Growing algae to produce biofuel is becoming a popular option to meet the US future energy needs. Currently, the two main ways to grow algae are open pond systems and algae bioreactors. Open pond systems are large outdoor ponds, which usually consist of a closed loop channel that continually circulates the water/algae mix. The algae bioreactor grows algae in a controlled closed-loop system that uses water, sun, carbon dioxide, and nutrients. The algae are pumped through vertical clear plastic bags and/or transparent pipes to allow for continuous addition of carbon dioxide and nutrients. From the research I’ve done, growing algae in an open pond system is possible, but the bioreactor is a much better alternative to implement large scale production.
Algae bioreactors and open pond systems have the same core idea of producing algae to create biofuel, but the bioreactor can do so much more effectively. Compared to open pond systems, bioreactors are much better at controlling the growing conditions, which allow the bioreactors to grow enhanced strains of algae that result in higher productivities. One study estimated that bioreactors can produce nearly 30 times the biomass concentration than general open pond systems, which results in a larger amount of oil per acre produced compared to open ponds.
The algae bioreactor is also able to use the greenhouse emissions from industrial plants to help the algae grow more efficiently, while also breaking down harmful greenhouse gases. By having a power plant pump its carbon dioxide into the bioreactor, the carbon dioxide will actually be fueling the creation of an energy source, rather than directly polluting the atmosphere. An independent testing firm measured the gas composition going into and out of the algae bioreactor at MIT. Their results showed that there was an 85.9% reduction in NOx emissions, an 82.3% reduction in CO2 emissions on sunny days, and a 50.1% reduction on rainy overcast days.
Lastly, bioreactors are able to produce biofuel cheaper than open pond systems. According to one source, when both systems are producing 100,000 kilograms of biomass, “the estimated cost of producing a kilogram of microalgal biomass is $2.95 and $3.80 for photobioreactors and [open pond] raceways, respectively.”