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Had a similar idea quite a few years back (still in school). The circuit I built took about 4 components and powered a small LED light while close to a RF source (CRT screen worked quite well ;-).
I always had this idea of building a thousand of these devices and connecting their output together, thus providing enough power to charge a battery ...
Yes, have a look at a basic crystal radio - that's where my ideas originally came from if I remember correctly!

Seems like such a low power system could have energy harvested from the motion of the human body just as easily without having an RF source nearby. Most hard hats have the outer shell and an inner "net" that sits on your head and provides a shock absorbing feature. Placing piezoelectric materials at these junctions would probably accomplish the same task.