Prof. Vilas Shamrao Patil

1. 2.SOLAR ENERGY
SYLLABUS
Solar energy, Solar energy spectrum (UV, visible and IR),
Thermal route, Photovoltaic route, Essential sub-systems in
solar energy plant, Solar constant, Clarity index, Solar
isolation, Solar energy from satellite station through
microwave to earth station, Solar photovoltaic systems, Merits
and limitations of solar PV systems, Prospects of solar PV
systems, Power of solar cell and solar PV panel.
2.1.INTRODUCTION :
We have seen in the previous chapter that solar energy is ever
available never ending renewable resource and is available free
of cost. It is periodic and is at present costly, because its
conversion into electrical energy is not that efficient. If proper
technology is developed, solar energy can be the most convenient
cheap resource of energy having no danger of depletion of
reserves. We are going to see about solar energy, its chains and
the present state of availing and utilization of the energy.
Solar energy is a real substitute for energy and pollution
free environment. It is everlasting source of energy for human
being in particular and life in general on the earth surface.
Due to depletion of conventional sources like coal, oil, gas
etc. on low scale and at rapid rate, and at the same time due to
increase in demand of energy exponentially with time, solar
energy is the most promising source of energy. Further, solar
energy is the origin of all other sources of energy. It is clean,
cheap, pollution-free, silent, renewable. abundantly available,
present everywhere and remote area friendly source ill energy.
Hence it is definitely the significant source of energy in future.

2. However, solar energy has limitations of variation from place to
place,vanation in time and season, high cost though the sun rays
are freely available, low efficiency of solar cells, etc. Efforts are
needed to overcome 'lime limitations to reap the advantages of
solar energy.
This chapter deals with the various aspects of solar energy,
especially route and photovoltaic route.
2.2 SOLAR ENERGY:
Solar energy is produced in enormous amount by the fusion of
hydrogen atoms into helium atoms at high temperature and is
radiated as heat and light. Solar energy is clean (pollution free),
cheap, constant and abundantly available renewable energy,
hence is very important. The sun radiates solar energy as heat
and light. It consists of infrared rays which cause heating, some
ultraviolet rays and visible light consisting of wavelengths of the
range 3000 A.U. (Violet) to 7500 A. U. (Red). The dominant
component is 6000 A.U. waves (Yellow).
Natural effects of solar energy : Radiation from the sun is
absorbed in the atmosphere, ocean and the earth and brings
about following effects. [Fig. (2.1)].

3. Usable (secondary) energy from the solar radiation is obtained
(i) directly as heat, (ii) on conversion into electrical energy by
photo-voltaic cells etc. and (iii) indirectly from the intermediate
resources of energy caused or produced by the solar energy.
Fig. 2.2
Solar energy received on the earth is periodic (cyclic),
intermittent (e.g. no energy in cloudy sky) and of low intensity
with low power density from 0 to 1 kW/m2
. It also depends upon
the orientation of the sun w. r. t. the earth during a day at a place

4. and also on the longitude, latitude of the place and on the clarity
of the atmosphere.
The Sun, one of the stars in milky way galaxy, is a sphere of
light gases namely hydrogen and helium at very high
temperature of about5800o
K at its surface. The temperature at
its core is still higher. At such a WO temperature, nuclear fusion
occurs in which lighter hydrogen isotopes combine to form
helium along with huge amount of energy released in the form
of heat and light. This heat and light energy reaches to the earth
which is at mean distance of 1.496 x 108
km from the sun.
The diameter of the sun is 1.39 x 106
km and that of the earth
is 1.27 x 106
km. As the sun is at very large distance from the
earth and though the sun is very large, it subtends an angle of 32
minutes (0.53°) at the earth surface. Due to this, the beam of
radiation received from the sun on the earth is almost parallel.
The brightness of the sun varies from its centre to its edge, but
it is assumed to be uniform over the solar disc.
The power of solar radiation received on the earth is
about 1.8 x 1011
MW which is many thousand times larger than
the present consumption rate on the earth. Hence, solar energy
could supply all the present and future needs of energy of the
world. Therefore, solar energy is most promising non-
conventional energy source. Further, solar energy is clean, free
of cost and everlasting source of energy. It is available on all parts
of the world where people live.
However, there are many limitations to the use of solar
energy. Among others, the two major problems with solar

5. energy are (i) it is dilute source of energy and (ii) availability of
this energy widely varies with time and place.
The source of solar energy is very dilute because even in the
hottest regions on the earth the solar radiation flux received on
the earth surface is about 1 kW/m2
and the total radiation over
a day is about 7 kWh/m2
These are very low values for practical
purpose and hence different types of solar collectors are
required which increases the cost.
The variation in solar energy is observed from place to place
and from time to time on the earth surface. The variation in
availability of solar energy on the earth surface is because of day-
night cycle and also seasonal changes occurring due to the
earth's motion around the sun. The climatic changes such as
clouds, storms also hamper the availability of the solar energy.
Besides this, there are other problems with solar energy which
are discussed in other sections in this chapter.
The solar energy is responsible for balance of temperature
on the earth and hence is responsible for sustaining life on the
earth. All other forms of energy are ultimately the
manifestation of soar energy. The
production of oxygen and organic matter in biological plants
by a process of photosynthesis is possible due to solar energy.
The different forms of energies that are resulting from solar
energy are shown in Fig. 2.3.

6. Fig. 2.3: Different energy forms resulting from solar energy
Winds are formed due to unequal heating of land and water
surfaces. Photosynthesis gives biomass along with oxygen and
organic chemicals. Heating of ocean water gives ocean
thermal energy. Winds are iesponsible for ocean waves giving
ocean wave energy. The ocean tidal vnergy is due to the
gravitational forces. Animals and plants buried deep inside the
earth for thousands of years give fossil fuels.
2.3. SOLARENERGYSPECTRUM(UV,VISIBLEANDIR)
It is the variation of solar energy flux with wavelength of
radiation. I lw solar energy flux received from the sun outside
the earth's iumosphere is constant. However, the spectral
distribution of this energy Ilux outside the earth's atmosphere is
called as solar energy spectrum. It Is shown in Fig. 2.4.
Solar spectrum is variation of solar energy flux with
wavelength of Idlotions. These radiations received outside the
earth's atmosphere are expressed in terms of solar constant.
The solar constant is the rate at which energy is received per

7. unit area normal to the rays of the sun at moon distance of the
earth from the sun. The earth rotates around the tun In an
elliptical orbit having very small eccentricity and the sun is at
one of the foci. Due to this, distance between the earth and the
sun varies slightly throughout the year. Hence, the extra
terrestrial energy flux also varies. This variation is about ± 3.3 per
cent over a year.
Fig. 2.4 : Spectral distribution of solar radiation outside the
earth's atmosphere.
The salient features of the solar energy spectrum are as
follows :
1. The solar energy spectrum mainly consists of three parts.
First is the near ultraviolet radiations (region) ranging from
wavelengths 0.3 to 0.39 pm. Second is the visible region, that
human eye can see, which has wavelengths ranging from 0.4 um
(violet) to 0.76 um (longest visible red). The third region is near
infrared (heat) radiation region whose wavelength varies from

8. 0.76 um to 1.0 p.m. These radiations viz ultraviolet, visible and
infrared are called light radiations.
2. Most of the solar energy is available in the visible region with
maximum energy flux at wavelength 0.5 .tm (yellow), where 1
µm = 10-6 m = 1 micron.
3. From Fig. 2.2, it is seen that the solar energy flux first
increases sharply with wavelength and passes through maximum
and then decreases asymptotically to zero.
4. About 95% of solar radiations (energy) are present between
the wavelengths 0.3 urn and 2.0 um and about 50% of radiations
are obtained upto visible region i.e. upto wavelength 0.76 um.
5. The radiation spectrum shown in Fig. 2.2 (curve-I) is almost
same as black body radiation (curve-HI). This can be shown from
Planck's law and Stefan-Boltzmann law. Therefore, radiations
emitted by the sun are essentially black-body radiations.
6. Solar radiations from the sun are scattered back to space by
the earth's atmosphere to some extent. The remaining radiations
enter into the atmosphere where it travels in the direction of
incidence (beam or direct radiations) and reach the earth
surface. Some part of these radiations entering the atmosphere
is absorbed and some part is scattered. These scattered
radiations can reach the earth surface which are called diffused
radiations. Thus the radiations reaching the earth surface are the
sum of direct radiations (beam radiations) and diffused
radiations. These are called total or global radiations received on
the earth surface shown by curve-I1 in Fig. 2.2. This shows that
the solar energy flux at the earth surface (solar insolation - curve
II) is less than the extra-terrestrial radiations (curve-I).

9. 7. The area under the curve-I has dimensions of (W/m2-mm) x
= W/m2. It represents the extraterrestrial solar power density
(also known as solar radiations intensity) and is a constant called
solar constant.
8. The area under the curve-II gives solar power density
(W/m2) received on ground surface. It varies with atmospheric
conditions, angle of the sun in the sky, latitude of the location on
the earth etc.
9. Curve-I1 shows some absorption bands due to absorption of
solar radiations by water vapour (H20), carbon dioxide (CO2) etc.
This absorption is of waves in infrared range (wavelength more
than 0.76 micron).
10. The ultraviolet waves (wavelengths less than 0.39 um) are
absorbed mostly due to ozone in the upper atmosphere of the
earth. Primarily the short wave (X < 0.3 urn) UV radiations are
absorbed in upper atmospheric ozone.
11. Different wavelengths in the sun light spectrum are
summarised in Table 2.1. This shows that the infrared region
is wider than ultraviolet region which is again wider than
visible region.
Table 2.1 Solar Light Spectrum.
Colour/region Wavelength
(tun)
Frequency (Hz)
Ultraviolet region
0.005 to
0.39
6 x 1016
- 7.69 x
1014
Violet 0.40 to 0.45
7.5 x 1014
- 6.6 x
1014
Blue 0.45 to 0.50
6.6 x 1014
- 6.0 x
1014

10. Green 0.50 to 0.57
6 x 1.014
- 5.27 x
1014
Yellow
Visible
region
0.57 to 0.59
5.27x1014
-5.01
x1014
Orange 0.59 to 0.61
5.01x1014
-4.92
x1014
Red 0.61 to 0.76
4.92 x 1014
- 3.9
x1014
Infrared region
0.76 to
4000
3.9 x 1014
- 7.5
x 1010