Solar Radiation
Introduction
The sun radiates energy uniformly in all direction in the form of electromagnetic waves.
Solar energy can be utilized directly in two ways. they are 1) by collecting the radiant heat and using it in directly into electrical energy using photovoltaic system. the former is referred to as "solar thermal and the latter as " solar photovoltaic " system.
Since the sun is expected to radial at an essentially constant rate, it may as an in - exhaustable source of useful energy.
solar energy is also used by various well natural effect, and appears in nature in some other form of energy .
These are indirect forms of solar energy. thus solar energy is the mother of all forms of energy. i.e. conventional or non conventional, renewable or non renewable , the only exception being nuclear energy.
The wavelength of electromagnetic wave radiated by sun is un range of 0.2 to 4.0.
The solar energy reaching the top of the earth atmosphere consist of about 8% ultraviolet radiation, 46% visible light and 46% infrared radiation.
Solar radiation at the earth's surface
The sun is the largest member of the solar system with a diameter of 1.39*10^9m, and an average distance of 1.495*10^11m from the earth is entirely different due to various reasons, discussed below.
Beam and diffuse solar radiation
The solar radiation that penetrates the earth's atmosphere and reaches the surface differs in both amount and character. Part of the radiation is reflected back into the space.
Part of the radiation is scattered by the atmospheric molecules.
Solar radiation that reaches the ground directly the sun without absorption or scattering is called radiation.
The total solar radiation received at any point on the earth's surface is the sum of the direct and diffuse radiation.
This is referred to as insolation at that point. Insolation defined as the total solar radiation energy received on a horizontal surface of unit area on a ground in unit time.
The insolation at a given location on the earth's surface depends on the altitude of the sun in the sky.
Altitude is the angle between the sun's direction and the horizontal.
Since the sun's altitude changes with the date and time of the day, the rate of arrival of solar radiation on the ground is varied. The position of the sun directly over head is called zenith.
Attenuation of beam radiation
The variation in solar radiation reaching the earth and received at the outside of the atmosphere is due to absorption and scattering in atmosphere.
1) Absorption
As solar radiation passes through the earth's atmosphere, the short wave ultraviolet rays are absorbed by the ozone in the atmosphere and long wave infrared rays are absorbed by the ozone in the atmosphere and long wave infrared rays are absorbed by the carbon dioxide and moisture in the atmosphere. By this effect the solar radiation is in narrow range.
2) Scattering
As solar radiation passes through the earth's atmosphere, the components of atmosphere such as water vapour and dust, scatter the portion of radiation.
A portion of this scattered radiation always reaches the earth's surface as diffuse radiation. Thus the radiation finally received at the earth's surface as diffuse radiation.
Thus the radiation always reaches the earth's surface as diffuse radiation. Thus the radiation finally received at the earth's surface consist partly of beam radiation and partly of diffuse radiation. The scattering must be attending the radiation.
Solar radiation measurements
Measurements of solar radiation are important because of the increasing number of solar heating and cooling applications.
The solar radiation instruments will measure the heating effect of direct solar radiation and diffuse solar radiation.
Beam radiation type instruments will measure the solar radiation received from a very small portion of the circumference solar sky.
A total radiation type instruments may be used for measuring the diffuse radiation alone by shading the sensing element from the sun's directly rays.
There basic types of instruments are employed for solar radiation measurements.
a) Pyrheliometer
A pyrheliometer collimates the radiation to determine the beam intensity as a function of incident angle.
pyrheliometer is an instrument used for measuring beam radiation. In pyrheliometer a sensing element is located at the base of the tube, whose axis is aligned with the direction of the sun rays.
The pyrheliometer contains a long collimator tube to collect beam radiation whose field of view is limited to a solid angle using appropriate diaphragms inside the tube.
The inside of the tube is blackened to absorb radiation incident at angle outside the collection solid angle.
b) Pyranometer
A pyranometer is an instrument which measures total or global radiation. A precision pyranometer is designed to respond to the radiation of all wavelength and hence measures accurately the total power in the incident spectrum.
In most pyranometers, the sun's radiation is allowed to fall on a black surface to which the hot junction of a thermopile are attached.
The cold junctions of the thermopile are located in such way they do not receive the radiation. The supporting stand keeps the black surface in proper position.
The temperature difference between the hot and cold junction is the function of the radiation falling on the sensitive surface.
The sensing element is covered by two concentric hemispherical glass domes to shield it from wind and rain. This also reduces the convection currents.
A radiation shield surrounding the outer dome and coplanar with the sensing element, prevents element direct solar radiation from heating the base of the instrument.
c) Sunshine recorder
This instrument measures the duration of the bright sunshine in hours during the course of the day.
It consist of a glass sphere mounted on a selection of metal bowl, having grooves for holding a recorder card strip and glass sphere.
The bowl and glass sphere is arranged in such a way that sun's rays are focused sharply in a spot on a card held in a groove in the bowl.
The card is prepared from a speed paper bearing a time scale. whenever there is a bright sunshine, the image formed is intense enough to burn a spot on the card strip.
Through the day, the sun moves across the sky the image moves along strip. Thus a burnt sspace whose length is proportional to the duration of sunshine is obtained on the strip.
Three overlapping pairs of grooves are provided in the spherical segment to take care of the different seasons of the year.
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