Study Finds Net Energy of Cellulosic Ethanol from Switchgrass Much Higher Than Expected

7 January 2008

Comparison of net energy yield (NEY) from switchgrass fields managed as a bioenergy crop; low-input, high-diversity, human-made prairies (LIHD) on small plots; and low-input switchgrass (LI-SW) small plots. Click to enlarge.

A five-year trial of switchgrass on farmland in the Midwestern United States found that the crop produces 540% more renewable energy as a biomass energy crop for cellulosic ethanol than energy consumed in its production. Previous estimates, based on small scale research plots (<5m² and estimated inputs) suggested switchgrass would yield a net energy production of about 343%.

Estimated average greenhouse gas (GHG) emissions from cellulosic ethanol derived from switchgrass were 94% lower than estimated GHG from gasoline. Kenneth Vogel at the US Department of Agriculture and the University of Nebraska, Lincoln, and his colleagues report their findings in an open access article in the current issue of the Proceedings of the National Academy of Sciences.

Estimated displacement (%) of GHG emissions by replacing conventional gasoline (baseline) with cellulosic ethanol derived from switchgrass. Minimum (grey), mean (blue), and maximum (green) percent GHG displacement for each switchgrass harvest year is based on actual production data from 10 switchgrass fields. Click to enlarge.

The research team managed switchgrass as a biomass energy crop in field trials of 3–9 ha (1 ha = 10,000m²) on marginal cropland on 10 farms to determine net energy and economic costs based on known farm inputs and harvested yields.

The annual biomass yields of established fields averaged 5.2 -11.1 Mg·ha^-1 with a resulting average estimated net energy yield (NEY) of 60 GJ·ha^-1·y^-1. Switchgrass monocultures managed for high yield produced 93% more biomass yield and an equivalent estimated NEY than previous estimates from human-made prairies that received low agricultural inputs.

One of the prime reasons for the improved yield was the actual lower energy inputs for biomass reported in comparison to the estimates previously reported. This highlights, the team noted in their paper, the “discrepancies that can occur when analyses are based on small-scale research plots and misassumptions.”

Cooperating farmers in the project were paid for their work and land use and documented all production operations and field biomass yields. The study provided five years of production and management information from each farm, which the researchers used to estimate net energy, petroleum inputs to ethanol outputs, and GHG emissions.

For an alternative transportation fuel to be a substitute for conventional gasoline, the alternative fuel should (i) have superior environmental benefits, (ii) be economically competitive, (iii) have meaningful supplies to meet energy demands, and (iv) have a positive NEV. The results of this study demonstrate that switchgrass grown and managed as a biomass energy crop produces >500% more renewable energy than energy consumed in its production and has significant environmental benefits, as estimated by net GHG emissions as well as soil conservation benefits.

It is expected that biomass conversion rates will be improved in the future because of both genetic modifications of biomass feedstocks and improvements in conversion technology, which should result in improvement in net energy for switchgrass.

Only a fraction of the research effort that has produced...significant improvements in corn genetics and management has been available for switchgrass and other potential perennial herbaceous biomass species. This is a baseline study that represents the technology available for switchgrass in 2000 and 2001, when the fields were planted.

...It is expected that further improvements in both genetics (hybrid cultivars, molecular markers) and agronomics (production system management practices and inputs) will be achieved for dedicated energy crops such as switchgrass, which will further improve biomass yields, conversion efficiency, and NEV. As an indicator of the improvement potential, switchgrass biomass yields in recent yield trials in Nebraska, South Dakota, and North Dakota (36–38) were 50% greater than achieved in this study.

Resources

• Schmer, M. R., Vogel, K. P., Mitchell, R. B. & Perrin, R. K. “Net energy of cellulosic ethanol from switchgrass”Proc. Natl. Acad. Sci. USA 105, 464-469 (2008)