here's the math http://publications.lib.chalmers.se/records/fulltext/local_128089.pdf it's aint heat! now anyone can make em?
There are countless devices on the market already in many shapes and sizes that directly convert light into an electrical current. For example, here's a link to the datasheet of a photo diode, which can be bought for about 20 cents a piece at mouser. http://www.optekinc.com/datasheets/OP980.pdf High tech is often remarkably cheap. Don't let that fool you though. It's still amazing stuff.
That photo diode said it worked with "near infrared frequencies", which got me thinking, if a photoelectric were made which ran off of infrared, could you, say, power these bright LEDs that they've got now -off of your own body heat?
The problem with that idea is that you have to keep the photo diode cool to get any usable net power. When the photo diode is at the same temperature as your body, photons will still "liberate" electrons at the pn junction, but equally many free electrons will get knocked back to where they came from, emitting infrared light. Essentially, the same radiation laws apply to your body and the photo diode.
