A unique bioreactor at the Energy Department's National Renewable Energy Laboratory (NREL) can help find the ideal locations for farms to produce algae that could someday compete with renewable diesel, cellulosic ethanol, and other petroleum alternatives as transportation fuel.
It does so by revealing the intricate biochemical rearrangements that algae undergo when grown in different locations in the United States. The bioreactor has also demonstrated that algae grown in ideal climates and given the optimal amount of nutrients can produce not just lipids, but proteins and carbohydrates that can be turned into diesel, butanol, ethanol, and other fuels useful to industry.
Aeration helps algae grow and helps replicate real-life conditions in the Simulated Algal Growth Environment (SAGE) reactor at NREL. The reactor controls light and temperature, helping scientists determine not just what strain will grow the best, but where in the United States it may do so. Ideal strains can be harvested for their lipids, proteins, and sugars for use in biofuels. Photo by Dennis Schroeder, NREL
"We've almost doubled the fuel we can get out of the same amount of biomass by using these additional components," said NREL Senior Scientist Lieve Laurens.
The Simulated Algal Growth Environment (SAGE) reactor resides in the Field Test Laboratory Building on NREL's campus in Golden, Colorado.
The reactor so precisely controls light, temperature, and delivery of carbon dioxide that it can mimic conditions anywhere. If scientists want to know how algae will grow in brackish water in the southwestern United States under mostly sunny skies, the dials can be set for that precise meteorology. NREL's bioreactor is unique because despite its large size and greatly increased capacity, it can replicate entire locations and climates—day, night, sunlight intensity, and temperature profiles. Light intensity is programmed, as is carbon dioxide delivery and uptake.
"We can model how a particular strain would grow in different locations around the United States, and how this impacts the oil productivity," as well as the general biochemical composition of the biomass, Laurens said.
"Feed and Starve" Approach Yields More Potential Fuel
Algae use the chlorophyll in their cells to turn sunlight into energy through photosynthesis. Add in nutrients and carbon dioxide, and the algae can produce sugars while rapidly reproducing.
Nick Sweeney (left) inoculates algae being grown in a tent reactor as Will Long looks on. Developed at NREL, the reactor controls light, carbon dioxide delivery, and temperature to establish baseline growth curves and recreate real-world growth conditions, allowing researchers to grow denser cultures in less time. Photo by Dennis Schroeder, NREL
Three "champion" strains of algae grow in the NREL reactor, which allows about five times the culture volume of other similar commercially available controlled-environment reactors. With its large size and ability to recreate real-world conditions, NREL's reactor allows researchers to grow dense cultures in far less time.
Optimal conditions — nutrients, carbon dioxide, and sunlight — lead to the extra energy capable of doubling the algae's productivity.
NREL has also come up with better and faster ways to analyze and characterize the components in the algae conversion process. "We harvest every other day," yielding maximum information about the changes in an algae population that can double in size twice a day, Laurens said.
Algal cells absorb the sunlight, which, together with carbon dioxide, grows the colony. For the strains being evaluated, the more humid air and warmer nights of Arizona look more conducive to optimal growth than the high desert climate and very cold nights of New Mexico. This makes sense because these strains have been cultivated extensively outdoors in Arizona. Algal strains adapted to New Mexico's climate may not grow as well in Arizona.
Laurens and NREL Bioconversion Engineer Nick Nagle can also trick the algae into doing more useful things than simply multiplying in number. For example, if they stint on the amount of nitrogen available, the algae go into storage mode, rather than multiplying mode. Some algae double in size a couple times a day when enough nutrients are available. When the nutrients run out, the algae stop growing so fast. But carbon dioxide and sunlight are still present, so in the interest of self-preservation, the algae store that carbon dioxide as high-energy organic molecules. It's a change that can be seen over the course of a few days.
"Once the algae run out of nitrogen, they turn into couch potatoes," Nagle said. "The cells start eating a lot of carbon and sitting around, putting on weight. That weight is literally in the form of carbohydrates and oils" — just what the biofuels industry needs from the little critters.
The "feed and starve" rotation is one useful pathway to getting more oils out of the algae.
Industry Wants All Parts of the Algae Now
NREL is working with Arizona State University to examine how champion strains of algae convert sunlight into biomass, keep nutrients viable, and respond to light energy. Early on, the protein content is high; then carbohydrates have their day in the sun, followed later by lipids.
"From the feedstock, we get these different products — lipids, proteins, carbohydrates," Nagle said. A few years back, industry would approach research labs with a specific request — how to use algae to get diesel oil. Now, they want it all: protein to turn into butanol or methane; lipids to turn into diesel; carbohydrates to turn into a third fuel.