Study: net energy from switchgrass based cellulosic ethanol much higher than thought
A large scale five-year trial of switchgrass as a bioenergy crop on farmland in the Midwestern United States found that when the perennial is converted into cellulosic ethanol it yields 540% more renewable energy than the non-renewable energy put into producing the fuel. This is a considerbly higher yield than previous estimates which were based on small scale research plots (smaller than 5m²) and on estimated inputs, suggesting switchgrass would result in a net energy production of about 343%. The results from the large scale field trials also prove that mere simulations made by researchers like Pimental and Patzek, often quoted by biofuel critics, are highly inaccurate.
Based on the new results, the scientists write that switchgrass based cellulosic ethanol is a highly energy efficient biofuel, results in strong net GHG emission reductions and provides major other environmental benefits such as soil conservation. 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 [*.pdf] in the current issue of the Proceedings of the National Academy of Sciences.
The research team managed switchgrass as a biomass energy crop in field trials of 3–9 hectares on marginal cropland on 10 farms to determine net energy and economic costs based on known farm inputs and harvested yields. 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.
Inputs and Net Energy Value
Agricultural energy inputs - fertilizer, herbicides, diesel fuel, seed - for the switchgrass fields based on actual farm inputs were lower than in previous switchgrass life cycle analysis studies, because diesel usage, fertilizer requirements, electricity rates, and machinery costs in the previous studies were largely based on estimated values, not on real trials.
The NEV (output energy–input energy) from switchgrass in the Great Plains varied with year of production and ethanol yield but exceeded 14.5 MJ/liter ethanol for all harvest years. NEV were consistent across locations, averaging 21.5 MJ/liter ethanol. These results were intermediate to previously simulated switchgrass energy balance studies. Ethanol yield was sensitive to climatic conditions and stand age more than agricultural inputs, which differs from prior studies that assumed a linear response of switchgrass ethanol yield to agricultural inputs.
Switchgrass, a perennial, does not achieve full biomass yield potential until one to two growing seasons after establishment. Proper agronomic practices with normal climatic conditions can result in establishment year biomass yields of 50% of full yield potential. Switchgrass, in long-term evaluations (more than 10 years), has been shown to have consistent biomass yields over time when stands are mature.
Bioenergy efficiency was also evaluated as an ethanol output (MJ)/petroleum input (MJ) ratio (PER) for the production, refining, and distribution phases. All previous switchgrass studies have reported that, under most ethanol yield projections, the amount of energy from ethanol produced from switchgrass biomass exceeds petroleum consumed. In this multifarm trial, switchgrass produced an estimated average 13.1 MJ ethanol for every MJ of petroleum input. The new analysis showed that at ethanol yields of 3500 liter/ha, PER surpassed all previous estimates. Establishment and second-year stands had the lowest PER, a result of tillage, seeding, and harvesting energy costs with reduced biomass yields. There was a linear relationship between ethanol yield and PER for all harvest years. However, linear trends by harvest year declined over time, suggesting that, on mature fields, PER will be consistently high and vary little by ethanol yield (figure 1, click to enlarge).
Ethanol Yield and Net Energy Yield
The 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/year (figure 2, click to enlarge). 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 notes in its paper, the discrepancies that can occur when analyses are based on small-scale research plots and misassumptions:
energy :: sustainability :: biomass :: bioenergy :: biofuels :: cellulosic ethanol :: energy balance :: efficiency :: switchgrass ::
GHG emissions
Life-cycle analysis models have quantified the amount of either GHG emitted from ethanol or GHG displaced by shifting to an ethanol energy source from a petroleum energy source. For switchgrass, studies have estimated the amount of GHG displaced by the amount of harvested material that is converted to ethanol. Others have determined the amount of GHG displaced by the amount of harvested material and by the amount of carbon dioxide sequestered into the soil profile.
The amount of soil carbon sequestration by reintroduction of perennial grasses to a field depends on existing soil C concentration, soil type, climate, precipitation, management, and annual biomass production. Soil carbon levels on low-input switchgrass fields (29 soil types) have been shown to increase over time, across soil depths, and are higher than adjacent cropland fields in the Northern Plains.
Switchgrass managed for bioenergy on multiple soil types in the Northern Plains was carbon-negative, sequestering 4.42 Mg C ha/year into the soil profile. In the new analysis, the amount of GHG emissions displaced using ethanol from switchgrass over conventional gasoline was estimated based on biomass yields by both fossil fuel displacement and the estimated carbon dioxide sequestered as soil C for 100 yr by switchgrass on converted cropland.
Life-cycle analysis estimated that ethanol from switchgrass averaged 94% lower GHG emissions than from gasoline (figure 3, click to enlarge). Switchgrass fields were GHG-positive, -neutral, or -negative, depending on agriculture input amounts (mainly N fertilization) and subsequent biomass yields. Three of the 5 harvest yr showed farms averaging near-GHG neutral levels. GHG emissions of ethanol from switchgrass, using only the displacement method, showed 88% less GHG emissions than conventional gasoline.
Based on the results, the scientists write that switchgrass based cellulosic ethanol appears to be a highly efficient and green energy source. 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:
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. The new baseline study represents the technology available for switchgrass in 2000 and 2001, when the fields were planted.
The researchers expect 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.
The scientists conclude by saying that:
Figure 1. Energy estimates for 10 switchgrass fields managed for bioenergy for the establishment year (filled circle) and second (open circle), third (yellow square), fourth (open square), and fifth years (red triangle), using input and biomass production data from 10 farms in the EBAMM model. (a) Comparison of net energy values (Mj/liter) from the fields based on known agricultural inputs with estimates from two simulated switchgrass studies. NEV are not shown for one study, because they were negative for switchgrass at all ethanol yields due to the misassumption that non-renewable energy will be used for all biorefinery energy needs. (b) PER, which is the biofuel output (MJ) divided by the petroleum (MJ) requirements for the agricultural, biorefinery, and distribution phases, for the 10 fields compared with three simulated studies (blue line, green line, and red line representing the often criticized study of Pimental and Patzek).
Figure 2: Comparison of estimated ethanol yield and NEY from switchgrass fields managed as a bioenergy crop; low-input, high-diversity, human-made prairies (LIHD) on small plots (19); low-input switchgrass (LI-SW) small plots (19); and corn grain yields (ref. 20; 2000–2005) from Nebraska and South and North Dakota). (a) Mean ethanol yield (liter/ha) was greater for the three farms with low mean ethanol yields, mean ethanol yields of all farms, and three farms with high mean ethanol yields (>2 year after seeding) or established switchgrass plots (>9yr after seeding) grown in a higher precipitation zone and was comparable to corn grain ethanol yields for the three states. Conversion of corn grain and cellulosic biomass to ethanol was estimated at 0.4 liter/kg, and 0.38 liter/kg respectively. (b) NEY from established switchgrass fields for all farms was consistently higher than human-made prairies or low-input switchgrass (19) grown in a higher precipitation zone.
Figure 3: 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. Estimated GHG values include the amount of CO2 sequestered in the soil (100 yr) by switchgrass, which was estimated to be 138.1 kg of CO2 Mg aboveground biomass per year.
References:
Schmer, M. R., Vogel, K. P., Mitchell, R. B. & Perrin, R. K. "Net energy of cellulosic ethanol from switchgrass" [*.pdf, open access], PNAS USA 105, 464-469 (2008).
Biopact: Study of energy crops shows miscanthus twice as productive as switchgrass - July 10, 2007
Based on the new results, the scientists write that switchgrass based cellulosic ethanol is a highly energy efficient biofuel, results in strong net GHG emission reductions and provides major other environmental benefits such as soil conservation. 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 [*.pdf] in the current issue of the Proceedings of the National Academy of Sciences.
The research team managed switchgrass as a biomass energy crop in field trials of 3–9 hectares on marginal cropland on 10 farms to determine net energy and economic costs based on known farm inputs and harvested yields. 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.
Inputs and Net Energy Value
Agricultural energy inputs - fertilizer, herbicides, diesel fuel, seed - for the switchgrass fields based on actual farm inputs were lower than in previous switchgrass life cycle analysis studies, because diesel usage, fertilizer requirements, electricity rates, and machinery costs in the previous studies were largely based on estimated values, not on real trials.
The NEV (output energy–input energy) from switchgrass in the Great Plains varied with year of production and ethanol yield but exceeded 14.5 MJ/liter ethanol for all harvest years. NEV were consistent across locations, averaging 21.5 MJ/liter ethanol. These results were intermediate to previously simulated switchgrass energy balance studies. Ethanol yield was sensitive to climatic conditions and stand age more than agricultural inputs, which differs from prior studies that assumed a linear response of switchgrass ethanol yield to agricultural inputs.
Switchgrass, a perennial, does not achieve full biomass yield potential until one to two growing seasons after establishment. Proper agronomic practices with normal climatic conditions can result in establishment year biomass yields of 50% of full yield potential. Switchgrass, in long-term evaluations (more than 10 years), has been shown to have consistent biomass yields over time when stands are mature.
Bioenergy efficiency was also evaluated as an ethanol output (MJ)/petroleum input (MJ) ratio (PER) for the production, refining, and distribution phases. All previous switchgrass studies have reported that, under most ethanol yield projections, the amount of energy from ethanol produced from switchgrass biomass exceeds petroleum consumed. In this multifarm trial, switchgrass produced an estimated average 13.1 MJ ethanol for every MJ of petroleum input. The new analysis showed that at ethanol yields of 3500 liter/ha, PER surpassed all previous estimates. Establishment and second-year stands had the lowest PER, a result of tillage, seeding, and harvesting energy costs with reduced biomass yields. There was a linear relationship between ethanol yield and PER for all harvest years. However, linear trends by harvest year declined over time, suggesting that, on mature fields, PER will be consistently high and vary little by ethanol yield (figure 1, click to enlarge).
Ethanol Yield and Net Energy Yield
The 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/year (figure 2, click to enlarge). 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 notes in its paper, the discrepancies that can occur when analyses are based on small-scale research plots and misassumptions:
energy :: sustainability :: biomass :: bioenergy :: biofuels :: cellulosic ethanol :: energy balance :: efficiency :: switchgrass ::
GHG emissions
Life-cycle analysis models have quantified the amount of either GHG emitted from ethanol or GHG displaced by shifting to an ethanol energy source from a petroleum energy source. For switchgrass, studies have estimated the amount of GHG displaced by the amount of harvested material that is converted to ethanol. Others have determined the amount of GHG displaced by the amount of harvested material and by the amount of carbon dioxide sequestered into the soil profile.
The amount of soil carbon sequestration by reintroduction of perennial grasses to a field depends on existing soil C concentration, soil type, climate, precipitation, management, and annual biomass production. Soil carbon levels on low-input switchgrass fields (29 soil types) have been shown to increase over time, across soil depths, and are higher than adjacent cropland fields in the Northern Plains.
Switchgrass managed for bioenergy on multiple soil types in the Northern Plains was carbon-negative, sequestering 4.42 Mg C ha/year into the soil profile. In the new analysis, the amount of GHG emissions displaced using ethanol from switchgrass over conventional gasoline was estimated based on biomass yields by both fossil fuel displacement and the estimated carbon dioxide sequestered as soil C for 100 yr by switchgrass on converted cropland.
Life-cycle analysis estimated that ethanol from switchgrass averaged 94% lower GHG emissions than from gasoline (figure 3, click to enlarge). Switchgrass fields were GHG-positive, -neutral, or -negative, depending on agriculture input amounts (mainly N fertilization) and subsequent biomass yields. Three of the 5 harvest yr showed farms averaging near-GHG neutral levels. GHG emissions of ethanol from switchgrass, using only the displacement method, showed 88% less GHG emissions than conventional gasoline.
Based on the results, the scientists write that switchgrass based cellulosic ethanol appears to be a highly efficient and green energy source. 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 more than 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. The new baseline study represents the technology available for switchgrass in 2000 and 2001, when the fields were planted.
The researchers expect 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.
The scientists conclude by saying that:
The Green Revolution greatly enhanced the capacity of agriculture to increase food supplies throughout the world by the use of improved genetics and management inputs. Green energy goals of nations likewise can be met in part through improved genetics and agronomics. The environmental and ecological effects of the conversion of cropland to CRP were largely positive. It is expected that results will be similar for conversion of land to perennial grasses such as switchgrass for bioenergy. However, environmental and ecological assessments should continue to be made at both the micro and macro scales.
Figure 1. Energy estimates for 10 switchgrass fields managed for bioenergy for the establishment year (filled circle) and second (open circle), third (yellow square), fourth (open square), and fifth years (red triangle), using input and biomass production data from 10 farms in the EBAMM model. (a) Comparison of net energy values (Mj/liter) from the fields based on known agricultural inputs with estimates from two simulated switchgrass studies. NEV are not shown for one study, because they were negative for switchgrass at all ethanol yields due to the misassumption that non-renewable energy will be used for all biorefinery energy needs. (b) PER, which is the biofuel output (MJ) divided by the petroleum (MJ) requirements for the agricultural, biorefinery, and distribution phases, for the 10 fields compared with three simulated studies (blue line, green line, and red line representing the often criticized study of Pimental and Patzek).
Figure 2: Comparison of estimated ethanol yield and NEY from switchgrass fields managed as a bioenergy crop; low-input, high-diversity, human-made prairies (LIHD) on small plots (19); low-input switchgrass (LI-SW) small plots (19); and corn grain yields (ref. 20; 2000–2005) from Nebraska and South and North Dakota). (a) Mean ethanol yield (liter/ha) was greater for the three farms with low mean ethanol yields, mean ethanol yields of all farms, and three farms with high mean ethanol yields (>2 year after seeding) or established switchgrass plots (>9yr after seeding) grown in a higher precipitation zone and was comparable to corn grain ethanol yields for the three states. Conversion of corn grain and cellulosic biomass to ethanol was estimated at 0.4 liter/kg, and 0.38 liter/kg respectively. (b) NEY from established switchgrass fields for all farms was consistently higher than human-made prairies or low-input switchgrass (19) grown in a higher precipitation zone.
Figure 3: 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. Estimated GHG values include the amount of CO2 sequestered in the soil (100 yr) by switchgrass, which was estimated to be 138.1 kg of CO2 Mg aboveground biomass per year.
References:
Schmer, M. R., Vogel, K. P., Mitchell, R. B. & Perrin, R. K. "Net energy of cellulosic ethanol from switchgrass" [*.pdf, open access], PNAS USA 105, 464-469 (2008).
Biopact: Study of energy crops shows miscanthus twice as productive as switchgrass - July 10, 2007
3 Comments:
Patzek and Pimental were Wrong?
Get Out!
P&P have done the biofuels people a great favor: we now know that simulations must be met with extreme care, and that nothing can replace real field trials.
The difference between the two results is pretty extreme. Makes one wonder why P&P published their estimations in the first place...
Jonas
Patzek is founder, and current director of University of California OIL CONSORTIUM. The oil companies pay his organization millions.
No wonder, here.
Pimental, I'm also a little suspicious of.
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