Comment by wildzzz
2mW would require about 1.5m² of area. I could maybe see this being used to pull a little bit of energy out of a room to charge batteries or capacitors to run sensors or other low power devices in short bursts. But you could harvest more power just by turning a hand crank generator for a few minutes. An average solar panel is like 10k times more efficient. I could still see some sort of use for it, maybe for being embedded into floors and ceilings of buildings to passively run sensors. There would be a small cooling effect on a room with enough of it.
but theoretically: because this does not require heat gradient (I assume that's impossible, they just base on micro gradients) you could fold this hundreds times to get much more from area