Solar Radiation

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Solar radiation principles

Solar radiation is the radiation (a form of energy) emitted from the sun. It travels in waves in the visible and near visible (ultraviolet and near infrared) spectrum. This spectrum is important for daylighting as well as for solar energy applications.

Overcast sky – diffuse radiation
clear sky – direct radiation

Direct solar radiation is coming directly from the sun, is directional and can cast shadows. Outside the earth's atmosphere, solar radiation has an intensity of approximately 1370 watts/meter². This is called solar constant. The surface of the earth does not receive the same amount of energy, under normal conditions around 35% of the direct solar radiation is reflected back into space, absorbed or scattered in the atmosphere. This percentage however can vary significantly due to cloud cover and other meteorological conditions as well as latitude and elevation of the location, season, pollution of the atmosphere and time of the day. The fraction of the radiation from the sun which is transmitted through an overcast sky is called diffuse radiation. In contrast to direct solar radiation it has no defined direction.

Generally solar radiation covers a wide range of wavelengths, such as ultraviolet radiation (or light), visible radiation, and infrared radiation. Irradiance is a measure (W/m²) for the incident solar radiation on a surface. Daylight design as well as the dimensioning of solar energy applications is usually based on irradiance measured at a certain location. The values can be obtained from related solar radiation maps.

Section Key Resources
  • Sörensen, B. (2004): Renewable energy, its physics, engineering, use, environmental impacts, economy and planning aspects, Elsevier Science
  • Quaschning, V. (2005): Understanding renewable energy systems, Carl Hanser Verlag GmbH & Co KG

Lead Author(s): Aris Tsangrassoulis

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