Virginia Tech professor proposes simple biomass-to-wheel efficiency analysis to inform decisions on biomass/biofuel/powertrain combinations; the advantage of sugar fuel cell vehicles
|Comparison of biomass-to-wheel (BTW) efficiency for different biomass utilization scenarios. Sugar Fuel Cell Vehicles and BEVs charged with electricity from a fuel cell (green bars) were the most efficient. Huang and Zhang. Click to enlarge.|
The potential role of biomass-derived fuels as a substitute for petroleum has engendered a great deal of research into different fuel pathways and possible powertrain combinations. Numerous life cycle analyses (LCAs) have attempted—and continue to—gauge the potential sustainability and impacts of these possible pairings.
However, notes Virginia Tech Prof. Y-H Percival Zhang in an open access paper published in the journal PLoS ONE, such analyses rely heavily on numerous assumptions, uncertain inputs (e.g., fertilizers, pesticides, farm machinery), energy conversion coefficients among different energy forms and sources, system boundaries, and so on. As a result, conflicting conclusions have been made even for well-known corn ethanol biorefineries, he points out. Zhang proposes using a simple biomass-to-wheel (BTW) energy efficiency analysis to help make a more informed decision for how to utilize (limited) biomass resources more efficiently.
The BTW efficiency (ηBTW) analysis methodology involves three elements: biomass-to-fuel (BTF), fuel distribution, and fuel-to-wheel (FTW). BTW efficiency is a ratio of the kinetic energy of an automobile’s wheels to the chemical energy of delivered biomass just before entering biorefineries.
Conducting this BTW analysis is simple and straightforward because it not only avoids uncertainties or debates for (i) biomass production-related issues, (ii) feedstock collection and transport, and (iii) land use change, but also excludes water consumption issues and greenhouse gas emissions in the whole biosystem. Therefore, energy efficiency analysis (but not life cycle analysis) may not only be helpful in narrowing down numerous choices before more complicated LCA and techno-economic analyses are conducted, but may also increase the transparency of such analyses.—Huang and Zhang
Using this BTW method, Zhang and his colleague Wei-Dong Huang assessed combinations of different biomass-to-biofuel approaches and their respective powertrain systems and compared them to a baseline corn-ethanol-ICE.
They ran different scenarios of fuel production through sugar, syngas, and steam platforms as well as six different powertrains viz. internal combustion engine vehicle (ICE); hybrid electric vehicle-gasoline (HEV-gas); hybrid electric vehicle-diesel (HEV-diesel); (hydrogen) fuel cell vehicle (FCV); battery electric vehicle (BEV); and sugar fuel cell vehicle (SFCV).
The combination of 12 kinds of biofuel production approaches and 6 kinds of advanced powertrains for passenger vehicles results in more than 20 scenarios (shown in the figure below). In the current paper, they calculated 14 scenarios.
|Scenarios of the production of fuels from biomass and their respective fuel power train systems. Solid lines represent the scenarios analyzed; the dotted lines represent possible scenarios not analyzed. Huang and Zhang. Click to enlarge.|
Among the findings:
The current corn ethanol/ICE scenario has ηBTW value of ~7%—i.e., only 7% of the chemical energy in corn kernels is converted to the kinetic energy on wheels, implying a great potential in increasing biomass utilization efficiency.
An ethanol HEV-gas system would double ηBTW values to 14–18%, suggesting the importance of developing hybrid electric vehicles based on available liquid fuel distribution system.
There is no significant difference in ηBTW between butanol and ethanol, but butanol may have other important future applications, such as powering jet planes.
The ηBTW values of methane/HEV-gas and methanol/HEV-gas are 19% and 17%, respectively, higher than those of ethanol and butanol, mainly due to higher product yields.
For ester-diesel, a significant amount of energy is lost during aerobic fermentation due to thermodynamic and bioenergetic limits, resulting in low ηBTW values.
Although (hydrogen) fuel cell vehicles (FCVs) have higher ηFTW efficiencies than ICE-gas and ICE-diesel, the H2/FCV scenario shows ~46% and ~15% ηBTW enhancements over ethanol HEV-gas and DME HEV-diesel, respectively, because significant energy loss in hydrogen distribution discounts FCV’s advantages over HEV-diesel.
The sugar/SFCV scenario would have very high ηBTW values of approximately 27% due to lower energy consumption in fuel transport and heat recapture in the sugar-to-hydrogen biotransformation, compared to the H2/FCV scenario.
BEV scenarios are among the highest ηBTW values, from 20% to 28%, with increasing electricity generation efficiencies from direct combustion, BIGCC, to FC-power.
Conducting energy efficiency analysis is simpler, faster, and less controversial than conducting life cycle analysis because the latter heavily depends on so many different assumptions and uncertain inputs. Here we present a straightforward energy efficiency analysis from biomass to wheels for different options, which contains three elements. Each element can be analyzed separately and adjusted individually; most of which have data well-documented in literature. Because of the same input and output in all cases, an increase in energy conversion efficiency nearly equals impact reductions in carbon and water footprints on the environment. Most of the results obtained from this biomass-to-wheel analysis were in good agreement with previous, more complicated life cycle analyses, supporting the validity of this methodology.
...Both the sugar/sugar fuel cell vehicle (SFCV) and fuel cell (FC)-power/ battery electric vehicle (BEV) scenarios would have [ηBTW values] nearly four times that of corn ethanol/ICE-gas, implying the importance of enhancing BTW efficiency in each conversion element.—Huang and Zhang
The Sugar Fuel Cell Vehicle (SFCV). Zhang proposed the concept of the SFCV in a paper in the RSC journal Energy & Environmental Science in 2009 to address problems such as high-density hydrogen storage in FCV, low-cost sustainable hydrogen production, costly hydrogen distribution infrastructure, and safety. The SFCV concept uses renewable sugar (carbohydrate) as a high hydrogen density carrier (gravimetric density of 8.33% mass H2, volumetric density of >100 g H2 per liter).
|Conceptual sugar-to-electricity system. Zhang 2009.
Click to enlarge.
|Conceptual hybrid power train system including on-board sugarto-hydrogen converter, PEM fuel cell and rechargeable battery. Zhang 2009. Click to enlarge.|
Transportation and distribution of the sugar/water slurry or sugar slurry could easily use available infrastructure.
This hypothetical SFCV would contain a sugar tank and an on-board sugar-to-hydrogen bioreformer, with a combined sugar tank and bioreformer volume that is much smaller than a compressed hydrogen tank or other hydrogen storage approaches. The on-board biotransformer would convert the sugar solution to high-purity hydrogen and carbon dioxide using a stabilized enzyme cocktail and a small-size hydrogen storage container would serve as a buffer, balancing hydrogen production and consumption.
Feeding a mixture of CO2/H2 or pure hydrogen in the proton exchange membrane (PEM) fuel cells would decrease system complexity and greatly increase system operation performance, and the waste heat release from PEM fuel cells would be coupled to the heat needed by the bioreformer.
When extra kinetic energy is needed for acceleration or start-up, electrical energy stored in the rechargeable battery would be released, as in a hybrid electric vehicle. The on-board bioreformer in SFCVs, mediated by the thermoenzyme cocktails under modest reaction conditions may be capable of providing high-purity hydrogen at a rate of ~23.5 g H2/L/h or higher, Huang and Zhang say. Given a bioreformer size of 42.8 L, one kg of hydrogen per hour could then be produced to drive the PEM fuel cell stack.
High-speed biohydrogen production rates have been implemented by high cell-density microbial fermentation. It is widely known that enzymatic reactions usually are at least one order-of-magnitude faster than microbial fermentations because the former has no cellular membrane to slow down mass transfer and much higher biocatalyst loadings, without the dilution of other biomacromolecules (e.g., DNA, RNA, other cellular proteins)...We expect that enzyme deactivation in the biotransformer will be solved through infrequent service maintenance, similar to the oil/air filter change for gasoline/ ICE vehicles.
Several technical obstacles of SFCVs include poor enzyme stability, labile and costly coenzymes, low reaction rates, and complicated system configuration and control. A huge potential market (e.g., nearly one trillion of US dollars per year) provides the motivation to solve these issues within a short time. Current progress includes the discovery of thermostable enzymes from extremophiles and low-cost production of recombinant enzymes, engineering redox enzymes that can work on small-size biomimetic cofactors, and accelerating hydrogen generation rates.—Huang and Zhang
Since, under the BTW analysis, the SFCV would have ~3.4 times the FTW efficiency (ηFTW) of ethanol/ICE-gas, one kg of sugar (i.e., 17 MG/kg) would release more kinetic energy than one kg of gasoline (i.e., 46.4 MJ/kg) from ICE-gas, Huang and Zhang said.
Assessment of any energy system is really challenging because it involves so many factors. Generally speaking, efficiency and cost are usually the two most important criteria. Since thermodynamics (energy efficiency) determine economics in the long term, SFCVs and FC-power/BEV seemed to be long-term winner candidates, but SFCVs have other important advantages. Currently and in the short term, costs mostly determine market acceptance and dominance.
But cost analysis is more complicated than energy efficiency analysis, because the former involves direct costs (e.g., fuel, vehicle, etc.), indirect costs (e.g., vehicle service, taxes, subsidies, infrastructure costs for repairing and rebuilding, resource availability, etc.), and hidden costs (e.g., safety, toxicity, waste treatment, greenhouse gas emissions, military expenditures, etc.). In the short term, cellulosic ethanol plus HEV-gas and methane-HEV-gas may be the most promising options.—Huang and Zhang
Huang W-D, Zhang Y-HP (2011) Energy Efficiency Analysis: Biomass-to-Wheel Efficiency Related with Biofuels Production, Fuel Distribution, and Powertrain Systems. PLoS ONE 6(7): e22113 doi: 10.1371/journal.pone.0022113
Y.-H. Percival Zhang (2009) A sweet out-of-the-box solution to the hydrogen economy: is the sugar-powered car science fiction? Energy Environ. Sci., 2, 272-282 doi: 10.1039/B818694D
Zhang Y-HP, Evans BR, Mielenz JR, Hopkins RC, Adams MW (2007) High-Yield Hydrogen Production from Starch and Water by a Synthetic Enzymatic Pathway. PLoS ONE 2(5): e456. doi: 10.1371/journal.pone.0000456