The economics of closed bioreactors: a back-of-the-envelope calculation

The economics of closed bioreactors: a back-of-the-envelope calculation

Algenol Biofuels in Naples, Florida, has metabolically enhanced cyanobacteria, or blue-green algae, to directly synthesize ethanol—one of the few, if only, companies working with direct production of this fuel.

The company has designed enclosed photobioreactors and in June announced it had licensed its technology to Biofields of Lomas de Chapultepec, Mexico. Biofields says it has committed $850 million to building the facility on 102,000 acres in the Sonoran Desert, where by 2012 it intends to produce a whopping one billion gallons of ethanol per year.

Algenol estimates that its technology can produce 10,000–12,000 gal/ac/yr of ethanol in the near term. But a simple calculation makes it hard to believe those yields will make Algenol’s bioreactors economical. The company’s current bioreactor design looks like a soda bottle turned on its side, and is about 3 feet wide and 50 feet long, says Paul Woods, CEO. At that size, the company could at most fit 290 bioreactors on an acre. At 12,000 gal/ac/yr, each bioreactor would produce 41 gallons per year. If the company could sell that ethanol for $5 a gallon—more than the going rate—it would make $205 per bioreactor per year.

Surely each of those bioreactors costs more than $205 per year to operate, especially if the cost of designing and building them is included? Woods’ answer: no. He says he hopes they can maintain the bioreactors for next to nothing. The company’s bioreactors will require less maintenance than other designs, he says, because the algal culture is static. Rather than harvesting algae, squeezing out the oil and growing new culture, as with lipid oils, the company waits until the ethanol is released from the organisms naturally and evaporates into the head space of the bioreactors, and then collects it.