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The end of the power cable?

Earlier this month I wrote about the genius of Nikola Tesla and how his inventions had paved the way for the information age. One of his lesser known experiments was based on his vision for a wireless power transmission network. Even today, the idea of transmitting power wirelessly over long distances seems a highly technical venture. But last week it was announced a US team had successfully taken up the baton from Tesla. So will his dream of wireless power transmission come true?

US company, Witricity, last week impressed audiences at the TED Global Conference in Oxford when showcasing their system by wirelessly charging mobile phones. The Witricity system requires modifications to the appliance being powered but ultimately its success is based on the increased efficiency of energy transfer when resonant frequencies are applied.

Two coils with exactly the same resonant frequency are needed; one embedded in the appliance and one plugged into the mains. When the coil in the mains plug is switched on, its magnetic field resonates with that of the second coil, allowing it to absorb energy wirelessly through the air.

Witricity claim the system is entirely safe as the energy is transmitted predominantly through magnetic fields, namely low frequency electromagnetic waves, which are harmless to humans who are non-magnetic in nature.

The potential for this is immense, though the technology is still on a small scale. According to Witricity’s own website, they are currently working on a system that operates in the range of 100 watts and have already had success with powering a 60 watt bulb from seven feet away. This was achieved with a 40% efficiency.

The world has quickly become used to information being transmitted wirelessly, but how quickly will wireless power grow? It has obvious advantages over batteries in that the device will effectively be on constant charge, but how much of a millstone will the subject of range be?

I can imagine in applications such as site work, wireless power will be a godsend. Although batteries for devices such as power tools have improved immeasurably, but they are expensive and they still run down. Also, a reduction in the amount of trailing cables would improve safety and a reliable, constant stream of power to equipment would increase productivity.

Tesla’s system operated on different physics, but essentially offered the same end product. Power without wires. I wonder ,if he was alive now, whether he would be happy that other people were working on realising his dream or if he would simply be contemplating why it had taken us so long?

Enjoy the newsletter,

Richard Scott
01732 359990

Your Comments:

Dear Richard,

End of the power cable? I doubt it

About 11 years ago I designed a noncontact charging system for a secure cash transfer box which we are still using. Basically the system comprised of two tuned coils separated by a gap and operating at moderately lowfrequency low 100'sKHz) , which I would guess from experience from an earlier article on this wireless power I've seen, is in the frequency range that Witricity are using. Although for our design the gap between the coils was comparatively short, about 1cm, the principle is the same.

In view of this and with 10+ years experience of this type of system I would have serious reservations regarding Witricity’s statement/claim:

“Witricity claim the system is entirely safe as the energy is transmitted predominantly through magnetic fields, namely low frequency electromagnetic waves, which are harmless to humans who are non-magnetic in nature”

The problem I believe is that although “humans who are non-magnetic in nature” they do not generally walk around in the buff and they could well be wearing items that respond to magnetic fields, such as metal necklaces or bracelets. A simple coil/loop of wire with no magnetic materials in it can act as a tuned circuit, a metal necklace dissipating 100w would probably give you a serious burn before you realised what was happening.

From my experience even flat metal sheets can absorb a significant amount of the transmitted power due to induced eddy currents, thus there is the potential for significant losses from objects within the room/range of the power transmitter. From their own admission, the 60W light bulb experiment was only 40% efficient indicatiing that the losses we about 90W, that is over 10x the standby power of a traditional telly (5W typ) and with the imminent banning of traditional incandescent light bulbs and other energy efficiency directives I would say this technology would represent a backward step in energy use.. My informed guess is that in a real environment 40% efficiency represents the upper end of what can be achieved.

Consequently I would be surprised if such a system would ever be endorsed by the authorities either from the safety or energy efficiency standpoint. There may be some niche applications for low power applications, a few watts perhaps such as a LED bedside light (if the iron bedstead or the electric blanket don’t cause problems), but much below this power level, energy harvesting or batteries (rechargeable) may be a better option.

Nick Cook
Technology Analyst / Electronics engineer



I think the likely fate of this wireless power transfer system is contained in this innocent sounding claim:-

“Witricity claim the system is entirely safe as the energy is transmitted predominantly through magnetic fields, namely low frequency electromagnetic waves, which are harmless to humans who are non-magnetic in nature”

Witricity appear to be totally ignoring the furore presently going on over the supposed carcinogenic effects of power line magnetic fields. In the USA even the EPA admits that:-

"Based on studies about the incidence of childhood leukemia involving a large number of households, NIEHS found that power line magnetic fields are a possible cause of cancer." and "The International Agency for Research on Cancer (WHO) reached a similar conclusion."

This is for very low level magnetic fields at 50/60Hz. When you are deliberately transmitting at a power level of typically 120W at a much higher frequency around 10MHz into someone's living room then the energy level, and potential for carcinogenic effects from long term exposure, is thousands of times higher.

Does Witricity have the capital backing to fund long term trials of possible carcinogenic effects over the next 10 years before they can sell their product? Even if they did where would they find volunteers willing to expose themselves to the possible risks? Would you volunteer? I know I wouldn't.

The public will reject this as being far too risky to touch with a bargepole. And with good reason as this supposed pure magnetic device as currently described radiates an EM power of 5W, roughly 10x the average power of a cellphone. Nobody is going to want the equivalent of a cellphone base station sitting in their front room transmitting constantly.

Basically this is a nice science fair demonstration for "the home of the future" but no more. As a long term, mass market product it has precisely zero chance of success.

Chris Green

Technical Director

PS The Science magazine paper referenced on the Witricity website shows that the actual frequency used is 9.9MHz not the 10-100KHz assumed by the previous respondent. This is well into the RF spectrum with a wavelength of 30m. Near field effects, i.e. this is all magnetic field, cannot be assumed when the seperation of the coils is 2m or 1/15 of a wavelength.

I forgot to mention the new EU Directive 2004/04/EC on magnetic field exposure due to come into effect in 2012. Among other things this will have the effect of banning MRI scanners. The device in the Science paper, with 8A/m at 20cm, has 50x the EU permissible magnetic field strength of 0.16A/m at 10MHz. Even with proposed improvements there is no way this device is ever going to meet the EU directive.

These comments reflect the personal opinions of readers and not necessarily those held by EPA Magazine or IML Group plc

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