Eek! I have ants on my desk running around on my computer!

Why are they here? What do they want? My computer's very hot. Perhaps they are stealing heat from my computer?

It's a thought. Could some kind of micro-machines (BeamRobotics) be built to go out and collect energy in the form of heat / light? And bring it back to the nest? How would they store the energy? Could they be made so efficient that they collected more energy than they consumed? Ants work, but because food is high energy. If we could create machines with this efficiency, we could send swarms of them to harvest waste heat and otherwise unused light.

Does the size have to be VERY small to make this efficient?

Perhaps they couldn't store the energy directly (eg. batteries) but by some other process eg. cracking water molecules from the air to make hyydrogen (or hydrogen rich material). Maybe to charge internal flywheels (but then what about the weight of carrying them?)

Counter Arguments

X : There are no mobile photosynthesizers! When did you last see a lettuce colony?

Y : But maybe there weren't rich concentrated sources of energy in nature eg. powerstation cooling towers / car exhausts.

X : Volcanoes?

Perhaps you need something more like a yeast? Or bacterial process? (WikiComposting)