Letter Re: Converting Body Motion and Heat Into Electricity

The piece that you linked to was simply copied from GizMag. (They did so with credit, as if that makes it okay.) It looks like volumatrixgroup.com is one of those sites that uses stolen blog posts to get hits for ads.

The technology itself is a scam under a thin veneer of science. Their “20W” figure is about three orders of magnitude too high, for one thing. That figure describes all of the energy losses in walking, including the energy losses inside muscles and joints and the energy that goes into warming the surface under the shoe. The part that goes into the shoe itself is probably somewhere around 5% to 15% of that, or 3W at most for both shoes.  You’ve probably heard of the Carnot efficiency limit, which shows that small differences in temperature are very difficult to use as a source of power. Near body temperature, each degree of difference allows about 0.35% of conversion efficiency, so the tiny amounts of thermal energy they’re proposing to convert, which normally cause your shoes to be a few degrees warmer than they’d otherwise be, can be converted to electricity at a maximum theoretical efficiency of maybe one or two percent. Much less, of course, after being processed by any real mechanism. 

So starting with maybe a few watts of thermal energy going into the shoe, the Carnot efficiency equation defines a limit around 60 mW of theoretical yield, and a good mechanism might produce 20 mW of useful electricity.  By comparison, other researchers came up with slightly better numbers in a relatively easy implementation– piezoelectric energy harvesting in the shoulder straps of military backpacks.   But it’s still silly. – P.N.G.