Thermodynamics of the corn-ethanol biofuel cycle

Jul 2006

In this paper the author defines sustainability, sustainable cyclic processes, and quantifies the degree of non-renewability of a major biofuel: ethanol produced from industrially-grown corn.
First, he demonstrates that more fossil energy is used to produce ethanol from corn than the ethanol’s calorific value. Analysis of the carbon cycle shows that all leftovers from ethanol production must be returned back to the fields to limit the irreversible mining of soil humus. Thus, production of ethanol from whole plants is unsustainable. In 2004, ethanol production from corn generated 11 million tonnes of incremental CO2, over and above the amount of CO2 generated by burning gasoline with 115% of the calorific value of this ethanol.
Second, the author calculates the cumulative exergy (available free energy) consumed in corn farming and ethanol production, and estimates the minimum amount of work necessary to restore the key non-renewable resources consumed by the industrial corn-ethanol cycle. This amount of work is compared with the maximum useful work obtained from the industrial corn-ethanol cycle. It appears that if the corn ethanol energy is used to power a car engine, the minimum restoration work is about 7 times the maximum useful work from the cycle. This ratio drops down to 2.4, if an ideal (but nonexistent) fuel cell is used to process the ethanol.
Third, the author estimates the U.S. taxpayer subsidies of the industrial corn-ethanol cycle at $3.3 billion in 2004. The parallel subsidies by the environment are estimated at $1.9 billion in 2004.
The latter estimate will increase manifold when the restoration costs of aquifers, streams and rivers, and the Gulf of Mexico are also included.
Finally, he estimates that (per year and unit area) the inefficient solar cells produce ~100 times more electricity than corn ethanol. He concludes that there is a need to rely rather on sunlight, the only source of renewable energy on the earth.

By: T. W. Patzek (University of California)

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