I don't think algae can take care of itself as well as higher plants, so growing ponds would have to be built, needing constant maintenance of perfect conditions. Kelp might be interesting.
http://www.sarid.net/technology/051027-food-fuel-compete.htm
" For net energy yield, ethanol from sugarcane in Brazil is in a class all by itself, yielding over 8 units of energy for each unit invested in cane production and ethanol distillation. Once the sugary syrup is removed from the cane, the fibrous remainder, bagasse, is burned to provide the heat needed for distillation, eliminating the need for an additional external energy source. This helps explain why Brazil can produce cane-based ethanol for 60¢ per gallon.
Ethanol from sugar beets in France comes in at 1.9 energy units for each unit of invested energy. Among the three principal feedstocks now used for ethanol production, US corn-based ethanol, which relies largely on natural gas for distillation energy, comes in a distant third in net energy efficiency, yielding only 1.5 units of energy for each energy unit used."
http://www.sarid.net/technology/051027-food-fuel-compete.htm
" For net energy yield, ethanol from sugarcane in Brazil is in a class all by itself, yielding over 8 units of energy for each unit invested in cane production and ethanol distillation. Once the sugary syrup is removed from the cane, the fibrous remainder, bagasse, is burned to provide the heat needed for distillation, eliminating the need for an additional external energy source. This helps explain why Brazil can produce cane-based ethanol for 60¢ per gallon.
Ethanol from sugar beets in France comes in at 1.9 energy units for each unit of invested energy. Among the three principal feedstocks now used for ethanol production, US corn-based ethanol, which relies largely on natural gas for distillation energy, comes in a distant third in net energy efficiency, yielding only 1.5 units of energy for each energy unit used."