There was a  good article in "QST" a year or so ago about Peltier
Junctions being used in the reverse mode of what they are usually used for.
Someone had taken a P.J. which was intended for use as the active part of
a mobile refrigerator and had run a small amateur radio transmitter from the
difference in temperature between a bucket of ice water and another of hot
water.  It works until the temperatures equalize.

        I don't remember the numbers, but I think the transmitter may have
been one Watt or less.  There are the usual energy transfer inefficiencies,
but one might be able to scrape together enough power to run a PIC-based
control system off of the waste heat of an engine on a boat or something
like that in which something is always going to be hot like the engine and
something else will always be cold like the water.

        I would think one would want a _GOOD_ brown out protective circuit
and probably a switching regulator to make the power supply as stable as it
can be in such a variable world.  One would want it either up and running
or off and not sort of trying to run which is what you would get when the
temperatures were equalizing after the engine stopped or when it was just
starting.

Martin McCormick