>This heats up the nanotubes, which then emit visible photons

I'm a bit skeptical that that would work. Otherwise you could build a perpetual motion machine by having the nanotubes in a hot room and running your perpetual motor off a solar cell powered by the light.

My guess is when the article says emit light they mean emit infra red light / radiation rather than visible. The article doesn't really make sense to me - I imagine someone got confused by the physics rather than it being an actual breakthrough in energy generation. I note the article says "propose a way to build" rather than having actually built something that works.