Thermodynamics of the Corn-Ethanol Biofuel cycle

January 13, 2005

In this paper [see link] I define sustainability, sustainable cyclic processes, and quantify the degree of non-renewability of a major biofuel: ethanol produced from industrially-grown corn.

First, I demonstrate 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 will generate 8 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, I calculate the cumulative exergy (available free energy) consumed in corn farming and ethanol production, and estimate 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 exergy is used to power a car engine, the minimum restoration work is about 6 times the maximum useful work from the cycle. This ratio drops down to 2, if an ideal (but nonexistent) fuel cell is used to process the ethanol.

Third, I estimate the U.S. taxpayer subsidies of the industrial corn-ethanol cycle at $3.8 billion in 2004. The parallel subsidies by the environment are estimated at $1.8 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, I estimate that (per year and unit area) the ineffcient solar cells produce ~100 times more electricity than corn ethanol. We need to rely more on sunlight, the only source of renewable energy on the earth.

» Full Paper (PDF – 800KB)

Tad Patzek

Tad Patzek is Professor of Petroleum and Chemical Engineering at the Earth Sciences Division and Director of the Ali I. Al-Naimi Petroleum Engineering Center in KAUST, Saudi Arabia. Between 2008 and 2014, he was the Lois K. and Richard D. Folger Leadership Professor and Chairman of the Petroleum and Geosystems Engineering Department at The University of Texas at Austin. He also held the Cockrell Regents Chair #11. Between 1990 and 2008, he was a Professor of Geoengineering at the University of California, Berkeley. Prior to joining Berkeley, he was a researcher at Shell Development, a research company managed for 20 years by M. King Hubbert of the Hubbert peaks. Patzek’s current research involves mathematical and numerical modeling of earth systems with emphasis on fluid flow in soils and rocks that can be hydrofractured. He is working on the thermodynamics and ecology of human survival, and food and energy supply for humanity. His current emphasis is the use of unconventional natural gas as a fuel bridge to the possible new energy supply schemes for the world. Patzek is a coauthor of over 200 papers and reports, and a book.

Tags: Biomass, Food, Renewable Energy