Photobioreactors, the production systems used to grow algae, seem to operate on a simple concept: place photosynthetic microorganisms in a liquid growth medium and add light.
But Dennis Vigil, associate professor of chemical and biological engineering at Iowa State, and his research partner, Michael Olsen, professor of mechanical engineering, know that photobioreactors are much more complex systems than they seem. These researchers seek a better understanding of photobioreactors, which may prove beneficial in the race to find alternative fuels.
A better way to biofuels
Vigil and Olsen are part of a larger scientific effort to find alternatives to fossil fuels, which includes methods to produce biofuel from cultivated algae. In turn, researchers worldwide have designed a variety of photobioreactor systems to grow algae over the past thirty years.
However, economically viable commercial-scale methods to convert solar energy into biofuel via algae remain elusive.
To that end, Vigil and Olsen are leading a $350,000, three-year National Science Foundation project to understand the best way to design and optimize highly efficient photobioreactors.
“The technical and economic feasibility of large-scale production of biofuels from algae is going to require two major advancements,” said Vigil. “One is the bioengineering of elite microorganisms, and the other –my area of research– is improved design of the process equipment including photobioreactors.”
Finding an accurate model
Vigil’s and Olsen’s research project proposes to construct reliable computational models that can accurately describe the physical behavior of photobioreactor systems, which in turn can be used in the design of more productive, efficient systems. To validate the computational models, the team will be conducting fluid dynamics and light transport experiments.
The first challenge of the research is to accurately describe the multi-phase flow occurring in a photobioreactor.
“You have a liquid growth medium circulating through the system,” explained Olsen. “In addition to the turbulent fluid flow, you have gas bubbles that are reacting to the turbulence and causing their own turbulence. And then there are the microorganisms as well.”
The second complexity, Vigil explained, is the transmission, scattering, and absorption of light as it goes through the algae suspension.
“Not all light is equally effective,” he said. “For photosynthesis, algae need certain wavelengths of light. We are learning that photobioreactor designs that were thought to receive plenty of light may be getting too much of the wrong wavelengths for optimal algae production.”
Measuring flow and light
The team will use lasers and high-speed cameras to track the positions of tiny “seed” particles in the flow of the bioreactor, a method called particle image velocimetry.
“Think of the particles as dust in the wind. We can capture images of the particle at a certain location, and then at a later time at a different location. We do a statistical dot-to-dot to tell us exactly how they behaved in the flow,” said Olsen.
Measuring light penetration into the reactor will allow the researchers to develop spectral models that describe varying amounts of different wavelengths of light, something Vigil said is a novel approach.
Vigil and Olsen said the pairing of the computation fluid dynamics and the radiation model will provide powerful insight into the way photobioreactors work because it allows for precise predictions concerning the light exposure experienced by algal cells.
“We can see an algae cell as it goes around in the reactor and observe what sort of radiation it is receiving. We can better understand why it is producing at a certain rate. These are the kind of deep, quantitatively accurate details that can be used to predict how much algae different reactor designs are likely to produce, and how they’ll perform,” Vigil explains.
A better design
Their insight is already giving Vigil and Olsen some ideas about tailoring reactor design. From previous research, Vigil noted that the optimal amount of light exposure algae need in relation to its entire photosynthesis process may be short, and that the random light exposure caused by bubble-induced mixing in most photobioreactor designs is not ideal.
“Essentially, if you want to speed up photosynthesis and use light more efficiently, the best thing possible is to have the microorganism capture photons, and then move it to a dark region of the reactor where the slow thermochemical reactions can proceed,” said Vigil.
That led Vigil to develop a bioreactor design using Taylor vortices, which occur when fluid is contained in between two rotating cylinders.
“Organizing the fluid flow creates a sort of conveyor belt system,” said Olsen. “It shuttles the organisms from light to dark in a much more orderly way.”
Ultimately, the goal of computational models is streamlining development of production systems, such as photobioreactors.
“We want to simplify this problem as much as we can with models that are fast enough and accurate enough so that engineers can use it in the design process,” said Olsen.
June 28, 2013 by Laura Millsaps