INDEX

ABSTRACT

Witricity or wireless electricity is the method of transmission of electricity without the use of

man-made conductors. This is the modern and hi tech way of transmitting electrical energy. It is

mostly used in cases when it is difficult or impossible to transfer electricity using traditional

methods. Generally conducting wires are used to transmit electricity over an area.one of the most

common way of achieving wireless transmission of electricity is direct induction and resonant

magnetic induction. There are other ways of achieving this too, like electrical conduction and

electromagnetic radiation. In direct induction, two conductors are coupled such that changes in

current in one conductor induce voltage in the other conductor.

This happens due to electromagnetic induction. Transformers are this method. In resonant

magnetic induction, the energy is transferred between two wires which are tuned at the same

frequency. The machine used for this purpose is known as resonance transformer. These two

methods done in the given order help in transmission of electricity in wireless fashion.

Electromagnetic radiations used are microwaves or lasers. Electromagnetic induction can also be

used for wireless transmission. It can be magnetic or capacitive in nature. Capacitive coupling is

also known as an electrostatic induction method. With laser or microwave method, power

transmission can be made more directional.

INTRODUCTION

Wireless electricity or witricity is the transfer of electric energy or power over a distance without

the use of wires. In our present electricity generation system we waste more than half of its

resources. Especially the transmission and distribution losses are the main concern of the present

power technology. Much of this power is wasted during transmission from power plant

generators to the consumer. The resistance of the wire used in the electrical grid distribution

system causes a loss of 26-30% of the energy generated. This loss implies that our present

system of electrical distribution is only 70-74% efficient. We have to think of alternate state - of

- art technology to transmit and distribute the electricity. Now- a- days global scenario has been

changed a lot and there are tremendous development in every field. If we don’t keep pace with

the development of new power technology we have to face a decreasing trend in the

development of power sector. The transmission of power without wires may be one noble

alternative for electricity transmission.

ORIGINATION and CONDITION

Wireless power transmission is not a new idea. Nickolas Tesla demonstrated transmission of

electrical energy without wires in early 19th century. Tesla used electromagnetic induction

systems. William C Brown demonstrated a micro wave powered model helicopter in 1964. This

receives all the power needed for flight from a micro wave beam. In 1975 Bill Brown transmitted

30kW power over a distance of 1 mile at 84% efficiency without using cables. Researchers

developed several technique for moving electricity over long distance without wires. Some exist

only as theories or prototypes, but others are already in use. onsider an example, in this electric

devices recharging without any plug-in. The device which can be recharged is placed on a

charger. Supply is given to the charger and there is no electrical contact between charger and

device. The recharging takes place in following 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 the device placed on 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.

In order for witricity charging to be able to be used there are some conditions that are necessary.

The first condition necessary is that charging must be able to occur through physical objects. If

the witricity cannot be passed through physical objects then the charging can only occur in a

perfect unobstructed environment. This is not realistic because there are objects all around us

that can interfere with the transmission of the energy. Research thus far has found that witricity

can be transmitted through wood, gypsum wallboard, plastics, textiles, glass, brick, and concrete.

The second condition necessary is that charging must be safe and not pose any sort of threat or

safety hazard to humans or animals. Since this type of energy transfer is non-radioactive it is safe

for humans and animals. The third condition necessary is that witricity charging must be able to

provide electricity to remote objects without the use of wires. Therefore, the energy must be

transferable from the transmitter to the capturing device over a certain distance. For example,

traditional magnetic induction that is used in an electric toothbrush needs physical contact, or

their needs to be a fairly short distance between the energy transmitter and the energy receiver.

Thus, magnetic induction is not a suitable technology for witricity transfer. According to

WiTricity, a company that has developed the wireless electricity technology, the distance

between the power source and the capturing device can range from a centimeter to several meters

between the power source and the capturing device, depending on the size of the device, how

efficient the transfer is desired, and the amount of power that needs to be transferred. According

to a team of researchers at MIT, two resonant objects of the same resonant frequency tend to

exchange energy more efficiently while dispersing little energy to off-resonant objects, and a

power transfer can be multi-directional and efficient. The team also calculated the efficiency of

wireless power transfer using strong coupled magnetic resonators to be the amount of useable

electrical energy at the power receiving device over the amount of energy sent by the power

sources. The fourth condition necessary is that the witricity is able to transfer a meaningful

amount of energy, or else it would defeat the purpose of the energy transfer. WiTrcity.com states

their technology is able to transfer energy ranging from miliwatts to several kilowatts of power.

The fifth condition necessary is that the witricity technology should not use a lot of memory. In

order for a program to be loaded on the sensors it cannot take a lot of memory, since the sensors

have limited memory capacity.

PARAMETERS Witricity charging protocols have many parameters. The first parameter is the degree of

charging. There are three different degrees in which a sensor can be charged. The first is perfect

charging, this type of charging charges all the batteries of the sensors until they are all full. This

is achieved by using an external power source such as a base station or using designated power

supply devices. The second way is limited charging, this type of charging charges the batteries of

the sensors that are at or above a certain threshold. It could have an external power source, but if

it does not it will try to maximize the network lifetime without creating a blind spot in the

wireless sensor network. The network lifetime is measured by the amount of data transmissions

necessary until the WSN has its first blind spot or until the WSN has a given percentage of the

WSN field in a blind spot. The third type of charging is no charging this is a wireless sensor

network without charging.

The second parameter is the type of charging algorithm. There are two different charging

algorithms; distributed and centralized. The distributed algorithm calculates the amount of

energy that needs to be transferred between a node and its direct neighbor based on information

received from its direct neighbor. The centralized algorithm calculates the amount of energy that

needs to be transferred between all the sensors in the network. The amount of energy that needs

to be transferred is represented in a matrix. In a two-dimensional WSN with n sensors, the matrix

is n x n for a three-dimensional WSN with n sensors the matrix is n x n x n.

The third parameter is the type of power supply. There is a base station and either all of the

sensors or some of the sensors are indicated as a percentage between 0 and 100%. It also allows

the base station to select a set of certain sensors. There are also designated power supply devices

that are non-sensor power supply devices that are deployed in the WSN. The next parameter is

the type of power capturing device. There is a base station and the sensors are indicated as

percentages too, the difference is the devices and base station might be a power capturing device

or act as an intermediate power relay device. Another parameter is the mobility of the base

station. The base station can either be stationary or mobile which has a velocity and direction.

The mobility of designated power supply devices is another parameter. The power supply

devices can either be stationary or mobile also with velocity and direction, or a combination of

both. The mobility of the sensors is another parameter. The sensors can be stationary, mobile

with velocity and direction, or a combination of both.

THEORY

Witricity works on the basic principle of Electromagnetism which transfers power between two

devices having magnetic resonance coupling and dissipate little energy to the environment.

1 Resonance

The behavior of system which oscillates at high amplitude at specific range with respect to other

called as resonance. Under resonance condition, the inductor and capacitor 1have minimum

series impedance and maximum parallel impedance whereas the inductive and capacitive

reactances are equal in amount.

=1/Wc

=1/√Wl

Where, w is the resonant frequency.

2 Resonant Energy Transfer

Short range wireless energy transmission technologies such as WiTricity, which use magnetic

fields for the transmission, function on the principle of resonant energy transfer. A magnetic field

is less hazardous to human beings as compared to electric fields. Wireless energy transmission

techniques use magnetic field which is having two coils with same resonant frequency. For the

better efficiency and less losses, the coils should have high quality factor Q and is used to avoid

iron losses and called as a resonant transformer.

3 Resonant Coupling

The job of primary coil is to produce a magnetic field and that of secondary coil to capture that

field as much as possible. For this process, the coil must have magnetic core in between the two

coils. If the distance between two coils is large then the power cannot be transfer efficiently and

can cause resistive losses in the primary coil. By using resonance coupling the efficiency can be

enhanced considerably.

WiTricity Mechanisms

There are various methods of transferring electricity wirelessly, basically they are classified as:

1) Near Field Transfer (Non-Radiative): These are wireless transmission techniques over

distances comparable to, or a few times the diameter of the device(s).

a) Inductive Coupling: Inductive coupling is the action of electrical transformer is the simplest

instance of wireless energy transfer. The primary and secondary circuits of a transformer are not

directly connected. The transfer of energy takes place by electromagnetic coupling through a

process known as mutual induction. The receiver must be very close to the transmitter or

induction unit in order to inductively couple with it.

b) Resonance Coupling: The idea of such mid-range induction was given by Marin Soljacic for

efficient wireless transfer. The reason behind it is that, if two such resonant objects are brought

in mid-range proximity, their near fields (consisting of so-called 'evanescent waves') and can

allow the energy to transfer from one object to the other within times much shorter than all loss

times, which were designed to be long, and thus with the maximum possible energy-transfer

efficiency. Electromagnetic resonance induction works on the principle of a primary coil

generating a predominantly magnetic field and a secondary coil being within that field so a

current is induced within its coils, when both of these are made to resonate at same frequency

they become much efficient.

2) Far Field Transfer (Radiative): Far Field transfer refers to methods achieving longer range

transfers, often multiple kilometre ranges, where the distance is much greater than the diameter

of the device(s).

a) Laser/Microwave Transmission: Such power transmissions can be made effective at long

range power beaming, with shorter wavelengths of electromagnetic radiation, typically in the

microwave range. A rectenna may be used to convert the microwave energy back into electricity.

These provide 95% efficiency. A new company, Powercast introduced wireless power transfer

technology using RF energy this system is applicable for a number of devices with low power

requirements. Currently, it achieves a maximum output of 6 volts for a little over one meter.

Fig.Example of European Space Agency Aeolus Satellite with laser beampointing at the Earth's atmosphere. Credits: ESA/AOES Medialab.

Energy Transmission via laser is an efficient way for long range, except for it requires a proper

line of sight for power beaming. In the case of light, power can be transmitted by converting

electricity into a laser beam that is then fired at a solar cell receiver. With such laser beam

efficiencies it is planned to build a solar panel grid in space & transferring the solar energy to

earth receivers via laser methods.

III. RESONANT COUPLED SYSTEM OVERVIEW

The resonant coupled system is formulated on the basis of coupled mode theory. A high

frequency power source drives power through a transmitting antenna. The transmitting antenna

sends power wirelessly using electromagnetic resonance coupling to the receiving antennas. Near

field coupling using evanescent field drives the receiving antennas. The distance of transmission

can be increased using coupled source antennas which work as resonators. This allows the

transmission to follow a curved transmission path in space.

IV. COMPARISON WITH INDUCTIVE COUPLING

Traditional inductive coupling methods have limited transmission distance due to weak coupling

between the source and loads. This occurs in the charging of conventional electric toothbrushes.

The tooth brush with the receiving coil is placed on the source cradle for getting charged. The

efficiency is as low as 1-2%. Using magnetic resonance the transmitting source coil frequency

exactly matches the frequency of the receiving coils at resonance. Since the energy transfer is

maximum at resonance, magnetic resonance coupling is found to have an efficiency of about

45% as has been demonstrated by the MIT team.

THEORY AND DESIGN OF THE PROPOSED SYSTEM

As a new wireless power transfer technology, WiTricity is based on the concept of near-field and

strongly coupled magnetic resonance. The fundamental principle is that resonant object scan

exchange energy efficiently, while nonresonant objects only interact weakly. Fig. shows the

basic design of the WiTricity system consists of source and device resonators, a driving loop,

and an output loop. The source resonator is coupled to the driving loop which is linked to an

oscillator that supplies energy to the system. The device resonator coil is coupled to the output

loop to provide the power to an external load.

Fig.Basic Design of WiTricity System.

In Inductive Coupling due to its large physical separation, wireless inductive coupling

transformers have large leakage inductances and small mutual inductance. Thus the coupling

rates are very small, quite often less than 0.1, while those for conventional transformers are

between 0.95–0.98. Judging on this aspect, inductive coupling technology is impractical. For the

WiTricity system, the coupling rate can however be as high as 0.7–0.9 by virtue of the strong

resonant frequency coupling between primary and secondary windings.

To achieve high coupling rate and transmission efficiency, sources with certain resonant

frequency (in the MHz range) is fed to the primary windings. Common formulas in the low

frequency range to predict the performance of the system are inapplicable as the values of

inductances and resistors vary greatly as frequency changes, especially at high frequency.

CIRCUIT ANALYSIS

The proposed WiTricity system uses a pair of rectangular spiral copper windings with the same

shape and structure to achieve wireless energy transfer. The amplitude of the exciting voltage,

which is given by an amplifier, is 25 V at the frequency range from 0.01 MHz to 10 MHz. The

induced electric field in the receiver is generated by variations in the magnetic flux produced by

the transmitter coil. Particular attention is paid to the analysis of the voltage received by the

receiver with different distances and different frequencies in order to find the energy transfer

pattern.

FEATURES AND APPLICATIONS OF WITRICITYA. Features of WiTricity

1) Highly Resonant Strong Coupling Provides High Efficiency Over Distance: WiTricity mode

of wireless power transfer is highly efficient over distances ranging from centimeters to several

meters. Efficiency may be defined as the amount of usable electrical energy that is available to

the device being powered, divided by the amount of energy that is drawn by the WiTricity

source. In many applications, efficiency can exceed 90%. And WiTricity sources only transfer

energy when it is needed. When a WiTricity powered device no longer needs to capture

additional energy, the WiTricity power source will automatically reduce its power consumption

to a power saving idle state.

2) Energy Transfer via Magnetic Near Field Can Penetrate and Wrap Around Obstacle: The

magnetic near field has several properties that make it an excellent means of transferring energy

in a typical consumer, commercial, or industrial environment. Most common building and

furnishing materials, such as wood, gypsum wall board, plastics, textiles, glass, brick, and

concrete are essentially transparent to magnetic fieldsenabling WiTricity technology to

efficiently transfer power through them. In addition, the magnetic near field has the ability to

wrap around many metallic obstacles that might otherwise block the magnetic fields. WiTricity

applications engineering team will work with you to address the materials and environmental

factors that may influence wireless energy transfer in your application.

B. Applications of WiTricity

WiTricity wireless power transfer technology can be applied in a wide variety of applications

and environments. The ability of our technology to transfer power safely, efficiently, and over

distance can improve products by making them more convenient, reliable, and environmentally

friendly.

WiTricity technology can be used to provide:

1) Automatic Wireless Power Charging: 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. When a device with rechargeable batteries charges itself

while still in use or at rest, without requiring a power cord or battery replacement.

This mode is for a mobile device that may be used both in and out of range of its WiTricity

power source.

2) Consumer Electronics

a) 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.

b) Direct wireless powering of stationary devices (flat screen TV‘s, digital picture frames, home

theatre accessories, wireless loud speakers, etc.) eliminating expensive custom wiring, unsightly

cables and wall-wart power supplies.

c) Direct wireless powering of desktop PC peripherals: wireless mouse, keyboard, printer,

speakers, display, etc. eliminating disposable batteries and awkward cabling.

3) Industrial

a) Direct wireless power and communication interconnections across rotating and moving joints

(robots, packaging machinery, assembly machinery, machine tools) eliminating costly and

failure-prone wiring.

b) Direct wireless power for wireless sensors and actuators, eliminating the need for expensive

power wiring or battery replacement and disposal.

4) Transportation

a) Automatic wireless charging for existing electric vehicle classes: golf carts, industrial

vehicles.

b) Automatic wireless charging for future hybrid and allelectric passenger and commercial

vehicles, at home, in parking garages, at fleet depots, and at remote kiosks.

c) Direct wireless power interconnections to replace costly vehicle wiring harnesses and slip

rings.

5) Medical Application: Wireless charging systems are being developed for implanted medical

devices including Left ventricular assist device (LVAD) heart assist pumps, pacemakers, and

infusion pumps. Using highly resonant wireless power transfer, such devices can be efficiently

powered through the skin and over distances much greater than the thickness of the skin, so that

power can be supplied to devices deeply implanted within the human body. The HRWPT

technique eliminates the need for drive lines that penetrate the human body, and for surgical

replacement of primary batteries.

6) Military Application:

Designers of defence systems are able to utilize wireless charging to improve the reliability,

ergonomics, and safety of electronic devices. The Talon Teleoperated robot shown in Figure 9 is

being equipped with wireless charging so that it can be recharged while it is being transported by

truck from site to site. Helmet mounted electronics, including night vision and radio devices can

be powered wirelessly from a battery pack carried in the soldier’s vest, eliminating the need for

disposable batteries or a power cord connecting the helmet to the vest mounted battery pack.