Assessing Climate to Improve Solar Design. Energy Efficiency and Renewable Energy Clearinghouse (EREC) Brochure Page: 3 of 8
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homeN prvd alsoitseeryAssessing climate can
help you better define
system requirements
for solar heating and
solar electric systems.Solar Resource Assessment
Calculating the amount of sunlight avail-
able for a solar energy system is complex
and challenging. Atmospheric conditions,
the changing position of the sun, and local
terrain interact to moderate the solar
resources at a given time and place. Solar
engineers rely on historical data about
weather, climate, and air pollution to
design a solar energy system for a specific
site. They must consider the sun's position
throughout the year. They must take care
to use published information about solar
radiation that applies to tilted, south-fac-
ing collectors. And they must understand
not only how seasonal weather patterns
affect sunlight, but also how they affect
solar energy system requirements.
Whether sunlight is direct or diffuse can
significantly affect solar energy system
performance. Solar electric systems
require direct sunlight. Collectors for solar
heating systems, including passive solar
buildings, capture both direct and diffuse
solar radiation. In some areas, solar
energy systems will capture significant
amounts of sunlight reflected from nearby
white surfaces such as snow, light-colored
sand, or salt flats. Solar designers need to
account for such surface reflection effects
where they exist.3
The 23.50 tilt in Earth's axis of rotation is a
more significant factor in determining the
amount of sunlight striking the Earth at a
particular location. Tilting results in
longer days in the northern hemisphere
from the spring (vernal) equinox to the
fall (autumnal) equinox and longer days
in the southern hemisphere during the
other 6 months. Days and nights are both
exactly 12 hours long on the equinoxes,
which occur each year on or around
March 23 and September 22.
Countries like the United States, which lie
in the middle latitudes, receive more solar
energy in the summer not only because
days are longer, but also because the sun
is nearly overhead. The sun's rays are far
more slanted during the shorter days of the
winter months. The result is a big differ-
ence in the amount of direct sunlight avail-
able for a solar energy system. Cities like
Denver (near 400 N latitude) receive nearly
three times more solar energy in June than
they do in December. That's why winter is
so much colder than summer.
The rotation of the Earth is responsible for
hourly variations in sunlight. In the early
morning and late afternoon, the sun is low
in the sky. Its rays travel further through
the atmosphere than at noon when the sun
is at its highest point. On a clear day, the
greatest amount of solar energy reaches a
solar collector around solar noon.z
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Phillips, J. Assessing Climate to Improve Solar Design. Energy Efficiency and Renewable Energy Clearinghouse (EREC) Brochure, book, August 29, 2001; Golden, Colorado. (https://digital.library.unt.edu/ark:/67531/metadc724999/m1/3/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.