In my last posting, I worked out how much land and water would be required to grow the sugar crop and ferment from it sufficient hydrogen to replace the U.K. current use of oil-based fuels for transportation. I concluded that the option is a complete non-starter, without an extreme curtailing of transportation use, per se. I now present a similar calculation for ethanol production from sugar which though "better" than hydrogen, still requires using more than twice the amount of arable land there is in the U.K., and hence bioethanol production on this scale would overrun our means for food production.
I shall make a direct comparison between gasoline and ethanol, assuming they are both intended to be burned in internal combustion engines. The efficiency of ethanol in terms of "tank to wheel" might be improved using fuel cells, but this is still firmly in the experimental stage. Currently, the U.K. uses 54 million tonnes of oil (equivalent), which provides:
54 x 10*6 x 42 x 10*9 = 2.268 x 10*18 Joules of energy (J).
Ethanol may be considered as a partially combusted form of fuel (since it contains oxygen, with oil doesn't, being entirely hydrocarbon), and so it delivers less energy when burnt. Specifically, burning one mole of ethanol (46 grams) releases 326.68 kilocalories of energy, and so one tonne of ethanol would provide (10*6/46) x 326.68 x 4.18 = 2.967 x 10*7 kJ = 29.67 Gigajoules (GJ).
This may be compared directly with the figure of 42 GJ quoted for burning one tonne of oil equivalent. Hence we see immediately that ethanol packs around 30% less of a punch than gasoline does, or put another way, a tank full of ethanol will take the car 30% less miles than an equivalent tank filled with gasoline.
We need, therefore, 2.268 x 10*18/29.67 x 10*9 = 76.4 million tonnes of ethanol, which might be produced by fermenting sugar, according to the process:
C6H12O6 --> 2C2H6O (ethanol) + 2CO2.
The process is supposed to be CO2 neutral because the same amount of CO2 produced in the fermentation and ultimate combustion steps will be absorbed by next year's sugar crop (in essence, although in practice the situation is not that good). Assuming that the process is 100% efficient, we can expect to get (2 x 46)/180 - that is the ratio of the molecular weights of ethanol to sugar - or 0.511 tonnes of ethanol per tonne of sugar.
Sugar cane yields a crop of 87 tonnes per hectare (ha), that produces 19% of its weight of sugar, which is 87 x 0.19 = 16.53 tonnes. Hence this should give us 16.53 x 0.511 = 8.449 tonnes of ethanol. Since the density of ethanol is 0.789 kilograms/litre, this would occupy a volume equal to: 8.449 x 1000 /0.789 = 10,706 litres.
The actual production figure is around 6,718 litres/ha, and so the process is 6,718/10,706 = 63% efficient. Indeed this is similar to the efficiency of the fermentation process designed to produce hydrogen from sugar.
6,718 litres of ethanol weighs 0.789 x 6.718 giving a yield = 5.3 tonnes/ha. Hence the sugar crop would require 76.4 x 10*6/5.3 = 14,415,094 ha = 144, 151 square kilometers (km*2). Sugar beet comes in slightly better at 19.1 tonnes/ha and so an equivalent crop would need (16.53/19.1) x 144,151 = 124,755 km*2.
Since the area of arable land in the U.K. is about 65,000 km*2 even of we used all of it and grew no food, we could just about meet half our current fuel requirements from ethanol. Perhaps if we could "seed" more land, we would still need around half the entire area of the U.K. mainland of 244,000 km*2 to produce it!
The message is once again that without severe cuts in transportation use, the situation is hopeless. My figures are rough, and the situation will perhaps be improved by new technologies - but only slightly. Using "bio" fuels to break the hold that imported oil has on us, is really a non-starter, at our current levels of fuel consumption. It is these we need to reduce first and foremost, but that will entail living quite differently ... and probably far more frugally. The option of a Die-Off in human population as energy resources run-out is far more uncomfortable, however.