After $100 Million, Exxon Backs Off Algae as Fuel

[Scootaloo]

36. Yes, I am

And thanks, I know how photosynthesis works. You're missing the point.

It takes energy to grow the algae - can't do it with sun alone. You need whatever it takes to fertilize and aerate the stuff - the manufacture of which requires energy. And of course, the construction of the tanks themselves take energy. And then the harvesting takes energy. The processing takes energy. The refining takes energy. The storing takes energy. The transport takes energy. Every bit of manufacture at each step, takes energy.

At each stage, energy is lost as well - no machine operates at 100% energy efficiency, it is just impossible (perhaps you believe in perpetual motion machines?)

What this means is that using biofuels to create more biofuels results in diminishing returns in terms of energy output. Even in a fantasy dreamland where 100% energy efficiency could be achieved, 100% output is all you could get - that is, equal to what you put in. Never more.

This is the same for fossil fuels as well, by the way; using fossil fuels to harvest and process fossil fuels is inefficient in the same way. The trick there is that fossil fuels are the collected energy storage of millions of years. we're not starting from scratch as with manmade biofuel. We're essentially transferring stored energy from millions of years ago, to the modern day, creating the illusion of over 100% efficiency in production. As fossil fuel stores shrink however, that efficiency declines, because more work is needed to get lesser quality material like the tar sands. if it started out at 200% efficiency, it shrinks down to 150% efficiency, then 120%, 107%, etc. We'll eventually reach the same problem here as with biofuels - the energy output will not justify the energy input. We hit that point faster with biofuels because we're "starting from scratch" rather than drawing on eons of stored energy.