On Sun, 21 Oct 2018 11:26:38 -0700 (PDT), wrote:

On Saturday, October 20, 2018 at 3:15:28 PM UTC-7, Jeff Liebermann wrote:
On Sat, 20 Oct 2018 13:39:27 -0700 (PDT), wrote:

Running on body heat would give you sort of unlimited run time.

There are problems trying to utilize body heat. The worst is the lack
of an easy cold junction. In order to produce electricity, one needs
a temperature DIFFERENCE. If the body is the warm junction, where's
the cold junction? Simply heating something will work if you use a
thermocouple, but the efficiency is very low and little power will be
produced. For example, using the ambient air as a cold junction will
produce a fabulous 0.3 microwatts with this contrivance:
https://www.dailymail.co.uk/sciencetech/article-4368596/Incredible-powercell-converts-BODY-HEAT-electricity.html

There are other ways that seem to be better:
https://en.wikipedia.org/wiki/Energy_harvesting

Remember that you can use either junction as the cold one. You
don't care in what direction the current flows. All you need is
a differential.

Agreed. Realistic thermal generators use either a really hot source,
such as radioactive decay, or a really cold sink, such as the water
temperature at the bottom of an ocean. However, the human body
doesn't offer many useful extremes. At a nominal 38.6C (98.2F) body
temperature, the best you can do is maybe 5 degrees difference to some
other body part which is too little to do anything useful. One can
probably make something work using air temperature, but if that
sometimes passes through the nominal body temperature, the
differential will be zero and no energy can be scavenged.

Using germanium you can get away with really low voltages.

Not really low. You can work a little above the bandgap voltage,
which is 0.67V for germanium (and 1.14v for silicon). Actually, one
needs a bit more than that to deal with some series resistive losses.
Lead sulphide is even lower at 0.4V, but has such a high dielectric
constant that these only run at very slow data rates. The latest rage
are lead sulphide nanoparticles which will soon revolutionize
everything the promoters are promoting. If you want to try it, tear
apart a common lead sulphide near IR detector, sandwich the crystal
between two metal plates, apply some differential heat, and see what
you can produce.

Incidentally, the common mountain bicycle has a fairly good mechanism
available for scavenging electric power. The up and down motion of
the bicycle does nothing to produce forward motion. It's literally
wasted and usually dissipated as heat. Put a generator in the seat
tube or shock absorber(s), and the up and down motion could be used to
produce power. A magnet inside a coil of wire would suffice. There
was a patent issued for this idea, but I'm too lazy to find it now.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558