Scientists create metamaterial that can absorb light completely and turn it into heat

Minor nitpick: it's not true that "a body will emit exactly as much as it absorbs" in general. That's only true once it reaches some equilibrium temperature, which may not be room temperature. If you stick a room-temperature metal sheet outside on a sunny day, it will heat up until it reaches a steady state where the rate of heat absorption is equal to the rate of heat dissipation -- this can be significantly higher than room temperature, which is why pieces of metal outside on a hot day are really really hot.

The steam engine and Otto cycle both convert waste heat to kinetic energy. They turned out to be marginally useful.

I wasn't suggesting coating objects with an expensive material to collect ambient infrared radiation. I was thinking of solar or solar thermal power, since the article advertises the material as 'tunable black'.

Oh, I had assumed that was an electron micrograph. But you're right, now I come to read the article again it only works over a narrow frequency range.