Bright Days for Solar

Continued from page 3

Unlike existing solar concentrators that use lenses and mirrors, sheets of luminescent glass or plastic wouldn't have to be pointed directly at the sun to absorb sunlight, eliminating the need for tracking. The sheets would also be lighter and easier to install than a comparable parabolic mirror or lens. The technology could even be built into homes, where windows "painted" with the transparent dyes could be edged with strips of solar cells.

Storing Light
Even if solar panels become extremely cheap and efficient, they still have a problem: they reach their peak power output for only a few hours a day and don't work at all at night. As long as solar energy provides only a small fraction of the world's electricity, other sources--often fossil fuels--can be relied upon to supply power after dark. But as solar grows, finding a way to store the energy it produces will become essential.

Researchers are investigating many ways to do this. At MIT, for example, Donald Sadoway, a professor of materials chemistry, is working on new kinds of high-capacity batteries. And ­Daniel ­Nocera, the Henry Dreyfus Professor of Energy, is developing inexpensive ways to use sunlight to split water into hydrogen and oxygen, which can be recombined in a fuel cell to generate electricity.

The standard process for splitting water, called electrolysis, is inefficient and expensive. That's why almost all the hydrogen produced today comes from fossil fuels, not water. But Nocera is taking a cue from nature, looking to photosynthesis for ideas about cheaper, more efficient techniques for splitting water.

In photosynthesis, plants use sunlight's energy to generate electrons and their positively charged counterparts, holes. An enzyme called the oxygen-evolving complex uses the holes to produce oxygen gas and hydrogen ions from water. Another enzyme combines the hydrogen ions with electrons to make hydrogen.

Until a couple of years ago, researchers didn't clearly understand the oxygen-evolving complex or how it works. When its structure was discovered, Nocera's lab flew into a frenzy of activity aimed at developing an artificial version of the enzyme that would be cheap and easy to make on a large scale. This spring, Nocera announced success: a new catalyst made from readily available cobalt can produce oxygen from water using sunlight.

More work remains, since converting the hydrogen ions into hydrogen gas is an extra step requiring a costly platinum catalyst. But Nocera says the discovery of a catalyst for making oxygen has supplied the missing link in the quest to mimic the action of the leaf.

"You can make fuels from a glass of water with sunlight now very cheaply," he said at a recent symposium. Nocera predicts that in 10 years, it could be practical for homeowners to install systems that use photovoltaics to generate power during the day and split water to make fuel at night.