Scientists search for green plant solar secrets

By Gary Raham
Nature Writer and Illustrator

It may not be nice to fool Mother Nature, but copying her is always a good bet.

Photosynthesizing bacteria and green plants tap the solar system's ultimate power source, the sun, with 95 percent efficiency while human technologists struggle to squeeze somewhere between 20 percent and 40 percent efficiency from silicon-based solar cells. As research continues to reveal the secrets of nature's elegant, though complex "wetware," technologists still endeavor to create profitable hardware truly worthy of the title "green."

Solar thermal technologies harness heat energy from the sun, which is used to heat fluids that in turn heat water to run conventional steam-turbine generators. Solar collectors must often be placed in deserts, and electricity must travel over conventional power lines.

Solar-photovoltaic technologies offer power on the spot and don't require water or transmission lines, but currently they are the most expensive of the noncarbon-based power alternatives.

The efforts to mimic nature's techniques involve producing devices that will compete with the current silicon-based photoreceptors used in traditional solar-voltaic cells. The new technologies can't yet compete without subsidies in the efficiency category--probably a relatively short-term deficiency. But they will bypass the use of silicon, a component becoming expensive and hard to get, and they can be incorporated into building materials, clothing and portable electronics in ways that may transform the future.

Nature's technique

Step 1: When light energy strikes photo-reactive pigments in a leaf (like chlorophyll), it knocks loose a cascade of electrons as water splits into its hydrogen and oxygen components.

Step 2: Those electrons go to work building sugars as they make and break various chemical bonds in a cyclic metabolic process.

Scientists are trying to imitate Step 1 while highjacking the moving electrons (which, by definition, is an electric current) to run toasters and PCs. In plants, the element magnesium in chlorophyll plays a key role in the process, and it all happens in the soggy interior of a plant cell.

Some scientists are looking for new catalysts that will work in dry, electronics-friendly environments. Others are finding ways to merge things like spinach chloroplasts into workable solar cells. Some efforts, already commercial products, use nanocrystals as key components of dye-sensitized solar cells.

Recent discoveries

MIT announced a year ago (July 2008) that Daniel Nocera, Henry Dreyfus Professor of Energy at that institution, has developed a "revolutionary" process that uses a cobalt-phosphate catalyst combined with a platinum catalyst in a thin film on an electrode to split water into oxygen and hydrogen much as in photosynthesis. The hydrogen and oxygen can be stored and later combined into a fuel cell that could generate power when the solar cells (used to power the electrode) are offline.

Nocera hopes that within 10 years a system based on this technology could allow individual homes to be completely off a central power grid. His work is part of the Solar Revolution Project funded by the Chesonis Family Foundation.

In 2004, Shuguang Zhang, associate director for the Center for Biomedical Engineering at MIT, announced a molecular electronic device made from glass sandwiched with a protein-gold-spinach cell complex that yielded 12 percent efficiency in converting light to electrical power, compared with 20 or 24 percent for silicon cells. One hundred thousand of Zhang's bioelectronic circuits, however, fit on the head of a pin.

In March, Heinz Frei, a chemist with Berkeley Lab's Physical Biosciences Division, and his research associate, Feng Jiao, announced that they had used nanometer-sized (billionth of a meter) crystals of cobalt oxide to mimic the water splitting skills of photosynthetic organisms. Micron (millionth of a meter)-sized particles of the same catalyst were far less efficient.

Scientists are still grappling with the differences size makes in the operation of photochemical processes like photosynthesis. Quantum effects emerge that challenge the intuition brought to bear in understanding physical and chemical processes at human scale.

Dye-sensitized solar cells use low-cost organic dyes and titanium dioxide nanoparticles to mimic photosynthetic cells. Jerusalem-based 3GSolar currently competes with Australia's Dyesol, Japanese Sharp and British company G24I to create building materials like rooftop tiles and window glass that can generate power for building occupants. Various off-grid areas in Africa, India and parts of China serve as current clients, but the technology is poised to service much broader applications.

Future prospects

It's relatively easy to imagine the construction of buildings with parts that generate their own power. Now imagine a hat, coat or other fashion accessory that might power iPods, small computers or biomonitors of various sorts. Truly copying nature might involve adopting the photosynthetic cells of algae or bacteria and giving them a home on human skin.

This would not be a first, of course. Lichens are fungi that collect water and nutrients to share with algal symbionts. A Texas A & M biologist recently found a sea slug (a kind of mollusk) that is partially solar-powered by the algal food it eats. It digests the cytoplasm of the cells except for the photosynthesizing plastids, which it retains as microscopic sugar factories. The slug can go without feeding for nine months at a time.

Imagine going really green and just sunbathing with algal buddies when it's time to eat.