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UNIT II
Solar EnergyM.Tech. 1st Semester
By:
S. S. Joshi
Lecturer
Electrical Engg Dept.Electrical Engg Dept.
SOT PDPU
Sun Earth Relationships
Solar Constant• The solar constant is defined as the total energy received
from the sun per unit time on a surface of unit area kept
perpendicular to the radiation, in space just outside the
earth's atmosphere when the earth is at its mean distance
from the sun.
• Solar constant have approximately value is 1370 W/m2.• Solar constant have approximately value is 1370 W/m2.
Solar Radiation at the Earth’s surface
• The solar radiation that penetrates the earth's atmosphere
and reaches the surface differs in both amount and
character from the radiation at the top of the atmosphere.
• In the 1st place, part of the radiation is reflected back into
the space, especially by clouds.
• Further more the radiation entering the atmosphere is• Further more the radiation entering the atmosphere is
partly absorbed by molecules in the air.
• Oxygen and ozone(O3), formed from oxygen, absorb nearly
all the ultraviolet radiation and water vapor and carbon
dioxide absorb some of the energy in the infrared range.
Direct or Beam Radiation• Solar radiation that has not been absorbed or
scattered and reaches the ground directly
from the sun is called "direct(or Beam)
radiation".
• Diffuse Radiation:The diffuse radiation is that solar radiation received from the
sun after its direction has been changed by reflection and
scattering by the atmosphere.
• Total or Global Radiation:The total solar radiation received at any point on the earth's
surface is the sum of the direct and diffuse radiation.
Irradiance, W/m2 (G):• The rate at which radiant energy is incident
(occur) on a surface, per unit area of thesurface.
• The symbol G is used for solar irradiance, withappropriate subscripts for beam, diffuse ortotal.total.
Elliptical orbit of earth’s
revolution
Autumn
7% more radiation
Winter
Spring
Summer
Irradiation, J/m2 :The incident energy per unit area on a surface,
found by integration of irradiance over a
specified by time, usually an hour a day.
IRRADIANCE= POWER IRRADIANCE= POWER
(Wm2)
IRRADIATION= ENERGY
( Wh/m2)
Extraterrestrial radiationSolar radiation incident on the outer atmosphere of the earth is
called extraterrestrial radiation.
A circle of constant longitude passing through a given place on the earth's
surface and the terrestrial poles.
Terrestrial radiation
• Terrestrial radiation is heat that is radiated
from the earth, solar radiation is heat radiated
from the sun.
Beam radiation
• Solar radiation along the line joining the
receiving point and the sun is called beam
radiation.
Important terms
Air mass:
• A term called air mass (AM) is often used as a measure of
the distance traveled by beam radiation through the
atmosphere before it reaches a location on the earth's
surface. The air mass is the ratio of the path of the sun's
rays through the atmosphere to the length of path when the
sun is at zenith.sun is at zenith.
• The zenith is an imaginary point directly "above" a
particular location, on the imaginary celestial sphere
Basic Earth-Sun angles
Earth's Equator:
It is an imaginary great circle normal to the earth's axis,
dividing the distance between the earth's poles along its
surface into two equal parts. The equator divides the earth
into two hemispheres called Northern and Southern
hemispheres.
Basic Earth-Sun angles
Meridian:
It is necessary to select some reference location on the earth for
helping in locating a particular position. An imaginary great circle
passing through the point and the two poles, intersecting the
equator at right angle is called prime meridian. The location through
which the prime meridian is passing is Greenwich (0o Longitude,
England.)
Longitude:
It is the angular distance of location, measured east or west
from the prime meridian. For example longitude of Bhopal
is 77o30' E.
Latitude:
It represents the angular location north or south of the
equator, north positive. The latitude of a point on theequator, north positive. The latitude of a point on the
surface of the earth is its angular distance north or south of
the equator measured from the center of the earth. Denoted
by φ
Declination angle (δ):
• The declination is theangular distance ofthe sun's rays north(or south) of theequator. It is theangle between a lineextending from theextending from thecenter of the sun andthe center of theearth and theprojection of this lineupon the earth'sequatorial plane.
Hour angle:
The hour angle is the angular distance between the meridian
of the observer and the meridian whose plane contains the
sun. The hour angle is zero at solar noon (when the sun
reaches its highest point in the sky) and increases by 15oevery
hour, morning negative, afternoon positive. The hour angle ishour, morning negative, afternoon positive. The hour angle is
the angle through which the earth must turn to bring the
meridian of a point directly in Line with the sun's rays.
Solar Altitude angle:
• The angle between the horizontal and line to the sun.
• Denoted by Sα
Solar Zenith angle:
• Angle between the vertical and line to the sun, i.e.
complement of the solar altitude angle
• Denoted byZθ
Solar Azimuth angle:
• It is the angle between the projection of sun’s ray to the point
on the horizontal plane and line due south passing through
that point.
• Value of azimuth angle is taken +ve when it is measured from
south towards west.
• Denoted by γ• Denoted by Sγ
Solar altitude angle
Surface azimuth angle:
The deviation of the
projection on a horizontal
plane of the normal to the
surface from the local
meridian, with zero due
south, east negative andsouth, east negative and
west positive;
Denoted by ;
γ oo 180180 ≤≤− γ
Slop or Tilt angle:
• The angle between inclined slop and horizontal plane.
• Denoted by
β
SOLAR THERMAL TECHNOLOGIES
Solar collector
Evacuated tube collector
Thermosyphon water heater
Forced circulation system
SWH- An Israil scene
Anticipated Savings from Solar Water
Heating Systems
• It is estimated that about 15 billion units of
electricity could be conserved over a period
of 20 years @750 kwh/sq.m of collector area,
year through deployment 1 million sq meter
of collector area implying a life cycle saviof collector area implying a life cycle savi
ng of over Rs.7,500 crore @ Rs.5/- per unit
of electricity (or Rs.75,000/ sq.m collector
area over a 20 year life cycle).
Anticipated Savings…cont’d
• Furthermore, one million sq.m of collector
area is capable of providing a theoretical
maximum peak saving of 500 MW. 1 sq.m
collector solar area can save around 60 litres collector solar area can save around 60 litres
of diesel per year or 900 litres over a 15 year
life cycle or Rs.27,000 worth of diesel
@Rs.30/litre. The payback period for such
systems is estimated at about 4–5 years.
A typical domestic SWH savings
Region North East South West
No of
Days
200 200 250 250
Days
Elc’Y
saving
(Units)
950 850 1200 1300
Solar power plant- parabolic system
Solar Tower power generation
1.291 mirrored heliostats and a 54 story high
tower the World's largest solar power tower
plant located near Seville in Spain in now on line
generating 20 megawatts (MW) of electricity,
enough to supply 10,000 homes.
Solar Chimney
What is the area required for a solar PV
power plant, per MW? For solar CSP?power plant, per MW? For solar CSP?
Area required fpr solar power generation•About 5 acres/ MW for solar PV (crystalline) and for 7-12
acres
for solar CSP (depending on the type of technology used)
•Within solar PV, it is assumed at 4-5 acres for crystalline
silicon (c-Si) technology and 7-8 acres per MW for thin film
solar (a-Si or CdTe) technology.solar (a-Si or CdTe) technology.
In reality, it depends on other parameters like cost of land,
Ground Coverage Ratio (GCR) (to avoid inter array shading,
GCR can be 0.45 to 0.65 and generation will vary based on
GCR) and choice of sun tracking systems
(with sun trackers the land required will be about 6 acres per
MW for crystalline solar modules).
Cost of solar power plants
• The capital investment for solar PV ranges from Rs
14 cr to 16 cr per MW depending on the technology.
The capital costs have come down significantly in
the last few years, and this cost is expected to
decrease further with technological advancements.decrease further with technological advancements.
• Capex for solar CSP is about Rs 12-13 crores MW,
but this is an approximate number, as the estimate
can differ widely based on the technology used.
Portable solar cooker
Concentrating cookers
Comm’y concentrating cooker
World’s largest solar cooker at Shirdi
PV Cell
• The physics of the PV cell is very similar to that
of the classical diode with a pn junction.
• When the junction absorbs light, the energy of
absorbed photons is transferred to the electron–absorbed photons is transferred to the electron–
proton system of the material, creating charge
carriers that are separated at the junction.
• The charge carriers may be electron–ion pairs in a
liquid electrolyte or electron–hole pairs in a solid
semiconducting material.
PV effect converts the photon energy
into voltage across the PN junction
• The charge carriers in the junction region create a
potential gradient, get accelerated under the
electric field, and circulate as current through an
external circuit.external circuit.
• The origin of the PV potential is the difference in
the chemical potential, called the Fermi level.
• What is it?????
• When they are joined, the junction approaches a new
thermodynamic equilibrium. Such equilibrium can be
achieved only when the Fermi level is equal in the
two materials.
• This occurs by the flow of electrons from one
material to the other until a voltage difference is
established between them, which has a potential just
equal to the initial difference of the Fermi level.
• This potential drives the photocurrent in the PV
circuit.
Basic construction of PV cell with
performance-enhancing features
SPV Technology
MODULE AND ARRAY
Merits of PV system
• Use of clean, cheap., noiseless, safe, renewable energy to
produce electricity at the location of utilization.
• Suitable for remote loads away from main electrical
network and at places where other fuels are scare and
costly. Cost of distribution lines can be eliminated.costly. Cost of distribution lines can be eliminated.
• Suitable for portable mobile loads eg radios, cars, buses,
space crafts etc.
• Reliable service, long 15 years life.
• Modest maintenance.
Limitations of PV system
• Irregular, intermittent supply of solar energy.
• Need for storage battries.
• High capital cost (Rs/kW) due to larger number of PVcell, low output power, low efficiency and hightechnology involved.technology involved.
• Not economical for central power plants of MW ratingdue to very large area of PV panels and very large energystorage system.
• Require storage batteries in large amount and dieselgenerator sets in cloudy wheather.
• Installed at roof tops only…
• Low efficiency.
EQUIVALENT ELECTRICAL CIRCUIT
Rs = 0 (no series loss), and Rsh = ∞ (no leakage to ground)
Rs varies from 0.05 to 0.10 Ω and Rsh from 200 to 300 Ω.Rs varies from 0.05 to 0.10 Ω and Rsh from 200 to 300 Ω.
The open-circuit voltage Voc of the cell is obtained when the load current is
zero, i.e., when I = 0, and is given by the following:
Voc = V + Irsh
The diode current is given by the classical diode current expression:
where
ID = the saturation current of the diode; Q = electron charge = 1.6×10–19 C
A = curve-fitting constant, k = Boltzmann constant = 1.38×10–23 J/°K
T = Temperature on absolute scale °K
)1(
Rsh; IgnoreInitially
0 =−−⇒
−=
IeII
III
L
t
qV
ph
dphL
k
d
γ )1ln(γ
−+−
=⇒ IRI
II
q
tV LS
LphkL
Mathematical Proof
)1ln(
)()1ln(
)1(
)1(
0
0
)(
0
0
+−
=+⇒
+=+
−⇒
=−−⇒
=−−⇒
+
I
II
q
tIRV
t
IRVq
I
II
IeII
IeII
LphkLSL
k
LSLLph
L
t
IRVq
ph
Lph
k
LSL
γ
γ
γ
much.... that reduced
not is Voc halfedIph if
ln(Iph), Voc As
)1ln(
0 circuit open if
)1ln(
0
0
α
γ+=⇒
==>
−+=⇒
I
I
q
tV
I
IRIq
V
phkOC
L
LSL
Rsh; Including
Mathematical Proof
L
sh
SLLRIVA
ph
L
sh
DAV
ph
shdphL
IR
RIVeII
IR
VeII
IIII
SLL
D
=+
−−−⇒
=−−−⇒
−−=
+)1(
)1(
)(
0
0
Current vs. voltage (I-V) characteristic of the PV
module in sunlight and in the dark
Power vs. voltage (P-V) characteristic of
the PV module in sunlight
I-V characteristic of a 22-W PV module at
full and half sun intensities
THE PV I–V CURVE UNDER
STANDARD TEST CONDITIONS (STC)
The I –V curve and power output for a
PV module
The maximum power point (MPP) corresponds to the biggest
rectangle that can fit beneath the I –V curve. The fill factor (FF) is
the ratio of the area (power) at MPP to the area formed by a
rectangle with sides VOC and ISC.
Fill factors around 70–75% for crystalline silicon solar modules
are typical, while for multi junction amorphous-Si modules, it is
closer to 50–60%.
Physics of Shading
Effect of Shading
• Consider the case when the bottom n − 1 cells
still have full sun and still some how carry
their original current I so they will still produce
their original voltage Vn−1. This means thattheir original voltage Vn−1. This means that
the output voltage of the entire module VSH
with one cell shaded will drop to,
Hot Spot Heating
If the operating current of the overall series string
approaches the short-circuit current of the "bad" cell,
the overall current becomes limited by the bad cell.
Example
Bypass Diodes for Shade Mitigation
• Figure shows a typical situation.
• In Fig. (a) a solar cell in full sun operating in
its normal range contributes about 0.5 V to the
voltage output of the module, but in thevoltage output of the module, but in the
equivalent circuit shown in (b) a shaded cell
experiences a drop as current is diverted
through the parallel and series resistances.
In full sun a cell may contribute around 0.5 V to the module
output; but when a cell is shaded, it can have a large voltage drop
across it
Need of bypass diode
• The voltage drop problem in shaded cells couldbe to corrected by adding a bypass diode acrosseach cell, as shown in Figure.
• When a solar cell is in the sun, there is a voltagerise across the cell so the bypass diode is cut offrise across the cell so the bypass diode is cut offand no current flows through it—it is as if thediode is not even there.
• When the solar cell is shaded, however, the dropthat would occur if the cell conducted any currentwould turn on the bypass diode, diverting thecurrent flow through that diode.
Mitigating the shade problem with a bypass diode.
In the sun (a), the bypass diode is cut off and all the normal
current goes through the solar cell. In shade (b), the bypass diode
conducts current around the shaded cell, allowing just the diode
drop of about 0.6 V to occur
How improved in I – V curve
Showing the ability of bypass diodes to mitigate
shading when modules are charging a 65 V battery.
Without bypass diodes, a partially shaded module
constricts the current delivered to the load (b). With constricts the current delivered to the load (b). With
bypass diodes, current is diverted around the shaded
module.
Blocking Diodes
• Bypass diodes help current go around a shaded or
malfunctioning module within a string. This not only improves
the string performance, but also prevents hot spots from
developing in individual shaded cells.
• When strings of modules are wired in parallel, a similar• When strings of modules are wired in parallel, a similar
problem may arise when one of the strings is not performing
well.
• Instead of supplying current to the array, a malfunctioning or
shaded string can withdraw current from the rest of the array.
By placing blocking diodes (also called isolation diodes) at the
top of each string as shown in Fig., the reverse current drawn
by a shaded string can be prevented.
Blocking diodes prevent reverse current from flowing
down malfunctioning or shaded strings