Artificial Photosynthesis
As smart as human beings can be, nature almost always does it better — possibly because nature has had hundreds of millions of years to get it right. Take photosynthesis for example. Plants with green leaves are able to capture the sun's energy and turn it into useful chemical fuel in a process that is much, much more efficient than our best photovoltaic solar panels.
That's why there are a number of scientists working on creating artificial photosynthesis; it was even a major plot point in Solar, the English writer Ian McEwan's global warming-themed novel. Daniel Nocera, an energy expert at the Massachusetts Institute of Technology, is pushing a form of artificial photosynthesis that would create electricity that would then be harnessed to produce hydrogen for use in fuel cells. That's only one way to harness photosynthesis, but already startups like Joule Biotechnologies are looking for ways to take it commercial. The future has to be solar-powered; the question will be how best to harness that free source of energy. The trees might have the best idea.