On renewable resources:
- Wind power is fundamentally a combination of solar power and the rotational energy of the Earth. Both are limited in the end, although perhaps not on any human time-scale.
- Tide power draws from the rotational energy of the Earth-Moon system. Also not truly renewable, but the magnitude of the source and the time-scale for drawing it down are so large that we mayfly humans may as well think of it that way.
- Solar power is, of course, limited by the life of the Sun. Or of whatever star we may eventually draw on. See previous comments.
- All of the above appear to be limited by the half-life of the proton. Lower limit thereof is currently though to be around 6.6x10**33 years [see wikipedia for references]
Wearing my engineering hat for a moment, I note that "renewable" power sources simply draw on power with larger supplies. They are not fundamentally different from saying "we'll never run out of {coal, wood, oil, uranium, whatever}". Only the timescale differs.
Xenophon
P.S. I admit that the magnitudes involved appear to make the limits meaningless from a human point of view. But that's what people believed about coal a few hundred years ago...