varun report

8/4/2019 Varun Report

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A SEMINAR REPORT ON

ENERGY SCAVENGING FOR WIRELESS

SENSOR NODES

Submitted in partial fulfilment of the requirements for the

award of the degree of

BACHELOR OF TECHNOLOGY

In

ELECRICAL AND ELECTRONICS ENGINEERING

Submitted by

VARUN GOPINATH: 08 413 026

DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING

LOURDES MATHA COLLEGE OF SCIENCE AND

TECHNOLOGY, LOURDES HILLS, KUTTICHAL

KERALA 695 574


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AUGUST 2011

LOURDES MATHA COLLEGE OF SCIENCE AND TECHNOLOGY

KUTTICHAL 695 574

DEPARTMENT OF ELECTRICAL AND ELECTRONICS

ENGINEERING

CERTIFICATE

Certified that seminar work entitled ENERGY SCAVENGING FOR WIRELESS SENSOR

NODES is a bonafide work carried out in the seventh semester by VARUN GOPINATH in partial

fulfilment for the award of Bachelor of Technology in ELECRICAL AND ELECTRONICSENGINEERING from University of Kerala during the academic year 20011-2012. who carried out the seminar

work under the guidance and no part of this work has been submitted earlier for the award of any degree.

GUIDE HEAD OF THE DEPARTMENT

SREEKALA DEVI , DINU THOMAS ,

Asst: professor,

Department of electrical and Department of electrical andelectronics engineering, electronics engineering,

Lourdes matha college of science Lourdes matha college of scienceand technology, and technology,kuttichal, kerala 695 574 kuttichal, kerala 695 574

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ACKNOWLEDGEMENTS

I take immense pleasure in thanking Mrs.DINU THOMAS , Department of electrical and

electronics engineering, LOURDES MATHA COLLEGE OF SCIENCE AND

TECHNOLOGY, our beloved HEAD OF THE DEPARTMENT for having permitted me to

carry out this seminar work.

I wish to express my deep sense of gratitude to my Guide, Mrs. SREEKALA DEVI,

Asst: professor, LOURDES MATHA COLLEGE OF SCIENCE AND TECHNOLOGY for her

able guidance and useful suggestions, which helped me in completing the seminar, in time.

Needless to mention that Mrs. SWAPNA . M, Asst:: professor, LOURDES MATHA

COLLEGE OF SCIENCE AND TECHNOLOGY, who had been a source of inspiration and

for her timely guidance in the conduct of my seminar work.

Finally, yet importantly, I would like to express my heartfelt thanks to my beloved parents

for their blessings, my friends/classmates for their help and wishes for the successful

completion of this seminar.

VARUN GOPINATH

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ABSTRACT

The vast reduction in size and power consumption of CMOS circuitry has led to a large

research effort based around the vision of ubiquitous networks of wireless communication nodes.

As the networks, which are usually designed to run on batteries, increase in number and the

devices decrease in size, the replacement of depleted batteries is not practical. Methods of

scavenging ambient power for use by low power wireless electronic devices have been explored in

an effort to make the wireless nodes and resulting wireless sensor networks indefinitely self-

contained.

Most wireless sensor nodes are powered by primary or secondary (rechargeable) batteries.

These take up a large proportion of the size and weight, and often the cost, of the nodes, and

furthermore the need to replace or recharge them creates a significant maintenance burden.

Maintenance free power provision would greatly increase the feasibility of networks with very

large numbers of, or very widely distributed, nodes. Recently the scavenging of energy from the

environment, in the form of heat, motion, light or other electromagnetic radiation, has been

actively researched as a possible solution to this problem. In this paper the progress and ultimate

potential of such power sources is reviewed, with an emphasis on motion and vibration

scavenging.

The power levels achievable are examined, and applications are considered in which such

sources are attractive to substitute for or supplement batteries. Energy harvesters provide a very

small amount of power for low-energy electronics. While the input fuel to some large-scale

generation costs money (oil, coal, etc.), the energy source for energy harvesters is present as

ambient background and is free. Energy harvesting devices converting ambient energy into

electrical energy have attracted much interest in both the military and commercial sectors . After a

broad comparison of potential energy scavenging methods, the conversion of ambient vibrations to

electricity was chosen as a method for further research.

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TABLE OF CONTENTS

Sl.no Contents Page no

1

2

3

4

4.1

4.2

4.2.1

4.2.2

4.2.3

5

6

7

8

9

10

ABSTRACT

TABLE OF CONTENTS

INTRODUCTION

OVERVIEW OF THE PROBLEM AND POTENTIAL

SOURCES OF POWER

MOTIVATION: WIRELESS SENSOR ANDACTUATOR NETWORKS

THREE METHODS OF POWERING WIRELESS

SENSOR NETWORKS

IMPROVE THE ENERGY DENSITY OF STORAGE

SYSTEMS.DEVELOP NOVEL METHODS TO DISTRIBUTE

POWER TO NODES.

DEVELOP TECHNOLOGIES THAT ENABLE A

NODE TO GENERATE OR SCAVENGE ITSOWN POWER.

ENERGY SCAVENGING METHODS

WIND OR AIR FLOW PRESSURE VARIATIONS SOLAR TEMPERATURE

PASSIVE HUMAN POWER

ACTIVE HUMAN POWER

VIBRATION AND MOTION

ULTIMATE POWER EQUATION

APPLICATION

COMPARISON OF ENERGY SCAVENGINGTECHNOLOGIES.

CONCLUSION

REFERENCE

4

5

6

8

9

10

10

11

13

14

1414

15

1819

19

20

25

31

33

35

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3. INTRODUCTION

Wireless sensors are emerging as viable instrumentation techniques for industrial

applications specially in condition monitoring applications. Condition monitoring provides

information on the health and maintenance requirements of industrial machinery and is widely

being adopted as an alternative to the more conventional preventive and breakdown maintenance

strategies. Measurements and monitoring of parameters such as vibration, temperature, noise

level and power consumption could help to detect the trends from developing faults and

determine the sources of problems. This can be used to schedule maintenance effectively to

avoid unnecessary maintenance and catastrophic failures. The selection of right sensors is the

key to effective condition monitoring. At present, most of the sensors are physically wired. The

wires provide both power and communications paths. However, in many applications, wired

sensors are impractical or inconvenient. In these situations, wireless sensor networks could be a

possible solution. These networks can be used in remote locations and also offer inexpensive and

flexible installation. In a very short span of time, wireless sensors have emerged as the sensing

technology of choice in a variety of industry instrumentation techniques because of their

flexibility, non-intrusive operation, safety and their low cost, low power characteristics. Wireless

sensors can be installed inside the mechanical devices, much closer to the phenomena of interest.Since communication is integrated into the sensor setup, they can be installed on moving parts as

well as static components. Energy harvesters provide a very small amount of power for low-

energy electronics. While the input fuel to some large-scale generation costs money (oil, coal,

etc.), the energy source for energy harvesters is present as ambient background and is free. For

example, temperature gradients exist from the operation of a combustion engine and in urban

areas, there is a large amount of electromagnetic energy in the environment because of radio and

television broadcasting.

Batteries are the primary power supply for current wireless sensor nodes. However, when

the battery is exhausted, the sensor node becomes non-operational till the battery is replaced.

This finite and often limited operational life makes it a burden for plant maintenance. Energy

scavenging for wireless sensors thus becomes more important and has attracted considerable

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research interest in recent years. Glynne-Jones 2001 reviewed a range of self-powered system

which uses vibration power sources, optical power sources, thermoelectric and radio power

sources and so on. He also published the design of vibration powered, electromagnetic miniature

generator in 2003. In 2005, Paradiso published a survey of energy scavenging for mobile and

wireless electronics which introduced various human-powered systems. Several groups of

researchers such as Ottman 2002 and Roundy 2003 have published designs for piezoelectric

generators. These convert ambient vibrations to electrical signals. Leland also published an

energy scavenger constructed from cantilever mount piezoelectric bimorphs and magnets for

household electrical monitoring in 2006.

Energy Scavenging is perhaps the most attractive of the three options because the lifetime

of the node would be limited only by failure of its own components. Energy harvesting is the

process of acquiring energy from the surrounding environment. However, it is also potentially

the most difficult method to exploit because each use environment will have different forms of

ambient energy, and therefore, there is no one solution that will fit all, or even a majority, of

applications. Energy harvesting devices converting ambient energy into electrical energy have

attracted much interest in both the military and commercial sectors. Some systems convert

motion, such as that of ocean waves, into electricity to be used by oceanographic monitoring

sensors for autonomous operation. Future applications may include high power output devices

(or arrays of such devices) deployed at remote locations to serve as reliable power stations for

large systems. Another application is in wearable electronics, where energy harvesting devices

can power or recharge cellphones, mobile computers, radio communication equipment, etc. All

of these devices must be sufficiently robust to endure long-term exposure to hostile environments

and have a broad range of dynamic sensitivity to exploit the entire spectrum of wave motions.

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4. OVERVIEW OF THE PROBLEM AND POTENTIAL

SOURCES OF POWER

WHAT IS ENERGY SCAVENGING?

C Converting power from ambient sources

C Other terms:

C Parisitic energy

C Energy harvesting

C Microgeneration

C Unlimited self-sustenance

C Power sources already existC Running out of power can occur, but can charge itself back up again

WHY WE NEED ENERGY SCAVENGING??

Most of the sensor nodes typically run on batteries, but as the networks increase in

number and devices decrease in size, the replacement of depleted batteries will not be practical.

Another approach is to use a battery large enough to survive the entire lifetime of the

wireless sensor device, but again this will dominate the overall cost and size.

C Currently fed from batteries

C Replacement/rechargeing is difficult

C Too numerous in the future

C Location may be unreachable

C Running out of power can occur

C Sensor size limited by battery size

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4.1 MOTIVATION: WIRELESS SENSOR AND ACTUATOR

NETWORKS

The past several years have seen an increasing interest in the development of Wireless

sensor and actuator networks. Such networks could potentially be used for a Wide variety of

applications. A few possible applications include: monitoring Temperature, light, and the location

of persons in commercial buildings to control the Environment in a more energy efficient manner,

sensing harmful chemical agents in high Traffic areas, monitoring fatigue crack formation on

aircraft, monitoring acceleration and Pressure in automobile tires, etc. Indeed, many experts

foresee that very low power Embedded electronic devices will become a ubiquitous part of our

environment, Performing functions in applications ranging from entertainment to factory

automation (Rabaey et al 2000, Gates 2002, Wang et al 2002, Hitach mu-Chip 2003). Advances in

IC (Integrated Circuit) manufacturing and low power circuit design And networking techniques

(Chandrakasan et al, 1998, Davis et al, 2001) have reduced The total power requirements of a

wireless sensor node to well below 1 milliwatt. Such Nodes would form dense ad-hoc networks

transmitting data from 1 to 10 meters. In fact, For communication distances over 10 meters, the

energy to transmit data rapidly Dominates the system (Rabaey et al 2002). Therefore, the

proposed sensor networks Would operate in a multi-hop fashion replacing large transmission

distances with multiple Low power, low cost nodes. The problem of powering a large number of

nodes in a dense network becomes Critical when one considers the prohibitive cost of wiring

power to them or replacing Batteries. In order for the nodes to be conveniently placed and used

they must be small, Which places severe limits on their lifetime if powered by a battery meant to

last the Entire life of the device. State of the art, non-rechargeable lithium batteries can provide

Up to 800 WH/L (watt hours per liter) or 2880 J/cm3. If an electronic device with a 1 cm3 Battery

were to consume 100 W of power on average (an aggressive goal), the device Could last 8000

hours or 333 days, almost a year. Actually, this is a very optimistic Estimate as the entire capacity

usually cannot be used due to voltage drop. It is worth Mentioning that the sensors and electronicsof a wireless sensor node will be far smaller Than 1 cm3, so, in this case, the battery would

dominate the system volume. Clearly, a Lifetime of 1 year is far from sufficient. The need to

develop alternative methods of Power for wireless sensor and actuator nodes is acute.

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4.2THREE METHODS OF POWERING WIRELESS SENSOR

NETWORKS

There are three possible ways to address the problem of powering the emerging wireless

technologies:

1. Improve the energy density of storage systems.

2. Develop novel methods to distribute power to nodes.

3. Develop technologies that enable a node to generate or scavenge its own power.

4.2.1 Improve the energy density of storage systems.

Research to increase the storage density of both rechargeable and primary batteries has

been conducted for many years and continues to receive substantial focus. The past few years

have also seen many efforts to miniaturize fuel cells which promise several times the energy

density of batteries. Finally, more recent research efforts are underway to develop miniature heat

engines that promise similar energy densities to fuel cells, but are capable of far higher

maximum power output. While these technologies promise to extend the lifetime of wireless

sensor nodes, they cannot extend their lifetime indefinitely.

PRIMARY BATTERIES:

Energy storage is in the form of electrochemical energy stored in a battery, most

predominant means of power supply. They have a fairly stable voltage, electronic devices can

often be run directly from the battery without any intervening power electronics.Regardless of

the form of the energy storage, the lifetime of the node will be determined by the fixed amountof energy stored on the device.

Zinc-air batteries, Lithium batteries, and Alkaline batteries are commonly using

primary batteries.

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Zinc-air batteries have the highest energy density, but their lifetime is very short.

Lithium batteries have excellent energy density and longevity, but costly.

Alkaline batteries combination of fair energy density and low cost, but their lifetime

is very short.

SECONDARY BATTERIES:

Rechargeable batteries. They requires another primary power source, to charge them.

One item to consider when using rechargeable batteries is that electronics to control the

charging profile must often be used.

So again it requires looking for alternate power sources.

4.2.2 Develop novel methods to distribute power to nodes.

The most common method (other than wires) of distributing power to embedded

electronics is through the use of RF (Radio Frequency) radiation. Many passive electronic

devices, such as electronic ID tags and smart cards, are powered by a nearby energy rich source

that transmits RF energy to the passive device, which then uses that energy to run its electronics.

However, this method is not practical when considering dense networks of wireless nodes

because an entire space, such as a room, would need to be flooded with RF radiation to power

the nodes. The amount of radiation needed to do this would probably present a health risk and

today exceeds FCC (Federal Communications Commission) regulations.

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As an example, the Location and Monitoring Service (LMS) offered by the FCC operates

between 902 and 928 MHz and is used as, but not limited to, a method to automatically identify

vehicles (at a toll plaza for example) (FCC 2002). The amount of power transmitted to a node

assuming no interference is given by Pr = Po2/(4R2) where Po is the transmitted power, is

the wavelength of the signal and R is the distance between transmitter and receiver. If a

maximum distance of 10 meters and the frequency band of the LMS are assumed, then to power

a node consuming 100W, the power transmitter would need to emit 14 watts of RF radiation.

In this band the FCC regulations state that person should not be exposed to more than 0.6

mW/cm2 (FCC 2002). In the case just described, a person 1 meter away from the power

transmitter would be exposed to 0.45 mW/cm2, which is just under federal regulations.

However, this assumes that there are no reflections between the transmitter and receiver. In a

realistic situation, the transmitter would need to far more than 14 watts, which would likely put

people in the vicinity at risk. The FCC also has regulations determining how much power can

be radiated at certain frequencies indoors. For example, the FCC regulation on ceiling mounted

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transmitters in the 2.4 2.4835 GHz band (the unlicensed industrial, scientific, and medical

band) is 1 watt (Evans et al 1996), which given the numbers above is far too low to transmit

power to sensor nodes throughout a room.

Solution works well where there is a high power scanner or other sources in very near

proximity to the wireless device. Less effective in dense regions, where a large area must be

flooded with RF radiation to power many wireless sensor nodes.

Eg: Given a 1 watt transmitter, and a 5 meter maximum distance the power

received at the node would be 50W, which is probably the borderline of being really useful.

4.2.3 Develop technologies that enable a node to generate or scavenge its

own power.

The third method, in which the wireless node generates its own power, has not been

explored as fully as the first two. The idea is that a node would convert ambient sources of

energy in the environment into electricity for use by the electronics. This method has been

dubbed energy scavenging, because the node is scavenging or harvesting unused ambient

energy. Energy scavenging is the most attractive of the three options because the lifetime of the

node would only be limited by failure of its own components. However, it is also potentially the

most difficult method to exploit because each use environment will have different forms of

ambient energy, and therefore, there is no one solution that will fit all, or even a majority, of

applications. Nevertheless, it was decided to pursue research into energy scavenging techniques

because of the attractiveness of completely self-sustaining wireless nodes. The driving force for

energy scavenging is the development of wireless sensor and actuator networks. In particular,

this research aims to develop a small, flexible wireless platform for ubiquitous wireless data

acquisition that minimizes power dissipation. The PicoRadio project researchers have developed

some specifications that affect the exploration of energy scavenging techniques that will be used

by their devices. The most important specifications for the power system are the total size and

average power dissipation of an individual Pico Node (an individual node in the PicoRadio

system is referred to here as a Pico Node). The size of a node must be no larger than 1 cm3, and

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the target average power dissipation of a completed node is 100 W. The power target is

particularly aggressive, and it is likely that several generations of prototypes will be necessary to

achieve this goal. Therefore, the measure of acceptability of an energy scavenging solution will

be its ability to provide 100 W of power in less then 1cm3.

This does not mean that solutions which do not meet this criterion are not worthy of

further exploration, but simply that they will not meet the needs of the PicoRadio project. Thus,

the primary metric for evaluating power sources used in this research is power per volume,

specifically W/cm3, with a target of at least 100 W/cm3.

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5. ENERGY SCAVENGING METHODS

5.1 WIND / AIR FLOW

Wind power has been used on a large scale as a power source for centuries. Large windmills

are still common today.

However there has not been any successful effort so far to generate power using this at very

small scale.

Large scale windmills operate at maximum efficiencies of about 40%. Efficiency is

dependent on wind velocity, and average operating efficiencies are usually about 20%.

Windmills are generally designed such that maximum efficiency occurs at wind velocities

around 8 m/s (or about 18 mph). At low air velocity, efficiency can be significantly lower than

20%.

As there are many possible applications in which a fairly constant air flow of a few meters

per second exists, it seems that research leading to the development of devices to convert

air flow to electrical power at small scales is warranted.

5.2 PRESSURE VARIATIONS

Variations in pressure can be used to generate power..

Atmospheric pressure varies throughout the day. The change in energy for a fixed volume of

ideal gas due to a change in pressure.

An average temperature variation over a 24 hour period would be about 10 C.

If 1cm3 of helium gas were used, a 10 C temperature variation would result in a pressure

change of 1.4 MPa. The corresponding change in energy would be 1.4 J per day, which

corresponds to 17 W/cm3.

While this is a simplistic analysis and assumes 100% conversion efficiency to electricity, it

does give an idea of what might be theoretically expected from naturally occurring pressure

variations.

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5.3 SOLAR ENERGY

Solar energy is abundant outdoors during the daytime. In direct sunlight at midday, the

power density of solar radiation on the earths surface is roughly 100 mW/cm3.

Silicon solar cells are a mature technology with efficiencies of single crystal silicon

cells ranging from 12% to 25%. Thin film polycrystalline, and amorphous silicon solar cells are

also commercially available and cost less than single crystal silicon, but also have lower

efficiency. As seen in the table, the power available falls off by a factor of about 100 on overcast

days. However, if the target application is outdoors and needs to operate primarily during the

daytime, solar cells offer an excellent and technologically mature solution. Available solar

power indoors, however, is drastically lower than that available outdoors. Measurements taken

in normal office lighting show that only several W/cm3 can be converted by a solar cell, which

is not nearly enough for the target application under consideration.

Table below shows power from a cadmium telluride solar cell at various distances from

a 60Watt incandescent bulb under standard office lighting conditions.The data clearly show that

if the target application is close to a light source, then there is sufficient energy to power a Pico

Node, however in ambient office lighting there is not. Furthermore, the power density falls off

roughly as 1/d2 as would be expected, where d is the distance from the light source.

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Crystalline Silicon:

Best efficiency: 15% - 20%

Outdoor use only.

Most expensive

Polycrystalline Silicon:

Efficiency: 10 - 13%

Outdoor use only.

Thin Film:

Efficiency: 8% - 10%

Good Indoor and Outdoor applications.

Cheapest

On a sunny day, the incident light on earths surface has a power density of roughly

100mW/cm3.

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Single crystal silicon solar cells exhibit efficiencies of 15-20%.

Common office lighting provides 100 W/cm2 at the surface of the desk.

Thin film amorphous silicon or cadmium telluride cells better efficiency indoors, still they

offer only 10% efficiency.

Solar cells are generally used to charge a secondary battery. They can be directly connected to

rechargeable batteries through a simple series diode to prevent the battery from discharging

through the solar cell.

For longer lifetime of rechargeable batteries, a controlled charging profile will be needed but it

will require more electronics which will use power themselves.

136 mW/cm2 is average light power on a sunny summer day.

Photovoltaic [PV] energy harvesting wireless technology offers significant advantages over

wired or solely battery-powered sensor solutions: virtually inexhaustible sources of power with

little or no adverse environmental effects. Indoor PV harvesting solutions have to date been

powered by specially tuned amorphous silicon (aSi)a technology most used in Solar

Calculators. In recent years new PV technologies have come to the forefront in Energy

Harvesting such as Dye Sensitized Solar Cells DSSC. The dyes absorbs light much like

chlorophyll does in plants. Electrons released on impact escape to the layer of TiO2 and from

there diffuse, through the electrolyte, as the dye can be tuned to the visible spectrum much

higher power can be produced. At 200 lux DSSC can provide over 15 micro watts per cm2.

Example:

Q. Inside, there is currently 1 mW/ square cm of light incident... How much power can we

get out of that with a 1 cm x 1cm cell?

A: Assume we are using a thin-film PV in order to match best with indoor light. Using a 9%

efficiency, we can get 0.09x1 = 90uW.

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5.4 TEMPERATURE VARIATIONS

Naturally occurring temperature variations can also provide a means by which energy

can be scavenged from the environment. Stordeur and Stark (Strodeur and Stark, 1997) have

demonstrated a thermoelectric micro-device capable of converting 15 W/cm3 from a 10 C

temperature gradient. While this is promising and, with the 9 improvement of thermoelectrics,

could eventually result in more than 15 W/cm3, situations in which there is a static 10 C

temperature difference within 1 cm3 are very rare. Alternatively, the natural temperature

variation over a 24 hour period might be used to generate electricity. It can be shown with fairly

simple calculations, assuming an average variation of 7 C, that an enclosed volume containing

an ideal gas could generate an average of 10 W/cm3. This, however, assumes no losses in the

conversion of the power to electricity. In fact some commercially available clocks, such as the

Atmos clock, operate on a similar principle. The Atmos clock includes a sealed volume of fluid

that undergoes a phase change right around 21 C. As the liquid turns to gas during a normal

days temperature variation, the pressure increases actuating a spring that winds the clock.

While this is very interesting, the level of power output is still substantially lower than other

possible methods.

Naturally occurring temperature variations can also provide a means by which energy

can be scavenged from the environment.

Maximum efficiency of power conversion is given by Carnot efficiency.

= (ThighTlow )/Thigh

Many researchers are using thermoelectric generators that exploit the Seebeck effect to

generate power.

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5.5 PASSIVE HUMAN POWER

A significant amount of work has been done on the possibility of scavenging power off

the human body for use by wearable electronic devices (Starner 1996, Shenck and Paradiso

2001). The conclusion of studies undertaken at MIT suggests that the most energy rich and

most easily exploitable source occurs at the foot during heel strike and in the bending of the

ball of the foot. This research has led to the development of the piezoelectric shoe inserts

referred to in the table. The power density available from the shoe inserts meets the constraints

of the current project. However, wearable computing and communication devices are not the

focus of this project. Furthermore, the problem of how to get the energy from a persons foot

to other places on the body has not been satisfactorily solved. For an RFID tag or other

wireless device worn on the shoe, the 10 piezoelectric shoe inserts offer a good solution.

However, the application space for such devices is extremely limited, and as mentioned, not

very applicable to wireless sensor networks.

5.6 ACTIVE HUMAN POWER

The type of human powered systems investigated at MIT could be referred to as passive

human powered systems in that the power is scavenged during common activities rather than

requiring the user to perform a specific activity to generate power. Human powered systems

of this second type, which require the user to perform a specific power generating motion, are

common and may be referred to separately as active human powered systems. Examples

include standard flashlights that are powered by squeezing a lever and the Free play wind-up

radios (Economist 1999). Active human powered devices, however, are not very applicable

for wireless sensor applications.

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5.7 VIBRATIONS

A combination of theory and experiment shows that about 300 W/cm3 could be

generated from vibrations that might be found in certain building environments. Vibrationswere measured on many surfaces inside buildings, and the resulting spectra used to calculate

the amount of power that could be generated. However, without discussing the details at this

point, it does appear that conversion of vibrations to electricity can be sufficient for the target

application in certain indoor environments. Some research has been done on scavenging power

from vibrations, however, it tends to be very focused on a single application or technology.

Therefore, a more broad look at the issue is warranted.

Vibration-to-electricity conversion offers the potential for wireless sensor nodes to be

self sustaining in many environments.

Low level vibrations occur in many environments including large commercial

buildings, automobiles, aircraft, ships, trains, and industrial environments.

Table below shows sources of vibrations and its effect produced:

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A few representative vibration spectra are shown in Figure 2.1. In all cases, vibrations

were measured with a standard piezoelectric accelerometer.

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EXISTING METHODS FOR VIBRATION ENERGY

SCAVENGING

1) ELECTROMAGNETIC (INDUCTIVE) POWER CONVERSION:Electromagnetic power conversion results from the relative motion of an electrical

conductor in a magnetic field. Typically the conductor is wound in a coil to make an

inductor. The relative motion between the coil and magnetic field cause a current to flow

in the coil.

There are a couple of significant strengths to electromagnetic implementation.

First, no separate voltage source is needed to get the process started as in electrostatic

conversion. Second, the system can be easily designed without the necessity of

mechanical contact between any parts, which improves reliability and reduces

mechanical damping. In theory, this type of converter could be designed to have very

little mechanical damping.

But is difficult to integrate this type of device with standard

microelectronics. For one thing, a strong magnet has to be manually attached to the

device. Additionally, just how much this magnet and its motion would affect electronics

in extremely close proximity is an open question.

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2) ELECTROSTATIC (CAPACITIVE) POWER CONVERSION:

Electrostatic generation consists of two conductors separated by a dielectric (i.e. a

capacitor), which move relative to one another. As the conductors move the energy

stored in the capacitor changes, thus providing the mechanism for mechanical to

electrical energy conversion. Change in capacitance causes either voltage or charge increase.

The primary disadvantage of electrostatic converters is that they require a separate

voltage source to initiate the conversion process because the capacitor must be charged

up to an initial voltage for the conversion process to start. Another disadvantage is that

for many design configurations mechanical limit stops must be included to ensure that the

capacitor electrodes do not come into contact and short the circuit. The resulting

mechanical contact could cause reliability problems as well as increase the amount of

mechanical damping.

This type of harvesting is based on the changing capacitance of vibration-dependent varactors.

Vibrations separate the plates of an initially charged varactor (variable capacitor), and

mechanical energy is converted into electrical energy. An example of a electrostatic energy

harvester with embedded energy storage is the M2E Power Kinetic Battery. Another example

is CSIROs Flexible Integrated Energy Device (FIED)[23] Yet another example is the Tremont

Electric nPower PEG.[24] Finally, there is the Regenerative shock absorber.

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3) PIEZOELECTRIC POWER CONVERSION:

Piezoelectricity is the ability of certain materials to produce a voltage when subjected to

mechanical stress.Piezoelectric materials are materials that physically deform in the presence

of an electric field, or conversely, produce an electrical charge when mechanically deformed.

This effect is due to the spontaneous separation of charge within certain crystal structures

under the right conditions producing an electric dipole. At the present time, polycrystalline

ceramic is the most common piezoelectric material. Polycrystalline

ceramic is composed of randomly oriented minute crystallites. Each crystallite is further

divided into tiny domains, or regions having similar dipole arrangements. Initially the

polar domains are oriented randomly, resulting in a lack of macroscopic piezoelectric

behavior.

Strain in piezoelectric material causes a charge separation (voltage across capacitor).

If a voltage is applied in the same direction as the dipoles (the direction of the poling (electric

field), the material elongates in that direction. The opposite effect is also present, specifically if

a mechanical strain is produced in the direction of the dipoles, a charge separation across the

material (which is a dielectric) occurs, producing a voltage.The use of piezoelectric materials

to harvest power has already become popular. Piezoelectric materials have the ability to

transform mechanical strain energy into electrical charge. Piezo elements are being embedded

in walkwaysto recover the people energy of footsteps. They can also be embedded in shoes

to recover walking energy.

| P a g e

Loa

dV

s

CR

s

Piezoelectric

generator

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MOTION:

Mechanical motion is another energy source which has attracted considerable attention. Such

motion sources generally fall into two clear classes low frequency, high amplitude motion

such as human body motion, and high frequency, low amplitude motion such as machinevibration. In the first category, the motion amplitudes are typically on the order of or greater

than the desired device dimensions, while in the latter the reverse is generally true.

6.ULTIMATE POWER LIMITS:

The extraction of energy from motion may be by direct application of force to the

mechanism, such as foot strike in a shoe mounted device. More common, and more universallyapplicable, are the inertial mechanisms. In these, the generator need only be attached to the

moving host at a single point, as illustrated in Fig. 1.

A proof mass is suspended within the generator such that internal motion is induced

when the device frame moves along with the host; electrical power is then generated by a

transduction mechanism which acts to damp this internal motion. Most devices described in the

literature consider the case of linear internal motion driven by linear source motion. However,

devices with rotating masses also exist, particularly for the generation of power in wrist

watches.

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The power levels theoretically achievable from linear inertial scavengers are limited by

four parameters: the proof mass and range of internal travel range of the device, m and Z, and

the amplitude and frequency of the source motion, Yo and (assuming harmonic source

motion). The maximum frame acceleration for harmonic motion is simply Yo; unless the

damping force per unit mass is below this level, the proof mass will move together with the

frame, so there will be no relative motion and thus no work done against the damper. This

places an upper limit on the force of m2Yo, and thus on the energy per transit of :

Umax = m YoZ (1)

It is worth noting that this maximum requires use of the full internal travel range,

whereas for high frequency sources this may be significantly greater than the excitation

amplitude of the source. In these cases, resonant oscillation of the proof mass on its suspension

is typically used to obtain the required internal amplitude. For low frequency, high amplitude

sources, on the other hand, non-resonant devices can be exploited, and these have the

significant advantage of operating effectively over a wide range of source frequency without

requiring active tuning. If energy is extracted in both directions of travel, then the maximum

power is simply twice Umax divided by the period 2/, giving:

Pmax = m YoZ/ (2)

Consequently, since mass is proportional to volume and maximum displacement to

linear dimension, maximum power scales as linear dimension to the fourth power, or as

Volume4/3. Thus power density reduces as device size decreases, obviously an undesirable

feature for miniaturization. Furthermore, the very strong dependence on frequency means that

for the low frequency group of applications, such as body-mounted sensors, the power density

is poor.

Although (2) gives the level of maximum power for harmonic excitation, it is derived

using the assumption that the damping force per unit mass can approach the maximum external

acceleration throughout the motion cycle. This, however, also implies that the mass makes

| P a g e 27

2

2

3


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each internal transit in negligible time, since the peak external acceleration is by definition only

present instantaneously. If we require the internal motion also to be harmonic, the maximum

power is reduced by a factor of /4.

Pmax = m Yo Z /4 (3)

If we do not require harmonic internal motion, we can allow the proof mass to make

each internal transit in less than half a cycle (resting at either end between transits). This allows

a larger force closer to the maximum peak value m2Yo to be employed, and brings the

achievable power closer to that of (2). The possible improvement is greater for cases where

Yo >> Zl. As stated above, inertial scavengers may also use rotating masses. Typically these

are unbalanced (e.g. semi-circular) so that they may be driven by linear motion. In an analysis

is presented which shows that the power limit of such a device, for a semi-circular proof mass

m of radius R, is given by:

Pmax = 0.27m YoR (4)

This is nearly identical to (4), except with the proof mass radius taking the place of the

internal travel range Zl. Thus the choice between a linear and a rotating internal mass is likely

to be based on practical considerations, such as ease of manufacture, cost or reliability, rather

than ultimate power limit.

| P a g e 28

3

3


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GENERIC VIBRATION-TO-ELECTRICITY CONVERSION

MODEL

One can formulate a general model for the conversion of the kinetic energy of a vibrating mass

to electrical power based on linear system theory without specifying the mechanism by which

the conversion takes place. This model is described by equation 2.1

.

Figure 2.4: Schematic of generic vibration converter

where:

z = spring deflection

y = input displacement

m = mass

be = electrically induced damping coefficient

bm = mechanical damping coefficient

k = spring constant

The term be represents an electrically induced damping coefficient. The primary idea behind

this model is that the conversion of energy from the oscillating mass to electricity (whatever

the mechanism is that does this) looks like a linear damper to the mass spring system.

| P a g e 29

ymkzzbzbzm em =+++


30/39

The power output is proportional to the square of the acceleration magnitude of the driving

vibrations.

Power is proportional to the proof mass of the converter, which means that scaling down the

size of the converter drastically reduces potential for power conversion.

The equivalent electrically induced damping ratio is designable, and the power output is

optimized when it is equal to the mechanical damping ratio.

For a given acceleration input, power output is inversely proportional to frequency. (This

assumes that the magnitude of displacement is achievable since as frequency goes down, the

displacement of the proof mass will increase.)

Finally, it is critical that the natural frequency of the conversion device closely

matches the fundamental vibration frequency of the driving vibrations.

| P a g e

222

3

3

2

21

+

=

nT

n

n

eYm

P

2

23

4T

eYm

P

=

emT +=

2

2

4 T

eAm

P

=

Power in terms of magnitude and

frequency ofinput.

Power assuming = n.

Power in terms of acceleration magnitude.

= Electro kinetic potential

30


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7.APPLICATION

BIOMECHANICAL

ENERGY HARVESTERS:The main application of energy scavenging

from vibarion is biomechanical harvester.

Vibrations are vastly available energy soure

and also it is creatable. Biomechanical

energy harvesters are also being created.

One current model is the

biomechanical energy harvester of Max

Donelan which straps around the knee.

Devices as this allow the generation of 2.5

watts of power per knee. This is

enough to power some 5 cell phones.

AERODYNAMIC EFFECT:

Micro wind turbine are used to harvest wind energy readily available in the environment in the

form of kinetic energy to power the low power electronic devices such as wireless sensor

nodes. When air flows across the blades of the turbine, a net pressure difference is developed

between the wind speeds above and below the blades. This will result in a lift force generated

which in turn rotate the blades. This is known as the aerodynamic effect.

WIRELESS CORROSION MONITORING SYSTEMS:

| P a g e

vibrating structure

frequency

Accel.PSD

mechanical energy

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Electroactive polymers (EAPs) have been proposed for harvesting energy. These polymers

have a large strain, elastic energy density, and high energy conversion efficiency. The total

weight of systems based on EAPs is proposed to be significantly lower than those based on

piezoelectric materials.

Nanogenerators, such as the one made by Georgia Tech, could provide a new way for

powering devices without batteries. As of 2008, it only generates some dozen nanowatts,

which is too low for any practical application.

Noise harvesting NiPS Laboratory in Italy has recently proposed to harvest wide spectrum low

scale vibrations via a nonlinear dynamical mechanism that can improve harvester efficiency up

to a factor 4 compared to traditional linear harvesters.

| P a g e

Self-powered Wireless Corrosion Sensor

corrosion sensorenergy harvester

33

low power

wireless transceiver


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8. Comparison of Energy Scavenging Technologies:

A broad survey of potential energy scavenging methods has been taken. The results of

this survey are shown in Table 1.1. The table also includes batteries and other energy storage

technologies for comparison. The upper two rows of the table contains energy storage

technologies in which, because they contain a fixed amount of energy, the power available to

the node decreases with increased lifetime. The lower half of the table contains pure power

scavenging sources and thus the amount of power available is not a function of the lifetime of

the device. As is the case with all power values reported in this thesis, power is normalized per

cubic centimeter to conform to the constraints. The values in the table are derived from a

combination of published studies, experiments performed by the author, theory, and

information that is commonly available in data sheets and textbooks. While this comparison is

by no means exhaustive, it does provide a broad cross section of potential methods to scavenge

energy and energy storage systems. Other potential sources were also considered but deemed

to be outside of the application space under consideration or to be unacceptable for some other

reason.

As we have seen some sources are

fundamentally characterized by

energy density (such as batteries)

while others or characterized by

power density (such as solar cells) a

direct comparison with a single

metric is difficult.

Furthermore, a battery that is large

enough to last the lifetime of the

device would dominate the overall system size and cost, and thus is not very attractive.

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CONTINUOUS POWER VS LIFETIMECONTINUOUS POWER VS LIFETIME

A brief explanation and evaluation of each source listed in Table 1.1 follows.

| P a g e

Continuous Power / cm3 vs. Life Several Energy Sources

0

1

10

100

1000

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5Years

microWatts

Lithium

Alkaline

Lithium rechargeableZinc air

NiMH

Solar

ibrations

35


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Three types of vibration to electricity converters have been considered:

electromagnetic, electrostatic, and piezoelectric. After a preliminary investigation, only

piezoelectric and electrostatic were pursued in detail. Both types of converters have been

modeled, designed, and fabricated. While solar cell based power systems have also been

developed for the target wireless sensor nodes, this has been more of a development and

benchmarking effort than a research effort.

Summary of conclusions:

Almost all wireless sensor nodes are presently powered by batteries. This situation

presents a substantial roadblock to the widespread deployment of wireless sensor

networks because the replacement of batteries is cost prohibitive.

We have looked at a wide variety of such sources. It can be concluded that no single

alternative power source will solve the problem for all, or even a large majority of

cases.

Both solar powered and vibration powered systems are being actively pursued and will

be up and running shortly.

Both solar power and vibration based energy scavenging look promising as methods to

scavenge power from the environment. In many cases, perhaps most cases, they are not

overlapping solutions because if solar energy is present, it is likely that vibrations are not, and

vice versa. It was, therefore, decided to pursue both solar and vibration based solutions for the

sensor nodes under development. Solar cells are a mature technology, and one that has been

profitably implemented many times in the past. So, while solar power based solutions have

been developed, the main focus of the research and development effort has been vibration

based power generators.

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10.REFERENCES

[1] S. Roundy, E. Leland, J. Baker, E. Carleton, E. Reilly, E.Lai, B. Otis, J. Rabaey, V.

Sundararajan and P.K. Wright,Improving Power Output for Vibration-Based Energy

Scavengers, IEEE Pervasive Computing, Vol 4, No 1,2005, pp 28 36.

[2] Joseph A. Paradiso, Thad Starner. Energy Scavenging for Mobile and Wireless

Electronics, IEEE Pervasive Computing, 4(1): 18-27,2005.

[3] Geffrey K. Ottman, Heath F. Hofmann, Archin C. Bhatt,George A. Lesieutre.

Adaptive Piezoelectric Energy Harvesting Circuit for Wireless Remote Power Supply.

IEEE Transactions On Power Electronics, 17(5): 669-676, 2002.

[4] Steven A. Macintyre, Magnetic Field Measurement from book measurement,

instrumentation, and sensor handbook, IEEE, 199.

[5] S. Roundy, D. Steingart, L. Frechette, P. Wright, and J.Rabaey, "Power sources for

wireless sensor networks,"in Wireless Sensor Networks, Proceedings, vol. 2920,

Lecture Notes in Computer Science, 2004, pp. 1-17.

[6] N. S. Shenck and J. A. Paradiso, "Energy scavenging with shoe-mounted

piezoelectrics,"IEEE Micro, vol. 21,pp. 30-42, 2001.

[7] E. M. Yeatman, "Advances In Power Sources For Wireless Sensor Nodes,"

presented at 1st International Workshop on Body Sensor Networks, April 6-7, London,

2004.

[8] E. S. Leland, E. M. Lai, and P. K. Wright, "A Self-Powered Wireless Sensor for

Indoor Environmental Monitoring," presented at 2004 Wireless Networking

Symposium, University of Texas at Austin Department of Electrical & Computer

Engineering, 2004.

[9] P. Glynne-Jones, M.J. Tudor, S.P. Beeby, N.M. White. An electromagnetic,

vibration-powered generator for intelligent sensor systems, Sensors and Actuators A

110 (2004) 344-349.

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