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Biofuel (Rev #1, changes)

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Biofuel refers to any sort of fuel that is derived from biomass, meaning material from living, or recently living organisms. Biofuels include solid biomass, liquid fuels and various biogases.

Agrawal and Singh

Since biofuels can be thought of as a mechanism for converting solar energy to usable fuel, their efficiency should be compared with solar power. The following paper makes some quantitative comparisons:

  • Rakesh Agrawal and Navneet R. Singh, Solar energy to biofuels, Annual Review of Chemical and Biomolecular Engineering 1 (July 2010), 343–364.

Overall, biofuels come out looking rather inefficient. However, it should be remembered that biofuels have the additional benefit of coming in liquid form, rather than the form of electricity. For certain applications, notably transportation, this can be advantageous.

Here are some notes on the above paper:

  • A crop growing at the rate of 1 kilogram per square meter per year in the United States, with an energy content of 17 megajoules per kilogram, captures only 0.28% of the average incident solar energy of 6307 megajoules per square meter per year (1752 kWh m-2 yr-1).

(Why do they say “in the United States?” Are they just being nationalistic, or are they attempting to estimate the amount of sunshine in that general location? Isn’t that pretty different in New Mexico versus, say, Alaska?)

  • The fast growing crop of Miscanthus (a kind of tall grass) is expected to have a growth rate of 3.7 kg per square meter per year.
  • Even the highly efficient sugarcane crop stores only 1% of the annual incident light as biomass.

  • Zhu et al. have estimated that the maximum conversion efficiency of solar energy to biomass under today’s atmospheric CO2 concentration of 383 ppm and at 30°C is 4.6% for C3 photosynthesis and 6% for C4 photosynthesis. But the highest efficiencies observed across a full growing season for C3 and C4 crops are 2.4% and 3.7%, respectively.

Note here that C3 photosynthesis is used by plants that get moderate sunlight and temperatures. C3 plants predate C4 plants, going back to the Mesozoic or earlier. They still make up 95% of plants we see. C4 photosynthesis developed when grasses migrated from the shady forest undercanopy to more open environments in the Oligocene (early Cenozoic). C4 plants do better in conditions of drought, high temperatures, or nitrogen shortage.

  • Algae are considered efficient collectors of solar energy. Under ideal growing conditions, yields in the range of 3–7 kilograms per square meter per year have been reported. A joint venture between ExxonMobil and Synthetic Genomics Inc. aims at getting 1.8 kilograms of fuel (oil) per square meter per year from algae. This is still just 0.9% of the incident solar energy. According to recent projections, it may be possible to grow algae at a rate of 12 kilograms per square meter per year with 30% oil content by mass. Assuming that the oil portion of the algae has the high energy density of 33 megajoule per kilogram, the resulting annual solar energy conversion efficiency of 4.2% is more than twice what’s been demonstrated so far.

  • By contrast, solar power can be turned into heat energy at efficiencies of up to 70%!

  • Or, electricity can be generated from solar power either by a solar-thermal process or a photovoltaic module with efficiencies in the range of 10 to 42%. This is much higher, but we also need to take production costs into account.

  • Commercial photovoltaic modules with efficiencies approaching 20% are already available, and lab-scale multijunction tandem cells have shown efficiencies slightly greater than 40%.

  • H2 can be produced from water by an electrolyzer with electricity to H2 efficiency in the neighborhood of 50%.

(But now we’re talking about converting electricity we’ve already made to hydrogen, so we need to multiply this 50% by the efficiency of the method we used to turn solar power into electricity.)

  • A cute little summary chart.

  • “Because… crop–based liquid fuels, when compared with [using solar power to make] electricity or H2, have so much lower solar-to-fuel efficiencies, we encounter a liquid fuel conundrum”. Namely, the high energy density, 32–36 megajoule/liter, of liquid fuels is tremendously attractive, but making these fuels out of crops is not a very efficient way of harvesting solar power.

(But again, one needs to do some sort of economic analysis to compare the full cost of planting, harvesting, and processing some sort of crop to the cost of alternative methods of harvesting energy! The efficiency percentages don’t tell the whole story.)