FOOTSTEP ENERGY HARVESTING SYSTEM USING PIEZOELECTRIC CONCEPT
KAVIRAJ A/L JAYARAMAN (SCSJ - 0010355) MOHAMAD SYAZWAN BIN SHARI’AT (SCSJ - 0010341) TELVINDERJIT SINGH (SCSJ - 0010235) MANRIT SINGH
SEGI COLLEGE SUBANG JAYA
1
FOOTSTEP ENERGY HARVESTING SYSTEM USING PIEZOELECTRIC CONCEPT
KAVIRAJ A/L JAYARAMAN (SCSJ - 0010355) MOHAMAD SYAZWAN BIN SHARI’AT (SCSJ - 0010341) TELVINDERJIT SINGH (SCSJ - 0010235) MANRIT SINGH
SEGI COLLEGE SUBANG JAYA
A report submitted in partial fulfillment of the requirements for the award of the Diploma in Mechanical Engineering
Faculty of Engineering and Information Technology
SEGi College Subang Jaya APRIL, 2013
“We declare that we have read this work and in our opinion this project is qualified in terms of scope and quality for purpose of awarding the Diploma in Mechanical Engineering”
Student
:
....................................................... KAVIRAJ A/L JAYARAMAN (SCSJ - 0010355)
....................................................... MOHAMAD SYAZWAN BIN SHARI’AT (SCSJ - 0010341)
....................................................... TELVINDERJIT SINGH (SCSJ - 0010235)
....................................................... MANRIT SINGH
Date
Supervisor
:
22nd April 2013
: .............................................................. NUR HASALLI BINTI HJ IBRAHIM
i
A special appreciation to our families for their encouragements and guidances;
A special thanks to Mr. Sri Jaiandran for the idea of Piezoelectricity;
It is our pleasure to receive advices and consultations from: Ms. Nur Hasalli Binti Hj Ibrahim Ms. Ida Fahani Binti Md. Jaye Mr. Gehjanthiran Mr. Manimaran
A special thanks to Sivaganesh Vadiveloo for the advices on circuits and electronic components
And to all of our understanding friends.
ACKNOWLEDGEMENT
The success of this thesis would not have been possible without the constant encouragement, advice and support from a vast number of people.
Appreciation is extended to our supervisor, Ms. Nur Hasalli Binti Hj Ibrahim, for her assistance and guidance throughout the whole project. From the beginning of the study, she has been generous with her time, guidance and supportive. Without her interest and encouragement, this project would never have been completed.
We would like to convey our millions of thanks to our beloved parents and families for all of their support and blessing that made us who we are today. Without their involvement and encouragement, this project would have not have been a realilty.
We would also like to express our sincere appreciation to all others who have played their part directly or indirectly by contributing for the success of this study. May GOD bless you.
ABSTRACT
CONTENTS
CHAPTER
DESCRIPTION
DECLARATION
i
DEDICATION
ii
ACKNOWLEDGEMENT
iii
ABSTRACT
iv
CONTENTS
v - vi
LIST OF FIGURE
vii
NOMENCLATURE
viii
LIST OF APPENDIXES
1
2
INTRODUCTION 1.1
Background of study
1.2
Problem Statement
1.3
Objectives of the Study
1.4
Scope of the Study
1.5
Significance of the Study
LITERATURE REVIEW 2.1
Introduction to Piezoelectricity
2.2
Ceramic Piezoelectric Disc
2.3
Material used
2.4
Polarization
2.5
Voltage and Current Output 2.5.1 Full Wave Bridge Rectifier
2.6
PAGES
Connection of Piezo 2.6.1 Voltage 2.6.2 Current
2.7
Battery
2.8
Capacitor
ix
1-4
5 - 13
2.9
3
Circuit Board
METHODOLOGY 3.1
Introduction
3.2
Materials and Tools
3.3
Product Preparation
14 - 26
3.3.1 Design 3.3.2 Building the Wood Frame 3.3.3 Building the Circuit and Piezoelectric Arrangement
4
3.4
Assembly of the Whole Circuit and the Wood Frame
3.5
Measuring the Electrical Output
3.6
Finishing Touch
RESULTS AND ANALYSIS 4.1
Introduction
4.2
Design Circuit
4.3
Equations/Simulations/General Circuits
27 - 35
4.3.1 AC/DC Converter 4.3.2 Capacitor Charging/Discharging
5
CONCLUSION AND DISCUSSION 5.1
Conclusion
5.2
limitations and Assumptions
5.3
Recommendation
36 - 37
6
REFERENCES
38 to 39
7
Appendix
40 - 42
LIST OF FIGURES
2.1
Piezoelectric Disc
2.1
Main components on the disc
2.2
Polarization of Piezoelectric Disc
2.3
Relationship between footstep and output voltage
2.4
Ac to DC graph after rectification
2.5
Full wave bridge rectifier
2.6
Footstep Energy Harvesting Concept by Pavegen
3.1
Methodology of study flow chart
3.2
List of material
3.3
Group discussion and planning
3.4
Measuring the wood before cutting process
3.5
Cutting wood process
3.6
Assembling wood part using nails
3.7
Soldering the component to the circuit board
3.8
Connecting the circuit to Piezoelectric
3.9
Assembling the circuit and wood frame
3.10
Arrangement of the Piezoelectric
3.11
Using multimeter to determine the output produced
3.12
Testing the product
3.13
Sand the wood surface using sandpaper
3.14
Painting the product
NOMENCLATURE
Physic Units N
=
Newton
V
=
Volt
A
=
Ampere
kg
=
kilogram
W
=
Watt
Ah
=
Ampere-hour
mm
=
milimetre
F
=
Farad
S
=
second
Chemical Formulae PZT
=
Lead Zirconate Titanate
SiO2
=
Silicon Dioxide
LIST OF APPENDIXES
Appendix 1 – Orthographic Projection of the project
Appendix 2 – Dimension Drawing
Appendix 3 – Schematic Diagram of circuit
CHAPTER I
INTRODUCTION
1.1
Background of study Piezoelectricity was discovered in 1880 by Pierre and Paul-Jacques Curie, who found
that when they compressed certain types of crystals including quartz, tourmaline, and Rochelle salt, along certain axes, a voltage was produced on the surface of the crystal. This effect is known as piezoelectric effect. [1]
Piezoelectricity is a form of electricity created when certain crystals are bent or otherwise deformed. These same crystals can also be made to bend slightly when a small current is run through them, encouraging their use in instruments for which great degrees of mechanical control are necessary. [2]
The property of piezoelectricity is dictated by both the atoms in the crystal and the particular way in which that crystal was formed. Some of the first substances that were used to demonstrate piezoelectricity are topaz, quartz, tourmaline, and cane sugar. Today, there are many crystals which are piezoelectric, some of which can even be found in human bone.
A piezoelectric crystal consists of multiple interlocking domains which have positive and negative charges. These domains are symmetrical within the crystal lattice, causing the lattice as a whole to be electrically neutral. When stress is put on the crystal, the symmetry is slightly broken, generating voltage. In a large dimension piezoelectric sensor, even a tiny bit of piezoelectric crystal can generate voltages in the thousands.
Even though a piezoelectric crystal never deforms by more than a few nanometers when a current is run through it, the force behind this deformation is extremely
1|Page
high, on the order of meganewtons. This deformational power is used in mechanics experiments and for aligning optical elements many times heavier than the piezoelectric crystal itself. [2]
A piezoelectric sensor is a device that converts pressure directly into electricity by the polarization effect. Piezo is derived from the Greek word piezein, “to squeeze.” Piezoelectric materials produce a voltage when strained. Assemblies of metals, ceramic (SiO) and lead zirconate titanate. Piezoelectricity is the field of technology that involves the application of piezoelectric sensors in producing electricity for practical use.
Over time, piezoelectric sensors have proven to be versatile tools for the measurement of various processes. They are used for quality assurance, process control and also in automotive industry to manufacture internal combustion engine. [3]
Piezoelectricity serves as a secondary power source. Electricity produced using piezoelectric sensor is environmental-free – no pollution. But, the only pollution produced is during the manufacturing of these devices in factories, transportation of the goods, and installation.
Now, the modern energy processing methods using generators such as to those on power plants, can be very sounds polluting and also at times the safety becomes a threat. Instead, energy is produced from piezoelectric sensor very quietly. One of the great pros of piezoelectricity is the ability to harness electricity in remote locations that are not linked to a national grid [4]. In today’s world, piezoelectricity is widely being used in many fields. For an instance, piezoelectric crystals have been used in phonographs as a stylus to create electrical signals that reflect the sound stored in grooves on a record. Phonograph is a device used for recording and reproduction of sound recordings. It was introduced in the late 19th century. [5]
Besides that, piezoelectric is also being used to light up street lamps. There is also a hot shower mechanism that has piezoelectric nano-tubes installed in the lining of the tube. Due to water pressure, the sensor is used to generate electricity to heat up the water. [5]
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Piezoelectric sensors can be installed in the soles of shoes or on the circumference of car tyre which eliminate the problem of finding the required space for piezoelectric sensor installation. Another advantage of piezoelectric is the cost. Initially, during installation, the cost may be high but in the long term a large sum can be saved. Directly, we can reduce the dependency of the humans on the natural resources as the main form of fuel. This would in a way conserve the mother nature.
1.2
Problem Statement
Now, electricity production has become one of important causes of global warming. This is due to the immense heat being dissipated during the process. This heat contribute to the gradual increase in Earth temperature. Furthermore, burning of fossil fuels also produces an end product of carbon dioxide and sometimes carbon monoxide which are harmful to human health. Carbon dioxide tends to radiate heat back into earth.
In addition, the usage of fossil fuel for generation of electricity is very costly. A large sum could be saved by using a renewable energy. This would enable invest in future projects rather than just spending it on purchasing fossil fuels alone.
By reviewing and analysing all these critical problem, designing new form of renewable energy such as piezoelectricity would be a perfect option in which it is useful for domestic application. Piezoelectricty, on the contrary, is clean and environmentally friendly. Piezoelectric components require little maintenance, and the initial investment can be recovered within a relatively short time.
1.3
Objectives of the Study
The aim of this research is to harvest energy from footstep using piezoelectric disk based on the concept of polarization. The objectives of the study are as follow :-
1) To produce renewable electricity from footstep using piezoelectric disk placed along a pathway. 2) To reduce the cost for power generation besides increasing the efficiency of power generation. 3|Page
1.4
Scope of the Study Piezoelectric sensors would be arranged in two orders, series and parallel. This is to
ensure there would be sufficient generation of electricity. The output be measured using a multimeter and a row of 5 light emitting diode would be placed to indicate the presence of electricity.
1.5
Significance of the Study
At the end of this research, we would be able to produce electricity in a minimal scale. This energy would be used as a secondary power sources to battery. Beside that, we can reduce the total cost of electricity application. In addition, the maintenance of piezoelectricity is minimal. The life span of a piezoelectric disk is also relatively long.
4|Page
CHAPTER II
LITERATURE REVIEW
2.1 Introduction to Piezoelectricity In this section, the fundamental theories that are related to the working principles of the footstep energy harvesting method would be thoroughly explained. The explanation that follows would include the principles of ceramic piezoelectric disc, functional output voltage and current, polarization method in the piezoelectric disc and a few other related concepts. Furthermore, research was conducted on the previous or current technologies being used in the real world. These case studies were analyzed and a slight improvement was made based on the case study to improve the activation of capacitor charging with a battery in a shorter period of time. The resulting effect was still visible despite being minimal. Single-Layer Piezoelectric Generators and Multi-Layer Piezoelectric Generators are the two main types of piezoelectric generators. Single-layer piezoelectric generator (Figure 1.1) was used in this project. This type of sensor is known as single layer due to the single layer of ceramic on the electrode. [6]
Multi-Layer Piezoelectric Generators consist of a stack of
Figure 2.1 Piezoelectric Disc
very thin (sub-millimeter-thick) piezoelectric ceramics alternated with electrodes. The electrical energy produced by a multilayer piezo generator is of a much lower voltage than is generated by a single-layer piezo generator. On the other hand, the current produced by a multilayer generator is significantly higher than the current generated by a single-layer piezoelectric generator. [6]
5|Page
2.2
Ceramic Piezoelectric Disc A disc shaped solid sensor that uses the piezoelectric effect by the means of pressure
to produce electrical charge through the concept of polarization. The pressure need to be applied on the piezoelectric disc in the form of mechanical movement. For instance, walking or running. This continuous application of pressure on the sensor produces an electric charge surge that can be used relatively for low power input appliances such as Light Emitting Diode (LED). [7]
2.3 Material used PZT, or lead zirconate titanate, is one of the world’s most widely used piezoelectric ceramic materials. PZT is composed of the two chemical elements lead and zirconium combined with the chemical compound titanate. The underside of the electrode is made up of plastic. A very thin layer of ceramic (SiO2) is deposited on the outermost layer of the plastic. The ceramic element converts the mechanical energy of compression into electrical energy. The ceramic is a conductor material that acts as the negative terminal while the metal acts as the positive terminal. [6] The interaction between the ceramic and the metal works in two ways: if outside electrical current is supplied, then the piezo will produce sounds. This property allows them to be used as buzzers and simple speakers. However, if no outside current is supplied and instead the piezo is subjected to changes in pressure (such as by the vibrations of guitar strings), then the interaction of the metal and ceramic actually creates a small amount of electrical current. The ceramic layer acts as the negative terminal meanwhile the metal acts as positive terminal.
Figure 2.7 Main components on the disc
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2.4 Polarization The piezoelectric disc used in this project works on the principle of polarization. Piezoelectric material consists of polarized ions within the crystal. The top and bottom layer of the piezoelectric disc is positively charged while the side wall is negatively charged. Mechanical compression or tension on a poled piezoelectric ceramic element changes the dipole moment, thus creates a voltage surge. [4] This results in the displacement of the ions within the crystal position. Tension along the direction of polarization, or compression perpendicular to the direction of polarization, generates a voltage with polarity opposite that of the poling voltage (Figure 1.4c). When the mechanical stress is applied, the polarization is negative for tensile and positive for compressive strain on the lead zirconate titanate layers. Values for compressive stress and the voltage (or field strength) generated by applying stress to a piezoelectric ceramic element are linearly proportional up to a material-specific stress. The same is true for applied voltage and generated strain. [6] If a voltage of the same polarity as the poling voltage is applied to a ceramic element, in the direction of the poling voltage, the element will lengthen and its diameter will become smaller (Figure 1.4d). If a voltage of polarity opposite that of the poling voltage is applied, the element will become shorter and broader (Figure 1.4e).
Figure 2.8 Polarization of Piezoelectric Disc
7|Page
2.5 Voltage and Current Output Piezoelectric do not produce a constant steady flow voltage when a mechanical force is applied on it. An alternating current in produced indeed. The power output by the piezoelectric harvester is not in a form which is directly usable by normal household appliances which the harvester powers. The voltage and current output by the harvester needs to be conditioned and converted to a form usable by the load circuits. The power conditioning and converting circuits should also be able to extract the maximum power available out of the piezoelectric energy harvester.
Figure 2.9 Relationship between footstep and output voltage
Commonly used analog and digital circuits require a regulated supply voltage to operate from. Since the piezoelectric harvester outputs a sinusoidal current, it first needs to be rectified before it can be used to power circuits. Full wave bridge rectifier circuits are used commonly to change the the output from AC to DC discussed below.
2.5.1 Full Wave Bridge Rectifier Full-bridge rectifiers are commonly used as rectifier circuits to convert the AC output of a piezoelectric harvester into a DC voltage. The rectifying circuits consist of 4 diodes. The voltage needs to rectified due to the need for constant supply of voltage light up the series of LED placed in parallel.
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Figure 2.10 Ac to DC graph after rectification
Figure 2.11 Full wave bridge rectifier
2.6 Connection of Piezo Initially, two possible connections were tested - parallel and series connections. The parallel connection did not show significant increase in the voltage output. Series connection, resulted in increased of voltage output but not in linear proportion. This may be due to the non-linear modification of internal impedance of the system. Despite parallel circuit yielding a poor outcome, the discs had to be connected in parallel in order to not disrupt the flow of charge. This is because, even when a few disc is stepped, the electrical charge should flow still through to the output. This same phenomenon does not takes place if the connection is in series. Practically, its impossible for one to step on all the discs at one time.
9|Page
2.6.1 Voltage In this particular project, a total of 37 piezoelectric disc have been used on a 1ft wide and 3ft length platform. Of the 37 piezos, 27 of it were connected in parallel while the remaining ones were connected in series, in a group of 3 each. The discs connected in series are placed at 3 spots considered to be ideal in order to boost the output voltage. [8] As we know,
For parallel circuit :
Thus, by connecting the discs in parallel, the voltage supplied at the output is equivalent to the voltage produced from one single disc. The recorded voltage within a range of 1.2V < Vave < 2 V. This amount of voltage was not sufficient to power and light up a series of 5 LED’s brightly.
Therefore, 3 patched of disc in series connection were placed among the parallel piezos.
For series circuit : V
= V1 + V2 +… + Vn ………………………..[8]
As expected the result was very much improved as it was possible to obtain voltage ranging up to 19V. But, the point to note here is, the hike in voltage solely depends on the mass of the particular person applying mechanical stress (footstep) on the disc.
2.6.2 Current As of current, the maximum current output of one single disc is 1600 µA. The output current from the parallel piezos were ‘0.0432 A’ which is relatively low but theoretically and practically it was sufficient. Since in series the current is the same for all elements,
For parallel circuit :
I = I1 + I2 + … = In ………………………….[8] 10 | P a g e
For series circuit
:
the current output from the series circuit were constant having a range of not more than 1800 µA. All the voltage and current readings were measured using an digital multimeter. By default, every circuits have resistance. Resistance is the controlling factor that limits the amount of current flowing through the circuit when the circuit is closed. For piezoelectric, the resistance is known as piezoresistive effect. Piezoresistive effect is described as the changes in the electrical resistivity of a semiconductor when mechanical stress is applied without affecting the electrical potential. [9] *Assumption : Mass of 70kg was used as a control for the calculations.
2.7 Battery Pavegen is a leading company specialized in large scale manufacturing and installation of piezoelectric systems. Upon applying pressure on the piezoelectric, the charge is used to light up pedestrian walk lamps and also at the same time charge the battery. When the no longer mechanical force on the piezo, the charge from the battery is used to continue lighting up the lamp for a certain period of time. Refer to Figure 1.7. The sensor Pavegen developed is capable of producing power up to 8 Watt. 80% of the sensor was made up of recycled materials.
11 | P a g e
Figure 2.12 Footstep Energy Harvesting Concept by Pavegen
Based on this idea, the project was carried out. In this project, the piezoelectric served as a secondary source of power. The main source of power that lights up the LED’s and also charges the capacitor is the battery. A 12 V- 2.3 Ah lithium ion rechargeable battery was used for this purpose. The sole reason behind this was that, piezoelectric disc is not capable of producing a high amount of voltage that could light up 5 LED’s. Therefore, it was merely not possible to light up the LED’s with just the charge from piezo alone because the amount of voltage being produced is very minimal. Series circuits were also installed to increase the voltage of the whole system, but the output requirement was relatively high.
2.8 Capacitor Capacitor is a component that stores charge from external power source. It consist of two conducting plates separated by an insulating material called the dielectric. The stored charge is then slowly used up by discharging. A 10000 V - 25µF capacitor was used to light up the LED. This storage capacity enables the light to discharge over the time span of 12 seconds. The capacitor is charged by the battery on a single contact. 12 | P a g e
To activate the process, a ‘press-on’ switch was used. The switch was placed beneath the walking path on the wood frame. As mentioned before, the capacitor was not able to be charged with piezo because the charge produced by the piezo is very small. Therefore, charging the capacitor with the charge from piezo was not idealistic.
2.9 Circuit Board A circuit board is used in electronic works for component soldering. Components for the particular system is solder onto the circuit board according to the schematic diagram. Here the circuit board was used to hold the resistors, LEDs, capacitor and diode. All the components were soldered onto the circuit board. Before the components were soldered onto the circuit board, a schematic diagram was drawn. This is to get a clearer picture of the positions of the components.
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CHAPTER 3
METHODOLOGY
3.1
Introduction The focus of this research is to harvesting green energy from footstep. This research is
carried out by planning and identifying the methodology process since this is very important as reference and guide to achieve the goal. Figure 3.1 is the flow chart as a guideline of the study.
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Start
`
Failed
Failed
Preparing
Wood
Circuit
frame
board
Maximum
Testing :
Measuring with multimeter
load test
Sturdiness
Output
Data Collection
Analysis
Result and Discussion
Conclusion and Recommendation
End
Figure 3.1: Methodology of study flow chart 15 | P a g e
3.2
Materials and Tools
Table 3.1: Materials and Tools No
Materials and Tools
1.
Piezoelectric
2.
Wood
3.
Circuit board
Figure
Capacitor 4.
5.
Resistor
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6.
Light-emitting diode (LED)
7.
Diode
8.
Wire
9.
Saw
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10.
Hammer
11.
Nail
12.
Soldering gun
13.
Flux
14.
Hot glue gun
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15.
Digital Multimeter
16.
Sandpaper
17.
Paint and brush/roller
Figure 3.2
List of materials
19 | P a g e
3.3
Product Preparation 3.3.1 Design 1) From discussion among group member, decide the size of the product and the circuit that will be used.
Figure 3.3 Group discussion and planning
2) Draw the design by using design software (e.g. CAD, SolidWork, etc.).
Refer Appendix 1 for the Orthographic Projection View
Refer Appendix 2 for Dimension Drawing
Refer Appendix 3 for schematic diagram of circuit.
20 | P a g e
3.3.2 Building the wood frame 1) Mark the wood with the proper dimensions.
Figure 3.4 Measuring the wood before cutting process
2) Based from the mark that was done, cut the wood using saw or any other proper tools.
Figure 3.5 Cutting wood process
3) Assemble all the part that was cut based from the design. 21 | P a g e
Figure 3.6 Assembling wood part using nails
3.3.3 Building the circuit and Piezoelectric arrangement 1) Solder the component to the circuit board based from the schematic diagram.
Figure 3.7 Soldering the component to the circuit board
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2) Connect the circuit to the Piezoelectric
Figure 3.8 Connecting the circuit to Piezoelectric
23 | P a g e
3.4
Assemble the whole circuit and the wood frame 1) Put the circuit in the compartment inside the wood frame.
Figure 3.9 Assembling the circuit and wood frame
2) Adjust the Piezoelectric location so maximum pressure would be applied on it when being stepped on.
Figure 3.10 Arrangement of the Piezoelectric
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3.5
Measuring the Electrical Output 1) Alligator clips will be fasten to the two wires from the circuit to make electrical contact to the finished device.
Figure 3.11 Using multimeter to determine the output produced
2) The black (–) wire of the multimeter will be attached to the blue wire from the circuit board (negative). 3) The red (+) wire of the multimeter will be attached to the red wire from the circuit board (positive). 4) Step on the platform and observe the electrical output.
Figure 3.12 Testing the product 25 | P a g e
3.6 Finishing touch 1) Polish the product with sandpaper to obtain smooth surface.
Figure 3.13 Sand the wood surface using sandpaper
2) Paint the product so it appears attractive.
Figure 3.14 Painting the product
26 | P a g e
CHAPTER 4
RESULTS AND ANALYSIS
4.1
Introduction
When starting doing the project there are many different circuits in mind to accomplish the goals. The project was started with a circuit that used an amplifier to boost the electrical signal that could be from the mechanical signal. However, the theory on this circuit was flawed, as piezoelectric do not behave this way. Thus, in order to accomplish the project, the operation of piezoelectric must so that the design of new efficient circuit can be determine to control the generated energy. The project was started by looking at a fly-converter with a transformer output. However, with this design the circuit could not control the temporary storage so it did not meet our performance requirements. However, the next design, which utilizes a low power, was the MOSFET for voltage sensing. The MOSFET ended up being the deciding component as we were able to procure a low power MOSFET, as most MOSFETs require larger power needs than we could not be able to generate, in order to activate their switching. So, this was not the correct circuit to be installed in the project. Finally, the last design had led us to begin investigating about the voltage-sensing switches. It is consisted of a system, which had a converter and load. However in between, the usage of combined temporary storage and voltage sensing circuit. This was mainly composed of diodes and a few inductors, along with the capacitors for storage. Therefore, we found that this circuit was practical for our project to work from 27 | P a g e
input low power to high input power which it can light the led brightly from the charging and discharging of the capacitor and the piezoelectric.
4.2
Design circuit
Figure 4.1: Equivalent circuit for direct discharge architecture The piezoelectric source model is connected in parallel to the bucket capacitor a variable resistor
and
. When the output regulator stage of such a system is turned OFF,
represents only the leakage current through the dielectric of
and the quiescent current of
the regulator and load stage. When the load is exerted, however,
depends upon the impedance of the load and the
loss in the regulator. Finally, the voltage source “2
” models the diode drops experienced
through a diode bridge rectifier. Typically, a circuit would require a number of current pulses from the source to charge at start-up before the load stage is turned ON and
begins to draw a significant amount of
current. Once a specified threshold is reached for the voltage
, the load stage is activated
and
is greater than that provided
begins to discharge assuming the current required by
by the source. Moreover,
is allowed to discharge to specified voltage and the cycle repeats.
There are a few important aspects of this conditioning scheme which demand attention: Energy is transferred from the source capacitor to the sum of the load voltage
(t) and 2
only after
(t) has reached
and after the diode bridge begins to conduct. With
28 | P a g e
each current pulse from the source capacitor quickly to the sum of
(t) and 2
, the voltage across the capacitor will rise
, and remain there for the remainder of the current pulse.
Due to the relationship Q=CV holds for both source and bucket capacitors, Q is fixed for each current pulse and the ratio between
and
is necessarily high (
/
= ~103), the
voltage across
rises (over the same period) at one thousandth of the rate at which the
voltage across
would have risen were it not loaded. These observations lead to a general
conclusion that the direct-discharge method clamps the source voltage quite low in comparison to the natural tendency of a piezoelectric source. [12] Now, given that the energy delivered to
during each current pulse is Q=VI, and the
total charge Q liberated during each current pulse is fixed and relatively constant, clamping the voltage is equivalent to clamping the energy over a cycle, or the power, transferred to the load stage. Even if the capacitors perfectly matched, only one quarter of the original energy can be transferred from one capacitor to another, and half of the total energy lost, if no other energy storage elements are placed in between. Moreover, the most efficient way to transfer energy off of a charging capacitor is by allowing it reach a maximum voltage and then leak off the charge through exponential voltage decay. That is what happens during an RC decay and, in a sense, is why supplementing a capacitive load with a matched inductance is advantageous in ac systems. To further illustrate this point and to quantify the inefficiency of such a system, the following mathematical model is derived. Using the equivalent circuit in Fig 3.1, and assuming an average load voltage <
> over the charge/discharge cycle:
29 | P a g e
(3.1)
where:
and
The previous equations describe the source and load voltages during the brief period before the rectifier diodes turn ON. The charge source is assumed to ramp linearly with each footfall and subsequent heel lift, thereby providing an approximately square pulse of current to
and
. The magnitude of this pulse is
, which has length L and period T in the
following derivation. The next series of equations result in a function describing load voltage
throughout
a current pulse cycle: pulse cycle:
30 | P a g e
Now in the Laplace domain,
solving for
, and letting
=
+
and Re =
|| ,
Or
31 | P a g e
At the end of the current pulse, the charge across
is nearly drained, so it is assumed that
negligible charge leaks through the diodes and onto the bucket capacitor after t = a. Moreover, the voltage
(t) begins to decay though
, or:
Where
And
Now, the energy transferred to the load is found using the following equation:
Or
And, integrating the previous expression,
32 | P a g e
33 | P a g e
4.3
Equations/Simulations/General Circuits 4.3.1
AC/DC converter [12]
Figure 4.2: Bridge rectifier circuit
34 | P a g e
4.3.2 Capacitor charging/discharging
Figure 4.3: Voltage and current simulation on a capacitor
: 15V,
: 2.3A, and
:
25A. Power from one capacitor
Power from power source
P=I*V
P=I*V
P= 4.6A*5V
P=2.3A*12V
P=23W
P=27.6W
Efficiency of capacitor = Output power/Input power *100% = 23W/27.6W *100% = 83.33% From simulation it is shown to be very efficient to use 10000µF in the project circuit.
35 | P a g e
CHAPTER 5
5.1 Conclusion and Recommendation
Although the theory developed in this report justifies the use of switching techniques in efficiently converting that energy to a usable form, there are obviously some practical limitations to the systems presented. Measurements of source current into the primary and load current transferred from the secondary reveal that very little current gain truly occurs between the input and output ports of the switch in the forward converter hybrid. Further, similar results were encountered when one examines the energy transferred through the series switch and inductor in the buck converter.
In addition, based on the results gathered in this investigation, the final prototype design does fulfill the objective of generating electricity from piezoelectric disk. Due to the low cost design of the piezoelectric system it is a practical product which could increase the operating period of most common products. The data collected is capable of extending the operational lifespan per charge of portable electronic devices.
5.2 Limitations and Assumptions
In the process of testing the viability of using piezoelectricity as a human generated source of electricity, there were several assumption that were made. Steps were taken to control the consistency of isolation data collected. There was care taken to ensure that the data collected from voltage tests carried out on the piezoelectric material was reliable by maintaining a consistent mass being applied on the piezoelectric material and the piezoelectric disk was also fixed to a wooden plank to prevent slip and unintended voltage spikes due to secondary impulses. Precautions taken to preserve consistency of footstep data include the data from multiple individuals being compared and an equal number of lead up steps up to the force plate. By comparing different footsteps against each other, it is possible to make more generalizable conclusions by getting a wider sample size.
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The final product does assume that real world conditions are accurately mimicked by test conditions. When electrical data was collected from the output terminal the environment and walking circumstances were made to be as similar to real life applications as possible.
5.3 Recommendation
Through testing of the final prototype, several issues became evident that needed rectification before the production unit. In addtion, investigation into the use of more sustainable or recycled materials should be considered if possible. Availability of material may be an issue too, but it is more feasible at large scale. White crystalline LEDs were used due to their brightness.
In conjunction,seals between tiles may be necessary to prevent debris or water from entering. In fact,doubling the height of the piezoelectric stack will double the performance of the tile, so maximizing the amount of piezoelectric material per unit area is critical during the final sourcing of components. The lower the lost, the more feasible it is to implement more per tile.
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REFERENCES
1) Piezoelectric effect. http://seminarprojects.com/Thread-project-on-piezoelectric-car#ixzz2QoCxRi2y, accessed on 15th January 2013. 2) What is piezoelectricity? 2003 – 2013. http://www.wisegeek.com/what-is-piezoelectricity.htm, accessed on 15th January 2013.
3) Piezoelectricity. http://en.wikipedia.org/wiki/Piezoelectricity, accessed on 16th February, 2013.
4) Fundamentals of piezoelectricity. 2012. http://www.piezo.ws/piezoelectric_actuator_tutorial/Piezo_Design_part2.php, accessed on 20th February, 2013.
5) Uses of Piezoelectric sensor. 2012. http://answers.yahoo.com/question/index?qid=20071212185616AAEAZMV, accessed on 22nd February,2013.
6) Piezoelectric. APC International Limited. 2013. http://www.americanpiezo.com/product-service/custom-piezoelectricelements/shapes-sizes.html, accessed on 25th February,2013.
7) Piezoelectric ceramics. 2013. IIT-EXELIS. http://www.exelisinc.com/capabilities/piezoelectrics/Pages/default.aspx, accessed on 25th February,2013.
8) Series and parallel circuits. http://en.wikipedia.org/wiki/Series_and_parallel_circuits, accessed on 26th February, 2013. 9) Piezoresistive effect-Impedance. 2013. 38 | P a g e
http://www.ionizationx.com/index.php?topic=1232.0, accessed on 26th February, 2013. 10) Pavegen-Pioneer in piezoelectricity. http://www.pavegen.com/, accessed on 26th February, 2013. 11) MOSFETs. 2013. http://www.fairchildsemi.com/products/mosfets/, accessed on 22nd February,2013. 12) Ramadass, Y.K., and A.P. Chandrakasan. “An Efficient Piezoelectric Energy Harvesting Interface Circuit Using a Bias-Flip Rectifier and Shared Inductor.” SolidState Circuits, IEEE Journal Of 45.1 (2010)
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Appendix 1 – Orthographic Projection View
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Appendix 2 – Dimension Drawing
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Appendix 3- Schematic Diagram
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