Self-Resonant Cooper CoilA month ago I wrote a post “World Without Wires” about the unique material developed by Japanese scientists that can transmit electrical energy to nearby devices without the need for direct contact. Now in today’s issue of Science magazine, an MIT research team led by Prof. Marin Soljacic reported its version of the “world without wires” - “Wireless Power Transfer via Strongly Coupled Magnetic Resonances“.

The team was able to wirelessly transfer power to a 60W light bulb from 7 feet away with 40% efficiency but unlike the Japanese method, it neither involves the pentacene based organic transistors, nor the magnetic induction.

Prof. Marin Soljacic decided to reexamine a century old problem of wireless electricity when he was awakened for the sixth time during the night in a single month by his beeping cell phone that had run out of charge - “It occurred to me that it would be so great if the thing took care of its own charging.”

The most known method of the wireless power transfer is electromagnetic radiation (radio waves); however, the efficiency of this method is extremely low because the radiation spreads omnidirectionally and the majority of power is wasted in space. But the directional or laser approach is not acceptable solution either, since it would require an uninterrupted line of sight between the source and the device, and it can be potentially harmful.

In the 1910s, Nikola Tesla dedicated a lot of time attempting to create a system that could transfer power wirelessly; however, the large coils that he used in his experiments emitted very strong electromagnetic fields and hence could not find practical applications.

The MIT research team’s newly proposed method, which they refer to as “WiTricity”, is based on a near-field magnetic resonance principle - where two resonant objects of the same resonant frequency tend to exchange energy efficiently, while interacting weakly with extraneous none-resonant objects. A good trivial example of the acoustic resonance coupled system is a room with 100 wine glasses filled to various levels, and hence, having different resonance frequencies. If an opera singer sings a note with the frequency that matches one of the glasses, then only that glass will resonate and could potentially explode if she sings powerful enough without affecting the rest.

” In any system of coupled resonators there often exists a so-called “strongly coupled” regime of operation. If one ensures to operate in that regime in a given system, the energy transfer can be very efficient.”

Soljacic’s team investigated a system of two electromagnetic resonators coupled through their magnetic fields - two copper coils: one coil attached to the power source that emits non-radiative magnetic field oscillating at MHz frequencies, and the other receiving unit that resonates in the field. It turned out that the strongly coupled regime in this system is achieved when the distance between the coils was several times larger than their sizes.

Image © Science

At first glance, such a power transfer is reminiscent of the usual magnetic induction; however, note that the common non-resonant induction is extremely inefficient for mid-range applications where the efficiency drops to 1 millionth.

The resonant nature of the process ensures the strong interaction between the sending unit and the receiving unit, while the interaction with the rest of the environment is weak. With this design, power transfer has a limited range, and the range would be shorter for smaller-size receivers. Still, 13″ coils are capable to sufficiently power a laptop and can transfer electricity over room-sized distances nearly omnidirectionally and efficiently, irrespective of the geometry of the surrounding space, even when objects completely obstruct the line-of-sight between the two coils.

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