Study questions energy balance of bioethanol and biodiesel
23 July 2005
Growing corn, soybeans, or other plants and converting them into biofuels uses more energy than the resulting bioethanol or biodiesel generates, concluded a recent study by David Pimentel from Cornell University and Tad W. Patzek from University of California-Berkeley. Thus, using today’s technologies, liquid fuels from biomass would not be a renewable energy source. The paper was published in Natural Resources Research, Vol. 14:1, 65-76.
In terms of energy output compared with energy input for ethanol production, the study found that:
- Corn requires 29% more fossil energy than the fuel produced;
- Switch grass requires 45% more fossil energy than the fuel produced; and
- Wood biomass requires 57% more fossil energy than the fuel produced.
In assessing inputs, the researchers considered such factors as the energy used in producing the crop (including production of pesticides and fertilizer, running farm machinery and irrigating, grinding and transporting the crop) and in fermenting/distilling the ethanol from the water mix.
Pimentel also provided a critique of certain assumptions and input data in the studies by the US Department of Agriculture (USDA). A 2002 study by the USDA reported a positive energy return for corn-based ethanol of 34%. This figure was revised up to 67% in a 2004 USDA study. As illustrated by the large discrepancies, these analyses are complex and their results depend on a large number of assumptions.
In terms of energy output compared with the energy input for biodiesel production, the study found that:
- Soybean plants require 32% more fossil energy than the fuel produced (considering the energy value of soy meal, a byproduct in the production of biodiesel, soy biodiesel still produced a net energy loss of 8%), and
- Sunflower plants require 118% more fossil energy than the fuel produced.
The above soybean results differ significantly from the estimates by the US Department of Energy (DOE). A 1998 DOE study concluded that manufacturing 1 MJ of biodiesel requires 0.311 MJ of fossil energy, thus representing a 69% energy surplus.
The main source of discrepancy appears to be in accounting for energy consumption in soy agriculture, which was responsible for 21% of the overall fossil fuel demand in biodiesel production according to the DOE analysis. Pimentel, on the other hand, considers a much higher agricultural energy input—including labor, machinery, diesel, gasoline, and LPG fuels, fertilizers, lime, seeds, herbicides, electricity, and transport—attributing as much as 66% of the energy input in biodiesel manufacturing to soybean production. The biggest agricultural energy input (36% of all agricultural demand) was lime, assumed to be used in large quantity in soybeans production.
Pimentel criticized government policies aimed at stimulating ethanol production from corn. “The government spends more than $3 billion a year to subsidize ethanol production when it does not provide a net energy balance or gain, is not a renewable energy source or an economical fuel. Further, its production and use contribute to air, water and soil pollution and global warming,” said Pimentel. He also pointed out that the vast majority of the subsidies do not go to farmers but to large ethanol-producing corporations.
Cornell University press release
It is possible (although complicated) to download the full text of the paper: Browse to Natural Resources Research, find the paper in the Volume 14 Number 1 issue, register (for free) and log-in to view full text.
National Biodiesel Board response