Study Finds Bioelectricity Better Option Than Liquid Biofuels for Transportation Output and GHG Emissions
|Kilometers per crop hectare for switchgrass feedstock with a small SUV. Campbell et al. (2009) Click to enlarge.
A new life cycle assessment comparing the performance of bioelectricity and ethanol from a variety of pathways with respect to transportation kilometers and GHG offsets achieved per unit area of biofuels cropland concludes that bioelectricity used to charge a battery electric vehicle outperforms ethanol for a combustion engine across a range of feedstocks, conversion technologies, and vehicle classes.
The study by University of California, Merced, Assistant Professor Elliott Campbell along with Christopher Field of the Carnegie Institution’s Department of Global Ecology and David Lobell of Stanford University, found that bioelectricity produces an average 81% more transportation kilometers and 108% more emissions offsets per unit area cropland than cellulosic ethanol. A paper on the work appeared in the 8 May issue of the journal Science.
The authors point out their study looked at only two criteria, kilometers travelled and greenhouse gas offsets, but did not examine the performance of electricity and ethanol for other policy-relevant criteria such as water consumption, air pollution or economic costs.
|“We found that converting biomass to electricity rather than ethanol makes the most sense for two policy-relevant issues, transportation and climate.”
In the assessment, the team used The Energy and Resources Group Biofuel Analysis Meta-Model (EBAMM) to consider scenarios covering a range of feedstocks and energy conversion technologies including corn and cellulosic ethanol, and four different vehicle classes: small- and mid-size cars and small and full-size SUVs. Co-product credits in EBAMM favor the ethanol pathway by accounting for ethanol co-products but not potential bioelectricity coproducts including steam for heat and fly-ash for cement, noted the researchers in their paper.
The net transportation output per hectare is larger for the bioelectricity case. With BEVs and ICVs of similar size, one can travel farther on biomass grown on a hectare of land when it is converted to electricity than when it is converted to ethanol...For this case, the gross transportation output per hectare is 85% greater for bioelectricity than cellulosic ethanol. This is largely due to fact that the small SUV BEV has an electric motor that is 3.1 times as efficient as the internal combustion engine of the small SUV ICV for highway driving.—Campbell et al. (2009)
Among the other findings:
For the gross transportation distance, the bioelectricity output is on average 112% greater than the ethanol output for the full range of feedstocks, energy conversions, and vehicle efficiencies.
For the net transportation distance, several of the corn ethanol cases result in negative distances because the distance that could be traveled with the net fuel cycle inputs (petroleum via ICV and electricity, coal and natural gas via BEV) is greater than the distance that could be traveled with the gross ethanol output.
The average net transportation distance for the switchgrass feedstock was 81% larger (SE = 21%) for bioelectricity than ethanol.
While bioelectricity generally performed better than ethanol, the bioelectricity and ethanol pathways had similar results for highway driving with the small car and full-size SUV. The two BEVs tested by the EPA for these vehicle classes had particularly low highway efficiencies and low ranges (<166 km). This suggests that these specific BEVs were not designed for highway driving, as opposed to the midsize car BEV and small SUV BEV which perform well for city and highway driving.
While the relative efficiency of these pathways may be altered in the future with new powertrain technologies, heating co-products, and electricity storage approaches, the researchers said, based on the efficiencies of deployed bioelectricity technologies and emerging cellulosic ethanol technologies, the bioelectricity pathway consistently produces more transportation kilometers than the ethanol pathway.
In terms of greenhouse gas emissions, they found that the average net offset for bioelectricity generated from switchgrass is 108% greater (SE = 28%) than the offset for switchgrass ethanol. For both pathways, these GHG offsets could only be achieved if land use impacts are avoided. GHG offsets for bioelectricity could be greatly increased by accounting for the steam co-products during electricity generation. Applying carbon capture and sequestration (CCS) technologies with bioelectricity could result in a carbon-negative energy source.
These results provide further support for general bioelectricity applications which are already thought to have greater climate mitigation benefits than ethanol. Electric transportation may also provide a bridge that connects transportation to future renewable energy sources such as solar and wind power....On the other hand, electric transportation also provides a bridge to the use of conventional coal energy for transportation. These results do not indicate that bioelectricity is the preferred pathway over ethanol as there are numerous other criteria that need to be evaluated such as impacts on regional water resources, battery toxicity and recycling, air pollution, and economic constraints.
The optimal pathway for biomass will also depend on how efficiently other feedstocks can be converted to both liquid fuels and electricity. Specifically, the competitiveness of biomass ethanol depends on the cost of petroleum, whereas the competitiveness of biomass electricity depends on the cost of coal, wind, hydro, solar, and nuclear. These results do suggest, however, that alternative bioenergy pathways have large differences in how efficiently they use the limited available land to maximize transportation and climate benefits.—Campbell et al. (2009)
J. E. Campbell, D. B. Lobell, C. B. Field (2009) Greater Transportation Energy and GHG Offsets from Bioelectricity Than Ethanol. Science doi: 10.1126/science.1168885