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Algae-based biofuel can have 4X carbon footprint of petro-diesel, study says

University of Cambridge researchers say closed air-lift bioreactors require more energy to produce algae-based biofuels than needed for fossil fuels.
Written by Andrew Nusca, Contributor on

One of the more simple ways to move to alternative energy sources is algae-based biofuels, which do not require a complete infrastructure overhaul to power vehicles and other machines.

For many, it's a no-brainer switch, which is why big companies such as DuPontandHoneywell as well as startups such as Verenium and Cobalt are working on a solution. (And have the U.S. military interested.)

But according to a new study published in the journal Energy and Fuels, it all depends on how you grow your algae.

According to the study, algae grown in closed air-lift bioreactors made of transparent tubes uses more energy than would be saved by using the biofuel over a fossil fuel -- more than three times as much, in fact.

Why? University of Cambridge researcher Anna Stephenson writes that all that energy is used when the algae is moved around to get enough sunlight, giving it a carbon footprint of 320 grams per megajoule equivalent of fuel. That compares to 86 grams for petro-diesel.

That's not to say algae-based biofuel can't be efficient -- Stephenson notes that algae grown in open ponds achieves a carbon footprint of 19 grams per megajoule, about four times smaller than petro-diesel.

She tells the New Scientist:

If you use tubular bioreactors, frictional losses mean the energy required to pump the culture around is so high that the biodiesel would have a much greater greenhouse gas emission than fossil diesel.

The big takeaway here is that growing algae in open ponds works.

There are two drawbacks, though. First, the water in the ponds tends to evaporate, which means they could require more water (a precious resource that's dwindling) than biofuel crops.

Second, open ponds have a smaller yield than tubes, because the tubes in the end expose the algae to more sunlight. (It just takes energy to do it.)

That's why Cambridge chemical engineer Ben Taylor is reportedly working on a closed reactor consisting of a system of tubes containing baffles to direct water and algae in the most energy-efficient way possible. The New Scientist notes that Taylor's system uses just 4 percent of the energy consumed by conventional tubular reactors.

In the end, it's all about price parity. Can algal biofuels soon be produced as cheaply or more cheaply than petroleum-based fuels? According to this study, you can't truly know without proper life-cycle assessment.

Photo: Algae tubes. (Steve Jurvetson/Flickr)

This post was originally published on Smartplanet.com

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