WASHINGTON — Scientists who are seeking new sources of clean energy are trying to mimic the way plants and trees do it, by converting sunlight into fuel.
Unlike standard solar panels on rooftops or arrays of solar collectors in the desert, this is a form of ``artificial photosynthesis.'' It tries to imitate the elaborate system that microbes, algae and green plants developed over 3 billion of years of evolution.
If it works, artificial photosynthesis could help reduce the world's dependence on fossil fuels without generating climate-warming greenhouse gases.
``A perfect solution to the energy problem is to mimic the natural system which has served us so well,'' James Barber, a biochemist at Imperial College, London, wrote in an e-mail.
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Natural photosynthesis captures less than 3 percent of the sunlight that reaches Earth, but that's enough to provide all the energy that living creatures need. The process converts carbon dioxide (CO2) and water (H2O) into oxygen and carbohydrates such as glucose, the sugary fuel that powers our bodies.
``When we say artificial photosynthesis, we mean trying to do what nature does in green leaves: absorb light at room temperature and ultimately convert the light into chemical fuels,'' said Catherine Murphy, a chemist at the University of South Carolina, Columbia. Murphy will lead a researchers' seminar on artificial photosynthesis this winter in Ventura, Calif.
Various approaches to artificial photosynthesis are under way in the United States, Europe, Japan and Australia.
For example, Tom Mallouk, a professor of chemistry and physics at Pennsylvania State University in University Park, has built an experimental device that uses light to launch a daisy-chain of tiny molecules that pass electrons — the particles that carry electrical energy — from one to another. When the electrons reach the end of the chain, they take part in a chemical process that generates hydrogen, which can be stored for use later as a fuel, he explained.
Mallouk's molecular clusters are about 2 nanometers (billionths of a meter) in size. They float amid red-orange dyes that absorb sunlight and use its energy to split water into its basic elements, oxygen and hydrogen.
``It's like natural photosynthesis on a chip,'' Mallouk said. It's a ``synthetic chemical system that's designed to do what photosynthesis does.''
Another researcher, Song Jin, a chemist at the University of Wisconsin, in Madison, is experimenting with nanometer-scale wires instead of clusters of molecules to convert solar energy (light) into chemical energy (fuel).
Jin declines to call his process ``artificial photosynthesis,'' but he agrees that the outcome will be similar to the products of natural photosynthesis. He said that his nanowires, a few billionths of a meter thick, ``might help the efficiency, performance and cost of energy conversion applications.''
Yet another preliminary technique is being tested by an international team of scientists, headed by Leone Spiccia of Monash University in Victoria, Australia, and Charles Dismukes at Princeton University in New Jersey. They use a molecular cluster containing atoms of manganese, a chemical used in plant photosynthesis to help break water molecules apart into hydrogen and oxygen.
This is ``the most difficult first step in the splitting of water into hydrogen and oxygen,'' Dismukes said. Later, the oxygen and hydrogen may be recombined in a fuel cell, creating carbon-free electricity to power a house or electric car, day or night.
``We have copied nature, taking the elements and mechanisms found in plant life and re-creating one of those processes in the laboratory,'' Spiccia said in a statement issued by Monash. ``The production of hydrogen using nothing but water and sunlight offers the possibility of an abundant, renewable, green source of energy for the future.''
Artificial photosynthesis differs from photovoltaics, the method used in solar panels, which generates an electrical current that can't be stored but must be loaded onto the electrical grid.
``Sometimes it's more useful to make fuel which can be stored indefinitely and used where you make it — for example, in your house or to run your car — rather than be sent through a power grid,'' Mallouk said.
Practical artificial-photosynthetic systems are at least 10 years away, Mallouk said. There are many technical problems to be solved to equal the efficiency of nature's way.
``Ask me again in 10 years and I'll probably say another 10 years,'' he said.
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