MIT Demos Wireless Power Transfer

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A team from MIT has experimentally demonstrated an important step toward accomplishing the goal of wireless power transfer. This brings them closer to their goal: smartphones, laptops, MP3 players and other portable electronics capable of charging themselves without ever being plugged in.

This group was able to supply power to a 60W light bulb from a source seven feet away. There was no physical connection between the source and the appliance.

How It Works

The MIT team refers to its concept as "WiTricity" (as in wireless electricity).

WiTricity is based on using coupled resonant objects. Two objects of the same resonant frequency tend to exchange energy efficiently, while interacting weakly with extraneous off-resonant objects.

Specifically, the MIT team focused on one particular type: magnetically coupled resonators. They explored a system of two electromagnetic resonators coupled mostly through their magnetic fields; they were able to identify the strongly coupled regime in this system, even when the distance between them was several times larger than the sizes of the resonant objects. This way, efficient power transfer was enabled.

Magnetic coupling is particularly suitable for everyday applications because most common materials interact only very weakly with magnetic fields, so interactions with extraneous environmental objects are suppressed even further. "The fact that magnetic fields interact so weakly with biological organisms is also important for safety considerations," Andre Kurs, a graduate student in physics on the team, points out.

The investigated design consists of two copper coils, each a self-resonant system. One of the coils, attached to the power source, is the sending unit. Instead of irradiating the environment with electromagnetic waves, it fills the space around it with a non-radiative magnetic field oscillating at MHz frequencies.

The non-radiative field mediates the power exchange with the other coil (the receiving unit), which is specially designed to resonate with this field. 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.

Robert Moffatt, an MIT undergraduate in physics, explains: "The crucial advantage of using the non-radiative field lies in the fact that most of the power not picked up by the receiving coil remains bound to the vicinity of the sending unit, instead of being radiated into the environment and lost." With such a design, power transfer has a limited range, and the range would be shorter for smaller-size receivers.

Powering a Laptop

The MIT group says that their discovery allows power levels more than sufficient to run a laptop to be transferred over room-sized distances nearly omni-directionally and efficiently, irrespective of the geometry of the surrounding space, even when environmental objects completely obstruct the line-of-sight between the two coils.

Prof. Peter points out: "As long as the laptop is in a room equipped with a source of such wireless power, it would charge automatically, without having to be plugged in. In fact, it would not even need a battery to operate inside of such a room."

The team believes their invention could, in the long run, reduce our society’s dependence on batteries, which are currently heavy and expensive.

Cast of Characters

The group behind this discovery is made up of members from MIT’s Department of Physics, Department of Electrical Engineering and Computer Science, and Institute for Soldier Nanotechnologies (ISN). The team members are Andre Kurs, Aristeidis Karalis, Robert Moffatt, Prof. Peter Fisher, and Prof. John Joannopoulos (Francis Wright Davis Chair and director of ISN), led by Prof. Marin Soljacic.

Their work has been reported in the latest issue of Science Express, the advance online publication of the journal Science.

It was funded by the Army Research Office (Institute for Soldier Nanotechnologies), National Science Foundation, and the Department of Energy.




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