1. Chapter 5B: WINDOW SHADING AND IMPROVEMENTS
Agami Reddy (rev Dec 2017)
Solar radiation and wall orientation
Effect of solar path
Exterior shading by overhangs
Solar profile angle
Case study
Interior shading devices
High performance glazing
Smart windows
HCB-3 Chap 5B: Window Shading and Improvements

2. Solar Radiation and Wall Orientation
Compare solar radiation hitting south-facing vs. east-west windows by month
Solar heat gain thru south windows maximum in winter and minimum in summer- good
What about solar heat gain thru east-west facing windows?
From
Randolph
And Masters:
Energy for
Sustainability
HCB-3 Chap 5B: Window Shading and Improvements

3. HCB-3 Chap 5B: Window Shading and Improvements
Solar Orientation
For typical house with long side facing south, solar exposure is maximized
For those with long N-S orientation, reverse effect
To control solar heat gains: use overhangs and other shading devices such as side fins
From Randolph
And Masters
HCB-3 Chap 5B: Window Shading and Improvements

4. Effect of Sun’s Path on Incident Angles
The practical impact of solar angles is its effect on how the sun hits our buildings
We can design buildings to maximize solar gain in winter and minimize in summer
From Randolph
And Masters
HCB-3 Chap 5B: Window Shading and Improvements

5. External Shading: Fixed Overhangs
Solar zenith angle:
Figure 5.14
Shading of a south-facing window
from a horizontal overhang.
If we neglect end effects of horizontal overhangs, analysis is much simpler
For south facing windows at solar noon:
<0
>0
HCB-3 Chap 5B: Window Shading and Improvements

6. HCB-3 Chap 5B: Window Shading and Improvements
- At solar noon
Eqs and 5.20
HCB-3 Chap 5B: Window Shading and Improvements

7. Profile Angle is used To analyze general cases,
This angle takes into account
combined effect of solar altitude
and solar-wall azimuth
(applies to direct radiation) 1’ 1 2’ S 2
vertical
Note: S-2 is the width of the overhang
HCB-3 Chap 5B: Window Shading and Improvements

8. HCB-3 Chap 5B: Window Shading and Improvements
Cylindrical projection
Figure 4.6
Cylindrical projection sun-path diagram: solar altitude angle (=90° − θs) versus azimuth ϕs. Time in legends is solar time. (a) Latitude λ = 30°
HCB-3 Chap 5B: Window Shading and Improvements

9. HCB-3 Chap 5B: Window Shading and Improvements
Figure 5.16 Variation of profile angle with solar time for south facing surface at 40oN latitude.
Practical implications is that if we design shade for noon for summer (which has simple equations), then the overhang will also provide shade for several hours (upto 3 hours) before and after noon
Note that the variation in profile angle is constant for the equinox day and for the hours about solar noon for the other days (9: :00 hours)
Practical implications?
HCB-3 Chap 5B: Window Shading and Improvements

10. HCB-3 Chap 5B: Window Shading and Improvements
The difference between the profile angle for south-facing and slightly off-south facing walls is also flat
Figure 5.17 Variation of profile angle with altitude angle and solar- wall azimuth angle.
HCB-3 Chap 5B: Window Shading and Improvements

11. HCB-3 Chap 5B: Window Shading and Improvements
Figure 5.15 Coordinates of the
shade S = (x, y, z) cast by a point P
(in this example, P is the corner of a rectangular window recess).
5.23
5.24
HCB-3 Chap 5B: Window Shading and Improvements

12. HCB-3 Chap 5B: Window Shading and Improvements
Example 5.6: Horizontal overhang over west-facing window
Eq. 5.22
HCB-3 Chap 5B: Window Shading and Improvements

13. HCB-3 Chap 5B: Window Shading and ImprovementsEq
Fraction of height which is shaded
Eq. 5.23
This example illustrates the fact that a major cause of cooling loads is solar radiation on east- or west-facing windows. In summer, there are long periods when the sun can reach these facades with fairly small angles of incidence. The range of incidence angles is so wide as to make it impossible to block all this radiation, short of eliminating the windows completely. But partial blocking can be achieved by combining horizontal overhangs with vertical shades to the south of each window.
HCB-3 Chap 5B: Window Shading and Improvements

14. HCB-3 Chap 5B: Window Shading and Improvements
Eq. 5.11
Eq. 5.25
This example illustrates the fact that a major cause of cooling loads is solar radiation on east- or west-facing windows. In summer, there are long periods when the sun can reach these facades with fairly small angles of incidence. The range of incidence angles is so wide as to make it impossible to block all this radiation, short of eliminating the windows completely. But partial blocking can be achieved by combining horizontal overhangs with vertical shades to the south of each window.
HCB-3 Chap 5B: Window Shading and Improvements

15. HCB-3 Chap 5B: Window Shading and Improvements
Fixed Vertical Fins in Windows
External fins
Internal louvers
HCB-3 Chap 5B: Window Shading and Improvements

16. HCB-3 Chap 5B: Window Shading and Improvements
Utilizing Passive Solar Control Strategies to Mitigate Solar Heat Gain Through Glazing
Case Study:
Without and With Vertical Fins on an Actual Building in Phoenix
One option evaluated
Actual
ASU’s Barett Honors College Residence Halls Lounge/Reading Room
Sept. 21
12:30 pm
HCB-3 Chap 5B: Window Shading and Improvements

17. HCB-3 Chap 5B: Window Shading and Improvements
80% reduction of solar load
on June 21, 12:30 PM
70% reduction of solar load
on December 21, 12:30 PM
70% reduction of solar load
on June 21, 12:30 PM
75% reduction of solar load
on December 21, 12:30 PM
80% reduction of solar load on June 21, 12:30 PM
70% reduction of solar load on December 21, 12:30 PM
HCB-3 Chap 5B: Window Shading and Improvements

18. HCB-3 Chap 5B: Window Shading and Improvements
Window Orientation
But don’t windows LOSE heat in the winter?
- Yes, but the NET energy depends on U-value and SHGF and location
What is the balance?
Most modern double-paned south-facing windows have positive net balance over course of winter
HCB-3 Chap 5B: Window Shading and Improvements

19. HCB-3 Chap 5B: Window Shading and Improvements
Internal Shading
Draperies reduce heat gain by reflecting back some of the solar radiation. This effect is captured by introducing an Interior Attenuation Coefficient (IAC)
Figure 5.18 Draperies reduce
heat gain by reflecting back
some of the solar radiation.
Then solar heat gains are expressed as
HCB-3 Chap 5B: Window Shading and Improvements

20. HCB-3 Chap 5B: Window Shading and Improvements
Consider Example 5.6 where the solar heat gains through a west facing window with a horizontal overhang were computed.
The effect of introducing interior opaque white roller shades (IAC=0.40) would reduce solar heat gains to x 0.4= 70 W/m2.
The combined effect of both the external overhang and the interior shading device has reduced the solar hear gains of the plain window of Example 5.3 by ( /216.7) = 0.677, i.e., by about 68%.
HCB-3 Chap 5B: Window Shading and Improvements

21. HCB-3 Chap 5B: Window Shading and Improvements
High Performance Glazing
PANELITE: Simple Technology
– Plastic honeycomb structure enclosed between two panes ¼” glazing, ½”air space with honeycomb, ¼” glazing Total: 1 inch thick SHGC = 0.14 during mid-day U-values = 0.29 Btu/(h.ft2.F)
HCB-3 Chap 5B: Window Shading and Improvements

22. HCB-3 Chap 5B: Window Shading and Improvements
Smart Windows 1/2
There are essentially four types:
 (a) thermochromic materials- transmissivity varies with temperature
As outdoor temperature increases, visible light transmittance decreases, and vice a versa
Gels sandwiched between glass and plastic switch from a clear state when cold to a more diffuse, white, reflective state when hot
Widespread commercial development hindered chemical leakage around the edges, and degrading the optical properties over time.
 (b) photochromic materials- transmittivity changes with incident light intensity
glass automatically adjusts its visible transmittance as exterior light changes
One drawback is that the glazing dims when exposed to winter sun, thereby increasing heating loads
Large sizes of windows are not commercially available.
HCB-3 Chap 5B: Window Shading and Improvements

23. HCB-3 Chap 5B: Window Shading and Improvements
Smart Windows 2/2
(c) liquid crystal display technology (widely used in wrist watches)- transmittivity varied by passing electric current
very thin layer of liquid crystals sandwiched between two transparent electrical conductors deposited on heat-treated glass
requires continuous power supply for the glass to remain clear ( V AC or 0.5 W/ft2 of glass area).
(d) electrochromic (EC) coatings on glass or plastic- optical and thermal properties change with voltage
samples have been produced whose transmissivity varies from 0.2 to 0.8
though EC window prototypes have been installed in a number of buildings in Japan, Europe and the United States, cost is still the primary drawback
This technology is the most promising of all “smart” window technologies.
HCB-3 Chap 5B: Window Shading and Improvements

24. HCB-3 Chap 5B: Window Shading and Improvements
Air Curtain Windows
Could be used in two ways:
Exhaust air from room is passed thru the two panes to offset some of the heat gain thru the window (shown in figure)
Ventilation air can be drawn thru the two panes and be preheated a little (cold climates)
However, such windows are not widely used- expensive
Fig. 5.19
HCB-3 Chap 5B: Window Shading and Improvements

25. Example of Dynamic Windows
Utilizing Passive Solar Control Strategies to Mitigate Solar Heat Gain Through Glazing
Example of Dynamic Windows
September 21st without and with passive shading systems at 12:30 pm (solar time):
Fixed shading devices lack flexibility for good control of heat gains
Moveable devices are better but are expensive
Solar Shutters, Biokatalyse Laboratory Building,
Technical University of Graz, Austria
The outer skin of the south façade consists of solar shutters and a cavity of one meter wide. Moving and rotating shutters allows for indoor daylight distribution to be altered.
HCB-3 Chap 5B: Window Shading and Improvements

26. HCB-3 Chap 5B: Window Shading and Improvements
Outcomes
Understand how solar radiation on vertical surfaces varies seasonally and implications towards window heat gains
Be able to solve problems with fixed external overhangs and recessed windows
Understanding the concept of profile angle and how it varies seasonally for a given location
Familiarity with internal shading fixtures such as curtains and the concept of IAC
Familiarity with different types of fixed and movable window shades
Familiarity with high performance glazing and smart windows
HCB-3 Chap 5B: Window Shading and Improvements