CHAPTER 1

Wireless Power Transmission

1.1 Introduction

Power is very important to modern systems. From the smallest sensors, bionic

implants, laptops, consumer products to satellites and oil platforms, it is important to

be able to deliver power means other than classical wires or transmission lines.

Wireless transmission is useful in cases where instantaneous or continuous energy

transfer is needed, but interconnecting wires are inconvenient, hazardous, or

impossible some times. In the case of biological implants, there must be a battery or

energy storage element present that can receive and hold energy. This element takes

up valuable space inside a person body. In the case of satellites, UAVs and oil

platforms, solar panels, fuel cells, or combustion engines are currently used to supply

power. Solar panels take up a great deal of weight and bulk in terms of energy density

and must have a tracking system to maximize exposure to the sun. Fuel cells and

combustion cells needs fuel and maintenance to be delivered on site.

Wireless Power Transmission (WPT) is the efficient transmission of electric

power from one point to another trough vacuum or an atmosphere without the use of

wire or any other substance. The power can be transmitted using Inductive coupling

for short range, resonant induction for mid-range electromagnetic wave power

transfer. WPT is a technology that can transport power to locations, which are

otherwise not possible or impractical to reach. The objective of this report is to design

and implement a method to transmit wireless electrical power through space. The

system will work by using resonant coils to transmit power from an AC line to a

resistive load. Investigation of various geometrical and physical form factors

evaluated in order to increase coupling between transmitter and receiver. [2]

1.2 Why Do We Go For WPT System

1.2.1 Losses in the Wired System

Professor Rauscher showed that the earth’s magnetosphere contains sufficient

potential energy (at least 3 billion kilowatts) so that the resonant excitation of the

earth-ionosphere cavity can reasonably be expected to increase the amplitude of

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natural “Schumann” frequencies, facilitating the capture of useful electrical power.

Tesla also knew that the earth could be treated as one big spherical conductor and the

ionosphere as another bigger spherical conductor, so that together they have parallel

plates and thus, comprise a “spherical capacitor.” Dr. Rauscher calculates the

capacitance to be about 15,000 microfarads for the complete earth-ionosphere cavity

capacitor. W.O. Schumann is credited for predicting the “self-oscillations” of the

conducting sphere of the earth, surrounded by an air layer and an ionosphere in 1952,

without knowing that Tesla had found the earth’s fundamental frequency fifty years

earlier. In comparison to the 3 billion kW available from the earth system, it is

possible to calculate what the U.S. consumed in electricity. In 2000, about 11 Quads

(quadrillion Btu) were actually used by consumers for electrical needs, which is equal

to 3.2 trillion kWh. Dividing by the 8760 hours in a year, we find that only 360

million kW are needed on site to power our entire country. This would still leave 2.6

billion kW for the rest of the world! The really shameful U.S. scandal, unknown to the

general public, is that out of the total electrical power generated using wire

transmission (about 31 Quads), a full 2/3 is totally wasted in “conversion losses.” .[2]

Fig 1.1: Losses in the Wired System

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No other energy production system of any kind in the world has so much

wastefulness. Instead of trying to build 2 power plants per week (at 300 MW each) for

the next 20 years (only to have a total of additional 6 trillion kWh available by 2020),

as some U.S. government officials want to do, we simply need to eliminate the 7

trillion kWh of conversion losses in our present electricity generation modality.

Tesla’s wireless transmission of power accomplishes this goal, better than any

distributed generation.

1.3 Historical Revolution Of Wireless Power Transmission

Fig 1.2 : Wireless Power Transmission

In 1864, James C. Maxwell predicted the existence of radio waves by means

of mathematical model. In 1884, John H. Poynting realized that the Poynting vector

would play an important role in quantifying the electromagnetic energy. In 1888,

bolstered by Maxwell's theory, Heinrich Hertz succeeded in showing experimental

evidence of radio waves by his spark gap radio transmitter. The prediction and

evidence of the radio wave in the end of 19th century was start of the wireless power

transmission.

During the same period of Marchese G. Marconi and Reginald Fessenden who

are pioneers of communication via radio waves, Nicola Tesla suggested an idea of the

wireless power transmission and carried out the first WPT experiment in 1899. He

said “This energy will be collected all over the globe preferably in small amounts,

ranging from a fraction of one to a few horse powers. One of its chief uses will be the

illumination of isolated homes”. He actually built a gigantic coil which was connected

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to a high mast of 200 ft. with a 3 ft. diameter ball at its top. He fed 300 Kw power to

the Tesla coil resonated at 150 kHz. The RF potential at the top sphere reached 100

MV. Unfortunately, he failed because the transmitted power was diffused to all

directions with 150 kHz radio waves whose wave length was 21 km.

To concentrate the transmitted power and to increase transmission efficiency,

we have to use higher frequency than that used by Tesla. In 1930s, much progress in

generating high power microwaves, namely 1-10 GHz radio waves, was achieved by

invention of the magnetron and the klystron. After World War II, high power and high

efficiency microwave tubes were advanced by development of radar Technology. We

can concentrate a power to receiver with microwaves. We call the wireless power

transmission with microwaves as microwave power transmission (MPT). Based on

the development of the microwave tubes during the World War II, W. C. Brown

started the First MPT research and development in 1960. First of all, he developed a

rectenna, rectifying antenna which he named, for receiving and rectifying

microwaves. The efficiency of the first rectenna developed in 1963 was 50 % at

output 4WDC and 40% at output 7WDC, respectively.[8]

Fig 1.3 : MPT Demonstration With Helicopter By W.C.Brown

With the rectenna, he succeeded in MPT experiments to wired helicopter in 1964 and

to free flied helicopter in 1968 (Fig. 1.3). In 1970s; he tried to increase DC RF

transmission RF DC total efficiency with 2.45 GHz microwave. In 1970, overall DC

total efficiency was only 26.5 % at 39WDC in Marshall Space Flight Centre.

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Fig 1.4 : 1st Ground To Ground MPT Experiment In 1975 At The Venius Site.

In parallel, he and his team succeeded in the largest MPT demonstration in

1975 at the Venus Site of JPL Goldstone Facility (fig 1.4). Distance between a

transmitting parabolic antennas, whose diameter was 26m, and a rectenna array,

whose size was 3.4 m x 7.2 m, was 1 mile.

After 1990s, many MPT laboratory and field experiments were carried out in

the world. We often use 2.45 GHz or 5.8 GHz of the ISM band (ISM=Industry,

Science, and Medical) for the MPT system. A Canadian Group demonstrated fuel free

airplane flight experiment with MPT in 1987 which was called SHARP (Stationary

High Altitude Relay Platform) with 2.45 GHz.

Fig 1.5 : Stationary High Altitude Relay Platform.

In USA, there were many MPT research and development projects after W. C.

Brown: for instance, retro directive microwave transmitters, rectenna , and microwave

circuit technologies.

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In Japan, there were many field MPT experiments such as fuel free airplane

flight experiment with MPT phased array with 2.411 GHz in 1992, ground to ground

MPT experiment with Power Company and universities in 1994-95.

Fig 1.6 : Ground To Ground MPT Experiment In Japan In 1994-95.

In the early 1970s, experiments with RFID tags began and by the early 2000’s

Professor She Yuen (Ron) Hui and S.C. Tang developed a charger to provide resonant

power transfer for small electronics. Today wireless power is used for everything

from industrial motors to charging smartphones and tablets.

Researchers predict that wireless power will be making a significant

contribution to energy supplies by the end of this decade [3].

1.4 What Is WPT? WPT is nothing but wireless electricity. Transmission of electrical energy

from one object to another without the use of wires is called as WPT. WPT will

ensure that the cell phones, laptops, iPods and other power hungry devices get

charged on their own, eliminating the need of plugging them in. WPT technology is

transferring electric energy or power over distance without wires. With the basics of

electricity and magnetism, and work our way up to the WPT technology. Even better,

because of WPT some of the devices won't enquire batteries to operate. No, this

concept of wireless electricity is not new. In fact Nikola Tesla used resonance

magnetic fields to transfer wireless power. [5]

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1.5 How Does Wireless Power Work? The basics of WPT involves the transmission of energy from a transmitter to a

receiver via an oscillating magnetic field. To achieve this, Direct Current (DC)

supplied by a power source, is converted into high frequency AC by specially

designed electronics built into the transmitter. The AC energizes a copper wire coil in

the transmitter, which generates a magnetic field. Once a second (receiver) coil is

placed within proximity of the magnetic field, the field can induce an alternating

current in the receiving coil.

Electronics in the receiving device then converts alternating current back into direct

current, which becomes usable power. The diagram FIG. 1.8 simplifies this process

into four key steps.

1.6 Need Of WPT Now a days there is a Rapid development of autonomous electronics like Laptops,

Cell phones, House hold robots and all those devices typically relay on chemical

energy storage(Battery) As they are becoming daily needs to present generation,

Wireless energy transfer would be useful for many applications and they need

midrange energy.

Fig 1.7 : Wireless Energy Transfer

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Fig. 1.8 : Simple Wireless Power Transmission Diagram

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CHAPTER 2

The Basic Idea of Transforming Electricity to WPT2.1 Electricity The flow of electrons (current) through a conductor (like a wire), or charges

through the atmosphere (like lightning). A convenient way for energy to get from one

place to another.

Fig 2.1 : An Illustration Representing The Earth's Magnetic Field

2.2 Magnetism A fundamental force of nature, which causes certain types of materials to attract or

repel each other. Permanent magnets, like the ones on your refrigerator and the earth‘s

magnetic field, are examples of objects having constant magnetic fields. Oscillating

magnetic fields vary with time, and can be generated by alternating current (AC)

flowing on a wire. The strength, direction, and extent of magnetic fields are often

represented and visualized by drawings of the magnetic field lines.[1]

2.3 Electromagnetism A term for the interdependence of time varying electric and magnetic fields. For

example, it turns out that an oscillating magnetic field produces an electric field and

an oscillating electric field produces a magnetic field.

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Fig 2.2: B RepresentsMagnetic Field When Current Flows Through A Coil.

As electric current, I, flows in a wire, it gives rise to a magnetic field, B,

which wraps around the wire. When the current reverses direction, the magnetic field

also reverses its direction.

2.4 Magnetic InductionA loop or coil of conductive material like copper, carrying an alternating

current (AC), is a Very efficient structure for generating or capturing a magnetic field.

If a conductive loop is connected to an AC power source, it will generate an

oscillating magnetic field in the vicinity of the loop. A second conducting loop,

brought close enough to the first, may capture some portion of that oscillating

magnetic field, which in turn, generates or induces an electric current in the second

coil. The current generated in the second coil may be used to power devices. This type

of electrical power transfer from one loop or coil to another is well known and

referred to as magnetic induction. Some common examples of devices based on

magnetic induction are electric transformers and electric generators. [1]

2.5 Energy/Power Coupling An electric transformer is a device that uses magnetic induction to transfer

energy from its primary winding to its secondary winding, without the windings being

connected to each other. It is used to transform AC current at one voltage to AC

current at a different voltage. Energy coupling occurs when an energy source has a

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means of transferring energy to another object. One simple example is a locomotive

pulling a train car the mechanical coupling between the two enables the locomotive to

pull the train, and overcome the forces of friction and inertia that keep the train still

and, the train moves.

Fig 2.3 : Magnetic Induction

Magnetic coupling occurs when the magnetic field of one object interacts with

a second object and induces an electric current in or on that object. In this way,

electric energy can be transferred from a power source to a powered device. In

contrast to the example of mechanical coupling given for the train, magnetic coupling

does not require any physical contact between the object generating the energy and

the object receiving or capturing that energy. [1]

2.6 WPT TechnologyWPT power sources and capture devices are specially designed magnetic

resonators that efficiently transfer power over large distances via the magnetic near

field. These proprietary source and device designs and the electronic systems that

control them support efficient energy transfer over distances that are many times the

size of the sources/devices themselves.

The WPT power source, left, is connected to AC power. The blue lines

represent the magnetic near field induced by the power source. The yellow lines

represent the flow of energy from the source to the WPT capture coil, which is shown

powering a light bulb. Note that this diagram also shows how the magnetic field (blue

lines) can wrap around a conductive obstacle between the power source and the

capture device.

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Fig 2.4 : WPT Power Source

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CHAPTER 3

Theories On WPT

3.1 Near Field TheoryNear field is wireless transmission techniques over distances comparable to, or

a few times the diameter of the device(s), and up to around a quarter of the

wavelengths used. Near field energy itself is non radiative, but some radiative losses

will occur. In addition there are usually resistive losses. Near field transfer is usually

magnetic (inductive), but electric (capacitive) energy transfer can also occur.

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

fields enabling WPT 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.[9]

3.1.1 Electrical Conduction Principle

Electrical energy can be transmitted by means of electrical currents made to

flow through naturally existing conductors, specifically the earth, lakes and oceans,

and through the upper atmosphere starting at approximately 35,000 feet (11,000 m)

elevation — a natural medium that can be made conducting if the breakdown voltage

is exceeded and the constituent gas becomes ionized. For example, when a high

voltage is applied across a neon tube the gas becomes ionized and a current passes

between the two internal electrodes. In a wireless energy transmission system using

this principle, a high-power ultraviolet beam might be used to form vertical ionized

channels in the air directly above the transmitter-receiver stations. The same concept

is used in virtual lightning rods, the electro laser electroshock weapon and has been

proposed for disabling vehicles. A global system for "the transmission of electrical

energy without wires" dependent upon the high electrical conductivity of the earth

was proposed by Nikola Tesla as early as 1904.[6]

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"The earth is 4,000 miles radius. Around this conducting earth is an atmosphere.

The earth is a conductor; the atmosphere above is a conductor, only there is a little

stratum between the conducting atmosphere and the conducting earth which is

insulating. Now, you realize right away that if you set up differences of potential at

one point, say, you will create in the media corresponding fluctuations of potential.

But, since the distance from the earth's surface to the conducting atmosphere is

minute, as compared with the distance of the receiver at 4,000 miles, say, you can

readily see that the energy cannot travel along this curve and get there, but will be

immediately transformed into conduction currents, and these currents will travel like

currents over a wire with a return. The energy will be recovered in the circuit, not by a

beam that passes along this curve and is reflected and absorbed, but it will travel by

conduction and will be recovered in this way."

Researchers experimenting with Tesla's wireless energy transmission system

design have made observations that may be inconsistent with a basic tenet of physics

related to the scalar derivatives of the electromagnetic potentials, which are presently

considered to be nonphysical. The intention of the Tesla world wireless energy

transmission system is to combine electrical power transmission along with

broadcasting and point-to-point wireless telecommunications, and allow for the

elimination of many existing high-tension power transmission lines, facilitating the

interconnection of electrical generation plants on a global scale. One of Tesla's patents

suggests he may have misinterpreted 25–70 km nodal structures associated with

cloud-ground lightning observations made during the 1899 Colorado Springs

experiments in terms of circum globally propagating standing waves instead of a local

interference phenomenon of direct and reflected waves. Regarding the recent notion

of power transmission through the earth-ionosphere cavity, a consideration of the

earth-ionosphere or concentric spherical shell waveguide propagation parameters as

they are known today shows that wireless energy transfer by direct excitation of a

Schumann cavity resonance mode is not realizable. "The conceptual difficulty with

this model is that, at the very low frequencies that Tesla said that he employed (1-50

kHz), earth-ionosphere waveguide excitation, now well understood, would seem to be

impossible with the either the Colorado Springs or the Long Island apparatus (at least

with the apparatus that is visible in the photographs of these facilities)." [4]

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Fig 3.1 : Tesla Coil Transformer Wound In The Form Of A Flat Spiral. This Is The

Transmitter Form As Described In U.S. Patent 645,576.

On the other hand, Tesla's concept of a global wireless electrical power

transmission grid and telecommunications network based upon energy transmission

by means of a spherical conductor transmission line with an upper three-space model

return circuit, while apparently not practical for power transmission, is feasible,

defying no law of physics. Global wireless energy transmission by means of a

spherical conductor “single-wire” surface wave transmission line and a propagating

TM00 mode may also be possible.

3.1.2 Induction Principle (Non Resonant Energy Transfer)

The action of an 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. (An added benefit is the capability to step the

primary voltage either up or down.) The Battery chargers of a mobile phone or the

transformers on the street is examples of how this principle can be used. Induction

cookers and many electric toothbrushes are also powered by this technique. The main

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drawback to induction, however, is the short range. The receiver must be very close to

the transmitter or induction unit in order to inductively couple with it.[4]

3.1.3 Electrostatic Induction Principle

FIG 3.2 : (Tesla Illuminating Two Exhausted Tubes By Means Of A Powerful,

Rapidly Alternating Electrostatic Field Created Between Two Vertical Metal Sheets

Suspended From The Ceiling On Insulating Cords.)

The "electrostatic induction effect" or "capacitive coupling" is an electric field

gradient or differential capacitance between two elevated electrodes over a conducting

ground plane for wireless energy transmission involving high frequency alternating

current potential differences transmitted between two plates or nodes. The

electrostatic forces through natural media across a conductor situated in the changing

magnetic flux can transfer energy to a receiving device (such as Tesla's wireless

bulbs).Sometimes called "the Tesla effect" it is the application of a type of electrical

displacement, i.e., the passage of electrical energy through space and matter, other

than and in addition to the development of a potential across a conductor.

Tesla stated, "Instead of depending on [electrodynamic] induction at a distance to

light the tube . . . the ideal way of lighting a hall or room would . . . be to produce

such a condition in it that an illuminating device could be moved and put anywhere,

and that it is lighted, no matter where it is put and without being electrically

connected to anything. I have been able to produce such a condition by creating in the

room a powerful, rapidly alternating electrostatic field. For this purpose I suspend a

sheet of metal a distance from the ceiling on insulating cords and connect it to one

terminal of the induction coil, the other terminal being preferably connected to the

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ground. Or else I suspend two sheets .each sheet being connected with one of the

terminals of the coil, and their size being carefully determined. An exhausted tube

may then be carried in the hand anywhere between the sheets or placed anywhere,

even a certain distance beyond them; it remains always luminous.” And “In some

cases when small amounts of energy are required the high elevation of the terminals,

and more particularly of the receiving-terminal D' may not be necessary, since,

especially when the frequency of the currents is very high, a sufficient amount of

energy may be collected at that terminal by electrostatic induction from the upper air

strata, which are rendered conducting by the active terminal of the transmitter or

through which the currents from the same are conveyed."[2]

3.1.4 Electro-Dynamic Induction (Resonant Inductive Coupling Method/

Evanescent Coupling Method)

An inductive transformer is a device commonly used in power circuits and

electromechanical motors (for example electrical toothbrushes and chargers). A

transformer typically operates up to mid-kHz frequencies. It essentially transfers

electrical energy from one circuit to another via induction: the time-varying magnetic

flux produced by a primary coil crosses a secondary coil and induces in it a voltage.

The primary and the secondary coils are not physically connected, hence the method

is wireless. Transformers can be very efficient but the distance between the coils must

be very small (typically a few millimetres). For distances a few times the size of the

coils, the efficiency drops significantly. Part of the underlying physics for most of the

existing methods for the wireless transfer of electricity is the fundamental principle of

resonance: the property of certain physical systems to oscillate with maximum

amplitudes at certain frequencies. It follows that, for any type of excitation

(mechanical, acoustic, electro magnetic, nuclear) with a given frequency, a receiver

will pick up the transmitted energy efficiently only when designed to resonate at the

excitation frequency. Only then do successive excitations after each oscillation period

add coherently in phase and lead to a build up of energy within the receiver. To

illustrate, consider 100 glasses filled with wine at different levels so that they support

acoustic resonances at different frequencies. Now let an electric-guitar player produce

and sustain a very well-defined note. Only one of the glasses, the one resonant with

the frequency of this note, will respond to the excitation, to the extent that it may even

break, while the rest will remain unaffected. Similarly, we tune the electromagnetic

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antenna of a radio to be resonant with the frequency of the station we want to listen to.

Many transformers used in power circuitry and elsewhere are also designed to employ

resonance to enhance the power transmission.[1]

3.1.5 Experimental Demonstration Of Evanescent Coupling At MIT

Based on their theory, they started experiments in late 2006. The main

challenges consisted of designing a driving circuit that would operate in our desired

low-MHz regime and constructing coils that would resonate with high enough value

of Q. After a trial-and-error phase, they realized that a simple coil design without a

separate capacitor, but using the coil’s self-capacitance to achieve resonance, was best

option in terms of Q. We made two copper-pipe coils with 60 cm diameters and with

five turns, such that they resonate at 10MHz and have Q=1000. A 60 W light bulb

was the chosen device, since it operates at tested frequencies (and what can be a

clearer sign of the functionality of a system than the switch on of a light bulb?). They

suspended the coils from the ceiling with fishing wire, at distance of 2m from each

other, tuned them up, turned them on and…there was light. At an efficiency of 45%,

this was, to our knowledge, the first-ever demonstration of midrange efficient wireless

energy transfer.

Fig 3.3 : Lightening A 60 Watt Bulb Around 2m Away From The Source At MIT

3.2 Far Field Theory

Far field methods achieve longer ranges, often multiple kilometer ranges,

where the distance is much greater than the diameter of the device(s). With radio

wave and optical devices the main reason for longer ranges is the fact that

electromagnetic radiation in the far-field can be made to match the shape of the

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receiving area (using high directivity antennas or well-collimated Laser Beam)

thereby delivering almost all emitted power at long ranges. The maximum directivity

for antennas is physically limited by diffraction. The Raytheon Company did the first

successful WPT experiment in 1963. In this experiment energy was transmitted with a

DC-to-DC efficiency of 13%. This company also demonstrated a microwave-powered

helicopter in 1964. The Jet propulsion lab of NASA carried out an experiment and

demonstrated the transfer of 30 kW over a distance of 1 mile in 1975. They used an

antenna array erected at the Goldstone facility. This test demonstrated the possibilities

of wireless power outside the laboratory. Rockwell International and David Sarnoff

Laboratory operated in 1991 a microwave powered rover at 5.86 GHz. Three

kilowatts of power was transmitted and 500 watts was received.

3.2.1 Microwave Power Transmission (MPT)

In order to transport electricity is has to be transformed into a suitable energy

form. For wireless transmission, this has to be a form that can travel through air.

Microwave frequencies hold this ability. The microwave spectrum is defined as

electromagnetic energy ranging from approximately 1 GHz to 1000 GHz in

frequency, but older usage includes lower frequencies. Most common applications are

within the 1 to 40 GHz range. A complete microwave transmission system consists of

three essential parts:

Electrical power to microwave power conversion

Absorption antenna that captures the waves

(Re)conversion to electrical power

Fig 3.4 : Microwave Transmitter And Rectenna

The components include a microwave source, a transmitting antenna and a

receiving antenna. The microwave source consists of an electron tubes or solid-state

devices with electronics to control power output. The slotted waveguide antenna,

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parabolic dish and micro strip patch are the most popular types. Due to high

efficiency (>95%) and high power handling capacity, the slotted waveguide antenna

seems to be the best option for power transmission. The combination of receiving and

converting unit is called rectenna. The rectenna is a rectifying antenna that is used to

directly convert microwave energy into DC electricity. It is an antenna includes a

mesh of dipoles and diodes for absorbing microwave energy from a transmitter and

converting it into electric power. Its elements are usually arranged in a multi element

phased array with a mesh pattern reflector element to make it directional. The

Rayleigh criterion dictates that any beam will spread (microwave or laser), become

weaker, and diffuse over distance. The larger the transmitter antenna or laser aperture,

the tighter the beam and the less it will spread as a function of distance (and vice

versa). Therefore, the system requires large transmitters and receivers. The used

power density of the microwave beam is normally in the order of 100 W/m2. This is

relative low compared to the power density of solar radiation on earth (1000 W/m2)

and chosen this way for safety reasons.

3.2.2 Applications

3.2.2.1 WPT For Space Solar

The largest application for microwave power transmission is space solar

power satellites (SPS). In this application, solar power is captured in space and

converted into electricity. The electricity is converted into microwaves and

transmitted to the earth. The microwave power will be captured with antennas and

converted into electricity. NASA is still investigating the possibilities of SPS. One of

the problems is the high investment cost due to the space transport. The current rates

on the Space Shuttle run between $7,000 and $11,000 per kilogram of transported

material. Recently the idea of Space Solar Power caught again the public attention e.g.

by the Obama transition team and The Economist .

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Fig 3.5 : Space Solar Power Satellite

3.2.2.2 Power Transfer, Bridging Applications

Using a powerful focused beam in the microwave or laser range long distances

can be covered. There are two methods of wireless power transmission for bridging

application. First is the direct method, from transmitting array to rectenna. A line of

sight is needed and is therefore limited to short (< 40 km) distances. Above 40

kilometers, huge structures are needed to compensate for the curvature of the earth.

The second method is via a relay reflector between the transmitter and rectenna. This

reflector needs to be at an altitude that is visible for both transmitter and rectenna.

Next one bridging applications of WPT are discussed.

Alaska '21WPT can be an option for power supply to rural areas. In 1993 was a project

presented about wireless power supply in Alaska. Because of limited infrastructure,

hundreds of small rural communities in Alaska must provide their own electricity.

These systems can be expensive not standard or just not available. At the moment, the

small communities produce their own power with mainly diesel engines. These

produce noise and pollution. Also the needed fuel has to be transported over long

distances. All this results in an electricity price in excess of $40 /kWh .Cable

connections trough water is no option because of ice. With the help of WPT, the

needed power production of the communities can be combined. This can reduce noise,

pollution and transportation of fuel. WPT may be capable of transmitting electrical

energy to Alaska’s remote villages. To investigate these possibilities, a pilot project

was conducted named "Alaska'21”. The system used for the pilot project consisted of

a2.45 GHz phased array design. The distances that should be bridged are between 1

and 15 miles. The status of the project is unknown.

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Fig 3.6 :(Alaska’21 ).

3.2.3 Safety Issues

Bio-Effects

A general public perception that microwaves are harmful has been a major

obstacle for the acceptance of power transmission with microwaves. A major concern

is that the long-term exposure to low levels of microwaves might be unsafe and even

could cause cancer. Since 1950, there have been thousands of papers published about

microwave bio-effects. The scientific research indicates that heating of humans

exposed to the radiation is the only known effect. There are also many claims of low-

level non thermal effects, but most of these are difficult to replicate or show

unsatisfying uncertainties. Large robust effects only occur well above exposure limits

existing anywhere in the world. The corresponding exposure limits listed in IEEE

standards at 2.45 or 5.8 GHz are 81.6 W/m2 and 100 W/m2 averaged over 6 minutes,

and 16.3 or 38.7 W/m2 averaged over 30 minutes .

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Fig 3.7 : Graph Showing Safety Level To Different Range Of Frequencies

This low compared to average solar radiation of 1000 W/m2. A clearly

relevant bio-effect is the effect of microwave radiation on birds, the so-called "fried

bird effect". Research is done on such effect at 2.45 GHz. The outcome showed slight

thermal effects that probably are welcome in the winter and to be avoided in the

summer . Larger birds tend to experience more heat stress then small birds . The

overall conclusion of bioeffects research is that microwave exposures are generally

harmless except for the case of penetrating exposure to intense fields far above

existing exposure limits .

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CHAPTER 4

Transmitters, Antennas And Receivers

4.1 Antennas For Microwave Power Transmission

All antennas can be applied for both the MPT system and communication

systems, for example, Yagi Uda antenna, horn antenna, parabolic antenna, micro strip

antenna, phased array antenna or any other type of antenna.

To fixed target of the MPT system, we usually select a large parabolic

antenna, for example, in MPT demonstration in 1975 at the Venus Site of JPL

Goldstone Facility and in ground to ground MPT experiment in 1994-95 in Japan. In

the fuel free airship light experiment with MPT in 1995 in Japan, they changed a

direction of the parabolic antenna to chase the moving airship. In some MPT

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experiments in Japan, the phased array antenna was adopted to steer a direction of the

microwave beam (FIG. 4.1).

All SPS is designed with the phased array antenna.

Fig 4.1 : Phased Array Used In Japanese Field MPT Experiment

4.2 Recent Technologies For Transmitters

The technology employed for generation of microwave Radiation is an

important subject for the MPT system. We need higher efficient generator/amplifier

for the MPT system than that for the wireless communication system. For highly

efficient beam collection on rectenna array, we need highly stabilized and accurate

phase and amplitude of microwaves for phased array system for the MPT .There are

two types of microwave generators/amplifiers. One is a microwave tube (magnetron)

and the other is semiconductor amplifier.

4.2.1 Magnetron

Magnetron is a crossed field tube in which electrons emitted from the cathode

take cyclical path to the anode. The magnetron is self-oscillatory device in which the

anode contains a resonant RF structure. The magnetron has long history from

invention by A. W. Hull in 1921.

The practical and efficient magnetron tube attracted worldwide interest only

after K. Okabe proposed divided anode type magnetron in 1928. Magnetron

technologies received a boost during the World War II, especially with the Japanese

Army. The magnetrons were also useful for microwave ovens. As a result, the

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magnetron of 500 – 1,000 W is widely in use for microwave ovens in 2.45 GHz, and

is a relatively inexpensive oscillator (below $5). There is a net global capacity of

45.5GW/year for all magnetrons used in microwave ovens whose production is 50–

55 millions. It was W. C. Brown who invented a voltage controlled oscillator with a

cooker type magnetron in PLL.

4.2.2 Semiconductor Amplifier

After 1980s, semiconductor devices became dominant in microwave world

instead of the microwave tubes. This was driven by advances in mobile phone

networks. The semiconductor device is expected to Expand microwave applications,

for example, phased array and active integrated antenna (AIA), because of its

manageability and mass productivity. After 1990s, some MPT experiments were

carried out in Japan with phased array of semiconductor amplifiers.

Typical semiconductor devices for microwave circuits are FET (Field Effect

Transistor), HBT (Hetero junction Bipolar Transistor), and HEMT (High Electron

Mobility Transistor). Present materials for the semiconductor devices are Si for lower

frequency below a few GHz and GaAs for higher frequency. It is easy to control

phase and amplitude through the microwave circuits with semiconductor devices, for

example, amplifiers, phase shifters, modulators, and so on. Currently, new materials

are under development to enable semiconductor devices yield increased output power

and efficiency.

4.3 Wireless Power Transmission – Receivers And Rectifiers

Point to point MPT system needs a large receiving area with a rectenna array

because one rectenna element receives and creates only a few W. Especially for the

SPS, we need a huge rectenna site and a power network connected to the existing

power networks on the ground. On contrary, there are some MPT applications with

one small rectenna element such as RF ID.

4.3.1 Recent Technologies Of Rectenna

The word “rectenna” is composed of “rectifying circuit” and “antenna”. The

rectenna can receive and rectify a microwave power to DC. The rectenna is passive

element with a rectifying diode, operated without any power source. The circuit,

especially diode, mainly determines the RF DC conversion efficiency. Silicon Scotty

barrier diodes were usually used for earlier rectenna. New devices like SiC and GaN

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are expected to increase the efficiency. The rectenna with FET or HEMT appeared

recently. The single shunt full wave rectifier is always used for the rectenna. It

consists of a diode inserted in the circuit in parallel, a λ/4 distributed line, and a

capacitor inserted in parallel. In an ideal situation, 100% of the received microwave

power should be converted into DC power.

4.3.2 Recent Technologies Of Rectenna Array

The rectenna will be used as an array for high power MPT because one

rectenna element rectifies a few W only. For usual phased array antenna, mutual

coupling and phase distribution are problems to solve. For the rectenna array, problem

is different from that of the array antenna because the rectenna array is connected not

in microwave phase but in DC phase. When we connect two rectenna in series or in

parallel, they will not operate at their optimum power output and their combined

power output will be less than that if operated independently. This is theoretical

prediction.

4.3.3 Efficiency

We classify the MPT efficiency roughly into three stages;

DC RF conversion efficiency which includes losses caused by beam forming.

Beam collection efficiency which means ratio of all radiated power to collected

power on a receiving antenna, and

RF DC conversion efficiency.

4.3.3.1 RF DC Conversion Efficiency

The RF DC conversion efficiency of the rectenna or the CWC is over 80 % of

experimental results as shown. Decline of the efficiency is caused by array connection

loss, change of optimum operation point of the rectenna array caused by change of

connected load, trouble of the rectenna, and any losses on the systems, for example,

DC/AC conversion, cables, etc. However, it is easier to realize higher efficiency than

that on the other two

4.3.3.2 Beam Collection Efficiency

The beam collection efficiency depends on the transmitter and receiver

aperture areas, the wavelength, and the separation distance between the two antennas.

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Fig 4.2: Efficiency Of Rectenna Element.

CHAPTER 5

Recent Technological Trends

There are several technological trends which are going to be concerned

for wireless power transmission which are as follows:

5.1 Retro Directive Beam Control

A microwave power transmission is suitable for a power transmission from/to

moving transmitters/targets. Therefore, accurate target detection and high efficient

beam forming are important. Retro directive system is always used for SPS.

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A corner reflector is most basic retro directive system. The corner reflectors

consist of perpendicular metal sheets, which meet at an apex. Incoming signals are

reflected back in the direction of arrival through multiple reflections off the wall of

the reflector. Van Atta array is also a basic technique of the retro directive system.

This array is made up of pairs of antennas spaced equidistant from the centre of the

array, and connected with equal length transmission lines. The signal received by an

antenna is re-radiated by its pair, thus the order of re-radiating elements are inverted

with respect to the centre of the array, achieving the proper phasing for retro

directivity.

Usual retro directive system have phase conjugate circuits in each

receiving/transmitting antenna, which play same role as pairs of antennas spaced

equidistant from the centre of the array in Van Atta array. The signal is called a pilot

signal. We do not need any phase shifters for beam forming. The retro directive

system is usually used for satellite communication, wireless LAN, military, and so on.

5.2 Environmental Issues

One of the characteristics of the MPT is to use more intense microwave than

that in wireless communication systems. Therefore, we have to consider MPT safety

for humans.

5.3 Interaction With Atmosphere

In general, effect of atmosphere on microwaves is quite small. There are

absorption and scatter by air, rain, and irregularity of air refraction ratio. In 2.45 GHz

and 5.8 GHz, the absorption by water vapour and oxygen Dominate the effect in the

air. Especially, it is enough to consider only absorption by the oxygen in the

microwave frequency. It is approximately 0.007 dB/km. In the SPS case, the amount

of total absorption through the air from space is approximately 0.035 dB.

5.4 Interaction With Space Plasmas

When microwaves from SPS propagate through ionospheric plasmas, some

interaction between microwaves and the ionospheric plasmas occurs. It is well known

that refraction, Faraday rotation, scintillation, and absorption occur between weak

microwave used for satellite communication and the plasmas. However, influence on

the MPT system is negligible. It is nonlinear interaction between intense microwave

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and the space plasmas that we have to investigate before the commercial SPS. We

theoretically predict that the following may occur: heating of the plasmas, plasma hall

effect, thermal self-focusing effect of the microwave beam, and three wave

interactions and excitation of electrostatic waves in MHz bands. These interactions

don’t occur in existent satellite communication systems because microwave power is

very weak.

Generating power by placing satellites with giant solar arrays in

Geosynchronous Earth Orbit and transmitting the power as microwaves on the earth

called Solar Power Satellites (SPS) will be the largest application of WPT .[8].

CHAPTER 6

Applications, Advantages And Future

6.1 Applications

WPT 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. WPT technology can be used to provide:

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6.1.1 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 WPT 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 WPT power source.

6.1.2 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 speakers, etc.) eliminating

expensive custom wiring, unsightly cables and wall wart power supplies.

Direct wireless powering of desktop PC peripherals: wireless mouse,

keyboard, printer, speakers, display, etc… eliminating disposable batteries and

awkward cabling.

6.1.3 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 expensive power wiring or

battery replacement and disposal.

6.1.4 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.

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Direct wireless power interconnections to replace costly vehicle wiring

harnesses and slip rings.

6.2 Other Applications

a) Direct wireless power interconnections and automatic wireless charging for

implantable medical devices (ventricular assist devices, pacemaker,

defibrillator, etc.).

b) Automatic wireless charging and for high tech military systems (battery

powered mobile devices, covert sensors, unmanned mobile robots and aircraft,

etc.).

c) Direct wireless powering and automatic wireless charging of smart cards.

6.3 Advantages And Disadvantages

There are so many advantages with this WPT concept, some of those are:

a) Unaffected by the day night cycle, weather or seasons.

b) This is an ecofriendly.

c) It is a boon for the devices which use midrange power.

d) At places where economic competition is not the prime consideration, it can

be an option. Wireless power transmission can supply power to places that are

difficult to reach. Especially small communities in rural areas could be

supplied with power using WPT.

e) First of all the wireless electrical transmission of data removes the need for

physical infrastructure like grids and towers. In this way the cost associated

with deploying towers and cables can be saved.

f) During the rains and after natural disasters it is often hard to manage the

cables and towers .by using WPT technology this problem can be eliminated.

g) The transmission and distribution loss associated with traditional electricity

grids can be overcome.

h) Today two words are ruling the world “efficiency” and “speed”. These two

words have become the base for the development in the technology.

i) The electricity generation using microwaves is more environments friendly.

Moreover it does not involve any emission of carbon gases.

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j) The monthly electricity bills using conventional electricity supply can be cut

to very low.

k) Use of battery for charging electrical and electronics devices can totally be

eliminated.

Some of the disadvantages are as follows:

1. High capital cost for practical implementation of wireless power transmission.

2. Another potential disadvantage is the interference of the microwaves with the

present wireless communication system.

3. The effect of microwave radiations at high doses received is not suitable to

human health.

6.4 Safety And Future Scope 6.4.1 Is WPT Technology Safe?

WPT technology is a non radiative mode of energy transfer, relying instead on

the magnetic near field. Magnetic fields interact very weakly with biological

organism’s people and animals and are scientifically regarded to be safe. Professor Sir

John Pendry of Imperial College London, a world renowned physicist, explains: The

body really responds strongly to electric fields, which is why you can cook a chicken

in a microwave. But it doesn't respond to magnetic fields. As far as we know the body

has almost zero response to magnetic fields in terms of the amount of power it

absorbs." Evidence of the safety of magnetic fields is illustrated by the widespread

acceptance and safety of household magnetic induction cook tops. Through

proprietary design of the WPT source, electric fields are almost completely contained

within the source. Thus WPT technology doesn‘t give rise to radio frequency

emissions that interfere with other electronic devices, and is not a source of electric

and magnetic field levels that pose a risk to people or animals. Limits for human

exposure to magnetic fields are set by regulatory bodies such as the FCC, ICNIRP,

and are based on broad scientific and medical consensus.

6.4.2 Future Scope Of WPT

MIT's WPT is only 40 to 45% efficient and according to Soljacic, they have to

be twice as efficient to compete with the traditional chemical batteries. The team's

next aim is to get a robotic vacuum or a laptop working, charging devices placed

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anywhere in the room and even robots on factory floors. The researchers are also

currently working on the health issues related to this concept and have said that in

another three to five years’ time, they will come up with a WPT system for

commercial use.

WPT, if successful will definitely change the way we live. Imagine cell

phones, laptops, digital camera's getting self-charged! Wow! Let's hope the

researchers will be able to come up with the commercial system soon. Till then, we

wait in anticipation! Human beings or other objects placed between the transmitter

and receiver do not hinder the transmission of power. However, does magnetic

coupling or resonance coupling have any harmful effects on humans? MIT's

researchers are quite confident that WPT's 'coupling resonance' is safe for humans.

They say that the magnetic fields tend to interact very weakly with the biological

tissues of the body, and so are not prone to cause any damage to any living beings.

CONCLUSION

Today we do live in the “wireless age”, in which the air that we

breathe probably contains more information than oxygen. However, this

is also an age where mobile phones, MP3 players, laptop computers and

domestic robots exist alongside old-fashioned power wires and bulky

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batteries. Unlike information, electrical energy is still physically confined

to these borderline anachronistic appliances. Overcoming these last

obstacles would finally make this a truly wireless world.

Transfer of power via microwaves has long been the focus of study

and experimentation. In the early 1900s, Nikola Tesla experimented with

transmission of power wirelessly, through microwaves. His work,

however, was largely left unimplemented, as his experiments were vastly

ahead of their time and the technology did not yet exist to make WPT via

microwaves feasible. Advances in wireless technologies since Tesla,

however, have made possible that which was not in the early 20th

century. Described in this section are the details of those technologies

behind MPT as a mechanism for WPT.

REFERENCES

1. Tesla, N. “Apparatus for transmitting electrical energy.” U.S. patent number

1,119,732, issued in December 1914.

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2. Ka-Lai, L., Hay, J. W. and Beart, P. G. W. “Contact-less power transfer.”

U.S. patent number 7,042,196, issued in May 2006. (SplashPower Ltd.,

www.splashpower.com)

3. Hirai, J., Kim, T.-W. and Kawamura, A. “Wireless transmission of power and

information and information for cableless linear motor drive.” IEEE Trans.

on power electronics 15, 21 (2000).

4. G. Landis and R. Cull, “Integrated Solar Power Satellites: An Approach to

Low-Mass Space Power,”Space Power, Vol. 11, No. 3–4, 303–218 (1992);

presented at SPS-91: Power From Space, Aug. 27–30, 1991, Paris France, pp.

225–232

5. www.WPT.com

6. http://www.cse.wustl.edu/~jain/cse574-14/ftp/power.pdf }

7. Electricity Flow Chart 1999, which contains US DOE/EIA data, updating the

Toby Grotz article in this book.)

8. http://ijtir.hctl.org/vol8/IJTIR_Article_201403012.pdf

9. http://www.seminarreports.in/2014/09/wireless-electricity-or-witricity.html

10. http://www.collegelib.com/t-witricity-wireless-electricity-seminar-abstract-

report.html

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