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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.

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Can part of the radiation, the object inside the cave emits, partly be bundled and focused onto a point/small area?

Yes. Typically reflection is used rather than refraction since many optically reflective materials also reflect infrared but many optically transparent materials are opaque to infrared.

Can the focused radiation onto that point be turned into some useful energy to power some device?

Yes, provided there is a heat sink at a lower temperature. In this case assuming that the temperature of the object is greater than 15 C.

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.

The temperature is more important than the size. The power increases as the fourth power of the temperature but only as the first power of the surface area.

Which material/shape/form would be best suited?

You would want something with a high heat capacity, good heat conductivity, and uniform emissivity.

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  • $\begingroup$ "Yes, provided there is a heat sink at a lower temperature. In this case assuming that the temperature of the object is greater than 15 C. " - Why would the object emitting the radiation(also absorbing) be at greater temperature than 15C? Where in my experimental setup would i indicate such a thing? Also, why would the heatsink need to be at a lower temperature? What if it wasn't? What would happen to all that focused infrared radiation hitting it? And no, there is no heatsink at a lower temperature. That would be trivial in such a case and we would not need any infrared rad to extract energy. $\endgroup$ – pZombie Jan 28 at 6:23
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    $\begingroup$ Without a temperature gradient you will not be able to convert thermal energy into useful energy. A focusing system will change only the rate of energy flow, not its direction. If everything is the same temperature then there will be no energy flow regardless of your focusing system. This is required by the second law of thermodynamics $\endgroup$ – Dale Jan 28 at 18:26
  • $\begingroup$ The idea was to target some material with the focused infrared radiation of the large object. A material which absorbs the frequency of the emitted and focused IRR well such that the material would heat up or do something else which would allow one to extract energy of it. If that is not possible, then what would happen instead when some material would be subject to the focused IRR from the large object? Would the radiation just pass through it without getting absorbed or reflected or what else? $\endgroup$ – pZombie Jan 28 at 18:48
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    $\begingroup$ “what would happen instead when some material would be subject to the focused IRR from the large object?“ If they are the same temperature already then nothing would happen. They are already in thermal equilibrium so there can be no net energy transfer. $\endgroup$ – Dale Jan 28 at 20:07
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    $\begingroup$ “As far as the material getting hit by the focused IRR is concerned, it only "knows" that it is getting hit by more IRR than other objects/materials within the cave.” Except that if everything is at 15 C then it isn’t getting hit by any more IR than anything else in the cave. Regardless of how you focus anything it gets the exact same amount of radiation. The object gets more radiation from whatever is at the other end of the focus, but less from the remainder of the surroundings for no net increase or decrease. Focusing only gives more energy if there is a temperature difference. $\endgroup$ – Dale Jan 29 at 0:38

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