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Energy Systems Engineering
Student/Group Name(s):
Brian Finan
Student ID:
09548734
Year: (e.g. 1st)
4th Year
Subject Code and Name:
EG460, Final Year Project
Lecturer Name:
Prof. Ger Hurley
Title of Assignment:
Progress Report
Submission Date:
19-12-12
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Brian Finan
09548734
Brian Finan
Abstract
The title of my project is “Maximum power point tracking for solar power applications with
partial shading”. In simple terms this project’s objective is to have a solar panel in the best
possible position all of the time, the purpose of this is to allow the panel access to the
maximum amount of solar irradiance possible and hence output the maximum power
possible. This study consists of examining solar energy as a viable option and obtaining the
maximum amount from the ‘green’ energy source, this means getting the maximum power
from a solar array at any given time. This is obtained by keeping the solar array at its
Maximum Power Point continually. On researching this it has been found that some major
factors come into account when the Maximum Power Point is trying to be located
continually. These factors include partial shading of the solar array, which may be due to
clouds, location of the solar arrays, branches of trees etc. This project looks at issues of
maximum power point tracking under partially shaded conditions, and comes up with a valid
solution to overcome the problem and obtain the Maximum Power Point without
interruption. To achieve Maximum Power Point Tracking (MPPT) there are a number of
options available to date, these are the Perturb and Observe (P&O) MPPT method and
Incremental Conductance MPPT method. I have carried out extensive research on these two
MPPT methods and some of my findings are documented below. It has been found that
solar arrays under partially shaded conditions can have power losses as high as 70%.
Introduction
Background to Solar Power
o Solar energy has become much more important globally in recent years due to the
global energy crisis the world faces.
o Solar panels use light energy from the sun to generate electricity through the
photovoltaic effect.
o A connected assembly of solar cells is known as a photovoltaic (PV) panel. Solar
energy is said to be very reliable as it is easy to predict how much energy can be
produced with PV solar panels.
o Photovoltaic is a method of generating electrical power by converting solar radiation
into direct current (DC) electricity, using semiconductors that exhibit the PV effect.
o Solar panels do not emit any greenhouse gases in operation unlike conventional
sources of energy i.e. fossil fuels.
o Solar tracker applications are widely used to maximise the angle of incident between
the incoming light of the sun and the panel. This angle should always be kept as near
as possible to 90 degrees. Figure 1 shows how much available power is lost if the
angle of the solar array differs from 90 degrees.
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o Driven by advances in technology and increase in manufacturing scale and
sophistication, the cost of this energy has declined steadily since the first solar cells
were manufactured.
Figure 1: Percentage of available power which is lost if the angle between the solar panel
and the sun differs from 90 degrees.
Basic operation of a Photovoltaic (PV) cell
A Photovoltaic cell is made of silicon, which is purified, melted and then crystalized. The
majority of the cell has a slightly positive electrical charge, with a thin layer, at the top,
having a slightly negative charge. A thin grid of metal is placed on the top of the cell which
allows adequate amounts of sunlight to be admitted but also had the ability to carry
electrical energy. Sunlight, sometimes described as particles called ‘photons’, hits the PV cell
and move into the cell. Photons strike electrons and dislodge them, these then become
loose and start to move to the top of the cell, this can be seen in figure 2. The greater the
amount of photons that are admitted by the cell results in a greater flow of electrons
towards the top of the cell. These then flow into the external electrical circuit through the
grid of metal placed on top of the cell. The electric fields in the solar cell put these free
electrons in directional current, from which the metal contacts on top of the cell can
generate electricity.
Therefore, cells produce current and voltage, the amount of current produced depends on
the area of the cell whereas the amount of voltage produced does not depend on the cells
area. Both the voltage and current are affected by the resistance of the circuit the cell is
present in. The light level and the temperature available to the cell affect the amount of
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current and voltage produced respectively, which will have a direct effect on the power
output, this can be seen clearly from figure 4.
Figure 2: Photovoltaic Cell
Maximum Power Point Tracking (MPPT)
This is a method used in Solar Photovoltaic (PV) arrays to expose uniform solar irradiance
and maintain a maximum power output for a period of time. In figure 3 the maximum
power output can clearly be seen at the ‘knee’ of the curve. This is the position that is most
sought after and is achieved when maximum voltage and maximum current are achieved at
the same time. MPPT is a method to ensure that maximum voltage and maximum current is
reached as much as possible. This is done by having the solar array track the path of the sun
and also by making sure that none of the solar array becomes partially shaded at any stage
due to cloud, branches of trees etc., and if this does occur a system is in place to adjust the
panel and get it back to output the maximum current and voltage and hence the maximum
output power. Details of the two methods used to track the maximum power point are
given below; the method I am going using in my project is the Perturb and Observe (P&O)
method.
If irradiance levels differ throughout the solar array, this results in multiple local maxima
points being produced. This results in nonlinearity of the PV characteristic curves, which
means there is more than one ‘knee’ in the P-V curve. Multiple local maxima are not good
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for tracking as it reduces the effectiveness of the tracking system, and these results in
overall loss in power output.
Figure 3: Curve showing voltage vs. current
Figure 4: These graphs show how power output increases as the temperature and irradiance
levels increase.
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Perturb and Observe method of Maximum Power Point Tracking
The purpose of this is to track the Solar Photovoltaic (PV) modules maximum power point.
The P&O tracking process is carried out by observing the array output power and
determining the next action, either to increase or decrease the array operating voltage. In
recent times this method has been widely used to achieve the maximum amount of power
from a solar panel. The presence of multiple local maxima, these occur when an entire PV
array do not receive uniform solar irradiance, reduce the effectiveness of this method
greatly. The reason for a solar array not receiving uniform solar irradiance may be due
clouds, trees, and buildings and when these factors affect solar irradiance, the array is said
to be under Partially Shaded Conditions (PSC).
If the operating voltage of a PV array is perturbed in a given direction and if the power
drawn from the PV array increases, this means that the operating point has moved towards
the Maximum Power Point and, therefore, the operating voltage must be further perturbed
in the same direction. Otherwise, if the power drawn from the PV array decreases, the
operating point has moved away from the Maximum Power Point and, therefore, the
direction of the operating voltage perturbation must be reversed. This is explained in the
form of a flowchart below, figure 6.
If the system increases the operating voltage and the power output increases, the system
will continue to do this until the power output decreases. Then the voltage is decreased to
get back to the system outputting its maximum power output. This continues indefinitely
which results in the power output value oscillating up to the Maximum Power Point
continually and never stabilizing. This can be seen in figure 5.
The advantages of this method include:
o Very simple and easy to implement.
o Most commonly used so information is widely available.
o Provides predictive and accurate solutions to maximum power point tracking under
partially shaded conditions.
The disadvantages of this method include:
o Power obtained oscillates just below the Maximum Power Point in steady state
operation.
o Under rapidly varying irradiance & load conditions the system can track in the wrong
direction.
o The size of the change in operation voltage chosen determines the speed &
convergence of the Maximum Power Point and the range of oscillation.
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Power
Output
Oscillating just
below max
power output
Time
Figure 5: Oscillation of Power just under maximum power output.
Start
Decrease
Operating Voltage
Increase Operating
Voltage
If Incorrect
If Correct
If Correct
If Incorrect
Power >
Previous Power
Power >
Previous Power
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Figure 6: Flowchart of P&O method.
Incremental Conductance method of Maximum Power Point Tracking
This method always adjusts the array terminal voltage according to the maximum power
point voltage. This method computes maximum power and directly controls the extracted
power from the PV cell. This method uses a DC-DC converter which is controlled by an
incremental conductance algorithm. This system offers great performance under quick
changing circumstances and can be implemented using low cost microcontrollers. The
method can track the maximum power points accurately at high speeds and greatly increase
the power output of a solar array under PSC. This method computes the maximum power
point by comparing (ΔI/ΔV) (Incremental conductance) to (I/V) (array conductance).
IL
Figure 7: Circuit diagram of Incremental conductance proof.
PL=IL2RL
IL=VS/Ro+RL
P= (VS/RL+RO)2(RL)
What value of RL will give me maximum load?
Find: dP/dRL
Which equals to: 1=2RL/RL+Ro
Therefore: Ro=RL
This means the maximum power transfer occurs when the load resistance equals the source
resistance
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Details of Microcontroller being used
The microcontroller I am going to use to implement the required algorithm is the Arduino. I
made the decision to use this microcontroller after carrying out research on it and also on
the 8051 microcontroller. The Arduino is relatively simple and is perfectly able to implement
the type of algorithm I am going to use. On researching the 8051 microcontroller I realised
that it is a lot more complicated than the Arduino and may prove hard to implement the
algorithm, if it was chosen.
The main reasons I chose the Arduino is:
o Inexpensive, most Arduino starter kits cost between €50-70.
o The Arduino will work on Windows, Mac and Linux.
o Simple clear and open source programming environment. Software for the
programming of the Arduino can be obtained online for free and the programming
environment is very similar to C and Java for ease of use.
Some characteristics of the Arduino:
o
o
o
o
o
o
6 analogue inputs.
Operating voltage of 5 volts.
DC current i/o pin 40mA.
Flash memory of 32kb (0.5kb used by the boot loader).
Input voltage maximum of 6-20 volts, recommended to use 7-12 volts.
14 digital outputs, 3 of which are pulse width modulators (providing 8 bit pulse width
modulation).
o Board can be powered by the USB port from a computer, 2.1mm centre positive plug
in the board, or the 5 volt and the 3.3 volt connection on the board.
o Clock speed of 16MHz.
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Figure 8: Arduino
Figure 9: Arduino pin locations
Aim of this Project
When this project is completed and built I hope to be able to shadow a part of a solar array
resulting in the array turning into the area of higher solar irradiance where the maximum
power point is present. The materials needed to complete this project are:
o
o
o
o
o
o
A solar array, last year’s project may be used.
Current sensor.
Voltage sensor.
DC-DC converter.
Microcontroller, Arduino.
Wiring, circuitry.
A simple layout of the purposed project is given below in figure 10.
Power
Solar Panel
DC-DC Converter
Voltage Sensor
Power
Current Sensor
MPPT Controller
(Arduino)
Load
Controlled via DAC
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Figure 10: Layout of proposed system.
DC-DC converter
This is a power electronics circuit that convert a DC voltage to a different DC voltage level
and often provides a regulated output. Switch mode DC-DC converters operate by storing
the input energy temporarily and then releasing that energy to the output at a different
voltage and current. A DC-DC converter could be compared to a transformer as they both
carry out much the same role changing input energy into a different impedance level, with
some energy being used by the converter when it is being passed through it. The converter
presents an electrical load to the solar panel that varies as the output voltage varies. This
load variation in turn causes a change in the operation point (current and voltage
characteristics) of the panel. Thus by intelligently controlling the operation of the DC-DC
converter, the power output of the panel can be intelligently controlled and made to output
the maximum possible power.
Voltage sensor
This will measure the voltage provided by the solar panel by using two resistors in parallel
with the solar panel and acting as a voltage divider. Voltage across these resistors will be
read and put into the Maximum Power Point Tracker’s digital controller.
Current sensor
This will measure the current provided by the solar panel. For this a single resistor will be
placed in series between the solar panel and the DC-DC converter. This current will also be
put into the MPPT digital controller.
Digital to Analogue Controller
This is used to convert digital code to an analogue signal, binary code to current, voltage or
electric charge.
Progress made to date
Parallel and Series connected solar arrays
Connecting solar cells in series means you connect the positive terminal of one solar cell to
the negative terminal of another. This results in the voltages of the cells being added
together while the amps stay the same. Connecting solar cells in parallel means you connect
the positive terminal of one cell to the positive terminal of another and the negative
terminal of one cell to the negative terminal of another. This results in the voltage
remaining the same but the currents of the cells will be added together. When an array of
solar cells is connected in series and parallel, significant problems exist with parallel
connections. Shadow effects can shut down the weaker parallel string (string that
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experience partial shading) which would cause a big power loss and some damage to the
weaker string. This damage is caused by excess reverse bias applied to the shadowed cell by
the cells which are receiving full solar irradiance.
In an array like the one shown below in figure 10, each solar cell has a specific maximum
voltage and maximum current. In the array shown each solar cell has a current of 3 amps
and a voltage of 6 volts, therefore this gives us a system with a voltage of 12 volts and a
current of 9 amps. If one of these paths becomes blocked due to partial shading or some
other factor, this will have a major effect. For example if the middle path was blocked the
voltage would remain at 12 volts but the current would be reduced from 9 amps to 6 amps.
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Figure 11: Example of a solar array of solar cells in series and parallel.
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Tasks to Overcome
In the coming weeks I expect my project to take shape. The next stage for me to complete is
to order the parts I need and start to write the required code for the Arduino. When I have
completed this I hope to start testing my work. I have no doubt that I will run into some
problems but with the research I have carried out and further research, if necessary, and the
help of my supervisor, I am confident these will be overcome. I plan to use PSPICE
simulation to model circuits to include in my final report. The system must be then tested
with variable voltage source to represent the panel simulator for a real 50W panel.
References
Figure 1: http://upload.wikimedia.org/wikipedia/en/e/ed/SolarPanel_alignment.png
(website last viewed on 17/12/2012)
Figure 2: http://www.polarpowerinc.com/info/operation20/operation23.htm (Last viewed
on 18/12/2012)
Figure 3: http://www.lettingchiboboshine.org.au/pages/activities/investigating-thecharacteristics-of-photovoltaic-solar-panels.php (last viewed on 18/12/2012)
Figure 4: http://sargosis.com/articles/science/how-pv-modules-work/calculating-the-iv-andpv-curves-for-a-solar-module/ (last viewed on 16/12/2012)
Figure 8: http://arduino.cc/en/uploads/Main/ArduinoUno_R3_Front.jpg (website last
viewed on 17/12/2012)
Figure 9: http://arduino.cc/en/Hacking/PinMapping (website last viewed on 17/12/2012)
The following journals & websites were used for vast research of the subject matter:
o A new approach to achieve maximum power pointtracking for PV system with a vari
able inductor
Zhang, Longlong; Hurley, William Gerard; Wolfle, Werner
Power Electronics for Distributed Generation Systems(PEDG), 2010 2nd IEEE International
Symposium on
Topic(s): Components, Circuits, Devices & Systems ; Power, Energy, & Industry Applications
Digital Object Identifier: 10.1109/PEDG.2010.5545758
Publication Year: 2010 , Page(s): 948 – 952
o Maximum Power Point Tracking Scheme for PVSystems Operating Under Partially S
hadedConditions
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Patel, H.; Agarwal, V.
Industrial Electronics, IEEE Transactions on
Volume: 55 , Issue: 4
Publication Year: 2008 , Page(s): 1689 - 1698
o Incremental conductance MPPT method for PVsystems
Safari, A.; Mekhilef, S.
Publication Year: 2011 , Page(s): 000345 – 000347
o High-Performance Adaptive Perturb and ObserveMPPT Technique for PhotovoltaicBased Microgrids
Abdelsalam, A.K.; Massoud, A.M.; Ahmed, S.; Enjeti, P.N.
Power Electronics, IEEE Transactions on
Volume: 26 , Issue: 4
Publication Year: 2011 , Page(s): 1010 - 1021
o http://www.polarpowerinc.com/info/operation20/operation23.htm
o http://www.lettingchiboboshine.org.au/pages/activities/investigating-thecharacteristics-of-photovoltaic-solar-panels.php
o http://www.solarchoice.net.au/blog/partial-shading-is-bad-for-solar-panels-powersystems/
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