NON-CONVENTIONAL ENERGY RESOURCES AND UTILISATION 67mous amount of energy. This energy is emitted as radiations of different forms in the electromagnetic
spectrum.
Out of these X-rays, gamma rays and most of ultraviolet rays do not pass through the earth's
atmosphere. But heat energy and light energy are the main radiations that reach the earth. This energy
is the basis for the existence of life on earth.
Sun is a sphere of intensely hot gaseous matter with a diameter of 1.39e^9 m and 1.5e^11 m away
from earth. Sun has an effective black body temperature of 5762 K and has a temperature of 8e^6 K to
40e^6 K. The sun is a continuous fusion reactor in which hydrogen (4 protons) combines to form helium
(one He nucleus). The mass of the He nucleus is less than that of the four protons, mass having been
lost in the reaction and converted to energy. The energy received from the sun on a unit area perpen-
dicular to the direction of propagation of radiation outside atmosphere is called solar constant, and has
a value 1353 Wm– 2. This radiation when received on the earth has a typical value of 1100 Wm– 2 and
is variable. The wavelength range is 0.29 to 2.5 micro meters. This energy is typically converted into
usual energy form through natural and man-made processes. Natural processes include wind and biomass.
Man-made processes include conversion into heat and electricity.
2.16.1 Solar Radiations
Radiation from sun on entering the earth’s atmosphere gets scattered by the atmospheric gas
molecules and dust particles and received on earth from all directions and is called diffuse radiation.
The portion of radiation received on earth from sun without change in original quality is called beam or
direct radiation.
The earth revolves about the sun in an approximately circular path, with the sun located slightly
off center of the circle. The earth’s axis of rotation is tilted 23.5 degrees with respect to its pane of
revolution about the sun, the position of the earth relative to the sun’s rays at the time of winter solstice
when the North Pole is inclined 23.5 degree away from the sun. All points on the earth’s surface north
of 66.5 N latitude are in total darkness while all regions within 23.5 degree of the South Pole receive
continuous sunlight. At the time of the summer solstice, the situation is reversed. At the time of the two
equinoxes, both poles are equidistant from the sun and all points on the earth's surface have 12 hours of
daylight and 12 hours of darkness. The sun’s ray passing through the center of the earth lies in the
equatorial plane at the time of equinoxes. From vernal equinox to autumnal equinox, the rays lie north
of the equatorial plane. From autumnal equinox to vernal equinox, the rays lie south of the equatorial
plane. The average direction of the sun’s rays for the entire year lies in the equatorial plane. Accord-
ingly to intercept maximum amount of solar energy over the whole year, a solar collector in the north-
ern hemisphere should be tilted and face due south.
The Nature and Availability of Solar Radiation. Solar radiation arrives on the surface of the
earth at a maximum power density of approximately 1 kilowatt per metre squared (kWm– 2). The actual
usable radiation component varies depending on geographical location, cloud cover, hours of sunlight
each day, etc. In reality, the solar flux density (same as power density) varies between 250 and 2500
kilowatt hours per metre squared per year (kWhm– 2 per year). As might be expected the total solar
radiation is highest at the equator, especially in sunny, desert areas. Solar radiation arrives at the earth's
outer atmosphere in the form of a direct beam. This light is then partially scattered by cloud, smog, dust
or other atmospheric phenomenon. We therefore receive solar radiation either as direct radiation or
scattered or diffuse radiation, the ratio depending on the atmospheric conditions. Both direct and diffuse
components of radiation are useful, the only distinction between the two being that diffuse radiation
cannot be concentrated for use.