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(ReadWith) BioDiesel

Many the cars in Brazil (including ours) use alcohol rather than petrol. But the price fluctuates depending on how the weather affects the sugar-cane harvest. (This year it's up.) I wonder how fast that could be ramped up to cover ordinary car use (which of course ignores electricity generation, plastics(!!), and industrial use.)

Crunching the numbers :

Economist says the proportion of Brazilian cars is less than the third I originally heard :

Still, it's upbeat.

[Rup3rt] Alcohol cars are more difficult to start and more unreliable in colder climates and so are less popular in the South and in Sao Paulo. Bicombustivel cars, now available from GM and Fiat, should address this and a tricombustivel (Alcohol, Petroleum, Natuaral Gas) is on its way.

Brazil's present method of making ethanol fuel from sugar leads to net savings of about 50% in greenhouse-gas emissions per kilometre travelled, compared with running cars on petrol.

Recently a guy at a party told me that even Brazilian sugar-cane alcohol actually consumed more energy to grow and process than you got. Anyone know where I could check this?

[Rup3rt] There are two studies:

This one is inconclusive

This one has detailed conclusions I'll paste here for the moment

"In the end, an average Brazilian sugarcane plantation for energy sequesters 130.4 GJ/ha-yr (0.41 W/m2) as the net chemical exergy of its anhydrous ethanol product and 3.3 GJ/ha-yr as electricity (0.0001 W/m2). This output is 2 times higher than the 66.8 GJ/ha-yr in ethanol produced from an average cornfield in the U.S. (Patzek, 2004). The main reason is that corn in the U.S. grows less than 6 months/year and sugarcane in Brazil grows 12 months/year before each harvest. If these 130.4 GJ/ha-yr in sugarcane ethanol are used to power a 35% Carnot engine (an efficient Toyota Prius car), one obtains 45.6 GJ/ha-yr (0.14 W/m2) of shaft work. If a 60% efficient fuel cell could be used, one would obtain 78.2 GJ/ha-yr (0.25 W/m2) of shaft work. Note that the cumulative exergy consumption in ethanol production (not counting the Biological Oxygen Demand (BOD) removal from distillery wastewater) is equal to the ethanol's chemical exergy.

Per kilogram of output biofuel (anhydrous ethanol), the average sugarcane plantation produces 10.37 MJ as useful work from a 35%-efficient Carnot engine. This efficiency should be compared with the 5.59 MJ/kg of wood pellets as electricity from the example Acacia plantation in Indonesia. Therefore, sugarcane ethanol is 85% more efficient as a source of shaft work than the acacia wood pellets."

Thermodynamics of Energy Production from Biomass, Tad W. Patzek and David Pimentel p.56

See also :