Solar Energy

The sun is the source of the vast majority of the energy we use on earth. Most of the energy we
use has undergone various transformations before it is finally utilized, but it is also possible to tap this
source of solar energy as it arrives on the earth’s surface. There are many applications for the direct use
of solar thermal energy, space heating and cooling, water heating, crop drying and solar cooking. It is a
technology, which is well understood and widely used in many countries throughout the world. Most
solar thermal technologies have been in existence in one form or another for centuries and have a wellestablished manufacturing base in most sun-rich developed countries.
The most common use for solar thermal technology is for domestic water heating. Hundreds of
thousands of domestic hot water systems are in use throughout the world, especially in areas such as the
Mediterranean and Australia where there is high solar insulation (the total energy per unit area received
from the sun). As world oil prices vary, it is a technology, which is rapidly gaining acceptance as an
energy saving measure in both domestic and commercial water heating applications. Presently, domestic
water heaters are usually only found amongst wealthier sections of the community in developing
countries. Other technologies exist which take advantage of the free energy provided by the sun. Water
heating technologies are usually referred to as active solar technologies, whereas other technologies,
such as space heating or cooling, which passively absorb the energy of the sun and have no moving
components, are referred to as passive solar technologies. More sophisticated solar technologies exist
for providing power for electricity generation. We will look at these briefly later in this fact sheet.

Sun is the source of many forms of energy available to us. Do you know how energy is obtained
from the sun? The most abundant element in sun is hydrogen. It is in a plasma state. This hydrogen at
high temperature, high pressure and high density undergoes nuclear fusion and hence releases an enormous 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 1.39e9 m and 1.5e11 m away
from earth. Sun has an effective black body temperature of 5762 K and has a temperature of 8e6 K to
40e6 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 perpendicular
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.

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. Accordingly
to intercept maximum amount of solar energy over the whole year, a solar collector in the northern
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.Solar radiation arriving from the sun reaches the earth’s surface as short wave radiation.                                                                                                                                                                     All of the energy arriving from the sun is eventually re-radiated into deep space otherwise the temperature of
the earth would be constantly increasing. This heat is radiated away from the earth as long-wave radiation.
The art of extracting the power from the solar energy source is based around the principle of capturing the short wave radiation and preventing it from being reradiated directly to the atmosphere. Glass and other selective surfaces are used to achieve this. Glass has the ability to allow the passage of short wave radiation whilst preventing heat from being radiated in the form of long wave radiation. For storage of this trapped heat, a liquid or solid with a high thermal mass is employed. In a water heating system this will be the fluid that runs through the collector, whereas in a building the walls will act as the thermal mass. Pools or lakes are sometimes used for seasonal storage of heat.