RCA (which is not the old and reputable company I remember, but has gone out of business and its name sold to anyone with the right amount of cash) recently announced a device called the Airnergy harvester, which supposedly simply soaks up the RF energy emitted by WiFi devices in the neighborhood and uses it to charge portable batteries. Wow, what an idea…but a moment’s thought makes it clear it can’t work. My local wireless router simply can’t be pumping out that much energy, or it would an awesomely wasteful device, and there can’t be that much power floating free in every few cubic inches of my home. Fortunately, one of the commenters at that site did the math and made it explicit. Don’t you just love math? It’s so powerful and so handy.
Here’s some math. Long story short, by my calculations, 100% efficiency and absorption at 5 feet away from a 100mW home router, (reasonable figures), it would take 34.5 years to charge that blackberry battery.
It’s not a Dyson Sphere, so you only get the power that hits the antenna.
Surface of a sphere = 4pir^2, r = 60″ (5 feet).
Surface area of a 5′ sphere = 45,216 square inches.
The device appears about 2″ x 3″ = 6 square inches.
The device then picks up, best case, 0.000133 of the power out from the router, which is 100mW, so.. 0.0133mW
If you leave it there for 24 hours, 0.0318 mWh are stored.
According to Will’s battery, it has ~4,000 mWh capacity.
So, it would take 12,579 days, or 34.5 years, to charge your blackberry battery once, presuming 100% absorption, no losses.
I call BS. Even adding up all the laptops, cell phones, routers, portable phones, everything, all the noise in the RF spectrum that could hit that device, I don’t see it charging the internal battery even in a week.
For a dose of unreality, though, read through the comments there. The earliest are all fast explanations of the lack of plausibility of the device, and then what happens? It alternates between clueless dopes saying, “Awesome! I want one of those!” and exasperated skeptics saying, “Read the comments up top, it can’t work!”