The sun generates energy through nuclear fusion in its core. This energy radiates outward through different layers of the sun and is finally emitted from the photosphere as electromagnetic radiation. About half of the solar energy that reaches the top of Earth's atmosphere reaches the surface, with the remaining energy being reflected or absorbed by gases, aerosols and clouds in the atmosphere. Instruments such as pyranometers and pyrheliometers are used to measure different components of solar radiation, including global, diffuse and direct radiation.

4.  The sun’s energy is created in the core by fusing hydrogen into helium. This
energy is irradiated through the radiative layer, then transmitted by convection
through the convective layer, and finally radiated through the photosphere
which is the part of the sun that we see.

5. The sun generates energy in its core in a process called nuclear fusion. During
nuclear fusion, the sun's extremely high pressure and hot temperature cause
hydrogen atoms to come apart and their nuclei (the central cores of the atoms)
to fuse or combine. Four hydrogen nuclei fuse to become one helium atom.
https://www.youtube.com/watch?v=-qzBpn2q8WQ

7. The sun radiates energy uniformly in all directions in the form
of Electromagnetic waves.
The earth’s outer atmosphere intercepts about one two-billionth
of the energy generated by the sun or about 1.5 quintillion
(1.5× 𝟏𝟎𝟏𝟖
) kilowatt-hours per year.
Due to Reflection, Scattering, absorption of gases and aerosols
in the atmosphere, however only 51% or approximately 0.76
quintillion (0.76× 𝟏𝟎𝟏𝟖
) kilowatt-hours per year reaches the
surface of the earth.

9. Characteristics
 Travels through space (vacuum) at a
speed of light.
 In the form of waves: Electromagnetic
waves.
 In stream of particles (Photons)
 Releases heat when absorbed
Electromagnetic spectrum
From short wavelength, high energy
(gamma rays) to long wavelength, low
energy (radio waves).
Solar spectrum

10.  The Sun’s energy distribution spectrum (yellow) roughly aligns with a
blackbody spectrum at a temperature of 5250 C (black line).
 However when travelling through the atmosphere results in absorptive losses
and less irradiance reaches the surface (red).

11.  The atmosphere absorbs extraterrestrial radiation at certain wavelengths,
resulting in an altered spectral distribution for terrestrial radiation.
Terrestrial Solar Spectrum

12. The radiative energy from the sun actually reaching the earth can be calculated:
Area of the Sun = 4𝜋𝑅2
= 4*𝜋*(7 × 108)2
= 6.16 × 1018 𝐦𝟐
Total power, P = 𝜎𝐴𝑆𝑇4
= 5.67 × 10−8*6.16 × 1018*57804
= 3.88 × 1026 𝐰att

13. We use the distance from the sun to obtain the
flux at the earth
The earth is Ri ≈1.5 × 1011 m from the sun
The area irradiated is
Ri
Ai
 The flux in space above the earth is called the insolation.
 The insolation is the power coming from the sun divided by the total area at the
radius of the earth.
Solar constant =
Power
Area of the sphere
=
3 .88 ×1026 watt
2.83×1023 𝑚2
≈1370 𝒘𝒂𝒕𝒕
𝒎𝟐
𝐴𝑖= 4𝜋𝑅2
𝐴𝑖= 2.83 × 1023
𝒎𝟐
Solar constant

15.  Solar radiation incident on the outer atmosphere of the earth is the known as
Extra-terrestrial Radiation.
 Solar radiation that reaches earth surface after passing through the earth’s
atmosphere is known as Terrestrial Radiation.
Solar Constant:
Entry point into atmosphere;
Intensity≈1370 W/𝒎𝟐

16.  There is a very small variation in the extraterrestrial solar radiation with
different periodicities and variation related to sunspot activities. For
practical/engineering applications and due to variability of atmospheric
transmission, the energy emitted by the sun can be considered as fixed.
 However due to variation in the earth-sun distance there is a variation of 3
percent in the extraterrestrial radiation flux and the same is shown in figure with
time of year and can also be calculated from the following equation.














365
360
cos
033
.
0
1
n
I
I sc
on
where Ion is the extraterrestrial radiation measured on the plane normal to the
radiation on the nth day of the year and Isc is the solar constant.
Variation of Distribution of Extraterrestrial Radiation

17. Variation of extraterrestrial solar radiation with time of year
1,300
1,350
1,400
1,450
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Irradiance
(W/m
2
)
Month

18. −𝛿
 The declination of the sun is the angle between the equator and a line drawn
from the centre of the Earth to the centre of the sun.
 If the Earth were not tilted on its axis of rotation, the declination would
always be 0° at the spring and autumn equinoxes. However, the Earth is tilted
by 23.45° and the declination angle varies plus or minus this amount.
(Winter solstice)

19. Annual variation of the declination angle
𝛿 = 23.45 sin
360
365
(284 + 𝑛)
d = Declination angle
n = 𝑛𝑡ℎ
day of the year (n=1 for
1st January)

20.  The earth rotates around its axis.
 The movement around the sun is elliptical.
 From this figure it can be seen that solar declinations vary from +23.5° on June 21 to
0° at the equinoxes( March 21 & Sept. 23) to -23.5° on Dec. 21.
-23.5° ≤ 𝜹 ≤ +23.5°

21.  Light from the Sun is spread out over a larger area, so that area isn't heated as
much.
 The Sun’s rays strike the Earth’s surface more directly, this causes
the solar radiation to be concentrated over a smaller area, causing more
intense heat.

22. Winter Solstice
 The Sun shines down most directly on the Tropic of Capricorn in the Southern
Hemisphere on the occasion of winter solstice.
 At the winter solstice, December 21 or 22,When it is winter solstice in the
Northern Hemisphere, it is summer solstice in the Southern Hemisphere.

23. Summer Solstice
 On the occasion of summer solstice, the Sun shines down most directly on the
Tropic of Cancer in the Northern Hemisphere, making an angle (𝛿 = +23.5°)
with the equatorial plane.
 At the summer solstice, June 21 or 22,When it is summer solstice in the Northern
Hemisphere, it is winter solstice in the Southern Hemisphere.
 We take delta as +ve whenever the Sun’s rays reach O by passing through the
Northern Hemisphere.

24. Latitude is distance north or south of the equator. The Equator is the line of 0°
latitude, the starting point for measuring latitude. The latitude of the North Pole is
90° N, and that of the South Pole is 90° S. The latitude of every point in between
must be some degree north or south, from 0° to 90°.

25. Longitude is distance east or west of the prime meridian. Longitude is measured in
degrees east or degree west of the prime meridian. This means one half of the world
is measured in degrees of east longitude up to 180°, and the other half in degrees of
west longitude up to 180°.

26. z

s
s


North
South
West
East
Zenith
𝜃𝑍 = Zenith angle
s = Solar altitude angle
= Surface azimuth angle
s = Solar azimuth angle
 = Collector slope
= Angle of incidence
Solar angles

27. Earth
 Sun light travels through the atmosphere, the light from the sun interacts with
the molecules in the air and are scattered or absorbed, causing the radiation
energy to weaken.
 The shortest distance travelled by the sun light in the atmosphere is when the
sun is at the Zenith and is longest when the sun is rising or setting.
Air mass ‘m’ is defined as :
AC
AB
atmosphere
the
of
depth
vertical
travelled
length
path
actual
m 
 = cosec  = Sec 𝜽Z

28. Solar Time
Time based on the apparent angular motion of the sun across the sky with solar
noon denoting the time, the sun crosses the meridian of the observer.
(Deviation in min.)
Where,
𝐋𝐬𝐭𝐝 = Standard meridian for the local time zone
(Standard meridian for India: 82.5°E Passing
through Allahabad)
𝐋𝐥𝐨𝐜= Longitude of the location
E = Equation of time, min.

29.  The angular displacement of the sun east or west of the local meridian due to the rotation of
the earth
 Denoted by (𝜔)
 15 per hour – noon is zero, so morning negative, afternoon positive
 Depends on Apparent Solar Time(ST)
Hour angle

30. INSTRUMENTS USED
 GLOBAL SOLAR RADIATION:
Direct + Diffuse radiation on horizontal surface- PYRANOMETER
 DIFFUSE SOLAR RADIATION:
Short wave radiation from entire hemispherical sky- PYRANOMETER
WITH SHADING DISK/RING
 DIRECT RADIATION
Direct radiation from sun- PYRHELIOMETER
 REFLECTED SOLAR RADIATION
Short wave radiation reflected from ground- PYRANOMETER FACING
DOWNWARDS