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Matthew Traum

MIT Team Invents “WiTricity” Wireless Energy Transfer

The scientific discovery necessary for a truly wireless existence has just been made! All laptop owners know that while long battery life and wireless Bluetooth peripherals provide the illusion of portable computing, the un-tethered laptop is really just a fish out of water. Eventually, batteries die and peripherals must be recharged. The laptop inevitably returns to its cage of wires after each brief stint of freedom.

That is all about to change.

An MIT team led by Prof. Marin Soljacic demonstrated lighting a 60W bulb from a power source seven feet distant with no physical connection between the source and the receiver. The concept, called “WiTrcity” (short for Wireless Electricity), was reported in the June 7 issue of Science Express. Additional details are posted in an MIT press release, “Goodbye Wires…”.

WiTricity has attracted significant buzz on the Internet. However, many people are asking why this technique is considered new. Based on the comments I’ve seen, everyone but me has built their own energy beaming station; including one guy who apparently wrapped a bunch of wire around his water tower to steal stray electricity from the power company, which built a high-voltage line across his property against his wishes.

So, what is new about the MIT discovery? Are they not simply “beaming” energy from point to point like everyone else? Since the late 19^th Century, various researchers have demonstrated energy “beaming” techniques. The prevailing technologies include broadcasting electromagnetic waves from a power source to a distant receiver (as demonstrated by Hertz, Tesla, and Marconi) or confining energy in a beam of photons and aiming it at a receiver (as demonstrated by NASA and others).

Unfortunately these solutions have drawbacks. While excellent for wireless information transmission (i.e., radio communication), electromagnetic waves are omni-directional and dissipative, making this technique inefficient for useful energy “beaming”. Confined energy beams require line-of-sight contact between emitter and receiver, and existing methods of creating beams of photons (i.e., lasers) are too inefficient to make this technique practical either.

Enter “resonant evanescent coupling (REC)”, a special form of evanescent wave coupling.

One way to think about REC is to note that wave phenomena include both primary waves and evanescent waves. The later are not normally considered in engineering analysis because they usually decay rapidly very close to the object that created them. Although evanescent waves are often ignored by engineers, they do result in very real and sometimes counterintuitive behavior. For example, tunneling, the phenomenon associated with elections spontaneously leaping through impenetrable barriers, results from the electron’s evanescent waves.

The novelty of WiTricity is that Soljacic and his MIT team intentionally built energy emitters and receivers whose electromagnetic evanescent waves radiated great distances without significant decay. In addition, these evanescent fields were tuned to resonate with each other. In other words, the evanescent fields of the emitter and receiver “see” each other, but they can’t see extraneous objects with different resonances, even if those objects directly block the line-of-sight between emitter and receiver. The coupling of evanescent waves also makes this technique substantially more efficient than “beaming” energy via conventional electromagnetic radiation because these waves don’t dissipate energy to the environment.

According to Soljacic’s Science Express paper, the energy transfer efficiency of REC is about 40% over a 2-meter distance, and the MIT researchers estimate that REC can transfer 1,000,000 times more energy wirelessly than conventional electromagnetic waves. Importantly, transferring identical quantities of energy over the same distance using a copper wire of ample diameter would incur negligible losses. Nonetheless, while it can’t compete with wired energy transfer, WiTricity represents a quantum leap in wireless energy transmission.

Thus, I predict that in the not-too-distant future, coffee shops will be offering both WiFi and WiTricity (in some form) to their wireless patrons. Laptops and peripherals with embedded WiTricity receivers will finally be set free from their power cord tethers.

By the way, this posting would not be complete without mention of a competing wireless energy transmission technology. Ligonier, PA based Powercast, LLC has pioneered RF energy transmission and harvesting techniques derived from RFID tag energy scavenging technology. I will leave coverage of this company and their technology for a future post.