Polymath A (mostly) technical weblog for Archivale.com

March 29, 2010

The Fallacy of “Energy Deficits”

Filed under: Materials — piolenc @ 9:03 am

Biofuels can pay even if they deliver less energy than went into making them

Some time ago, there was a big dustup, on-line and off, about a study of alcohol production from corn that argued that less energy was derived from  the alcohol than was used in making it. This elicited a storm of protest and contestation from alcohol advocates, who believed that, if true, this fact would condemn their favorite renewable motor fuel to oblivion. My reaction was slightly atypical:

I said “who cares?” and turned the page.

It does seem obvious that, if you get less energy out than you put in, the process must not be viable, but a little thought will demonstrate that this is not necessarily so. To set the stage for clearer thinking, let’s use a concrete example.

Suppose that you have a hunting lodge off in the mountains, too far from civilization and its amenities to bring in electricity from the grid. You like your comforts, though, so over the years you’ve built up a sizable array of photovoltaic panels, producing several hundred watts of output in full sun. Unfortunately, most of your electricity consumption takes place at night, so you’ve also had to build up a battery of Edison or Ironclad cells to store energy during the day for use at night. But wait. When you put three kilowatt-hours of energy into the battery, you only get two or so back. You have an energy deficit; your installation is not viable! Fortunately, you’ve never heard of energy deficits, so you go on happily enjoying the light of your compact fluorescents and the sound of your radio and CD player well into the night. You can even talk to the missus using your cell phone and that booster antenna in the treetops, and get your email using that satellite dish on the roof and your laptop computer.

Some will object that the foregoing example is irrelevant, but in fact it is completely on point. Fuel is not an energy source—it is an energy carrier, just like the battery bank in your hypothetical hunting lodge. Like the battery, it allows energy from an intermittent and diffuse source—the Sun—to be concentrated and  stored for later use. In both cases, if the process—battery storage or biofuel production—were not implemented, the result would be that energy would go to waste. In the hunting lodge example, the wasted energy would be the incident solar energy at midday, when you were out hunting and the lodge was only using a few watts of standby power. In the farm alcohol example, the wasted energy is that stored in the waste biomass, usually cornstalks, used to fire the mash heater and the still.

It’s clear, then, that mere quantity of energy does not tell the whole story;  the nature of the input also matters. If, instead of solar panels, you had used a diesel-fired generator to charge your battery bank, the result could be different; you might save diesel fuel by using the generator to power your loads directly, instead of first storing the energy in a battery bank. Likewise, if Farmer Brown used diesel fuel or heating oil to fire his mash tank and still, it might matter very much if the alcohol that he produced had less heating value than the high-grade fuel that he consumed.

I say “might” instead of “would,” because the relative market value of the input and output also matter. Suppose for instance that Farmer Brown was selling into a market dominated by fuel-cell-based cogeneration. It’s relatively easy to make a fuel cell that will consume lower alcohols, but feeding one on a mixture of higher hydrocarbons like fuel oil usually requires a fuel reformer—an expensive piece of equipment that takes a big bite out of efficiency. In that event, Farmer Brown might very well be able to get more for the clean alcohol that he produces than he paid for the fuel oil that he used, even if the heating value of the fuel oil was greater.

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