Algae are real interesting critters, they can continue to grow in the dark over night, to the best of my understanding.
They are part plant, part animal, IIRC, in that they convert sunlight and CO2 into O2 during the day, and they do the opposite at night, convert O2 to CO2 at night, but they continue to grow and live day and night. I would need to read up to see how much of their mass growth or production of new cells depends on sunlight, or if they continue to grow at night. I do know they continue to breath at night, and they convert O2 to CO2 at night in ponds. In fact that O2 conversion to CO2 at night (this happens when they get no sunlight), is what drops the O2 levels in lakes and ponds and results in huge fish kills out in the wild where algae gets a grip, and that is the reason EPA regulates the pollutant runoff that breeds algae, like phosphate soap and fertilizer run off.
They may need the day/night cycles to achieve optimal growth? Most of my experience with algae has been trying to kill it, to keep it out of waste water treatment systems, as it adds to the waste water pollution in a municipal or industrial waste water treatment system (raises the COD and BOD), and they muck with the pH balance as the sunlight comes and goes, and that messes with the bacteria that treat the waste water as they need O2 and do not like wild pH changes!
Back to your question, I don't know the "optimal" answer, but my experience and education tell me that surface area of the tubes, and transparency can go a long way, and all BEHLs competitors units that I have seen so far have a vertical structure of some kind, so they seem to think they can get higher density per square foot, at a higher cost per square foot with out sacrificing light coverage. I am surprised that none of them are using smaller diameter tubes to increase the surface area for light to meet the water and to increase mixing and turbulence. I would think thin walled tubes about 1/16" diameter with a nice delta P across the tube inlet and outlet would make more sense, borrowing from what I know about water treatment. The tubes could be hit with reverse pressure periodically if needed to unclog the inlets.
While writing this it has dawned on me that one achillies heal for algae is that it does convert CO2 to O2 at night, only at day so we can forget direct operation on a power plants exhaust. The CO2 would need to be captured and stored for day time feeding only. Also locations near the equator would be the most land efficient locations as they get the most daylight year round.
It would be most interesting to see the net mass balance on CO2 to O2 conversion versus O2 conversion to CO2 as Algae go from day to night time modes. Probably other variables involved like nutrients, pH, temperature, mixing, and the species of algae used.
