There are many approaches
A lot of people are working on algae as a possible biofuel source, and like you say, a lot depends on what the crop is and what you're using.
There's an infinite amount of sunlight, and water is pretty straightforward (particularly for salt-water algae) so the bottleneck is where you grow it, and where the other elements of photosynthesis come into it. I have no doubt solutions will be found. I do know that corn isn't going to be the final answer.