Gizmodo reported about a Gadget shown at CES 2010 that supposedly harvests energy from a wireless hotspot. The “RCA Airnergy WiFi Hotspot Power Harvester” consists of a small battery, a USB connector and some circuitry that is supposed to convert wireless signals into DC power to top up the battery. The gadget can then be used to recharge or power any device that can draw power from a USB port, such as a cell phone or iPod.

A claim was made in a Youtube video on the Gizmodo site that the gadget will charge a Blackberry mobile phone from 30% to fully charged in 90 minutes. That may well be true, if the internal battery of the gadget starts off fully charged and is big enough. The big question is, how much energy can this wireless harvester actually draw out of thin air to replenish its internal battery, if any?

The whole thing reminds me of the hoax of the Japanese “car that runs on water” demonstrated in June 2008 by now apparently defunct company Genepax Co. Ltd. (their website went offline the following year). That car turned out to have a set of lead-acid batteries that — fully charged — could have powered the car some distance even if the proprietary fuel cell announced by Genepax was completely dysfunctional. In any case, the quoted power output of the fuel cell of only 300W was completely inadequate to power a car, meaning the batteries (the real power source) would have had to be recharged from a wall socket before too long.

Likewise, the amount of power available from a WiFi hotspot is nowhere near enough to run a computer or mobile phone. Take my cheap Samsung mobile phone with a 880 mAh 3.7 V Li ion battery (a battery capacity of 3.2 Wh) that I normally need to charge every other day or so. 3.2 Wh over 48 h works out as about 67 mW, which is not that much. However, the maximum power at which a wireless access point may transmit under FCC regulations without needing a broadcast license is a mere 100 mW. Even if the “Hotspot Harvester” could convert 2/3 of the radio energy into usable DC power, it would have to suck up 100% of all energy radiated by the access point, which would have to broadcast at full blast all the time instead of just when there is traffic, just to keep my cell phone charged.

In reality, there is no way the harvester can grab 100% or 10% or even 1% of the energy from the hotspot, which radiates wireless signals in all directions. The gadget can only harvest the small fraction of the airwaves that cross its antenna, which is only a few centimetres by a few centimetres in size, while the hotspot may be metres or tens of metres away. The numbers simply don’t add up.

What that device is then is just a glorified spare battery that will need to be recharged by plugging it into a wall socket or the USB port of a mains-powered computer. The “energy harvesting” function can make no meaningful contribution to the battery charge – unless maybe you happen to put it inside a microwave oven and radiate it with 1000W of power (boys, don’t try this at home! 😉 ).

The sad thing is how many websites and blogs have given free publicity to these claims, without doing the math to check if they make any sense at all.