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Words: Mike Farish
Many people know what the term Wi-Fi means – the ability to link a computer to the internet without having to connect a wire to the device to transfer data into and out of it. It is something that seemed astonishing just a few years ago but which is now taken for granted, whether it takes the form of “hotspots” in airport lounges or a wireless modem in the home.
But what about “Wi-Tri” – the ability to run a computer without a power lead connecting it to a mains electricity supply or a cumbersome on-board battery? That term is not yet part of the language because, although a computer can now access almost any type of information from the internet without that need for a physical connection, the same is not true for the electricity needed to power it. Instead, it still needs to be connected to a mains supply by a plug and lead or, alternatively, draw power from a battery that will nevertheless become exhausted after just a few hours.
But what if that need for a power line or battery could be abolished as decisively as the need for a data cable? What if a device that relies on electricity could be continuously powered in an apparently virtual manner, as if it were literally drawing electricity out of the air around it?
Wireless electricity is not a fantasy
The consequences might be many and varied – apart from possibly adding a new word to everyday vocabulary. People in their own home might be relieved of the irritation of finding that a device such as a remote controller for a television set was suddenly and frustratingly inoperable. Factory workers would not find a forklift truck grinding to a halt because someone had omitted to recharge its batteries the previous night. The need for specialist recycling facilities to deal with the chemicals contained in defunct batteries might be drastically reduced.
Wi-Tri, as it turns out, is not a fantasy. Within the past few months a team of researchers at the Massachusetts Institute of Technology (MIT) in the US East Coast city of Boston has illuminated a 60-watt light bulb without a mains connection or a battery via a power source that was roughly two metres away. They have done so with the aid of a technique they have dubbed “WiTricity” and though the development is still no more than a laboratory experiment, all the potential applications that stem from that achievement have started to edge towards becoming a practical reality.
The roots of this potentially highly significant development lie in a familiar and mundane incident. Just a few years ago, Professor Marin Soljacic found himself woken in the middle of the night by his mobile telephone warning him that the device’s batteries were running low. “It was probably the sixth time that month that I was awakened by my cell phone beeping to let me know I had forgotten to charge it,” he recalls. But that night, his irritation at having his sleep disturbed prompted him to wonder if there could be a technology that would prevent it happening again. Moreover, given that Professor Soljacic teaches physics at MIT, he was in a position to turn his question into a scientific research project and he duly formed a team to pursue the issue. 
Brain storms and design hurdles
The team’s objective was to find a way to transmit electrical energy efficiently and without wires between a power source and a device that would use that energy. Their starting point was the recognition that the wireless transmission of power is in itself a well-known phenomenon. The best-known example in the modern world is television and radio broadcasting, in which data is transmitted as electromagnetic radiation. Another example, once the stuff of science fiction but now so commonplace that it is taken for granted, is a laser, which focuses light into a beam of concentrated energy so intense that it can burn through metal. Yet another is the transformer in which an electric current flowing through a coil of wire creates an electromagnetic field that induces a current to flow in a similar coil that is co-located, but not actually in physical contact with it.
But all three of those examples also illustrate the major problems that any system of practicable wireless electricity supply would have to overcome. Broadcasting is omni-directional – the signals from an antenna spread out uniformly in all directions – an obvious necessity when the aim is to enable people in multiple, scattered locations to tune in to a programme, but hopelessly inefficient as a means of transmitting power to a specific device because so much of the energy involved would be scattered and lost.
A laser, in contrast, does provide a direct means of energy transmission between two locations but requires a direct, unimpeded line-of-sight between them and is also difficult to keep targeted when either of them is in motion.
Meanwhile a transformer, though a robust and proven piece of equipment capable and highly efficient energy transfer, requires that the transmitter and receiver be far too close together to enable the charging of devices separated by distances such as the typical height or width of a room in a house.
So the basic specification for a technology that could meet all the requirements almost wrote itself. It had to permit the point-to-point transmission of power even when one of the locations involved might be moving and do so across a distance that could be measured in metres rather than centimetres. In addition, there could be no effect on other devices (or people) who were in the vicinity.
Finding the solution
It seemed a tall order. But as the team thought about the problem they came to realise that a solution might lie in the phenomenon of resonance: the means by which a singer can shatter a glass simply by singing a particular note at high volume. What happens is that when the singer’s voice hits the natural frequency of the glass, (the frequency at which it will vibrate most easily if subjected to an external stimulus) there is a resonant interaction between them of such force that glass is shattered.
In other words, resonant systems exchange energy between themselves with great efficiency. So if the team could find a way of making a transmitting and a receiving device for electromagnetic fields resonate at exactly the same frequency, they might have a means of transmitting electric power between them over a considerable distance.
Moreover, other objects or devices in the vicinity with different frequencies would be completely unaffected. Doctorate student at MIT and research team member Aristeidis Karalis explains why: “It is just as if you had numerous identical glasses all filled with different amounts of liquid. They would all have different natural frequencies and only the one that resonated with the singer’s voice would shatter. All the rest would be unaffected.”
The team found their solution by using transmitting and receiving devices, each of which contained both a coil of copper wire for generating an electromagnetic field and a capacitor, a device that is able to store and then discharge electrical energy. Each of these combinations of devices then forms a self-contained resonant system with the result that power can be transferred between them with great efficiency. Aristeidis Karalis, again, explains why this is so. “With this set-up,” he says, “you get a “strong coupling” as opposed to the much weaker coupling you get with a conventional transformer, even though that is superficially similar.”
Wi-Tri in your living room
The consequence is quite astonishing. The configuration of devices developed by the MIT team can transfer energy between the transmitter and receiver roughly a “million times” more efficiently than a transformer could across a similar distance. But now that the theory has been proven the race is on to find a commercial application for the development. Aristeidis Karalis says that the team is working on ideas that may result in real life applications within “three to five years”.
What those applications could be is anyone’s guess. With the obvious limitation that they will take place close to mains power sources all sorts of possibilities suggest themselves – video recorders that never fail to record a pre-set programme because someone has accidentally switched them off, smoke detectors that are always alert for danger because they do not have batteries that can run down and, of course, mobile phones that do not bleep in the early hours, causing their owners to think about ways of ensuring a good night’s sleep.
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