Caught part of a TV show the other night documenting the algae growth experiment at the Redhawk electricity plant in Arizona. Some details for the experiment can be found here and here.
The idea is that CO2 and H2O emissions are diverted to an algae farm near the power plant. Currently that farm appears to be quite small, as it's still experimental (I think they had 8-10 upright tanks, probably containing a few hundred gallons each). They plan to produce biodiesel, ethanol, and protein supplements (they suggested for cattle feed).
Towards the end of the program, they showed one of the new greenhouses they're building to expand the experiment. Looked to me like it would expand total capacity about ten- to twenty-fold.
Then finally they answered the question that had been running through my head the entire show: how much CO2 can actually be processed with this technique? According to the program, two acres' worth of tanks would process the emissions from approximately one generated megawatt. This assumes a certain level of operating efficiency (of which the Arizona sun is a big contributor).
Time for some math: the Redhawk power plant (which is natural gas-fueled) generates up to 1040 MW's. Thus to handle the full capacity of the plant, you'd need 2080 acres of algae tanks.
That's over three square miles! This of course raises some practical questions...
1. What if space isn't immediately available next to whatever plant? Wouldn't you have to either build the tanks underground or somehow transport the CO2 emissions off-site?
2. What do you do in regions that aren't as optimal as Maricopa County Arizona? Wouldn't less optimal regions require even more algae tanks to process the same CO2 output?
3. How easily does the process adapt to other types of power plant? Natural gas is an important fuel for electrical generation, but it doesn't compare to coal for total output.
4. This fact sheet says one acre produces 6000 gallons of ethanol and 5000 gallons of biodiesel each year. The fact sheet doesn't mention it but the TV show pointed out that the algae doesn't directly produce the fuel--it has to be further refined. Knowing that, how much labor and energy is required to harvest the algae from a 2000 acre farm and refine it into usable fuel?
Bottom line, this sounds like a good idea. But IMO there are still some pretty big hurdles to cross before this technology will have a real impact on CO2 emissions.
BTW: 11000 gallons/acre of fuel per year sounds okay until you do the math. That's only 200 barrels, meaning the full production from a Redhawk-sized plant would be 400K barrels per year. That's just not very much compared to current crude oil production and refinement.