joe-90 said:
Wood is a commodity with energy already built in and it was enough for all our needs up until a few centuries ago. Along with windmills and water wheels - which were formidable machines in their time - it's how our not so ancient ancestors harvested solar power.
Coal and oil are just solar energy in a more concentrated form. Coal made the industrial revolution possible but oil, although very convenient, was never so essential. If it wasn't easily accessible we would be running our cars on a liquid or gaseous fuel made from coal - and making all our plastics too. Fossil fuels have sustained us well enough but, as you rightly point out, they can't do so forever. Fortunately there are other things with energy built in.
Uranium has energy built in. Weight for weight it has roughly a million times more built-in energy than any chemical fuel. Actually lots of elements have energy built in. Those beyond bismuth have so much that they are inclined to self destruct.
Some, like astatine, do so with great gusto but those ones are exceedingly rare. They only exist at all because of three long-lived nuclides that have been here since our solar system first formed. If you want to get energy out of natural radioactive decay you need a lot of material and a very big container to put it in. We do have one and you're probably standing on it right now! 
We have about half the uranium 238 and most of the thorium 232 that our planet started out with and there will still be lots left when our local star goes out and takes us with it.We also have a modest amount of the third nuclide, uranium 235 and this can be persuaded to give up its stored energy a lot faster - inconveniently fast if you go about it the wrong way! Furthermore, the fission neutrons can be used to turn the otherwise useless U238 into a nuclear fuel thus: U238 + n = U239 -> Np 239 -> Pu 239. (Actually, free neutrons can be used for all sorts of transmutations - but turning lead into gold isn't one of them.
) Nuclear power stations all over the world are running on uranium or plutonium or both. I don't know how much uranium we have left but a little goes a long way.The biggest problem with nuclear fission is the waste it produces. If you thought coal smoke was dirty then, to quote Bachman Turner Overdrive, "You ain't seen nothin' yet!" The fission products are many and varied and overloaded with neutrons. Some have inconveniently long half-lives. We are piling them up in long term storage in the hope that we will eventually figure out how to get rid of them.
Our best hope for the future is fusion. It's not immediately obvious that the humble hydrogen atom has energy built in but it does. That single proton would be so much happier snuggled up in a foursome as helium.
That's how stars work. The difficult bit is making it happen here on Earth in a controlled fashion. It has been done though we haven't exactly perfected the process yet.But do we need it?
Some would say no, and they have a point, but I would object. On the one hand the low energy lifestyle is very appealing. I would happily spend most of my time consuming minimal energy - but not all of it. That's because I'm a physicist (amongst other things) and I want to know what's inside a proton and what lies beyond Pluto. Windmills and water wheels will never provide enough power for the CERN synchrotron, still less the Starship Enterprise!As a species we are nothing if not inquisitive. That's why we're still here while other hominids are extinct. Environmentalists might not like it but we will eventually find ourselves a new source of energy and we will consume it on scale that makes today's gigawatt power stations look like so many old-fashioned water wheels!
