Solar Shakti Fundas Garg

Prof. (Dr.) H.P. Garg
IREDA Chair Emeritus Professor
Centre for Energy Studies
Indian Institute of Technology, Hauz Khas,
New Delhi-110016, India
Tel. No. 91-11-2659 1249 (office)
91-11-2508 7744 (res.)
Mob. 98180 00984
Fax: 91-11-2659 1249 / 2658 1121
E-mail: garghp@ces.iitd.ernet.in
hpgarg01@rediffmail.com
FUNDAMENTALS OF
SOLAR ENERGY

Energy related issues for India?
• Wider access to electricity
• Significant investment needs
• Choice of current & emerging
technology that address
• Environmental protection:
climate concerns & increasing
CO2 emissions
• Energy security
• Economic growth
• Institutional, financial and
technological barriers
• Affordability issues
I
S
S
U
E
S

The SUN
The SUN
Source of
Source of
all Energy
all Energy
Produces
Produces
Energy
Energy
from H
from H2
2

What is Solar Energy?
• Originates with the
thermonuclear fusion
reactions occurring in the
sun.
• Represents the entire
electromagnetic spectrum
(visible light, infrared,
ultraviolet, x-rays, and
radio waves).
• The sun is, in effect, a
continuous fusion reactor
with its constituent gases
as the ‘containing vessel’
4H
4H1
1
→He
→He4
4
+ 2
+ 2β
β+
+
+ 2
+ 2ν
ν + 25 MeV
+ 25 MeV
E = mc
E = mc2
2
retained by gravitational forces. The fusion reaction in
which hydrogen (i.e. four protons) combines to form
helium (i.e. one helium nucleus) accompanied by a 0.7
percent loss of mass and converted to energy is the
source of energy in the SUN.

Global Solar Energy Balance
(TeraWatts)
100
Captured by Plant Photosynthesis
40,000
Used to Evaporate Water (Weather)
82,000
Absorbed and Then Reflected as Heat
53,000
Reflected to Space Immediately
50-
2000
Solar Energy Potential (Practical)
120,000
Solar Energy Potential (Theoretical)
13
Total Energy Used by Human Society
178,000
Solar Energy Input

Solar Radiation
• We are concerned about the portion of the
electromagnetic radiation emitted from the run in
the wavelength range of 0.25 – 3.0 µm (micron).
• We are also concerned about the solar geometry
i.e. sun and its position in the sky, the direction of
direct (beam radiation) on variously inclined and
oriented surfaces.
• We are also concerned about the extraterrestrial
radiation on a horizontal surface which is the limit
of the solar radiation on the surface of the earth.
• We are also concerned about the earth; its
motion, orientation and tilt with respect to the sun
effecting the availability of solar radiation.
• We are also concerned about the earth’s
atmosphere responsible for the reduction due to
absorption, scattering and reflection of solar
radiation.

The Sun’s Structure
 The sun is, in effect, a continuous fusion
reactor with its constituent gases as the
‘containing vessel’ retained by gravitational
forces. The most accepted fusion reaction is
in which hydrogen (i.e. four protons)
combines to form helium (i.e. one helium
nucleus); the mass of the helium nucleus is
less that of the four protons, mass having
 The sun is a sphere of Intensely hot gaseous
The sun is a sphere of Intensely hot gaseous
matter with a diameter of 1.39
matter with a diameter of 1.39 ×
× 10
109
9
m and is,
m and is,
on an average, 1.5
on an average, 1.5 ×
×10
1011
11
m from the earth.
m from the earth.
 The sun has an effective
The sun has an effective
blackbody temperature of
blackbody temperature of
5777k. The temperature in the
5777k. The temperature in the
central interior region is
central interior region is
variously estimated at 8
variously estimated at 8 ×
×10
106
6
to
to
40
40×
×10
106
6
K and the density is
K and the density is
estimated to be about 100 times
estimated to be about 100 times
that of water.
that of water.

• The Sun is 333,400 times more massive than the Earth
and contains 99.86% of the mass of the entire solar
system
• It consist of 78% Hydrogen, 20% Helium and 2% of other
elements
• It is estimated that 90% of the energy is generated in the
region of 0 to 0.23 R (where R is the radius of the sun),
which contains 40% of the mass of the sun and density is
about 105
kg/m3
.
• At a distance 0.7 R from the centre, the temperature
drops to about 130,000 K and density drops to 70 kg / m3
;
and the zone from 0.7 to 1.0 R is known as convective
zone, where temperature drops to about 6000 K and
density to about 10-5
kg/m3

• The outer layer of the
convective zone is called the
photosphere, whose edge is
sharply defined, opaque, gases
here are strongly ionized and is
the source of most radiation.
• The emitted solar radiation is
the composite result of several
layers that emit and absorb
radiation of various
wavelengths.

• Outside the
photosphere is a
layer of cooler gases
several hundred
kilometers deep
called the reversing
layer and after this
10,000 km deep layer
called
Chromosphere.
• Further there is
Corona with very low
density and of very
high temperature.

The Earth
• The earth is shaped as an oblate spheroid – a sphere flattened at the
poles and bulging in the plane normal to the poles. For most
practical purposes we consider the earth as a sphere with a diameter
of about 12,800 km and a mean density of about 5.517 g/cm3
.
• Earth has a central core of about 2560 km in diameter which is more
rigid than steel. Beyond Central Core is the mantle, which forms
about 70 percent of the earth’s mass, and beyond this is the outer
crust which forms about 1 per cent of the mass.
• The earth describes an ellipse round the sun, with the later at one of
the foci. The apparent path of the sun as seen from the earth is
known as the ecliptic.
• The eccentricity of the earth’s orbit is very small (e=0.01673), so that
the orbit is in fact very nearly circular. The shortest distance is
Rp = a(1-e)=147.10×106
km
and longest distance is
Ra = a (1+e) = 152.10 × 106
km
Where ‘a’ is the semi-measure axis of the earth’s orbit.
• The mean earth – sun distance is the mean of Rp and Ra and its
numerical value is 149.5985 × 106
km.
• On January 1, the earth is closest to the sun and on July 1 the earth is
most remote to the sun.

• The earth makes one rotation about its axis every 24
hrs and completes a revolution around the sun in a
period of 365.25 days approx.
• The earth’s axis of rotation is tilted 23.5 deg. with
respect to its orbit about the sun. In its orbital
movement, the earth keeps its axis oriented in the
same direction.
• This tilted position of the earth, alongwith the earth’s
daily rotation and yearly revolution, accounts for the
distribution of solar radiation over the earth surface,
the changing length of hours of daylight and night
length, and the changing of the seasons.
The Earth
(contd.)

Earth Data
Mean distance from the Sun: 1.496 x 108
km
Maximum distance from the Sun: 1.521 x 108
km
Minimum distance from the Sun: 1.471 x 108
km
Mean orbital velocity: 29.8 km/s
Sidereal period: 365.256 days
Rotation period: 23.9345 hours
Inclination of equator to orbit: 230
26’
Diameter (equatorial): 12,756 km
Mass: 5.976 x 1024
kg
Mean density: 5520 kg/m3
Escape speed: 11.2 km/s
Surface temperature range: Maximum: 60 0
C
Mean: 20 0
C
Minimum: - 90 0
C

A a
A a
Internal Structure of the Solid Earth

The Solar Constant
• The geometry of the sun - earth relationship is
schematically shown in the figure.
• The eccentricity of the earth’s orbit is such that the
distance between the sun and earth (1.495 × 1011
m)
varies by 1.7 per cent.
• The sun substends an angle of 32' at the earth because
of its large size and distance.
• The radiation emitted by the sun reaches unattenuated
upto the outside of the atmosphere and thus is a fixed
intensity.
• The solar constant (Ion) is the energy received from
the sun, per unit time, on a unit area of surface
perpendicular to the direction of radiation, at a mean
earth-sun distance, outside the earth atmosphere.
• The latest value of solar constant is 1366.8 ± 4.2
watts/m2
or 433 Btu/ft2
hr or 4.921 MJ/m2
hr or 1.960
cal/cm2
min.

The Solar Constant (contd.)
• In olden days when rocket or space craft facilities were not
available, solar radiation measurements were made on
ground and at different heights of mountains and
extrapolations and corrections for attenuations produced
by different constituents of the atmosphere for different
portions of the solar spectrum were made and value of solar
constant was determined.
• Pioneering studies were done by C.G. Abbot in Smithsonian
Laboratories who gave a value of 1322 W/m2
which got
revised by F.S. Johnson (1954) to 1395 W/m2
.
• Later with the availability of very high altitude aircraft,
balloons, and space craft, direct measurement of solar
radiation outside the earth atmosphere was made and
reported by several scientists like A.J. Drummond, M.P.
Thekaekara, C.Frohlick etc. and gave a value of 1353 W/m2
with an error of ± 1.5 per cent.
• Later C. Frohlick reexamined the value of 1353 W/m2
in view
of new pyrheliometric scale and with some additional space
craft measurements and with three rocket flights the World
Radiation Centre (WRC) adopted a new value of solar
constant as 1367 W/m2
.

Spectral Distribution of Extraterrestrial Radiation
• In addition to the total energy in the solar spectrum (i.e.
the solar constant), it is useful to know the spectral
distribution of the extraterrestrial solar radiation, that is,
the solar radiation that would be received in the absence
of the atmosphere.
• A standard spectral irradiance curve based on high
altitude and space measurements is shown here which is
found to be similar to the 5777K blackbody spectrum.
• From this figure following observations are made:
– The peak solar intensity is 2028.8 w/m2
at a wavelength
of 0.48 µm.
– The solar spectrum varies from 0.2 – 3.0 µm,
– The energy in various spectral ranges is as follows:
(0.78 – 3.0 µm)
623
46
(0.38 – 0.78 µm)
656
48
0.2 – 0.38µm)
88
6
Wavelength
Energy (W/m2
)
Percent
Infrared
Visible
Ultravoilet

The WRC standard spectral irradiance
curve at mean earth-sun distance

Variation of Distribution of Extraterristrial Radiation
• There is a very small variation in the extraterrestrial
solar radiation with different periodicities and
variation related to sunspot activities. For practical
and 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
extraterristrial 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 entraterristrial radiation
measured on the plane normal to the radiation
on the nth
day of the year and Isc is the solar

Scattering of Light
Scattering of Light
Solar radiation
Solar radiation
passing through
passing through
earth's
earth's
atmosphere is
atmosphere is
scattered by
scattered by
gases, aerosols,
gases, aerosols,
and dust.
and dust.
At the horizon
At the horizon
sunlight passes
sunlight passes
through more
through more
scatterers,
scatterers,
leaving longer
leaving longer
wavelengths
wavelengths
and redder
and redder
colors revealed.
colors revealed.

Depletion of Solar Radiation by the Atmosphere
• The earth is surrounded by an atmosphere containing
various gases, dust and other suspended particles,
water vapour and clouds of various types. The solar
radiation during its passage in the atmosphere gets
partly absorbed, scattered and reflected in different
wavelength bands selectively.
• Radiation gets absorbed in water vapor, Ozone, CO2 ,
O2 in certain wavelengths.
• Radiation gets scattered by molecules of different
gases and small dust particles known as Rayleigh
scattering where the intensity is inversely proportional
to the fourth power of wavelength of light (l α 1/λ4
).
• If the size of the particles are larger than the
wavelength of light then Mie Scattering will takes
place.
• There will be a reflection of radiation due to clouds,
particles of larger size and other material in the
atmosphere.
• Considerable amount of solar radiation also gets
absorbed by clouds which are of several types.

• Some radiation gets reflected back in the
atmosphere due to reflection from the ground, from
the clouds, and scattering. This fraction of radiation
reflected back is called albedo of the ground and on
an average the albedo is 0.3.
• The solar radiation which reaches on the earth
surface unattenuated (after scattering, reflection and
absorption) is called direct radiation or beam
radiation.
• The radiation which gets reflected, absorbed or
scattered is not completely lost in the atmosphere
and comes back on the surface of the earth in the
short wavelength region and called sky or diffuse
solar radiation.
• The sum of the diffuse and direct radiation on the
surface of the earth is called global or total solar
radiation.
Depletion of solar radiation by the atmosphere (contd.)

The distance travelled by the sunbeam in the earth’s atmosphere is responsible
for the amount of scattering, absorption and reflection of solar radiation. The
shortest distance travelled by the sunbeam in the atmosphere is when the sun is
at the Zenith and is longest when the sun is rising or setting. Airmass ‘m’ is
defined as :
AC
AB
atmosphere
the
of
depth
vertical
travelled
length
path
actual
m =
=
= cosec α = Sec φZ
m = 0 when outside the earth atmosphere
m = 1 when sun is at the Zenith
m = 2 when Zenith angle is 60°

• Moon (1940) has proposed standard curves for
calculating transmittence.
• For Indian conditions a standard atmosphere
composed of following conditions is defined as:
Standard atmosphere : p =760 mm
ω =20 mm
d =300 / cm3
ozone = 2.8 mm
For m = 0 to 5 for Indian atmosphere
2
/
)
3135
.
0
(
1
1246
m
w
m
IDN
+
=
This equation in India is used extensively for computing direct
radiation at normal incidence for several stations.
Depletion of solar radiation by the atmosphere

Basic Earth – Sun Angles
• For calculating solar radiation and designing solar
devices, the knowledge of sun’s path in the sky, on various
days in a year at a particular place is a pre-requisite.
• Solar altitude angle (α) and solar azimuth angle (Az) are
the two coordinates locating the sun in the sky.
• The apparent solar path on a particular day is shown in the
figure thereby showing sun’s zenith angle (θz), altitude (α)
and azimuth angle (Az) at a particular position of the sun.
• The altitude angle of the sun (α) is defined as the angle in
a vertical plane between the sun’s rays and the horizontal
projection of the sun rays.
• The azimuth angle (Az) is the angle in the horizontal plane
measured from the south (northern hemisphere) to the
horizontal projection of the sun rays. Displacements east
of south are negative and west of south are positive.
• The zenith angle (θz) is the angle between sun’s rays and
the line perpendicular to the horizontal plane i.e. the angle
of incidence of beam radiation on a horizontal surface (α +
θz = π/2)

Solar zenith, altitude and azimuth angles (northern hemisphere),
θz = zenith angle, α=solar altitude, Az=solar azimuth

• To specify the location of a place on the earth, two angles
the latitude (L) and longitude angle (φ) are r eq uir ed.
• To understand L and φ, please see the figure in which, the
polar axis is shown by NOS, the earth’s centre being at 0.
The great circle ABDA, normal to the polar axis, is known
as equator.
• Latitude (L) of a place (say C in figure) is the angle between
the lines joining the place with the centre of the earth and
the equator with the centre of earth or it is the angular
displacement of the place north or south of the equator,
north positive, -90°≤ L ≤90°.
• The angle between the prime meridian (a semicircle
passing through the poles and observatory at Greenwich,
UK) and the meridian (a similar semicircle passing through
the place, C, and the poles) is called longitude, φ, of that
place. In the figure NGJS represents the prime meridian
and NCBS represents the meridian of the place. The prime
meridian has zero longitude. In the figure the longitude of
the point C is φ°1, east and that of point D is φ°2 west and
written as φ1°E and φ°2W respectively.
Basic Earth – Sun Angles

• From this figure it can be seen that solar
declinations (defined as the angular displacement of
the sun from the plane of the earth’s equator), vary
from +23.5° on June 22 to 0° at the equinoxes
(March 21 and September 24) to -23.5° on December
22.
• The values of sun’s declaration, δ, can be found out
from the table or figure as shown here and given as:
Basic Earth – Sun Angles (contd.)











 +
=
365
284
360
sin
45
.
23
n
δ
• Where n is the day of the year. The exact value of δ
for a particular day can be read from Nautical
Almanak since the declination varies slightly to the
same day from year to year.
-23.45°≤δ≤+23.45°
For a day declination may be assumed constant and
for practical purposes the values as shown
graphically can be conveniently used.

• The position of a point P on the earth’s surface with
respect to the sun’s rays can be determined at any
instant if the latitude of the place L, hour angle w and
the sun’s declination δ are known as shown in the
figure.
• Point P in the figure represents a place in the northern
hemisphere. The hour angle is the angular
displacement of the sun east or west of the local
meridian due to rotation of the earth on its axis at 15°
per hour, morning negative and afternoon positive.
• At solar noon the sun is highest in the sky and at that
time hour angle is zero. The hour angle express the
time of day with respect to solar noon. One hour of
time equals 15° of hour angle.
Basic Earth – Sun Angles (contd.)

SOLAR TIME AND EQUATION OF TIME
• Solar time is the time used in all sun-angle relationship and it
does not coincide with local clock (standard time) time. Two
corrections are required to convert standard time to solar time.
The first correction is due to difference in longitude (L) between
observer’s meridian (longitude, φloc) and the meridian on which
the local standard time is based (φst). The sun takes 4 minutes to
traverse 1 deg. of longitude.
• The second correction is due to equation of time (E in minutes),
which takes into account the perturbations in the earth’s rate of
rotation which affect the time the sun crosses the observer’s
meridian. The difference in minutes between solar time and
standard time is :
Solar time – Standard time = 4 (φst - φloc) + E
Solar noon =
For India φst = longitude of standard meridian = Allahabad = 82.5°
• Equation of time as shown in the figure can be represented as :
E = 9.87 Sin 2B – 7.53 cos B - 1.5 Sin B
where
B = 360 (n-81) / 364
E
st
loc
−





 −
−
15
12
φ
φ

Angle of Incidence on Horizontal and Inclined Planes
• Since, most solar equipments (e.g. flat-plate collectors) for
absorbing radiation are tilted at an angle to the horizontal, it
becomes necessary to calculate the solar flux that falls on a tilted
surface. This flux is the sum of the beam and diffuse radiations
falling directly on the surface and the radiation reflected on the
surface from the surroundings.
• Although the earth's path around the sun is elliptical and the solar
day is not 24 hours, the position of the sun at any instant relative to
a place on the spinning earth can be easily determined in terms of
various angles as described below. Some angles used are:
L = latitude of place north or south of equator (north positive)
δ = declination of sun (north positive)
ω = hour angle from solar noon (morning positive and afternoon
negative)
θz = zenith angle
α= altitude of sun
β = tilt of plane from horizontal
φ = longitude of place
Az= azimuth of sun from south
Azs= azimuth of surface from south, east positive and west negative
θi = angle of incidence of beam or direct radiation on a surface.

It is also seen in the figure that a surface located
at the latitude L, tilted towards the equator at an
angle β from the horizontal surface is parallel to
a horizontal surface at the latitude (L-β) degrees.
Thus Eq.(1) can be written as:
cos θt = cos(L-β) cos δ cos ω + sin (L-β) sin δ (2)
sin α = cos L cos δ cos ω + sin L sin δ (1)
Angle of Incidence on Horizontal and Inclined
Planes (contd.)
From the figure one can easily calculate the altitude
(α) of the sun at any given point of time, place and
day as given below:
Where
Where θ
θt is the angle of incidence on an
t is the angle of incidence on an

• At the time of solar noon, the altitude of the
sun, αn, can be determined by putting ω=0 in eq.
(1):
αn = 90° - (L-δ) (3)
• Sunrise hour angle or sunset hour angle, ωs,
can also be determined from Eq.(1) by putting α
=0.
Cos ωs = - tan L tan δ (4)
• Day length or possible sunshine hours, N, is
given by
Angle of Incidence on Horizontal and
Inclined Planes (contd.)
)
tan
tan
(
15
2
15
2 1
δ
ω
L
Cos
N s
−
=
= −
(5)

For an inclined plane cos ω’s = - tan (L-β) tan δ, where
ω’s is the sunrise or sunset hour angle for an inclined
plane.
As we have derived the expression for sin α, similarly an
expression for cos AZ can also be derived:
cos AZ cos α = sin L cos δ ω - cos L sin δ (6)
and also
sin AZ cos α = cos δ sin ω (7)
and also,
ω
δ
ω
sin
tan
cos
cos
sin
cot
L
L
AZ
−
= (8)
Angle of Incidence on Horizontal and
Inclined Planes (contd.)

The general expression for angle of incidence (θi) of the sun’s rays
on any surface can be derived and is given as:
cos θi = (cos L cos β + sin L sin β cos Azs)
cos δ cos ω + cos δ sin ω sin β sin Azs
+ sin δ (sin L cos β - cos L sin β cos Azs) (9)
Now the intensity It incident on a given plane is given by
It = IN cos θi
or It = IN [(cos L cos β + sin L sin β cos Azs)
cos δ cos ω + cos δ sin ω sin β Azs)]
+ sin δ (Sin L Cos β - Cos L Sin β Cos Azs (10)
The intensities and the angle of incidence on horizontal and vertical
surfaces can be obtained by putting β = 0 (for horizontal) and β = 90
Angle of incidence on horizontal and inclined
Planes (contd.)

Factors Governing availability of solar
energy on the earth
• Earth sun distance
• Tilt of the earth’s axis
• Atmospheric Attenuation
Factors Affecting Solar Energy availability on a
Collector Surface
• Geographic location
• Site location of collector
• Collector orientation and tilt
• Time of day
• Time of year
• Atmospheric conditions
• Type of collector

Radiation Instruments
• Pyranometer
• Pyrheliometer
• Pyrgeometer
• Net Radiometer
• Sunshine Recorder
These instruments are classified either as
first class or second class or third class
depending on their sensitivity, stability and
accuracy.

Solar Radiation Components
• DIRECT RADIATION
Direct transmission of solar radiation to earth
surface
• DIFFUSE SOLAR RADIATION
Scattered by molecules and aerosols on
entering the earth’s atmosphere
• GLOBAL SOLAR RADIATION = DIRECT
RADIATION + DIFFUSE SOLAR RADIATION
 Concentrators use Direct Radiation plus a Small
Portion of Scattered Radiation
 Flat Plate collectors use Direct and Diffuse
Solar Radiation and also reflected Radiation

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 RING
• DIRECT RADIATION
Direct radiation from sun PYRHELIOMETER
• REFLECTED SOLAR RADIATION
Short wave radiation reflected from ground
PYRANOMETER FACING DOWNWARDS
• LONGWAVE RADIATION
(i) Emitted from ground (upward direction)
(ij) Atmospheric radiation (Downward direction)
PYRGEOMETER & NET PYRADIOMETER

DETECTORS FOR RADIATION MEASUREMENT
CALORIMETRIC SENSORS
• The radiant energy is incident on a high conductivity
metal coated with a nonselective black paint of high
absorptance.
THERMOMECHANICAL SENSORS
• The radiant flux is measured through bendings of a
bimetallic strip.
THERMOELECTRIC SENSORS
• Consists of two dissimilar metallic wires with their
ends connected.
PHOTOELECTRIC SENSORS
• Photovoltaic instruments are most numerous in the
field of solar radiation measurement. A photovoltaic
device is made of a semiconducting material such
as silicon.

Radiation Measurement in India
All Instruments should be periodically calibrated
1. Systematic measurement of solar and terrestrial
radiation in India started during IGY 1957-58
2. National Radiation Centre, POONA has absolute cavity
radiometer which is used as primary standard.
3. IMD National Radiation Centre, POONA not only serves
as National Radiation Centre but also as a WMO
Regional Radiation Centre for Asia.
4. IMD National Radiation Centre maintains a set of
reference, working and travelling standard instruments
for ensuring the accuracy of radiation measurements on
a National and Regional level.

MEASUREMENT OF DIFFUSE RADIATION
• Same Instrument as used for the Measurement of Total
or Global Radiation
• A Suitable Device (Disc or Shadow Ring) is used to
prevent Direct Solar Radiation from reaching the receiver
(Pyranorneter).
Factors Affecting the Accuracy are given below:
• Multiple Reflection within the Glass Cover Affects the
Accuracy of the Measurement.
• In Calculating the Correction Factor, it is Assumed that
the Sky is Isotropic.
• A Part of the Circumsolar Radiation is also prevented
from reaching the receiver by the Shading Device.
• The Dimensions of the Receivers are not Adequately
Standardized.

PARAMETERS OF PYRANOMETERS
Important parameters associated with a
pyranometer includes the following:
• SENSITIVITY
– Sensitivity, R is Ratio of Output Signal, ‘S’, to the
received irradiance, I.
R = S/I, UNIT : mV / W/m2
• TEMP. COEFFICIENT OF SENSITIVITY
100
/
×
∆
∆
=
T
R
R
C
θ
cos
l
100
Cosine
η
×
=
r
Pyranomete
of
reading
Actual
Error
UNIT : °C-1
• COSINE ERROR

• AZIMUTHAL ERROR
Variation in output of the pyranometer as
Azimuthal Angle alone is changed.
• LINEARITY
Output of the Pyranometer should be
Proportional to the intensity of the
Irradiance but it is not so in the true sense.
PARAMETERS OF PYRANOMETERS
(contd.)

PARAMETERS OF PYRANOMETERS (contd.)
• TILT ERROR
Calibration Factor Changes if the tilt of the Instrument is
changed from 0° to any other value.
Eppley PSP model shows no tilt error.
• SPIRIT LEVEL
If the detector is not horiozntal, it will record the radiation
higher or lower than the actual value. Horizontality is assured
by spirit level.
• TIME CONSTANT
Reponse of pyranometer to a step function.
• STABILITY
Variations of calibration factor with time. Coating peels off,
with time.
• SPECIAL RESPONSE
Response should be uniform over 0.3 to 3.0 µm range.
• RELATED SITUATIONS
MOISTURE Silica Gel
DEPOSITION Frost, Dew, Bird
NEGATIVE VALUES Detector irradiates at night
READING EXCEEDS(Ion) Deflection from cloud or building

Absent
Absent
High
High
High
V. Good
Good
Poor
Bad
Bad
Low
Good
High
High
high
All
5
2
1.2
0.4 – 0.75
Calorimetric
Thermoelectric
Photoelectric
Photographic
Visual
Selectivity
Linearity
Sensitivity
Wave length
(µm)
Effect used
General characteristics of sensors for radiant
energy measurements

Classification of pyrheliometers
• STANDARD PYRHELIOMETERS
Absolute cavity radiometer
Angstrom electrical compensation pyrheliometer
Abbot silver – disk pyrheliometer
• FIRST – CLASS PYRHELIOMETER
Michelson bimetallic pyrheliometer
Linke – Feussner iron – clad pyrheliometer
New eppley pyrheliometer (temperature compensated)
Yanishevsky thermoelectric pyrheliometer
• SECOND CLASS PYRHELIOMETERS
Moll – Gorczynski pyrheliometer
Old Eppley pyrheliometer (not temperature compensated)
The smithsonian water – flow pyrheliometer was omitted from the list
of standard instrument, but it has been one of the primary standard of
the United States.

A PYRANOMETER SHOULD HAVE THE
FOLLOWING CHARACTERSTICS
 The calibration factor must be independent of
temperature
 It should not be wavelength-selective
 Absence of zero drift
 Calibration factor must be independent of the intensity
 Response time should be as small as possible
 Calibration Factor must be independent of time
 Temperature response should be minimum
 Cosine and azimuthal response or spatial variation in
the sensitivity of the detector should be minimum
 Sensitivity should be as large as possible

Typical thermopile used in pyranometers

Measurement
of
global
and
diffuse
solar
radiation
on
horizontal
surface

Measurement of Direct radiation at normal incidence

NORMAL INCIDENCE PYRHELIOMETER

Global radiation availability in India

16
80.30
12.13
Chennai
559
73.85
18.48
Pune
6
88.45
22.60
Kokatta
216
77.33
28.63
New Delhi
Height above
sea level (m)
Longitude
(°E)
Latitude
(°N)
Station
Geographical parameters for four typical Indian Stations

18.04
17.21
19.11
20.70
19.69
18.36
20.09
22.64
24.33
25.63
24.95
21.16
HT
15.52
15.37
17.78
20.16
19.84
18.79
20.84
23.40
24.30
24.44
22.54
18.47
H
Chennai
21.56
22.17
23.00
19.37
16.20
15.77
20.45
24.69
25.56
25.95
25.60
23.00
HT
17.10
18.22
20.38
18.76
16.42
16.24
21.49
25.96
25.56
24.19
21.92
18.61
H
Pune
19.40
20.63
18.36
16.02
16.16
15.77
16.45
21.78
22.32
21.96
20.84
19.19
HT
14.65
16.16
15.95
15.37
16.42
16.49
17.28
22.68
22.10
20.09
17.46
14.96
H
Kolkatta
19.83
22.10
23.43
21.31
17.64
17.89
21.38
24.91
25.10
24.76
22.50
19.61
HT
13.82
16.27
19.26
20.16
18.18
19.19
23.54
26.21
24.95
22.07
18.00
14.33
H
New Delhi
Dec
Nov
Oct
Sept
Aug
Jul
Jun
May
Apr
Mar
Feb
Jan
Daily global radiation on horizontal surface and on optimum
tilt for four different Indian Stations (Unit: MJ m-2 day-1)
H = daily global radiation on horizontal surface
HT = daily global radiation at annual optimum tilt

Variation of Radiation with Tilt for a South
Facing Surface
Fixed Surface
Annual
mean
daily
solar
radiation
(M
J
/
m
2
day
1
)
New
Delhi
Pune
Kolkat
ta
Chenn
ai
Tilt of Surface (degrees)

DURATION OF SUNSHINE HOURS
• A knowledge of the daily and hourly records of the amount of
sunshine is necessary for estimating global solar radiation
values using regression equations and for optimizing the
design of a particular solar collector. This measurement is
simpler and sunshine recorders are far less expensive than
solar radiation measuring equipments.
• The sunshine hours are extensively measured all over the
world using Campbell Stokes sunshine recorders. It consists
essentially of a glass sphere about 10 cm in diameter with an
axis mounted in a section of a spherical bowl parallel to that of
the earth, the diameter of which is such that the Sun's rays are
focused sharply on a card held in grooves in the bowl.
• The sphere acts as a lens and the focused image moves on a
specially prepared paper bearing a time scale. Bright sunshine
burn a path along this paper. The method of supporting the
sphere differs according to whether the instrument is required
for operation in polar, temperate or tropical latitudes.
• Three overlapping pairs of grooves are provided in the
spherical segment to take cards suitable for different seasons
of the year. The chief requirement of the sphere is that it should
be of uniform, well annealed and colourless glass.

The Campbell-Stokes sunshine recorder

Estimation of Average daily global solar radiation
Angstrom proposed the following empirical correlation for computing the average
daily global radiation on a horizontal surface:
p
a
c S
S
b
a
H
H
'
'+
=
where H = monthly average daily radiation on a horizontal surface,
c
H
a' , b' = empirical constants,
a
S = monthly average daily actual hours of sunshine,
p
S = monthly average daily possible sunshine hours
c
H
There is an ambiguity in defining clear day and hence to get ,
o
H
p
a
o S
S
b
a
H
H
+
=
= average clear sky daily radiation for the location and month in question,
the above formula was modified using extraterristrial
solar radiation,
(1)
(2)

where Ho is the extraterristrial solar radiation on a horizontal surface and
can be calculated as:
Estimation of Average daily global solar
radiation (Contd ... )
where Ws in the sunset hour angle in degrees, n is the average day for the
whole month and π is in radians
is measured value of actual sunshine hours and measured using
Campbell Stokes sunshine recorder. The possible sunshine hours, Sp,
can be calculated for a place using the formula






+
= n
lon
H
365
360
cos
33
.
0
1
24
0
π






+ δ
π
δ sin
sin
180
sin
cos
cos L
W
W
L s
s
)
tan
tan
(
cos
15
2
15
2 1
δ
L
W
S s
p −
=
= −
(3)
a
S
(4)

Estimation of Average daily global solar
radiation (Contd ... )
Equation (2) can be used for calculating average daily global radiation at
a location when data on actual sunshine hours, Sa , possible sunshine
hours, Sp , extraterrestrial solar radiation, H0 and values of a and b are
known for a nearby location with a similar climate. The constants a and
b for a place is found out by plotting a graph between known values of
H / H0 and Sa / Sp, as follows:
Slope b
a
Sa / Sp
0
H
H

0.44
0.30
Chennai
0.42
0.28
Calcutta
0.43
0.31
Pune
0.57
0.25
New Delhi
b
a
Location
The regression constants a and b for few
Indian stations are:
Estimation of Average daily global
solar radiation (Contd ... )

Uses of Solar Energy
• Heating of Water
• Heating of Houses (active
systems)
• Distillation of Water
• Cooking of Food
• Greenhouse Heating
• Drying of Food
• Power Generation
• Refrigeration and Airconditioning
• Passive Heating and Cooling
• Production of Very High
Temperatures
• Industrial Process Heat Systems
• Pumping of Water
• Direct Conversion of Electricity
(PV)

FLAT PLATE COLLECTORS
• The flat plate collector forms the heart of any solar
energy collection system and can be employed to heat
fluid (liquid or air) from ambient to near 100°C.
• The term ‘flat plate’ is slightly misleading since the
absorbing surface may not necessarily be flat but may
be grooved and other shapes.
• Flat plate collectors are under investigation for the last
300 years. The first reported flat plate collector was
demonstrated by Mr. H.B. Saussure, a Swiss scientist
during the second half of the seventeenth century.
• During the last six decades scientists in several
countries mainly in USA, UK, Australia, Israel, Germany,
South Africa, China and India have built, tested, studied
and optimized different types of flat plate collectors
mainly liquid heating flat plate collector.

 Pioneering work on solar flat-plate collectors have
been done by Hottel, Whillier and Bliss in USA who
mathematically modelled the collector and gave
Hottel-Whillier-Bliss equations to understand the
collectors.
 Later Prof. H.Tabor in Israel has done significant
work on understanding the behaviour of collectors
and gave several original ideas like convection-
suppression, selective black coatings and evacuated
collectors.
 Significant work on flat-plate collectors was done by
Prof. H.P.Garg in India and gave methodology for
optimizing the collector configuration, designing the
collector, thermal rating procedure of collectors,
thermal loss optimization, collector tilt optimization
and dirt correction factor, etc.

• Flat plate collectors are of two type: liquid heating
type and air heating type,
• The most obvious difference between the two is the
mode of heat transfer between the absorber plate
and the heated fluid,
• In the best type of liquid – plate collector, which
generally makes use of a fin-tube construction, heat
absorbed is transferred to the tubes by conduction,
• In a conventional flat-plate air heater there is a duct
(passage) between the absorbing plate and rear
plate. Thus the difference being in the heat transfer
exchanger design.
• Other components like glazing, insulation, casing,
orientation, tilt, exposure, etc. remain the same.

Schematic cross-section of a typical flat plate solar
collector illustrating the major functional parts

Flat Plate Collectors
• The main purpose of the collector is to absorb the sun’s
energy and transfer this energy efficiently to the liquid
flowing in it. There is a great variety of flat plate
collectors, but a tube in plate type of collector, is widely
used. The collector can be all metallic or plastic, single
glazed or double glazed, selectively coated or ordinary
black painted depending on the temperature of
operation and outside climatic conditions.
• As is seen earlier, a flat plate collector has the following
components:
– A blackened or selectively coated flat – absorbing
plate, normally metallic, which absorbs the incident
solar radiation, convert it into heat and conducts the
heat to the fluid passages.
– Tubes, channels or passages attached to the
collector absorber plate to circulate the fluid required
to remove the thermal energy from the plate.

COMPONENTS OF FLAT PLATE COLLECTOR (contd.)
• Insulation material provided at the back and sides of the
absorber plate whose principal function is to reduce heat
loss from the back and sides of the absorber plate.
• A transparent or translucent cover or covers whose
principal functions are to reduce the upward heat losses
and to provide weather proofing.
• An enclosing box whose principal functions are to hold
the other components of the collector and to protect the
collector plate and insulation material from the weather.
Collectors generally available in the market, although
confirming to the above general design, have some
differences between them. The components most often
changed are the absorber plate configuration, the black
coating on the absorber plate, and the glazing.

Improving Efficiency of a Flat-Plat Collector
The efficiency can be improved by:
• Improving transmittance - absortance product,
• Reducing thermal losses (conduction,
convection and radiation),
• Improving heat transfer coefficient from
absorbing plate to the working fluid,
• Optimizing collector configuration for better heat
exchanger efficiency,
• Optimizing tilt, orientation and exposure of
collector

Transparent Cover Plate
The function of cover plates are:
• Transmit maximum solar radiation,
• Minimize upward heat loss from absorber plate to the
environment,
• Protecting the absorber plate from weather.
The most critical factors for the cover plate materials are:
– Strength
– Durability
– Non-degradability
– Cost
– Solar-energy and thermal energy transmittance
Tempered glass is the most common cover material for
collectors because of its proven durability and stability
against UV radiation. Tempered glass cover, if properly
mounted, is highly resistant to breakage both from thermal
cycling and natural events.

Antireflective coatings
• All transparent materials (like glass) reflect some light from
their surfaces. By using a thin film having a refractive index
between that of air and transparent medium, the reflectance of
the interfaces can be changed. For normal incidence, the
fraction of light reflected is given by:
2
1
2
1
2








+
−
=
n
n
n
n
R
Where n2
and n1
are the refractive indices of the transparent
sheet and the medium respectively. Coating the surface
with a non-absorbing film will reduce the reflectance.

Insulation materials for Flat-Plate
Collectors
• Several thermal insulating materials which can be
used to reduce heat losses from the absorbing plate
and pipes are commonly available.
• The desired characteristics of an insulating material
are:
– Low thermal conductivity,
– Stability at high temperature (upto 200°C),
– No degassing upto around 200°C,
– Self-supporting feature without tendency to settle,
– Ease of application,
– No contribution in corrosion, and
– Low cost.
• Some of the good insulating materials are: glass wool,
fibre glass, rock wool, polyurethane, cork etc.

SELECTIVE BLACK COATINGS
• For efficient collection of solar radiation, the
absorber surface should absorb more solar
radiation and emit less thermal radiation.
• This selective behavior is possible since solar
radiation is in the wavelength range of 0.2 – 2.5
µm while thermal radiations emitted from a
surface at temperature more than 100°C is
above 5.0 µm.
• An ideal selective coating would be one with
absorptance (α) = 1 in the range of 0.2 – 2.5 µm
and emittance (ε)=0 in the operating
temperature range (above 100°C or 3.0 – 7.0 µ
m wavelength range).
• Practical selective black coating will have α/ε as
high as possible.

SELECTIVE BLACK COATINGS
(contd.)
• There are four principal types of selective
surface (opague).
• The first is one which absorb and emit as much
radiation as possible at all wavelengths and is
known as black body.
• The second surface will absorb more solar
radiation and emit less radiation. The example
is nickel black on a polished substrate.
• The third surface will absorb less solar radiation
and emit more radiation. The example is white
paint on a metal sheet.
• The fourth surface will absorb less solar
radiation and emit less radiation. The example is
aluminium foil.

Reflectance of selective coatings

1. Integral construction
• Tube wall should be thick to withstand fluid pressure and
prevent corrosion.
• Here tube thickness is one half the plate thickness
resulting in an ultra thick weight and costs 50% more
than tube and fin absorber.
Collector – Plate configuration
2. Tube and Fin construction (Mechanical Jointing)
• Simple construction but shows poor bonding resulting in
poor heat transfer.
• Therefore the contact area should be large and joint should
be uniformly tight.
3. Tube and Fin construction (Adhesive or soldered bonding)
• This type of jointing is better than mechanical jointing but suffers from
low thermal conductivity.
• For better heat flow large contact area, and thin and continuous layer of
bonding material are necessary.
• The bonding material may deteriorate with aging and thermal cycling.
4. Tube and Fin construction (metallurgical bond)
• A good joint from mechanical strength point of view but shows
low thermal conductivity compared to solder bonding.
• High plate thickness required.
5. Tube and Fin construction (Forge welding )
• Tube and Fin of different materials can be used.
• High thermal conductivity.

The useful energy derived from a flat plate collector is the difference
between the energy absorbed and the energy lost from the collector.
For a flat plate collector of area Ac the energy balance equation is
written as :
a
ic
u
e
Tt q
d
d
q
q
T
I =
+
+
=
τ
α 1
)
( (1)
Where
(Tα)e = effective transmittance-absorptance product of the absorber
given as
d
ρ
α
τα
)
1
(
1 −
−
=
ENERGY BALANCE ON A FLAT PLATE COLLECTOR
The flat plate collectors are always oriented and tilted (fixed) so that they
receive maximum solar radiation during the desired season of use. But the
solar radiation is generally measured on the horizontal surfaces so these
values require conversion to use on tilted surfaces.
In unit time, an unit area of the absorber will absorb energy qa given by
DS
R
R
I
R
I
R
I
I
q R
R
R
Th
d
d
d
dh
D
D
D
dh
Th
a ]
)
[ α
τ
α
τ
α
τ +
+
−
= (2)

• Under steady state conditions, the heat balance of the absorber is
given by the simple equation:
(useful heat collected) = (heat absorbed by the plate) - (heat losses)
qu = ITt(τα)e - UL(Tp - Ta) (3)
(Cont.)
• Usually the plate temperature Tp given in equation (3) is not
known and is difficult to calculate or measure since it is a
function of several parameters discussed earlier.
• More useful for design is a relation in which Tp is replaced by
the inlet fluid temperature Ti and the whole right hand side is
multiplied by a term FR, the heat removal efficiency factor,
which depends on collector design details and fluid flow rate.
qu = FR[ITt(τα)e - UL(Ti - Ta)] (4)
• The three design factors, FR, (τα)e and UL are measures of thermal
performance and combine to yield overall collector efficiency in terms
of the operating variables of temperature and insolation.

Tt
c
u
c
I
A
q
=
η
(5)
Usually, the efficiency is computed over a finite time period, τ, and
therefore the expression for average efficiency is as follows:
∫
∫
= τ
τ
τ
τ
η
o
Tt
c
o
u
c
d
I
A
d
q
(6)
where τ is the time period over which the performance is averaged.
Thus instantaneous efficiency using equation 4 & 5 of the flat plate
collector is given as:
T
a
L
R
e
R
I
T
T
U
F
F i
)
(
)
(
−
−
= τα
η (7)
The instantaneous efficiency of a collector, ηc is simply the ratio of the
useful energy derived to the total solar energy falling on the collector,
or
(Cont.)

Indicating that if η is plotted against (Ti – Ta)/IT a straight line will result,
with a slope of FRUL and y- intercept of FR(τα)e. This is the way actual
performance data for solar collectors are presented. The collector
heat removal factor may be calculated from the following equation :
















−
−
=
p
p
c
L
c
L
p
R
C
m
F
A
U
A
U
C
m
F


exp
1 (8)
(Cont.)
re
temperatu
fluid
local
at the
is
surface
absorber
entire
the
if
collected
energy
useful
collected
energy
useful
actual
=
p
F
Where, Fp= collector plate efficiency factor.
The Eq. (3) can now be written as:
)
(
)
(
[ a
m
L
e
T
p
c
u
T
T
U
I
F
A
q
−
−
= ατ (9)
Where, Tm is the average fluid temperature









−
+
+
+
+
=
F
D
W
D
U
C
DK
m
Dh
w
U
I
F
L
b
t
t
fi
L
p
)
(
[
1
1
1
/
π
π
(10)
Where
w = centre-to-centre tube spacing
D = outside diameter of the tube
hfi= tube-to-fluid (film) heat transfer coefficient
Kt = thermal conductivity of tube
Cb = bond conductance ( = Kb b/t)
Kb = bond material thermal conductivity
b = bond width
t = bond thickness
mt = tube thickness
F = fin efficiency factor given as:
2
/
)
(
]
2
/
)
(
tanh[
D
w
a
D
w
a
F
−
−
= (11)
The plate efficiency factor (Fp) for a tube in plate type of collector may be
calculated from the following equation:
(Cont.)

=
dhc
wUL
π
heat transfer resistance from inner surface of tube to
the fluid,
=
t
t
L
dK
m
wU
π
conduction of heat from outside wall to inside wall
of tube,
=
b
L
C
wU
conduction of heat from the fin to the tube through the
tube fin bond,
=
−
+ ))
(
'
( b
w
F
b
U
wU
L
L
conduction of heat along the fin towards the
pipe,
(Cont.)
where

The overall heat loss coefficient UL =Ql/(Tp-Ta) is made up of three
components – top loss coefficient Ut, the bottom loss coefficient Ub, and
the edge loss coefficient Ue:
UL = Ut + Ub + Ue …………..(12)
The bottom loss coefficient, Ub, is simply the ratio of the thermal
conductivity of the insulation (Ki) beneath the absorber plate to the
thickness li:
Ub = Ki / li Ac ………….. (13)
Likewise, the edge loss coefficient is the ratio of the thermal conductivity
of the insulation at the edge to the thickness, times the ratio of the area
of edge Ae to the collector effective aperture area Ac:






×
=
Ac
Ae
Ue
edge
at the
insulation
of
thickness
edge
at
insulation
of
ty
conductivi
thermal ……..(14)
LOSS COEFFICIENT OF FLAT PLATE
COLLECTORS

The modified equation as given by Garg for Ut is :
[ ]
C
m
w
N
f
N
N
Ta
Tp
Ta
Tp
hw
L
f
N
Ta
Tp
L
Tp
N
Ut
O
g
p
p
2
2
2
252
.
0
3
/
1
2
)
1
(
0425
.
0
1
)
)(
(
1
/
)
/(
)
(
cos
)
/
429
.
204
(
1










−
−
+
+
−
+
+
+
+










+
+
−
=
ε
ε
ε
σ
β
……(15)
where f = (9/hω - 30/h2
ω) (Ta/316.9) (1+0.091 N)
where Tp = absorber plate temperature (k)
LOSS COEFFICIENT OF FLAT PLATE COLLECTORS
(Cont.)

Ta = ambient temperature (k)
N = number of transparent cover plates
εp = thermal emissivity of absorver plate surface
εg= thermal emissivity of the cover plate (for glass, εg= 0.88)
β = Collector slope (degrees)
σ = Stefan – Boltzman constant = 5.67 ×10-8
W/m2
k4
hw = convective heat transfer coefficient due to wind (w/m2
°C)
= 2.8 + 3.0 V
V = Wind speed (m/sec)
LOSS COEFFICIENT OF FLAT PLATE COLLECTORS
(Cont.)

COLLECTOR CONFIGURATION
• The collector system considered here is of the pipe and fin
type as shown below:
• Which is supposed to be the best choice for domestic as well
as industrial water heating requirements. The possible
materials of the fin (Kp) may be copper, aluminum, steel or
galvanized iron of thickness (mp) 0.091 cm, 0.071 cm, 0.056
cm, 0.046 cm and 0.038 cm. Similarly the pipe may be of
copper, aluminum, steel or galvanized iron of inner diameter
(d) as 1.27 cm, 1.91 cm and 2.54 cm, spaced (w) at 2.5, 5.0,
7.5, 10.0, 12.5, 15.0, 17.5, or 20.0 cm. The bond conductance
is taken as 10, 20, 30, 40 (W/m°C).

COLLECTOR CONFIGURATION
• Thus from all above description we conclude that the tube
spacing, its diameter, its material; fin material and its
thickness; heat transfer coefficient; bond conductance; heat
loss coefficient are all directly related to the system
performance.
• Therefore the aim of the designer should be the best cost
effectiveness which is a function of efficiency and cost. The
main scope for reducing the cost lies in selecting the
optimum combination of pipe spacing and fin thickness for a
particular material of pipe and fin. Material cost will be
reduced by increasing the spacing between pipes and by
making the plate thinner.
• However this leads to a reduction in fin efficiency, plate
efficiency factor and overall system performance. Therefore
the aim should be to determine the combination of pipe
spacing and plate thickness, which will minimize the ratio of
cost to useful energy collected by the system

Optimization of collector configuration
• Optimization of collector configuration means the selection
of best combination of plate and pipe materials pipe to give
maximum efficiency at minimum cost.
• Several parameters and combinations of material that can
be used for a flat-plate collector as shown in the equation of
plate efficiency factor have been used along with the
associated cost of each combination and minimum value of
C/Fp (cost/efficiency) for each geometry calculated.
• The optimized configuration for a minimum value of
cost/efficiency is for the following specifications of flat-plate
collector:
Plate material : Aluminum
Thickness of plate : 28 SWG
Tube material : Galvanised Iron
Tube diameter : 19 mm
Tube to tube spacing : 10 cm

The photograph of an optimised collector plate

Optimization of Collector Tilt and Orientation
• A flat-plate collector is always titled and oriented (fixed) in such a
way that it receives maximum solar radiation during the desired
season of use.
• Since in northern hemisphere such as in India, sun appears to be
moving from east to west via south, the collector should face
exactly towards the south. Deviation of 5-10 degrees from south
towards east or west will not effect the performance much. The
exact south at a place can be determined at solar time using plumb
line.
• A detailed scientific analysis for finding out optimum tilt for flat
plate collectors was conducted by Prof. H.P.Garg considering, direct
and diffuse solar radiation separately, transmittance of glass cover
with angle of incidence; place(L), date(δ) and time of day(ω) and
derived an expression of optimum tilt(βopt).
• Based on this equation and curves developed for different Indian
stations, following thumb rules are derived for collector tilt:
– For Winter performance (November-February), the collector tilt
can be latitude of the place plus 15 degrees (L+150
),
– For summer performance (March-October), the collector tilt can
be latitude of the place minus 15 degrees (L-150
),
– For year round performance (January-December), the collector
tilt can be 0.9 times the latitude(0.9L0
).

There are variety of solar collectors and each behave
differently under different climatic conditions, operating
parameters and design variables.
Hence there was a need of unified approach for thermally
rating the collectors for finding out instantaneous efficiency,
effect of angle of incidence of solar radiation and
determination of collector time constant (a measure of
effective heat capacity).
National Bureau of Standards (NBS) of USA in 1974
developed the first procedure for testing and thermal rating of
collectors (as proposed earlier by Garg & Gupta) which was
later modified by ASHRAE in 1977 and is known as ASHRAE
Standard 93-77. The ASHRAE 93-77 was adopted with some
minor changes in many countries of the world including India.
THERMAL TESTING OF SOLAR COLLECTORS

Qu = Ac FR [ITt (τα)e – UL (Ti-Ta)] (2)
Tt
I
T
T
U
F
F
I
A
Q a
i
L
R
e
R
Tt
c
u
i
)
(
)
(
−
−
=
= τα
η
Tt
c
i
o
p
i
I
A
T
T
C
m )
( −
=

η
These equations are the basis of the standard test procedures.
The collector performance equation as discussed earlier are:
)
( i
o
p
u T
T
C
m
Q −
=  (1)
(3)
(4)
(contd.)

(contd.)
The general test procedure is to operate the collector in the test
facility under nearly steady conditions, measure the data to
determine Qu from Equation (1), and measure ITt, Ti, and Ta which
are needed for analysis based on Equation 3. Of necessity, this
means outdoor tests are done in the midday hours on clear days
when the beam radiation is high and usually with the beam radiation
nearly normal to the collector. Thus the transmittance – absorptance
product for these test conditions is approximately the normal
incidence value and is written as (τα)n.
Tests are made with a range of inlet temperature conditions. To
minimize effects of heat capacity of collectors, tests are usually made
in nearly symmetrical pairs, one before and one after solar noon, with
results of the pairs averaged. Instantaneous efficiencies are
determined from ηi=mCp(To)/AcITt for the averaged pairs, and are
plotted as a function of (Ti-Ta)/ITt). A sample plot of data taken at five
test sites under conditions meeting ASHRAE 93-97 specifications, is
shown in figure.

If UL, FR, and (τα)n were all constant, the plots of ηi versus
(Ti-Ta)/ITt would be straight lines with intercept FR (τα)n and
slope – FR UL. However, they are not, and the data scatter.
We know that UL is a function of temperature and wind speed,
with decreasing dependence as the number of covers
increases. Also, FR is a weak function of temperature.
And some variations of the relative proportions of beam,
diffuse, and ground-reflected components of solar radiation
will occur.
Thus scatter in the data are to be expected, because of
temperature dependence, wind effects, and angle of
incidence variations. In spite of these difficulties, long time
performance estimates of many solar heating systems,
collectors can be characterized by the intercept and slope [i.e.
(contd.)

Performance curve of a solar collector

Longterm Average Performance of Flat-Plate Collectors
• Generally the performance of solar collectors is given by instantaneous
efficiency on clear days.
• The true performance of solar collector will depend on cloudiness of
atmosphere and varying angle of incidence.
• Longterm performance can help in optimizing the design and evaluation
of economics.
Two methods are generally employed for longterm performance:
ix) Computer simulation method using longterm weather data
ii) Utilizability (Φ) method as given by Liu and Jordan using monthly
average hourly radiation and temperature data
• Using Hottel-Whillier-Bliss equations and longterm monthly average
solar radiation and ambient temperature data, utilizability curves were
produced for various cloudiness indices or cities of USA.
• Using the same analogy design curves of several Indian stations both
for summer months and winter months were produced by Garg for flat-
plate liquid heating collectors.

Design curves for Flat Plate Collector
for winter use for summer use

A typical air-heating solar collector

• A Conventional air heater is typically a flat passage
between two parallel plates. One of the plates is
blackened to absorb incident solar radiation. One or
more transparent covers are located above the
absorbing surface. Insulation around the sides and base
of the unit is necessary to keep heat losses to a
minimum.
• There are eight variables that a designer concerns
himself with in the construction of an air heater;
– Heater configuration is the aspect ratio of the duct and the
length of the duct through which the air passes.
– Airflow: Air must be pumped through the heater; increasing the
air velocity results in higher collection efficiencies, but also in
increased operating costs.
– The type and number of layers of glazing must be considered
and spectral transmittance properties must be examined.
Flat plate air heating collectors

– Absorber plate material: although selective surfaces
can significantly improve the performance of solar air
heaters by increasing the collector efficiency, black-
painted solar heaters are commonly used due to the
cost of selective surfaces. The absorber need not be
metal, since the air to be heated is in contact with the
entire absorbing surface This means that the
thermal conductivity of the absorber plate is relatively
unimportant.
– Natural convection barriers: a stagnant air gap
interposes a high impedance to convective heat flow
between the absorber plate and the ambient air. The
losses, both of radiation and convection, can be
reduced to low values by the use of multiple covers or
honeycombs, but the consequent reduction in
transmission of solar radiation makes more than one
air gap of doubtful value.
(contd.)

– Plate-to-air heat transfer coefficient: the absorber
can be roughened and coated to increase the
effective coefficient of heat transfer between the air
and the plate. The roughness ensures a high level of
turbulence in the boundary layer of the flowing air
steam. For this reason, crumpled or corrugated
sheets and wire screens are attractive as absorbing
materials.
– Insulation is required at the absorber base to
minimize heat losses through the underside of the
heater.
– Solar radiation data corresponding to the site are
needed to evaluate heater performance.
(contd.)

TUBULAR SOLAR ENERGY COLLECTORS
There are two methods for improving the
performance of solar collectors. The first
method increases solar flux incident on the
absorber by using some type of concentrators.
The second method involves the reduction of
heat loss from the absorbing surface.
Tubular collectors or evacuated tube collectors
(ETC) with their inherently high compressive
strength and resistance to implosion, are the
most practical means for eliminating
convection losses by surrounding the absorber
with a vacuum of the order of 10-4
mm of Hg.

• Tubular collectors have several advantages.
They may be used to get small
concentration ratio (1.5-2.0) by forming a
mirror from part of the internal concave
surface of a glass tube. This reflector can
focus radiation on to the absorber inside the
tube.
• Performance may also be improved by
filling the envelope with high-molecular-
weight noble gases. External concentrators
of radiation are generally used in an
evacuated receiver for improvement of its
performance.
TUBULAR SOLAR ENERGY
COLLECTORS (contd.)

Several versions of evacuated tube
collectors are manufactured by industries
such as Philips in Holland and Sanyo in
Japan. With the recent advances in
vacuum technology. evacuated tube
collectors are reliably mass produced
mainly in China. Their high temperature
effectiveness is essential for the efficient
operation of solar air-conditioning
systems and process heat systems and
now even for water heating.
TUBULAR SOLAR ENERGY COLLECTORS (contd.)

Schematic diagram of concentric-tube collector
optics; (b) cut-way view of evacuated tube solar
collector manufactured by Owens-Illinois, Inc., USA

Chinese Solar Tubes
Borosilicate Glass (3.3)
Glass-glass seal (not metal to glass)
Selective absorber coating (sputtered)
Thermal absorption of 92%
Excellent thermal insulation =
performance
Passively track sun throughout the day
Silver (barium getter) vacuum indicator
Strong (excellent hail resistance)
Long lasting performance
Cheap and easy to replace if damaged

SOLAR POND
• A solar pond is a body of water that collects and stores solar energy. Solar
energy will warm a body of water (that is exposed to the sun), but the
water loses its heat unless some method is used to trap it. Water warmed
by the sun expands and rises as it becomes less dense. Once it reaches
the surface, the water loses its heat to the air through convection, or
evaporates, taking heat with it. The colder water, which is heavier, moves
down to replace the warm water, creating a natural convective circulation
that mixes the water and dissipates the heat. The design of solar ponds
reduces either convection or evaporation in order to store the heat
collected by the pond.
• A solar pond can store solar heat much more efficiently than a body of
water of the same size because the salinity gradient prevents convection
currents. Solar radiation entering the pond penetrates through to the lower
layer, which contains concentrated salt solution. The temperature in this
layer rises since the heat it absorbs from the sunlight is unable to move
upwards to the surface by convection. Solar heat is thus stored in the
lower layer of the pond.
• The solar pond works on a very simple principle. It is well-known that
water or air is heated they become lighter and rise upward. Similarly, in an
ordinary pond, the sun’s rays heat the water and the heated water from
within the pond rises and reaches the top but loses the heat into the
atmosphere. The net result is that the pond water remains at the
atmospheric temperature. The solar pond restricts this tendency by
dissolving salt in the bottom layer of the pond making it too heavy to rise.
A shematic view of a solar pond is given in Figure.

Salt gradient solar pond with heat exchanger

Built in 1980.
Problems like
leaking, algae growth
& mineral impurities
were observed.
Experience,
material behaviour,
monitoring &
modeling.
2.0
100
Pondicherry
(India)
Supplying process
heat to a dairy
Operating
experience, material
behaviour and
possible applications
3.0
6000
Bhuj
(India)
Getting heated,
designed to supply 20
KW. Rankine cycle
turbines.
Operating
experience and
applications for
power production.
2.3
1600
Bhavnagar
(India)
Max. Temp. 800
C in
1972. Worked for
two years.
Operating
experience and
behaviour of
materials
1.2
1210
Bhavnagar
(India)
Achievements
Main Objectives
Depth
(m)
Area (m2
)
Location
MAJOR SALT – GRADIENT SOLAR PONDS (in
India)

Asia’s largest solar pond of 6000 m2
area at Bhuj, Gujarat in 1990/91

Solar Concentrators
• Solar concentrators are optical devices which increase the
flux on the absorber surface as compared to the flux incident
on the concentrator aperture. Optical concentration is
achieved by the use of reflecting or refracting elements
positioned to concentrate the incident flux onto a desired
absorber surface.
• A solar concentrator usually consists of (i) an optical device
to focus solar radiation (ii) a blackened metaliic absorber
provided with a transparent cover, and (iii) a tracking device
for continuously following the sun.
• Temperatures as high as 3000°C can be achieved with such
devices and they find applications in both photothermal and
photovoltaic conversion of solar energy.

Classifications
• Solar concentrators may be broadly classified into three
categories, namely,
(i) point focusing
(ii) line focusing, and
(iii) line focusing of limited extent
Point focusing concentrators have circular symmetry and are
generally used when high concentration is required. These
systems requiring two axis tracking can generate temperature in
the range 800-3000°C. Point focusing concentrators are being
used for solar thermal power generation purposes.
Line focus concentrators have cylindrical symmetry and are
generally used when intermediate concentration is required to
meet the demand of a desired task. Temperatures in the range
of 100-350°C can be generated using line focus concentrators.
These systems can be utilised for solar thermal power
generation as well as for industrial process heat applications.
Solar Concentrators (contd.)

Schematic diagrams of different solar concentrators
(a) Flat absorber with flat reflectors, (b) Parabolic cencentrator,
(c) Compound parabolic concentrator, (d) Fresnel lens,
(e) Cylindrical parabolic concentrator

THERMODYNAMIC LIMITS TO CONCENTRATION
The concentration has an upper limit that depends on whether the
concentrator is a point focus (three dimensional geometry) or line focus
(two dimensional geometry) type. The maximum possible concentration
achievable with a concentrator that only accepts all the incident sunlight
within an acceptance half angle Qm is given by
m
D
Sin
C
θ
2
)
3
(
max
1
=
m
D
Sin
C
θ
1
)
2
(
max =
Where θm is the half of the angular substance of the sun at any point on
the earth ( = 16' ).
The maximum achievable concentration for these two types of
concentrators are about. 45,000 and 215 respectively.
In practice, however, these levels of concentration are not achievable
because of tracking errors and presence of surface imperfections in the
surface of reflecting or refracting element.
Solar Concentrators (contd.)

POINT FOCUSING CONCENTRATIONS
To achieve high efficiencies at high temperatures one needs concentrations
producing point focus. These concentrations require two axis tracking.
Concentrator designs which fall in this category are – a paraboloid of
revolution, central tower receiver system and circular freshnel lens etc.
Paraboloid of Revolution
The surface produced by rotating a parabola about its optical axis is called a
paraboloid. With perfect optical surfaces, a parallel beam of light produces a
point focus. However, a somewhat enlarged focal point or image is
produced due to finite angular substance of the sun.
The concentration ratio for a paraboloid can be determined easily from basic
geometry but depends on the shape of the absorber. For a spherical
absorber it is given by
0
2
2
4 ξ
θ
Sin
Sin
C r
sph =
Where θr is the rim angle of the parabola.
Maximum concentration is achieved for
2
π
θ =
r

Parabolic Trough Concentrator
• Linear concentrators with parabolic cross section have
been studied extensively both analytically and
experimentally, and have been proposed and used for
applications requiring intermediate concentration
ratios and temperatures in the range of 100 to 500°C.
Figure shows a collector of this type which is part of a
power generation system in California. The receiver
used with this concentrator is cylindrical and is
enclosed in an evacuated tubular cover ; flat receivers
have also been used with reflectors of this type.
• Designed in a power range of 30 – 150 MW.
• Working Principle:
– Solar Receiver consists of a large array of parabolic
trough reflectors that reflects the sunlight to a
receiver tube located along the trough’s focal line.
Heat transfer fluid (HTF) flowing in the tube is heated
and then transported to a heat exchanger /
evaporator for steam and power generation.

Tracking
System
Edge Angle
Focal Length
Absorber Diameter
Reflector (Parabolic Trough)
Aperture
Parabolic Trough
Parabolic Trough
Concentrator
Concentrator
T t

Rs. 2500/m2
Rs. 4000/m2
Collector Cost
37%
30%
Turbine Cycle efficiency
80MW
10MW
Unit Capacity
78%
65%
Optical Efficiency
400°C
200°C
Operating Temperature
90m
20m
Length
5.76m
1.8m
Aperture
To
From
Feature
Improvements in the parabolic trough
concentrators and systems since 1982

A large area solar dish has been developed to provide
process heat for milk pasteurization at a dairy of
Maharashtra Rajya Sahakari Dugdh Mahasangh Maryadit
(MRSDMM), Maharashtra under a R&D project sponsored
by MNRE to IIT Bombay jointly with M/ s. Clique
Developments Pvt. Ltd. (CDPL), Mumbai. The solar dish
has been installed and commissioned.
The technical specifications of the solar system are
Aperture Area 160 m2
Reflector area 123 m2
Thermal power (annual average) 50-70 kWth
Annual operating hours 3200-3350 hours/ year
Annual fuel savings (Furnace oil) 16 to 24 kilo litre/ year
Operating wind speed up to 54 kmph
Survival wind speed up to 140 kmph
Aerial clear space required for the dish 25 m x 20 m x 18 m
height Clear area required on ground / roof 3 m x 3 m
Tracking power 500 W
T t
Large Area Solar Dish at Milk Dairy at
Latur, Maharashtra

Solar Water Heating
• Solar Water Heaters (SWH) have been extensively
used for the last more than 8 decades.
• The countries where these are extensively studied are
USA, Australia, U.K., Israel, South Africa and India.
• The countries in which Solar Water Heaters are
extensively used are : USA, Australia, U.K., Germany,
India, Jordan, Israel, Cyprus, China, Greece, Japan,
Sweden and several other countries.
• In recent years considerable knowledge has been
developed about solar hot water systems.
• Basically solar water heaters are either for domestic
applications, large applications or swimming pool
water heating applications.

TYPES OF SOLAR WATER HEATER
• Built-in-storage type Solar Water Heater
(Integrated – collector storage type)
• Domestic Solar Water Heaters
(Natural Circulation type / thermosyphon type)
• Large Size Solar Water Heater
(Industrial type)
• Swimming Pool Water Heater

Many different designs of solar water heaters are possible and they
may be classified in many ways. Each type has its own advantages
and disadvantages, and depending on the situation a particular
design is recommended. Some of the solar water heating
configurations are as follows :
 A direct natural circulation solar water heater.
 An indirect natural circulation solar water heater.
 An indirect forced circulation type solar water heater.
 A single cylinder indirect forced circulation solar water heater.
 An indirect system with air heat collectors.
In general it can be said that a solar water heating system consists of
the following components :
 Flat plate collectors
 Storage tank
 Heat exchanger
 Automatic control
 Pumps, pipe work, valves and fittings
Domestic Solar Water Heaters

Conventional Domestic Solar Water Heater
Working Principle of Solar Water Heating System

Natural circulation type solar water heater
(Schematic)

It has been experimentally observed that in a
SWH, the inlet (Ti) and outlet (To) water
temperature rise for a collector is nearly
constant and generally it is about 10°C. Thus
Simple model for Natural Circulation Type SWH
(To-Ti) = 10°C
Thus we can calculate the natural flow rate )
(m
 using collector equation
[ ]
)
(
)
( Ta
Ti
U
e
H
A
F
Q L
C
R
u −
−
= τα
Tf
Cp
m
Ti
To
Cp
m
Qu ∆
=
−
= 
 )
(
and
Tf
Cp
Ta
Ti
U
e
H
A
F
m L
C
R
∆
−
−
=
)
(
)
(
[ τα

Thus
Substituting the values of FR
we get,
40
=
m
 litres /m2
hr

Collector inlet (Ti) and outlet (To) temperature for a
natural circulation water heater
(Ti)
(To)

THE STORAGE TANK
The storage tank stores the heat collected during the day
for use when needed. For the storage of hot water,
copper, steel, galvanized iron, aluminium, concrete, plastic,
and sometimes wooden tanks are used. The tank should
be sized to hold between 1.5 and 2 days supply of hot
water. The auxiliary heating arrangements may be electric
or gas booster and thermostat should be fitted in the
central part of the tank and not in the bottom of the tank.
For domestic purposes, the thermostat setting is done
between 50-60°C. There are many variations in the tank
design and a few are listed below :
o Vertical or horizontal type
o Pressure or non-pressure type
o Gas, electric or solid fuel booster, off-peak or continuous
tariff, or
o Internally or externally mounted.
There is very little information available on system
performance for the above storage types.

Some Common Liquid to Liquid Heat Exchanger Designs for Solar Energy Use

Recommended Way of Connecting Bank of Collectors
Positioning of Differential Controller

Schematic of forced circulation solar hot water system
with 3 different schemes for supplying auxiliary energy

Solar Water Heaters
• Hot water at 60-80o
C for hotels,
hospitals, restaurants, dairies, industry
and domestic use.
• System comprises one or more
collectors, storage tank, piping etc.
Heat exchanger and pumps added, if
necessary.
• About 2.15 million sq.m. collector area
installed.
• BIS standard for collectors introduced
in 1990/1992. Standards updated
recently.
• 60 BIS approved manufacturers with
production capacity of over 300,000
sq. m. collector area per annum.

 As boiler feed water for steam generation
Godavari Fertilizers & Chemicals : 1,20,000 lpd
Ltd., Kakinada
Quinn India Ltd., Hyderabad : 75,000 lpd
Shivamrut Dudh Utpadak : 30,000 lpd
Sahakari Sangh Ltd., Akluj
 Hot water for multistoried residential complex
DS Kulkarni Developers Ltd., Pune : 56,400 lpd at 60o
C
120,000 LPD CAPACITY SOLAR WATER HEATER
AT GODAVARI FERTILISER & CHEMICALS LTD.

SOLAR DOMESTIC HOT WATER SYSTEMS IN ISRAEL
SOLAR DOMESTIC HOT WATER SYSTEMS IN Pune (India)

Why Solar Cookers ?
• High cost or Unavailability of
commercial fuels – Kerosene, Coal,
Gas, Electricity
• Deforestation caused by Increasing
Firewood Consumption
• Use of Dung and Agricultural Waste as
Fuels Instead of for Soil Enrichment
• Diversion of Human Resource for Fuel
Gathering

Types of Solar Cookers
• Direct or focusing type solar cooker
– In these cookers some kind of single or multifacet solar energy concentrator
(parabolic, spherical, cylindrical, fresnel) is used which when directed towards the
sun focus the solar radiation on a focal point or area where a cooking pot or frying
pan is placed. In these cookers the convection heat loss from cooking vessel is
large and the cooker utilizes only the direct solar radiation.
• Indirect or Box type Solar Cooker
– In these cookers an insulated hot box (square, rectangular, cylindrical)
painted black from inside and insulated from all sides except window
side which is double glazed is used. Single plane or multiple plane
reflectors are used. Some times these are also known as oven type solar
cookers. These can be electrical cum solar cookers and some cookers
utilize a kind of latent heat storage material.
• Advanced type Solar Cooker
– In these cookers, the problem of cooking outdoors is avoided to some
extent. The cookers use either a flat plate collector, cylindrical (PTC)
concentrator, or a multifacet or large parabolic (mosaic type)
concentrator which collect or focuses the solar heat and transfers or
reflect from a secondary reflector to the cooking vessel. The cooking in
some cases can either be done with stored heat or the solar heat is
directly transferred to the cooking vessel in the kitchen.

Dull black painted stable upto
250°C
Very good adhesive
characteristics
1.2 mm thick
Two pots – dia 200 m
Two pots – dia 150 mm
Depth of pots – 67 mm
•Aluminium alloy
sheet
•Stainless Steel sheet
Cooking
Containers
Reflectivity > 85%
Scratch resistant
Resistant to solar radiation
and atmospheric variation
4 mm thick
54 x 54 cm
Silvered or
Glass aluminized
Reflector
(Mirror)
Double glass system must be
air tight
Transmittance > 85%
3-4 mm thick
50 x 50 cm size
spacing between sheets
1 cm
Water white glass
(Temperated /
toughned)
Glazing (Double
glass lid)
Free from resin binders Stable
upto 250°C
5 cm or more thick
k = 0.052 W/m K
Glass fibres in the
form of pads
Insulation (Back
and side)
Painted dull black
Should not touch outer body
0.56 mm thick
(50 x 50 x 10 cm)
Aluminium
Inner Cooking
Box
Resistant to ultraviolet
radiation and atmospheric
variations
0.48 mm thick
(60 x 60 x 17 cm)
0.56 mm thick
(60 x 60 x 17 cm)
2 mm thick
(60 x 60 x 17 cm)
•Galvanished iron
•Aluminium
•FRP
Outer Box
Requirements / Remarks
Thickness / size
Material
Component
Solar Box – type Cooker : Design Details

Solar Box-type Cooker : Cooking
Time for Recipes
It takes about 2 – 2.25 hours for cooking
depending upon the kind of food and
season. Different items like dal, rice,
vegetables etc. are normally cooked
simultaneously in separate containers.
The time taken for cooking is less in
summer than in winter.

SK - Type Solar Cookers
(SK-10, SK-12, SK-14, SK-98)
• SK – Solar Cooker uses parabolic reflector
made of thin, hard aluminium sheets with
protected, high reflecting surface mounted at a
rigid basket structure.
• Reflector with short focal distance for safety
reasons, long tracking intervals and high
efficiency.
• Cooking pot in a standard 12 – litres pot of black
enameled steel with a diameter of 28 cm.
• Tracking is done by moving the whole cooker
(azimuth) and by turning the reflector around
the horizontal axis (elevation), adjustment of the
reflector to the sun by use of a shadow indicator.

Technical Data (SK Type Solar Cooker)
• Reflector diameter : 140 cm
• Nominal effective power : 0.6 kW
• Pot capacity : 12 litres
• Pot diameter : 28 cm
• Max. temperature : 200°C
• Capacity : Boils 48 litre
of water in a day
• Tracking : Manual
• Cost : INR Rs. 6000/-
• Cooking Food : 10-15 people at a time

Parabolic solar cooker
Parabolic solar cooker,
,
not only for cooking …
not only for cooking …
… but also for
baking, frying,
conserving,
and much
more …

Parabolic Domestic Solar Cooker (SK 14)

World's Largest Solar Steam Cooking
System at Tirupati, Andhra Pradesh
Location
• Installed at the temple town of Tirumala, Andhra Pradesh with nearly
50 percent funding from MNRE.
System
• Employs automatic tracking solar dish concentrators to convert
water into high pressure steam which Is used for cooking purpose
in the community kitchen.
Technical Details
• Solar dish concentrators (106 Nos) with total reflector area of about
1000m2
.
• Modular in nature and consists of several units (parallel & series)
connected to central pipe-line system.
• Each dish consists of scheffler mirrors with an aperture area of 9.4
sq.m.
• Generates 4,000 kg of steam per day at 180°C and 10 Kg/cm2
.
• Cook meals for around 15,000 persons per day.
• The cooker saves about 1,20,000 litres of diesel per year.
• The total cost of the system is about Rs. 110 lakh.
Implementing Agency
• Ministry of New & Renewable Energy (MNRE).

World’s Largest Solar Steam Cooking System

WORLD’S LARGEST SOLAR STEAM
COOKING SYSTEM AT TIRUPATI

Solar Steam Cooking System at Army
Mess, Ladakh, Jammu & Kashmir
(Installation 12.04.05)

Solar Bowl Cooking Concentrator
• Developed at Centre for Scientific Research,
Auroville
• Capable of Cooking food for 1000 people.
• System consists of :
– 15 m. diameter non-tracking solar Bowl concentrator
– Automatic tracking receiver
– Use of thermic fluid to transfer energy collected by
receiver for generating steam
– Heat storage tank with heat exchanger
– Double jacketed cooking pots

Bowl Concentrator (15 m dia) for Community Cooking
(1000 people) at Auroville, Pondicherry

Reasons for the non-acceptance of the solar cookers
• Too expensive for individual family ownership
• Incompatible with traditional cooking practices
• too complicated to handle
• cooking can be done only in the direct sun
• can not cook at night
• can not cook in cloudy weather
• can not cook indoors
• danger of getting burned or eye damage
• are not locally available
• less durable; needs repair or replacement of parts which are not
easily available
• The cooker needs frequent adjustment towards the sun and
exposure of the cooking pot to the blowing dust and sand
effected the food taste
• Easy availability of alternative cooking fuels like wood and fuel
wood
• There is no provision of storing the heat therefore cooking of
food was not possible where there are clouds or sun is not
strong
• No proper education, training and involvement of women folk

Technical issues need attention for the
wider use of solar cookers
• Reliability
• Efficiency
• Quality
• Durability
• Utility
• Maintenance
• Weight
• Servicing
• Affordability
• Cost effectiveness
• Compatibility with food habits
• Training and education
• Micro level financing
• Marketing strategy
• Local availability
• Involvement of rural folk
• Dedication and commitments
• Provision of storage material
• Cooking indoors

• The function of a Building or a house is to provide shelter to its
occupants from weather.
• Since weather conditions vary from one place to another and vary
widely over the year, and humans feel comfortable within certain
range of temperatures and humidities, the house are made to provide
everyday living comfort.
• The heating of house in winter and cooling in summer to provide
comfort using solar energy or other natural concepts is an ancient
concept and is in use since men started to build habitations.
• Basically solar heating or cooling systems are of two types : Passive
heating and cooling and active heating & cooling.
• Passive systems do not need any mechanical system and are
designed such as the glazed area, walls and roofs are made use of
collecting, storing and distributing the heat indoors by natural
processes of convection, conduction and radiation.
• Five basic concepts of passive heating are : direct gains, collector
storage wall, sunspace collector - storage roof and convective zone.
• Components of active heating system are : (I) solar collector, (ii)
storage device, (iii) auxiliary heating system (iv) Distribution system
including fan, duct and controls.
• To provide near comfort conditions the most cost effective method is
to Judiciously make use of both passive and active systems.
Solar Buildings

• Everybody needs a comfortable house where activities like sitting,
sleeping, dinning, food preparation, storing, studying, recreation,
bathing, hobbies, etc. can be conducted.
• Building site and location is very important. The natural topography
and micro climate may significantly effect the performance.
• The three thermo physical properties, the thermal resistance, heat
capacity and solar absorption of surface are very important.
• There is no fixed thumb rule to find out the optimum combination of
various requirements or features. This can be done by using
economic methodologies, and performance prediction methods
using computer simulation.
• Several climatic parameters effecting the performance of the
building are solar radiation, air temperature its diurnal variation and
extreme, air humidity, precipitation its quantity and distribution,
wind its speed and direction, incoming and outgoing radiation, sky
temperature and sky conditions, sunshine duration, day length and
night length.
• There are several factors which are responsible for thermal comfort
such as air temperature, mean radiant temperature, air humidity, air
motion, clothing and activity level.
• Apart from Climatic parameters and thermophysical properties of
materials used in the buildings, the Building site, shape, location,
orientation, plan, elevation, topogtaphy, microclimate, etc.
significantly effect the performance.
Solar Buildings (contd.)

Solar Passive Building of Solar Energy Centre

The Solar Passive Building of Punjab
Energy Development Agency at

SOLAR PASSIVE BUILDING
STATE BANK OF PATIALA, SHIMLA

PARAMETERS FOR SOLAR DRYING
• The drying of product depends on external variables like
temperature, humidity and velocity of air stream and internal
variables which is a function of drying material and depends on
parameters like surface characteristics (rough or smooth surface),
Chemical composition (sugar, starch, etc.), physical structure
(porosity, density, etc.). and size and shape of the product. The rate
of moisture movement from the product inside to the air outside
differ from one product to another and very much depends weather
the material is hygroscopic or non-hygroscopic. Non- hygroscopic
materials can be dried to zero moisture level while the hygroscopic
materials like most of the food products will always have a residual
moisture content.
• The design of a solar dryer depends on : solar radiation,
temperature of air, relative humidity of air, moisture content of the
product, amount of product to be dried, time required for drying,
availability of auxiliary energy, material of construction of dryer
and the resource availability.

PHYSICS OF SOLAR DRYING
• Heat by convection and radiation to Surface of
product
→ Goes to interior of product
• Increase in temperature
• Formation of water vapour
→ Evaporation of moisture from Surface
Drying can be accelerated by:
• Increasing flow rate of air
• Increasing temperature of drying air
• Initial Drying - Surface drying, later on drying
depends on type of materials.
• Non hygroscopic- drying possible upto zero
moisture content.
• Hygroscopic - grains, fruit, food stuff have residual
moisture.

RATIONALE FOR CONTROLLED DRYING
1. Grain
• Improves product quality,
• Improves storage capability,
• Reduces time and space requirement for drying,
• Facilitates quick preparation of fields for next cropping,
• Facilitates wet season harvesting and storage,
• Improves drying hygene.
2. Timber
• Improves product quality,
• Reduces period capitoltied up in drying stock,
• Improves low expertise, low capital, improved drying options,
• Expands range of usable timber species,
• Improves attainable drying level.
3. Fruits, Vegetables & Fish
• Reduces product seasonability,
• Improves marketing control of farmer,
• Reduces spoilage,
• Improves drying hygene,
• Improves storage capability,
• Reduces nutritional fluctuations.

CLASSIFICATION OF SOLAR DRYERS
• DIRECT TYPE DRYERS: In direct or natural convection type
dryers, the agricultural product is placed in shallow layers in a
blackened enclosure with a transparent cover. The solar
radiations are directly absorbed by the product itself. The food
product is heated up and the moisture from the product
evaporates and goes out by the natural convection.
• INDIRECT TYPE DRYERS: In these dryers the food product is
placed in a drying chamber. The air is heated in solar air
heaters and then blown through the drying chamber. In some
of the designs, dryers receive direct solar radiations and also
heated air from solar air heaters. In these dryers manipulation
of temperature, humidity and drying rate is possible to some
extent.
• FORCED CIRCULATION TYPE DRYERS: In these dryers, hot air
is continuously blown over the food product. The food product
itself is loaded or unload continuously or periodically. These
kind of dryers are comparatively thermodynamically efficient,
faster and can be used for drying large agricultural product.
These dryers can be of cross-flow type, concurrent flow type or
counter-flow type.

(c) Forced circulation type solar dryers
(a) Direct type solar
dryers
(b) Indirect type solar
dryers
TYPE OF SOLAR DRYERS

585
Glendale Tea Factory, Coonoor, TN
390
Guernesy Tea Factory, Brookland, Coonoor, TN
320
Pandiar Tea Factory, Near Gudalur, TN
320
Parkside Tea Factory, Near Coonoor, TN
250
Kilkothagiri Tea Factory, Milkothagiri, TN
220
Kavukal Tea Factory, Kothagiri, TN
100
UPASI Demonstration Tea Factory, Coonoor, TN
112
Golden Hills Tea Factory, Near Coonoor, TN
130
Manjolai Tea Factory, Tirunelveli, TN
Collector
area (m2
)
Location
Details of few Solar Drying Systems for
Tea Drying in India

212 m2
(Glazed) + 424 m2
(unglazed)
Flat Plate
Galvanized Iron with black paint
4 mm thick tempered glass
5 – 5.5 kg s-1
Solar Collector
Total Area
Type
Absorber
Glazing
Air Flow
11°N
77°E
1950 m
Site
Latitude
Longitude
Altitude
Details of a Roof Integrated Solar Air Heating
System Installed at Coornoor, Tamil Nadu
• In the period 1991-95 nine such units, having a total collector area of
about 2700 m2
, were installed in South Indian Tea Factories.
• It is possible to save annually an average of 25% of the fossil fuel used
in the tea factories.
• The payback period for the system is less than 2 years

Leather Dryer with Roof mounted Solar Air Heaters
(4 x 167m2
area) at M.A. Khizar Hussain & Sons,
Ranipet, Chennai

LEATHER DRIER WITH SOLAR HOT AIR DUCTS
AT M/S M.A. KHIZAR HUSSAIN & SONS, RANIPET

Important Conclusions
• Experience over the past four decades has shown that
inspite of high potential of solar drying it has not taken off.
Some of the reasons are;
• Systematic work on solar dryer has been done only in few
countries.
• Solar dryer has not been developed as a system.
• In industralized countries, there is great interest towards
solar drying. However, neither the temperature nor the
heat requirement can be achieved with solar collector.
• Solar drying is considered more applicable to low
temperature in-storage type drying in tropical and
subtropical countries.
• Pre-healing of drying air in batch dryers has been
demonstrated to be techno-economically viable.
• Solar drying should be disseminated for medium and low
scale farmers for drying cash crops.
• To popularise solar drying, pilot demonstration followed
by training and workshop will have to be intensified.

SOLAR DESALINATION TECHNIQUES
30,000 – 50,000 ppm
Sea Water
2,000 – 2,500 ppm
Underground
Saline Water
100-125 litres / person / day
(NEW)
15-25 litres / person / day
(OLD)
Demand of Potable
Water
Rivers, Lakes, Ponds, Wells etc.
Sources of Potable
Water
Domestic, Industries and
Agriculture
Requirement
Less than 550 ppm
Potable Water

WATER DESALINATION TECHNOLOGY
• Potable water (fresh water) suitable for human
consumption should not contain dissolved salts
more than 500 ppm.
• For agricultural purposes, water containing salt
content of 1000 ppm is considered as the upper
limit.
• Potable water is required for domestic, agriculture
and industries.
• Some applications in industries like cooling
purposes, sea water is feasible despite the corrosion
problems while other industries use higher quality
water than is acceptable for drinking water. Modern
steam power generation plant need water with less
than 10 ppm.
• Potable/fresh water is available from rivers, lakes,
ponds, wells, etc.
• Underground saline/brackish water contains
dissolved salts of about 2,000-2,500 ppm.

METHODS OF CONVERTING BRACKISH
WATER INTO POTABLE WATER
• DESALINATION: The saline water is evaporated using
thermal energy and the resulting steam is collected and
condensed as final product.
• VAPOR COMPRESSION: Here water vapour from boiling
water is compressed adiabatically and vapour gets
superheated. The superheated vapor is first cooled to
saturation temperature and then condensed at constant
pressure. This process is derived by mechanical energy.
• REVERSE OSMOSIS: Here saline water is pushed at high
pressure through special membranes allowing water
molecules pass selectively and not the dissolved salts.
• ELECTRODIALYSIS: Here a pair of special membranes,
perpendicular to which there is an electric field are used and
water is passed through them. Water does not pass through
the membranes while dissolved salts pass selectively.
In distillation; thermal energy is used while in vapour
compression, reverse osmosis, electrodialysis, etc. some
mechanical and electrical energy is used.

Types of Solar Still
• Single Effect Basin Solar Still
• Tilted Tray Solar Still
• Multibasin Stepped Solar Still
• Regeneration Inclined Step Solar Still
• Wick Type Solar Still
• Multiple Effect Diffusion Solar Still
• Chimney Type Solar Still
• Multi-Tray Multiple Effect Solar Still
• Double Basin Solar Still
• Humidification Dumidification Distiller
• Multistage Flash Distiller
• Solar – Assisted wiped film Multistage Flash Distiller

COMPONENTS OF SINGLE
EFFECT SOLAR STILL
• Basin
• Black Liner
• Transparent Cover
• Condensate Channel
• Sealant
• Insulation
• Supply and Delivery System

BASIC REQUIREMENTS OF A GOOD
SOLAR STILL
• Be easily assembled in the field,'
• Be constructed with locally available materials,
• Be light weight for ease of handling and
transportation,
• Have an effective life of 10 to 20 Yrs.
• No requirement of any external power sources,
• Can also serve as a rainfall catchment surface,
• Is able to withstand prevailing winds,
• Materials used should not contaminate the
distillate,
• Meet standard civil and structural engineering
standards, and,
• Should be low in cost.

Double sloped experimental solar still

SOLAR STILL OUTPUT DEPENDS
ON MANY PARAMETERS
1. Climatic Parameters
• Solar Radiation
• Ambient Temperature
• Wind Speed
• Outside Humidity
• Sky Conditions
2. Design Parameters
• Single slope or double slope
• Glazing material
• Water depth in Basin
• Bottom insulation
• Orientation of still
• Inclination of glazing
• Spacing between water and glazing
• Type of solar still

1. Operational parameters
• Water Depth
• Preheating of Water
• Colouring of Water
• Salinity of Water
• Rate of Algae Growth
• Input Water supply arrangement (continuously
or in batches)
SOLAR STILL OUTPUT DEPENDS ON
MANY PARAMETERS Contd…

Main Problems of Solar Still
• Low distillate output per unit area
• Leakage of vapour through joints
• High maintenance
• Productivity decreases with time for a
variety of reasons
• Cost per unit output is very high

CONCLUSIONS ON BASIN- TYPE SOLAR STILL
 The solar still output (distillate) is a strong function of solar
radiation on a horizontal surface. The distillate output
increases linearly with the solar insolation for a given
ambient temperature. If the ambient temperature increases or
the wind velocity decreases, the heat loss from solar still
decreases resulting in higher distillation rate. It is observed
for each 10°C rise in ambient temperature the output
increases by 10 percent.
 The depth of water in the basin also effects the performance
considerably. At lower basin depths, the thermal capacity will
be lower and hence the increase in water temperature will be
large resulting in higher output. However, it all depends on
the insulation of the still. If there is no lnsulatlon, increase in
water temperature will also increase the bottom heat loss. It
has been observed that if the water depth increases from 1.2
cm to 30 cm the output of still decreases by 30 percent.

Solar Shakti Fundas Garg

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