Solar Energy: UC explores the next generation

With California moving aggressively to reduce greenhouse gas emissions, solar power plays a growing role in alternative energy strategies. The state's utility companies have committed to producing 20 percent of their power from renewable energy sources by 2010 with solar energy expected to provide a significant supply of renewable energy.

In October, the first large solar thermal plant built in California in 18 years opened outside Bakersfield. Currently 34 large solar thermal projects have been proposed in the state.

The California Solar Initiative, through enticing consumer rebates, has a goal of having one million solar roofs in California by 2018. When Gov. Schwarzenegger signed legislation in 2006 to put the Million Solar Roofs Plan into action, his office estimated that installing a million photovoltaic solar systems on new and existing rooftops would provide 3,000 megawatts of additional power. And the greenhouse gas reductions would equate to taking a million cars off the road.

New federal tax credits and state solar credits make tapping the sun for power an attractive proposition for consumers and businesses. Yet before solar power can reach its full potential, UC energy experts believe the efficiency and cost of photovoltaic technology must improve – a goal they are pursuing.

Severin Borenstein, director of the University of California Energy Institute and a professor of business and public policy in the Haas School of Business at UC Berkeley, analyzed the cost of power generated through photovoltaic panels. He concluded that with the current PV technology, programs that subsidize residential installations are wasting money that could better be spent on new solar technology research.

"Based on the actual data of what these panels produce and when they produce it, the best estimate is solar power costs about 40 cents a kilowatt to replace power that cost less than half of that," said Borenstein.

Solar thermal power and wind power are a much more cost-effective renewable energy sources, he said. Photovoltaic, or PV, solar systems, convert sunlight directly into electricity through panels of cells made of expensive silicon. Those solar panels generate the most power during peak usage times on hot sunny days. The homes stay connected to the electric utility and can tap additional power if the system does not produce enough.

Solar thermal plants like the one in Bakersfield use giant mirrors to focus sunlight on tubes filled with water. When the water gets hot enough, it produces steam that turns a turbine to produce electricity, which is distributed through the electricity grid.

"Most Californians are considering the private economics: 'Will I save money?'" said Borenstein. "The answer is you might if you are a high user of electricity."

California's utility companies charge customers on an increasing-block pricing system. The price of each block of electricity rises as the customer's use increases. Big electricity consumers pay more, so they would reap more savings for going solar. His January study, titled "The Market Value and Cost of Solar Photovoltaic Electricity Product," concluded that the cost of solar PV is about 80 percent more that the value of the electricity it will produce. Still, he understands the consumer attraction to generating your own power from the sun.

"Solar panels are very sexy," said Borenstein. "Nobody can see the solar thermal plant in the desert. You can't see the wind turbines. The panels let your neighbors know you are producing power."

Borenstein is quick to emphasize that his economic analysis is based on the solar cells in use today made of expensive silicon. New technologies could change the whole economic picture. That's one of the research challenges UC scientists are pursuing: the cheaper, more efficient solar cell.

"There are really efficient solar cells out there made with silicon, but they're very expensive," said Adam Moule, a UC Davis assistant professor of engineering and materials science. "Then there are solar cells with polymers, and they're cheap but not that good. My goal is to take these ones that aren't very good but make them better and still cheap."

Moule has a three-year U.S. Department of Energy grant, part of the agency's Solar America Initiative to develop the next generation of solar devices. He is studying ways to assemble thin layers of plastics in a flexible, solar electricity-producing panel. If perfected, he said, his flexible panel technology could have applications such as producing military tents that could generate their own electricity rather than occupants relying on portable generators.

UC chemists and engineers on several campuses have been working on perfecting plastic solar cells. In July 2007, Nobel Laureate Alan Heeger, a professor of physics at UC Santa Barbara, published his discovery of a new organic tandem solar cell with increased efficiency. The tandem cells are developed from two multilayered parts that together can gather a wider range of solar radiation.

"This is the highest level (of efficiency) achieved for solar cells made from organic materials," Heeger said at the time his research was published in Science. "I am confident that we can make additional improvements that will yield efficiencies sufficiently high for commercial products." Heeger, who shared the Nobel Prize in Chemistry in 2000, worked on the tandem cell with Korean scientist Kwanghee Lee.

Recently, UC San Diego engineers have designed an experimental thin-film solar cell with nanowires that enhanced the measure of electric current by six to seven orders of magnitude more than the polymer-only thin-film devices. More technical hurdles must be overcome before the nanowire-enhanced cells could be commercialized.