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ARIZONA SOLAR CENTER, www.azsolarcenter.com/technology
Introduction to Solar Energy: The sun's energy arrives on earth in the primary form of heat and light. Other aspects of solar radiation are less easily perceived and their detection often requires sophisticated equipment. All solar radiation travels through space in waves, and it is the length of these waves (the shortest is less than a millionth of an inch, the longest more than a thousand yards) by which all solar radiation is classified. The aggregate of all radiation aspects of the sun is called the solar spectrum.
There are two important facets about the solar spectrum.
1. While the sun emits radiation in all wavelengths, it is the short wavelength radiation that accounts for the majority of energy in the solar spectrum. For example, the portion of the spectrum perceived as the visible light is a relatively small segment compared to the variety of spectrum wavelengths, yet accounts for 46 percent of the energy radiating from the sun. Another 49 percent, that which is perceived as heat, is derived from the infrared band of the spectrum.
2. The proportion of different wavelengths in the solar spectrum does not change and therefore the energy output of the sun remains constant. A measurement of this phenomena is known as the Solar Constant, defined as the amount of heat energy delivered by solar radiation to a square foot of material set perpendicular to the sun’s rays for one hour at the outer edge of the earth’s atmosphere. The Solar Constant measurement is about 429.2 BTU’s with minimal changes over the year. The energy measured as the Solar Constant is not a measure of the amount of solar energy that actually reaches the earth’s surface, since as much as 35 percent of all the solar radiation intercepted by the earth and its surrounding atmosphere is reflected back into space. Additionally, water vapor and atmospheric gases absorb another 15 percent. As a global average only about 35-40 percent of the solar radiation entering the atmosphere actually reaches the earth’s surface.
As a practical matter, global averages are of little interest. The essential point is that the atmosphere impacts on the amount of solar energy that actually reaches the earth’s surface - the more atmosphere solar radiation has to move through, the more is lost on the way...
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