The BTW efficiency (η_BTW) 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.

The scheme of energy efficiency analysis for the biomass-to-wheel efficiency (ηBTW) calculation. W is the kinetic energy transferred to wheels; EB is the chemical combustion energy of the biomass, where dry corn stover as a typical biomass contains ~65% carbohydrates (cellulose and hemicellulose, mainly); ~18% lignin; ~5% ash; ~12% other organic molecules; and the EB value is 16.5 MJ of low heating value/kg of corn stover; ηBTF is the biomass-to-fuel (BTF) efficiency through biorefineries or power stations without significant inputs or outputs of other energy. ηBTF can be calculated as EF/EB, where EF is the fuel produced in biorefineries or power stations. ηTDL is the fuel loss efficiency during its transport and distribution. ηTDL can be calculated as fuel consumed for its transport and distribution from biorefineries to vehicles [EC/(EC+ET)] where EC is the energy consumed in the process of fuel transport and distribution, ET is the fuel energy delivered to end users (i.e., powertrains), and EF = EC + ET. ηFTW is the fuel-to-wheel (FTW) efficiency from the fuel to kinetic energy through powertrain. ηFTW can be calculated as a ratio between the kinetic energy on wheels (W) and fuel energy in the tank (ET).

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 η_BTW 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 η_BTW 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 η_BTW between butanol and ethanol, but butanol may have other important future applications, such as powering jet planes.

• The η_BTW 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 η_BTW values.

• Although (hydrogen) fuel cell vehicles (FCVs) have higher η_FTW efficiencies than ICE-gas and ICE-diesel, the H₂/FCV scenario shows ~46% and ~15% η_BTW 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 η_BTW 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 η_BTW 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 [η_BTW 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 (η_FTW) 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

Resources

• 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