Biofuel from sources like switchgrass could cost $3 a gallon by 2012 while concentrated solar, which can store heat to create energy when the sun isn’t out, looks promising too. This from the Boston Consulting Group. They are less bullish about photovoltaic solar and wind because there is no easy way to store the energy as yet.
But given the huge advances in all forms of renewable energy, I bet someone solves the storage problem. Such stored energy needs to be instantly available, completely reliable, and of utility-scale, no easy task. But once solved, it will be a game-changer.