Saturday, December 17, 2011

Cutting the Cord: Wireless Power

Tesla tames the lightning; picture from here
At the beginning of the 20th Century, Nikola Tesla worked to broadcast electric power through the air, bypassing power lines and making it available to everybody within range of his transmitter.  He was largely ignored.  At the beginning of the 21st Century, the world still mostly made do with wires for transmissions. However, this changed quickly. For example, wireless routers, once a high-end item for hard-core techies, now can be found in living rooms across America.

Broadcasting encrypted data and sending power through the air are different goals, but there are signs that dependence on wires for the final leg of power transmission are crumbling, at least on the small scale. While nobody is currently working on the scale of Tesla, several companies are now offering commercial wireless electrical power solutions, with the clear potential for residential use in the relatively near future. None of them have the ability to keep a house on the grid without a wire, but two of them are able to reduce the proliferation of cords coming out of your wall.

WiTricity, based in Watertown, Massachusetts, uses resonant magnetic coupling to wirelessly send power through the air. MIT Professor Marin Soljačić thought about the idea late one night while his wife slept, her cell phone beeping its low-power warning. He asked himself why there wasn’t a way to tap into any of the electricity flowing through his house to simply recharge the phone without plugging it in. He worked out the math in the middle of the night, got a lab-full of people at MIT to help test the theory, and a short time later WiTricity was born.


TED Talk by Eric Giler, WiTricity CEO

The coupling works like this: a base station connects to a wall outlet, much like any other electrically-powered device. This station uses a small fraction of the power from the connection to generate a magnetic field oscillating at a particular magnetic frequency. The station sends power to receiver circuits, called “capture devices,” that resonate on the same magnetic frequency. The range of the transmission varies with the size of the circuit used in the base station, but a grid of receiver circuits, or spaced layers, can expand the range of transmission. The magnetic field is asymmetrical in shape, varying with the size and shape of the circuit. It can face interference from metal objects, but only if the metal can completely block the field. Because it is generated magnetically, instead of using broadcast energy, even a high-powered field is safer for human health than your average cell phone.

The base stations are not power sinks. They are designed to stay in a low-power ready state when no device needs to draw power--much like a modern computer or plasma TV--only typically with a lower power draw than either.

The WiTricity system is good at moving relatively large amounts of power. Current regulations cap the maximum power sent magnetically at 3.3 kilowatts, for safety purposes. While their power movement is good, and their maximum efficiency is 95% of moved power received, they do have a limiting factor of relatively low range, though it is much better than the traditional magnetic induction range measured in millimeters. Without a linked circuit extending the range, WiTricity electrical transfer has a practical limit of a few meters for large circuits and under a meter for small circuits. 

The company shows great potential. It is deep in talks with medical implant companies, looking at those whose devices are limited by a wired battery for recharging. Wires going into the body are an easy point of infection. Speaking at IDEAS Boston, CEO Eric Giler mentioned that infections enter the body through the entrance made by these wires, making wire-site infection the leading cause of death for people implanted with these otherwise-lifesaving devices; by eliminating the wire while allowing batteries to recharge, people whose lives have been saved by battery-powered medical implants should have an increased survival rate. 

Trickle-charging items from cell-phones to electric cars to medical devices can be done safely and securely, and the proper circuit can be added onto a battery. When I went to WiTricity's offices, marketing director Yinon Weiss showed how the receiver circuits can even be used to completely replace a battery. The wireless optical mouse felt much lighter without the batteries in it, but it worked just as well. To date, the main creative use of WiTricity has been for expansive lighting displays: some lighting designers decided that they could make wonderful displays and chandeliers even better by eliminating the wires used to power them. They are clearly on to something; I would enjoy a network that could charge my laptop and light my house.

Powercast is another company that sends out power without the need for wires between stations. Based in Pittsburgh, Pennsylvania, this company’s technology is based on transmitting power via radio waves, using spectrum that is currently mostly unused in the US. Their system is straightforward: a base station draws power from the wall and broadcasts it in a 60º arc to receiver stations. The company has been working to expand the range and efficiency of their units. While right now the system can manage approximately a 13 meter range, the power conversion from radio waves to direct current is currently at a maximum of 70%. 
Powercast kit for wireless sensors, model number P2110-EVAL-01
The company’s largest limitation is a legal one as opposed to a technological one. The US and Canada, the only places where Powercast currently does business, has strict limits on how much power may be broadcast using radio waves. The legal limit is 4 watts; the company keeps its technology to 3 watts, likely to avoid accidentally hitting the limit. The actual transmitter only pulls 1.5 watts to broadcast 3.

Three watts might not seem like a lot of power; in truth, it isn’t. This company shines with low power applications, such as a sensor network. While a laptop or even a cell-phone draw more power than the Powercast system can easily recharge, a wide-range fire alarm system is ideal: it does not use very much power but does require a fair amount of maintenance. According to Vice President Harry Ostaffe, by installing an appropriate grid of RF transmitters and the company’s “Powerharvester Receivers,” a business could put up a full suite of low-power sensors without the need to pay for hardwiring, or paying the costs of relatively high-maintenance batteries. The system would not be suitable for a lighting system, even one with low-power LEDs, but it would work well for low-power trickle charging for items like a remote control. The company focuses on items which currently use battery power that lasts from weeks to years.

Powercast is not sitting on its laurels. It keeps plugging away to improve its technology and keep it simple. Ease of use is a very important part of the company’s vision, with deployable plug-and-play solutions that do not require a licensed engineer to figure out. While currently it is working on business and military applications, home solutions are not out of the question sometime in the future. I for one would love to have fire alarms that never beep due to low power ever again.

If any companies have the know-how to make the age-old dream into a reality for the common man, WiTricity and Powercast are those companies. While Nikola Tesla would likely shake his head at the current state of wireless power, he would be happy that at least some people are finally taking it seriously.

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