One of the less-publicised but more-interesting product announcements to come out of this year’s CES conference is the Airnergy Wi-Fi energy harvester, brought to us by (surprisingly) the folks at RCA. Priced at $40 and planned for sale this summer, the little gizmo supposedly plucks wi-fi radio signals out of the air, then uses the energy to charge an internal battery which can then charge your mobile phone / MP3 player / etc.
While the gadget enthusiast in me wants to say “sign me up!” the electrical engineer in me says, “hold the phone…”
In the US, the maximum legal power transmitted from most wi-fi devices is 0.1 Watts. If you plug that into this RF power calculator and assume a distance of 5 meters with no line-losses and 3dB of gain in each antenna, the maximum harvestable power at the Airnergy will be about 0.00000157 Watts. This is plenty of energy to sling bytes of data through the air, but even if the charging circuit were (quite impossibly) 100% efficient, this input power is an order of magnitude less than even the self-discharge rate of most Lithium Ion batteries.
Even though RCA of-late seems to be more interested in selling their branding to marginal, no-name OEMs, the appearance of their logo on this gizmo is just about the only thing that gives me any hope that it might not be smoke and mirrors. The physics doesn’t add up, but who knows – 15 years ago, most folks probably wouldn’t have believed that 0.1 Watts at 2.4GHz was enough power to carry data at 100MB/s – yet you can happily do so at your local coffee shop. From your cell phone. Along with a half-dozen others.