How Far Can Wireless Power Reach?
If you still have to put your device in a specific place to charge, it’s not very convenient. If you only want a small amount of power – for a sensor, clock or a wireless light switch you could move around the room – harvesting RF or even ambient energy from vibrations (including pressing said switch) gives you power without wires, but what you really want is to beam the power around (safely).
MIT physicist Marin Soljacic, apparently driven by the incessant low-power alert of his wife’s Nokia phone, used a trick long known for improving radio signals to send power between two metal coils tuned to the same frequency – resonant coupling. Put power of that frequency into one coil and it’s converted into a magnetic field that’s captured by the other coil and turned back into electricity; the resonance allows the second coil to capture the power even at a distance. It’s like tuning fork vibrating when you play the right note (or a wine glass breaking when you play its resonant frequency).
As well as publishing the results, MIT set up a company called WiTricity (wireless electricity) to build products. Intel reproduced the original results at its 2009 developer conference, transmitting 60W of power a few feet across the room to power a lightbulb from two-foot coils and calling it Wireless Resonant Energy Link (WREL).
WiTricity is using a larger transmitter with a one-foot receiving coil at the other end of a room and plans to shrink the coil even further. In 2009 it powered a 1,000W light and a 3,000W electric car, and it has demo devices powering laptops, TVs and other home entertainment devices.
So far efficiency is low; 75% with the 60W version but as low as 20% in some cases. But if you charge two 20% efficient devices at once it goes up to 30%, because the receiving coils also resonate with each other. Add enough devices to charge and it could get close to 100%, although tuning the multiple coils is perhaps too tricky for a consumer product.