"IT'S BIZARRE.  IT'S TOTALLY ABSURD."

In the Dallas example, we presented a realistic assessment of the water requirements for an open photobioreactor, and the amount of water lost to evaporation was not insubstantial.  This appears to be in stark contrast to closed systems who claim minimal water loss as a major advantage.  But things are rarely as they seem, especially in the algae arena.

As we have mentioned before, maximizing the full potential productivity of algae requires year-round growth.   This, in turn, requires the photobioreactors to be maintained within a narrow temperature range, usually centered around 25C.  No matter where the bioreactors are located, either some heating is required in the cooler months or some cooling is needed in the warmer months, or both.

Until recently, we could find little information on the energy required to heat or cool closed photobioreactors.  Proponents of the various systems being proposed or developed either failed to address this issue (and its economic implications) or preferred not to disclose it.  In 2009, however, Bechet et al. published "Mechanistic Modeling of Broth Temperature in Outdoor [closed] Photobioreactors."   We finally have some data with which to reason.

We'll let you read the paper for a full explanation of how Bechet et al. reached their conclusions.  Suffice it to say that their model predicts that in Merced, California it would take around 18,000 GJ/year/hectare (6.9 x 10-9 BTU/year/acre) to cool a closed photobioreactor to 25C, assuming a photobioreactor with an illuminated area of 1 square meter. 

So where does the energy to cool the closed photobioreactor come from?  The literature contains a number of suggestions, the two most common being either a cold-water heat exchanger or an evaporative cooling system.  The former is capital intensive to build and operate while the latter consumes vast amounts of high-quality fresh water-- nearly 1 million gallons/acre/year in the Merced example.

Clearly, operating a full-scale closed photobioreactor may be as energy and water intensive as an open system, IF NOT MORE SO, particularly in areas where both heating and/or cooling of the broth are required.  

Next time you talk to a closed photobioreactor proponent, ask them how they intend to heat and/or cool their system at scale and at what cost.  But be prepared to hear "that's proprietary information."  This wouldn't be a bad question for the Department of Energy (DOE), either, as they continue to pour millions of taxpayer dollars into closed photobioreactor research.  On the whole issue of closed photobioreactors, we'll let Dr. John R. Benemann, "The Man Who Wrote the Book on Algal Biodiesel, "  have the last word on attempts to make algal production more economical by using enclosed bioreactors (rather than open ponds, as were used for the Aquatic Species Program).  "It's bizarre; it's totally absurd." -American Scientist, 2006