SOLAR ENERGY SYSTEMS

• Sun – largest member of solar system
• Sun – is a sphere of extremely hot gaseous matter with a diameter of 1.39 X 109 m and it maintains a
distance of 1.495 X 1011 m from earth.
• Sun’s high energy is maintained by enormous nuclear energy, released by the continuous fusion
reaction.
• The fusion reaction involves four hydrogen atoms combining to form one helium atom (4 x 1H1 
2He4 + 26.7 MeV).
• The sun as other hot bodies radiates heat energy uniformly in all directions. The radiated heat
energy moves out as electromagnetic waves. The radiated heat energy increases the temperature of
a body on its interception and absorption. This radiated heat energy from the sun is called solar
energy and it provides the energy needed to sustain life in our solar system.

RADIATION SPECTRUM FROM SUN AND EARTH
• Sun can be considered for all practical
purposes as a blacCk body having high
surface temperatureof 6000 K.
• The radiation spectrum consists of emission at
various wavelengths but more at shorter
wavelengths as shown in Figure 2.2(a).
• The maximum emissive power of the
radiation takes place at wavelength of 0.48
m.
• Similarly, the earth can also be considered a
black bodyat temperature of 288°C.
• The radiation spectrum from the earth
consists of emission generally of longer
wavelengths as shown in Figure 2.2(b). The
maximum emission is taking place at
wavelength of about 10 m.

RADIATION SPECTRUM FROM SUN AND EARTH (CONT..)
Irradiance
• It is the rate at which radiant energy is incidenting on a unit surface area.
• It is the measureof power density of sunlight falling per unit area and time.
• It is measured in watt per square metre.
• Heat energy is measured in joules and while watt or joules per second is unit of power.
Irradiation
• It is solar energy per unit surface area which is striking a body over a specified time.
• Hence it is integration of solar illumination or irradiance over a specified time (usually an
hour or kilowatt a day).
• It is measured in kilowatt-hour or kilowatt day per square metre.
• For example,if irradiance is 20 kW/m2 for 5 h, irradiation is 20 x 5 = 100 kWh/m2

EXTRATERRESTRIAL RADIATION AND SOLAR CONSTANT
• The earth revolves around the sun in an
elliptical orbit as shown in Figure 2.3.
• The earth is closest to the sun on 21 March
and 23 September.
• The earth is farthest from the sun on 21
June and 22 December.
• The mean distance of the earth from the
sun is 1.495 x 1011 m
• The intensity of solar radiation outside the earth's atmosphere reduces with distance and
itis dependent on the distance between the earth and the sun.
• In fact, the intensity of solar radiation reaching outside the earth's atmospheric varies with
the square of the distance between the centres of the earth and the sun.
• This is the reason why earth receives 7% more radiation on 21 March and 23 September
as compared to 21 June and 22 December.

EXTRATERRESTRIAL RADIATION AND SOLAR
CONSTANT (CONT..)
• The intensity of solar radiation keeps on attenuating as earth
propagates away from the surface of the sun, but the content
of wavelengths in the radiation spectrum does not change.
• The earth axis is tilted about 23.45° with respect to earth's
orbit around the sun as shownin Figure 2.4.
• Owing to this tilting of earth's axis, the northern hemisphere of
the earth points towards the sun in the month of June and it
points away from the sun in the month of December.
• However, earth's axis remains perpendicular to the imaginary
line drawn from the earth to the sun during the months of
September and March.
• The sun-earth's distance varies during earth's rotation around the sun, thereby varying the solar energy reaching
its surface during revolution, which brings about seasonal changes.
• The northern hemisphere has summer when the earth is tilting forwards the sun and winter when the earth is
tilting away from the sun.
• In the months of September and March, both the hemispheres are at the same distance from the sun and
receive equal sunshine. During the summer, the sun is higher in the sky, while the sun is lower in the sky
during winter for the northern hemisphere.

EXTRATERRESTRIAL RADIATION AND SOLAR
CONSTANT (CONT..)
Extraterrestrial radiation
• Solar radiation incident on the outer atmosphere of the earth is called extraterrestrial radiation. The
extraterrestrial radiation varies based on the change in sun-earth's distance arising from earth's elliptical orbit
of rotation. The extraterrestrial radiation is not affected by changes in atmospheric condition.
Solar constant
• It is defined as the energy received from the sun per unit time on a unit surface area perpendicular to the
direction of propagation of solar radiation at the top of earth's atmosphere when earth is at its mean distance
from the sun. The value of solar constant is taken as1367 W/m2.
The extraterrestrial radiation can be determined by using solar constant as follows:

EXTRATERRESTRIAL RADIATION AND SOLAR
CONSTANT (CONT..)
• Terrestrial radiation
• When radiation passes through earth's atmosphere, it is subjected
to the mechanism of atmospheric absorption and scattering
depending on atmospheric conditions.
• Earth's atmosphere contains various
constituents, suspended dust and solid
and liquid particles, such as air
molecules, oxygen, nitrogen, carbon
dioxide, carbon monoxide, ozone, water
vapour and dust.
• Therefore, solar radiation or intensity of radiation is depleted during its passage through the
atmosphere.
• The solar radiation that reaches earth's surface after passing through earth's atmosphere is called
terrestrial radiation.
• The propagation of solar radiation through earth's atmosphere isshown in Figure 2.5.

EXTRATERRESTRIAL RADIATION AND SOLAR
CONSTANT (CONT..)
• From extraterrestrial region, the solar radiation reaches earth's surface in two ways:
i. a part of sun's radiation travels through earth's atmosphere and its reaches directly, which is
called direct or beam radiation, and
ii. the remaining major part of the solar radiation is scattered, reflected back into the space or
absorbed by earth's atmosphere. A part of this radiation may reach earth's surface. This
radiation reaching earth's surface by the mechanism of scattering and reflecting, that is,
reradiation, is called diffuse or sky radiation.
• The diffuse radiation takes place uniformly in all directions and its intensity does not change with the
orientation of the surface.
• However, direct or beam radiation depends on the orientation ofthe surface.
• The beam radiation depends on the angle of incident on the surface and its intensity is maximum
when the solar radiation is falling normal to the surface.
• The solar radiation propagating normal to its direction is specified by In .

EXTRATERRESTRIAL RADIATION AND SOLAR
CONSTANT (CONT..)
Beam radiation
• Solar radiation along the line joining the receiving point and the sun is called beam radiation. This
is radiation has any unique direction.
Diffuse radiation
• It is the solar radiation which is scattered by the particles in earth's atmosphere and this radiation
does not have any unique direction.
Total or global radiation
• Total or global radiation at any location on earth's surface is the sum of beam radiation and diffuse
radiation.
Air mass (m)
• The radiation reaching earth's surface depends on (i) atmospheric conditions and depletionand (ii)
solar attitude. Air mass is the ratio of the path length through the atmosphere which the solar beam
actually traverses up to earth's surface to the vertical path length through the atmosphere (minimum
height of terrestrial atmosphere).

EXTRATERRESTRIAL RADIATION AND SOLAR
CONSTANT (CONT..)
• At sea level, the air mass is unity when the sun is vertically is in the sky (inclinationangle
90°).

LATTITUDE & LONGITUDE
• Any location on the earth can be described by two numbers-latitude and longitude.
Latitude
• On a globe of the earth, lines of latitude are circles of different sizes. The largest one is the circle at equator
(circle at equator with centre at earth's centre) whose latitude is taken as zero.
• The circles at the poles have latitude of 90° north and 90° south (or -90°) where these circles shrink to a point.
• To specify the latitude of some point "P" on the earth's surface, draw the radius OP to the point P from centre O.
• The elevation angle () of the point P
above the equator is called latitude  as
shown in the Figure 2.7(a).
• There are 180 circles, of which 90 in
each hemisphere specify the latitude of
various points on earth's surface as
shown inFigure 2.7(b).
• Each degree of latitude is about 111 km
apart. Latitude lines run horizontally and
these are parallel.

LATTITUDE & LONGITUDE
Longitude
• On the globe, vertical lines of constant longitude (meridians) extend
from pole to pole similarto the segment boundaries on peeled orange.
• Every meridian has to cross the equator and equator is a circle. Like
any circle, it has 360 degrees or divisions.
• Hence, longitude of a point is the marked value of that division where
its meridian meets the equator circle.
• The meridian passing through the Royal Astronomical Observatory at
Greenwich, UK had been chosen as zero longitude.
• The meridian passing through this location is called prime meridian.
• The prime meridian or longitude is considered zero longitude and there are 180 longitude lines or degrees at
cast (+180°) and 180 degrees at west (-180°) of Greenwich.
• The longitude lines meet at poles and these have wide separation at the equator (about 111 km).
• The longitudinal lines are shown in Figure 2.8. Solar noon is the time when the sun is at the longitude of the
place

BASIC SUN-EARTH ANGLES
Latitude or angle of latitude ()
• The latitude of a location on earth's surface is the angle
made by the radial line joining the specified location to
the centre of earth with the projection of this line on the
equatorial plane as shown in Figure 2.9. The latitude at
equator is zero and and it is 90° at poles.
Declination angle ()
• It is the angle made by the line joining the
centres of sun and earth with the equatorial plane
as shown in Figure 2.10
• The angle of declination varies when earth
revolves around the sun.
• It has maximum value of 23.45° when earth
achieves a position in its orbit corresponding to
21 June and it has minimum value of -23.45°
when earth is in orbital position corresponding to
22 December.

BASIC SUN-EARTH ANGLES (CONT..)
• The angle of declination is taken positive when it is measured above the equatorial plane in the northern
hemisphere.
• The angle of declination can be given by:
Hour angle ()
• The hour angle at any instant is the angle through which the
earth has to turn to bring the meridian of the observer directly
in line with sun's rays.
• It is an angular measure of time.
• It is the angle in degree traced by the sun in 1 h with
reference to 12 noon of the location.
• The convention of measuring it is that the noon-based calculated local apparent time (LAT) is positive in
afternoon and negative in forenoon as shown in Figure 2.11.
• The earth completes one rotation (360°) in 24 h.
• Hence, 1 h corresponds to 15° of earth's rotation.
• As at solar noon the sun rays is in line with local meridian or longitude, the hour angle at that instant is zero.
The hour angle can be given as follows:

BASIC SUN-EARTH ANGLES (CONT..)
Inclination or altitude angle ()
• It is the angle between sun's ray and its projection on
horizontal surface as shown in Figure 2.12.
Zenith angle (z)
• It is the angle between sun's ray and normal to the
horizontal plane as shown in Figure 2.12.
Solar azimuth angle (s)
• It is the angle between the projection of sun's ray to the point on the horizontal plane and line
due south passing through that point.
• The value of the azimuth angle is taken positivewhen it is measure form south towards west.

BASIC SUN-EARTH ANGLES (CONT..)
Angle of incidence ()
The angle of incidence for any surface is defined as the angle formed between the direction of the sun ray
and the line normal to the surface as shown in the Figure 2.13.
Tilt or slope angle ()
• The tilt angle is the angle between the inclined slope and the horizontal plane as shown in the Figure
2.13.
Surface azimuth angle ()
• It is the angle in horizontal plane between the line due south and the horizontal projection of normal to
the inclined plane surface. It is taken as positive when measured form southtowards west.