was determined using equations for circular cutting
methods defined by Ishikawa (1994) and verified by Sylvester and Haberl
(2000) ( Table 2 ). The transmittance, t, of the solar
screen is determined by a defined relationship between the area of
each unit hole, S, and the width of photovoltaic area between
each hole, W, yielding,
( 2 . 1 )
for the circular laser cutting method.
It is assumed that the solar screen material is designed to block
all light equally, visible and infrared. Thus, the solar screen
was given no spectral selectivity. The values used for the solar
screen properties are shown in Table 2 . Please note that the
calculated solar transmittance was 44% was used to adjust the original
laboratory transmittance data. A transmittance of 35% was used
because the sum of the transmittance (calculated) and reflectance
(measured) cannot be greater that 100%.
Table
2 . Solar Screen Thermal Properties Used in the
Simulation
| |
Transmittance1
|
Reflectance2
|
Absorbtance2
|
Emittance
|
|
Aluminized Surface
|
0.35
|
0.65
|
0.07
|
0.69
|
|
Black Surface
|
0.35
|
0.19
|
0.51
|
0.86
|
1 This value was
determined using the maximum allowable transmittance, which is less
than are equal to the calculated value.
2 The value was
determined with the indicated surface facing the sun.
As discussed earlier, three conditions
were simulated and compared. The base case used the single glazed
system with no solar screen. For the four alternative conditions,
the solar screen was added. The aluminized and black surfaces
were analyzed for summer and winter conditions when facing outward.
Figure
3 . Window Configuration
Base on installation requirement of
the In’flector solar screen, air was used as the gas within
a two-inch gap between the two surfaces, which it is not a sealed
space. Thus, some infiltration will occur, but it can only be
quantified within an energy simulation of a select building.
Overall, the simulated window is 48 inches in width and 72 inches
in height and contained no breaks in the framing system ( Figure 3
). The glazing and solar screen values used in the simulation
are shown in Table 3.
Table
3 . Window Data Used Within Simulation Program
|
|
Thick
|
Tsol1
|
Rsol
|
Tvis2
|
Rvis
|
Tir
|
Emis
|
Keff
|
|
|
|
|
1
|
2
|
|
1
|
2
|
|
1
|
2
|
|
|
Aluminized3
|
0.0312
|
0.35
|
0.650
|
0.190
|
0.35
|
0.650
|
0.190
|
0.000
|
0.690
|
0.860
|
0.07
|
|
Black3
|
0.0312
|
0.35
|
0.190
|
0.650
|
0.35
|
0.190
|
0.650
|
0.000
|
0.860
|
0.690
|
0.51
|
|
1/8 glazing
|
0.120
|
0.834
|
0.075
|
0.075
|
0.899
|
0.082
|
0.082
|
0.000
|
0.840
|
0.840
|
0.520
|
1 This
value was determined using the maximum allowable transmittance, which
is less than are equal to the calculated value.
2
This value was not measured in the laboratory experiment. Thus,
the study assumes it to be equal to the total solar.
3
The indicated surface faces outward for the listed values.
The results show that the glazing
performs best with the aluminized surface facing outward during the
summer for all thermal properties ( Table 4 ). In the case of
the winter condition, the glazing performs better with the black surface
facing outward. However, the thermal performance of the window
shows no significant change for the orientation of the window ( Table
5 ).
Table
4 . Analysis of Glazing Systems for Summer Conditions
|
Window Condition
|
Keff
(Btu/h-ft-F)
|
Width (in.)
|
Uc
(Btu/h-ft-F)
|
SCc
|
SHGCc
|
VTc
|
|
Base
|
-
|
0.120
|
1.03
|
1.00
|
0.86
|
0.90
|
|
Black
|
-
|
2.151
|
0.54
|
0.45
|
0.39
|
0.33
|
|
Aluminized
|
-
|
2.151
|
0.49
|
0.37
|
0.32
|
0.32
|
Table
5 . Analysis of Glazing Systems for Winter Conditions
|
Window Condition
|
Keff
(Btu/h-ft-F)
|
Width (in.)
|
Uc
(Btu/h-ft-F)
|
SCc
|
SHGCc
|
VTc
|
|
Base
|
-
|
0.120
|
1.11
|
-
|
-
|
0.90
|
|
Black
|
0.1544
|
2.151
|
0.50
|
-
|
-
|
0.33
|
|
Aluminized
|
0.1697
|
2.151
|
0.51
|
-
|
-
|
0.32
|
To further analyze the glazing, a
frame was added to factor a typical frame effect. This condition
will now be referred to as the window system ( Figure 4 ).
Figure
4 . Frame Values used in the Simulation
|
Type
|
Source
|
U-Value
|
Edge Corr1
|
Width
|
Abs
|
|
Aluminum with no breaks
|
ASH/LBL
|
1.9
|
1
|
2.250
|
0.90
|
1
The fifth
correlation is recommend when modeling single glazing (Arasteh, 1989).
More research of these correlations is required.
As expected, the frame effect negatively
affected the performance of the window system. Likewise, as
with the glazing, the results show that the window system performs
best with the aluminized surface facing outward during the summer
( Table 6 ). In the case of the winter condition, the window
system performs better with the black surface facing outward.
However, the window’s performance shows no significant change
for the orientation of the window ( Table 7 ).
Table
6 . Analysis of Window Systems for Summer Condition
|
Window Condition
|
U
(Btu/h-ft2-F)
|
SC
|
SHGC
|
VT
|
|
Base
|
1.16
|
0.95
|
0.82
|
0.76
|
|
Black
|
0.76
|
0.49
|
0.42
|
0.28
|
|
Aluminized
|
0.72
|
0.42
|
0.36
|
0.27
|
Table
7 . Analysis of Window Systems for Winter Condition
|
Window Condition
|
U
(Btu/h-ft2-F)
|
SC
|
SHGC
|
VT
|
|
Base
|
1.23
|
-
|
-
|
0.76
|
|
Black
|
0.72
|
-
|
-
|
0.28
|
|
Aluminized
|
0.74
|
-
|
-
|
0.27
|
As noted earlier the scope of this
study included simulation testing only base on previously measured
thermal properties of the In’flector solar screen and existing
optical data for the glazing.
As seen in the results, the data
shows that the heat gain attributed to the windows of a building
using the In’flector solar glazing will be significantly reduced.
Specifically, the U-value of the glazing improved by an average
of 54% for the winter condition and 50% for the summer condition
( Table 8 ; Table 9 ). It is also noted that the system performs
best with the aluminized system facing outward during the summer
and with the black surface facing outward during the winter.
When factoring framing, the U-value of the window system improved
by an average of 41% for the winter condition and 36% for the summer
condition ( Table 10 ; Table 11 ).
Table
8 . Effects of Solar Screen for Summer Conditions
on Glazing
|
Window Condition
|
Uc
(Btu/h-ft2-F)
|
SCc
|
SHGCc
|
VTc
|
|
Base
|
-
|
-
|
-
|
-
|
|
Black
|
47.6%
|
55.0%
|
54.7%
|
63.3%
|
|
Aluminized
|
52.4%
|
63.0%
|
62.8%
|
64.4%
|
Table
9 . Effects of Solar Screen for Winter Conditions
on Glazing
|
Window Condition
|
Uc
(Btu/h-ft2-F)
|
SCc
|
SHGCc
|
VTc
|
|
Base
|
-
|
-
|
-
|
-
|
|
Black
|
55.0%
|
-
|
-
|
63.3%
|
|
Aluminized
|
54.1%
|
-
|
-
|
64.4%
|
Like the glazing only condition,
the window system performs best with the aluminized system facing
outward during the summer and with the black surface facing outward
during the winter. In addition, the framing around the window
has a significant effect on the window’s overall performance
and requires a more detailed analysis and accurate framing data
to improve the predictive capability of the simulation for a select
building.
Table
10 . Effect of Solar Screen for Summer Conditions
on Window
|
Window Condition
|
U
(Btu/h-ft2-F)
|
SC
|
SHGC
|
VT
|
|
Base
|
-
|
-
|
-
|
-
|
|
Black
|
34.5%
|
48.4%
|
48.8%
|
63.2%
|
|
Aluminized
|
37.9%
|
55.8%
|
56.1%
|
64.5%
|
Table
11 . Effect of Solar Screen for Winter Conditions
on Window
|
Window Condition
|
U
(Btu/h-ft2-F)
|
SC
|
SHGC
|
VT
|
|
Base
|
-
|
-
|
-
|
-
|
|
Black
|
41.5%
|
-
|
-
|
63.2%
|
|
Aluminized
|
39.8%
|
-
|
-
|
64.5%
|
In review of these findings, the
researcher recommends three levels of analysis: 1) glazing specific
window analysis, 2) glazing and framing specific window analysis,
and 3) a thermal energy simulation of the selected building which
uses specific glazing and framing data to determine an overall effect
of the window glazing on the energy consumption of the building.
A thermal simulation of a select building would factor the building’s
materials, configuration, and mechanical systems.
Arasteh, D., 1989. An Analysis of
Edge Heat Transfer in Residential Windows. Presented at the ASHRAE/DOE/BTECC/CIBSE
Conference on Thermal Performance of the Exterior Envelopes of Buildings
IV, Orlando, Florida, December 4-7, 1989.
Sylvester, K. and J. Haberl. 2000.
Semi Transparent PV Glazing: Development of Window Properties for
Input Within the DOE-2 Window Library. Solar 2000 Conference Proceedings.
American Solar Energy Society. Madison, Wisconsin (June) (CD-ROM).
NFRC, 2002. NFRC 300: Standard Test
Method for Determining the Solar and Infrared Optical Properties
of Glazing materials and Fading Resistance Systems. National Fenestration
Rating Council.
Yellot, J. and D. Talt, 1982. Report
No. 8219-1. John Yellott Engineering Associates, Inc. Phoenix, AR.
E. Finlayson, D. Arasteh, C. Huizenga,
M. Rubin, S. Reilly (1993). Window 4.0: Documentation of Calculation
Procedures. LBL-33943, Windows and Daylighting Group, Lawrence Berkeley
Laboratory, Berkeley, CA 94720.
Ishikawa, N. 1994. Building Integrated
PV Mounting Technologies in Japan, Proceeding of the Third International
Workshop on Photovoltaics in Buildings. Cambridge, MA. (September).
Abs: frame absorptance for
solar radiation at normal incidence.
Emis: infrared emittance
of the glazing layer (exterior facing surface is 1, and interior
oriented surface is 2).
Keff: a conductivity or
effective conductivity of a window component (frame, divider, glass,
gap)
Rsol: solar reflectance
of the glazing layer (exterior-facing surface is 1, and interior-facing
surface is 2).
Rvis: visible reflectance
(exterior-facing surface is 1, and interior-facing surface is 2).
Rb: the back (interior)
surface reflectance of a glazing system
Rf: the front (exterior)
surface reflectance of a glazing system
SC: the shading coefficient
for the total window system representing the ratio of the solar
heat gain through the window system relative to that through 3 mm
(1/8") clear glass at normal incidence.
SCc: the shading coefficient
for the glazing system (center-of-glass).
SHGC: the solar heat gain
coefficient of the total window system representing the solar heat
gain through the window system relative to the incident solar radiation.
SHGCc: the solar heat gain
coefficient for the glazing system (center-of-glass) only.
Source: source of information
for a frame or divider element.
Thick: glass thickness (SI:mm,
IP:in).
Tir: thermal infrared transmittance
of a glazing layer.
Tsol: solar transmittance
of the glazing layer.
Tvis: visible transmittance of
the glazing layer.
U-value: the total heat
transfer coefficient for the window system (SI:W/m2-°C, IP:Btu/h-ft2-°F).
Uc: the U-value for the
glazing system (center-of-glass) only (SI:W/m2-°C, IP:Btu/h-ft2-°F).
VT: the total window system’s
visible transmittance at normal incidence.
Vtc: visible transmittance
of the glazing system (center-of-glass) only.
1.1.1 Black Surface Facing Outward
for the Summer Condition
WINDOW 4.1 Report
Page 1
06/07/02 12:36:36
ID:10
FrID: 1
Name:Black Summer
===============================
Mode:Design
||
||
EnvCond:3
||
||
|| GlzSysID: 15
||
Type:Picture
FrID: 1||
Div ID: 0 ||FrID:
1
Tilt: 90
||
||
Size:Fixed BB
||
||
Width: 48.00"
||
||
Height: 72.00"
||
||
Area: 24.00 ft2
===============================
FrID: 1
U-value:
0.757 Btu/h-ft2-F
SC: 0.486
SHGC: 0.418
Vt: 0.282
Data
for Glazing Systems
COG
ID
Name
Area #Lay Tilt Uc
SCc SHGCc Vtc RHG
ft2
Btu/h-ft2
---
--------------- ----- ---- ---- -------
---- ----- ---- -----
15
Black Summer 16.711 2
90 0.538 0.45
0.39 0.33 98
Glass
and Gas Data for Glazing System '15 Black Summer'
ID
Name
D(in) Tsol 1 Rsol 2 Tvis 1 Rvis 2 Tir 1
Emis 2 Keff
----
--------------- ----- ---- ---- ---- ---- ---- ---- ---- ---- ----
----
Outside
1 Infl-Winter 0.031 .350 .190 .650 .350
.190 .650 .000 .860 .690 .510
1 Air
2.000
.069
102
CLEAR_3.DAT 0.120 .834 .075 .075 .899 .082
.082 .000 .840 .840 .520
Inside
Frame
Data
Frame Edge
Location
ID Name
Source Area
Area Uframe Uedge
ft2 ft2 Btu/h-ft2-F
----------------
--- ------------- ------------- ----- -----
------- ------
Left
Jamb 1
Al no break ASH/LBL
1.090 1.129 1.9001 0.6317
Header
1 Al no break ASH/LBL
0.715 0.712 1.9001 0.6317
Right
Jamb 1 Al
no break ASH/LBL
1.090 1.129 1.9001 0.6317
Sill
1 Al no break ASH/LBL
0.715 0.712 1.9001 0.6317
WINDOW
4.1 Report
Page 2
06/07/02 12:36:36
Gas
Data
ID
Name
Cond dCond Visc dVisc
Dens dDens Pran dPran
Btu/ Btu/h- lb-s/ lb-s/ lb/ft3
lb/ft3-
h-ft- ft-F2 ft2 ft2-F
F
F
x e-5 x e-5 x e-8
___
______________ _____ ______ _______ _______ _______ _______
_____ ______
1 Air
.0139 2.4395 0.0361 0.1161 0.0805 -0.0002 .7200
.00100
Environmental
Conditions: 3 ASHRAE-SUMMR
Tout Tin WndSpd Wnd Dir
Solar Tsky Esky
(F) (F) (mph)
(Btu/h-ft2) (F)
----- ---- ------ -------- ------
---- ----
Uvalue
89.0 75.0 7.50 Windward
248.2 89.0 1.00
Solar
89.0 75.0 7.50 Windward
248.2 89.0 1.00
Frame
Library Data
U-value Edge GlzSys GlzSys Width
ID
Name
Source Frame Edge
Corr Width Uc (PFD) Abs
Btu/h- Btu/h- in
Btu/h- in
ft2-F ft2-F
ft2-F
___
_____________ ____________ ______ ______ ____ ______
______ _____ ____
1 Al no break ASH/LBL
1.90 N/A 1
var N/A 2.250 0.90
Divider
Library Data
U-value Edge GlzSys GlzSys Width
ID
Name
Source Div
Edge Corr Width Uc (PFD)
Abs
Btu/h- Btu/h- in
Btu/h- in
ft2-F ft2-F
ft2-F
___
_____________ ____________ ______ ______ ____ ______
______ _____ ____
No
Dividers for this Glazing System
WINDOW 4.1 Report
Page 3
06/07/02 12:36:36
Optical
Properties for Glazing System '15 Black Summer'
Angle 0 10 20
30 40 50 60
70 80 90 Hemis
Vtc : 0.332
Rf : 0.201
Rb : 0.637
Tsol : 0.307
Rf : 0.200
Rb : 0.550
Abs 1: 0.460
Abs 2: 0.034
Abs 3:
Abs 4:
Abs 5:
Abs 6:
SHGCc: 0.390
SCc:
0.45 Color Properties DomWL
Purity L*
a* b*
Tdw:
N/A Transmittance
um %
Tuv:
N/A Reflectance
um %
Temperature
Distribution (degrees F) for '15 Black Summer'
Condensation
Env.
Conditions: 3 ASHRAE-SUMMR
U-value RH
Solar
Outside Air
89.0
89.0
Outer Surface
112.6 N/A
112.6
Layer 1 Center
112.9
112.9
Inner Surface
112.8
112.8
Outer Surface
94.5
94.5
Layer 2 Center
94.3
94.3
Inner Surface 94.1
N/A
94.1
Inside Air
75.0
75.0
1.1.2 Black Surface Facing Outward
for the Winter Condition
WINDOW
4.1 Report
Page 1
06/07/02 12:36:46
ID:11
FrID: 1
Name:Black Winter
===============================
Mode:Design
||
||
EnvCond:2
||
||
|| GlzSysID: 13
||
Type:Picture
FrID: 1||
Div ID: 0 ||FrID:
1
Tilt: 90
||
||
Size:Fixed BB
||
||
Width: 48.00"
||
||
Height: 72.00"
||
||
Area: 24.00 ft2
===============================
FrID: 1
U-value:
0.723 Btu/h-ft2-F
SC: N/A
SHGC: N/A
Vt: 0.282
Data
for Glazing Systems
COG
ID
Name
Area #Lay Tilt Uc
SCc SHGCc Vtc RHG
ft2
Btu/h-ft2
---
--------------- ----- ---- ---- -------
---- ----- ---- -----
13
Black Winter 16.711 2
90 0.496 N/A
N/A 0.33 N/A
Glass
and Gas Data for Glazing System '13 Black Winter'
ID
Name
D(in) Tsol 1 Rsol 2 Tvis 1 Rvis 2 Tir 1
Emis 2 Keff
----
--------------- ----- ---- ---- ---- ---- ---- ---- ---- ---- ----
----
Outside
| 