Fake Trees, Real Potential
By mimicking a tree’s approach to photosynthesis, the Surface-Adhering Bioreactor grows crops of algae with unheard-of efficiency. Assistant professor of mechanical engineering Halil Berberoglu built it, along with graduate student Thomas Murphy and three NASA biologists. At just a couple inches high, the prototype is more bonsai than banyan, and it doesn’t look like a tree. “There is no trunk and there are no branches,” Berberoglu says. “It’s essentially a configuration of giant leaves.”
Leaves made of algae, that is. These simple organisms conduct photosynthesis, and they’ve long been studied for their ability to produce oxygen and biofuel and pull carbon from the atmosphere. But they’re surprisingly expensive to raise.
Normally algae are cultivated in artificial ponds or tanklike devices called photobioreactors. If you stir algae into water, add nutrients, and provide sunlight, they’ll capture the sun’s energy to do their work.
Unfortunately, those approaches require a lot of water and energy. As much as 99 percent of the volume of a photobioreactor is water, which is pumped constantly to keep the algae from settling. Harvesting the biofuels is also an energy-guzzling process, so they aren’t cheap enough to compete with oil. And though engineers would like to put algae to work recycling stale air in manned spacecraft, lifting their heavy tanks into orbit isn’t practical.
That’s why Berberoglu and Murphy started thinking about trees. Trees carry out photosynthesis with proportionately far less water, and they don’t exert energy getting that water to their leaves. The secret: evaporation from the leaves, which pulls water up through the tree’s vascular system like suction on a straw. It’s passive, but powerful: A mature pecan tree can lift up to 200 gallons of water a day.
The artificial tree works in much the same way. It contains a row of plastic plates that stand upright in a tray of water and fertilizer. Tiny channels run through each plate, like the veins of a leaf. As sunlight warms the device, liquid rises through the channels to the surface of the plates, where it evaporates. The plates are coated with filter paper, and it’s there that the algae gather and photosynthesize like leaves.
“Evaporation is a very neat way of doing this,” Berberoglu says. “You just use the heat part of the solar radiation coming in to drive the transport process.”
To harvest the artificial tree’s algal lipids, you can just sponge up the drops where they gather at the tips of the “leaves.” “It’s analogous to harvesting eggs from a chicken as opposed to eating a chicken,” Murphy says.
The team has applied for a provisional patent, and they’re sending a system to the International Space Station for further testing. If the artificial trees work in space, Berberoglu says, it will be a game-changer. It may not only produce fresh oxygen for astronauts, but could also replace old methods of culturing cells in space.
Here on Earth, it’s too soon to speculate if these algal products will replace fossil fuels. But Berberoglu says leaves up to 1.5 meters high are already doable, and we can “plant” as many as we need, side by side. If all goes well, we may one day see forests of these trees.
Top, The photosynthetic productivity of the biofilm is measured with a pulse amplitude modulate fluorometer. Bottom, Cultures of green algae, left, and cyanobacteria, right, in bottles use much more water than the algae grown as biofilms.
Photos courtesy Carol Grosvenor, Department of Mechanical Engineering.
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