From: grarpamp <grarpamp@gmail.com>
   >Free power of course...
   >[1]https://science.slashdot.org/story/16/07/14/2049246/cleanspace-co-s
   ensor-runs-on-freevolt-rf-harvesting
   >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.
   I don't think there's necessarily a contradiction here.  While there do
   exist CO sensors which need
   to keep the detector heated to 400 degrees C, and therefore benefit
   from being connected to
   the AC supply, "electrochemical cells" which have extremely low power
   consumption are now
   available. [2]https://en.wikipedia.org/wiki/Carbon_monoxide_detector
   The device needs merely to
    'wake up' occasionally, maybe every minute or so, interrogate the
   voltage on the electrochemical
   cell, and sound the alarm if the limit value is exceeded.
   The main concern is ensuring that the detector has enough power at the
   time it is necessary
   to alarm to sound that alarm.  The ongoing (non-alarm) power
   consumption is probably much less than the
    self-discharge rate of a battery.   Modern alkaline-cells have a
   shelf-life of in excess of 5 years, and
   perhaps as much as 10 years, although Wikipedia doesn't state this
   specifically.
   [3]https://en.wikipedia.org/wiki/Alkaline_battery
   [4]http://data.energizer.com/PDFs/non-rechargeable_FAQ.pdf
   In contrast, a tracking device must actively emit some sort of signal
   to alert some remote system to
   its current location, which takes power.  It must also somehow know
   where it is:  GPS receivers use
    much less power than they once did, but even that is far greater than
   that which a tiny battery could
   be expected to provide.
   And the amount of power available from "free RF" is obviously quite
   variable:  If you're 0.5 kilometer
   away from a 50Kw television transmitter, that's a lot of signal
   strength.  Compare that with out in
   the boonies, 1 km away from a cell-phone tower is not a lot.  It would
   make more sense to design
   a CO detector with solar cells on the bottom, and have people mount it
   on an outside window, with
   a rechargeable battery to store enough to get through the night.
                        Jim Bell

References

   1. https://science.slashdot.org/story/16/07/14/2049246/cleanspace-co-sensor-runs-on-freevolt-rf-harvesting
   2. https://en.wikipedia.org/wiki/Carbon_monoxide_detector
   3. https://en.wikipedia.org/wiki/Alkaline_battery
   4. http://data.energizer.com/PDFs/non-rechargeable_FAQ.pdf