Home' Technology Review : November December 2008 Contents FEATURE STORY
TECHNOLOGY REVIEW NOVEMBER /DECEMBER
the energy captured by the dyes would drive the water-splitting
reaction. Either way, solar energy would be converted into hydro-
gen fuel that could be easily stored and used at night---or when-
ever it's needed.
Nocera's audacious claims for the importance of his advance
are the kind that academic chemists are usually loath to make in
front of their peers. Indeed, a number of experts have questioned
how well his system can be scaled up and how economical it will
be. But Nocera shows no signs of backing down. "With this dis-
covery, I totally change the dialogue," he told the audience in May.
"All of the old arguments go out the window."
THE DARK SIDE OF SOLAR
Sunlight is the world's largest potential source of renewable energy,
but that potential could easily go unrealized. Not only do solar pan-
els not work at night, but daytime production waxes and wanes as
clouds pass overhead. That's why today most solar panels---both
those in solar farms built by utilities and those mounted on the
roofs of houses and businesses---are connected to the electrical
grid. During sunny days, when solar panels are operating at peak
capacity, homeowners and companies can sell their excess power
to utilities. But they generally have to rely on the grid at night, or
when clouds shade the panels.
This system works only because solar power makes such a tiny
contribution to overall electricity production: it meets a small
fraction of 1 percent of total demand in the United States. As the
contribution of solar power grows, its unreliability will become
an increasingly serious problem.
If solar power grows enough to provide as little as 10 percent
of total electricity, utilities will need to decide what to do when
clouds move in during times of peak demand, says Ryan Wiser,
a research scientist who studies electricity markets at Lawrence
Berkeley National Laboratory in Berkeley, CA. Either utilities will
need to operate extra natural-gas plants that can quickly ramp
up to compensate for the lost power, or they'll need to invest in
energy storage. The first option is currently cheaper, Wiser says:
"Electrical storage is just too expensive."
But if we count on solar energy for more than about 20 per-
cent of total electricity, he says, it will start to contribute to what's
called base load power, the amount of power necessary to meet
minimum demand. And base load power (which is now supplied
mostly by coal-fired plants) must be provided at a relatively con-
stant rate. Solar energy can't be harnessed for this purpose unless
it can be stored on a large scale for use 24 hours a day, in good
weather and bad.
In short, for solar to become a primary source of electricity, vast
amounts of a ordable storage will be needed. And today's options
for storing electricity just aren't practical on a large enough scale,
says Nathan Lewis, a professor of chemistry at Caltech. Take one
of the least expensive methods: using electricity to pump water
uphill and then running the water through a turbine to generate
electricity later on. One kilogram of water pumped up 100 meters
stores about a kilojoule of energy. In comparison, a kilogram of
gasoline stores about 45,000 kilojoules. Storing enough energy
this way would require massive dams and huge reservoirs that
would be emptied and filled every day. And try finding enough
water for that in places such as Arizona and Nevada, where sun-
light is particularly abundant.
Batteries, meanwhile, are expensive: they could add $10,000
to the cost of a typical home solar system. And although they're
improving, they still store far less energy than fuels such as gas-
oline and hydrogen store in the form of chemical bonds. The
best batteries store about 300 watt-hours of energy per kilo-
gram, Lewis says, while gasoline stores 13,000 watt-hours per
kilogram. "The numbers make it obvious that chemical fuels
are the only energy-dense way to obtain massive energy stor-
age," Lewis says. Of those fuels, not only is hydrogen poten-
tially cleaner than gasoline, but by weight it stores much more
energy---about three times as much, though it takes up more
space because it's a gas.
The challenge lies in using energy from the sun to make such
fuels cheaply and e ciently. This is where Nocera's e orts to
mimic photosynthesis come in.
In real photosynthesis, green plants use chlorophyll to capture
energy from sunlight and then use that energy to drive a series
of complex chemical reactions that turn water and carbon diox-
ide into energy-rich carbohydrates such as starch and sugar. But
what primarily interests many researchers is an early step in
the process, in which a combination of proteins and inorganic
catalysts helps break water e ciently into oxygen and hydro-
The field of artificial photosynthesis got o to a quick start.
In the early 1970s, a graduate student at the University of Tokyo,
Nocera's audacious claims
are the kind that academic
chemists are usually loath
to make in front of their
peers. But Nocera shows
no signs of backing down:
"With this discovery, I
totally change the dialogue.
All of the old arguments go
out the window."
Links Archive January February 2009 September October 2008 Navigation Previous Page Next Page