We can capture and convert solar radiation into useful forms of energy, such as heat and electricity, using a variety of technologies. The technical feasibility and economical operation of these technologies at a specific location depends on the available solar radiation or solar resource.
Basic principlesEvery location on Earth receives sunlight at least part of the year. The amount of solar radiation that reaches any one "spot" on the Earth's surface varies according to these factors:
The Earth revolves around the Sun in an elliptical orbit and is closer to the Sun during part of the year. When the Sun is nearer the Earth, the Earth's surface receives a little more solar energy. The Earth is nearer the Sun when it's summer in the southern hemisphere and winter in the northern hemisphere. However the presence of vast oceans moderates the hotter summers and colder winters one would expect to see in the southern hemisphere as a result of this difference.
The 23.5° tilt in the 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 six 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. Cities like Denver, Colorado, (near 40° latitude) receive nearly three times more solar energy in June than they do in December.
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.
Diffuse and direct solar radiationAs sunlight passes through the atmosphere, some of it is absorbed, scattered, and reflected by the following:
MeasurementScientists measure the amount of sunlight falling on specific locations at different times of the year. They then estimate the amount of sunlight falling on regions at the same latitude with similar climates. Measurements of solar energy are typically expressed as total radiation on a horizontal surface, or as total radiation on a surface tracking the sun.
Radiation data for solar electric (photovoltaic) systems are often represented as kilowatt-hours per square meter (kWh/m2). Direct estimates of solar energy may also be expressed as watts per square meter (W/m2).
Radiation data for solar water heating and space heating systems are usually represented in British thermal units per square foot (BTU/ft2).
More informationEvaluating your site's potential for solar water heating
Related category• SOLAR ENERGY AND POWER
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