photo: jurveston via flickr
Algae biofuels are probably the most touted future hope to replace large amounts of petroleum-based liquid fuels with a renewable source. However, a new study in Energy and Fuels shows that when you grow that algae in bioreactors made up of clear tubes, so much energy is required that the carbon footprint of the biofuel is over three times greater than the fossil fuel it would replace.According to Anna Stephenson from the University of Cambridge, when algae is grown in clear-tubed bioreactors the energy required to move the algae around so that it gets enough sunlight means that per megajoule equivalent of fuel, the algae fuel has a carbon footprint of 320 grams while petro-diesel takes 86 grams.
photo: Sustainable Initiatives Fund Trust via flickr
Ponds Offer Lower Carbon Footprint, Higher Water Consumption
Stephenson notes that when in grown in open ponds, however, the carbon footprint drops markedly, becoming 4.5 times lower than petro-diesel. New Scientist's summary of that:
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.
Here's the original study: Life-Cycle Assessment of Potential Algae Biodiesel Production in the United Kingdom: A Comparison of Raceways and Air-Lift Tubular Bioreactors
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