Some time ago, I proposed on this forum that the easiest way to store renewable energy (wind and solar) is to produce a carbohydrate like methane which then can be stored for some time and then if needed burned again. The German car company Audi has actually taken that idea one step further and produced diesel out of water and CO_2 collected from the atmosphere. If you want to know more, here is a link to the article giving more details:
Of course, the problem with that idea is that the amount of fuel produced that way can not be more than a small fraction of the solar energy hitting the earth. This means that only relatively few cars will be able to run on this type of fuel.