In my opinion, microalgae production for bulk purposes (i.e. biofuels rather than current pharma usages) has thus far failed because the existing cultivation systems are very energy intensive (pumps), labour intensive (cleaning), and inconsistent (cell growth and lipid concentrations are more or less non-deterministic). This all means that the cost of production is higher than what you can sell the raw output for in most cases.

Our novel approach was to engineer growth media aerosols (100 micron diameter microdroplets), and suspend microalage cells inside each microdroplet. Then we made a literal cloud (microdroplet air suspension) inside an environmentally controlled greenhouse volume. Doing this enables an extremely large surface area contact between the growth media and atmosphere (also CO2 enriched atmosphere), as well as energy efficient mixing and circulation processes. Our calculations showed that orders of magnitude energy efficiency improvements are possible compared to existing cultivation systems (raceways, tubular PBRs), and additionally scaling is inherently possible due to the ease of building out larger and larger volumes to contain such microdroplet atmospheric suspensions.

Existing systems cannot be effectively scaled due to e.g. pressure limitations associated with tall water volumes, risks of culture contamination, light penetration etc.

[0] https://skyfarmclimate.tech/