Following experimental setup
We go underground inside a gigantic cave.
Inside the cave is a huge object of a material and form which has to be decided by the answers still.
The air temperature inside the cave with an open entrance is always at about 15°C - This is NOT a closed system
As i understand it, an object which has a temperature above absolute zero will both emit and absorb infrared radiation.
edit: Clarifying that the object as all other objects inside the cave are not heated by any sources other than what the air and or ground/cave wall radiation provide all of which are at 15°C. I am assuming that even at 15°C the large object inside the cave would emit infrared radiation as well as absorb such at all times, being at an equilibrium at 15°C. Should this not be the case and should an object at "ambient" temperatures not emit any radiation nor absorb any at all times, then my question should be deleted.
Can part of the radiation, the object inside the cave emits, partly be bundled and focused onto a point/small area?
1) Can the focused radiation onto that point be turned into some useful energy to power some device?
2) How large would the object have to be in order to generate about 1w of power for our device. Rough estimates are fine, including the efficiency of the mirrors/dishes/lenses in question.
3) Which material/shape/form would be best suited? (real materials/shape/forms)
Keep in mind that this is NOT a closed system. The air temperature would always stay at about 15°C. The cave walls/ground would be at about the same temperature as well.