Well, growing algae is not much different than farming catfish or crawfish. Water plus sunlight and food. The sun and nature do all the heavy lifting. We do have the up-front infrastructure costs, but if implemented on a cookie-cutter massive scale, these costs plummet, and being what should be long-lived assets, payback will be much less than service life.
This is an element of transitioning from purely fossil to fossil and reasonable renewable. We have plenty of oil - gas - for at least a century (not to mention coal), BUT this is the century to make the transition for these liquid / pipeline fuels...
By 2100 or so, we ought to be in a different, secure, long-term balanced source-sink energy production mode, taking that off the table for the balance of mankind’s tenure - as long as it may be.
Live long and prosper!
One only need read the prognostications, predictions and prophecies of the 1950’s on what today would be like to find a caution on saying what may or may not be in another half century or so.
And where in the world do you learn stuff like, “..long-term balanced source-sink energy production mode,....”?