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DEVELOPMENT OF A MICROCONTROLLER
BASED SOLAR PHOTOVOLTAIC MPPT
CHARGE CONTROL SYSTEM
Using
INCREMENTAL CONDUCTANCE METHOD
A Thesis
Submitted in partial fulfillment of the
Requirement for the degree of
MASTER OF ELECTRONICS & TELE-COMMUNICATION ENGINEERING
(With Specialization in ELECTRON DEVICE)
By
TANUSREE DUTTA
Reg. No. 105231 of 2008-2009 Exam Roll No. M4ETC10-02
Class Roll No. 000810702003 of 1008-1009MAY 2010
Under the supervision of
PROF. H. SAHA
Department of Electronics & Tele-communication
Engineering
JADAVPUR UNIVERSITY,
KOLKATA- 700032, INDIA
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CERTIFICATE OF APPROVAL*
The foregoing thesis is hereby approved as a creditable study of
an engineering subject and
presented in a manner satisfactory to warrant acceptance as
pre-requisite to the degree for which
it has been submitted. It is understood that by this approval
the undersigned do not necessarily
endorse or approve any statement made, opinion expressed or
conclusion drawn there in but
approve the thesis only for which it is submitted.
Committee on final examination
For the evaluation of the Thesis
.
.
Examiners
* Only in case the thesis is approved
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A C K N O W L E D G E M E N T A C K N O W L E D G E M E N T A C
K N O W L E D G E M E N T A C K N O W L E D G E M E N T
It gives me imm ense pleasure to express my deepest sense of
grat itude and s incere thanks to m y highly
respected and esteemed supervisor Prof .Prof.Prof .Prof .
HiranmayHiranmayHiranmayHiranmay Saha,Saha,Saha,Saha,
Supervisor,Supervisor,Supervisor,Supervisor, IC Design &
fabricat ion Centre,IC Design & fabricat ion Centre,IC Design
& fabricat ion Centre,IC Design & fabricat ion Centre,
JJJJadavpur adavpur adavpur adavpur Univers
ityUniversityUniversityUnivers ity, for his revered supervis ion
throughout my dissertat ion work, which made this task a
pleasa nt jo b. It w as re al p leasu re to w ork under his su
per vis io n.
I extend my s incere thanks to Prof . Goutam Bhattacharya,
Ramkrishna Miss ion Vidyamandira, for his keen
interest, continuous encouragement and support.
I am also indebted to my mother, brother, sisters and well
wishers who are taking lot of pains for progress in
my life and for their sacrifices, blessing and constant prayers
for my advancement.
I express my special thanks to Prof . B.Gupta (HOD,Dept.of
ETCE,JU) and Prof . S.K.Sanyal , Dept. of
ETC E,JU, for their kindness and providing m e the faci l it ies
of the Labo ratory to use for my w ork.
I would also l ike to thanks al l the Research Scholars , staff
members and project students of IC Design &
Fabricat ion Center. Special thanks are due to Mr.A.Mondal ,
Mr.G.P.Mishra, Mr.A.Kindu. Dr.S.Roy
Choudh ury, Mrs.S.Roy, Ms.T.Majhi , Mr.A.Sengupta.
I am also thankful to Prof . R.N.Ghosh, St . Thomas Col lege of
Engg. & Tech., Kol . & Mr.Arup Sarkar, Agni
Power Electronics, Kol., for their support.
I also l ike to thank Dr.S.Mukhopadhya, Secretary, St . Thomas
Col lege of Engg. & Tech., Kol . Prof .Mrs.
S.Sen, Principal , St . Thomas Col lege of Engg. & Tech.,
kol , M r.Goutam Ban erjee, Registrar, St. Thomas
Col lege of Engg. & Tech., kol , for al lowing me to p
ursuing my M .E. in Electronics & Tele-commu nication at
Jadavpur Univers ity.
T A N U S R E E D U T T A T A N U S R E E D U T T A T A N U S R
E E D U T T A T A N U S R E E D U T T A
Reg. No. Reg. No. Reg. No. Reg. No.105231 of 2008105231 of
2008105231 of 2008105231 of 2008- ---09,09,09,09,
D A T E D A T E D A T E D A T E : ::: Rol l No.000810702003Rol l
No.000810702003Rol l No.000810702003Rol l No.000810702003 of 2008of
2008of 2008of 2008- ---09090909
Exam Roll No. M4ETC10Exam Roll No. M4ETC10Exam Roll No.
M4ETC10Exam Roll No. M4ETC10----02020202
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1.0 Introduction to Maximum Power Point Tracking (MPPT).
Introduction. 2Need for Maximum Power Point Tracking
How Maximum Power Point is Achieved 7
Methods of Maximum Power Point 7
Application of MPPT. . 8
2.0 Literature Review 11
Algorithms for MPPTPerturb & Observe
Incremental Conductance 17
Parasitic Capacitance 18
Voltage Based Maximum Power Point Tracking 18
Current Based Maximum Power Point Tracking 18
4.0 Block Diagram of MPPT System.
Basic Block diagram of MPPT 20
What is MPPT 20
Solar Photovoltaic Cell 22
DC-DC Converter. 30
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Introduction to Microcontroller 34Characteristics of Battery
36
5.0Hardware Description 39
6.0 Software Description... 62
7.0Experimental Setup. 75
8.0Result 82
9.0Conclusions & Future Scope. . 85
10.0References ... 87
11.0Annexure... 88
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ABSTRACT
Renewable energy sources play an important role in
electricity
generation. Various renewable energy sources like wind, solar,
geothermal, ocean
thermal and biomass can be used for generation of electricity
and for meeting our
daily energy needs. Energy from the sun is the best option for
electricity
generation as it is available everywhere and is free to harness.
On an average the
sunshine hour in India is about 6hrs annually also the sun shine
shines in India for
about 9 months in a year. Electricity from the sun can be
generated through the
solar photovoltaic modules (SPV). The SPV comes in various power
output to
meet the load requirement. Maximization of power from a solar
photo voltaic
module (SPV) is of special interest as the efficiency of the SPV
module is verylow. A maximum power tracker is used for extracting
the maximum power from
the SPV module .The present work describes the maximum power
point tracker
(MPPT) for the SPV module connected to a battery which is used
as a load. A
Microcontroller is used for control of the MPPT algorithm. The
power tracker is
developed and tested successfully in the laboratory.
Maximum power point tracking (MPPT) is used in photovoltaic
(PV)
systems to maximize the photovoltaic array output power,
irrespective of the
temperature and irradiation conditions and of the load
electrical characteristics. Anew MPPT system has been developed,
consisting of a Buck-type dc/dc converter,
which is controlled by a microcontroller-based unit. The main
difference between
the method used in the proposed MPPT system and other techniques
used in the
past is that the PV array output power is used to directly
control the dc/dc
converter, thus reducing the complexity of the system. The
resulting system has
high-efficiency, lower-cost and can be easily modified to handle
more energy
sources (e.g., wind-generators).
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`
CHAPTER 1.
Introduction to Maximum Power
Point Tracking (MPPT).
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Introduction
Develop a Microcontroller based dedicated MPPT controller for
solar PV module based on the
incremental conductance method. As people are much concerned
with the fossil fuel exhaustion
and the environmental problems caused by the conventional power
generation, renewable
energy sources and among them photovoltaic panels and
wind-generators are now widely used.
So Solar Energy is a good choice for electric power generation.
The solar energy is directly
converted into electrical energy by solar photovoltaic module.
Photovoltaic sources are used
today in many applications such as battery charging, water
pumping, home power supply,
swimming-pool heating systems, satellite power systems etc. They
have the advantage of being
maintenance and pollution-free but their installation cost is
high and inmost applications, they
require a power conditioner (dc/dc or dc/ac converter) for load
interface. Since PV modules still
have relatively low conversion efficiency, the overall system
cost can be reduced using high
efficiency power conditioners which, in addition, are designed
to extract the maximum possiblepower from the PV module.
The photovoltaic modules are made up of silicon cells. The
silicon solar cell which give output
voltage of around 0.7V under open circuit condition. When many
such cells are connected in
series we get a solar PV module. Normally in a module there are
36 cells which amount for a
open circuit voltage of about 20V. The current rating of the
modules depends on the area of the
individual cells. Higher the cell area high is the current
output of the cell. For obtaining higher
power output, the solar PV modules are connected in series and
parallel combinations forming
solar PV arrays. A typical characteristic curve of the called
current (I) and voltage (V) curve and
power (W) and voltage (V) curve of the module is shown is
fig.1.
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Fig.1 Characteristics of a typical Solar PV Module.
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Need for maximum power point tracking
Power output of a Solar PV module changes with change in
direction of sun, changes in solar
insolation level and with varying temperature as shown in the
fig.2 & 3.
Fig.2 Changes in the characteristics of the Solar PV module due
to change in the insolation
level.
As seen in the PV (power vs. voltage) curve of the module there
is a single maxima of power.
That is there exists a maximum power corresponding to a
particular voltage and current. We
know that the efficiency of the solar PV module is low about
13%. Since the module efficiency
is low it is desirable to operate the module at the maximum
power point so that the maximumpower can be delivered to the load
under varying temperature and insolation conditions. Hence
maximization of improves the utilization of the solar PV module.
A maximum power point
tracker (MPPT) is used for extracting the maximum power from the
solar PV module and
transferring that power to the load. A dc/dc converter (step
up/step down) serves the purpose of
transferring maximum power from the solar PV module to the load.
A dc/dc converter acts as an
interface between the load & module in fig.4.
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Fig.3 Change in the module characteristics due to the change in
temperature
By changing the duty cycle the load impedance as seen by the
source is varied and matched at
the point of the maximum power with the source so as to transfer
the maximum power.
Fig.4 Block diagram of a typical MPPT system
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How maximum power point is obtained.
As discuss in this chapter the maximum power point is obtained
by introducing dc/dc converter
in between the load and the solar PV module. The duty cycle of
the converter is changed till the
maximum power point is obtained considering a down converter is
used.
Vo=D*Vi ( Vo is output voltage and Vi is input voltage)
D is the duty cycle of the PWM.
Io = D*Ii
So the Output Power
Pout = Vo * Io
Input Power,
Pin = Vi * Ii
By varying the duty cycle of the PWM, maximum power point is
extract from the Solar PV
module by using a different algorithm.
Fig.5 DC/DC converter helps in tracking the maximum power
point.
Methods of Maximum Power Point Tracking.The maximum power is
reached with the help of a dc/dc converter by adjusting its
dutycycle.Now question arises how to vary the duty cycle and in
which direction so that maximumpower is reached. Whether manual
tracking or automatic tracking? Manual tracking is notpossible so
automatic tracking is preferred to manual tracking. An automatic
tracking can beperformed by utilizing various algorithms.a. Perturb
and observeb. Incremental Conductancec. Parasitic Capacitance
d. Voltage Based Maximum Power Trackinge. Current Based Maximum
power Tracking
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The algorithms are implemented in a microcontroller to implement
the maximum power point
tracking. The algorithm changes the duty cycle of the dc/dc
converter to maximize the power
output of the module and make it operate at the maximum power
point of the module.
Applications of Maximum Power Point Trackers.MPPT systems are
used mainly in systems where source of power is nonlinear. Such as
thesolar PV modules or the wind generator systems. MPPT systems are
generally used in solar PV
applications such as battery chargers and grid connected stand
alone PV systems.
a) Battery charging:Charging of battery (lead acid/NiCad) which
is used for the storage of
electrical energy. This energy if it comes from the solar PV
systems then fast charging of
the battery can be done with the help of the MPPT charge
controller.
Fig.6.Battery charging application of MPPT
b) Grid connected and standalone PV systems:In grid connected or
stand alone PV systems.Thesolar arrays supply power to the grid or
to the local load. A dc/dc converter is used as the
array voltage is dc and as grid voltage is ac an dc/ac converter
must be used.
Fig.7.Grid connected application using MPPT
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Before a dc/ac converter a dc/dc converter (normally step up) is
used which serves the purpose
of the maximum power point tracking as explained earlier. Due to
maximum power tracking
always the maximum power is transferred to the grid or the local
load.
c) Water pumping applications:Solar PV arrays can be used to run
dc motors which drive the
pump for supplying the water in the fields. By using the maximum
power point tracker the
power to the motor can be increased and so the output flow rate
of the pump will also increase.
Fig.8. Pumping application of the MPPT.
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CHAPTER 2.Literature Review.
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Fig.9 MPPT system with the incremental conductance control
method.
Fig.10 Feed-forward maximum power tracking control system.
In this method the power converter is controlled using the PV
array output power. The MPPT
control algorithm is based on the calculation of the PV output
power and of the power change
by sampling voltage and current values. The power change is
detected by comparing the present
and previous voltage levels, in order to calculate a reference
voltage which is used to produce
the PWM control signal.The dc/dc converter is driven by a
DSP-based controller for fast-
response and the overall system stability is improved by
including a PI controller which is so
used to match the array and reference voltage levels. However,
the DSP-based control unit
increases the implementation cost of the system.
2). Control of DC/DC Converters for Solar Energy System with
Maximum Power
Tracking.
Chihchiang Hua and Chihming Shen.
The object of this paper is to analyze and design DC/DC
converters of different types in a solarenergy system to
investigate the performance of the converters.A simple method
which
combines a discrete time control and a PI compensator is used to
track the Maximum power
points (MPP's) of the solar array. The system is kept to operate
close to the MPPT's, thus the
maximum possible power transfer from the solar array is
achieved. The implementation of the
proposed converter system was based on a digital signal
processor (DSP). Experimental tests
where carried out for buck, boost and buck-boost converters
using a simple maximum power
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point tracking (MPPT) algorithm. The efficiencies for the system
with different converters are
compared. The paper is use full in evaluating the response of
step up, step down converter for
the MPPT system. Paper proposes that the Step down converter is
the best option for the use in
the MPPT system as it give higher efficiency.
3) Maximum Power Tracking for Photovoltaic Power Systems.
Joe-Air Jiang1, Tsong-Liang Huang2, Ying-Tung Hsiao2* and
Chia-Hong Chen2.
The authors have developed a new MPPT algorithm based on the
fact that the MPOP(maximum
peak operating point) of a PV generator can be tracked
accurately by comparing the incremental
and instantaneous conductances of the PV array. The work was
carried out by both simulation
and experiment, with results showing that the developed
incremental conductance(IntCond)
algorithm has successfully tracked the MPOP, even in cases of
rapidly changing atmospheric
conditions, and has higher efficiency than ordinary algorithms
in terms of total PV energy
transferred to the load.
4) A New Algorithm for Rapid Tracking of Approximate Maximum
Power Point in
Photovoltaic Systems.
Sachin Jain, Student Member, IEEE, and Vivek Agarwal.
A robust oscillation method is used for implementing the maximum
power point tracking for
the solar arrays. The method uses only one variable that is load
current for detecting the
maximum power.This method is suitable for the battery charging
application where MPPT is to
be implemented.The algorithm is implemented through a simple
circuit.The paper gives detailed
discussion about design of a step down converter used for the
MPPT.
5). Microprocessor-Controlled New Class of Optimal Battery
Chargers for Photovoltaic
Applications.
Mohamad A. S. Masoum, Seyed Mahdi Mousavi Badejani, and Ewald F.
Fuchs.
The authors discuss a control system of a residential
photovoltaic system.The paper explains a
perturb and observe algorithm and how can it be implemented
using a microprocessor. This
paper is one of the basic papers which explains the Incremental
Conductance algorithm.Also
controller design using PI scheme obtained.
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6) Implementation of a DSP-controlled Photovoltaic Peak Power
Tracking system.
Chihchiang Hua,Member, IEEE, Jongrong Lin, and Chihming Shen
The corresponding authors have proposed a new kind of maximum
power point tracking
algorithm based on Incremental Conductance algorithm.The
algorithm is fast acting which
eliminate the ripple in the module voltage. The module voltage
and current that are taken forprocessing are not averaged but are
instantaneous this speed ups the process of peak power
tracking. Also the paper implements the new algorithm on the
real time platform.The software
used was DSP.
7). Comparative Study of Maximum Power Point Tracking Algorithms
Using an
Experimental, Programmable, Maximum Power Point Tracking Test
Bed.
D. P. Hohm, M. E. Ropp.
The authors have compares all the different kinds of algorithm
that are used for the maximum
power point tracking.This helps in proper selection of the
algorithm.Preliminary results indicatethat perturb and observe
compares favorably with incremental conductance and constant
voltage. Although incremental conductance is able to provide
marginally better performance in
case of rapidly varying atmospheric conditions, the increased
complexity of the algorithm will
require more expensive hardware and therefore may have an
advantage over perturb and
observe only in large PV arrays.
8) Theoretical and Experimental Analyses of Photovoltaic Systems
With Voltage and
Current-Based Maximum Power-Point Tracking.
Mohammad A. S. Masoum, Hooman Dehbonei, and Ewald F. Fuchs
Detailed theoretical and experimental analyses of two simple,
fast and reliable maximum
power-point tracking (MPPT) techniques for photovoltaic (PV)
systems are presented. Voltage-
based (VMPPT) and the Current-based (CMPPT) approaches.A
microprocessor-controlled
tracker capable of online voltage and current measurements and
programmed with VMPPT and
CMPPT algorithms is constructed.The load of the solar system is
either a water pump or
resistance. The paper has given a simulink model of the Dc/Dc
converter and a solar PV
module.
The literature review consists of vast survey of papers from the
various conferences. The
literatures give sufficient idea about the basics of the MPPT
algorithm and how the MPPtracking is takes place.
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CHAPTER 3.
Algorithms to track the Maximum
Power Point.
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Different algorithms help to track the maximum power point of
the solar pv module
automatically.
The various algorithms used are:
a) Perturb and Observe.
b) Incremental Conductance.
c) Parasitic Capacitance.
d) Voltage Based Peak Power Tracking.
e) Current Based peak power Tracking
a) Perturb and Observe method- In this algorithm a slight
perturbation is introduced in
the system. Due to this perturbation the power of the module
changes. If the power increases
due to the perturbation then the perturbation is continued in
that direction. After the peak power
is reached the power at the next instant decreases and hence
after that the perturbation reverses.
Fig.11 Perturb and observe algorithm
When the steady state is reached the algorithm oscillates around
the maximum point. In order to
keep the power variation small the perturbation size is kept
very small.The algorithm is
developed in such a manner that it sets a reference voltage of
the module corresponding to the
maximum voltage of the module. A Microcontroller then acts
moving the operating point of the
module to that particular voltage level. It is observed that
there some power loss due to this
perturbation also the fails to track the power under fast
varying atmospheric conditions. But still
this algorithm is very popular and simple.
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Implemented Methodb) Incremental conductance method:- The
disadvantage of the perturb and observemethod to track the maximum
power under fast varying atmospheric condition is overcome
byIncremental conductance method. The algorithm makes use of the
equation:
P=V*I(where P= module power,V=module voltage, I=module
current);
Diff. with respect to dV
dP/dV=I+V*dI/dV
Depending on this equation the algorithm work at maximum power
point
dP/dV=0
dI/dV=-I/V
Fig.12.Incremental conductance method.
If operating point is to the left of the power curve then we
have
dP/dV>0
dI/dV>I/V
By using this equation, algorithm works.
The incremental conductance can determine that the MPPT has
reached the MPP and stop
perturbing the operating point.If this condition is not met, the
direction in which the MPPT
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operating point must be perturbed can be calculated using the
relationship between dl/dV and -
I/V. This relationship is derived from the fact that dP/dV is
negative when the MPPT is to the
right of the MPP and positive when it is to the left of the MPP.
This algorithm has disvantages
over perturb and observe in that it can determine when the MPPT
has reached the MPP, where
perturb and observe oscillates around the MPP. Also, incremental
conductance can track rapidly
increasing and decreasing irradiance conditions with higher
accuracy than perturb and
observe.One disadvantage of this algorithm is the increased
complexity when compared to
perturb and observe method.
Others Method
c) Parasitic capacitances:- The parasitic capacitance method is
a refinement of
incremental conductance method that takes into account the
parasitic capacitances of the solar
cells in the PV array . Parasitic capacitance uses the switching
ripple of the MPPT to perturb the
array. To account for the parasitic capacitance, the average
ripple in the array power and
voltage,generated by the switching frequency, are measured using
a series of filters and
multipliers and then used to calculate the array conductance.The
incremental conductancealgorithm is then used to determine the
direction to move the operating point of the MPPT. One
disadvantage of this algorithm is that the parasitic capacitance
in each module is very small, and
will only come into play in large PV arrays where several module
strings are connected in
parallel. Also, the DC-DC converter has a sizable input
capacitor used filter out small ripple in
the array power.This capacitor may mask the overall effects of
the parasitic capacitance of the
PV array.
d) Voltage control maximum point tracker:- It is assumed that a
maximum power
point of a particular solar PV module lies at about 0.75 times
the open circuit voltage of the
module. So by measuring the open circuit voltage a reference
voltage can be generated and feedforward voltage control scheme can
be implemented to bring the solar pv module voltage to the
point of maximum power.One problem of this technique is the open
circuit voltage of the
module varies with the temperature. So as the temperature
increases the module open circuit
voltage changes and we have to measure the open circuit voltage
of the module very often.
Hence the load must be disconnected from the module to measure
open circuit voltage. Due to
which the power during that instant will not be utilize.
e) Current control maximum power point tracker:- The maximum
power of the
module lies at the point which is at about 0.9 times the short
circuit current of the module. In
order to measure this point the module or array is
short-circuited. And then by using the current
mode control the module current is adjusted to the value which
is approx 0.9 times the shortcircuit current. The problem with this
method is that a high power resistor is required which can
stain the short-circuit current. The module has to be short
circuited to measure the short circuit
current as it goes on varying with the changes in insolation
level.
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CHAPTER 4.BLOCK DIAGRAM OF MPPT SYSTEM
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Fig.13
What is MPPT ?
A MPPT, or maximum power point tracker is an electronic DC to DC
converter that
optimizes the match between the solar array (PV panels), and the
battery bank, utility power,
DC motor, or DC pump.
.Fig.14 Characteristic curve of solar photovoltaic MPPT
system
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what do we mean by "optimize"?
Most PV panels are built to put out a nominal 12 volts. The
catch is nominal. In actual fact,
most all are designed to put out from 16 to 36 volts. The
problem is that a nominal 12 volt
battery is pretty close to an actual 12 volts - 10.5 to 12.7
volts, depending on state of charge.
Under charge, most batteries want from around 13.2 to 14.2 volts
to fully charge , quite a bit
different than what most panels are designed to put out.This is
electronic tracking, and has
nothing to do with moving the panels. Instead, the controller
looks at the output of the panels,
and compares it to the battery voltage. It then figures out what
is the best power that the panel
can put out to charge the battery. It takes this and converts it
to best voltage to get maximum
AMPS into the battery. Most modern MPPT's are around 92-97%
efficient in the conversion.
You typically get a 20 to 45% power gain in winter and 10-15% in
summer. Actual gain can
vary widely depending weather,temperature, battery state of
charge, and other factors.
MPPT's are most effective under these conditions:
Winter, and/or cloudy or hazy days - when the extra power is
needed the most.
Cold weather - solar panels work better at cold temperatures,
but without a MPPT we are losing
most of that. Cold weather is most likely in winter - the time
when sun hours are low and you
need the power to recharge batteries the most. Low battery
charge - the lower the state of charge
in your battery, the more current a MPPT puts into them -
another time when the extra power is
needed the most. You can have both of these conditions at the
same time. The Power point
tracker is a high frequency DC to DC converter. They take the DC
input from the solar panels,change it to high frequency AC, and
convert it back down to a different DC voltage and current
to exactly match the panels to batteries. MPPT's operate at high
audio frequencies, usually in
the 20-80 kHz range. Most newer models of MPPT controllers
available are Microcontroller
based. They know when to adjust the output that it is being sent
to the battery, and they actually
shut down for a few microseconds and "look" at the solar panel
and battery and make any
needed adjustments.
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SOLAR PHOTOVOLTAIC CELL
Simple explanation
Photons in sunlight hit the solar panel and are absorbed by
semiconducting materials,such as
silicon.Electronics (negatively charged) are knocked loose from
their atoms, allowing them to
flow through the material to produce electricity. Due to the
special composition of solar cells,
the electrons are only allowed to move in a single direction.The
complementary positive
charges that are also created (like bubbles) are called holes
and flow in the direction opposite of
the electrons in a silicon solar panel.An array of solar cells
converts solar energy into a usable
amount of direct (DC) electricity.
Photogeneration of charge carriers
When a photons hits a piece of silicon, one of three things can
happen:
1)The photon can pass straight through the silicon this
(generally) happens for lower energy
photon.
2)The photon can reflect off the surface,
3)The photon can be absorbed by the silicon, if the photon
energy is higher than the silicon band
gap value.This generates an electron-hole pair and sometimes
heat,depending on the band
structure.
When a photon is absorbed, its energy is given to an electron in
the crystal lattice.Usually this
electron is in the valence band, and is tightly bound in
covalent bonds between neighboring
atoms, and hence unable to move far. The energy given to it by
the photon "excites" it into the
conduction band,where it is free to move around within the
semiconductor. The covalent bond
that the electron was previously a part of now has one fewer
electron this is known as a hole.
The presence of a missing covalent bond allows the bonded
electrons of neighboring atoms to
move into the "hole," leaving another hole behind, and in this
way a hole can move through the
lattice. Thus, it can be said that photons absorbed in the
semiconductor create mobile electron-
hole pairs
A photon need only have greater energy than that of the band gap
in order to excite an electron
from the valence band into the conduction band. However, the
solar frequency specturm
approximates a black body spectrum at ~6000 K, and as such, much
of the solar radiation
reaching the Earth is composed of photons with energies greater
than the band gap of silicon.
These higher energy photons will be absorbed by the solar cell,
but the difference in energy
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between these photons and the silicon band gap is converted into
heat (via lattice vibrations
called phonons) rather than into usable electrical energy.
Charge carrier separation
There are two main modes for charge carrier separation in a
solar cell:
1)driftof carriers, driven by an electrostatic field established
across the device.
2)diffusion of carriers from zones of high carrier concentration
to zones of low carrier
concentration (following a gradient of electrochemical
potential).
In the widely used p-n junction solar cells, the dominant mode
of charge carrier separation is by
drift. However, in non-p-n-junction solar cells (typical of the
third generation solar cell research
such as dye and polymer solar cell), a general electrostatic
field has been confirmed to be
absent, and the dominant mode of separation is via carrier
diffusion.
The p-n junction
Main articles: semiconductor and p-n junction.
The most commonly known solar cell is configured as a large-area
p-n junction made from
silicon. As a simplification,one can imagine bringing a layer of
n-type silicon into direct contact
with a layer of p-type silicon.In practice, p-n junctions of
silicon solar cells are not made in this
way, but rather, by diffusing an n-type dopant into one side of
a p-type wafer (or vice versa).
If a piece of p-type silicon is placed in intimate contact with
a piece of n-type silicon,then a
diffusion of electrons occurs from the region of high electron
concentration (the n-type side of
the junction) into the region of low electron concentration
(p-type side of the junction). When
the electrons diffuse across the p-n junction, they recombine
with holes on the p-type side. The
diffusion of carriers does not happen indefinitely however,
because of an electric field which is
created by the imbalance of charge immediately on either side of
the junction which this
diffusion creates. The electric field established across the p-n
junction creates a diode that
promotes charge flow, known as drift current,that opposes and
eventually balances out the
diffusion of electron and holes. This region where electrons and
holes have diffused across the
junction is called the depletion region because it no longer
contains any mobile charge carriers.
It is also known as the "space charge region".
Connection to an external load
Ohomic metal-semiconductor contacts are made to both the n-type
and p-type sides of the solar
cell, and the electrodes connected to an external load.
Electrons that are created on the n-type
side, or have been "collected" by the junction and swept onto
the n-type side, may travel
through the wire, power the load, and continue through the wire
until they reach the p-type
semiconductor-metal contact. Here, they recombine with a hole
that was either created as an
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electron-hole pair on the p-type side of the solar cell, or are
swept across the junction from the
n-type side after being created there.
The voltage measured is equal to the difference in the quasi
fermi levels of the minority carriers
i.e. electrons in the p-type portion, and holes in the n-type
portion.
Fig,15 The equivalent circuit of a solar cell
Fig.16 The schematic symbol of a solar cell
To understand the electronic behavior of a solar cell, it is
useful to create a model which is
electrically equivalent, and is based on discrete electrical
components whose behavior is well
known. An ideal solar cell may be modelled by a current source
in parallel with a diode; in
practice no solar cell is ideal, so a shunt resistance and a
series resistance component are added
to the model. The resulting equivalent circuit of a solar cell
is shown in fig. Also shown, on the
right,is the schematic representation of a solar cell for use in
circuit diagrams.
Characteristic equation
From the equivalent circuit it is evident that the current
produced by the solar cell is equal to
that produced by the current source, minus that which flows
through the diode, minus that
which flows through the shunt resistor:
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I=ILIDISH
where
I= output current (amperes)
IL= photogenerated current (amperes)
ID= diode current (amperes)
ISH= shunt current (amperes)
The current through these elements is governed by the voltage
across them:
Vj= V+IRS
where
Vj= voltage across both diode and resistorRSH(volts)
V= voltage across the output terminals (volts)
I= output current (amperes)
RS= series resistance ()
By the Shockley diode equation, the current diverted through the
diode is:
where
I0= reverse saturation current (amperes)
n= diode ideality factor (1 for an ideal diode)
q= elementary charge
k= Boltzmanns constant
T= absolute temperature
For silicon at 25C, volts.
By Ohms law, the current diverted through the shunt resistor
is:
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WhereRSH= shunt resistance
Substituting these into the first equation produces the
characteristic equation of a solar cell,
which relates solar cell parameters to the output current and
voltage:
An alternative derivation produces an equation similar in
appearance but with Von the left-hand
side. The two alternatives are identites; that is, they yield
precisely the same results.
In principle, given a particular operating voltage Vthe equation
may be solved to determine the
operating currentIat that voltage. However, because the equation
involvesIon both sides in a
transcendental function the equation has no general analytical
solution. However, even without
a solution it is physically instructive. Furthermore, it is
easily solved using numerical methods.
(A general analytical solution to the equation is possible using
Lamberts W function, but since
Lambert's W generally itself must be solved numerically this is
a technicality.)
Since the parameters I0, n, RS, and RSH cannot be measured
directly,the most common
application of the characteristic equation is nonlinear
regression to extract the values of these
parameters on the basis of their combined effect on solar cell
behavior.
Open-circuit voltage and short-circuit current
When the cell is operated at open circuit, I= 0 and the voltage
across the output terminals is
defined as the open-circuit voltage. Assuming the shunt
resistance is high enough to neglect the
final term of the characteristic equation, the open-circuit
voltage VOCis:
Similarly, when the cell is operated at short circuit, V= 0 and
the currentIthrough the terminals
is defined as the short-circuit current. It can be shown that
for a high-quality solar cell (low RS
andI0, and highRSH) the short-circuit currentISCis:
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Effect of physical size
The values of I0, RS, and RSH are dependent upon the physical
size of the solar cell. In
comparing otherwise identical cells, a cell with twice the
surface area of another will, in
principle, have double the I0 because it has twice the junction
area across which current can
leak. It will also have half the RSandRSHbecause it has twice
the cross-sectional area through
which current can flow. For this reason, the characteristic
equation is frequently written in terms
of current density, or current produced per unit cell area:
Where,
J= current density (amperes/cm2
)
JL= photogenerated current density (amperes/cm2)
Jo= reverse saturation current density (amperes/cm2)
rS= specific series resistance (-cm2)
rSH= specific shunt resistance (-cm2)
This formulation has several advantages. One is that since cell
characteristics are referenced to a
common cross-sectional area they may be compared for cells of
different physical dimensions.
While this is of limited benefit in a manufacturing setting,
where all cells tend to be the same
size, it is useful in research and in comparing cells between
manufacturers. Another advantage
is that the density equation naturally scales the parameter
values to similar orders of magnitude,
which can make numerical extraction of them simpler and more
accurate even with naive
solution methods.
A practical limitation of this formulation is that as cell sizes
shrink certain parasitic effects grow
in importance and can affect the extracted parameter values. For
example, recombination and
contamination of the junction tend to be greatest at the
perimeter of the cell, so very small cellsmay exhibit higher values
of J0 or lower values of rSH than larger cells that are
otherwise
identical. In such cases, comparisons between cells must be made
cautiously and with these
effects in mind.
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Cell temperature
Fig.17 Effect of temperature on the current-voltage
characteristics of a solar cell
Temperature affects the characteristic equation in two ways:
directly, via T in the
exponential term, and indirectly via its effect onI0. (Strictly
speaking, temperature affects all of
the terms, but these two far more significantly than the
others.) While increasing Treduces themagnitude of the exponent in
the characteristic equation, the value of I0 increases in
proportion
to exp(T). The net effect is to reduce VOC (the open-circuit
Voltage) linearly with increasing
temperature. The magnitude of this reduction is inversely
proportional to VOC; that is, cells with
higher values of VOCsuffer smaller reductions in voltage with
increasing temperature. For most
crystalline silicon solar cells the reduction is about 0.50%/C,
though the rate for the highest-
efficiency crystalline silicon cells is around 0.35%/C. By way
of comparison, the rate for
amorphous silicon solar cells is 0.20-0.30%/C, depending on how
the cell is made.
The amount of photogenerated currentILincreases slightly with
increasing temperature because
of an increase in the number of thermally generated carriers in
the cell. This effect is slight,however: about 0.065%/C for
crystalline silicon cells and 0.09% for amorphous silicon
cells.
The overall effect of temperature on cell efficiency can be
computed using these factors in
combination with the characteristic equation. However, since the
change in voltage is much
stronger than the change in current, the overall effect on
efficiency tends to be similar to that on
voltage. Most crystalline silicon solar cells decline in
efficiency by 0.50%/C and most
amorphous cells decline by 0.15-0.25%/C. The figure to the right
shows I-V curves that might
typically be seen for a crystalline silicon solar cell at
various temperatures.
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Series resistance
Fig.18 Effect of series resistance on the current-voltage
characteristics of a solar cell
As series resistance increases, the voltage drop between the
junction voltage and the terminal
voltage becomes greater for the same flow of current. The result
is that the current-controlled
portion of the I-V curve begins to sag toward the origin,
producing a significant decrease in the
terminal voltage Vand a slight reduction inISC, the
short-circuit current. Very high values of RSwill also produce a
significant reduction inISC; in these regimes, series resistance
dominates and
the behavior of the solar cell resembles that of a resistor.
These effects are shown for crystalline
silicon solar cells in the I-V curves displayed in the figure to
the right.
Shunt resistance
Fig.19 Effect of shunt resistance on the currentvoltage
characteristics of a solar cell
As shunt resistance decreases, the current diverted through the
shunt resistor increases for a
given level of junction voltage. The result is that the
voltage-controlled portion of the I-V curvebegins to sag toward the
origin, producing a significant decrease in the terminal
currentIand a
slight reduction in VOC. Very low values ofRSH will produce a
significant reduction in VOC.
Much as in the case of a high series resistance, a badly shunted
solar cell will take on operating
characteristics similar to those of a resistor.These effects are
shown for crystalline silicon solar
cells in the I-V curves displayed in the figure to the
right.
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Reverse saturation current
Fig.20 Effect of reverse saturation current on the
current-voltage characteristics of a solar cell
If one assumes infinite shunt resistance, the characteristic
equation can be solved for VOC:
Thus, an increase inI0produces a reduction in VOCproportional to
the inverse of the logarithm
of the increase. This explains mathematically the reason for the
reduction in VOCthat
acompanies increases in temperature described above. The effect
of reverse saturation current
on the I-V curve of a crystalline silicon solar cell are shown
in the figure to the right. Physically,
reverse saturation current is a measure of the "leakage"of
carriers across the p-n junction in
reverse bias. This leakage is a result of carrier recombination
in the neutral regions on either
side of the junction.
4.4) DC-DC CONVERTER
Introduction.
The power switch was the key to practical switching regulators.
Prior to the invention of the
Vertical Metal Oxide Semiconductor (VMOS) power switch,
switching supplies were generally
not practical.The inductor's main function is to limit the
current slew rate through the power
switch. This action limits the otherwise high-peak current that
would be limited by the switch
resistance alone. The key advantage for using an inductor in
switching regulators is that an
A linear regulator uses a resistive voltage drop to regulate the
voltage,losing power (voltage
drop times the current) in the form of heat.A switching
regulators inductor does have a voltage
drop and an associated current but the current is 90 degrees out
of phase with the voltage.
Because of this,the energy is stored and can be recovered in the
discharge phase of the
switching cycle.This results in a much higher efficiency and
much less heat.
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What is a Switching Regulator?
A switching regulator is a circuit that uses a power switch,an
inductor,and a diode to transfer
energy from input to output. The basic components of the
switching circuit can be rearranged tofrom a step-down(buck)
,step-up(boost).or an inverter (flyback). These design are shown in
fig.
21,22 ,23 & 24 respectively,where Figures 23 & 24 are
the same except for the transformer and
the diode polarity.Feedback and control circuitry can be
carefully nested around these circuits to
regulate the energy transfer and maintain a constant output
within nornmal operating conditions.
Fig.21 Buck converter topologies
Fig..22 simple boost converter
Figure 23. Inverting topology.
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Figure 24.Transformer flyback topology.
Why Use a Switching Regulator?Switching regulators offer three
main advantages compared to a linear regulators. First,switching
efficiency can be much better than linear. Second, because less
energy is lost in thetransfer smaller components and less thermal
management are required. Third, the energy stored
by an inductor in a switching regulator can be transformed to
output voltages that can be greaterthan the input (boost), negative
(inverter), or can even be transferred trough a transformer
toprovide electrical isolation with respect to the input.Linear
regulators provide lower noise and higher bandwidth ,their
simplicity can sometimesoffer a less expensive solution. These are
the advantages of the linear regulators.
There are, admittedly, disadvantages with switching
regulators.They can be noisy and require
energy management in the form of a control loop.The solution to
these control problems is
found integrated in modern switching modes controller chips.
Charge Phase
A basic boost configuration is depicted in fig.25. Assuming that
the switch has been open for a
long time and that the voltage drop across the diode is
negative, the voltage across the capacitor
is equal to the input voltage. When the switch closes, the input
voltage, +V IN, is impressed
across the inductor and the diode prevents the capacitor from
discharging +V OUT to ground.
Because the input voltage is DC, current through the inductor
rises linearly with time at a rate
proportional to the input voltage divided by the inductance.
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Figure 25. Charging phase: when the switch closes, current ramps
up through the inductor.
Discharge Phase
Fig.26shows the discharge phase. When the switch opens again,
the inductor current continues
to flow into the rectification diode to charge the output.As the
output voltage rises,the slope of
the current ,di/dt though the inductor reverses. The output
voltage rises until equilibrium is
reached or:
VL= Ldi/dt
In other words, the higher the inductor voltage, the faster
inductor current drops.
Fig.26 Discharge phade:when the switch opens,current flows to
the load through the rectifying
diode
In a steady-state operating condition the average voltage across
the inductor over the entire
switching cycle is zero. This implies that the average current
through the inductor is also in
steady state. This is an important rule governing all
inductor-based switching topologies. Taking
this one step further, we can establish that for a given charge
time ton and a given input voltage
and with the circuit in equilibrium, there is a specific time,
tOFF, for an output voltage. Because
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the average inductor voltage in steady state must equal zero, we
can calculate for the boost
circuit.
VIN tON= tOFF VL
and because:
VOUT= VIN+ VL
We can then establish the relationship:
VOUT= VIN (1 + tON/tOFF)
using the relationship for duty cycle (D):
tON/(tON+ tOFF) = D
Then for the boost circuit:
VOUT= VIN/(1-D)
Similar derivations can be had for the buck circuit:
VOUT= VIN D
and for the inverter circuit (flyback):
VOUT= VIN D/(1-D)
Introduction to MicrocontrollerAMicrocontroller has a CPU in
addition to a fixed amount of RAM,ROM,I/O ports, and a timer
all on a single chip.In other words,the
processors,RAM,ROM,,I/Oports,and timer are all
embedded together on one chip; therefore, the designer cannot
add any external memory, I/O,or
timer to it.The fixed amount of on-chip ROM,RAM and number of
I/O ports in microcontrollers
makes them ideal for many applications in which cost and space
are critical.In manyapplications,for example a TV remote
control,there is no need for the computing power of a 486
or even an 8086 microprocessor.
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Block Diagram of Microcontroller:
CPU RAM ROM
I/O Timer Serial COM Port
Criteria for choosing a microcontroller
The first and foremost criterion in choosing a microcontroller
is that it must the task at hand
efficiently and cost effectively. In analyzing the needs of a
microcontroller-based project,we
must first see whether an 8-bit,16-bit,or 32-bit microcontroller
can best handle the computing
needs of the task most effective
Among other considerations in this category are:
Speed
Packaging
Power consumption
The amount of RAM & ROM chip.
The number of I/O pins and the timer on the chip.
How easy to upgrade to higher-performance or lower
power-consumption version.
Cost per unit.
All these criterion are fulfill by using a AVR
ATMEGA8Microcontroller
Features of AVR ATMEGA8 Microcontroller
High-performance, Low-power AVR 8-bit MicrocontrollerAdvanced
RISC Architecture
130 Powerful Instructions Most Single-clock Cycle Execution
32 x 8 General Purpose Working Registers
Fully Static Operation
Up to 16 MIPS Throughput at 16 MHz
On-chip 2-cycle Multiplier
High Endurance Non-volatile Memory segments
8K Bytes of In-System Self-programmable Flash program memory
512 Bytes EEPROM
1K Byte Internal SRAM
Write/Erase Cycles: 10,000 Flash/100,000 EEPROM
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Data retention: 20 years at 85C/100 years at 25C(1)
Optional Boot Code Section with Independent Lock Bits
In-System Programming by On-chip Boot Program
True Read-While-Write Operation
Programming Lock for Software Security
Peripheral Features
Two 8-bit Timer/Counters with Separate Prescaler, one Compare
Mode
One 16-bit Timer/Counter with Separate Prescaler, Compare Mode,
and Capture
Mode
Real Time Counter with Separate Oscillator
Three PWM Channels
8-channel ADC in TQFP and QFN/MLF package
Eight Channels 10-bit Accuracy
6-channel ADC in PDIP package
Six Channels 10-bit Accuracy
Byte-oriented Two-wire Serial Interface
Programmable Serial USART
Master/Slave SPI Serial Interface
Programmable Watchdog Timer with Separate On-chip Oscillator
On-chip Analog Comparator
Special Microcontroller Features
Power-on Reset and Programmable Brown-out Detection
Internal Calibrated RC Oscillator
External and Internal Interrupt Sources
Five Sleep Modes: Idle, ADC Noise Reduction, Power-save,
Power-down,
andStandby
I/O and Packages
23 Programmable I/O Lines
28-lead PDIP, 32-lead TQFP, and 32-pad QFN/MLF
Operating Voltages
2.7 - 5.5V (ATmega8L)
4.5 - 5.5V (ATmega8)
Speed Grades
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0 - 8 MHz (ATmega8L)
0 - 16 MHz (ATmega8)
Power Consumption at 4 Mhz, 3V, 25C
Active: 3.6 mA
Idle Mode: 1.0 mA
Power-down Mode: 0.5 A
LOAD (BATTERY):
The current limiting method is required to charged the
liead-acid batteries.The charge time of a
sealed lead-acid battery is 12-16 hours (up to 36 hours for
larger capacity batteries). With higher
charge currents and multi-stage charge methods, the charge time
can be reduced to 10 hours or
less. Lead-acid cannot be fully charged as quickly. It takes
about 5 times as long to recharge a
lead-acid battery to the same level as it does to discharge.
A multi-stage charger first applies a constant current charge,
raising the cell voltage to a presetvoltage. Stage 1 takes about 5
hours and the battery is charged to 70%. During the topping
charge in Stage 2 that follows, the charge current is gradually
reduced as the cell is being
saturated. The topping charge takes another 5 hours and is
essential for the well being of the
battery. If omitted, the battery would eventually lose the
ability to accept a full charge. Full
charge is attained after the voltage has reached the threshold
and the current has dropped to 3%
of the rated current or has leveled off. Fig. shows the charging
characteristics of battery.
Fig.27 Charging Characteristics of Battery
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CHAPTER 5.HARDWARE DESCRIPTION
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CIRCUIT ANALYSIS & DESCRIPTIONTo track the Maximum power
point, a Hardware section is required which consist of
differentparts. Each part performs the different function.
The different hardware parts are:
Microcontroller
Buffer
Opto coupler
Transistor Amplifier
Buck converter
Current to Voltage converter using OP-AMP
Positive voltage to Negative Voltage Converter
Micro switch which connected to Port of microcontroller
LED which connected to Port of microcontroller
Positive 5V regulated Power Supply
LCD (Liquid Crystal Display)
Specification of the MPPT Solar Charge Controller:
Design for 1Amp Current
Consider the,Solar Photovoltaic Module Voltage = 25 Volt,
which is the Input of the Buck Converter so ,Vin = 25V
Required Voltage for Charging a 12V Battery about 13.5Volt,
So we consider the Output of the Buck converter,
Vout = 15Voltand consider the Output current Iout = 1Amp
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Now
DESIGNING OF BUCK CONVERTER
Fig.28 Buck converter using IC3524
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Vin = 25 Volt
Vout = 15Volt
Iout = 1Amp
f = 31.25 KHz
L =( 2.5 x 25 x 15)/31.25 x 1000 x ( 15 + 25 ) x 1
= 0.75 Mh
Consider, Vo = 50mV
Co = 1 x 15 / 0.05 x 31.25 x1000 (15 +25)
= 240 FNearest available value is 220F, 35V.
To reduce the ripple component a 0.1 F Non-electrolytic
capacitor is connected in parallelwith the electrolytic
capacitor.
Now Enhancement of the Output Current
Design for 5Amp CurrentVin = 25 Volt
Vout = 15Volt
Iout = 5Amp
f= 31.25 KHz
L =( 2.5 x 25 x 15)/31.25 x 1000 x ( 15 + 25 ) x 5
= 0.15mH
Ripple Voltage
Vo = 50mV
Co = 5 x 15 / 0.05 x 31.25 x1000 (15 +25)
= 1200 FNearest available value is 1000uF, 35V & 220F.These
two are connected in parallel to get 1220F
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BLOCK DIAGRAM OF THE CIRCUIT
SWITCH(micro)LED
BUFFER
AVR ATMEGA8 MICROCONTROLLER
OPTO-COUPLER
SOLAR PVARRAY
LOADBUCKCONVETERAMPLIFIER
LCDDISPLAY
Fig.29
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CIRCUIT DIAGRAM OF MPPT CHARGE CONTROLLER
1K
1N4148
+5V
100E
21
1000uF
1
PD1 3
+12V
1
10E
1N4148
+12V
0.1uF
10K
4
22
2
1
1N4148
1K
100nF
1K,1/2W
0.1uF
2
5PD2
20
3
2 TL084
-12V
+5V
1N4148
0.uF
470E
100uF
100uF
6PD3
9
4
3
1N4148
3
SPV
0.1uF
VI
PD4
10
5
+5V
2.2K
1K
+12V
0.1E
VO
+5V
0.75mH
PD58
6
11
1N4148
0.1E
+5V
1 3
TL084
LM7805
R17
PD6
7
7
C4
1n
12
1N4148
+5v
2
+
ATMEGA
8
MI
CRO
CONTROLLER
TC7660
47K
PD7
23
8
13
Batt.
-12V
1N4148
ADC0
-47K
PB0
10uF
14
330E
1K
10K
1K
+5V
PB1
ADC1 24
15
330E
5K
SW Push button
10K
10uF
PB2
SW Push button
ADC2 25
16
330E
1K
47K
+
CD4049 BA159
10K
10K
PB3
ADC3 26
17
LCD DISPLAY 16X2
47K
10KPC817
10K
ADC4 27
18
10K
10K
+5V
BD139
100uF
10uF
+5V
ADC5 28
19PB4
10K
1N4148
1K
SW Push button
IRF9640
100uH+12v
0
.1uF
PD0 2
1
PB5
Fig.30
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PIN CONFIGURATION OF AVR ATMEGA8 MICROCONTROLLER
PIN FUNCTION
PIN 1: PC6- Generic IO pin PC6(Port C6)
/RESET- Reset Pin for MCU, active at low
PIN 2: PD0- Generic IO pin PD0 (Port D0)
PIN 3: PD1- Generic IO pin PD1 (Port D1)
PIN 4: PD2- Generic IO pin PD2 (Port D2)
INTO- External interrupt source 0 to the MCU.
PIN 5: PD3- Generic IO pin PD3 (Port D3)
INT1- External interrupt source 1 to the MCU.
PIN 6: PD4- Generic IO pin PD4(Port D4)
T0- Timer/Counter0 clock sourceXCX- USART external clock
PIN 7: Vcc Power Supply(+5V)
PIN 8: GND Common Ground
PIN 9: PB6- Generic IO pin PB6(Port B6)
XTAL1-Pin for external clock source (crystal,resonator) for
MCU(input) TOSC1-Timer
Oscillator Pin1-clock source for asynchronous clocking of
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Timer/counter1
PIN 10: PB7- Generic IO pin PB6(Port B7) XTAL2-Pin for external
clock source
(crystal,resonator) for MCU(inTOSC2-Timer Oscillator Pin2-clock
source for asynchronous
clocking of Timer/counter1
PIN 11: PD5- Generic IO pin PD5(Port D5) T1-Timer/Counter1 clock
source.
PIN 12: PD6- Generic IO pin PD6(Port D6) N0: AIN0 Analog
comparator Positive
input.When configured as an input and with the internal MOS
pull-up resistor switched
off,thin film also serves as the positive input of the on chip
analog comparator.
PIN 13: PD7- Generic IO pin PD7(Port D6)AIN1: AIN1 Analog
comparator Negative input.
When configured as an input and with the internal MOS pull-up
resistor switched off, thin
film also serves as the negative.
PIN 14: PB0- Generic IO pin PB0(Port B0)
ICP1-Timer/Counter1 input capture pin.
PIN 15: PB1- Generic IO pin PB1(Port B1)
OC1A-Output Compare matchA output.The pin can serve as an
external output for theTimer/Counter1 output CompareA.The pin has
to be configured as an output to serve the
function. The OC1A pin is also the output pin for the PWM mode
timer function.
PIN 16: PB2- Generic IO pin PB2(Port B2)
/ss-slave select pin for using with SPI.
OC1A-Output Compare matchB output.The pin can serve as an
external output for the
Timer/Counter1 output CompareB.The pin has to be configured as
an output to serve the
function. The OC1A pin is also the output pin for the PWM mode
timer function.
PIN 17: PB3- Generic IO pin PB3(Port B3)
OC2-Timer/Counter2 output compare match output. The pin can
serve as an external output
for the timer/counter2 output compare.the pin has to be
configured as an output to serve thisfunction.The OC2 pin is also
the output pin for the PWM mode timer function.
PIN 18: PB4- Generic IO pin PB4(Port B4)
MISO-Data output pin for memory uploading or SPI.
PIN 19: PB5- Generic IO pin PB5(Port B5)
SCK-Clock input pin for memory up/downloading or SPI.
PIN 20: AVCC- Power supply for AD Converter.
PIN 21: AREF- Reference voltage for AD converter.
PIN 22: GND- Common Ground.
PIN 23: PC0- Generic IO pin PC0(Port C0)
ADC0-Analog to Digital input ADC0.
PIN 24: PC1- Generic IO pin PC1(Port C1)
ADC1-Analog to Digital input ADC1.
PIN 25: PC2- Generic IO pin PC2(Port C2)
ADC2-Analog to Digital input ADC2.
PIN 26: PC3- Generic IO pin PC3(Port C3)
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A simplified block diagram of the 8-bit Timer/Counter.
Fig.31
The Timer/Counter (TCNT2) and Output Compare Register (OCR2) are
8-bit registers.
Interrupt request.signals are all visible in the Timer Interrupt
Flag Register (TIFR).All interrupts
are individually masked with the Timer Interrupt Mask Register
(TIMSK).
Definitions:
Many register and bit references in this document are written in
general form. A lower case nreplaces the Timer/Counter number, in
this case. However, when using the register or bit
defines in a program, the precise form must be used (i.e., TCNT2
for accessing Timer/Counter2
counter value and so on).
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Timer/Counter
Clock Sources:
The Timer/Counter can be clocked by an internal synchronous or
an external asynchronous
clock source. The clock source clkT2 is by default equal to the
MCU clock, clk I/O. When the
AS2 bit in the ASSR Register is written to logic one, the clock
source is taken from the
Timer/Counter Oscillator connected to TOSC1 and TOSC2.
Counter Unit:
The main part of the 8-bit Timer/Counter is the programmable
bi-directional counter unit.
Counter Unit Block Diagram
Fig.32
Signal description (internal signals):
count Increment or decrement TCNT2 by 1.
direction Selects between increment and decrement.
clear Clear TCNT2 (set all bits to zero).
clkT2 Timer/Counter clock.TOP Signalizes that TCNT2 has reached
maximum
value.
BOTTOM Signalizes that TCNT2 has reached minimum
Value (zero)
Fast PWM Mode:
The fast Pulse Width Modulation or fast PWM mode (WGM21:0 = 3)
provides a high PWM
waveform generation option. The fast PWM differs from the other
PWM option by its single-
slope operation. The counter counts from BOTTOM to MAX then
restarts from BOTTOM. In
non-inverting Compare Output mode, the Output Compare (OC2) is
cleared on the Compare
Match between TCNT2 and OCR2, and set at BOTTOM.In inverting
Compare Output mode,
the output is set on Compare Match and cleared at BOTTOM.Due to
the single-slope operation,
the operating frequency of the fast PWM mode can be twice as
high as the phase correct PWM
mode that uses dual-slope operation. This high frequency makes
the fast PWM mode well
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suited for power regulation, rectification, and DAC
applications. High frequency allows
physically small sized external components (coils, capacitors),
and therefore reduces total
system cost.
Fast PWM Mode, Timing Diagram
Fig.33
This 8-bit PWM is connected to a buffer IC.
Digital Buffer:
Output of the NOT gate is the "complement" or inverse of its
input signal. For example, when
its input signal is "HIGH" its output state will NOTbe "HIGH"
and when its input signal is
"LOW" its output state will NOTbe "LOW", it inverts. Another
single input logical device used
a lot in electronic circuits and which is the reverse of the NOT
gate is called a Digital Buffer.
A Digital Buffer is another single input device that does no
invert or perform any type of
logical operation on its input signal as its output exactly
matches that of its input signal. In other
words, its Output equals its Input. It is a "Non-inverting"
device and so will give us the Boolean
expression of: A = Q.
Then we can define the operation of a single input Digital
Buffer as being:
"If A is true, then Q is true"
A Digital Buffercan also be made by connecting together two NOT
gates as shown below. The
first will "invert" the input signal A and the second will
"re-invert" it back to its original level.
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1 2
BUFFER
AA
1 2AA
1 2A'
Fig.34
Truth Table
A Q
0 0
1 1
HEX INVERTER IC CD4049 used as a buffer when two inverter are
connected i.e
output of the one inverter is connected to the other that
produces the buffer. Buffer is
used to boost up the PWM signal & also used for High
impedance matching at the
output.
OPTO-COUPLER
Output of the Buffer is connected to a Opto-coupler.
Opto-coupler is used as a
isolator. It isolate the digital section from the high voltage
power section. This isolator
is used to save the low voltage digital circuit from the high
voltage power circuit.
PC817 used as a Opto-coupler.
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Theory of Optocoupler
The optical coupler is a venerable device that offers the design
engineer new freedoms in
designing circuits and systems. Problems such as ground loop
isolation,common mode noise
rejection, power supply transformations, and many more problems
can be solved or simplifiedwith the use of an optical coupler.
Operation is based on the principle of detecting emitted light.
The input to the coupler is
connected to a light emitter and the output is a photodector,
the two elements being separated by
a transparent insulator and housed in a lightexcluding package.
There are many types of
optical couplers; for example, the light source could be an
incandescent lamp or a light emitting
diode (LED). Also,the detector could be photovoltaic cell,
photoconductive cell, photodiode,
phototransistor, or a lightsensitive SCR. By various
combinations of emitters and detectors, a
number of different types of optical couplers could be
assembled. Once an emitter and detector
have been assembled as a coupler, the optical portion is
permanently established so that device
use is only electronic in nature. This eliminates the need for
the circuit designer to have
knowledge of optics.
COUPLER CHARACTERISTICS
The PC817 is an optical coupler consisting of a gallium arsenide
(GaAs) LED and a silicon
phototransistor.
INPUT
For most applications the basic LED parameters IF and VF are all
that are needed to definethe input. Fig.36 shows these forward
characteristics, providing the necessary information to
design the LED drive circuit. Most circuit applications will
require a current limiting resistor in
series with the LED input.
OUTPUT
The output of the coupler is the phototransistor. The basic
parameters of interest are the
collector current IC and collector emitter voltageVCE.Figure37
is a curve of VCE(sat) versus
IC for two different drive level.
COUPLING
To fully characterize the coupler, a new parameter, the dc
current transfer ratio or coupling
efficiency () must be defined. This is the ratio of the
transistor collector current to diode
current IC/IF.
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RESPONSE TIME
The speed is fairly slow compared to switching transistors, but
is typical of phototransistors
because of the large basecollector area. The switching time or
bandwidth of the coupler is a
function of the load resistor RL because of the RLCO time
constant where CO is the parallel
combination of the device and load capacitances.
Fig.35 Opto-Coupler
Fig.36Input Characteristic of Opto-Coupler
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Fig.37 Output Characteristic of Opto-Coupler
AMPLIFIER
Output of the Opto-coupler is connected to a Transistor
Amplifier which is used to amplify
the DC level of the PWM signal. BD139 power transistor is used
as a Amplifier when
proper biasing is applied to the circuit.
Circuit Diagram of the Amplifier:
Opto-coupleroutput
1K,
1/2
W
2.2K100E
SPV
O/P of Amplifier
Fig.38
This Amplifier output is used to drive the Gate of MOSFET. So
this amplifier is used as a
driver circuit of the MOSFET.
MOSFET used as a switch for Buck Converter
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Why we use Buck Converter:We know that for charging a 12Volt
battery minimum required voltage is 13.5 Volt, but
Solar PV Module produces a 25Volt.So to step down the voltage
Buck Converter or Step-
Down converter is required.
BUCK CONVERTER3.3mH
IRF 9640MOSFET
100uF
0.1uF
Output ofBuck conve
1K,1/2 W
100E
Amplified PWM
SPV Voltage
Fig.39
Buck Converter:
In a regulator ,the average output voltage Va,is less than the
input voltage,Vs hence the name
buck, a very popular regulator. The circuit diagram of a buck
regulator using a MOSFET.The
circuit operation can be divided into two modes. Mode 1 begins
when MOSFET is switched on
at t=0. The input current,which rises,flows through filter
inductor L, filter capacitor C and loadresistor R.Mode2 begins when
MOSFET is switched off at t=t1.The freewheeling diode Dm
conducts due to energy stored in the inductor and the inductor
current continues to flow through
L,C,load and diode Dm. The inductor Current falls until MOSFET
is switched on again in the
next cycle. The equivalent circuits for the modes of operation
are shown in figure.39.The
waveforms for the voltage & current flows continuously in
the inductor L.It is assumed that the
current rises and falls linearly in practical circuits, the
switch has a finite, nonlinear
resistance.Its effect can generally be negligible in most
applications. Depending on the
switching frequency, filter inductance and capacitance the
inductor current could be
discontinuous.
The voltage across the inductor L is , in general
eL = L di/dt
Assuming that the inductor current rises linearly from I1to I2in
time t1,
Vs Va = L ( I2-I1/t1) = L I/t1
Or t1 = IL/(Vs- Va)
and the inductor current falls linearly from I2 to I1 in time
t2,
-Va = - L I/t2
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R2= 10K
R2/(RI+ R2) x 25 = 10K/(10K + 47K) x 25 V = 4.38V
This voltage is connected to a ADC port of the ATMEGA8
Microcontroller.
Similarly, Output of the Buck Converter is required to reduced
upto +5V.
Output of Buck Converter = 15V.
R1= 22K
R2= 10K
R2/(R1+ R2)X15 = 10K/(10K + 22K) x 15 V = 4.68V
22K
10K
Buck Converter O/P
1N4148
1N4148
1K
+5V
Fig.41
Current to Voltage Converter:
10K
-5V
0.1E
+5V
10K
3
2
7
4
6
1
5+
-
V+
V-
OUT
OS1
OS2
1K
3
2
7
4
6
1
5+
-
V+
V-
OUT
OS1
OS2
47K
12V,7A-h Battery
O/P of the Buck Converteror
Solar PV Voltage
+5V
-5V
Fig.42
It is an Inverting amplifier using Op-Amp TL084.
If Battery is charged by the 1A Current. We connect a
0.1resistance which is
connected in series with the Battey. So Maximum Voltage is
drpped across 0.1
is 0.1V.
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Now this Voltage is amplified by using a Inverting
amplifier.
For Inverting Amplifier,
Vo/Vin = -Rf/Rin
Vo should be maximum 5V.
Gain hould be Maximum (5/0.1)=50
We consider the Rin = 1k
Rf = 50K
We consider the 47K resistor because nearest available Value of
50k resistor is 47K.It is
Inverting amplifier so it produces a -4.7V. To obtain a +4.7V
output a similar amplifier is
designed whose gain is 1. So we consider Rf= Rin=10K.
So, all the Voltage & Current of a Solar PV Module &
Buck Converter are converted into
+5V.
Now all the voltage & Current of a Solar PV Module &
Buck Converter are connected to
a ADC Port of a Microcontroller.
Pin No.23,24,25,26 of a Microcontroller used as a ADC
Channel.
The Micro Switches are connected to a Port D4 & D5 to Set or
Reset the Microcontroller.
Pin No.6,11 of a Microcontroller used as a Port D4 & D5.
Light Emitting Diode (LED) are connected to a Port B0,B1,B2 to
indicate the differentoperating condition.
Pin No.14,15,16 of a Microcontroller used as a Port B0, B1 &
B2.
Maximum Power Point Tracking (MPPT) Method:
Maximum Power Point Tracking is performed by using a very
advanced method
(Incremental Conductance Method) which is independent of
temperature, other
atmospheric condition that is used to track the maximum Power
Point very quickly.
Maximum Power Point is tracked by using a microcontroller. So,
to track the maximum
power point, Solar PV Module voltage is connected to a source of
the p- channel MOSFET
and also connected to a ADC of the Microcontroller. Solar PV
module Current is also
converted into a Voltage and is connected to a ADC of the
microcontroller.
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Similarly, Buck converter output Voltage is connected to a ADC
of the microcontroller.
The output of the Buck is used to charging a Battery.So the
current of the Buck Converter
is converted to a voltage which is connected to a ADC of the
microcontroller.
For Maximum Power Point tracking Solar PV Module voltage is
connected to a Buck
Converter which is controlled by the duty cycle of the PWM. The
PWM is generated from
the AVR ATMEGA 8 Microcontroller.
So, to track the Maximum Power Point ,
For a particular duty cycle of the PWM,
Measure the
Solar PV Module Voltage ( Vin1)
Solar PV Module Current (Iin1)Buck Converter Output Voltage
(Vout1)
Buck Converter Output Current (Iout1)
Now increase the duty cycle by 1
Measure the
Solar PV Module Voltage ( Vin2)
Solar PV Module Current (Iin2)
Buck Converter Output Voltage (Vout2)
Buck Converter Output Current (Iout2)
Now measure the difference between the Voltage & current to
obtain the conductance
Vin = Vin2 Vin1
Iin = Iin2 Iin1
Vout = Vout2 Vout1
Iout = Iout2 Iout1
So the Incremental Conductance (S1) = Iin / Vin
And instanteneous Conductance (S2) = - ( Iin2/ Vin2)
Then the Maximum Power Point will tracked that means MPPT will
performed.
So to track the Maximum Power Point a Software Programming is
required to adjust the
duty cycle automatically & extract the Maximum Power from
the Solar PV Module for
every change in Voltage & Current of the PV module.
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CHAPTER 6.
SOFTWARE DESCRIPTION.
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SOFTWARE SECTION
The Solar Photovoltaic Maximum Power Point Tracking charge
Controller is controlled by the
variation of the duty cycle of the PWM which is used to control
the Buck Converter.
This control is done by the microcontroller. There are different
programming language which is
used in Microcontroller such as Assembly, C language ,Basic
etc.
Here, a special type of Software is used to compile the
programmed in microcontroller. Name
of the Software is AVR BASCOM. In the BASCOM software we can
write the program in
different languages such as Assembly, C language ,Basic etc.
But in this project Program is written by the AVR BASCOM BASIC
language. This language
is very easy to write and also very easy to debug the error.
So to write the Software coding, a algorithm is required. Here a
special type of algorithm is
used to track the Maximum Power Point. The Algorithm is
Incremental Conductance method.
This algorithm is independent of temperature, other atmospheric
condition.
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DESCRIPTION OF THE ALGORITHM (Incremental Conductance
Method):
Detetc the Solar PV Module Volage V1 & Output Current I1 for
a particular duty cycle of the
PWM.
Measure the Power P1 =V1xI1.
Now increase the duty cycle by 1 and measure the Solar Module
Voltage V2 & O/P Current I2
.
Measure the Power P2 =V2xI2
Measure V = V2 V1I = I2 I1
If V = 0 then I = 0
If I = 0 then output voltage remains the same
If I 0 & If I > 0 decrease the duty cycle by 1
If I < 0 increase the duty cycle by 1
If V0 then I/V = - (I2/ V2)
If I/V = - (I2/ V2) then output remains same
If I/V - (I2/ V2) then if I/V > - (I2/ V2)
Increase the duty cycle by 1
if I/V < - (I2/ V2) then decrease the duty cycle by 1
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FLOW CHART OF MPPT using (INC)
YES
NO
YES YES
NO NO
YES YES
NO NO
NO
Detect
V(k) &I(k)
Compute dV & dI
dV=V(k)-V(k-1)
dI=I k -I k-1
START
dV=0
Renew V(k) & I(k)
V(k-1)=V(k)
I(k-1)=I(k)
dI/dV
= -I/V
dI/dV
> -I/V
dI>0
Decrease O/P
voltage
Increase O/P
Voltage
Decrease
O/P Voltage
dI=0
O/P Voltage remains
the same
Increase O/P
voltage
O/P
Volt.remains
the same
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Coding of Incremental Conductance Method
(Using AVR BASCOM BASIC Language)
'$sim
$regfile = "m8def.dat"
$crystal = 8000000
'
/////////////////////////////////////////////////////////////////////////////////////////////'
Config Adc = Single , Prescaler = Auto , Reference = Avcc
Start Adc
'//////////////////////////////////////////////////////////////////////////////////////////////'
Dim W1 As Word 'spv voltage1'
Dim W2 As Word 'o/p current1'
Dim W3 As Word 'spv voltage2'
Dim W4 As Word 'o/p current2'
Dim S1 As Integer 'change in voltage'' 'Dim S2 As Integer
'change in current''
Dim R1 As Single 'change in conductance'
Dim R2 As Single 'instanteneous conductance'
'////////////////////////////////////////////////////////////////////////////////////////////////////////
Config PortB.3 = Output
Config Timer2 = Pwm , Prescale = 8 , Pwm = On , Compare Pwm =
Clear Up
'///////////////////////////////////////////////////////////////////////////////////////////////////////////
Dim Gp As Integer 'duty cycle of Pwm'
Gp = 1
Main:
Compare2 = Gp
Waitms 200
B:
W1 = Getadc(2)
Waitms 200
W2 = Getadc(3)
Waitms 200
'//////////////////////////////////////////////////////////////////////////////////////////////'
Gp = Gp + 1 'duty cycle is increased by 1'Compare2 = Gp
Waitms 200
Start:
W3 = Getadc(3)
Waitms 200
W4 = Getadc(2)
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'///////////////////////////////////////////////////////////////////////////////////////////'
S1 = W3 - W1
S2 = W4 - W2
'/////////////////////////////////////////////////////////////////////////////////////////
If S1 = 0 Then
Goto C
Else
Goto D
End If
'/////////////////////////////////////////////////////////////////////////////////////'
C:
If S2 = 0 Then
Goto EElse
Goto F
End If
'////////////////////////////////////////////////////////////////////
///////////////////////
D:
R1 = S2 / S1
R2 = W4 / W3
'/////////////////////////////////////////////////////////////////////////////////////////
If R1 = R2 Then
Goto GElse
Goto H
End If
'///////////////////////////////////////////////////////////////////////////////////////////
'
E:
Gp = Gp
Compare2 = Gp
Goto End
F:
If S2 > 0 Then
Goto I
Else
Goto J
End If
'////////////////////////////////////////////////////////////////////'
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G:
Gp = Gp
Compare2 = Gp
Goto End
H:
If R1 > R2 Then
Goto K
Else
Goto L
End If
'/////////////////////////////////////////////////////////////////////////'
I:
Gp = Gp + 1Compare2 = Gp
If Gp > 200 Then
Gp = 200
Compare2 = Gp
End If
Goto End
'/////////////////////////////////////////////////////////////////////////////////////////////////'
J:
Gp = Gp - 1
Compare2 = GpIf Gp = 255 Then
Gp = 0
End If
Goto End
K:
Gp = Gp + 1
Compare2 = Gp
If Gp > 200 Then
Gp = 200
Compare2 = Gp
End If
Goto End
'////////////////////////////////////////////////////////////////////////////////////////////////////////////////////'
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L:
Gp = Gp - 1
Compare2 = Gp
If Gp = 255 Then
Gp = 0
End If
Goto End
End:
W3 = W1
W4 = W2
Goto Start
'
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Description of the Algorithm (Modified Incremental Conductance
Method):
Vmax & Imax are the maximum desiarable voltage &
Current.
At First detect the SPV Voltage(Vi1) & SPV Current (Ii1)for
a particular Duty Cycle of the
PWM.
And detect the Buck Converter Output Voltage (Vb1) & Battery
Current (Ib1).
Now Increase the Duty Cycle by 1.
Again, detect the SPV Voltage(Vi2) & SPV Current (Ii2).
And detect the Buck Converter Output Voltage (Vb2) & battery
Current (Ib2).
Now Again Increase the Duty Cycle by 1.
Detect the SPV Voltage(Vi3) & SPV Current (Ii3).
And detect the Buck Converter Output Voltage (Vb3) & Battery
Current (Ib3).
Compute the Change in Voltages & Currents.
dV1=Vb2-Vb1
dI1=Ib2-Ib1
dV2= Vb3-Vb2
dI2=Ib3-Ib2
Now Compute the Change in Conductance
S1=dI1/dV1
S2=dI2/dV2
If S2>S1 then Duty Cycle is increased by 1
And similarly, If S2
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,,,,,
,,,
+
,
+
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Coding of modified Incremental Conductance Method
'$sim
$regfile = "m8def.dat"
$crystal = 8000000
'
//////////////////////////////////////////////////////////////////////////////////////////////
Config Adc = Single , Prescaler = Auto , Reference = Avcc
Start Adc
'//////////////////////////////////////////////////////////////////////////////////////////////
Dim Vb1 As Word buck o/p voltage1
Dim Vb2 As Word buck o/p voltage2
Dim Vb3 As Word buck o/p voltage3
Dim Ib1 As Word battery current1
Dim Ib2 As Word battery current2
Dim Ib3 As Word battery current3Dim Vmax As Word 'max O/P
voltage'
Dim Imax As Word 'max O/P current'
Dim Vi As Word 'spv voltage
Dim Ii As Word spv current
Dim Wi As Word spv power
Dim P1 As Word 'change in voltage1'
Dim P2 As Word 'change in voltage2'
Dim Q1 As Word 'change in current1'
Dim Q2 As Word 'change in curent2'
Dim S1 As Word 'change in conductance1'Dim S2 As Word 'change in
conductance2'
'
///////////////////////////////////////////////////////////////////////////////////////////////////////
Config Portb.3 = Output
Config Timer2 =
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