Wednesday, July 17, 2013

NatGeo on Biofuels

What breakthroughs do biofuels need? asks National Geographic.

The real breakthrough would be recognition that biofuels simply cannot reasonably be expected to address a significant fraction of our energy needs. The only breakthrough worth having would be a tenfold increase in the solar efficiency of photosynthesis, which would only be possible through advanced genetic engineering, is very far beyond our current capabilities, and would be vehemently opposed by environmentalists.

The basic problem is that photosynthesis is horribly inefficient in converting sunlight to usable fuel. The maximum theoretical energy conversion efficiency of sunlight to biomass, not useful fuel, is just 6% [Zhu et al., 2008]; real efficiencies are considerably lower.

Brazil's sugarcane ethanol production is the absolute gold standard for large-scale biofuel production. The Brazilians started this bandwagon in the 70's and have aggressively optimized their ethanol production for 40 years. The most bullish prediction is that — if they keep improving at the same rate as they have in the past — they'll be able to average 9,000 liters of ethanol per hectare per year by 2018 [Goldemberg, 2008].
Brazilian Ethanol Yield Over Time
What's the energy content of 9,000 liters of ethanol? The highest value I could find is 23.4 MJ/l (megajoules per liter), corresponding to the HHV (higher heating value) of anhydrous ethanol. This means that 9 kl (kiloliters) of ethanol yields at most 211 GJ (gigajoules) of thermal energy on combustion.

Now, how much sunlight falls on a hectare? In energy slang a “sun” is 1 kW/m^2 (kilowatt per square meter). That's about the peak insolation (energy density on the ground from the sun) at noon at the equator, but of course, the sun doesn't shine at night, insolation falls with latitude, and there's seasonal variation. An insolation map of Brazil suggests that a reasonable value for the sum of the insolation over a year may be up to 2,000 kWh/m^2 (kilowatt hours per square meter), so let's be super-generous to biofuels and use a stingy figure of 1,000 kWh/m^2, which is more like the correct value for Ireland than Brazil, and equates to 36,000 GJ/ha.
Yearly Sum of Global Irradiance

So, every hectare gets at least 36,000 GJ of sunlight and produces at most 211 GJ of ethanol. That's an energy conversion efficiency of, at best, 211/36000 or less than 0.6%. A fair figure (using the LHV of ethanol, production of 7kl/ha, and 1,750 kWh/m^2) would be less than half of that. I think it's fair to say that the solar-to-liquid-fuel energy conversion efficiency of sugarcane ethanol production is currently no more than one quarter of one percent, less by the time the input energy necessary to grow, harvest, ferment, and distill the ethanol (at least one tenth of the energy produced) is accounted for.

You can do far better than this generating hydrogen in your backyard using a modern solar PV panel (20% efficient) to power a commercially available electrolyser (73% efficient). Under reasonable assumptions, today you can produce hydrogen via solar panels and electrolysis with more than 50 times the efficiency of sugarcane ethanol production. I'm not arguing for hydrogen-powered cars, merely illustrating the horrible inefficiency of ethanol production.

Speaking of cars, to ram the point home, let's suppose that we could magically transform all of the vehicles in the United States into flex-fuel vehicles capable of running on 100% ethanol. Let's further suppose that we could out-do the future Brazilians at their own game and obtain fantastic yields of 10 kl/ha (or 1 l/m^2). Let's further suppose that we could substitute ethanol 1:1 for gasoline, meaning that we would need 500 billion liters of ethanol per year just for gasoline, never mind other energy needs. How much land would that require? About 50 million hectares. That's about half the total area of the United States or about three times its arable land area. If you use actual figures for US corn ethanol production (about 3,750 l/ha)? You need 8 times the arable land area of the USA.

So when I see headlines like this, I say, “So what, it's 1% efficient?”

I don't know how we can address our need for a gasoline substitute (easily transportable, high energy density, short “recharge” time, etc.), but it seems pretty clear that biofuels are merely a distraction.

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