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Solar-energy research heats up in Santa Cruz
Together, the scientists — among a handful of solar-technology researchers in the San Francisco Bay Area — are helping respond to the challenge thrown down Sept. 12 when Gov. Gray Davis signed Senate Bill 1078 requiring California to generate 20 percent of its retail energy electricity from renewables by 2017. The bill, the strongest of its kind in the country, would double the existing use of wind, solar and other renewable power sources. Lauded by environmentalists, the bill is coupled with SB 1038, which promises $62.5 million to solar research and development in the next five years. "This is the largest renewable energy fund by far in the U.S.," said Ralph Cavanagh of the Natural Resources Defense Council. "It accounts for about half of all the other states combined. It’s a substantial investment." After the oil crisis of 1973, research on solar technology exploded and with 25 years of research and improvements on the manufacturing process, the cost of silicon photovoltaics has been reduced dramatically from $15 per kilowatt hour in 1977 to about 24 cents per kwh today. It’s still not low enough to compete with coal, nuclear or natural gas, however — an average PG&E customer pays between 10 and 24 cents per kwh. The challenge to cheaper solar-generated electricity is not how well solar cells, or photovoltaics, convert sunlight to electricity, said Smestad. Solar cells only need to be about 5 percent efficient to provide a reasonable balance between the size of the panel and the power it produces. "We have 15 percent efficient commercial solar cells," said Smestad. "The challenge is to figure out the physics so that we can make it cheaper." This challenge may require a switch to completely new types of materials. The vast majority of solar panels today are made of silicon, the workhorse of the semiconductor industry. It makes for highly efficient, stable solar cells, but conventional silicon must be fabricated and deposited at high temperatures and low pressures, an inherently expensive process. In response, researchers are working on a variety of cheaper materials. Some are transparent or semi-transparent, offering the possibility of windows that can produce electric power. Others are flexible, like plastic, and so may be used in clothing or rolled out in sheets. "We have a tendency to think, the public, that we have everything that can be had in terms of technology," said Smestad. Carter works with polymers — plastics — that can conduct electricity. It is surprising, when you think about it, that a plastic could conduct electricity at all; your credit card doesn’t conduct, neither do the tires of your car. Wires are insulated with rubber and plastics, both polymers, because these materials do not conduct. But specially designed polymers behave in interesting ways. The consistency of flour or fluffy hair, they dissolve in solvents to form liquidy solutions that can be spread out into films thinner than a soap bubble and deposited on flexible plastic sheets. These materials can be tweaked to emit or absorb almost every color that can be seen, and may form the basis of the next generation of flat-panel televisions, cell phone displays and such futuristic applications as electronic paper. This summer, Philips launched the first such product — the Philishave — an electric razor with a polymer display that indicates battery life. Polymers also hold great promise for solar cell applications, because they’re cheap. Polymers are liquid and may be screen-printed or ink-jet printed at room temperature and pressure using the same basic technology as we use to make T-shirts, said Carter. This brings the hypothetical cost to 2 to 7 cents per kwh, as compared to 10 to 24 cents for fossil fuels. So why aren’t polymer photovoltaics coating our homes and businesses? At this point, said Carter, they aren’t tough enough. These materials degrade in strong sunlight and so last a year or two. Commercial solar panels for residential applications need to be stable for about 10 years, said Smestad. Polymer cells are also plagued by problems of efficiency – how well they convert sunlight to electric power. They must improve by about four times over their current performance to be commercially viable, Carter said. While Carter struggles with improving the polymer cells, Smestad, formerly of the Monterey Institute for International Studies and his colleague, Jin Zhang of UC Santa Cruz, are working on a different type of solar device. Smestad and Zhang were awarded one of only a few research grants available through the California Energy Commission to improve the performance of Gretzel cells. Instead of polymers or silicon, a Gretzel cell uses a semiconductor commonly found in toothpaste and sunscreen – titanium dioxide. A glass sheet is coated with bump nanoparticles of titanium dioxide and then dipped in die. A special liquid known as an electrolyte, which helps replenish the supply of electrons, is poured over this bumpy surface and soaks in like a sponge. The whole cell is then sealed. When light knocks an electron free from the titanium dioxide, it travels to the positive side of the cell, creating an electric current. The construction is so simple that Smestad has developed a kit for students to make a cell out of berry juice, cheap titanium dioxide and commercial iodine. The cell is strong enough to power a tiny fan. So, why use petroleum instead of Gretzel cells? The trouble is that even with the best seal, eventually, liquid evaporates. Polymer and Gretzel cells are not the only technologies that promise to reduce energy costs. At UC Berkeley, physicist Paul Alivisatos is investigating a hybrid approach to get around the efficiency problem faced by Carter’s group. He blends nanoparticles — particles on the order of one-billionth of a meter — of silicon-like semiconductors with polymers. Once incoming light knocks an electron free from an atom, the electron’s journey across the cell is hastened by the presence of the nanoparticles. Smestad is adamant that the future of solar technology depends upon a concerted research effort, and government funding to support that research. Current government spending on renewables is just pennies to every dollar spent on coal or nuclear energy, not to mention the defense budget. "Every cell that is coming out is showing us one thing — that we haven’t scratched the surface with what can be done with low-cost solar cells," he said. "I’m not getting any younger, and there are new discoveries that need to be made." Contact Stephanie Chasteen at jcopeland@santa-cruz.com. For more information on Gretzel cells and solar cell kit, visit http://www.solideas.com/solrcell/cellkit.html.
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