Green Energy

Short:
Coal will become cleaner and greener, via better scrubbers, carbon capture and gasification. It is still the cheapest and most abundant energy source we have.
Cellulose ethanol is a lot more promising than corn ethanol which is is a big threat to biodiversity and the poor.
Reducing transmission loss would save us 20%.

Long:
Nuclear Fusion could be the answer.
Probably not hydrogen, as it is too expensive and too explosive.
I heard that 100 square miles of solar in space would be enough for all our needs, how to get the power back to us is the problem.