In my WIP (webcomic in progress) mentioned in my other posts, there is an earth-originated colony based on an alien planet in a semi-post-apocalyptic society with limited technology and resources. because of this, even the mining villages will not notice the underground civilization of the native sentient beings until late in the comic. I have contemplated many aspects of the civilization and I ran into the problem of power. Of course thermal energy is an easy solution but I don't think its enough.
What I'm looking for is a way to move solar energy from the surface to the underground settlement without things like giant solar panel fields giving away the their presence. (edit: the idea of power 'plants', pun intended, is good and elaboration/variations on that are great, but if there are any other ideas id love to hear them.)
(more detail: these natives' civilization is very old, and they have extremely complex and advanced bio-technology and engineering as well as bionic machinery. the reason they're underground is to relieve the surface and life of their impact and pollution, hence why fossil-fuels are a no-no).