ABSTRACT:
The technology used for wireless power transmission is known as witricity. Wireless power transmission is not a new idea, Nikola Tesla proposed theories of wireless power transmission in the late 1800s and early 1900s. Tesla's work was impressive, but it did not immediately lead to wide spread practical methods for wireless power transmission. Since then many researchers have developed several techniques for moving electricity over long distances without wires. Some exist only as theories or prototypes, but others are already in use. In 2006 researchers at Massachusetts Institute of Technology led by Marine Soijacic discovered an efficient way to transfer power between coils separated by a few meters. They have dubbed this technology as “witricity”. The physical phenomenon of long-lifetime resonant electromagnetic states with localized slowly-evanescent field patterns used to transfer energy efficiently over non-negligible distances like 8 times the radius of the coil, even in the presence of extraneous environmental objects was experimentally demonstrated.
HISTORY OF WIRELESS POWER:
Wireless power transmission is not new for us. Various methods of Transmitting power wirelessly have been known for centuries. In 1899, Sir Nikola Tesla proposed a method to transmit energy over long distances wirelessly using ElectroMagnetic radiation principle, but such radiation is excellent for wireless transmission of Information, it is not feasible to Power transmission.
Directed radiation modes like Lasers or highly-directional antennas can be efficiently used for energy transfer even for long distances but require existence of an uninterruptible line-of-sight and a complicated tracking system in the case of mobile objects and also not Eco friendly. So these procedures are insufficient to Wireless power transmission.
Witricity is a new technology used for wireless power transmission. The inventors of witricity are the researchers from Massachusetts Institute of Technology (MIT). This method is mainly based on the transmission of magnetic energy by taking two Magnetically Resonant objects in Strongly Coupled regime. The source and load are coupled to the coils which are magnetically resonant and transmit energy in the magnetic form in strongly coupled regime. As they are in magnetic resonance, there is no possibility of external disturbances.
NEED OF WITRICITY:
Now a days there is a Rapid development of autonomous electronics like laptops, cell-phones, house-hold robots and all the above devices typically rely on chemical energy storage(Battery) .As they are becoming daily needs to present generation, Wireless energy transfer would be useful for many applications as above and they need midrange energy.
If we adopt this witricity concept to these devices, we may eliminate the problems caused by the batteries. We may use this by fixing on our room ceiling to charge our devices wirelessly.
BASIC PRINCIPLE:
The basic concept behind Witricity-Wireless Electricity is Magnetic Resonance. Two resonant objects of the same resonant frequency tend to exchange energy efficiently, while dissipating relatively little energy in extraneous off-resonant objects. In systems of coupled resonances, there is often a general “Strongly Coupled” regime of operation. If one can operate in that regime in a given system, the energy transfer is expected to be very efficient. Midrange power transfer implemented in this way can be nearly omni directional and efficient, irrespective of the geometry of the surrounding space, with low interference and losses into environmental objects. The above considerations apply irrespective of the physical nature of the resonances.
THEORETICAL MODEL FOR SELF RESONANT CIRCUITS:
The experimental realization of the scheme consists of two self-resonant coils.
One coil (source coil) is coupled inductively to an oscillating circuit, the other (device coil) is inductively to a resistive load. Self resonant coils relay on the interplay between distributed inductance to achieve resonance.
The two coils are made up of an electrically conducting wire of total length l and cross sectional area a, wound into a helix of n turns, radius r & height h.
When turned on ,the sender coil emits electricity in the form of a magnetic field, oscillating at a specific frequency in few MHz.
The receiver coil picks up the transmission which must also be resonating at same frequency, while the rest of the environment is unaffected.
Energy not transferred to the devices is re-absorbed by the source coil.
Figure shows the schematic of the experimental setup. A is a single copper loop of radius 25 cm that is part of the driving circuit, which outputs a sine wave with frequency 9.9 MHz. S and D are respectively the source and device coils referred to in the text. B is a loop of wire attached to the load (light bulb). The varioussrepresent direct couplings between the objects indicated by the arrows. The anglebetween coil Dand the loop A is adjusted to ensure that their direct coupling is zero. Coils S and D are aligned coaxially. The direct couplings between B and A and between B and S are negligible.
ANALYSIS:
Coil is resonant, the current and charge density profilesare/2 out of phase from each other, meaning that the real partof one is maximum when the real part of the other is zero. Equivalently,the energy contained in the coil is at certain points in timecompletely due to the current, and at other points it is completelydue to the charge. Using electromagnetic theory, we can definean effective inductanceLand an effective capacitanceCforeach coil as follows:
/ (2)
where the spatial currentJ(r) and charge density(r) are obtained respectively from thecurrent and charge densities along the isolated coil, in conjunctionwith the geometry of the object. As defined,LandChave theproperty that the energyUcontained in the coil is given by
With this relation and the equation of continuity,the resulting resonant frequency isf0= 1/[2(LC)1/2]. This coil can be treated as a standard oscillator in coupled-modetheory by defininga(t)=[(L/2)1/2]I0(t) according to the coupled mode theory.
The power dissipated can be estimated by noting that the sinusoidalprofile of the current distribution implies that the spatialaverage of the peak current squared is |I0|2/2. For a coil withnturns and made of a material with conductivity,the standard formulas for ohmic (Ro) and radiation (Rr) resistancecan be modified accordingly:
The first term in Eq. 5 is a magneticdipole radiation term (assumingr< 2c/, wherecis thespeed of light); the second term is due to the electric dipoleof the coil and is smaller than the first term for our experimentalparameters. The coupled-mode theory decay constant for the coilis therefore=(Ro+Rr)/2L, and its quality factor isQ=/2.
The coupling coefficientDScan be found from the powertransferred from the source to the device coil, assuming a steady-statesolution in which currents and charge densities vary in timeas exp(it):
WhereMis the effective mutual inductance,isthe scalar potential,Ais the vector potential, and the subscriptS indicates that the electric field is due to the source. Wethen conclude from standard coupled-mode theory arguments thatDS=SD==M/[2(LSLD1/2)]. When the distanceDbetween thecenters of the coils is much larger than their characteristicsize,scales with theD–3dependence characteristic ofdipole-dipole coupling. Bothandare functions of the frequency,and/and the efficiency are maximized for a particular valueoff, which is in the range 1 to 50 MHz for typical parametersof interest. Thus an appropriate frequency for a givencoil size,playsa major role in optimizing the power transfer.
In order for the power transfer to be efficient, we design the system such that the rate of energy transfer between the emitter and the receiver is greater than the rate of energy dissipation.This way the device can capture the energy and use it for useful work before too much of it get wasted away.
We determine the efficiency of transfer of energy taking place between the source coil and the load by measuring the current at the midpoint of the self resonant coils with a current probe.This gives a measurement of the current parameters Is and Id
Then the efficiency can be calculated PS,D=L|IS,D|2 and obtain the efficiency from =PW/(PS+PD+PW)
ADVANTAGES:
· The main advantages of this system is that we can get electricity anywhere without wires.
· The nature of power delivery is omni directional i.e. in every direction.
· Magnetic resonances are particularly suitable for everyday application because most of the common materials do not interact with magnetic fields, so interactions with environmental objects are suppressed even further.
· Significant decluttering of office space
· No need for meter rooms and electrical closets.
· Reduction of e-waste by eliminating the need for power cords
· Negative health implications - By the use of resonant coupling wave
lengths produced are far lower and thus make it harmless.
· Less costly - The components of transmitter and receivers are cheaper. So this system is less costly.
DISADVANTAGES:
· Wireless power transmission can be possible only in few meters.
· Efficiency is only about 40%.
· Retrofitting old equipment or purchasing new equipment could become a very expensive endeavor
· Possibility of “energy theft”. Wi Fi, someone can be using your internet or your power.
As witricity is in development stage, lot of work is done for improving the efficiency and distance between transmitter and receiver.
PRACTICALITY AND SAFETY:
· Witricity demonstration is still too recent, and too focused, to produce any definite conclusions.
· With the witricity method ,they anticipate transmitting power over distances about a meter, which is much less than ideal but still very impressive.
· If the technology is improved and honed to a point where it can be “productized", it stands to turn any number of industries on their respective ears.
· The MIT team said its discovery is different from all previous effort because it uses “magnetically coupled resonance", which means it will not only be safe but it will be fairly efficient.
APPLICATIONS:
Witricity has a bright future in providing wireless electricity. There are no limitations in witricity power applications. Some of the potential applications are powering of cell phones, laptops and other devices that normally run with the help of batteries or plugging in wires. Witricity applications are expected to work on the gadgets that are in close proximity to a source of wireless power, where in the gadgets charges automatically without necessarily, having to get plugged in. By the use of witricity there is no need of batteries or remembering to recharge batteries periodically. If a source is placed in each room to provide power supply to the whole house
Witricity has many medical applications. It is used for providing electric power in many commercially available medical implantable devices.
Another application of this technology includes transmission of information. It would not interfere with radio waves and it is cheap and efficient.
CONCLUSION:
Wireless Electricity concept is a boon for devices which uses midrange energy. The Power transfer is explained with the help of Magnetic resonance and Coupled mode theory.
The power transfer is not visibly affectedas humans and various everyday objects, such as metals, wood,and electronic devices large and small, are placed between thetwo coils—even in cases where they completely obstructthe line of sight between source and device.External objects have a noticeable effect only when they arewithin a few centimetres from either one of the coils. Some materials mostlyjust shift the resonant frequency, which can in principle beeasily corrected with a feedback circuit; other materials lowerQwhen placed closer thana few centimetres from the coil, thereby lowering the efficiencyof the transfer.
Witricity is in development stage, lots of work is to be done to use it for wireless power applications. Currently the project is looking for power transmission in the range of 100w. Before the establishment of this technology the detailed study must be done to check whether it cause any harm on any living beings.
REFERENCES:
An article published in the Science Magazine as “Wireless Power Transfer via Strongly Coupled Magnetic Resonances” by Andre kurs, Science 317,83(2007);Dol:10.1126/science.1143254.
H. Haus , “Waves and Fields in Optoelectronics”, Prentice hall Publishers, Englewood.
http://web.mit.edu/newsoffice
A. Karalis, J. D. Joannopoulos, M. Soljai,Ann. Phys., 10.1016/j.aop.2007.04.017 (2007).
N. Tesla, U.S. patent 1,119,732 (1914).