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Pond Scum That Makes Fuel Year-Round

Could geothermal heat boost biofuel output?
February 2, 2009

Algae growing in a heated pond at the University of Nevada test site. Credit: John Bebout

Green algae, or common pond scum, have been held up as a renewable energy panacea. Highly refined strains of the fast-growing biomass can absorb CO2 straight from a power plant’s smokestacks, thrive in brackish water, and have the potential to yield far more biofuel per acre than corn does. One promising method of algae production involves nurturing the green goo in decidedly low-tech, open-air ponds. But the approach is plagued by a number of issues, including a fivefold drop in yields in cold winter weather.

Now a team from the University of Nevada has shown that simply cranking up the heat can avoid much of the seasonal production decrease. In late November, John Cushman and his colleagues inoculated an outdoor pond with a “starter” culture of halophytic (salt-loving) algae cells. Since then, they have circulated water heated by natural gas through the pond to keep it at a constant 29 °C (85 °F), despite subzero winter temperatures–an approach that simulates the use of heat from geothermal vents. Three weeks later, they harvested approximately five pounds of algae by dry weight–just half the yield anticipated for summer.

“This will allow us to move from a seasonal crop to optimal production 365 days a year,” says Cushman of the potential to combine algae production with geothermal heating. If the scheme proves a success, Nevada could be in a unique position to capitalize. The state is bathed in sunlight, has vast tracks of open desert, and sits on top of little-utilized saline aquifers and geothermal resources.

But even with the addition of geothermal heat, Al Darzins, head of the National Renewable Energy Laboratory’s (NREL) recently reinstated algae biofuel research program, questions whether current production methods can be cost competitive. “The price range of algal oil that could currently be produced, from open ponds to closed bioreactors, may be $10 to $40 per gallon,” Darzins says. “And that’s even before you turn it into fuel.”

While geothermal heat might increase production, Darzins says that the added investment could be significant. “You still have to put in pipes to transfer the heat to your algae ponds, and that comes at a cost.”

The open-air facilities are also susceptible to contamination by lower-yield strains of algae and other organisms. Darzins says that the highly saline environment–the salinity of the University of Nevada test pond is roughly twice that of seawater–would help limit outside contamination, but he admits that the problem is likely to persist. “What’s to say some protozoan that just loves to eat algae might take over the pond? There are ways of preventing their growth, but everything has a cost, and it has to be dirt, dirt cheap.”

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