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When algae are farmed in perspex tubes, she says, the energy needed to pump the algae around to ensure adequate exposure to sunlight results in a carbon footprint of 320 grams per megajoule equivalent of fuel. This compares with 86 g/MJ to extract, refine and burn regular diesel (Energy and Fuels, DOI: 10.1021/ef1003123).
"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," she says.
Her model shows that growing algae in open ponds offers "a lot more potential to produce an environmentally sustainable fuel" - the footprint of biodiesel produced this way is only 19 g/MJ. But open ponds have one major drawback, namely that the water tends to evaporate, making them potentially more water-hungry than biofuel crops. What's more, the yield of diesel from open ponds tends to be lower than from growing tubes, where the algae have better exposure to sunlight.
Just an idea, why not have the ponds set up like a green house? The water that evaporated would be contained and captured to return to the pond. Thus reducing the loss and need for high water usage.
If you look at every one of the alternative fuel proposals they cost more in money and CO2 out put then they save.