The idea is straightforward: grow algae in large quantities and harvest the energy dense byproducts as an alternative to fossil fuels. Like larger plants, microalgae use solar energy to fix carbon dioxide into energy dense molecules, which can then be used to synthesize transportation fuels, or produce bio-plastics and other materials.

But the current processes are largely inefficient, requiring large water and energy inputs. In order to scale up algal biofuel production, the end-to-end process must be made more efficient.

Dr. Halil Berberoglu and Thomas Murphy, researchers in the Mechanical Engineering Department at The University of Texas at Austin, are taking clues from natural processes to improve algae cultivation by designing synthetic “tree” structures. Their system, called the Surface Adhering Bioreactor (SABR), mimics the way trees deliver nutrients and transport sap while allowing for finer control over inputs and growing conditions than traditional cultivation methods (UT Austin):

In this concept, algae cells are grown as photosynthetic biofilms on porous surfaces that keep them hydrated and provide them with the nutrients they need for growing to maturity. Once the biofilm is matured, the supply of certain nutrients is stopped and the growth of cells is inhibited. At this point, the algae are provided with the necessary inputs to carry on photosynthesizing and secreting out energy dense molecules, such as free fatty acids. These are carried away from the cells in small channels mimicking the veins in plants and concentrated using evaporation-driven flows.

These concentrated energy-dense molecules can then be converted to a wide variety of biofuels. Once the algal biofilm reaches the end of its productive life over several months, it is removed, a new biofilm is grown to maturity, and the cycle continues. In this way, the available solar energy, water, and nutrients are directed more towards the production of fuel precursors and less towards growth, achieving a higher solar energy conversion and resource utilization efficiency.

In experiments, Dr. Berberoglu and Mr. Murphy have seen improved efficiency over traditional cultivation methods. Mr. Murphy explains in an email:

“In one experiment, we ran a SABR side by side with an identically sized suspended growth photobioreactor to compare their water and energy efficiencies. The working water volume of the SABR was 25 times less than that of the conventional reactor. Further, about 40 Watts per cubic meter of culture volume were required to mix the suspended growth reactor, whereas this power requirement was eliminated for SABR. In certain regions of the SABR, the growth rate was four times greater than that of the conventional reactor. Averaged over the entire SABR, the growth rate was about equal to that of the conventional reactor."

Even with these improvements, maximizing productivity while minimizing water loss remains a significant challenge. Mr. Murphy again:

“Since evaporation provides the driving force for delivering nutrients and water to the organisms, faster nutrient delivery requires faster evaporation. Real trees face this challenge as well. When it gets hot, leaves close their stomata, thereby preventing excessive evaporation but also retarding delivery of nutrients from the soil. This is an effective water management strategy, but then again, trees don't grow very quickly. Right now we are working on strategies for maximizing productivity while minimizing the water loss rate.”

The UT Austin research team intends to submit their methodology and results for peer review next month. For more information on Dr. Berberoglu's research, visit his faculty page here.

Video credit: Faculty Innovation Center, Cockrell School of Engineering, The University of Texas at Austin.