Choose from 18 presentations and forums on
passive solar design ( wref2012.org).
The mediocre performance of shading systems
or any other energy-saving feature is no longer
acceptable — preserving the planet depends on
buildings of high performance.
Figure 5. A north-south section through the center of the sky dome helps to explain the logic of drawing
sun rays on a section through a south window.
on March 21. Because the sun makes the same path across the sky on March 21 and Sept. 21, a fixed overhang can’t let the sun enter a window on March 21 and keep it out on Sept. 21; the sun angles are the same.
Placing Glazing Based on
the Sun’s Annual Path
Both the sky dome and sun path diagrams
clearly show that the summer sun spends the
whole morning in the eastern sky, the noon
hours high overhead and the afternoon in the
western sky. See the red shaded area in figure 4
(left image). As a result, solar-responsive design
should minimize east windows, skylights and
west windows. Any necessary east or west windows should be heavily shaded, and south-facing clerestories (e.g., dormers) should be used
instead of skylights to keep out the high summer
sun while harvesting the winter sun. The clerestories should face north in climates that need
no heating in the winter. These tools also clearly
show that the winter sun spends the whole day
in the southern sky, making south windows
the most desirable wherever winter heating is
required (figure 4, right image). Note that the
sun rises due east and sets due west only on the
equinoxes (March 21 and Sept. 21). Half of the
year the sun rises north of east (during summer),
and the other half of the year it rises south of east
(during winter).
To further understand solar geometry, we
can view the sky dome in section. Figure 5 (this
page) shows a north-south section through the
sky dome at 32 degrees north latitude. At every
latitude at noon, the Dec. 21-altitude angle of
the sun is 23. 5 degrees below the equinox angle
(March and Sept. 21) and 23. 5 degrees above
the equinox angle on June 21. Thus every location on the planet experiences a 47-degree ( 2
x 23. 5) change in altitude angle during a year.
Not by coincidence, the tilt of the Earth’s axis
of rotation is 23. 5 degrees from the plane of the
Earth’s orbit around the sun. The view in figure
5 is valuable because it is similar to a north-south
section through a south-facing window.
With a basis in solar geometry, effective
passive solar heating, shading and daylighting
are possible and likely. This basis also helps
in placing active solar systems for maximum
exposure to the southern sun by avoiding
shading from trees, neighboring buildings and
the site building itself. In the June issue, we’ll
continue this series by examining daylighting,
which is more understandable with knowledge
of solar geometry. ST
