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October 14, 2009 > TechKnow Talk: Wireless Electricity

TechKnow Talk: Wireless Electricity

Unplugging the Future

Arriving home late after a long flight from a business trip, Janet drops her purse on the kitchen counter, her laptop on the floor nearby, and her DVD player and headphones on the table. Exhausted, she heads directly to bed. In the morning, her electronics are all fully charged, including the cell phone and GPS receiver in her purse.

This seemingly far-fetched scenario may become a reality quite soon. We usually think of electricity as something that comes from a socket in the wall, but wireless power is already in limited use in many homes and businesses and new research is yielding promising results for a future with far fewer wires.

Nikola Tesla, one of the great geniuses of modern science, demonstrated transmission of electrical energy through the air in the 1890s. The theories he evolved with such experiments are being applied today to develop the wireless power of tomorrow.

Wireless electricity relies on a relatively simple technology called inductive coupling. If an electrical current is passed through a wire, a circular magnetic field is generated around the wire: the stronger the current, the stronger the magnetic field. Conversely, a magnetic field will generate an electrical current in a wire placed within it: the stronger the magnetic field, the stronger the current.

A wire wrapped into a coil and powered with electricity produces a magnetic field many times stronger than a single strand. Now imagine a second coil of wire placed near the first. The magnetic field created from the electricity passing through the first (primary) coil interacts with the second coil and induces an electrical current through that coil. This inductive coupling is, in basic terms, how a transformer works.

A common application of this technology to consumer products is the electric toothbrush, utilizing a primary, powered coil in a cradle and a secondary coil in the base of the toothbrush. The cradle is plugged in, creating a current in its coil. The resultant magnetic field induces a current in the coil in the toothbrush, which in turn charges a battery in the handle.

Other applications are also available, such as a placemat-sized mat containing primary coils. This mat is plugged into a standard outlet, and any compatible devices placed on the mat are automatically charged. Electricity transfer from such a device is at least 80% as energy efficient as wired charging. In some situations it may be more efficient, as several devices can be charged at once and the mat can be set to power off when devices have reached full charge.

A possible future application of inductive coupling is to recharge electric cars. Placing a large mat containing a primary coil on the garage floor, the owner would simply park over it, eliminating the need to plug in the car. But recharging the batteries of an electric vehicle would require much more sizable coils, and currents, than a toothbrush or a cell phone!

In fact, there are two important limitations to this handy technology. First, the coils must be compatible or "matched" for inductive coupling to generate the proper electrical current in the secondary coil. Second, the magnetic field is relatively weak, so the coils must be placed very close together for inductive coupling to occur. Both of these problems may be solved soon.

In December 2008, the Wireless Power Consortium was formed. This body, composed primarily of consumer electronics companies, has the stated mission of creating an international wireless electricity standard to promote interoperability of rechargeable electronic devices. Devices manufactured in conformance to this standard will be marked with a stylized "Qi" symbol.

While this standard will resolve the compatibility issue, it will take technology to solve the problem of proximity. The magnetic field extends in all directions, so only the section of it occupied by the secondary coil is actually employed to recharge the device. To generate a sufficiently large magnetic field using simple inductive coupling to charge Janet's cell phone ten feet from the primary coil would require a huge coil and massive amounts of power. This would be tremendously inefficient and very possibly unsafe.

The solution is likely a technology variously called resonant induction, magnetically coupled resonance, or resonant inductive coupling. Coils used for this purpose are configured as open rings, with capacitance plates at each end. The concept involves tuning the primary coil to a very specific electromagnetic frequency that excites a resonance in the secondary coil over considerable distance.

Acoustic (sound) waves are sometimes used as an imperfect analogy for this phenomenon. A singer can shatter a wineglass with a note of sufficient volume at the precise resonant frequency of the glass. The glass vibrates, ultimately violently, when exposed to this tone.

It turns out that a properly constructed wire coil is similarly sensitive to a very specific frequency of the electromagnetic field. Though the actual mechanism of energy transfer is more closely related to an effect atomic physicists call quantum tunneling than to the transmission of an acoustic wave through the air.

Tuned to this resonant frequency, the primary coil can produce a magnetic field capable of stimulating electrical current in secondary coils some distance away, with a tiny fraction of the field strength that would be necessary using simple inductive coupling.

For Qi devices built to produce and respond to the standard resonant frequency, a primary coil could be embedded in Janet's kitchen ceiling or mounted on the wall like a smoke alarm, and simultaneously charge all the electronics in the vicinity. Because a magnetic field is not impeded by furniture or other small objects, devices would not require "line of sight" to the primary coil.

While a working prototype of this technology has not yet been adequately demonstrated, some very promising advances have been achieved in recent months. One company has even predicted consumer availability in less than a year.

Questions have been raised as to the safety of resonant coupling devices in the home or office. While low-level magnetic fields are in general not dangerous to living organisms, studies will need to be performed to ensure the highly tuned field does not present any health risks for humans.

Assuming the remaining bugs will be worked out we can look forward to the convenience of fewer tangled wire bundles. The environment should benefit as well. Replacement of devices using removable batteries with wirelessly rechargeable products may eliminate the need for some of the 40 billion disposable batteries currently discarded each year.

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