Wireless Power Transmission
Evolution and Developmental Prospects

Rachit Sethia
Department of Electrical & Electronics
Oriental Institute of Science & Technology
Bhopal, India rachitsethia19@gmail.com Sahil Saxena
Department of Electronics & Communication
Lakshmi Narain College of Technology
Bhopal, India saxena.sahil.16@gmail.com Abstract— In the present paper, wireless power transmission is shown as the righteous alternative for present day power distribution system. Various method of wireless power distribution system are acquainted which can be ramified for power distribution, showing electrodynamic induction as the befitting one. Many concepts, research papers, patents are available on wireless power transmission and are fruitfully materialized by Witricity by magnetic resonant coupling method. This paper is mainly concentrated on:
1. The most popular concept i.e. Electrodynamic induction.
2. Microwave transmission method.
The paper also discusses the possible ways to get useful and practical results out of all research carried out so far elsewhere.
Keywords- Wireless power transmission, Electromagnetic induction, Nikola Tesla, Witricity, Electrodynamic Induction.
Introduction
Our present power distribution system encompasses various losses and flaws which blemishes the same. Recent canvas have shown that existing power transmission system see a considerably high power loss that in 26-30% in developed countries and about30-40% in developing or underdeveloped countries. The primary reason for the losses during transmission and distribution of power is the resistance of the wire used for grid. Therefore, inefficiency in power transmission along with ever increasing need of electricity as a basic necessity calls for an efficacious and efficient transmission method. Since, wireless power transmission system provides much higher efficiency, low transmission cost and avoids power theft, therefore wireless power transmission seems obligatory.
Wireless Power Transmission
Wireless power transmission is the transfer of electrical energy from the source of power to the load by omitting the application of artificial conductor or wires between them. Microwave power transmission and electromagnetic induction method are some promising technologies and may be righteous alternative for efficient power transmission. In early 19th century, the realization that electricity could be wheedled to light up a bulb actuated an idea in the minds of noetic people to determine the best way to distribute it. At the head of the pack was inverter Nikola Tesla. He figured out that a vast infrastructure of wires extending into every city, building or room was onerous and thus wireless power transmission seemed opposite. For wireless power transmission the most feasible method according to Tesla was `AIR CONDUCTION METHOD’.
AIR CONDUCTION METHOD falls into two categories:

1. Closed circuit system 2. Open circuit system

Closed circuit system consist of large tesla coil transmitter, an ionized path connecting the transmitter to the upper atmosphere and the second ionized path connecting the upper atmosphere back down to the receiving end and a receiver placed at a receiving end. This circuit back to the transmitting end from the receiving end is completed through earth.

Figure 1. Tesla's Air Conduction Method

The upper atmosphere is not an ohmic conductor but will conduct electricity when broken down, i.e. ionized. This conductor path would then conduct current like neon tube with planetary proportion. It would require a certain amount of energy to maintain electrical discharge through it For this he drew up plan for a tower about 57 meters tall called `WARDENCLYFFE TOWER’. He claimed that it would transmit power to places miles away and started building it in shore ham, long island.

Figure 2. Wardenclyfee Tower

This theory theoretically appeased the concerned but was not promising from a practical viewpoint because to reach to useful distances, enormous voltages were needed. Also, the power required to ionize the air path would be mandatory and large.

Impracticalities observed in Nikola Tesla’s method led to the evolution of following types of wireless transmission techniques –

1. Electromagnetic Radiation

a) Microwave Power Transmission b) Laser Power Transmission

2. Electromagnetic Induction

a) Electrostatic Induction b) Electrodynamic Induction

Microwave Power Transmission

In microwave method of power transmission radio waves are beamed over long distances with shorter wavelengths of electromagnetic

Figure 3. Block Diagram of Microwave Power Transmission

radiation that falls in microwave range. William C. Brown pioneered this method of wireless transmission by developing a unit that demonstrated the transmission of power through free space using microwaves. The primary components of this unit are depicted in the above diagram.

Microwave Power Generator
Microwave transmitting devices are classified as microwave vacuum tubes and microwave power modules and semiconductor microwave transmitters. The magnetrons operate on the frequencies of 2.5 GHz, 5.8 GHz, 8.5 GHz, 10 GHz, 35 GHz and among these 2.5 GHz frequency has the highest efficiency of over 90 %.

Transmitting Antenna The slotted wave guide transmitting antenna is ideal for power transmission due to its high aperture efficiency and power handling capability. The other 2 types include the micro strip patch and parabolic dish antenna.

Rectenna
Rectenna at the receiving side receives the transmitted power and converts the microwave power into D.C power. The antenna used in rectenna may be dipole, Yagi-uda or parabolic dish antenna. Among these patch dipole or dipole has the highest efficiency.
Very high capital cost and interference with present communication systems are the reasons which impeach the practical implication of this method.

LASER POWER TRANSMISSION

Laser generates phase-coherent electromagnetic radiations at optical and infrared frequencies. One of the methods of laser power transmission is solar pumping. In solar pumping the solar energy is concentrated and then injected into the laser medium to produce laser beams.
Laser method of power transmission is generally known as power beaming because the power is beamed at a receiver that can convert it to usable electrical energy.
In case of laser power transmission method conversion of electrical energy into light is inefficient and the conversion of light into electrical is also inefficient. Atmospheric absorption causes losses and this method requires a direct line of sight with the target.

ELECTROMAGNETIC INDUCTION
Electrostatic Induction
Electrodynamic Induction

Electrostatic Induction

In this method electrical energy passes through the dielectric. When a high frequency alternating current power supply is provided, an electric field is generated by charging the plates with high potential.
Nikola Tesla illuminated a bulb by usuing this method of wireless transmission of power. For this he suspended a metal sheet on the insulating chord at a distance from the ceiling. He connected one terminal to the induction coil and the other terminal to the ground.
In another arrangement, he suspended two sheets. Each sheet of which was connected to the two terminals of the coil. The exhausted tube was carried in hand or kept anywhere between the sheets.

Figure 4. Tesla showing Power Transmission through Electrostatic Induction

Drawbacks

For wireless power transmission number of wires used should be nominal. In this method of wires are required to charge and maintain potential difference between the plates. Thus both the plates are connected to wires.
Also, to develop this electric field over a large distance, Large plates would be required that would eventually make this system gigantic.

In quest for the optimum method of wireless power transmission in today’s world, electrodynamic induction or resonant inductive coupling method seemed nifty to the versed.

Electrodynamic Induction

Introduction
Inductive coupling in electrodynamics induction is an old and well understood method of wireless power transfer. A primary coil is driven by the source which creates a sinusoidally varying magnetic field which in turn induces voltage across the terminals of a secondary coil and thus power is transferred wirelessly to the load. This is the same mechanism which is responsible for power transfer in transformer and hence it can also be shown that transfer is also possible in air. When this mechanism is used in general or in advanced applications high voltages are needed by the device to be powered and this is achieved by adding a parallel capacitor to the secondary coil to form a resonant circuit at the particular operating frequency.
History
In 1984, resonant coupling was used by Nikola Tesla to light up phosphorant and incandescent lamps. In 1987 a high voltage resonance transformer was patented by him, which came to be known as Tesla coil. This Tesla coil transferred electrical energy from primary to the secondary coil by resonant induction and was capable of producing high voltages at high frequencies. This theory saw practical applications in 1960’s when resonant inductive wireless energy transfer was applied successfully in implantable devices. The separation between the induction coils was less than 20 cm. Therefore, today this technique provides electric power in many commercially available medical implantable devices.

Recent works:
In November 2006, Marin Solijocic with a team of researchers at M.I.T. applied the electromagnetic theory for wireless power transmission which included the use of strongly coupled resonators.
Statement:
During preparation of theoretical analysis their demonstrations showed that if we can design electromagnetic resonators in which loss due to radiation and absorption is minimal and they also have a near field with mid-range extent (i.e. a few times the size of resonators), mid range efficient wireless transfer of energy is possible. Considering two such resonant circuits as mentioned above and tuning them to the same frequency within a fraction of wavelength, the near fields associated with them get coupled by means of evanescent wave coupling which can be explained with the help of quantum tunneling.
For maximum possible energy transfer frequency the lost times are designed to be very long so that when oscillating fields develop between inductors, the time taken by them to transfer energy from one object to another is shorter than all the lost times. Problems can arrive when there are extraneous objects in vicinity but since resonant wavelength being much larger than resonator causes the field to circumvent the objects and thus the line of sight in case of mid range energy transfer is not required. Using this magnetic resonance coupling method power can be delivered from a large source coil to one or many smaller coils. For this the coil’s terminals are provided with lumped capacitors which are used to match resonant frequencies for the coils.
EXPERIMENTAL ANALYSIS:

Figure [ 5 ]

In the figure 5 shown, there is a resonant coupling system in which a signal generator is delivering power to two illuminating L.E.Ds. This source is operating at 8.3 MHz of resonating frequency and is directly connected across the terminals of lower coils amongst the two large green colored coils which are 30 cm in diameter and are spaced 3.8 cm apart. The upper coil terminated by an adjustable lumped capacitor is inductively coupled to the lower coil. The center of the upper coil contains two small receivers and each of the receivers embeds a coil of diameter 1.3 cm which is in turn connected to adjustable lumped capacitors as described above. These two coils, each in one of the two receivers are wrapped in a black electrical tape. Thus a total of six coils are associated with the two loads. A high Q resonant coupling is established considering the fact that sources and loads are both connected to the coils that are inductively coupled to each other but at the same time remain distinct from the coils that have termination by lumped capacitances.

Figure [ 6 ]

In the single-receiver system of Fig. 6, the source drives a large single-turn coil, labeled L1 that is inductively coupled to a large multiturn resonant coil L2 of the same diameter. The small resonant coil L3 is inductively coupled to a small coil of the same diameter, labeled L4 , that is terminated by a load element. Lumped capacitors C2 and C3 respectively terminate the resonant coils L2 and L3 . The resistances R1 , R2,R3 , and R4 are the small resistances of the coils themselves, while RS is the internal resistance of the source, and RL is the load resistance.

The work described here used two identical open-circuited “self-resonant” coils, with a resonant frequency based upon the distributed inductance and distributed capacitance of each coil.
Here, with completely different source and receiver coils, the lumped capacitances are chosen so as to yield identical resonant frequencies,

(Experimental Analysis from reference [19] )

This alteration provides a simple means to achieve resonant coupling between a large source coil and one or several small receiving coils.

Transmitter Circuitry:

Figure 7. Transmitter Circuitry

Clock Generator 4CG:

Any resonant circuit and an amplifier can be used as a clock generator. The resonant circuit can be simple like an RC oscillator or a more complex arrangement like a quartz piezo-electric oscillator. The amplifier feeds a portion of signal back to the oscillator to maintain oscillations.

Phase Shift Network 4PH:

The phase shift network in Figure generates a phase difference between the clock signal 4CG and the VCO 5V output 5Va. The leading edge of the clock signal sets Latch 1 5L1, while the leading edge of the VCO output 5Va turns off the output of Latch 1. The pulse width of Latch 1 is thus proportional to the phase difference between the clock generator input 4CG and VCO output 5Va. The phase detector output of Latch 1 is then smoothed by a low pass filter, which is then passed the error amplifier, which then adjusts the VCO such that the phase detector output is equal to the phase control input..

(Transmitter and Receiver Circuitry from reference [17] )

Figure [ 8 ]

Receiver Circuitary:

Referring to Figure, the receiver control electronics circuitry 6R is equipped with a voltage regulator, which is unique to each receiving device 6D, which regulates the voltage to a specified value.

Figure [ 9 ]. Receiver Circuitry

Advantages

1. This will lead to reduction of cumbersome and ugly webs of power chords. 2. This will be highly reliable as product failure rates will reduce and convenient as no charging, changing of battery will be required. 3. No requirement of disposable batteries and will enhance the usage of electric grid power rather than battery charging.

APPLICATIONS 1. Wireless charging of mobile goods even when in use and in motion will be possible; for instance automatic charging of a laptop in a car. Direct wireless powering of stationary goods will be accomplishable which will lead to reduction of wires and cords used. 2. This technology will prove as an asset for industries using equipment where wire connections may prove hazardous, also this will eliminate expensive wiring ultimately reducing the initial cost. 3. Automatic wireless charging will eliminate the need of battery replacements. This technology can be extensively used for electric vehicle charging and moreover for future vehicles and hybrid models which could be charged while parked in a garage. 4. Wireless power interconnections and charging will serve as a boon in medical field. It finds its uses in implantable medical devices (e.g.: pacemakers).
POSSIBILITIES:

With the prolificacy in this technology, a day can be prognosticated when wireless electricity can be so common, omnipresent and ubiquitous that government can provide transmitters at regular and veritable intervals such that even common man can harness it.
“We definitely construe the instauration of what can be precisely called as ‘live wireless electricity spots’ where any one can walk in and be able to charge any sort of electronic device, wirelessly of course.”
We obviously ruminate the instauration of disjoined or separate charging zones in commercial ececis like malls, restaurants, offices, cafes etc. to make such livespots viable.
The team of Witricity, an organization working on this possibility lead by Marin Solijacic, this technology should be twice as efficacious as its today to grapple away high use of traditional batteries. The team's next aim is to get a robotic vacuum or a laptop working, charging devices placed 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 WiTricity system for commercial use.
AcknoWlEdgement
The authors are grateful to the management of OIST and LNCT, Bhopal for their overwhelming support. The authors gratefully acknowledge the support and constant encouragement of Mrs. Deepika Masand, HOD Electrical and Electronics of OIST and Mrs. Soni Changlani, HOD Electronics and communications of LNCT.

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