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    WiTricity Explained
    Experiments, Studies and Background Information


    WiTricity Studies and Experiments
    WiTricity Explained

    WiTricity, stands for wireless electricity, is a term coined initially by Dave Gerding in 2005 and used by a MIT research team led by prof. Marin Soljačić in 2007, to describe the ability to provide electricity to remote objects without wires (wireless power transfer). This could be useful to power consumer and industrial electronics like cell phones, laptops, etc.

    Scientists and engineers have known for nearly two centuries that transferring electric power does not require wires to be in physical contact. Electric motors and power transformers contain coils that transmit energy to each other by the phenomenon of electromagnetic induction. A current running in an emitting coil induces another current in a receiving coil; the two coils are in close proximity, but they do not touch.

    Later, scientists discovered electromagnetic radiation in the form of radio waves, but transferring energy from one point to another through ordinary electromagnetic radiation is typically very inefficient: The waves tend to spread in all directions, so most of the energy is lost to the environment. Serious interest and effort was devoted in this direction, most notably by Nikola Tesla in the late 19th century, but with little success.

    Every technician working at a radio transmission plant knows that holding a fluorescent lamp a few meters from the antenna pole will light it. But of course this method is not efficient and safe since high radiation energy is involved. In any case, don't try this experiment by any means since it could be very dangerous.

    One can also 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.

    Soljačić realized that the close-range induction taking place inside a transformer - or something similar to it - could potentially transfer energy over longer distances, say, from one end of a room to the other. Instead of irradiating the environment with electromagnetic waves, a power transmitter would fill the space around it with a "non-radiative" electromagnetic field. Energy would only be picked up by gadgets specially designed to "resonate" with the field. Most of the energy not picked up by a receiver would be reabsorbed by the emitter.

    Non-radiative wireless power is based on evanescent waves that are found in the nearfield region within one-third wavelength (a few meters in our case) of any radio antenna. During normal operation, an antenna emits electromagnetic fields into the surrounding nearfield region, then a portion of the field energy (evanescent waves) decays since it is re-absorbed by the antenna, while the remainder is radiated into the environment as EM waves.

    The novelty of WiTricity is that Soljacic and his MIT team built energy emitters (transmitters) whose electromagnetic evanescent waves radiated greater distances without significant decay. In addition, the emitter and receiver resonate with each other (resonant evanescent coupling (REC)) and the energy transfer rate is high. And no energy is transferred to other objects with different resonances, even if those objects directly block the line-of-sight between emitter and receiver.

    Non-radiative wireless power would have limited range, and the range would be shorter for smaller-size receivers. But the team calculates that an object the size of a laptop could be recharged within a few meters of the power source. Placing one source in each room could provide coverage throughout your home.

    The MIT researchers successfully demonstrated the ability to power a 60 watt light bulb from a power source that was 2 meters (7 ft) away at roughly 40% efficiency. They used two capacitively loaded copper coils, 60 centimeters (24 in) in diameter, oriented along the same axis, The coils were designed to resonate together at 10 MHz (relatively safe for living tissue). One was connected inductively to a power source, the other to a bulb. The setup powered the bulb on, even when the direct line of sight was blocked using a wooden panel.

    As for now (July 2007), the researchers plan to miniaturize the setup enough for commercial use in three to five years and suggest that the radiated power densities can be brought below the threshold for FCC safety regulations.


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    Last updated: January 2011
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