Outdoor production of microalgae could be set on a sound industrial basis if solar energy were utilized at a much higher efficiency than presently obtained. Many types of photobioreactors have been developed in the past in an attempt to answer this challenge, but their photosynthetic efficiency has been rather similar to the basically inefficient open raceway commonly used today. Efficient utilization of the oversaturating solar energy flux mandates that reactors should have a narrow lightpath to facilitate ultra-high cell densities, be maximally exposed to sunshine, and have an efficient mixing system to create strong turbulent streaming to affect dark-light cycles of the highest possible frequency.