I've looked into algae a while back, something like a solar panel for algae sounds like it should be ideal. I skimmed a few papers, but apparently pumping it around is a major energy cost, and cleaning problematic. And you need to pump in air / CO2. And then the amount of light needs to be right, for example too much light or constant light is bad. But theoretically algae should be the ideal protein source.
It would be cool if you could genetically engineer different and better tastes though. Something like "pancake mix" coming directly from your roof through a pipe into your kitchen :D