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SARHAD UNIVERSITY OF SCIENCE &INFORMATION TECHNOLOGY,
PESHAWARfaculty of under Graduate Studies
Techno Economic Modeling And simulation of Off-GridPhotovoltaic
(PV) electricity generation system
Group Members :
Syed Bilal Ahmad Madni (SUIT-11-01-008-0008)
Muhammad Shoaib (SUIT-11-01-008-0005)
Mehran Khan (SUIT-11-01-008-0003)
Discipline: BS Electronics
Supervised by: Engineer. Danial Naeem
This report is submitted in partial fulfillment of the
requirements of the degree ofBS in Electronics field.
Faculty of Under Graduate studies, at Sarhad University of
science andinformation Technology, Peshawar 2015
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Acknowledgement: We truly acknowledge the cooperation and help
make by Mr Danial Naeem,and Waqas Ali Lecturer and supervisor at
Comwave institute Haripur. They have been a constant source of
guidance throughout the course of this project. We would also like
to thank Mr sajid Ali from Comwave intitute Haripur for his help
and guidance throughout thisProject.
We are also thankful to my friends and family whose silent
support led me to complete myProject.
1- Name of friend 2- Name of friend ...
Dated:
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TECHNO-ECONOMIC MODELLING &SIMULATION OF OFF-GRID
PHOTOVOLTAIC
(PV) ELECTRICITYGENERATION SYSTEM
This report was successfully submitted toSarhad University Of
Science And Information Technology
Peshawar
External supervisor Internal SupervisourName of external SUIT,
Peshawar DesignationName of organization
Declaration:
This is certify that, Students Name:Syed Bilal Ahmad Madni
Registration No: SUIT-11-01-008-000 Students Name: Mehran Khan
Registration No:SUIT-11-01-008-0008 Students name: Mohammad Shoaib
Registration No:SUIT-11-01-008-0008has successfully completed the
final project named as TECHNO-ECONOMIC MODELLING & SIMULATION
OF OFF-GRID PHOTOVOLTAIC (PV) ELECTRICITY GENERATION SYSTEM at the
Sarhad University of Science & Information Technology,
Peshawar, to fulfill the partial requirement of the degree of
___________________.
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Abstract:
Photovoltaic that is an important topic which can be studied and
researched in Pakistan because
of availability of solar energy potential at full day time. This
report deals with simulations and
designs of off -grid photovoltaic system for an electrical home
loads. It provides theoretical
studies of photovoltaic and modeling techniques using equivalent
electric circuits .the report
includes maximum obtains of solar power in simulation software
Power sim to verify the DC to
AC conversion with battery charge controller. The inverter model
that would be chosen for
generating square wave as an output to hold the 500watt load of
a house
In this report 7056 watt-hour /day energy load of a house. In
this system Poly crystalline PV
modules are used each module has rated power of 150watt and
inverter has rated output of
650watt, 1000KVA with 9kwh storage capacity of battery is
included in this system
The economic feasibility design of OFF-Grid PV system is
simulated by using RET screen
Based on economic evaluation grid tie system is as 22Rs/kwh and
the cost of energy generated
by stand-alone PV power generation system is 16Rs/kwh.
Chapter 1 Introduction:
The basic idea of a solar cell is to convert light energy into
electrical energy, the energy of light is transmitted by photons,
small packets or quantum of lights, electrical energy is stored in
electromagnetic fields, which in turn make a current of electrons
flow, thus the solar cells converts light, a flow of photons, to
electric current, a flow of electrons. The development of solar
cell technology begins with the 1839 research of French physicist
Antoine-Cesar Becquerel, he observed the photovoltaic effect while
experimenting with the solidelectrode in an electrolyte solution,
when he saw a voltage develop when light fell upon the
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electrode. In 1941, the silicon solar cell was invented by
Russell Ohl. In 1954, three American researchers Gerald Pearson,
Calvin fuller and Daryl Chapin designed a silicon solar cell
capable of a six percent energy conversion efficiency with direct
sun light the three inventors created an array of several strips of
silicon, (each about the size of a razor blade) Placed them in
sunlight, captured the free electrons and turned them into electric
current.
The world energy consumption and the resulting carbon dioxide
emission is increasing simultaneously and this increase puts in
danger the environmental stability of our earth, making this an
important topic in a society, both in political and social aspects.
The energy production has mainly been based on energy source like
oil, gas and coal. Which until recently looked upon as close an
inexhaustible. As World energy consumption is growing with a high
rateand the fossil fuels reserves are decreasing need of renewable
energy source is much important for research. The Sun is
non-polluting resource responsible for the sustained life on earth
and cangive us efficient renewable energy in Pakistan, because a
potential is available.
In this master thesis a standalone PV system will be studied. A
standalone PV system is used in the places where no electrical grid
is available. The PV system will utilize the solar energy as
thepower source and transfer the power into the battery through
conditioning by power electronics, after that the energy is stored
in battery then converted by another stage of power electronics to
be used in a home load. The power electronics is an essential part
of a PV system, and it is necessary to understand how to utilize
and control this part for optimization of thepower generation. This
issue will support teaching in control of power electronics,
through learning many control strategies and know the suitable
parameters to obtain more efficient performance for a standalone PV
system by using PSIM and RET Screen program and simulate this
system on real conditions. To achieve this objective the
mathematical models were studied which characterize each part of
system such as PV module, DC-AC converters,Charge
controller,battery and inverter. After that the suitable rated
value for all components in a standalone PV system was calculated
according to the energy consumption in kWh for a home load. In
addition, economical study wasmade to know the cost in Rs/ kWh for
a standalone PV system and compare it with other PV system which is
a grid tie PV system considering the feed in tariff. Photovoltaic
offers to consumers the ability to generate electricity in clean,
quiet and reliable way. Photovoltaic system is comprised of
photovoltaic cells, devices that convert light energy directly into
electricity because the source of light is usually the sun; they
are often called solar cells. The word photo meaning light and
Voltaic which refers to producing electricity, therefore the
photovoltaic process is producing electricity directly from
sunlight. Photovoltaic is also known as PV. In chapter one, the
study begins with an introduction to this thesis, it gives
information about objective, procedure and the main outline of the
research.
The model of PV system is implemented Using PSIM (Power
Simulation) software to study and simulate real Off-Grid PV
system.
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.1. Photovoltaic cells: The photovoltaic (PV) cell is basically
a pn junction with a central depletion region. At theend of each
zone an electrical contact is placed. The more heavily doped zone
is called theemitter zone and the other is the base zone. This last
region is also called the absorber regionbecause the great part of
incident light is absorbed here. Differently from a diode, the PV
cellis designed so to allow holeselectrons couples to be generated
inside the junction due toincident light. The aim of this section
is to define the law that ties voltage and current of a PV
cellincluding the dependence on incident light. Solar cells are
made from semiconductorMaterials (PN junction) usually silicon
which are specially treated to from an electric field,Positive on
one side (backside) and negative on the other (towards the sun).
When solarenergy (photons) hits the solar cell, electrons are
knocked loose from the atoms in thesemiconductor material, creating
electron hole-pairs.if electrical conductors are then attached to
the positive and negative sides, forming anelectrical circuit, the
electrons are captured in the form of electric current (photo
current)
Figure 1.1 Photovoltaic Cell
A typical PV cell made of crystalline silicon is 12 centimeters
in diameter and 0.25 millimeter thick, in full sunlight it
generates4 amperes of direct current at 0.5 volt or 2 wattsof
electrical power
.2 Types of Photovoltaic cell: There are essentially two types
of PV technology. Crystalline and thin-film. Crystalline can again
be classified into two types.
1. Mono crystalline cells
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2. Polycrystalline cells
Mono crystalline cells
These are made of cells cut from a signal cylindrical crystal of
silicon. While mono crystalline cells offers the highest efficiency
(approximately 18% conversion of incident sunlight), their complex
manufacturing process makes them slightly more expensive
Polycrystalline cellsthese are made by cutting micro-fine wafers
from ingot of molten re-crystallized silicon. Polies crystalline
are cheaper to produce, but there is a slight compromise on
efficiency (approximately14% conversion of incident sunlight) Thin
film PV is made by depositing an ultrathin layer of photovoltaic
material onto a substrate. The most common type of thin-film PV is
made from the material a-Si (amorphous-Silicon), but numerous other
materials such as CIGS (copper Indium/galliumselenide) CIS (copper
indium selenide, CdTe (Cadmium Telluride) The efficiency of this
types varies approximately in the range from 2% - 10% .
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a) Mono-crystalline PV b) Poly-crystalline PV c) amorphous PV
Figure 1.2 Mono crystalline PV
1.3 The Photovoltaic Array:
If an output voltage and a current from a single module is
smaller than desired, the modules can
be connected into arrays, the connection methods depends on
which variable that need to be
increased. For a higher output voltage the modules must be
connected in series while connecting
them in parallel in turns gives higher currents, it is important
to know the rating of each module
when creating an array, the highest efficiency of the system is
achieved when the MPP
(maximum power point) of each of the modules occur at the same
voltage level.
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Figure 1.3 Cell, Module and Array
2.4 Physics of Photovoltaic Cells:
2.4.1 The Photoelectric Effect
The transformation of the radiated energy coming from the Sun
into electrical energy implies thestudy of the interaction of
electromagnetic waves with matter. This mechanism can beunderstood
starting from the photoelectric effect in which electrons are
emitted from a materialwhen it is exposed to electromagnetic
radiation. In particular, it was observed that (using visiblelight
for alkali metals, near ultraviolet for other metals, and extreme
ultraviolet for non-metals)the energy of emitted electrons
increased with the frequency and did not depend on the intensityof
the radiation. This effect was first observed by Heinrich Hertz in
1887 and for several years itwas apparently in contrast with James
Clerk Maxwells wave theory of light; according to thistheory, the
electron energy would be proportional to the intensity of the
radiation. The followingmain experimental results, for given
material, were observed:
1. The rate at which photoelectrons are ejected is directly
proportional to the intensity of the
incident light
2. A threshold frequency, below which no photoelectrons are
emitted, exists
3. Above the threshold frequency, if the intensity of light is
increased, the number of
emitted electrons is increased as well but their maximum energy
does not vary; moreover
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very low intensity of incident light, with frequency greater
that the threshold, is able to
extract electrons;
4. Above the threshold frequency, if the frequency of incident
light is increased, the
maximum energy of photoelectrons is also increased; Albert
Einstein theorized, in 1905,
that light is composed of discrete quanta, now called photons,
and that the energy of a
quantum of light is given by the product of the frequency of the
corresponding wave
multiplied by a constant, later called Plancks constant.
2.4.2 Conductors, Semiconductors, Insulators:
In a single isolated atom, energy levels of electrons are
discrete. For hydrogen atom, the Bohrs
model gives:
(1.1)
Where mo is the free electron mass and q its charge, e2o is the
free space permittivity and n is a
positive integer known as principal quantum number. The
fundamental level corresponds to n=1
and the related energy is
If N atoms interact (for example in a crystal), N outer levels
have energy only slightly different
and thermal energy allows electrons to pass from one level to
another (the energy corresponding
to T = 300 K is kT & 0.026 eV). Resulting energy levels are
grouped in bands. Two main bands
are recognizable: conduction band and valence band. These two
bands are separated by a
forbidden region that is characterized by an energy value Eg.
This value makes the difference
among insulators, conductors, and semiconductors. In an
insulator, the forbidden band has a wide
energy (for example Eg = 9 eV for SiO2) neither thermal energy
nor an electric field is able to
raise the energy of an electron to send it into the conduction
band. Due to the absence of free
electrons for conduction, the material behaves as an insulator.
On the contrary, in a conductor the
conduction band is partially superimposed to the valence band.
As a consequence, there are many
electrons available for conduction and an electric field can
give them sufficient energy to
perform conduction.
In a semiconductor, the two bands are separated but the energy
of the forbidden band is low
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(Eg = 1.12 eV for Si at T = 300 K) and it is easy to give energy
to an electron to go into the
conduction band. In this case, the hole in the valence band
contributes to the conduction as well
as the electron in the conduction band.
The forbidden band amplitude varies with temperature, for Si the
amplitude is:
(1.2)
The temperature coefficient is negative, it means that the
forbidden band amplitude decreases
with temperature.
2.4.3 Absorption of Light:
The radiated energy interacts with the matter, including
semiconductors, as photons, whose
energy is , and momentum
The excitation of an electron from the valence band to the
conduction band is called fundamental
absorption and, as a consequence, a hole appears in the valence
band. Both the total energy and
the momentum must be conserved; in particular, for direct
band-gap semiconductors (GaAs,
GaInP, CdTe, and CU (InGa) Se2) a transition can occur remaining
constant the momentum of
the photons. The crystal momentum is equal to where l is the
lattice constant and it is bigger
than the photon momentum. Being the wavelength of sunlight of
the order of 10 -4 cm and the
lattice constant of 10-8 cm, it can be assumed that the
conservation law can be applied only to the
photon momentum. The probability of an induced transition from a
level E1 into the valence
band to a level E2 into the conduction band for a photon with
energy is given by a coefficient
that depends on the difference between the photon energy and the
forbidden band gap.
(1.3)
Some semiconductors allow only transitions with , in such
cases:
(1.4)In indirect band-gap semiconductor, like Si and Ge, the
maximum of the valence band and the
minimum of conduction band occur for different values of the
momentum .as shown below
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Figure 1.4 Energy versus momentum representation of the energy
band structure for indirect band-gap semiconductor
Conservation of the momentum implies in this case the emission
or the absorption of a phonon.3
In particular, if the photon energy is greater than the
difference between the starting electron
energy level in the valence band and the final level in
conduction band, a phonon is emitted. On
the contrary, if the photon energy is lower than the difference
between the starting electron
energy level in the valence band and the final level in
conduction band, a phonon is
Absorbed. The absorption coefficient is different depending on
absorption or emission
phenomenon.
(1.5)
(1.6)
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Where Eph is the phonon energy. It should be noted that for
indirect band-gap semiconductor,
the absorption of a photon depends on the availability of energy
states, and on the
absorbed/emitted phonons as well. This makes the absorption
coefficient for indirect transition
smaller than the corresponding one for direct transition. As a
result, light is able to penetrate
more inside an indirect band-gap semiconductor.
2.4.4 Doping: The conductivity of a semiconductor can be varied
by introducing specific dopants. it can benoted that phosphorous
has five valence electrons (3s23p3) whereas boron has three
valence
electrons (3s23p1). If phosphorous atoms are introduced in a
silicon crystal, one of its five
valence electrons becomes available for conduction, the
remaining four electrons are tied with
covalence bonds of silicon lattice. This kind of dopant is said
donor. In the same way by
introducing boron, its three valence electrons are tied
Figure 1.5 a) n-type doping with Phosphorous b) p-type doping
with Boron
with covalence bonds of silicon lattice and a hole remains as
shown in above fig 1.5 This kind of
dopant is said acceptor. From the point-of-view of energy
levels, the presence of donor
introduces additional energy levels near the conduction band
(within few kT), hence thermal
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energy can allow the added electron to move to the conduction
band. In the same way, the
presence of an acceptor introduces additional energy levels near
the valence band.
In case of donor introduction, electrons are the primary source
of conduction and the
semiconductor is said n-type, on the contrary, if an acceptor is
introduced, conduction is due to
hole, and the semiconductor is said p-type. The atoms of donors
(ND) or acceptors (NA) are
usually completely ionized, as a consequence for n-type
semiconductor and for p-type
semiconductor
This hypothesis will be maintained in the following, throughout
the chapter. The presence of
dopant changes the Fermi level compared to an intrinsic
semiconductor, this value can be
recalculated for an n-type semiconductor.
(1.7)Compared to an intrinsic semiconductor the Fermi level is
increased.For a p-type semiconductor.
(1.8)
and the Fermi level is lower compared to the intrinsic
semiconductor holds even for doped
semiconductors, for an n-type:
(1.9)
and the donors concentration can be expressed versus the Fermi
level for an intrinsic
semiconductor:
When is obtained by using below I and II equation with the
position of
(I) (1.10)
.... (II) (1.11)
Then for P-type we can get
(1.12)
In an n-type semiconductor, electrons represent majority
carriers and holes minority carriers.
Usually, if necessary, their concentration symbol includes a
pedex to indicate the semiconductor
type. Hence, in an n-type semiconductor there are majority
carriers and minority carriers. In a
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p-type semiconductor, there are pp majority carriers and np
minority carriers. If necessary, to
specify the equilibrium conditions a further pedex o can be
added. When double doping with
both donors and acceptors is performed, the type of the
semiconductor is determined by the
greatest impurity concentration.
2.4.5 PN Junction:A pn junction can be conceptually conceived as
a two doped semiconductor of n-type and p-
type that have a surface in common. When both semiconductors are
separated, they are
electrically neutral. As soon as they get in touch, majority
carriers of n-type semiconductors (the
electrons) begin to diffuse into the p-type semiconductor and
vice versa. As a result, near the
surface of separation between the two semiconductors, in n-type
semiconductor, holes coming
from p-type semiconductor tend to combine with electrons and the
positive charge of the
corresponding ionized donors is not more compensated by majority
carriers. Inside the n-type
region, near the junction, where there are no more majority.
Figure 1.6 Schematic representation of a pn junction
charges, a depletion is observed and the corresponding zone
remains with fixed positive charges.
In the same way, in p-type side, electrons coming from n-type
semiconductor tend to combine
with holes and the negative charge of the ionized acceptors is
not more compensated by majority
carriers. Inside the p-type region, near the junction, where
there are no more majority charges, a
depletion is observed and the corresponding zone remains with
fixed negative charges. As the
fixed charges are uncovered, an electric field is produced and
the diffusion process is slowed
down. A pn junction is drawn in Fig. 1.6 in 1D representation;
the origin (x = 0) is the junction
surface, xp and Wp are the depletion boundary at the end of
p-type region, while -xn and -Wn are
the depletion boundary at the end of n-type region. It should be
noted that, if a semiconductor is
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more doped than the other (usually indicated with apex +), the
greater quantity of free carrier
diffused in the other semiconductor cause a more extended
depletion. It is assumed a uniformed
and no degenerated doping and that dopants are fully ionized.
The whole zone in which there are
f ixed uncompensated charge is called depletion region or space
charge region. The remaining
zones can be considered as neutral (often called quasi neutral).
The electric field due to the fixed
charges origins an electrostatic potential difference called
built-in voltage. The Poissons Eq.)
can be rewritten as:
(1.13)
Where is the electrostatic potential, p0 and no are the hole and
electron equilibrium
concentration, is the concentration of ionized donors (positive
fixed charges), and is the
concentration of ionized acceptors (negative fixed charges).
Figure 1.7 Electric symbol and voltage versus current diode
characteristic
When the forward bias voltage approaches Vbi the depletion zone
tends to vanish and the current
is limited by the semiconductor and ohmic contact, as well. In
this case, the voltage versus
current characteristic is approximated by straight line. When a
reverse bias is applied, it means
that a positive voltage is applied to the n zone contact, can be
still utilized. As a matter of fact,
the exponential term is negligible and a reverse saturation
current is obtained. In this case, the
obtained small current is given only by carriers generated
inside the junction and it does not
depend on the applied reverse bias. the voltage versus current
characteristic of a diode. From
what explained above, it is clear that a diode allows the
current to pass from p zone to n zone
when it is forward biased. The ohmic contact belonging to the p
zone is called anode while the
ohmic contact belonging to the n zone is called cathode.
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2.4.6 Optical Generation Rate:As we know only photons with
wavelength can contribute to generate holeselectrons couples.
The generation rate depends on a grid shadowing factor s, on the
reflectance on the absorption
coefficient and on incident photon flux according to the
equation 1.14
( 1.14)
Physical Model of a PV Cell:On the basis of Kirchhoffs current
law (KCL) an equivalent circuit can be deduced. It represents
a physical circuit model of a PV cell. This circuit is drawn
in
Figure 1.8 Physical model of a PV cell
It should be noted that the output current is the sum of a
current given by a generator that
depends on solar irradiance minus the current that flows through
the two diodes.The first current
corresponds to
In equation ( 1.15)
(1.15)
and the second current corresponds to
and the third current corresponds to
As a matter of fact, the second and the third term of (Eq.1.15)
can be considered as Shockley
diode equations. Finally, the output voltage is obtained by the
diodes direct bias due to the
current generator. During operating conditions, when solar
radiation occurs, the generator current
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flows through the diodes and a voltage appears at the terminals.
If no load is applied this voltage
is an open circuit voltage, i.e., the voltage of a directly
polarized pn junction and it is the
maximum value achievable by a PV cell. If a load is connected, a
part of the current of the
generator flows into the load, voltage decreases and electric
power is supplied to the load. The
conversion process is completed. Starting from solar radiation,
electric energy has been obtained.
It should be noted that if the load is raised (it corresponds to
a lower resistance) current rises too
and voltage decreases; the supplied power reaches a maximum and
then decreases until the short
circuit condition.
Figure 1.9 a) structure of crystalline b) Monocrystalline c)
amorphous silicon
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When no solar radiation is present, the generated current is
null and consequently the voltage at
terminals. However, this does not correspond to a short circuit
behavior on the contrary; the PV
cell does not allow negative current flow imposed by external
circuits.
Crystalline Silicon:Crystalline silicon is considered as an
ideal structure where the pattern is regular throughout the
whole surface. All theory explained above is developed with
reference to this structure. The main
advantage consists of highest ratio solar irradiance produced
electric power. With
Monocrystalline silicon, power conversion efficiency ranging
from 20 to 24 % is expected, with
GaAs, power conversion efficiency ranging from 20 to 29 % is
expected. Crystalline silicon, on
the other hand, is expensive owing to manufacturing process. For
this reason, several alternative
cheaper silicon structures have been developed.
Multicrystalline:Multicrystalline and polycrystalline silicon
can be produced by a less sophisticated technique
compared with crystalline. However, in this case, the presence
of grain boundaries must be taken
into account. In particular, cell performance is reduced because
at the boundaries the carriers
flow is blocked, the level structure is altered, and the current
that would flow across pn junction
is shunted away. Some remedies have been devised as, for
example, the use of grains of few
millimeters to cover the entire distance from the back to the
front of the cell with minimum
number of grains. With Polycrystalline silicon, a power
conversion efficiency ranging from 13 to
18 % is expected.
Amorphous:Amorphous silicon presents a less regular structure
with unsatisfied bonds. These dangling
bonds are passivated by hydrogen by allowing doping (otherwise
impossible) and raising the
band gap form 1.1 eV of crystalline silicon to 1.7 eV; in this
way, photons of higher energy can
be absorbed and the required thickness of the material is lower.
As a consequence, amorphous
silicon can be used as a thin film form deposited on glass or
other substrates for low cost
applications. The band structure of amorphous materials is
similar to the crystalline material over
short distance and a mobility gap, in which conduction occur,
can be defined. However, there are
a great number of localized energy states within mobility
gap.
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2.5.1 Basic Structure of PV:
PV cells are the basic building blocks of the PV modules, for
almost all applications the one halfvolt produced by a single cell.
Therefore cells are connected together in series to increase
thevoltage. Several of these series strings of cells may be
connected together in parallel to increasethe current. So in basic
structure of PV model there must be include the effects of series
andparallel resistance of the PV, so when the first Kirchhoffs law
is applied to one of the nodes ofequivalent circuit, the current
supply by the PV, at a specified temperature.
PV Model in PSIM:
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Figure 1.10 PV model in PSIM
Chapter 3
Off-Grid PV System:
In the stand alone PV system the battery energy storage is
necessary to help get a stable and
reliable output from PV generator. In this case we need a
battery charge regulator to protect the
battery against overcharge and deep discharge which shorten the
battery life time.
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Figure 3.1 Charge controller
Above figure shows a charge controller system this system
consist a PV system, filter and battery
charger. In PV system block it contains a PV array, and buck
boost converter. Buck boost
converter is implemented to the system to maintain the output
voltage at the required value.
When the PV gets the max or min voltage from the sunlight, the
converter will use to control the
output voltage at the required value to charge the battery.
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Figure 3.2 Battery charger
Above figure (3.2) shows a Battery Charger which is consist of a
boost converter with controller.
The output connection of this circuit to the PV system is
usually dc-dc converter mainly boost
chopper in order to boost the voltage to the predefined levels.
Figure (3.3) show a boost
converter in battery charger block.
Figure 3.3 Power boost converter
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Off - grid inverter:In this below block we show design of an
Off-grid inverter Figure (3.4) shows the inverter
design for a PV system. First stage consists of a battery
regulator with the value 48 Volts and a
boost converter for amplication of the battery voltage to the
required value amount to 192 V dc.
The boost converter parameters are L=10010-3 H, C= 100010-6 F, f
switching=100 kHz, Duty
cycle=75% .
Figure 3.4 Inverter design for PV electricity generation
system
DC/AC Inverter Analysis:Here we analyzed, inverter
architectures, single phase wave form of inverter and six-step
of
three-phase will be analyzed for photovoltaic system. Detailed
modulation strategies of the space
vector modulation will be described for the three-phase
inverter.
Single phase full bridge DC/AC inverter:In photovoltaic system,
the DC/AC inverter is used to converts the power of the source
by
switching the DC input voltage (or current) in a pre-determined
sequence to generate AC voltage
(or current) output. Figure (3.5) shows the equivalent circuit
of single-phase inverter. This has
four switches that turn on and off to obtain a sinusoidal
output.
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Figure 3.5 Equivalent circuit of the full bridge single phase
inverter.
The load of the inverter is a single-phase AC load or connected
to single-phase grid power. The
topology of the single-phase inverter is shown in figure (3.6).
The single-phase inverter has four
switches and four anti-parallel protective diodes. It provide
path for the inductive current to flow
when the switches are open and protect the switches from the
large voltage by interrupting the
inductive current.
Figure 3.6 Topology of a single phase inverter with filter and
load
To generate an AC waveform in single-phase inverter, the
switches S1, S2 ON and S3, S4 off for
period t1 and t2 as shown in figure (3.7). For that period, the
output is a positive voltage.
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Figure 3.7 Output current for S1,S2 ON; S3,S4 OFF for t1 < t
< t2
For period t2 to t3 in figure (3.8), the switches S3, S4 are on
and S1 and S2 are off to obtain
negative voltage.
Figure 3.8 output current for S3, S4 ON;S1,S2 OFF for t2 < t
< t3
Switches S1 and S4 should not be closed simultaneously, the same
for switches S3 and S2.
Otherwise short circuit of the DC bus will occur. By following
the switching scheme, the inverter
output voltage will alternate between positive and negative as
figure (5.5), and the sinusoidal
fundamental component is obtained as shown in figure (5.6).
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Impact of solar radiation on I-V characteristic curve of
photovoltaic module:
Standard sunlight conditions on a clear day are assumed to be
(1000 W/m^2). This is
sometimes called one sun or a peak sun . Less than one sun can
reduce the current output of
the module by a proportional amount, for example if only
one-half sun (500w/m^2) is available,
the amount of current is roughly cut in half.
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Figure 3.1 I-V characteristics at t=25 C, By PSIM different
irradiances [y-axis: current (A): X-axis: voltage(volt)].
Impact of temperature on I-V characteristic Curve of
Photovoltaic Module:
The temperature of PV cell is an important parameter that has to
be taken into consideration inPV system operation. The PV cell has
given temperature coefficients for both the current and thevoltage
(). The current coefficient is mostly negligible; hence it is
mainly the voltagetemperature coefficient that is considered during
calculations. For Silicon based cells thecoefficient ()=2mV/C_ Per
cell.
Figure 3.2 I-V characteristic at 1000w/m2, By PSIM with
Different temperature [Y-axis: Current (A), X-axis: Voltage
(volt)]
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Impact of shading on I-V Characteristic curve of photovoltaic
module:
Solar Pv panel is a power source having non-linear internal
resistance. A major challenge inusing a PV source containing a
number of cells in series is to deal with its non-linear
internalresistance. The problem gets all the more complex when the
array receives non-uniforminsolationand covert it into heat. This
heat may damage the illuminated cells under certainconditions. To
relieve the stress on shaded cells, bypass diodes are added across
the module.
Battery:
Battery stores direct current electrical energy in chemical form
for later use. In PV system, theenergy is used at night and during
cloudy weather.
A battery charging when energy is being put in and discharging
when energy is being takenout .A cycle is considered one
charge-discharge sequence, when often occurs over a period ofone
day in residential PV systems. The following types of batteries are
commonly used in PVsystem.
1. Lead acid batteries
2. Liquid vented
3. Alkaline batteries
4. Nickel Cadmium
5. Nickel iron
The performance of storage batteries is described below.
Ampere-hour capacity: The number ofamp-hours a battery can deliver
is simply the number of amps of current it can discharge,multiplied
by the number of hours it can deliver that current. System
designers use amp-hourspecifications to determine how long the
system will operate without any significant amount ofsunlight to
recharge the batteries. This measure of "days of autonomy" is an
important part ofdesign procedures. Theoretically, a 200 amp-hour
battery should be able to deliver either 200amps for one hour, 50
amps for 4 hours, 4 amps for 50 hours, or one amp for 200
hours.
Charge and discharge rates: If the battery is charged or
discharged at a different rate thanspecified, the available
amp-hour capacity will increase or decrease. Generally, if the
battery isdischarged at a slower rate, its capacity will probably
be slightly higher. More rapid rates willgenerally reduce the
available capacity. The rate of charge or discharge is defined as
the totalcapacity divided by some number. For example, a discharge
rate of C/20 means the battery is
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being discharged at a current equal to 1/20th of its total
capacity. In the case of a 400 amp-hourbattery, this would mean a
discharge rate of 20 A.
Temperature: Batteries are rated for performance at 80oF. Lower
temperatures reduce amphour capacity significantly. Higher
temperatures result in a slightly higher capacity, but this
willincrease water loss and decrease the number of cycles in the
battery life.
Depth of discharge: This describes how much of the total amp
hour capacity of the battery isused during a charge-recharge cycle.
As an example, "shallow cycle" batteries are designed to discharge
from 10% to 25% of theirtotal amp-hour capacity during each cycle.
In contrast, most "deep cycle" batteries designed forphotovoltaic
applications are designed to discharge up to 80% of their capacity
withoutdamage. Even deep cycle batteries are affected by the depth
of discharge. The deeper thedischarge, the smaller the number of
charging cycles thebattery will lost.
Block Diagram :