The Green Benefits Of Wireless Power

Witricity has promised to delivering power without wires or batteries. We spoke to the chief executive, Eric Giler, about whether this could contribute to sustainable technology.

Last week, Witricity made a media splash, when Giler showed how its near-field indictive coupling system could power Google phones, iPhones and a TV, at the TEDglobal conference in Oxford.

That’s all very well, we asked, but does the world need a new – and probably more inefficient way to deliver electricity, at a time when there’s a pressing need to use less energy and use it more efficiently?

The question struck a nerve, and Giler phoned us up to explain more. The system can be more than 90 percent efficient, and if used sensibly can reduce the environmental impact of electrical devices, he said.

“We aren’t transmitting electricity through the air, and we aren’t using induction,” he said. The strongly-coupled magnetic resonance effects in the Witricity system are different from the magnetic induction used in a system like the Palm Pre’s Touchstone wireless charger, he explained.

While the system uses frequencies that are low for electromagnetic waves – from hundreds of kHZ up to tens of thousands of kHz – they are high for electrical devices, and well within the capabilities of normal conversion systems, he said.

Efficiency of more than 90 percent

Witricity’s system requires the source and the charging device to have magnetic loops that are resonant – responding to the same precise frequency, and the transfer of energy between them will be very efficient, he said: “It’s just a function of distance.”

An early public demonstration showed a 45 percent efficiency, but the process has improved a great deal since then, and his promise at TED global that there would be a 15 percent cost penalty for using Witricity instead of a wire, was simply worked out from the cost of adding transmitting and receiving coils to devices.

“When you’re close to the source, it’s pretty close to 100 percent efficient, but as you separate the coils you get a fall off,” he said. “At a distance of one coil diameter, you get about 90 percent efficiency, and when you pull away to five coil diameters, you typically have ten percent of the power.

Power that isn’t transmitted to the device is not dissipated in the air, or in other objects between the two, he said: “the balance of the power is held by the source and dissipated as heat in the coil structure.”

Charging cars and phones

How much this power loss matters depends on how much power you are transmitting, he explained. Witricity is working on wireless charging for cars, and these need a very high efficiency transfer because they would otherwise produce a lot of heat: “Car manufacturers want an efficiency of 95 percent or more.” Systems to charge cars would use position sensors so the device would net start charging until it is in a position that would get the required efficient transfer, perhaps when the car is over a charging pad.

Charging phones might be different, said Giler. The company has envisaged systems where a coil in a room will deliver power to wireless mice and keyboards, along with any phones or other devices in people’s pockects.

Phones take around 2W to charge, so even if the phone is being charged at ten percent efficiency, that would only result in 20W of heat being dissipated, he said. But Witricity doesn’t intend to be that wasteful, for two reasons.

Firstly, the system would not charge devices unless it could do so efficiently: “The system is only turned on when the power is required,” he said, but refused to describe how the system locates the devices to be charged before charging them. “That’s our secret sauce,” he said.

Secondly, he claimed that while the system is charging one device at low efficiency, it can charge others using the power that would otherwise be wasted, so in principle it could always operate at high efficiency if there are plenty of devices near the source.