Environmental Engineering Reference
In-Depth Information
5
Solar
Every hour, the sun radiates more energy onto the earth than the entire human population uses in
an entire year. The sun shines nearly everywhere in the United States nearly every day, with the
exception of Alaska during winter. Solar energy is available everywhere the sun shines, and it
can be used to generate both heat and electricity. Every square meter of the earth's surface, when
exposed to direct sunlight, receives about 1,000 watts (one kilowatt) of energy from the sun's
light. Depending on the angle of sunlight, which changes with the time of day and geographical
location, the power of the sun's light will be somewhat more or less than one kilowatt-hour.
On average every square meter exposed to direct sunlight receives about one kilowatt per hour
of solar energy. But because during the early and late hours of the day the angle of the sun's light
is low, sunlight will provide useful solar energy for only about six or seven hours per day. Thus, a
square meter of land in direct sunlight receives about six kilowatt-hours of solar energy per day. Of
course, the amount of sunlight that arrives at the earth's surface is variable, depending on location,
time of day, time of year, and weather conditions. Consequently, a square meter of land in direct
sunlight in the continental United States receives from 3.5 to 6.8 kilowatt-hours of solar energy per
day, depending on location. Areas in the southwestern United States receive the most solar radia-
tion, and areas in the northwest and northeast receive the least insolation, as illustrated in
Figure
5.1,
which shows the approximate daily insolation in kilowatt-hours per square meter falling on a
south-facing surface tilted at an angle equal to local latitude, for an average day in the year.
Figure 5.1
Photovoltaic Solar Resource of the United States
4.5
3.5
5
4
6
6.8
5
5.5
Source:
Zones interpolated from USEIA 2008.
Notes:
Numbers apply to areas between the isobars. Average annual kWh/m
3
/day.
104
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