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A Seminar Report
on
“WITRICITY”
(Wireless Power Transmission)
Submitted for partial fulfillment
of
Bachelor of Engineering
in
Electronics & Communication Engineering
Submitted to Submitted by
Ms. Shruti Kalra Supreet Kumar SinghAssistant Professor Roll # 118
VIII Semester
Jaipur Engineering College & Research Centre, Jaipur
Session : 2011 – 12
1
ABSTRACT
The technology used for wireless power transmission is known as witricity. Wireless
power transmission is not a new idea, Nikola Tesla proposed theories of wireless
power transmission in the late 1800s and early 1900s. Tesla's work was impressive,
but it did not immediately lead to wide spread practical methods for wireless power
transmission. Since then many researchers have developed several techniques for
moving electricity over long distances without wires. Some exist only as theories or
prototypes, but others are already in use. In 2006 researchers at Massachusetts
Institute of Technology led by Marine Soijacic discovered an efficient way to transfer
power between coils separated by a few meters. They have dubbed this technology as
witricity. Witricity is based upon coupled resonant objects. Two resonant objects of
the same resonant frequency tend to exchange energy efficiently, while not
interchanging the surroundings. The researchers demonstrate the ability to transfer
60W with approximately 40% efficiency over distance in excess of 2 meters.
Currently the project is looking for power transmission in the range of 100watts. As
witricity is in the development stage, lots of work is to be done in improving the
range of power transmission and efficiency.
2
CONTENTS
Sr.no . Content Pg no.
1 Introduction 4
2 Principle of working 5
3 Circuit and its component 11
4 Advantages and Disadvantages 15
5 Application 17
6 Conclusion 20
7 Bibliography 21
3
1. INTRODUCTION
If we are particularly organized and good with tie wrap then also a few dusty power
cord tangles around our home. We have even had to follow one particular cord
through the seemingly impossible snarl to the outlet hoping that the plug pull will be
the right one. This is one of the downfalls of electricity. While it can make people's
lives easier, it can add a lot of clutter in the process. For these reasons, scientists have
tried to develop methods of wireless power transmission that could cut the clutter or
lead to clean sources of electricity. Wireless power transmission is not a new idea.
Many researchers developed several methods for wireless power transmission. But
witricity is a new technology used for wireless power transmission. By the use of this
technology transmission of electrical energy to remote objects without wires can be
possible. The inventors of witricity are the researchers from Massachusetts Institute
of Technology (MIT). They developed a new technology for wireless electricity
transmission and this is based upon the coupled resonant objects. In this resonant
magnetic fields are used. So the wastage of power is reduced. The system consists of
witricity transmitters and receivers. The transmitters and receivers contain magnetic
loop antennas made of copper coils and they are tuned to the same frequency.
4
2. PRINCIPLE OF WORKING
A transformer is an example of wireless energy transfer, although it does not seem to
be so, due to the close proximity of the primary and secondary coils. If the coils are
moved further apart and have different cores, instead of a common core, you have
wireless energy transfer, due to the induction effect. However, the important issue
here is the efficiency with which energy is transferred. The farther apart the coils are
from each other, the less the transfer efficiency. If this figure can be improved to
reach e.g. 70% or 80% then we could have a practical way to transfer energy without
wires over long distances. If efficiency is ignored (or not important), as is the case of
supplying small appliances, wireless energy transfer can be a very practical way of
supply. However, if you are to supply thousands of watts to a circuit, things look a lot
more difficult.
2.1 Inductive Coupling
Inductive coupling uses magnetic fields that are a natural part of current's movement
through wire. Any time electrical current moves through a wire, it creates a circular
magnetic field around the wire. Bending the wire into a coil amplifies the magnetic
field. The more loops the coil makes, the bigger the field will be.
If you place a second coil of wire in the magnetic field you've created, the field can
induce a current in the wire. This is essentially how a transformer works, and it's how
an electric toothbrush recharges. It takes three basic steps:
1. Current from the wall outlet flows through a coil inside the charger, creating a
magnetic field. In a transformer, this coil is called the primary winding.
2. When you place your toothbrush in the charger, the magnetic field induces a
current in another coil, or secondary winding, which connects to the battery.
3. This current recharges the battery.
5
You can use the same principle to recharge several devices at once. For example, the
Splash power recharging mat and Edison Electric's Power desk both use coils to
create a magnetic field. Electronic devices use corresponding built-in or plug-in
receivers to recharge while resting on the mat. These receivers contain compatible
coils and the circuitry necessary to deliver electricity to devices' batteries.
2.2 Resonance and Wireless Power
Household devices produce relatively small magnetic fields. For this reason, chargers
hold devices at the distance necessary to induce a current, which can only happen if
the coils are close together. A larger, stronger field could induce current from farther
away, but the process would be extremely inefficient. Since a magnetic field spreads
in all directions, making a larger one would waste a lot of energy.
Research at MIT indicates that induction can take place a little differently if the
electromagnetic fields around the coils resonate at the same frequency. The theory
uses a curved coil of wire as an inductor. A capacitance plate, which can hold a
charge, attaches to each end of the coil. As electricity travels through this coil, the
coil begins to resonate. Its resonant frequency is a product of the inductance of the
coil and the capacitance of the plates.
Resonant transfer works by making a coil ring with an oscillating current. This
generates an oscillating magnetic field. Because the coil is highly resonant any energy
placed in the coil dies away relatively slowly over very many cycles; but if a second
coil is brought near to it, the coil can pick up most of the energy before it is lost, even
if it is some distance away. The fields used are predominately non- radiative, near
field (sometimes called evanescent waves), as all hardware is kept well within the 1/4
wavelength distance they radiate little energy from the transmitter to infinity.
6
FIG 2.1 The MIT wireless power project uses a curved coil and capacitive plates.
The wireless power transfer system involves resonating two identical coils with a
high frequency power source. The power is transmitted through magnetic resonance
coupling in between the two coils at the resonance frequencies. The power
transmitted to the receiving antenna is then utilized in the load Electromagnetic
resonance coupling involves creating an LC resonance, and transferring the power
with electromagnetic couplings without radiating electromagnetic waves. Hence, the
magnetic coupling and electric coupling can be represented as mutual inductance and
mutual capacitance respectively as shown in Fig. 2.
Zsource in Fig. 2 represents the characteristic impedance, and Zload is the impedance
of the load. In this system, they are both considered to be the same at Z0, 50_ the
default characteristic of most high frequency systems. The ohm loss and the radiation
loss of the antennas are represented by R. In this paper, the power is transferred via
magnetic coupling. Therefore the coupling can be represented by mutual induction
Lm. Next the resonance frequency is calculated based on the equivalent circuit. To
satisfy the resonance condition, the reactance of Fig.2 must be 0, as in equation (1).
The condition in equation (1) can be satisfied by two resonant frequencies as
calculated in equation (2) and (3).
7
2.2.1 Energy transfer and efficiency
The general principle is that if a given oscillating amount of energy (for example
alternating current from a wall outlet) is placed into a primary coil which is capacitive
loaded, the coil will 'ring', and form an oscillating magnetic field. The energy will
transfer back and forth between the magnetic field in the inductor and the electric
field across the capacitor at the resonant frequency. This oscillation will die away at a
rate determined by the Q factor, mainly due to resistive and radiative losses.
However, provided the secondary coil cuts enough of the field that it absorbs more
energy than is lost in each cycle of the primary, then most of the energy can still be
transferred.
8
The primary coil forms a series RLC circuit, and the Q factor for such a coil is:
,
Because the Q factor can be very high, (experimentally around a thousand has been
demonstrated with air cored coils) only a small percentage of the field has to be
coupled from one coil to the other to achieve high efficiency, even though the field
dies quickly with distance from a coil, the primary and secondary can be several
diameters apart.
2.2.2 Coupling coefficient
The coupling coefficient is the fraction of the flux of the primary that cuts the
secondary coil, and is a function of the geometry of the system. The coupling
coefficient is between 0 and 1.
Systems are said to be tightly coupled, loosely coupled, critically coupled or over
coupled. Tight coupling is when the coupling coefficient is around 1 as with
conventional transformers. Over coupling is when the secondary coil is close enough
that it tends to collapse the primary's field, and critical coupling is when the transfer
in the pass band is optimal. Loose coupling is when the coils are distant from each
other, so that most of the flux misses the secondary, in Tesla coils around 0.2 is used,
and at greater distances, for example for wireless power transmission, it may be lower
than 0.01.
2.2.3 Power transfer
Because the Q can be very high, even when low power is fed into the transmitter coil,
a relatively intense field builds up over multiple cycles, which increases the power
that can be received—at resonance far more power is in the oscillating field than is
being fed into the coil, and the receiver coil receives a percentage of that.
9
2.2.4 Voltage gain
The voltage gain of resonantly coupled coils is proportional to the square root of the
ratio of secondary and primary inductances
2.2.5 Transmitter coils and circuitry
Unlike the multiple-layer secondary of a non-resonant transformer, coils for this
purpose are often single layer solenoids (to minimize skin effect and give improved
Q) in parallel with a suitable capacitor, or they may be other shapes such as wave-
wound lit wire. Insulation is either absent, with spacers, or low permittivity, low loss
materials such as silk to minimize dielectric losses.
10
3. CIRCUIT AND ITS COMPONENETS
The various components in making the circuit for the project- Wireless transmission
of electric power using resonance circuit can be explained as:
1. Bifilar enhanced coil: 22 gauge copper wire
2. Capacitors 10. ‘1 µF’ capacitors are connected to give ‘.01 µf’capacitor.
3. LED
4. Function generator
3.1 Primary and Secondary coils
Both primary and secondary circuit consists of bifilar enhanced coils. These coils are
of copper wire of 22 gauge and have 41 turns on either side i.e. both primary and
secondary coils consists of 41 turns of 22 gauge copper wire each. The inductance of
both the coils is 2.788 micro Henry
3.2 Capacitors
A capacitor is a passive electronic component consisting of a pair of conductors
separated by a dielectric (insulator). When there is a potential difference (voltage)
across the conductors, a static electric field develops in the dielectric that stores
energy and produces a mechanical force between the conductors. An ideal capacitor
is characterized by a single constant value, capacitance, measured in farads. This is
the ratio of the electric charge on each conductor to the potential difference betwee
them.
Fig 3.1 A capacitor
11
Capacitors are widely used in electronic circuits for blocking direct current while
allowing alternating current to pass, in filter networks, for smoothing the output of
power supplies, in the resonant circuits that tune radios to particular frequencies and
for many other purposes.
In the circuit 10 – 0.1 micro Farad capacitors are connected in series to give a 0.01
micro farad capacitor on both secondary and primary circuit.
FIG 3.2 Series connection of Capacitors
3.3 Function Generator
A function generator is a piece of electronic test equipment or software used to
generate electrical waveforms. These waveforms can be either repetitive or single-
shot, in which case some kind of triggering source is required (internal or external).
Function Generators are used in development, testing and repair of electronic
equipment, e.g. as a signal source to test amplifiers, or to introduce an error signal
into a control loop.
Analog function generators usually generate a triangle waveform as the basis for all
of its other outputs. The triangle is generated by repeatedly charging and discharging
a capacitor from a constant current source. This produces a linearly ascending or
descending voltage ramp. As the output voltage reaches upper and lower limits, the
charging and discharging is reversed using a comparator, producing the linear triangle
wave. By varying the current and the size of the capacitor, different frequencies may
be obtained. Saw tooth waves can be produced by charging the capacitor slowly,
using a current, but using a diode over the current source to discharge quickly - the
polarity of the diode changes the polarity of the resulting saw tooth, i.e. slow rise and
fast fall, or fast rise and slow fall.
12
A typical function generator can provide frequencies up to 20 MHz RF generators for
higher frequencies are not function generators in the strict sense since typically
produce pure or modulated sine signals only.
In the project we have supplied a sinusoidal waveform of 30 V and 40kHz.
FIG 3.3 A picture of Function Generator
3.4 Light Emitting Diode
A light-emitting diode (LED) is a semiconductor light source. LEDs are used as
indicator lamps in many devices, and are increasingly used for lighting. Introduced as
a practical electronic component in 1962, early LEDs emitted low-intensity red light,
but modern versions are available across the visible, ultraviolet and infrared
wavelengths, with very high brightness.
When a light-emitting diode is forward biased (switched on), electrons are able to
recombine with electron holes within the device, releasing energy in the form of
photons. This effect is called electroluminescence and the color of the light
(corresponding to the energy of the photon) is determined by the energy gap of the
semiconductor. An LED is often small in area (less than 1 mm2), and integrated
optical components may be used shape its radiation pattern. LEDs present many
advantages over incandescent light sources including lower energy consumption,
longer lifetime, improved robustness, smaller size, faster switching, and greater
durability and reliability. LEDs powerful enough for room lighting are relatively
13
expensive and require more precise current and heat management than compact
fluorescent lamp sources of comparable output.
In the circuit LED is being used as an indicating device in the circuit of our project.
14
4. ADVANTAGES AND DISADVANTAGES
4.1 Advantages
Wireless Power Transmission system would completely eliminates the existing high-
tension power transmission line cables, towers and sub stations between the
generating station and consumers and facilitates the interconnection of electrical
generation plants on a global scale. It has more freedom of choice of both receiver
and transmitters. Even mobile transmitters and receivers can be chosen for the WPT
system. The cost of transmission and distribution become less and the cost of
electrical energy for the consumer also would be reduced. The power could be
transmitted to the places where the wired transmission is not possible. Loss of
transmission is negligible level in the Wireless Power Transmission; therefore, the
efficiency of this method is very much higher than the wired transmission. Power is
available at the rectenna as long as the WPT is operating. The power failure due to
short circuit and fault on cables would never exist in the transmission and power theft
would be not possible at all.
Eliminates the use of power transmission lines, cables and substations, thus
reducing the cost of electrical energy.
More freedom of choice of both receiver and transmitter.
Power can be transferred to the places where wired transmission is not
possible.
Low transmission loss, thus increasing the efficiency.
To make the recharge of electronic devices more convenient for consumer
especially for the elderly and the disabled
To reduce the Faults because of Short Circuit of wires and cables
15
4.2 Disadvantages
The Capital Cost for practical implementation of WPT seems to be very high and the
other disadvantage of the concept is interference of microwave with present
communication systems.
4.3 Biological Impacts
Common beliefs fear the effect of microwave radiation. But the studies in this domain
repeatedly proves that the microwave radiation level would be never higher than the
dose received while opening the microwave oven door, meaning it is slightly higher
than the emissions created by cellular telephones. Cellular telephones operate with
power densities at or below the ANSI/IEEE exposure standards. Thus public exposure
to WPT fields would also be below existing safety guidelines.
16
5. APPLICATION
Generating power by placing satellites with giant solar
arrays in Geosynchronous Earth Orbit and transmitting
the power as microwaves to the earth known as Solar
Power Satellites (SPS) is the largest application of WPT.
Another application of WPT is moving targets such as
fuel free airplanes, fuel free electric vehicles, moving robots
and fuel free rockets. The other applications of WPT are Ubiquitous Power Source
(or) Wireless Power Source, Wireless sensors and RF Power Adaptive Rectifying
Circuits (PARC).
Direct Wireless Power—when all the power a device needs is provided wirelessly,
and no batteries are required. This mode is for a device that is always used within
range of its WiTricity power source.
Automatic Wireless Charging—when a device with rechargeable batteries charges
itself while still in use or at rest, without requiring a power cord or battery
replacement.
Consumer Electronics
Automatic wireless charging of mobile electronics (phones, laptops, game
controllers, etc.) in home, car, office, Wi-Fi hotspots while devices are in use
and mobile.
Direct wireless powering of stationary devices (flat screen TV’s, digital
picture frames, home theater
accessories, wireless loud … eliminating expensive custom wiring, unsightly
cables and “wall-wart” power supplies.
17
FIG 5.1 Electronics Application
Direct wireless powering of desktop PC peripherals: wireless mouse,
keyboard, printer, speakers, display, etc … eliminating disposable batteries
and awkward cabling.
Industrial
Direct wireless power and communication
interconnections across rotating and moving
“joints” (robots, packaging machinery,
assembly machinery, machine tools) …
eliminating costly and failure-prone wiring.
Direct wireless power and communication
interconnections at points of use in harsh
environments (drilling, mining, underwater, etc.)
where it is impractical or impossible to run wires.
Direct wireless power for wireless sensors and actuators, eliminating the need
for power wiring or battery replacement
and disposal.
Transportation
Automatic wireless charging for existing
electric vehicle classes: golf carts,
industrial vehicles.
Automatic wireless charging for future
hybrid and all-electric passenger and
commercial
Vehicles, at home, in parking garages, at
fleet depots, and at remote kiosks.
Direct wireless power interconnections to replace costly vehicle wiring
harnesses and slip rings.
18
FIG 5.2 Industrial Application
FIG 5.3 Transportation Application
Other Applications
Direct wireless power interconnections and
automatic wireless charging for implantable
medical devices (ventricular assist devices,
pacemaker, defibrillator, etc.).
Automatic wireless charging and for high
tech military systems (battery powered
mobile devices, covert sensors, unmanned
mobile robots and aircraft, etc.).
Direct wireless powering and automatic
wireless charging of smart cards.
FIG 5.4 other application
wireless charging of consumer appliances, mobile robots, etc.
6. CONCLUSION
19
FIG 4.4 Othen
The concept of Wireless Power Transmission system is presented. The technological
developments in Wireless Power Transmission (WPT), the advantages,
disadvantages, biological impacts and applications of WPT are also discussed.
This concept offers greater possibilities for transmitting power with negligible losses
and ease of transmission than any invention or discovery heretofore made. Dr. Neville
of NASA states “You don’t need cables, pipes, or copper wires to receive power. We
can send it to you like a cell phone call – where you want it, when you want it, in real
time”. We can expect with certitude that in next few years’ wonders will be wrought
by its applications if all the conditions are favorable.
The transmission of power without wires is not a theory or a mere possibility, it is
now a reality. The electrical energy can be economically transmitted without wires to
any terrestrial distance. Many researchers have established in numerous observations,
experiments and measurements, qualitative and quantitative. Dr. Nikola Tesla is the
pioneer of this invention.
Wireless transmission of electricity have tremendous merits like high transmission
integrity and Low Loss (90 – 97 % efficient) and can be transmitted to anywhere in
the globe and eliminate the need for an inefficient, costly, and capital intensive grid of
cables, towers, and substations. The system would reduce the cost of electrical energy
used by the consumer and get rid of the landscape of wires, cables, and transmission
towers. It has negligible demerits like reactive power which was found insignificant
and biologically compatible. It has a tremendous economic impact to human society.
Many countries will benefit from this service. Monthly electric utility bills from old-
fashioned, fossil-fuelled, loss-prone electrified wire-grid delivery services will be
optional, much like “cable TV” of today.
7. BIBLIOGRAPHY
20
en.wikipedia.org/wiki/Wireless_energy_transfer
http://electronics.howstuffworks.com/wireless-power.htm
http://www.instructables.com/id/Wireless-Power-Transmission-Over-Short-Distances-U/
www.tfcbooks.com/articles/tws8c.htm
21