the past 150 years, burgeoning industrialization
has increased carbon in the atmosphere by 40 percent
and driven a continuing rise in global temperatures.
The trend won't stop soon. Among the consequences:
rising sea levels, increased air pollution, and more
hurricanes, floods, and droughts. Meanwhile, the
age of cheap oil and gas has come to an end.
In the short term humans urgently need to use energy
more efficiently, and we need to stop putting carbon
straight into the air. More important for the long
term, we need to find or create ways to use energy
that don't release any carbon at all.
after taking over as director of Lawrence Berkeley
National Laboratory in mid-2004, Steven Chu noted
that one of the Lab's greatest strengths lay in
its potential to mobilize multidisciplinary scientific
programs to develop carbon-neutral sources of energy.
In the spring of 2005 Chu convened a meeting of
Lab researchers and expert guests whom he invited
to tackle what he called the "hardest questions" confronting
the effort to turn sunlight into abundant, dependable,
cheap, and convenient chemical fuels and electricity
for human use. Following are a sampling of just some
of the ideas discussed at Solar to Fuel: Future Challenges
Some of the challenges concern matters of scale.
Energy storage, for example: steel cylinders may
be fine for the laboratory or the welding shop, but
as yet there's no safe way to store hydrogen that's
also convenient and cheap. Batteries are fine for
flashlights and electric vehicles, but there's no
good way to store a power plant's worth of electricity.
If energy doesn't come as a solid or liquid, how
do we store it?
more fundamental challenge is catalysis. Whether
it's splitting water into hydrogen and oxygen in
a sort of reverse fuel cell, fermenting the cellulose
in agricultural trash to make ethanol, or simply
growing plants more cheaply by equipping them to
fix nitrogen for themselves, there are many ways
to enhance the conversion of sunlight to fuel — but
none of these catalytic processes operate on a scale
that as yet makes economical sense.
Doin' what comes naturally
to solar-to-fuel conversion may be divided broadly
into those that modify nature — using the
techniques of synthetic biology, for example — and
those that start from scratch, using materials and
chemical processes from the inorganic world. In the
middle is a wide and promising range of tactics that
borrow from nature and artifice both.
What the natural processes have in common is photosynthesis,
a complex and very rapid sequence of reactions in
plants and some bacteria that converts incident solar
radiation into stored chemical energy. Two protein
assemblies are involved. Photosystem II uses light
to break molecules of water into oxygen, hydrogen
ions (protons), and free electrons. From a plant's
point of view the oxygen is waste, but the electrons
and protons enable photosystem I to convert carbon
dioxide to carbohydrates, fixing the carbon and energizing
In terms of net productivity the process isn't fast
or efficient, but it hardly matters: plants are cheap,
and they're everywhere. Two examples serve to illustrate
the many biologically based schemes for making their
stored energy more useful to humans:
One approach is to design organisms that can produce
fuels like ethanol or methane directly from sunlight,
air, water, and soil. Ethanol from corn is already
added to gasoline in this country, although as Tad
Patzek of Berkeley Lab's Earth Sciences Division
pointed out at the meeting, the energy content of
the gasoline and other fossil fuels required to fertilize,
cultivate, harvest, transport, and distill corn is
significantly greater (at maximum corn-conversion
efficiency, an extra five percent) than the energy
content of corn ethanol itself. Not to mention the
attendant pollution from all that extra fuel use.
Clearly a bad bargain.
take yeast, say — normally unresponsive to light — and
modify it to contain bacteriorhodopsin, a bacterial
relative of the "visual purple" protein that responds
to light in the retina of the eye. Stimulated by
sunlight, bacteriorhodopsin in yeast could make ATP,
which would fix atmospheric CO 2 , which the yeast
could convert directly to ethanol. Antón Vila-Sanjurjo
and Carlos Bustamante of the Lab's Physical Biosciences
Division (PBD) are at work on just such a synthetic
different route to biofuels is through more efficient
conversion of biomass, including the great mass
of trash from agriculture and forestry that now
goes to waste. Half the carbon in biomass is tied
up in various forms of cellulose, which forms the
stiff walls of plant cells. Made of polymerized
sugars, carbohydrates consisting almost entirely
of hydrogen, carbon, and oxygen, cellulose is broken
down by microbes that employ special enzymes — or
protein machines, like the large, complex cellulosomes
found in the membranes of some species of Clostridium
No existing organism works fast enough, or with
enough specificity, to efficiently produce hydrogen
or methanol from cellulose for human use, however.
Unlike yeast or E. coli , Clostridia are difficult
to engineer. Nor can cellulosomes easily be transplanted
to different species of microbes.
The solution may be to design a new organism from
the chromosomes out, with custom-tailored enzymes,
cellulose-degrading machinery, metabolic and signaling
pathways, and genetic control mechanisms optimized
for converting the cellulose in biomass, like sawdust
or rice straw, to high-quality fuels. It's a multidisciplinary
challenge of the kind that PBD's Jay Keasling has
already practiced in the field of medicine, by engineering
synthetic bacteria to produce valuable drug compounds
that are usually made by plants.
It's not unnatural
materials like metal and semiconductor compounds
are proven catalysts for splitting water or carbon
dioxide into their constituent elements — oxygen
plus hydrogen or carbon, including ions and free
electrons — when energized by ultraviolet or visible
light. Life has made similar use of metals for a
long time: Earth's first organisms probably fixed
carbon through reactions catalyzed by iron and sulfur.
In photosynthesis, higher plants and cyanobacteria
use photosystem II instead, splitting water with
a tiny molecular complex that incorporates manganese
and calcium, and which may very well have been borrowed
from minerals by early microbes.
Traditionally, scientists looking for a way to drive
solar production of chemical fuels with the aid of
solid catalysts haven't spent a lot of time studying
plants and microbes; they are concerned with mounting
the right inorganic catalyst in the right support
structure to produce an efficient, stable system
at the right price.