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Non-Radiative Wireless

Power Transfer So efficient power transfer is possible. But there are still other factors : Our energy-transfer application requires resonant modes of high Q=ω/2Γ for slow intrinsic-loss rates Γ, and hence this coupling is implemented using, not the lossy radiative far-field, but the non-lossy stationary near-field.

Strong (fast) coupling rate κ is required over distances larger than the characteristic sizes of the objects.

The extent of the near-field into the air surrounding a resonant object is set typically by the wavelength. Large wavelength => low frequency

We will operate at radio frequencies f ~ 1-10Mhz, 𝛌 ~ 30 – 300m Practical Considerations Why Near Fields Why not Radiation

Near and Far Fields

Inductive Coupling

Resonant Inductive Coupling

Strong Coupling and Efficiency

Alternative perspective on Strong Coupling

Other Factors affecting Power Transfer

Health and Safety Considerations

Applications

References OVERVIEW Simply put, freedom from wires. From all the charging cords and cables for laptops, mobiles, etc that populate our rooms and homes. Motivation(s) Aristeidis Karalis, John D. Joannopoulos, Marin Soljacic. “Efficient wireless non-radiative mid-range energy transfer”, Ann. Phys, volume 323, pp. 34-48, (2008).

A. Kurs, A. Karalis, R. Moffatt, J.D. Joannopoulos, P.Fisher, M. Soljacic “Wireless power transfer via strongly coupled magnetic resonances”, Science, volume 317, pp. 83-86, (2007).

F.Z. Shen, W.Z. Cui, W. Ma, J.T. Huangfu, L.X. Ran Circuit Analysis of Wireless Power Transfer by “Coupled Magnetic Resonance”

David Schneider; “A critical look at wireless power”

Wikipedia: Resonant Inductive Coupling, Near and Far Fields

http://www.witricity.com/pages/technology.html References Consider a source and a device at a big distance from each other. Therefore efficient power transfer is not possible between them. But, if I keep another coil between them, both the source and device can couple with them and thus,wireless transfer of energy becomes possible over larger distances and between perpendicularly oriented coils.

The distance over which energy can transferred can be extended with a high-Q resonating repeater - a tuned resonant coil that can be packaged as a very thin and flexible substrate. It is not connected by wire to an energy source; instead, it enables the magnetic field created by the source resonator to "hop" over a large distance from the source to a capture device. Energy Hopping – a wireless extension cord!! Non-Radiative energy transfer. Fields are generated at radio frequencies – Safe.

This method high magnitude magnetic fields. The electric field is concentrated in the region between the capacitor plates.

Magnetic fields interact very weakly with biological organisms - people and animals - and are scientifically regarded to be safe. Health Issues Strong Coupling : An alternative definition In 1894 Nikola Tesla used resonant inductive coupling, also known as "electro-dynamic induction" to wirelessly light up phosphorescent , incandescent lamps at the 35 South Fifth Avenue Lab, Chicago Springs Faraday’s Experiments (1830’s) : Magnetic fields can induce charge and force on current carrying wires, basis for transformers But What Advances ? Efficiency of the system depends largely on tuning the source and device to the same resonant frequency.

Hence, for a given source and an arbitrary object, the energy transfer is very weak i.e. no energy is lost to it.

Thus the system is weakly affected by presence of other objects in the vicinity. Effects of Extraneous Objects Realization – Lumped Capacitor Coils Only resonance is not enough to achieve high efficiency. The coils need to be ‘strongly coupled’.

Consider two coils S and D (none of them driven).

The Coupling equations are : Strong Coupling : An alternative definition But even with resonance as shown, what really matters is how much of the input energy we are able to get across to the load.

So calculating efficiency : Efficiency Quality Factor In general, the fields of a source in a homogeneous isotropic medium can be written as a multipole expansion.

Far away, fields decay in proportion to 1/r where r is the distance from the source. These are the radiating fields. Near and Far Fields : Briefly Experiment performed at MIT in 2007 60 W bulb

Lit at distance of 2m

Efficiency ~45 % Mid-Range Transfer We could have cars running on sources embedded

inside roads House without wires!! Applications In fact, Tesla had also achieved something similar in the early 1900’s with his Tesla Coils, although with a much larger power consumption To reduce the reactive power, the imaginary part should be eliminated in the equivalent impedance

One way is to add capacitances on both sides, in a manner exactly analogous to creating a resonating stand-alone LC oscillator. Resonant Inductive Coupling Faraday’s Law: Primary generates a magnetic field, inducing potential difference in a secondary coil . Inductive Coupling

For an isolated coil with a source,

Solving the two equations,

Overall efficiency of the system (with some load w):

On simplifying, efficiency essentially depends on the dimensionless quantity : Abhishek Khanna 10D070045

Puneet Arora 10D070010 Deeper Question :

With all the advances in understanding of EM technology since Maxwell and Tesla, why haven’t we still found a reliable, convenient and widespread method for transferring power wirelessly over human-sized distances (~1- 5 m) atleast ? Electric Car charging through inductive coils placed in garages

Close contact wireless chargers for mobile phones already being sold But all these technologies rely on extremely close

distances between source and devices Wastage For any Radiative field, once the energy is put into space it will continuously move outward , i.e. radiate.

So even if there doesn’t exist a device to receive energy, that energy is forever lost. In contrast , in a Reactive (near) field, the energy (ideally) continuously oscillates between the source and space, until it is absorbed by a device. Interference Given the dependent nature of the E and B fields, any material affecting one will also attenuate the other Since E and B can evolve separetly, one can exist even if the other is absorbed or redirected

Eg : non-ferrous materials like copper would stop the E field, but not the B field Safety Techniques exist to limit this power transfer to magnetic fields, which are usually less dangerous to human bodies

Moreover, until 'resonance' occurs very little power is transferred even with high magnitude of fields Any significant power transfer will involve high frequency fields which carry more energy and are morereadily absorbed by our bodies COUPLING What if there is an object between the device and source? How robust is the system? Such high energy fields are bound to have health hazards. INTRO FIELD ZONES PRACTICAL REALIZATION APPLICATIONS 2 ways to look at coupling The next term that becomes significant is proportional to 1/r^2 and is called the induction term. It can be thought of as the primarily magnetic energy stored in the field, and returned to the antenna in every half-cycle, through self-induction. Beyond this, all the terms decaying as 1/r^3, 1/r^4 ... can be clubbed together as an exponentially decayin wave, and are called the evanescent waves Device:

Subwavelength (𝛌 >> size of object) resonances can often be accompanied with a high radiation-Q, so this will typically be the appropriate choice for the possibly-mobile resonant device-object d.

Source:

The resonant source-object S will in practice often be immobile and with less stringent restrictions on its allowed geometry and size.

Therefore chosen large enough that the near-field extent is not limited by the wavelength.

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