Energy Density, and the Intersection of Biotech with Biofuels Production

I attended an MIT new ventures competition (the 100K competition) last night, and the mixer afterward I was asked the following question. after describing the link I see between biotech and energy production.

"Do you think the energy density of plant-produced biodeisel will be high enough to really replace fossil fuels?"

My answer was that as a liquid, biodiesel has a very high energy density, and is also very convenient for storage and transportation (certainly when compared to hydrogen gas). However, I realize that the question is deeper than that (as, of course, is the answer).

The energy density question is not only about the final fuel produced, but incorporates aspects of how efficiently that energy was produced. This is where I think that growing crops in order to convert them into fuels may have serious efficiency problems. What a crop does is convert solar energy into carbohydrates, proteins, nucleic acids, fats, and all the other components of a plant. How efficient is this process?

One gets a small number of harvests per season. Thus, the solar energy that lands on the field day in and day out, at roughly 1 kilowatt per square meter, is converted into the plant matter that is in turn harvested. Now, forgetting about the process of turning the plant into useful liquid fuel (which will require some energy), let's just imagine that we take all of the plant matter that grows on one square meter over the course of a 3 month growing period. Let's say that's 4 tall corn plants. Now, let's combust these plants in their entirety, capturing all of the energy possible. How much energy do we get? How many calories? How many Kilowatt hours? Now, I don't know the answer to this, but it's probably possible to figure it out. However, consider how much solar energy has fallen on that 1 square meter of land during the time these 4 corn plants were growing. Every day, this 1 square meter of land would produce about 5 kilowatt hours of electricity per day if in the sunbelt. 5 kilowatt hours per day times 90 days = 450 Kilowatt hours. Now, that's about half of what the average family uses each month, so each energy user would require approximately 6 square meters to produce enough electricity (by this as yet imaginary 100% efficient process).

But, that's not the point. This one patch of land can produce a half-month's electricity use for the average American family. That includes running the fridge, and the TV and all those battery recharges, etc. etc.. Does it seem plausible that burning (again at 100% efficiency) 4 corn plants could produce that much energy? I find it extremely unlikely. (I'd love to find the actual answer to this question. How much energy could you possibly get out of 1 square meter's worth of corn plants? I highly doubt it would be 450 Kilowatt hours.)

So, growing plants as a means of capturing and storing solar energy for later conversion into a biofeul seems an extremely inefficient process. Not only that, it's got other problems, it takes water, and fertilizer (which takes energy to produce). As an important energy source, plant-derived fuels will presumably be grown by the most intensive processes possible, thus increasing use of fertilizer and pesticides (since no one's going to eat the corn, who cares if it's poisoned). Not to mention the costs of planting harvest and refining, which must be added to the cost of each crop. The current ethanol market shows that corn for energy can in fact displace corn grown for food. That's a good thing in the sense that it forces people to equate the different forms of energy we are capturing from the environment. But it is obviously bad if it's merely a consequence of an inefficient process.

So, why not just cover the same farm with solar panels? Not a bad idea, except the output of solar panels is electricity, which is not nearly as easy to store and transport as is any liquid fuel. This is especially problematic if you want to capture solar energy in the areas where it's done most efficiently and transport the energy elsewhere.

A solution? A continuous, in-line, solar energy capture process that produces liquid fuel directly. Photons in, Biodiesel out (or at least something easily converted into biodiesel). The system would be a hybrid, photosynthetic oil producer. Some companies are looking to do this with algae which take in light and CO2, and convert it largely to fats in a continuous bioreactor. There are lots of engineering challenges here, of course. How to keep the algae alive under a variety of conditions, how and what to feed them under optimal conditions (it will take more than CO2 and photons), what is the maximum energy intensity that these organisms can withstand. But the principle is sound. Photons in, bio-fuel out.

These are challenges that can presumably be solved. However, another approach is to take a totally in-vitro approach, developing systems of purified photosynthesis proteins, and fat synthesis cascades in order to create a wholly bio-catalyzed process without living organisms. This will be much more difficult, but will presumably be more robust. Robustness may not be that big a problem, since as it produces a liquid fuel, the facility can always be placed where conditions are most suitable, the liquid fuel stored and shipped (as it is now from say the Middle East).

This is the intersection of biotech and fuels production. Going beyond the genetically engineered plant, all the way to a bio-process for the direct conversion of photons, water and CO2 into biofuels. Of course, this process won't be 100% efficient either, but even if it's only 1% efficient, it's probably still better than growing corn for fuel.