"The more I find out, the less I know."

Sun - February 10, 2008 at 12:47 PM in

Biofuels are not the answer


I've been gradually becoming less and less excited about the potential for biofuels (ethanol, biodiesel, etc.) to replace fossil fuels, and I've now come all the way around to the opposite opinion of what I used to believe.

I'm now convinced that biofuels are not the answer, either to global warming or our limited reserves of liquid fossil fuels.

Conversion Efficiency
Biofuels are a way of storing the energy from sunlight in (usually) liquid form, suitable for powering vehicles, heating homes, and similar purposes. The problem is that when you measure the net energy content of the resulting fuel (after subtracting the energy required to process it, such as planting, harvesting, fermentation, distillation, etc.), you find that the ethanol, biodiesel, etc. contains almost a laughably small percentage of the original energy of the sunlight which fell on the field. We're talking about hundredths of a percent or less--with some scientists arguing that some fuels (like corn ethanol) actually contain less energy than what it took to process the plants into fuel.

While there's plenty of sunlight available--in theory, just the sunlight hitting Nevada is orders of magnitude larger than what's needed to meet global energy demand--there is a limited amount of arable land on the planet, and much of that is already farmed for food. Given how inefficient biofuels are at capturing solar energy, growing enough biofuel to meet demand will exhaust the available supply of land suitable to current farming practices and fuel crops.

This also sets up an unhealthy competition between growing food and growing fuel, one in which the energy demands of wealthy countries could lead directly to famine in poorer places.

Of course, new technologies could change this calculus. There's plenty of "land" available in the middle of the Pacific ocean, and a (hypothetical, as-yet-to-be-invented) process for farming massive floating algae beds could be an economical way to grow biofuels without competing against land currently used for food.

As things stand right now, however, existing biofuel technology simply won't be sufficient.

Solar Power
Once you realize that biofuels are nothing more than a grossly inefficient form of solar power, however, the answer to a big part of the problem becomes obvious. Existing solar-electric (either photovoltaic or solar-thermal) technology is reasonably efficient, and even starting to come within shouting distance of coal-based electricity in cost. The conversion is efficient enough that our current energy needs can be met with a reasonable amount of collector area, and as a bonus, the best sites for solar collectors are often unsuitable for crops so there would be no food vs. energy competition.

The problem is that the most efficient processes we have for capturing solar power today all convert the solar power into electricity. Electricity is the best choice for many energy uses, but is hard to store (in a compact and lightweight form) for powering a vehicle. You need either an efficient way to convert solar power into liquid fuel, or a better battery.

Battery technology is slowly improving, and commercially available electric cars today can go about 20 miles on a charge. That's enough for a lot of people's daily commute, but doesn't even come close to sufficient for long-haul trucking. The idea that we might someday have battery-powered airliners is almost laughable, given the limited weight available for fuel on an airplane.

Direct conversion of sunlight into liquid fuel is pretty much a pipe dream at this point. There has been some interesting progress in conversion of sunlight into hydrogen gas, but hydrogen (despite the hype) is a poor vehicle fuel: it's hard to handle, and takes up too much weight and space for the necessary pressure tanks.

Transition Plans
Existing products and technology show a likely path for weaning ourselves from fossil fuels. First will come longer range electric vehicles and plug-in hybrids. Shorter trips (which account for a disproportionate share of fuel consumption) will become mostly all-electric, though petroleum will still be used to fuel long trips. A commercially available car capable of going 60 miles on electricity alone seems possible within five years, and for most people that will result in daily gasoline usage close to zero.

More and more of the electricity from the power grid will come from renewable sources, especially wind and solar, now that those are close to or less than the cost of fossil fuels for peak generating capacity. Since those sources are somewhat unreliable, you may get a discount from the power company for not charging your electric car on days when there is less solar or wind power available.

That alone will dramatically reduce our need for fossil fuels, and requires no new technology.

Replacing liquid fuels for long-distance travel (trucks and airplanes) is harder to foresee. It may be that biofuel is necessary for those applications, but at least it will represent much less impact than trying to run our entire transportation infrastructure on biofuel. Better, though, would be a reasonably efficient process for converting sunlight to liquid fuel (maybe with hydrogen as an intermediate step)--but I'm not aware of any promising technology, or even serious research, in that direction.

Biofuel Niches
I don't think biofuels are a sustainable way to replace all our liquid fuel needs: the majority of the burden has to come from solar power and electricity.

There are some niches where it makes sense, though. Long-distance transportation is one of those. Waste biomass conversion (i.e. taking biomass which would otherwise just be landfilled, such as downed trees, agricultural waste, etc., and either burning it for heat or converting it to liquid fuel) is also sensible, though there isn't enough waste biomass around to make a big dent in our fossil fuel use.

But fueling your Hummer with biodiesel is not the way to save the planet.

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