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Net energy of cellulosic ethanol from switchgrass
M. R. Schmer, K. P. Vogel, R. B. Mitchell, and R. K. Perrin
Proc. Natl. Acad. Sci. USA vol.105 no.2 p.464469 January 15, 2008

Abstract:
Perennial herbaceous plants such as switchgrass (Panicum virgatum
L.) are being evaluated as cellulosic bioenergy crops. Two
major concerns have been the net energy efficiency and economic
feasibility of switchgrass and similar crops. All previous energy
analyses have been based on data from research plots (<5 m2) and
estimated inputs. We managed switchgrass as a biomass energy
crop in field trials of 39 ha (1 ha=10,000 m2) on marginal cropland
on 10 farms across a wide precipitation and temperature gradient
in the midcontinental U.S. to determine net energy and economic
costs based on known farm inputs and harvested yields. In this
report, we summarize the agricultural energy input costs, biomass
yield, estimated ethanol output, greenhouse gas emissions, and
net energy results. Annual biomass yields of established fields
averaged 5.2-11.1 Mg/ha with a resulting average estimated net
energy yield (NEY) of 60 GJ/ha/y. Switchgrass produced 540%
more renewable than nonrenewable energy consumed. 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. Estimated average greenhouse gas (GHG) emissions from
cellulosic ethanol derived from switchgrass were 94% lower than
estimated GHG from gasoline. This is a baseline study that represents
the genetic material and agronomic technology available for
switchgrass production in 2000 and 2001, when the fields were
planted. Improved genetics and agronomics may further enhance
energy sustainability and biofuel yield of switchgrass.

http://www.pnas.org/cgi/reprint/0704767105v1.pdf (467K)

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