Here Comes the Sun
Abram Katz, Register Science Editor
07/01/2007
nhregister.com.
Every hour of sunlight carries enough energy to supply the world with electricity for about a year.
However, sunlight is spread over a wide area — one hemisphere of Earth — and there is no efficient way to collect it.
Also, cars, trucks, boats, rockets, emergency generators, and even camp lanterns, require fuel, not electricity.
And collecting solar radiation takes a lot of expensive photovoltaic cells.
Gary W. Brudvig, chairman of chemistry at Yale University, has a map of the United States with a square about the size of Iowa. That’s about how large an area would have to be covered with solar cells to generate enough electricity for the U.S.
Power consumption on a global scale is enormous. The world runs on about 15 terawatts, or 15 trillion watts, a year. Plastering almost 100,000 square miles around the globe with 10 percent efficient photo cells would generate about 20 terawatts.
A substantial amount of that electricity would be lost in power lines, and if a storm system happened to pass over an array, local power would drop instantaneously.
Photovoltaic cells are also environmentally dirty.
So the U.S. Department of Energy is seeking novel methods of providing renewable power.
Brudvig and colleagues have received a highly competitive DOE grant to find a way to convert solar energy directly into usable nonfossil fuel.
Yale and 13 other institutions were selected for funding from about 700 energy proposals, Brudvig said. Yale will receive about $1.44 million over the next three years.
"There is a lot of momentum to develop renewable energy. First the concentration was on hydrogen. Now it’s solar energy," Brudvig said.
That suits him well, because he has spent decades studying photosynthesis, the process with which plants transform sunlight, water and carbon dioxide into carbohydrates and oxygen.
Brudvig’s group is working on a simplified way to emulate green plants, minus the chlorophyll, leaves, roots, and carbon dioxide.
As a practical matter, solar energy through artificial photosynthesis could supply about 600 terawatts a year, he said.
Why not just grow more plants and make more ethanol?
The big drawback of fermentation is that yeast can only digest sugars. Consequently, only 3 percent of a corn plant can be fermented, Brudvig said. Other plants may have a higher sugar content, but a bulk of the sun’s energy is still locked up as cellulose and lignin.
Right now growing corn to produce fuel is a losing proposition, he said. More energy goes into growing and processing the corn than can be tapped from the final ethanol.
Plants are less than 1 percent efficient in converting sunlight into biomass, Brudvig said. "Plants are just trying to live and reproduce, so that’s fine for them," he said.
Photosynthesis would have to be about 10 times more efficient to help people with their energy needs, he said. Natural photosynthesis is a good interim measure, but Brudvig and colleagues decided to design an artificial system to harness sunlight with greater efficiency, he said.
"Our goal is artificial photosynthesis, to move electrons not using chlorophyll," he said.
Brudvig is experimenting with manganese complexes hooked onto nano particles of titanium oxide and suspended in water.
Nano particles are the material of choice because they maximize surface area, essential to a process that depends on light.
Both manganese and titanium are common in Earth’s crust and relatively inexpensive, he said. Earth also has a large supply of water.
The manganese molecule can absorb energy from sunlight and use it to split water molecules into oxygen and hydrogen. The plan is for the manganese to split a water molecule into one oxygen atoms, two positively charged hydrogen nuclei and two negatively charged electrons, he said.
These electrons are transported to the titanium oxide, where light boosts them into a higher energy state andconducts them to another reaction surface that create hydrogen, methane, methanol, and other fuels.
"Any fuel has excess electrons. We could transfer the moving electrons to make hydrogen, and then use hydrogen to turn carbon dioxide into methanol," Brudvig said. That is, in chemical symbols, CO2 + 6 e- + 6 H+ ---> (CH3 OH) + H2O.
Excess oxygen from the water would be released to the atmosphere.
Manganese has no trouble with salt water, either.
"We want to unravel the principles so we can make fuel from sunlight. We have to disentangle what are the rules of the game," he said.
Among the questions to be answered is how the electrons are transported into the titanium oxide particle, and how best to anchor the manganese complexes onto the nano particles.
Another challenge is to translate microscopic nano particle complexes into an industrial scale, Brudvig said.
The ultimate invention may resemble a solar panel, plus hoses to carry away fuel. Reactions would take place in a two-chambered photocatalytic cell.
Meanwhile, petroleum is expected to run out in about 50 years and coal in another200 to 300. To meet the anticipated planetary energy demand of 2050 would require opening a nuclear power plant every using nuclear power alone would require opening a nuclear power plant every day for the next 50 years, Brudvig said.
Solar energy is a more likely energy source, he said.
On this project Brudvig is collaborating with Victor S. Batista, Robert H. Crabtree, and Charles A. Schmutternmaer, all of Yale.
Abram Katz can be reached at akatz@nhregister.com or 789-5719. |
