[jacquesm]

If you pump enough energy into the source, sure. But your average coffeeshop is not going to replace their ceilingtiles with what would be really inefficient space heaters in order to charge some phones (and cook the inhabitants in the process).

That's the whole point of the exercise here: if the efficiency isn't there the whole thing is dead because you can't be pumping kilowatts into space in order to get a few watts (or milliwatts) back out. The difference between the two will get converted into heat!

So the only way this will work is if the efficiency is really high, much higher than seems to be feasible right now.