While scientists at the turn of the 20th century were experimenting with the wireless transmission of information, like radio, Croatian-born inventor Nikola Tesla had a grander vision (Fig. 1). He imagined the wireless transmission of power—to supply "light, heat, or motive power anywhere—on sea, or land, or high in the air," he told The New York Times in 1904.
With the help of the October 2006 film The Prestige, where a fictionalized version of the physicist helps an aspiring illusionist develop a magic trick, it's rumored that Tesla was able to light a field of 200 incandescent bulbs from a power source 25 miles away during early experiments at his Colorado Springs laboratory.
No one's sure that happened. Yet Tesla did hold several patents relating to the wireless transmission of energy, part of his wireless "World System," which would send both information and electricity around the globe.
Over time, Tesla's ideas were met with some skepticism as he became reclusive and made bizarre claims like being able to communicate with Mars. Even though the press came to rely on him solely for scientific prophecy, much of Tesla's vision for wireless power transfer is coming into fruition today, through startup companies and research institutions.
WIRELESS POWER The most widespread use of wireless power transfer, albeit on a very small scale, is in passive RFID tags. The tag has no power source, so it relies on the electrical current of an incoming radio frequency signal to power up its CMOS chip and transmit a response.
Bigger wireless-power initiatives debuted at the 2007 Consumer Electronics Show with companies like Powercast and Fulton Innovation's eCoupled technology. A Powercast transmitter can send juice up to 10 feet over RF signals. A receiver that's either built in or attached with an adapter converts the waves into electricity to charge small devices like phones or cameras.
Rather than radio frequency, eCoupled coils transfer energy through inductive coupling, which relies on a shared electromagnetic field (see "Charge Your Gadgets Without Plugging Them In."). Visteon has already used eCoupled induction coils to create wireless cell-phone and MP3 player chargers, which are expected to be released later this year.
While these applications are practical, researchers at the Massachusetts Institute of Technology have come much closer to Tesla's grander vision. At the same time that The Prestige was released last fall, the researchers—led by professor Marin Stoljacic, a great admirer of Tesla's—outlined a way to juice bigger devices without wires. By June, the team lit a 60-W light bulb from a power source seven feet away (Fig. 2).
MIT's "WiTricity" (for "wireless electricity") also relies on magnetic induction to transfer power, but with an added component—resonance. All objects resonate. When two objects resonate at the same frequency, they exchange energy efficiently. It's similar to how a wine glass bursts if it resonates in tune with an opera singer's voice.
The team used two copper coils. The sending coil gave off a non-radiative magnetic field oscillating at megahertz frequencies that resonated in unison with the receiving coil. The coils exchanged energy solely with each other, and any power not picked up by the receiving coil remained bound to the area of the sending coil. Power didn't radiate into the environment, and objects placed between the coils didn't hamper the energy transfer.
The researchers acknowledged that the transfer seems similar to standard magnetic induction used in transformers today, but they also said that such methods can only send power over short distances. Resonant coupling enables magnetic induction to work over greater distances.
And to those who argue Tesla already demonstrated resonant coupling in wireless power transfer, the team wrote in its paper that Tesla's incarnations, like Tesla coils, radiated energy inefficiently in all directions. Stoljacic and his team intend to demonstrate greater efficiency than the 40% that their experiment achieved.
Related to "Tesla's power initiatives," industry giant ABB has fully functional, and commercially available wireless power delivery systems used (at this time) for industrial wireless I/O (sensors, actuators, pneumatics, robotic I/O.) There are varied options with it, but one standard approach is to wirelessly power up to 360 sensors and other I/O (and their individual wireless comms transceivers) within a volume of space (workcell) about 10x10x20 cubic feet. The result is totally wireless field I/O - for those myriad applications where the field wiring is a liability. I think both Kristina Fiore and MIT's Marin Stoljacic would thoroughly enjoy learning more about it. ABB's product category "WISA" (Wireless Interface for Sensors and Actuators) would've fit well into your Sept 13th article, supporting all of your premises about Tesla's ideas. Let me know if you're interested and I'll put Electronic Design in touch with details and our inventors. Chris Jeffrey Business Development Area Manager ABB Low-voltage Products & Systems Wichita Falls, TX 76302 940-397-7113
Anonymous -December 03, 2007
Anyone interested in wireless power transfer should check out the work of Dr. Konstantin Meyl.
Anonymous -October 30, 2007
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