The basic problem with virtually all biofuel proposals is scale. Given present populations and energy utilization, biofuel simply doesn't add up.

The world uses 100 million barrels of petroleum daily, the US about 20 million (slightly less). Even small uses of petroleum such as aviation (about 2% of the total) represent vast amounts of fuel. And with projected population and energy utilization growth, demand will increase above this.

The basic maths are laid out in a paper I've found to be a tremendously useful reference, Jeffrey Dukes, "Burning Buried Sunshine": http://globalecology.stanford.edu/DGE/Dukes/Dukes_ClimChange...

Briefly: present human consumption of net primary productivity (plant growth) is about 14% of Earth's total. The fossil fuels consumed in 1997 (the date analyzed by the paper) comprise the equivalent of another 21% of NPP (and the actual period over which the fossil fuels accumulated is far greater: 400 years of growth, accumulated over ~5 million years, due to inefficiencies in the process of coal, oil, and gas formation and accumulation).

You can look at biofuels on an energy per unit area basis. There's roughly 1 KW/m^2 of incident sunlight. Current PV solar cells convert around 20% of this, and tend to capture the equivalent of 8 hours of sunlight per day, which is to say, a 30% capacity factor. Maximum PV efficiency (with far higher costs of production) is around 85%. But with present commercially available means, you can capture 20% of 1 kW for 8 hours each day, or roughly 1.6 kWh per day. A kilowatt (kW) is a unit of power, a kilowatt-hour is a unit of energy.

US electrical production of roughly 4,000 TWh would require 7800 km^2 of area to produce, or a square 88 km on a side.

That's for electricity.

Plants and algae have an efficiency of from 1-10%, with conventional crops and plants generally in the 1-3% range. We can do the same area calculation as above, or work from yields expressed as gallons of fuel per acre. A high-yield crop such as canola is generally given as 100 gal/acre, hemp advocates claim as much as 300 gal/acre (though this is generally disputed), and algae biofuels as much as 1000 gal/acre.

Given the 7.3 billion barrels of oil consumed, lets work out acreage requirements for algae. GNU Units is one of my favorite tools for this:

You have: 7.3 billion barrels / (1000 gallons/acre)
You want: million km^2
* 1.2407662

That is, we need 1.24 million km^2, or a square 1113 km on a side (that's 479,000 mi^2, or a square 692 miles on a side, or a bit more than 300 million acres of land.

There are presently slightly more than 400 million acres of land under agricultural production in the US. Three quarters of that would have to go to fuel production under one of the most productive biofuel processes we have.

Moreover, algae require ponds, which is to say, a LOT of water, pumping pesticide, fertilizer, and a lot of processing.

Say you figured out a way to move all of this off-shore, and could set up grow-ponds in the oceans off the Atlantic and Pacific coasts? They're roughly 1000 miles long each, so to figure on the width of our algae grow structure, we divide 479,000 by 2000 and find ... the ponds would have to extend only 239 miles off-shore. That's from Key West, FL, to Eastport, ME, from San Diego, CA, to Port Angeles, WA. And they'd have to survive freezing and nor'easters and hurricanes and ...

Yeah, sounds kinda tough.

If the US population and per-capita energy utilization were far less, say maybe 10% of present levels, this might be conceivable, but as things stand, not really. And then there's the rest of the world.

Even more modest proposals to, say, supply aviation fuel from halophyte + aquaponics farms, described by Boeing as the biggest breakthrough in biofuels, really doesn't add up: http://redd.it/1wo2hl

(Another interesting fact I discovered in researching that was that the US hit Peak Aviation Fuel in 1999, and present flight activity is more than 30% below what projections of that time suggested. Passenger miles are up somewhat given increased load factors -- more bumped flights and less legroom due to cramming more people onto fewer flights. Kopits points this out also in his presentation, confirming my observations.)

Which is why I find the NRL's project so interesting. The magnitude of its plant and scale requirements is far, far less than competing biofuel alternatives.