This isn't a viable solution, but it's sure to entertain. Remember as kids, cartoons showed large machines being fueled by electricity generated by hamsters on their little wheels? Believe it. While they don't generate a lot of power, scientists have found a way to harness this power.
http://www.sciencedaily.com/releases/2009/02/090213115020.htm Ridiculous? Yes. Funny? Yes. Nostalgic? Yes. But honestly, this is a forerunner to some interesting bio/nanotechnology. Scientists are now able to harness irregular energy from organisms (which is different from our static, normal electric flow generated from turbines and the like) and transform it into electricity. Think about it: while far away, our descendants could go out for a jog, come back, connect their jogging suit to a jack of sorts and get power from it.
Though I have to be honest, I'm also worried where this could go. I would hate for living things (obviously not humans) to become slaves to this. That would be terrible and against my moral code completely, but I hope that we are indeed better than that.