Solar Radiation Maps
Global Horizontal Irradiation (GHI)
The maps of Global horizontal irradiation for various countries and regions are available on this page to download. SolarGIS database is the source of solar data represented on the maps. Global horizontal irradiation is the most important parameter for evaluation of solar energy potential of a particular region and the most basic value for PV simulations.
The word “Solar Radiation” is the electromagnetic radiation or energy emitted by the sun. Sunlight is a portion of the electromagnetic radiation given off by the Sun, in particular infrared, visible, and ultraviolet light. On Earth, sunlight is filtered through the Earth's atmosphere, and is obvious as daylight when the Sun is above the horizon. One of the main cause of climate change is due to solar radiation. Depending upon the nature of the surface, the radiation will be absorbed, reflected or transmitted through the object. This take place when solar radiation strikes any object. When the direct solar radiation is not blocked by clouds, it is experienced as sunshine, a combination of bright light and radiant heat. When it is blocked by the clouds or reflects off other objects, it is experienced as diffused light.
The extraterrestrial radiation is the radiation which is incident outside the earth’s surface.
The extraterrestrial radiation is 1367 watts/m2.
Due to the change in distance between earth and sun, there is a seasonal variation in the extraterrestrial rate.
Terrestrial Solar Radiation:
It is the electromagnetic radiation which originates from earth and its atmosphere.
Terrestrial Radiation is a longer wavelength which is totally infrared.
When the terrestrial solar radiation reaches the earth’s surface,it is broken into two components i.e.,diffuse radiation and beam radiation.
Beam Radiation is the solar radiation which moves through the atmosphere in a straight line without being scattered, reflected or absorbed by particles in the air.
Diffuse Radiation is the solar radiation which is being scattered, reflected or absorbed by the particles while passing through the atmosphere but ultimately reaches the earth’s surface.
This diagram illustrates how the incoming solar radiation is divided up and recycled through the Earth System.
The Nature of Electromagnetic Radiation
Unlike convection or conduction, heat transfer by electromagnetic radiation requires no intervening medium to transmit it. Electromagnetic radiation travels through space in the form of waves. It's hard to imagine radiation moving as waves through empty space without a medium to transfer the wave form. The waves created when you drop a rock into a pool require molecules of water to propagate them, but not so for radiation.
Energy as electromagnetic waves
The quantity of energy carried in a wave is associated with the height or amplitude of the wave. Everything else being equal, the amount of energy carried in a wave is directly proportional to the amplitude of the wave. The type or "quality" of radiation depends on the wavelength, the distance between successive crests. The greater the distance between wave crests, the longer the wavelength.
Figure 4.3 The Solar radiation spectrum
The maximum wavelength at which a body emits radiation depends on its temperature. Wein's (pronounced "weens") Law states that the peak wavelength of radiation emission is inversely related to the temperature of the emitting body. That is, the hotter the body, the shorter the wavelength of peak emission. Figure 4.4 shows the wavelengths over which the Sun and Earth emit most of their radiation. The Sun being a much hotter body emits most of its radiation in the shortwave end and the Earth in the longwave end of the spectrum. The division between shortwave and longwave radiation occurs at about 3 micrometers.
Figure 4.4 Comparison of solar and earth radiation spectra
Radiation as particles
It's hard to imagine radiation moving as waves through empty space without a medium to transfer the wave form. For instance, the waves created when you drop a rock into a pool of water require molecules of H2O to propagate them. Though we describe electromagnetic radiation as invisible waves of energy, at the smallest scale it behaves as a particle, like when light is emitted by a single atom or molecule. When energy is given off there is a change in the orbital pattern of the electrons that surround the nucleus of an atom. As the orbit changes, a bundle of energy called a "photon" is released. However, particles of light differ from particles of matter: they have no mass, occupy no space, and travel at the speed of light, 2.9998 X 108 m s-1. The amount of energy carried by a photon varies inversely with wavelength, the shorter the wavelength, the more energetic the photon.
Figure 4.2 Wave properties
Any body that has a temperature is emitting electromagnetic radiation. There are an infinite number of wavelengths that make up the electromagnetic spectrum though we group them into a number of bands (Figure 4.3). The shortest wavelengths fall into the gamma rays, the electromagnetic radiation we can see with our eyes and processed by our brains falls into the visible band, and radio waves are comprised of the longest wavelengths.
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