Professor Marin Soljacic and team members Andre Kurs, Aristeidis Karalis, Robert Moffatt, Prof. Peter Fisher, and Prof. John Joannopoulos realized their theoretical prediction by lighting a 60 W light bulb from a power source more than 2m away with no physical connection between the source and the appliance. The MIT team refers to its concept of wireless electricity as “WiTricity.”
The Beep That Lit a Light Bulb
The initial idea came to Soljacic one night as he stared at his cell phone on the kitchen counter. “It was probably the sixth time that month that I was awakened by my cell phone beeping to let me know that I had forgotten to charge it,” he says. “It occurred to me that it would be so great if the thing took care of its own charging.”
Methods of transmitting power wirelessly have been known for centuries. Perhaps the best known example is electromagnetic radiation, such as radio waves. While such radiation is excellent for wireless transmission of information, it is not feasible for power transmission. Since radiation spreads in all directions, a vast majority of power would end up being wasted into free space.
We can envision using directed electromagnetic radiation such as lasers but this is not very practical and can even be dangerous. It requires an uninterrupted line of sight between the source and the device, as well as a sophisticated tracking mechanism when the device is mobile.
Magnetically coupled resonance seemed like a more efficient and much safer solution. WiTricity is based on using coupled resonant objects. Two resonant objects of the same resonant frequency tend to exchange energy efficiently, while interacting weakly with extraneous off-resonant objects, say the MIT scientists. So, imagine a room with 100 identical wine glasses, each filled with wine up to a different level, so they all have different resonant frequencies. If an opera singer sings a sufficiently loud single note inside the room, a glass of the corresponding frequency might accumulate sufficient energy to actually explode, while not influencing the other glasses. 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.
While these considerations are universal, applying to all kinds of resonances, the MIT team focused on one particular type: magnetically coupled resonators. The team 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.
The Tie That Binds
The scientists say 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,” explains Kurs, a graduate student in physics.
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 designed specially to resonate with the 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.
“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,” explains Moffatt, an MIT undergraduate in physics. “With such a design, power transfer has a limited range, and the range would be shorter for smaller-size receivers.”
Still, the experimenters suggest that for laptop-sized coils, power levels more than sufficient to run a laptop can be transferred over room-sized distances nearly omni-directionally and efficiently.
At first glance, say the scientists, such a power transfer is reminiscent of relatively commonplace magnetic induction, such as is used in power transformers, which contain coils that transmit power to each other over very short distances. An electric current running in a sending coil induces another current in a receiving coil. The two coils are very close, but they do not touch. However, this behavior changes dramatically when the distance between the coils is increased. As Karalis, a graduate student in electrical engineering and computer science, points out, “Here is where the magic of the resonant coupling comes about. The usual non-resonant magnetic induction would be almost 1 million times less efficient in this particular system.”
The work was funded by the Army Research Office (Institute for Soldier Nanotechnologies), National Science Foundation (Center for Materials Science and Engineering), and the Department of Energy.