SOLAR PHOTOVOLTAIC
B.Tech 8th Semester Date: 05/12/2017
Electrical Engineering Department
Visvesvaraya National Institute of Technology Nagpur
Solar Energy
The energy received on the earth from sun is termed as
‘Solar Energy’ and it reached in the form of radiation
The amount of solar energy available on the earth’s surface is
considerably smaller than that arriving at the top of the
atmosphere
Atmospheric constituents affect solar radiation by two
processes – absorption and scattering.
Depends on the composition of the atmosphere as well as on
the wavelength of the component of the solar spectrum
The Sun
The Sun is a sphere of hot gaseous matter with a diameter of
1.39 x 109 m and is on the average distance of 1.5 x 1011 m
from the earth
The Sun has effectively a black body temperature of 5762 K
and density 100 times that of water
The Sun is, in effect a continuous fusion reactor in
which four hydrogen atoms combines to form one helium
atom, mass of helium nucleus is less than that of hydrogen,
some mass having been lost in the reaction and converted to
energy
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The Earth
The earth as a sphere with a diameter of nearly 12800 km
The sun subtends an angle of only 32 minutes (0.53o) at the
earth surface
Thus, the beam radiation received from the sun on the earth
is almost parallel
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Ray of the Sun
The total power emitted from the sun is composed not of a single
wavelength, but is composed of many wavelengths
Different wavelengths show up as different colours, but not all the
wavelengths can be seen since some are "invisible" to the human eye.
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Solar constant
The sun’s output energy is the same, day to day, month to month,
year to year, century to century.
On any day of the year, at any particular location, the amount of
sunlight is the same as last year’s.
The energy flux received from the sun outside the earth’s
atmosphere is essentially constant
The Solar Constant is the rate at which energy is received from the
sun on a unit area perpendicular to the rays of the sun, at the mean
distance of the earth from the sun
Many experimental investigations suggest the value of solar
constant as 1367 W/m2
The solar constant includes all types of solar radiation and not just
the visible light
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Solar radiation through the
atmosphere
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Solar radiation at the earth’s surface
Solar radiation are subjected to the mechanisms of absorption
and scattering as it passes through the earth’s atmosphere
Absorption occurs primarily due to presence of ozone and
water vapour in the atmosphere and to a lesser extent due to
other gases (like CO2, NO2, CO, O2 and CH4) and
particulate matter
Scattering occurs due to all gaseous molecules as well as
particulate matter in the atmosphere
The scattering radiation is redistributed in all directions,
some going back into the space and some reaching the earth’s
surface
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Solar radiation or insolation is made up of various wavelengths mostly in the range of
0.22.5 microns with a peak at 0.5 microns.
mml@eee.vnit.ac.in http://commons.wikimedia.org/wiki/File:Solar_Spectrum.png
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Irradiance
It is an amount of solar energy received on a unit surface
expressed in units of kWh/m2
Solar irradiance is essentially the solar insolation (power)
integrated with respect to time
When solar irradiance data is represented on an average daily
basis, the value is often called PEAK SUN HOURS (PSH)
and can be thought of as the number of equivalent hours/day
that solar insolation is at its peak level of 1kW/m2.
The worldwide average daily value of solar irradiance on
optimally oriented surfaces is approximately 5 kWh/m2 or 5
PSH. Solar irradiance is denoted by ' H '.
Direct or beam radiation
Solar radiation received at the earth’s surface without change
of direction (i.e. in the line of sun), is called beam or direct
radiation
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Diffuse radiation
The radiation received at the earth’s surface from all parts of
the sky’s hemisphere (after being subjected to scattering in
the atmosphere) is called diffuse radiation
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Total or Global radiation
The sum of the beam and diffuse radiation is referred to as
total or global radiation
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Earth-Sun geometry
The Earth (and other planets) move round the
Sun in a elliptical orbit, a journey which takes a
year to complete.
Once a day, the Earth rotates around its axis.
The Earth's axis is tilted relative to the orbital
plane.
These factors combine to give us day, night, the
seasons
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Lambert's Cosine Law
The energy received by a point on the Earth's surface from beam
radiation is determined by the angle of the beam to the surface.
When θ is zero (sun overhead), cos(θ) is 1, the point receives
maximum radiation, when it 90 deg. (sunrise and sunset), cos(θ)
is zero and the point receives no radiation.
Thus to derive the solar radiation received, we need two
parameters, IB and θ.
I = IB cos(θ)
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The tilt of the earth’s spin axis with respect to the ecliptic
plane is what causes our seasons. “Winter” and “summer” are
designations for the solstices in the Northern Hemisphere
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Solar declination, δ.
The angle formed between the plane of the equator and a line
drawn from the center of the sun to the center of the earth is
called the solar declination, δ.
It varies between the extremes of ± 23.45◦
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Solar Declination δ for the 21st Day of
Each Month (degrees)
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A south-facing collector tipped up to an angle equal to its
latitude is perpendicular to the sun’s rays at solar noon during
the equinoxes
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The altitude angle of the sun at solar noon
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Ans= 40.4
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Two ways of using solar energy
Solar thermal systems
Direct heating system (solar cooker)
Heat exchange operating system
Photovoltaic systems
Solar cells
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Introduction to photovoltaic
A material or device that is capable of converting the energy
contained in photons of light into an electrical voltage and
current is said to be photovoltaic
A photon with short enough wavelength and high enough
energy can cause an electron in a photovoltaic material to
break free of the atom that holds it.
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History
Albert Einstein published a theoretical explanation of the
photovoltaic effect in 1904, which led to a Nobel Prize in 1923
In 1839 Edmond Becquerel accidentally discovered photovoltaic
effect when he was working on solid-state physics.
In 1878 Adam and Day presented a paper on photovoltaic effect.
1n 1883 Fxitz fabricated the first thin film solar cell.
In 1941 ohl fabricated silicon PV cell but that was very inefficient.
In 1954 Bell labs Chopin, Fuller, Pearson fabricated PV cell with
efficiency of 6%.
In the 1950s there were several attempts to commercialize PVs,
but their cost was prohibitive.
In 1958 PV cell was used as a backup power source in satellite
Vanguard-1. This extended the life of satellite for about 6 years.
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History .... (cont..)
Spurred by the emerging energy crises of the 1970s, late
1980s, higher efficiencies and lower costs brought PVs closer
to reality, and they began to find application in many offgrid
terrestrial applications such as pocket calculators, off-shore
buoys, highway lights, signs and emergency call boxes, rural
water pumping, and small home systems.
By 2002, worldwide production of photovoltaics had
approached 600 MW per year and was increasing by over
40% per year
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Why PV?
Its strong points are:
direct conversion of solar radiation into electricity,
no mechanical moving parts, no noise,
no high temperatures,
no pollution,
PV modules have a very long lifetime,
the energy source, the sun, is free, ubiquitous, and
inexhaustible,
PV is a very flexible energy source, its power ranging from
microwatts to megawatts.
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BASIC SEMICONDUCTOR PHYSICS
Photovoltaics use semiconductor materials to convert
sunlight into electricity.
The technology for doing so is very closely related to the
solid-state technologies used to make transistors, diodes
The starting point for photovoltaic devices, as well as almost
all semiconductors, is pure crystalline silicon.
fourth column of the periodic table
boron and phosphorus, from Groups III and V, are added to
silicon to make most PVs
Gallium and arsenic are used in GaAs solar cells, while
cadmium and tellurium are used in CdTe cells.
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The Portion of the Periodic Table of Greatest Importance for
Photovoltaics Includes the Elements Silicon, Boron, Phosphorus,
Gallium, Arsenic, Cadmium, and Tellurium
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BASIC SEMICONDUCTOR PHYSICS (Cont..)
Silicon has 14 protons and electrons as in (a).
A convenient shorthand is drawn in (b), in which only the four
outer electrons are shown, spinning around a nucleus with a +4
charge.
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BASIC SEMICONDUCTOR PHYSICS (Cont..)
Crystalline silicon forms a three-dimensional tetrahedral structure (a);
but it is easier to draw it as a two-dimensional flat array (b).
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BASIC SEMICONDUCTOR PHYSICS (Cont..)
Energy bands for (a) metals and (b) semiconductors. Metals have partially
filled conduction bands, which allows them to carry electric current easily.
Semiconductors at absolute zero temperature have no electrons in the
conduction band, which makes them insulators.
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BASIC SEMICONDUCTOR PHYSICS (Cont..)
The gaps between allowable energy bands are called
forbidden bands, the most important of which is the gap
separating the conduction band from the highest filled band
below it.
The energy that an electron must acquire to jump across the
forbidden band to the conduction band is called the band-gap
energy, designated
The units for band-gap energy are usually electron-volts
(eV), where one electron-volt is the energy that an electron
acquires when its voltage is increased by 1 V (1 eV = 1.6 ×
10−19 J).
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BASIC SEMICONDUCTOR PHYSICS (Cont..)
The band-gap Eg for silicon is 1.12 eV, which means an electron
needs to acquire that much energy to free itself from the
electrostatic force that ties it to its own nucleus—that is, to
jump into the conduction band.
The band gap (EG) is the gap in energy between the bound
state and the free state, between the valence band and
conduction band. Therefore, the band gap is the minimum
change in energy required to excite the electron so that it can
participate in conduction.
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BASIC SEMICONDUCTOR PHYSICS (Cont..)
A photon with sufficient energy can create a hole–electron pair
as in (a).
The electron can recombine with the hole, releasing a photon of
34 energy (b).
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BASIC SEMICONDUCTOR PHYSICS (Cont..)
When a hole is filled by a nearby valence electron, the hole
appears to move.
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BASIC SEMICONDUCTOR PHYSICS (Cont..)
An n-type material. (a) The pentavalent donor. (b) The representation of
the donor as a mobile negative charge with a fixed, immobile positive
charge.
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BASIC SEMICONDUCTOR PHYSICS (Cont..)
In a p-type material, trivalent acceptors contribute movable, positively
charged holes leaving rigid, immobile negative charges in the crystal lattice
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BASIC SEMICONDUCTOR PHYSICS (Cont..)
(a) When a p–n junction is first formed, there are mobile holes in the
p-side and mobile electrons in the n-side.
(b) As they migrate across the junction, an electric field builds up that opposes, and quickly
stops, diffusion.
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Diode
Id is the diode current in the direction of the arrow (A),
Vd is the voltage across the diode terminals from the p-side to the n-side (V),
I0 is the reverse saturation current (A),
q is the electron charge (1.602 × 10−19C),
k is Boltzmann’s constant (1.381 × 10−23 J/K), and
T is the junction temperature (K).
n is the ideality factor
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Diode
A junction temperature of 25◦C is often used as a standard, which results in the
following diode equation
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p–n junction when it is exposed
to sunlight
Photons are absorbed, hole-electron pairs are formed.
If these mobile charge carriers reach the vicinity of the junction, the
electric field in the depletion region will push the holes into the p-side
and push the electrons into the n-side,
The p-side accumulates holes and the n-side accumulates electrons, which
creates a voltage that can be used to deliver current to a load.
If electrical contacts are attached to the top and bottom of the cell,
electrons will flow out of the n-side into the connecting wire, through the
load and back to the p-side
Since wire cannot conduct holes, it is only the electrons that actually
move around the circuit.
When they reach the p-side, they recombine with holes completing the
circuit. By convention, positive current flows in the direction opposite
to electron flow
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p–n junction when it is exposed
to sunlight
When photons create hole–electron pairs near the junction, the electric
field in the depletion region sweeps holes into the p-side and sweeps
electrons into the n-side of the cell.
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p–n junction when it is exposed
to sunlight
Electrons flow from the n-side contact, through the load, and back to
the p-side where they recombine with holes. Conventional current I is in
the opposite direction.
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Photon energy
Photons with enough energy create hole–electron pairs in a
semiconductor.
Photons can be characterized by their wavelengths or their
frequency as well as by their energy; the three are related by
the following
c = λν
where c is the speed of light (3 × 108 m/s), v is the frequency
(hertz), λ is the wavelength (m), and
E = hν = hc/λ
Where E is the energy of a photon (J) and h is Planck’s constant
(6.626 × 10−34 J-s).
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Problem
Example : Photons to Create Hole–Electron Pairs in Silicon What
maximum wavelength can a photon have to create hole–electron pairs
in silicon? What minimum frequency is that? Silicon has a band gap of
1.12 eV and 1 eV = 1.6 × 10−19 J.
Solution:
the wavelength must be less than
the frequency must be at least
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Photons with wavelengths above 1.11 μm don’t have the 1.12 eV needed to
excite an electron, and this energy is lost. Photons with shorter wavelengths have more than
enough energy, but any energy above 1.12 eV is wasted as well.
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Theoretical efficiency
20.2% of the energy in the spectrum is lost due to photons
having less energy than the band gap of silicon (hν < Eg), and
another 30.2% is lost due to photons with hν > Eg.
The remaining 49.6% represents the maximum possible fraction of
the sun’s energy that could be collected with a silicon solar
cell.
That is, the constraints imposed by silicon’s band gap limit
the efficiency of silicon to just under 50%.
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Other factors that contribute to the
drop in theoretical efficiency
Only about half to two-thirds of the full band-gap voltage
across the terminals of the solar cell.
Recombination of holes and electrons before they can
contribute to current flow.
Photons that are not absorbed in the cell either because they
are reflected off the face of the cell, or because they pass
right through the cell, or because they are blocked by the
metal conductors that collect current from the top of the
cell.
Internal resistance within the cell, which dissipates power
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A simple equivalent circuit
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VI characteristics
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Variation of VI characteristics with
irradiance
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Effect of temperature
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Effect of temperature
The decrease in the band gap of a semiconductor with
increasing temperature can be viewed as increasing the
energy of the electrons in the material.
Lower energy is therefore needed to break the bond.
reduction in the bond energy also reduces the band gap.
Therefore increasing the temperature reduces the band gap
The open-circuit voltage decreases with increase in
temperature because of the temperature dependence of I0
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Fill Factor (FF)
The FF is defined as the ratio of the maximum power from
the solar cell to the product of Voc and Isc.
Graphically, the FF is a measure of the "squareness" of the
solar cell
It is the area of the largest rectangle which will fit in the IV
curve.
Ideally, the fill factor should be 1 or 100%. However, the
actual value of FF is about 0.8 or 80%. A
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Problem
For the simple equivalent circuit for a 0.005 m2
photovoltaic cell shown below, the reverse saturation
current is I0 = 10−9 A and at an insolation of 1-sun the
short-circuit current is ISC = 1 A,. At 25◦C, find the
following:
The open-circuit voltage.
The load current when the output voltage is V = 0.5V.
The power delivered to the load when the output voltage is 0.5 V.
The efficiency of the cell at V = 0.5V.
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Effect of series and shunt resistances
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Series resistance
The contact resistance between the metal contact and the
silicon; and
the resistance of the top and rear metal contacts.
The main impact of series resistance is to reduce the fill
factor, although excessively high values may also reduce the
short-circuit current.
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Schematic of a silicon solar cell.
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Shunt resistance
The shunt resistance (Rsh) is due to p-n junction non-
idealities and impurities near the junction, which cause
partial shorting of the junction, particularly near cell edges.
Low shunt resistance causes power losses in solar cells by
providing an alternate current path for the light-generated
current. Such a diversion reduces the amount of current
flowing through the solar cell junction and reduces the
voltage from the solar cell.
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Equivalent circuit of PV cell
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Mathematical model of PV cell
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Problem
The equivalent circuit for a PV cell includes a parallel resistance
of RP =10 . The cell has area 0.005 m2, reverse saturation current
of I0 = 10−9 A and at an insolation of 1-sun the short-circuit
current is ISC = 1 A, At 25◦C, with an output voltage of 0.5 V, find
the following:
The load current.
The power delivered to the load.
The efficiency of the cell.
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Types of solar cell
The most important parameters of a semiconductor material
for solar cell operation are:
the band gap;
the number of free carriers (electrons or holes) available for
conduction; and
the "generation" and recombination of free carriers (electrons
or holes) in response to light shining on the material.
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Types of solar cells
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http://mcensustainableenergy.pbworks.com/w/page/20638168/Solar%20Photovoltaics
Types of solar cells
Amorphous silicon /
thin film silicon cells
Monocrystalline silicon cell Polycrystalline silicon cell
Made from cells cut from an Made by depositing
Made using cells saw-cut
ingot of melted and silicon in a thin
from a single cylindrical
recrystallised silicon homogenous layer onto
crystal of silicon
a substrate
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Commercial solar cells technology,
materials and efficiency
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Modules and Arrays
http://pveducation.org/
EVA (ethyl vinyl acetate)
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Series and parallel connected solar cells
Combined IV characteristics ?
If characteristics of the cells are mismatched.
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Mismatching of solar cell
Mismatch losses are caused by the interconnection of solar cells
or modules which do not have identical properties or which
experience different conditions from one another.
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Shading of Solar Cell
If the series string is short circuited, then the forward bias across all of
these cells reverse biases the shaded cell. Hot-spot heating occurs
when a large number of series connected cells cause a large reverse
bias across the shaded cell, leading to large dissipation of power in the
poor cell. Essentially the entire generating capacity of all the good cells
is dissipated in the poor cell.
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Bypass and blocking diode
http://www.electronicshub.org/bypass-diodes-in-solar-panels/
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Examples of PV Module Performance Data Under
Standard Test Conditions (1 kW/m2, AM 1.5, 25◦C Cell
Temperature)
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AIR MASS RATIO
the length of the path taken by the sun’s rays through the
atmosphere to reach a spot on the ground, divided by the
path length corresponding to the sun directly overhead, is
called the air mass ratio,
air mass ratio of 1 (designated “AM1”) means that the sun is
directly overhead.
By convention, AM0 means no atmosphere; that is, it is the
extraterrestrial solar spectrum.
For most photovoltaic work, an air mass ratio of 1.5,
corresponding to the sun being 42 degrees above the
horizon, is assumed to be the standard.
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IMPACTS OF TEMPERATURE AND
INSOLATION ON I–V CURVES
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IMPACTS OF TEMPERATURE AND
INSOLATION ON I–V CURVES
As cell temperature increases, the open-circuit voltage
decreases substantially while the short-circuit current
increases only slightly
For crystalline silicon cells, VOC drops by about 0.37% for each
degree Celsius increase in temperature and ISC increases by
approximately 0.05%
net result when cells heat up is the MPP slides slightly
upward and toward the left with a decrease in maximum
power available of about 0.5%/◦C
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IMPACTS OF TEMPERATURE AND
INSOLATION ON I–V CURVES
To help system designers account for changes in cell
performance with temperature, manufacturers often provide
an indicator called the NOCT, which stands for nominal
operating cell temperature.
The NOCT is cell temperature in a module when ambient is
20◦C, solar irradiation is 0.8 kW/m2, and windspeed is 1
m/s.
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IMPACTS OF TEMPERATURE AND
INSOLATION ON I–V CURVES
where Tcell is cell temperature (◦C),Tamb is ambient temperature, and S is
solar insolation (kW/m2).
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Pyranometer
Pyranometers are instruments for measuring global radiation
(direct and diffuse).
The detectors of these instruments must have a response
independent of the wavelength of radiation over the solar
energy spectrum.
The detectors convert the solar radiation into an electrical
voltage, which is an indicator for the solar radiation.
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Pyrheliometer
A pyrheliometer is an instrument for measurement of
direct beam solar irradiance. Sunlight enters the instrument
through a window and is directed onto a thermopile which
converts heat to an electrical signal that can be recorded
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Sunshine recorder
It consists of a solid glass
sphere as a lens
A strip of inflammable paper is
mounted around the
appropriate part of the sphere,
and the solar image burns a
mark on the paper whenever
the beam radiation is above a
critical level.
The lengths of the burned
portions of the paper gives and
index of the duration of bright
sunshine.
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Sunshine Recorder ….(conti…)
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Solar PV inverters
They are classified into
Centralized inverters
String inverters
Multi string inverter and
Module integrated inverter.
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Centralized Inverters
A single, large inverter is connected to many PV modules
wired in series to form strings with up to 600 V/1,000 V of
open-circuit voltage. All the solar PV modules are connected
in strings, generating a sufficient high voltage to avoid
amplification and the strings are connected in parallel to
support high power to output.
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Drawbacks of Centralize Inverter
The involvement of high voltage DC cable from the strings to the inverter and
losses in string diodes.
This structure is also limited from Maximum Power Point (MPP) Tracking and
controlling mismatch between strings so individual PVs, resulting in low
efficiency and reliability.
This topology is not flexible and this makes it less appealing in mass production.
Central inverters cannot monitor the performance of individual PV modules, so
damaged or otherwise compromised modules often go undetected.
With all these issues, this technology is not used in new solar PV systems
installation.
Finally, central inverters limit the design and site selection of PVsystems,
particularly in residential applications.
They require co-planar module layouts and a lack of partial shading from
chimneys, trees, vent pipes, etc. PV installers may opt out of half or more of
potential sites due to these restrictions
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String Inverter
String inverters have to be connected
to a limited and relative high number
of PV modules.
The installation is easy and suitable for
modular expansion.
The energy harvest of each PV module
can be optimized because each PV
module performs at its MPP.
All DC connections are extremely
short and are loaded with very low
DC voltages and currents.
This reduces the risk of fire or the risk
of a short circuit during or after a fire.
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PV with Microinverter
One PV module works together with one PV inverter, often
named as PV micro inverter.
This micro inverter has already the most important
functionalities of a PV central or string inverter like MPP
Tracking and DC to AC conversion over a wide input voltage
range and a galvanic isolation.
The significant smaller input voltages and currents result
in low DC input power and AC power output power
and so in a device with small dimensions and low weight.
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PV with Microinverter
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PV module with power optimizers
Power Optimizers are small DC-DC converters - outside of
the PV inverter device- and act as MPP Trackers per PV
module. This is the
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The disadvantages of a PV micro inverter are
- higher initial equipment cost per Wp than the equivalent
power of a central or string inverter [2]
- increased installation time since each inverter needs to
be installed adjacent to its PV module
- more complicated maintenance, removing or replacing.
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Problem
A residential house has a power requirement of 400 W for 4 hours
every night. It is proposed to meet the requirement by using a PV
array, a battery storage system and an inverter. The whole system
is over designed to that it can meet one extra night’s requirment
even if there has been no sunshine during the day. Calculate the
number of PV modules and batteries required.
Given: i) Solar radiation is available for an average of six hours
daily and the average hourly global radiation flux incident on the
array is 650 w/m2.
ii) Battery rating = 12 V; 120 Ah. Depth of discharge = 0.7 and
charging and discharging efficiency = 0.9
ii) inverter efficiency at full load = 0.85
iv) module conversion efficiency = 10 %, module area 1.191 X
0.533
Problem
How many minimum number of batteries of rating 100 Ah will
be charged with 10 solar panels of the following rating for 6
hours average radiation of 1000 W/m2:
Pmax= 75 W, Vmax=15 V, Imax= 5 A, Dimensions: 1m × 0.7m,
efficiency 10 %
Assume battery charging voltage = 12 V, Charging efficiency of
the battery = 9
Problem
Two solar panels containing 36 single crystal silicon cells,
12.5 cm in diameter and conversion efficiency of 13.8 %
are connected to the battery system through a charge
controller. Calculate the number of batteries that can be
charged from these panels.
Given:
Battery rating: 10 Ah, 6 V
Battery charging and discharging efficiency of 90 %.
Charge controller efficiency is 94 %.
Average hourly global radiation flux incident on the array is 860
W/m2 for average six hours daily.
94 mml@eee.vnit.ac.in
Problem
For the simple equivalent circuit for a 0.005 m2
photovoltaic cell, the reverse saturation current is I0 =
10−9 A and at an insolation of 1-sun the short-circuit
current is ISC = 1 A. At 25◦C, find the following:
Open circuit voltage
The load current when the output voltage is V = 0.5V.
The power delivered to the load when the output voltage is 0.5
V.
The efficiency of the cell at V = 0.5V.
Given: Boltzmann’s constant = 1.38 × 10-23, Charge of
electron = 1.6 × 10-19
95 mml@eee.vnit.ac.in
MNRE roof top solar pv calculator
http://solarrooftop.gov.in/Grid/financial_tool/1
96 mml@eee.vnit.ac.in
97 mml@eee.vnit.ac.in
