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CleanSpace CO Sensor Runs On Freevolt RF Harvesting

Posted by BeauHD on from the nothing-is-free dept.

mspohr writes: A few years ago, a Kickstarter was set up to develop a locator tag powered by free radio frequency (RF) energy harvested from the environment. This was called a scam here on Slashdot and was shut down before it was funded on Kickstarter. However, it now appears that the concept is not as far-fetched as some predicted. A UK company CleanSpace has developed a carbon monoxide (CO) sensor which is powered by free RF. A review of the product has been posted on YouTube. It uses Freevolt technology to keep a battery charged and the CO sensor running. Since they have several thousand of these devices collecting data, they do appear to work and it seems to be in the 'not a scam' department.

2 of 110 comments (clear)

  1. I worked on this by Anonymous Coward · · Score: 5, Informative

    I've worked on this project and it doesn't work.

    Basically all it is is an antenna with a half bridge diode rectifier. This is pretty straightforward however the problem is that despite using 'zero drop' diodes the ambient RF energy does not produce a sufficient voltage (power out of the antenna with the line impedance at the diode) to switch the diode properly which causes a rapid drop off in efficiency. Basically the diode is still in the sloped part of the IV curve (zoom in on an IV curve at the knee). This combined with the fairly low amounts of power available means that the power output of these things is in the 10s of uW, which is less power than is consumed by the charge pump needed to top the battery up! So how does it work then? Check the tear down photos and you'll see a massive battery...

    The other interesting thing about this 'technology' is that you can calculate the energy density of a harvester, it's simply the mean time till failure x power output / volume of device. When you do this calculation you quickly realise that due to the size of the antenna needed to see any appreciable power the device would have to last for 10-20 years to begin to approach the energy density of even coin cells (including the casing)!

  2. Re:RF harvesting can work for power. by jiriw · · Score: 4, Informative

    Most rechargeable batteries have a substantial self discharge rate and will go to zero in a few months with charging.

    This used to be the case, but for a decade or so now there are rechargeable 'ready to use'/'low self-discharge' (LSD) NiMH batteries on the market that can hold a significant amount of their charge for several years (for good ones, 75% after 3 years). LSD NiMH's do have a bit smaller capacity per volume but that is maybe a 10-20% difference, at the most.

    But maybe these sensors have a rechargeable Li-Ion battery? I don't know about LSD types of those but Li-Ion have very bad charging characteristics when almost empty (high internal resistance - so it's harder to charge the emptier it is) so I don't think they are useful when recharging in these extremely low power conditions. And then there are supercapacitors, but they have a way too high self-discharge rate to be able to claim 5 years of operation on the 'battery' alone.