The 100 g/square meter/day productivity figure seems much too high.
The U.S. Dept. of Energy used a figure of 50 g/square meter/day as
their target. Even that was unobtainable on a continual basis in
outdoor ponds mainly because of low night temperatures. Over an 8
month period, they obtained 15 to 25 g/square meter/day.
The main advantage that algae offer compared to potatoes is that algae
could theoretically maintain maximal photosynthesis forever. Once an
algae monoculture had reached an optimal concentration (for maximal
photosynthesis) in the pond, small amounts could be withdrawn
periodically, and the optimal algae concentration could be maintained
indefinitely. Every crop of potatoes starts with a zero LAI and slowly
reaches an optimal LAI. Then the LAI goes back to zero at harvest. The
potato field will probably be fallow most of the year while algae
could be producing yearround.
Other theoretical advantages of algae for biodiesel are:
1. Algae could produce oil all the time, while potatoes form their
tubers only during the later stages of crop production.
2. Algae could be grown on nonagricultural lands in the desert while
potatoes require agricultural land.
3. Oil extraction from algae seems easier than fermentation of potato
starch to produce ethanol.
Despite all the theoretical advantages of algae, there are many
technical problems such that the energy costs to grow the algae and
extract the oil would probably exceed the energy in the oil they
1998. A Look Back at the U.S. Department of Energy's Aquatic
Species Program: Biodiesel from Algae.
David R. Hershey