8 June 2007

Wireless Power Transfer Revisited

by Kate Melville

In 1904, renowned electrical impresario Nikola Tesla proposed a method whereby electrical energy could be transmitted without wires by using high frequency alternating current potential differences between two plates. For some reason (possibly the inconvenience of having two enormous, highly charged slabs of metal on either side of a toaster to make it work) the idea never caught on, denying Tesla of his dream to wirelessly transmit electricity around the world.

But now, electricity sans wires has been revisited by a team from the Massachusetts Institute of Technology who describe, in the latest issue of Science, how they managed to light a 60W bulb from across a room using strongly coupled magnetic resonators. Lead researcher, MIT's Professor Marin Soljacic, has dubbed the new transmission method "WiTricity".

Soljacic explained that WiTricity is based on using two objects with the same resonant frequency, allowing them to exchange energy efficiently, while not interacting strongly with extraneous off-resonant objects. He provides the example of 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 even explode, while not influencing the other glasses. Such strongly coupled systems have the ability of allowing relatively efficient energy transfer.

The MIT team focused on one particular type of resonant system - magnetically coupled resonators. The team explored a system of two electromagnetic resonators coupled mostly through their magnetic fields. 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," co-researcher Andre Kurs noted.

The system consists of two copper coils, one a transmitter and the other a receiver, where each is a self-resonant system. The transmitter, instead of irradiating the environment with electromagnetic waves, 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 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.

Using a non-radiative field means 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. The team claims that for laptop computers, power levels more than sufficient to run the machine could be transferred over room-sized distances nearly omni-directionally and efficiently, irrespective of the geometry of the surrounding space, even when objects completely obstruct the line-of-sight between the two coils.

The team believes that society's current infatuation with all things wireless will prove fertile ground for WiTricity. "In the past, there was no great demand for such a system, so people did not have a strong motivation to look into it," they note. "[But] over the past several years, portable electronic devices, such as laptops, cell phones, iPods and even household robots have become widespread, all of which require batteries that need to be recharged often. Hopefully, we will be getting rid of some more wires - and also batteries - soon"

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Electronic Maladies
Chips Themselves Go Wireless

Source: Massachusetts Institute of Technology