Learner's Notes

          Sun is one of the most important Non-conventional source of energy. It emits both heat
and light radiations. The mechanism of converting light energy into electrical energy is known
as photovoltaic effect.
         Basically, a solar cell is a P-N junction that absorbs light, releases electrons and holes, creating a voltage in the cell. For larger applications an array of cells, connected in series or
parallel depending upon the voltage and current requirements, is used.
         Such an array is called a solar module. Solar cells are used to supply power to, Space
Applications, Rural Telephone Exchanges, Street Lighting, Mountain terrain mission control,
Remotely operated Robots, etc.,

PHOTOVOLTAIC EFFECT
         It is the principle of using light energy, and producing electrical ene- rgy. Solar cells are based on this effect. Photovoltaic effect was reported
initially in 1839 by Henry Becquerel who observed light dependent voltage
between electrodes immersed in an electrolyte.

SOLAR RADIATION
          The radiative energy from the sun derives from a nuclear fusion reaction. The intensity
of solar radiation in free space, at the average dist-
ance of the earth from the sun is defined as the so-lar constant and has a value of 1353W/m2. The solar constant at the earth's surface when the sun
is overhead, is 925 W/m2.
The difference (1353-925 = 428 W/m2) is caused
by the atmospheric attenuation of sunlight, mainly
due to ultravoilet absorption in the ozone, scatter-
ing by airborne dust and aerosols etc.,
A graph of Spectral irradiance Vs Wavelength of
light is as shown. The 'Air mass Zero' curve in the
graph indicates the amount of light with respect to
their wavelength in micrometres, in space. The 'Air mass One' curve in the graph indicates the
amount of light with respect to their wavelength in micrometres, after atmospheric attenuation.

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