Wouldn't it be great if your cell phone could recharge itself when you forget to plug it in? Based on experiments going on at MIT, that day may be coming soon. A team from MIT's Department of Physics, Department of Electrical Engineering and Computer Science, and Institute for Soldier Nanotechnologies (ISN) has demonstrated an important step toward achieving wireless power transfer. The team was able to light a 60 W light bulb from a power source seven feet away with no physical connection between the source and appliance. The MIT team refers to its concept as “WiTricity,” wireless electricity, the same idea Nikola Tesla worked on during the early 1900s.

Methods of transmitting power wirelessly have been known for centuries, such as the electromagnetic radiation used in radio waves. While this type of radiation works well for wireless information transmission, it's not feasible for power transmission. Since radiation spreads in all directions, the majority of power would end up being wasted into space. Another option is directed electromagnetic radiation, such as lasers, but this is impractical and dangerous. It also requires an uninterrupted line of sight between source and device and a sophisticated tracking mechanism when the device is mobile.

WiTricity is based on using coupled resonant objects. Two resonant objects of the same frequency exchange energy efficiently, while interacting weakly with extraneous off-resonant objects. 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, energy transfer can be very efficient.

The MIT team focused on magnetically coupled resonators. Researchers 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 objects.

Magnetic coupling is particularly suitable for everyday applications because most materials interact only weakly with magnetic fields, so interactions with environmental objects are suppressed even further, according to the MIT team. The team's design consisted of two copper coils, each a self-resonant system. The coil 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 designed to resonate with the field.

With this system, power sufficient to run a laptop computer can be transferred over room-sized distances, even when environmental objects obstruct the line of sight between the coils. As long as the laptop is in a room equipped with a source of this wireless power, it would charge automatically without having to be plugged in and wouldn't need a battery either. For more information, visit www.mit.edu.