Convertirea energiei solare

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University Politehnica of Bucharest

Faculty of Electronics, Telecommunications and InformationTechnology

Solar tracker system

Dissertation Thesis

submitted in partial fulfillment of the requirements for the Degree of

Master of Science in the the domain electronics, study program

Advanced Microelectronics

Thesis Advisor, Student,

Professor Vasile Alexandru Predescu Vlad

2014

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Table of Contents

1. Solar Energy .................................................................................................................9

1.1 History of Photovoltaic Cells ..................................................................................11

1.2 Incident Angle ..........................................................................................................12

2.Solar Trackers ...............................................................................................................13

2.2 Passive trackers ........................................................................................................15

2.2 Active trackers ..........................................................................................................15

2.2.1 Single axis trackers ...........................................................................................15

2.2.1 Dual axis tracker ...............................................................................................16

2.2.1 Chronological tracker .......................................................................................17

3.Experimental setup .......................................................................................................17

3.1 Hardware setup ................................................................................................17

3.1.1 Stepper motor ...............................................................................................18

3.1.2 Driver ...........................................................................................................22

3.1.3 Light sensors ................................................................................................24

3.1.3.1 Alignment ......................................................................................27

3.1.3.1 Principle of orientation ..................................................................28

3.1.4 Microcontroller ............................................................................................30

3.1.5 Electrical Circuit ...........................................................................................31

3.1.6 The mechanical assembly .............................................................................32

3.2 Software setup ..................................................................................................34

4.The results......................................................................................................................36

Conclusions ........................................................................................................................43

Appendix 1 Arduino Code .................................................................................................44

Bibliography ......................................................................................................................54

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List of figures

1. Fig 1.1 Solar cell section .............................................................................................10

2. Fig 1.2 Angle of Incidence to Solar Cell ....................................................................12

3. Fig 2.1 Sun trajectory during different seasons ..........................................................14

4. Fig 2.2 Single axis trackers .........................................................................................15

5. Fig 3.1 Stepper motor section .....................................................................................19

6. Fig 3.2 Wiring diagram ...............................................................................................20

7. Fig 3.3 Stepper motor signals sequence full step ........................................................21

8. Fig 3.4 Stepper motor signals sequence half step .......................................................21

9. Fig 3.5 H bridge concept .............................................................................................22

10. Fig 3.6 Bipolar Stepping-Motor Control ....................................................................23

11. Fig 3.7 Pin distribution of the driver ............................................................................24

12. Fig 3.8 Equivalent electrical circuit of a solar cell ......................................................25

13. Fig 3.9 The I-V curve of the solar cell ........................................................................26

14. Fig 3.10 Horizontal section of the top of the device ...................................................27

15. Fig 3.11 Vertical section of the top of the device .......................................................28

16. Fig 3.12 Different incident angles upon detectors ......................................................29

17. Fig 3.13 Equal incident angles upon detectors ...........................................................30

18. Fig 3.14 Voltage output on a full rotation ...................................................................30

19. Fig 3.15 Electrical Circuit ............................................................................................32

20. Fig 3.16 Section of the top ..........................................................................................34

21. Fig 4.1 The voltage characteristics of the detection cells ............................................36

22. Fig 4.2 The incident angle upon the detection cell ......................................................37

23. Fig 4.3 Voltage evolution of the Azimuthal detection cells ........................................41

24. Fig 4.4 Voltage evolution of the Elevation detection cells ..........................................42

25. Fig 4.4 Cloudy conditions ............................................................................................43

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List of tables

1. Table 2.1 Direct power lost (%) due to misalignment angle ................................. 16

2. Table 4.1 Voltage values of azimuthal detection cells ............................................ 38

3. Table 4.2 Voltage values of elevation detection cells ............................................ 41

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Introduction

In order to obtain the maximum energy out of photovoltaic panel it must be positioned to

face the sun throughout the day. This can be achieved using a solar tracker system. In this thesis

the dual tracker system will be analyzed and physically implemented. This dual tracker system is

characterized by the ability to track the sun both on azimuthal and elevation axis. This features

enable the tracker to follow the movement of the sun and assure an optimal incident angle upon

the photovoltaic panel. In this thesis the implementation of this tracker was divided into its

principle elements and analyzed. In the result section the tracker was tested under real conditions

and the data gathered was analyzed.

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1.Solar energy

In a world where pollution has a major impact, where energy demands are on a constant

growth, the search for new clean sources of energy is a requirement for a sustainable

development. The most abundant source of clean energy is the sun. In a single hour it beams

towards Earth enough energy to match up the global energy needs for a whole year.

The sun has an estimated temperature of 5500 C degrees and emits high amount of

energy in form of radiation that heats up all the planets of the solar system. The Earth absorbs

3850 000 exajoules per year from the sun1. The sunlight is composed by two types of beams

the direct ones which carries about 90 % of the solar energy and the diffuse beams .

During a year the average solar radiation arriving at the top of the atmosphere is around

1366 watts/ . As the suns beams are attenuated by the atmosphere the earth receive

approximately 1000 watts/ for a given surface perpendicular to the suns beam in a clear day

at sea level.

There are currently different ways to collect the solar energy: solar thermal collectors

are used to heat water or other material for domestic use, solar concentration systems using

mirrors to concentrate the light into a heat-collective element which heats up water or any other

heat-transferring fluid in order to generate steam which is used to spin turbines and generate

electricity.

The most common way to harness this type of energy is using a solar cell, an electrical

device that converts the energy of light directly into electricity. A solar cell is made of two layers

of silicon. Silicon atoms have 14 electrons arranged in three different shells. The first two shells

are completely full while the third has only four. In order to have a stable shell it shares its four

electrons with four nearby silicon atoms and in this way atoms of silicon fill their third shell and

form a crystalline structure. In this case silicon is a poor electrical conductor since none of its

electron is free to move around. To make out of the silicon a conductor the material has to be

doped with impurities. When the impurity has 5 electrons in the outer shell there will be an extra

electron that will wander around as a carrier. This type of material is called an N type

semiconductor. The usual impurities used for this kind of doping is phosphorous. When the

impurity has 3 electrons in the outer shell there will be a hole that will act as a carrier around the

material. This type of material is called P type semiconductor.

1 http://en.wikipedia.org/wiki/Solar_energy

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When putting together the two layers they form a P-N junction. At the junction of the two

layers, the holes and the electrons form a depletion zone. In this area the electrons and holes

combine and produce an electric field that acts as a barrier for the electrons of the N layer that try

to cross over to the P layer. When light hits the solar cell the photovoltaic effect is taking place.

The photovoltaic effect can be observed when the light is hitting a material surface and the

electrons present in the valence band absorb energy and jump to the conduction band and thus

free. In our case when the light is hitting the two layers of the solar cell, it will form an extra

electron / hole. If a current path is provided between the two layers, the electrons will flow to the

P side. In this manner the current is achieved.

In order to have a functional solar cell an antireflective coating must be applied due to

the fact that silicon is shiny material that reflects light. For protection reasons a glass is added on

top of the two layers.

Top contact

Junction {

Bottom contact

Fig 1.1 Solar cell section

N Type semiconductor

P Type semiconductor

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

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The light provided by the sun can be separated into different wavelengths with photons of

different energies. Some of them have dont have enough energy to brake an electron/hole out of

atom. Those photons just pass through the material. On the other hand there are photons with too

much energy. There is a certain amount of energy required to free an electron of crystalline

silicon and it is equal to 1.1 electron volts (eV).Any energy above this threshold is lost.This

amount of energy is called the band gap energy of the material. If the energy of the photon is

twice the required amount then it can create more than one electron/hole pair. Given the fact that

only specific photons can be used on a silicon material, the total loss of energy can go up to 70

%.

1.1 History of Photovoltaic Cells

In 1839 Alexandre Edmond Becquerel is the first to observe the photovoltaic effect. Using an electrode dived in a conductive solution he exposed it to the light. He observed a

current and a voltage across the electrode. This is the first observation of the sun producing

electrical power.

The first genuine solar cell was built around 1883 by Charles Fritts, who used junctions

made of coating selenium with a thin layer of gold.

The first practical photovoltaic cell was publicly demonstrated on Aril 25, 1954 at Bell

Laboratories .The inventors were Daryl Chapin, Calvin Souther Fuller and Gerald Pearson. The

cell was made out of diffused silicon p-n junction. In 1955 Hoffman Electronics- Semiconductor

Division came with the first commercial photovoltaic product with a 2 percent efficiency. The

cells cost 25 $ at 14 miliwatts / cell or the equivalent 1 785 $ / watt.

By mid 60s the efficiency has raised near 10 % but the costs remained prohibitive for

general use.

Many years due to its high cost/power ratio the only area where this technology was used

was the spacecraft industry. Solar cells were preferred instead of batteries and research has been

done to improve the efficiency of the cells. This development was done mainly by the

governments in the space programs which benefited of considerable budgets. On the other hand

private companies invested in the development of cheaper cells. Exxon SPC was a leading

company in solar cells production and has succeeded in producing a cheap solar cell with a cost

of 10$ / watt in 1973.

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Nowadays the most efficient solar cells are produced by the institute of Fraunhofer,

Berlin largest solar energy research institute in Europe. The efficiency of these solar cells is 44.7 %.This high efficiency is achieved due to its four different subcells. Each subcell absorb

different wavelength of the solar spectrum and in this way the cell can absorb more energy from

the sun.

Although the efficiency can go up to 44 %, the most common cells are in the range of 12-

15 % efficiency. In this range the cost is at an affordable level for public use. More and more

houses are equipped with solar panels which makes the household partially independent on an

energetic level.

1.2 Incident Angle

All this methods of collecting the solar energy have a common problem the sun is on a

constant movement and during night the systems are not gathering any energy. Assuming a fixed

solar cell the angle of an incident light ray () will vary throughout the day.

N

ormal S

Solar cell

Fig. 1.2 Angle of Incidence to Solar Cell

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Assuming a constant intensity of the light the power output of the solar cell will vary

according to the following formula:

W = A cos()

Where :

represents the light intensity

A represents the limiting conversion factor in the design of the panel.

-represents the incident angle of the ray

As the formula states, the maximum power will be generated when the incident ray is

hitting the solar cell perpendicular to the surface , parallel with the normal and there will be no

power generated when the incident ray will be parallel with the surface and perpendicular with

the normal. With a fixed solar panel there is a significant power loss during the day due to the

constant movement of the sun. One way to reduce that loss is mount the solar cell to face south

for the northern hemisphere. Depending on the season, the maximum height of sun will be higher

or lower. To compensate that, the tilt of the panel can be modified to have the sun rays hitting

perpendicular the panel when the sun is at its maximum height. Doing all this will assure a 70 %

of the optimum output of the panel.

The 25 % loss can be reduced using a tracking system that follows the movement of the

sun and assure a perpendicular incident ray on the surface of the solar cell at any given moment.

Using such systems, the amount of power gained will increase with 40%.

2. Solar Trackers

The sun is on a constant movement throughout the day and throughout the year. A solar

tracker must be able to follow this movement and assure that the panel is facing the sun. In order

to track the sun on its path along the sky the system must be able to shift its azimuthal angle and

elevation angle. In fig 2.1 the sun trajectory can be observed both on summer and during winter

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The elevation angle

The azimuthal angle

The main types of trackers are :

- active trackers

- passive trackers

- chronological trackers

- manual trackers

The active trackers use motors and gear trains to rotate the panel while passive

trackers use chemical properties of the materials to do that. The chronological move

the panel with a fixed number of steps according to the movement of the earth around

the sun. The manual

Fig 2.1 Sun trajectory

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2.1 Passive trackers

Passive trackers are systems using mechanical or chemical features of materials to track

the movement of the sun. One approach is to use a shape memory alloy on both sides of the solar

panel. When the panel is perpendicular with the sun rays, the two sides are in equilibrium. When

the sun moves, one side will be more exposed to heat and expand while the other will contract,

causing the panel to rotate. Another approach is to use a thermo hydraulic system composed of

two tube tanks placed at each side of the panel. If the panel is not perpendicular to the sun the

fluid in the tanks will heat differently. The resulting difference in terms of pressure will drive the

fluid through a connecting pipe into the tube with the lower temperature. The shift in weight will

cause the panel to rotate and face the sun. These kind of trackers have the advantages of low

cost, more reliable, no energy consumers and no need for motors. On the other hand these

solutions are not as efficient as the active ones ( 23% compared with 40-57% for active ) and

tend to be slow in the mornings.

2.2 Active trackers

Active Trackers are systems based on sensors, dc motors and logic circuit. The sensors

are gathering data about the sun position. Based on the data collected from the sensors the logic

circuit gives command to dc motors to turn in a specific direction a certain number of steps in

order for the solar panel to face the sun.

2.2.1 Single axis trackers

Depending on the axis on which the system can rotate the single axis trackers are divided

into horizontal trackers, vertical trackers, tilted trackers and polar aligned trackers. These

trackers can provide a significant power gain at low costs. In fig 1.2 there can be seen examples

of single axis tracker

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a. Tilted single axis b. Horizontal single axis

2 Fig 2.2 Single axis trackers

2.2.2 Dual axis tracker

The dual axis tracker has 2 degrees of freedom that act as axes of rotation.Due to this

flexibility it can track the sun on its path along the sky in every moment of the day. This tracking

system provides the best performance but with higher costs. In order to rotate on both azimuthal

and elevation axes it requires two different DC motors with different sensors for both directions.

For best performances stepper motors are used in order to rotate the system with a fixed angle in

the desired direction. A minor misalignment will cause power loss as it can be seen in fig

2.2.

Misalignment (angle ) Direct power loss (%)=1-cos()

5 0.4

10 2

15 4

20 7

25 10

60 >50

2 blog.pepperl-fuchs.us

Table 2.1 Direct power lost (%) due to misalignment (angle )

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2.3 Chronological tracker

The principle of the chronological tracker is to counteract the Earths rotation by turning

with the same angle as the earth but in opposite direction. For example: assuming the solar panel

is fixed the sun is rotating around the solar panel with 360 degree per day. That is 15 degrees per

hour. The system can be set to slowly turn 15 degrees per hour during day and then return to an

initial state in order to not twist the wires. A chronological tracker has a simple implementation

but it can be an accurate tracker

2.4 Manual tracker

This type of tracker is used in the less developed countries where the labor is not

expensive and the systems are replaced by operators who adjust the tracker.This type of tracker

has the advantage of providing an ongoing maintenance for the solar panels.

3. Experimental Setup

3.1 Hardware setup

Building a dual axis tracker from scratch brings up the opportunity to choose over a wide

range of components with different characteristics. Selecting those components was done starting

from the idea that the main solar cell will be able to charge a smartphone. The requirements for

the task are 5 V and 500 mA output. To do that the solar cell selected was a SRS-11 solar cell

with 8 V and 650 mA output. Due to its large output the dimensions of the solar cell are 180 x

220 mm . Starting from those data the whole project was designed and implemented.

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3.1.1 Stepper motor

A stepper motor (or step motor) is a DC electric motor that divides a full rotation into a

number of equal steps.

Inside of a stepper, there is a motor shaft connected to a permanent magnet and a

number of electromagnets surrounding it. When a magnetic field is induced around a particular

electromagnet the permanent magnet is getting aligned with the induced pole.

The torque of a stepper motor is produced when the magnetic fluxes between the rotor (

the permanent magnet ) and the electromagnet have a displacement between them.The torque of

the motor is proportional to the intensity of the magnetic flux generated when the winding is

energized.

The intenstity of the magnetic flux is defined by :

Where :

H- magnetic flux intensity

i- current

N- number of winding turns

l- magnetic flux path lenght

The magnetic field around the electromagnets is produced by the pulses applied at the

motor terminals. The frequency of these pulses determines the rotational speed of stepper. If the

frequency is higher than a threshold frequency the motor will not be able to keep up and will stop

moving.

In fig 3.1 there is a section of the stepper motor.

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Fig 3.1 Stepper motor section

Stepper motors are either unipolar or bipolar. These motors are similar excepting the fact

that the unipolar has a center tap on each winding. In order to drive a bipolar motor a full bridge

driver must be used while for the unipolar motor a simpler circuit can be used.

Electromagnet

Permanent magnet

Phase 1

Phase 2

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The unipolar stepper motor has the disadvantage that it has limited capability to energize

all windings at any time and as a result it has a lower torque compared to the bipolar motor. In

order to use the unipolar as a bipolar the center tap can be disconnected.

The advantages for this motor are the following :

- The rotation angle of the shaft is proportional with the number of steps

- The motor has full torque when standing still while its windings are being energized

- Precise positioning with an error less than 5 %.

- Since there are no contact brushes the motor is very reliable and the its life span

depend simply on the bearing

- A wide range of rotational speeds may be obtained depending on the frequency of the

pulses applied

The main disadvantages of this motor are that it is difficult to operate at very high speeds

and it resonance can occur.

For this project two DC two-phase stepper motors were used with inputs of 12 V and 200

steps.

With the help of the four cables of the motor, the two windings can be polarized as in fig 3.1

3

3 https://www.sparkfun.com/datasheets/Robotics/SM-42BYG011-25.pdf

Fig 3.2 Wiring diagram

20

In order to make the motor move in a desired direction a precise sequence of signals must be sent

to the motor. If the signals are not coming in this order the motor will not move in a single direction but

instead will move back and forth on the same position. The sequence can be seen in fig 3.2

Steps I II III IV

Red

Green

Yellow

Blue

Using this sequence these motors have reached a speed of 30 rotation per minute at a resolution

of 200 steps- (1.8 degrees per step ). Giving the fact that such speed is not required in a solar tracker

system the motors were controlled in half step. The half step control gives to possibility to divide a step in

two and double the resolution to 400 steps ( 0.9 degrees per step ) but in the same time decreasing the

maximum rotational speed . In order to do that the signal sequence increased to 8 steps. The sequence can

be seen in fig 3.3

Steps I II III IV V VI VII VIII

Red

Green

Yellow

Blue

Fig 3.3 Stepper motor signals sequence full step

Fig 3.4 Stepper motor signals sequence half step

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3.1.2 Driver

To control a bipolar stepper motor there is a need of a motor controller containing two H

bridges. The H bridge is an electronic circuit that enables a voltage to be applied across a load in

either direction. This is done using four switches. When the switches S1 and S4 (Fig 3.4) are

closed ( and S2 and S3 are open) a positive voltage will be applied across the motor. By opening

S1 and S4 switches (and S2 and S3 are closed ) the voltage across the load is reversed.

4

Typically the H bridge is constructed using transistors as switches. The upper switches

are implemented using p-channel MOSFETs transistors while the bottom switches are

implemented using N-channel MOSFETs transistors.

A variation of this circuit that uses two transistors on one side of the load is called a half

bridge. This circuit can drive a motor in a single direction only. The bottom MOSFET transistor

is used to electronically stop a motor from spinning. Without, it the motor will keep spinning

under its own momentum.

To control the two bipolar steppers two L293 drivers were used. These drivers are

quadruple high-current half H-drivers designed to provide bidirectional drive currents of up to 1

A at voltages from 4.5 to 36.

4 http://en.wikipedia.org/wiki/H_bridge

Fig 3.5 H bridge concept

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5

This circuit is configurable having three pins to configure the device. One is used to

enable the device while the other two are used to select how many half H bridges to use. The

selection pins are: pin one for the selection of half bridge one and two and pin nine for the

selection of half bridge three and four. To enable those pins, a 5 V voltage must be applied

across the pins.

The circuit has 4 input pins designed to receive transmitting control signals from a

microcontroller and 4 outputs to enable voltages across the motor windings.

In addition it has a input voltage that can range from 4.5 to 36 V that will be applied on

the motor windings. For this project a 12 V source was used to power up the motors.

In fig 3.6 is the pin distribution of the device

5 http://www.ti.com/lit/ds/symlink/l293d.pdf

Fig 3.6 Bipolar Stepping-Motor Control

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6

Pin 1 and 9: 1 and 2 Half bridge enable and 3 and 4 Half bridge enable.

Pins 2, 7, 10 and 15 : Inputs from the Microcontroler.

Pins 3, 6, 11 and 14 : Outputs to the motor.

Pin 16 : Device enable pin.

Pin 8 : Input voltage to be applied on the motor windings.

Pins 4, 5, 13 and 12 : Ground pins and they act as heat sink.

The output order of the motor wires, according the signal sequence for full step and half

step, is output 1 Red, output 2 Green, output 3 Yellow and output 4 Blue.

3.1.3 Light sensors

For this solar tracker the light sensors used to approximate the position of sun were

photovoltaic cells due to the fact that the output voltage is directly proportional with the intensity

of the incident light. When the solar cell is not illuminated it acts as a big diode. When light hits

the cell it starts to generate power.

The electrical circuit of the a solar cell is presented in fig 3.7

6 http://www.ti.com/lit/ds/symlink/l293d.pdf

Fig 3.7 Pin distribution of the driver

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7

Using Kirchoffs current law :

Where :

- is the light generated current

- is the voltage-dependent current lost due to recombination

- is the current lost due to the shunt resistance

- shunt resistance

- series resistance

is defined by the following formula :

[(

) ]

Where n is the ideality factor of the diode, the saturation current and and is the

thermal voltage.

is defined by the following formula:

The governing equation for the current of a single diode model is:

7 http://en.wikipedia.org/wiki/Theory_of_solar_cells

Fig 3.8 Equivalent electrical circuit of a solar cell

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[(

) ]

If the circuit is considered an open circuit, the equation becomes :

[(

) ]

- Open circuit voltage

If we consider high enough the last term can be neglected thus the equation

becomes:

For short circuit current is defined as follows ( given an output voltage = 0 )

[

]

If we assume that is higher that and is small compared to I

The I-V curve can be observed in fig 3.9

I-V curve of the solar cell Power from the solar cell

Voltage

Curr

en

t

Fig 3.9 The I-V curve of the solar cell

26

The nominal voltage of the detector cells used is 4 V and nominal current is 35 mA.

These specifications were imposed by the analog digital converter of the microcontroller. The

dimensions of the solar cells are 39 X 35 mm.. Due to the fact that the cells are identical the logic

responsible for the decision of movement is simplified.

3.1.3.1 Alignment

One solar cell was disposed on each of the four sides of the rectangular main cell. The

four cells were placed at an angle of 30 degrees to the surface plane of the main cell . In fig 3.7

there is a vertical section of the wood frame with the main solar cell attached to it. At the ends of

the solar cell there are two rectangular triangles with a angle of 30 degrees. On top of these

rectangular triangles are mounted the solar cells.

Detection Solar cell Detection Solar cell

Wood frame

Fig 3.10 Horizontal section of the top of the device

180 mm

250 mm

Main solar cell

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3.1.3.2 Principle of orientation

The tracker system is based on signals generated by the four tilted detection cells. The

cells are grouped two by two each pair being responsible for the orientation on one axis. The

generated voltage of the cells depend on the light intensity and the angle of incidence. Assuming

a single source of light, the voltages generated by the detection cells will vary only with the

angle of incidence. Given one axis, the two voltages of the detection cells will give different

values as long as the the two incident angles ( ) will be different. This situation is shown in

Fig 3.9

Detection Solar cell Detection Solar cell

Wood frame

Fig 3.11 Vertical section of the top of the device

220 mm

290 mm

Main solar cell

Detector solar cell

Detector solar cell

Fig 3.12 Different incident angles upon detectors

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Due to the the fact that the detection cells are placed on a 30 degree angles with respect

to the main solar cell the angles can be equal only if the light is perpendicular to the surface

of the main cell. This can be seen in fig 3.10.

The principle of this tracker is to move the device in the direction of the detector cell that outputs

a higher value until the values of the two detectors of the same axis are equal.In that moment it

means that the light is hitting the axis perpendicular. By doing this to both axis the cell become

perpendicular to the light. In fig 3.118 it can be seen the output of the detectors in a full rotation

of the motors. Blue and purple lines represent the output of 2 detectors on one axis while green

and yellow lines represent the output of the other 2 detectors on the second axis. The intersection

point represent the optimal spot in order to have perpedicular rays on the axis.

8 AUTOMATIC SYSTEM AND ENERGETIC EFFICIENCY OPTIMIZATION ALGORITHM FOR SOLAR PANELS ON MOBILE SYSTEMS

Alexandru Vasile, Andrei Drumea, Cristi Neacsu, Marinela Angel, Dan Alexandru Stoichescu

Detector solar cell

Light source

Detector solar cell

Fig 3.13 Equal incident angles upon detectors

Fig 3.14 Voltage output on a full rotation

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3.1.4 Microcontroller

This tracker uses a Arduino Leonardo board with its ATmega32u4 microcontroller for

decision making. The ATmega32u4 is a low-power CMOS 8-bit microcontroller based RISC

architecture. The core combines a wide range of instructions with 32 general purpose working

registers, all of them connected to the Arithmetic Logic Unit (ALU) .This allows two

independent register to be accessed in one single instruction executed in one clock cycle. This

architecture increase the speed of the microcontroller up to ten times compared to CISC

microcontrollers. Among the features of the microcontroller are: four flexible Timer/Counters,

buit-in USB communications , 10-bit Analog Digital Converter, calibrated temperature sensor ,

26 general purpose I/O lines, a memory of 32 KB amob which 4 are used for the bootloader, 2.5

KB of SRAM and 1KB of EEPROM.

The Arduino board has 20 digital input/output pins out of which 7 can be used as

pulse width modulation outputs with 8 bits resolution and 12 as analog inputs. Due to the ADC

of the microcontroller, the 12 analog inputs have 10 bits of resolution (1024 different values).

This high resolution is useful when building a tracker because the precision of the device

depends on the ability of the circuit to distinguish between different values of the detectors.

These pins work at 5 v and can provide and receive a maximum of 40 mA. This is the reason for

which those specific solar cells were used as detectors.

The Arduino Leonardo can be powerd either by a USB connection either by an external

supply.For this project the board was mainly powered by the USB but it can alsi be powered

from the 12 V charger .The board has a voltage regulator that can accept voltages up to 20 volts.

In order to have a stable circuit the voltage must be above 7 volts.The recomanded voltage range

is 7-12 volts. The board has 2 pins with regulated power supply ( 5 volts and 3.3 volts ).

The Leonardo board can be programmed using Arduino software. This soft allows

writing code with syntax similar to C++ and then it can upload it to the board. The debugging

can be also done using this Arduino software. An important tool for debugging is the Serial

Monitor feature in which the user can track different variables either code generate values either

analog inputs from the 6 analog pins.

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3.1.5 Electrical Circuit

In the picture below the electrical circuit can be observed

SC SC SC SC

Fig 3.15 Electrical Circuit

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SC solar cell used as solar detector

L293 the 2 drivers used to control the movement of the two stepper motors

A0 A3 analog inputs for the Arduino board. The analog inputs are transformed to

digital data with the help of the ADC located on the microcontroller.

Pins 0-7 output pins responsible for the sequence of signals required to move the

motors in the desired direction. The speed at which the Arduino board is transmitting the signals

is controlled using the software provided by the Arduino developers. With the increase of the

transmitting speed the rotational speed is also increased.

Components :

The electrical circuit of this dual axis tracker was done using the following parts :

- 2 DC two phases stepper motors - SM-42BYG011-25 with a voltage input of 12 V.

The current of the motors are 0.33 A with a maximum torque of 2.3 kg/cm. This

torque is achieved using full step while if the half step is used the torque is reduced.

- Two drivers l293d COMP-GEN-17

- Arduino board powered using a USB cable connected to a computer

- A 12 v charger

- A breadboard to connect the two drivers with the Arduino board and the motors

- ~50 connectors

3.1.6 The mechanical assembly

The mechanical part was done using wood as the main material. On the bottom of the

device there is a box where the Arduino board, the breadboard and one of the motors are located.

The motor in the box is responsible for the azimuthal rotation of the main solar cell. On top of

the box there is a tower connected to the azimuthal movement motor and the entire tower is

rotating when the motor is moving. Inside the tower is the second motor, the one responsible for

the elevation movement. The elevation movement is done in the following manner : On top of

the tower is located the main solar cell connected to the tower by two pieces of wood and a

screw that allows the main solar cell to swing on the screw axis. The solar cell center of mass is

positioned with a displacement to the screw axis which makes the solar cell unbalanced with a

tendency to lean on a certain direction. This is counterbalanced by a strap attached to the

32

elevation motor shaft and to the termination of the solar cell which is lighter. In fig 3.13 there

can be seen a section of the top of the device

When the detection cell in the lighter section is receiving more light and thus is

producing more voltage, the elevation motor will turn clockwise and the strap will wrap around

the shaft making the solar cell to tilt towards the lighter end. This is done until the two detection

cells are giving the same voltage output. If the situation is reversed the heavier end is receiving

more light, the elevation motor will turn counter-clockwise and unwrap the strap letting the solar

cell to tilt towards the heavier due to the gravity

Motor

Motor shaft

Screw

Main solar cell

Fig 3.16 Section of the top

Strap

Detection Cells

33

3.2 Software

The Arduino IDE ( integrated development environment ) is a cross platform application

developed in Java programing language. This programe is designed to introduce newcomers to

software development. It includes a code editor which makes writing code easier by using syntax

highlighting, brace matching or automatic indentation. Another feature that makes the

programming easier is the fact that uploading a program to the board can be done with a click. A

code written in Arduino IDE is called sketch and has as an extension the characters ino.

The code in Arduino IDE is written in C or C++ and it has a wide range of libraries.

Those libraries make many operations much easier. For example in order to make a runnable

cyclic executive program the users must define only two function: setup a function that runs

only once at the beginning of the program to initialize settings and loop - a function that is

called repeatedly until the board powers off or another program is uploaded into the board.

The Arduino IDE has a library designed to control the stepper motor and by providing

only the speed and the number of steps required to move the motor the program can do all the

control behind the scenes. In this project due to the need of control and the additional features

that have been implemented on the tracker the whole software design was done from scratch.

The idea behind the code is the following : the inputs of the code are the delay between two steps

( which gives the rotational speed ) and the number of steps inputs given by the user - and

inputs given by the detection cells. A timer located on the microcontroller is used to determine

the elapsed time since the last move. If it is grater then the imposed delay the motor will move

one step. The direction on which the motor will move will be determined based on the inputs

given by the detection cells . The detection cells will provide a voltage in the rage of 0 to 4.4 V.

This voltage is transformed into a digital value using the analog to digital converter of the

microcontroller . The ADC of the microcontroller has 10 bits of resolution which gives 1024

levels of voltage. So for any voltage in rage of 0 to 5 V there is a correspondent value in the

range of 0 to 1023. The formula for finding the digital value of the input voltage is

The input voltages of the detection cells are converted into digital values. The pair of

cells aligned to detect the azimuthal position are compared and if the difference between the two

of them is grater then 100 ( a value that corresponds to an approximate 0.5 V difference ) the

34

motor will rotate in the direction of the cell with a higher voltage. The same method is applied to

the pair of cells aligned to detect the elevation position.

After the direction is set the number of steps begin to be executed. The number of steps

differ from one motor to another because the motor responsible for the elevation angle is rotating

a strap on its shaft connected to the wood frame of the main solar cell. Because of this strap a full

rotation of the elevation motor corresponds to small angle displacement. The first motor is

rotating 8 steps, 7.2 degrees displacement each time there is a difference between the detection

cells placed on the azimuthal axis. The second motor is rotating 400 steps a full revolution -

when there is a difference between the detection cells placed on the elevation axis. The values of

the detection cells are monitored once every two seconds.

The rotational speed is set to 7.5 RPM. This is done by setting the delay between two

steps to 20 ms. The formula for setting up the speed of the motors is :

The speed can be doubled if the half step technique is used. For accuracy half step

technique was used but with the sacrifice of the speed.

The initial position of the tracker is with its main solar cell parallel to earth thus giving

the opportunity to detect the sun in any given spot. During the day the solar cell will move in

order to track the sun and at the sunset the solar cell will face west. In the morning the sun will

hit the solar cell at its back and there is a chance that the sun will not illuminate any of the

detection cells enough to give the signal to move. For this situation there is a variable in the code

that will keep in the memory the total movement of the device on both axis. When the detection

cells will have voltages less then 100 (voltages that are common when no light is hitting the solar

cells) the tracker will move in the opposite directions on which it moved until that moment. This

will cause the tracker to move to the initial position when it was activated, a position parallel to

the earth from which it can track the sun once again.

In appendix 1 the whole code can be reviewed.

35

4 The results

After setting up the circuit, the detection cells proved to have a different light voltage

characteristic. Three of them shared a similar light voltage characteristic while the fourth have

a significant difference. In fig 4.1 the characteristic of the fourth detection cell vs the rest of

three can be seen

0.3 V

2 V

4.4 V

The characteristic of three detection cells

The characteristic of the fourth detection cell

Voltage

250 500 700 1000 Light intensity

W/

4.1 The voltage characteristics of the detection cells

36

In this situation the detection cell number four is treated separately. In case its voltage is

between 1.56 and 3 volts ( digital 300 and 600 ) the voltage will be decreased with aprox 1.5

volts ( 300 in digital ) .This is done inside the code after converting the voltage into digital. By

doing this, the azimuthal pair of solar cells will be balanced and will be able to give relevant data

for the decision of movement.

The detection cell are placed on top of rectangular triangles with an angle of 30 degrees

with respect to the main cell. If the light is hitting perpendicular the main cell the detection cell

will be hit at an angle of 30 degrees.

The voltage of the detection cell will be :

Where

is the voltage generated by the detection cell when the light is hitting

perpendicular

When the light is hitting the main cell under a specific angle the detection cells will be

hit at the angles 1+ and 1 . The detection cells will generate the following voltages:

and

For this tracker a 2 % loss due to misalignment is considered an accepted error . and as a

consequence a 10 degrees misalignment is the threshold value for the movement of the tracker.

This implies that the voltages across the detection cells to be :

Fig 4.2 The incident angle upon the detection cell

37

and

for a sunny day is 4 V, the nominal voltage of the detection cell but can go down

to 1.75 - 2 volts for a cloudy day. For this tracker the was considered a average value of 3

volts.

In this case the and . Transformed into digital by the

ADC provided by the microcontroller these values are correspondent to 466 and 570. The

difference between the two of them ( 570 466 =104 100 ) represent the threshold digital

value for the movement of the motors.

In order to test the tracker the device was placed outside in order to track the sun over a

period of sixteen hours from 6 AM until 10 PM. The results are the following:

Table 4.1

Time Azimuthal detection cell 1 Azimuthal detection cell 2

2:10 PM 826

817

2:18 PM 910 841

2:26 PM 912

911

2:34 PM 909

905

2:42 PM 820

819

2:50 PM 886

876

2:58 PM 759

485

3:06 PM 917

912

3:14 PM 762

472

3:22 PM 787

827

3:30 PM 911 909

38

3:38 PM 907

896

3:46 PM 915

914

3:54 PM 906

902

4:02 PM 798

805

4:10 PM 778

499

4:18 PM 712

493

4:26 PM 672

489

4:34 PM 707

805

4:42 PM 700

492

4:50 PM 718

803

4:58 PM 711

804

5:06 PM 784

831

5:14 PM 772

828

5:22 PM 764

811

5:30 PM 770

813

5:38 PM 851

871

5:46 PM 911

916

5:54 PM 882

891

6:02 PM 888

900

6:10 PM 880

893

6:18 PM 875

888

6:26 PM 756 462

39

Table 4.2

Time Elevation detection cell 1 Elevation detection cell 2

2:10 PM 730 859

2:18 PM 884 917

2:26 PM 897 917

2:34 PM 903 908

2:42 PM 690 863

2:50 PM 852 894

2:58 PM 575 835

3:06 PM 908 917

3:14 PM 682 797

3:22 PM 693 856

3:30 PM 905 911

3:38 PM 898 902

3:46 PM 908 918

3:54 PM 901 907

4:02 PM 705 842

4:10 PM 681 832

4:18 PM 586 818

4:26 PM 565 803

4:34 PM 623 813

4:42 PM 615 800

4:50 PM 657 801

4:58 PM 663 804

5:06 PM 751 836

5:14 PM 750 824

5:22 PM 737 804

5:30 PM 762 793

5:38 PM 855 866

5:46 PM 915 917

Table 4.1 Voltage values of the azimuthal detector cells

40

5:54 PM 887 897

6:02 PM 896 989

6:10 PM 887 842

6:18 PM 881 854

6:26 PM 724 758

In fig 4.3 the voltage evolution of azimuthal detection cells can be observed

0

100

200

300

400

500

600

700

800

900

1000

2:10PM

2:26PM

2:42PM

2:58PM

3:14PM

3:30PM

3:46PM

4:02PM

4:18PM

4:34PM

4:50PM

5:06PM

5:22PM

5:38PM

5:54PM

6:10PM

6:26PM

Fig 4.3 Voltage evolution of azimuthal detection cells

Table 4.2 Voltage values of the elevation detector cells

41

In fig 4.4 the voltage evolution o elevation detection cells can be observed.

As it can be seen in fig 4.3 the has moved in the direction of the detection cell number two. The

voltage of this cell is constant higher due to the fact that the sun moving towards it and it tends to hit it

more perpendicular. When the voltage difference exceeds the threshold the tracker sends the signals to

the azimuthal motor in order to spin the device to face the sun.

In fig 4.4 it can be seen that the first elevation detection cell has a more steep response at a light

intensity difference then the second cell.The sun has moved in the direction of the second detection cell.

During the test, between 4 :00 PM and 5 :40 PM the sky was cloudy and this was recorded by

the detection cells. The voltage have decreased across all cells. This could be signal for entering a cloud

mode where the tracker could move on its own based on the previews steps. As the sun came out again

the voltage across all the detectors has risen .

0

200

400

600

800

1000

1200

2:10PM

2:26PM

2:42PM

2:58PM

3:14PM

3:30PM

3:46PM

4:02PM

4:18PM

4:34PM

4:50PM

5:06PM

5:22PM

5:38PM

5:54PM

6:10PM

6:26PM

Fig 4.4 Voltage evolution of elevation detection

cells

42

Conclusions

Building a solar tracker can be done in multiple ways. This flexibility offers the chance to

configure the system in almost every aspect. For this project the principle used was the one of

solar detectors placed on the edges of a main solar cell at tilted angle. This angle is responsible

for the accuracy of the tracker. As the angle increases, the accuracy does the same. In this project

the angle was set at 30 degrees which enabled a 2 % maximum misalignment. This error

threshold is obtained on a theoretical level. In practice due to the differences between the

detector cells this error is higher. Having a solar tracker with error positioning of less than 5 %

gives the main cell the capability of producing 99.5 % out of its maximal output. This high gain

produced by the tracker comes at low cost in terms of circuit electrical consume. In the same

time the cost for the device is small compared with the increase of the output making it a good

investment.

Fig 4.5 Cloudy conditions

Cloudy conditions

43

Appendix 1

// the rotational speed for motor 1 and motor 2 set by the step delay

unsigned long step_delay1=20;

unsigned long step_delay2=20;

// the number of steps to move for motor 1 = 4 and for motor 2 = 400

int steps_to_move1=4;

int steps_to_move2=400;

// the output of the code , pins 1 ,2 ,3 ,4 the outputs responsible for the control of motor 1

// the output of the code , pins 5 ,6 ,7 ,8 the outputs responsible for the control of motor 1

int pin1=1;

int pin2=2;

int pin3=3;

int pin4=4;

int pin5=5;

int pin6=6;

int pin7=7;

int pin8=8;

// steps_left1 and step_left2 variables that count the number of steps that each motor has to

make

int steps_left1;

int steps_left2;

// thisstep1 and thistep2 the current step on which the motor is

int thisstep1=0;

int thisstep2=0;

44

// pozitie1 and pozitie2 based on this variable it is decided what kind of signals are transmitted

int pozitie1;

int pozitie2;

// localizare1 and localizare2 variables that stores how many steps were taken in a general

direction

int localizare1=0;

int localizare2=0;

// directions1 and directions2 variables decide whether the motor will turn clockwise or anti

clockwise

int directions1;

int directions2;

//last_step_time1 and last_step_time2 variables that act as a time stamp for the last step

unsigned long last_step_time1;

unsigned long last_step_time2;

void setup()

{

// initialize the pin 1-8 as output pins

pinMode(pin1, OUTPUT);








Serial.begin(9600);

}

45

void loop()

{

last_step_time1=0;

last_step_time2=0;

thisstep1=0;

thisstep2=0;

int sensora=analogRead(A0);

float voltagea = sensora * (5.0 / 1023.0);

sensora=sensora;

Serial.println(sensora);

int sensorb=analogRead(A1);

if(sensorb>320&&sensorb 120)

{

directions1 = 0;

steps_left1 = steps_to_move1;

}

46

if (sensorb - sensora > 120)

{

directions1 =1 ;


}

if (sensorc - sensord > 100)

{ directions2 = 1;


}

if (sensord - sensorc > 100)

{

directions2 = 0;


}

if(sensora

47

{steps_left2=abs(localizare2);

directions2=1;

}

}

while(steps_left1 > 0 || steps_left2>0)

{

if (millis()-last_step_time1 >= step_delay1 && steps_left1>0) {

last_step_time1 = millis();

if (directions1 == 1) {

thisstep1++;

localizare1++;

steps_left1--;

}

else {

if (thisstep1 == 0) {

thisstep1 =abs(steps_to_move1);

}

thisstep1--;

localizare1--;

steps_left1--;

}

pozitie1=thisstep1%8;

switch (pozitie1) {

case 0: // 1010

digitalWrite(pin1, HIGH);

digitalWrite(pin2, LOW);



48

break;

case 1: // 0010





break;

case 2: // 0110





break;

case 3: // 0100





break;

case 4: //0101





break;

case 5: //0001



49



break;

case 6: //1001





break;

case 7: //1000





break;

}

}

// move only if the appropriate delay has passed:

if (millis()-last_step_time2 >= step_delay2 && steps_left2>0) {

last_step_time2 = millis();

if (directions2 == 1) {

thisstep2++;

localizare2++;

steps_left2--;

}

else {

if (thisstep2 == 0) {

50

thisstep2 =abs(steps_to_move2);

}

thisstep2--;

localizare2--;

steps_left2--;

}

pozitie2=thisstep2%8;

switch (pozitie2) {

case 0: // 1010





break;

case 1 : // 0010





break;

case 2: // 0110





break;

case 3: // 0100


51




break;

case 4: //0101





break;

case 5: //0001





break;

case 6: //1001





break;

case 7: //1000





break;

52

}

}

}

delay(2000);

}

53

Bibliography

Automatic system and energetic efficiency optimization algorithm for solar panels

on mobile Alexandru Vasile, Andrei Drumea, Cristi Neacsu, Marinela Angel, Dan

Alexandru Stoichescu

Hamilton, R. DC Motor Brush Life. IEEE Transactions on Industrial

Applications. 2000

Roth, P., Georgiev, A., Boudinov, H.. Cheap two Axis sun Following Device.

Energy Conversion and Management. 2005

http://www.pveducation.org/pvcdrom/solar-cell-operation/iv-curve

http://en.wikipedia.org/wiki/Solar_cell

http://en.wikipedia.org/wiki/Solar_energy

http://science.howstuffworks.com/environmental/energy/solar-cell.htm

http://www.pveducation.org/pvcdrom/properties-of-sunlight/solar-radiation-on-tilted-surface

http://en.wikipedia.org/wiki/Solar_tracker

http://www.solarpowerworldonline.com/2013/04/how-does-a-solar-tracker-work/

http://www.solartown.com/learning/solar-panels/beginners-guide-to-solar-

trackers-how-to-increase-output-for-your-home-solar-panel-system/

http://en.wikipedia.org/wiki/Stepper_motor

https://www.sparkfun.com/products/9238

http://arduino.cc/en/Tutorial/StepperBipolar

http://en.wikipedia.org/wiki/H_bridge

http://www.ti.com/lit/ds/symlink/l293d.pdf

http://arduino.cc/en/Main/arduinoBoardLeonardo

http://arduino.cc/en/main/software

http://www.atmel.com/devices/atmega32u4.aspx

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