Developing the Greenest and Most Efficient Car

1
Developing the Greenest and Most Efficient Car
Powered by Sugars Y.-H. Percival Zhang 1, and
Jonathan R. Mielenz 2 1Department of Biological
Systems Engineering, Virginia Tech, Blacksburg,
VA 24061 2 Life Science Division, Oak Ridge
National Laboratory, Oak Ridge, TN 37831. ,
corresponding author. Email ypzhang_at_vt.edu Tel
540-231-7414 (O), 540-231-0747 (L)
Three Birds are killed by one stone !!!
Challenges Unique Features
Likely lowest
hydrogen production costs (2/kg) Hydrogen
Production Mild reaction conditions (30-100oC, no
pressure) Highest yields from sugars (12
H2/glucose) Hydrogen Storage 14.8 H2-mass ,
108 kg H2/m3 Hydrogen Distribution Store and
distribute solid sugars
.
Abstract
The hydrogen economy offers a compelling
clean energy future but there are four main
obstacles hydrogen production, storage, and
distribution, as well as high costs of fuel
cells. We have invented a synthetic enzymatic
pathway consisting of 13 enzymes for producing
hydrogen from starch and water as C6H10O5 (l) 7
H2O (l) ? 12 H2 (g) 6 CO2 (g). The unique
features, such as mild reaction conditions (30oC
and atmospheric pressure), high hydrogen yields,
likely low production costs (2/kg H2), and a
high energy-density carrier starch (14.8 H2-based
mass), provides perfect opportunities for mobile
applications. With technology improvements and
integration with fuel cells, this technology also
solves the challenges associated with hydrogen
storage, distribution, and infrastructure in the
hydrogen economy We envision that future mobile
appliances will store solid starch, produce
hydrogen from starch and water via this reaction,
and then generate electricity by hydrogen fuel
cells at the same compact place. The prototype
zero-pollution car powered by sugars is under
development.
Conceptual Sugar Car
The supposed sugar-fuel cell (Figure 2)
would achieve the highest energy efficiency
(50-55) than any other technology (e.g.,
ICE-electrical hybrid, hydrogen-fuel cell) in the
world (7).
Rationale and Challenges
The future hydrogen economy is a linked
network of chemical processes that produces
hydrogen, stores hydrogen chemically or
physically, and converts the stored hydrogen to
electrical energy at the point of use. It would
solve the problem of finite fossil fuel reserves
and minimize the negative environmental impacts
of hydrocarbon fuel burning (1,2) because
hydrogen fuel cells have much higher energy
efficiency (50-80) than do internal combustion
engines (18-23), and do not produce any
pollutants (1,2). Four main technical
challenges for mobile hydrogen-fuel cell
applications were outlined by the Department of
Energy (DOE) 1) decreasing hydrogen production
costs via a number of means, 2) finding viable
methods for high-density hydrogen storage, 3)
establishing a safe and effective infrastructure
for seamless delivery of hydrogen from production
to storage to use, and 4) dramatically lowering
the costs of fuel cells. So far, all
hydrogen-producing methods based on less costly
biomass have been plagued with low energy yields
and/or severe reaction conditions and/or complex
processing requirements all solid hydrogen
storage methods have low energy storage densities
and are not suitable for mobile applications a
large scale hydrogen distribution infrastructure
does not exist.
Sugar Car
A
B
Our Invention
Figure 2. The conceptual design for the
integrated on-board bio-converter and fuel cell
(A) and sugar car (B).
Our idea is to utilize energy stored in
polysaccharides to break up water and release all
energy in the form of hydrogen by a novel
enzymatic technology (synthetic enzymatic pathway
engineering) C6H10O5 (l) 7 H2O (l) ? 12 H2
(g) 6 CO2 (g)
1 We designed an artificial
enzymatic pathway comprised of reversible
enzymatic reactions and pathways 1) a
chain-shortening phosphorylation reaction
catalyzed by a-glucan or b-glucan phosphorylases
(Equation 2) (3,4), 2) a conversion from
glucose-1-phosphate (G-1-P) to glucose-6-phosphate
(G-6-P) catalyzed by phosphoglucomutase
(Equation 3), 3) a pentose phosphate pathway
(Equation 4), and 4) hydrogen generation from
NADPH catalyzed by hydrogenase (Equation 5) (5)
(C6H10O5)n H2O Pi ? (C6H10O5)n-1 G-1-P
2 G-1-P ?
G-6-P
3 G-6-P
12 NADP 6 H2O ? 12 NADPH 12 H 6 CO2 Pi
4 12 NADPH 12 H ? 12 H2 12
NADP 5
Figure 1. The synthetic metabolic pathway for
complete conversion of glucan and water to
hydrogen and carbon dioxide. PPP, pentose
phosphate pathway. The enzymes are 1 GNP,
glucan phosphorylase 2 PGM, phosphoglucomutase
3 G6PDH, G-6-P dehydrogenase 4 6PGDH,
6-phosphogluconate dehydrogenase, 5 Ru5PI
phosphoribose isomerase 6 R5PI. ribulose
5-phosphate epimerase 7 TKL, transketolase 8
TAL, transaldolase 9 TPI, triose phosphate
isomerase 10 ALD, aldolase, 11 FBP,
fructose-1, 6-bisphosphatase 12 PGI,
phosphoglucose isomerase and 13 H2ase,
hydrogenase. The metabolites are G1P,
glucose-1-phosphate G6P, glucose-6-phosphate
6PG, 6-phosphogluconate Ru5P, ribulose-5-phosphat
e X5P, xylulose-5-phosphate R5P,
ribose-5-phosphate S7P, sedoheptulose-7-phosphate
G3P, glyceraldehyde-3-phosphate E4P,
erythrose-4-phosphate DHAP, dihydroxacetone
phosphate FDP, fructose-1,6-diphosphate F6P,
fructose-6-phosphate and Pi, inorganic phosphate
We (Virginia Tech, Oak Ridge National
Laboratory, University of Georgia) are working
together to increase reaction rates, reduce
enzyme costs, increase enzyme stability, and
develop the sugar car concept. Any
collaborations towards this ambitious project and
funding are welcome.
References

1. J. A. Turner, Science 305, 972 (2004).
2. B. C. H. Steele, A. Heinzel, Nature 414, 345
   (2001)
3. Y.-H. P. Zhang, L. R. Lynd, Appl. Environ.
   Microbiol. 70, 1563 (2004).
4. Y.-H. P. Zhang, L. R. Lynd, Proc. Natl. Acad.
   Sci. USA 102, 7321 (2005).
5. J. Woodward, M. Orr, K. Cordray, E. Greenbaum,
   Nature 405, 1014 (2000).
6. Y.-H. P. Zhang, B. R. Evans, J. R. Mielenz, R.
   Hopkins, M.M W. Adams. under review (2007).
7. M. I. Hoffert et al., Science 298, 981 (2002).

We have finished the proof-of-concept
experiment (data not shown). The manuscript is
under review (6).