Here comes the sun

Sticker shock at the gas pumps combined with concerns over the diminishing supply of nonrenewable energy sources like coal, oil and natural gas have added new urgency to the search for alternatives. Riding to the rescue are technology-based resources that will never run out, because they are renewable; are environmentally friendly, and do not add to the greenhouse effect; and are increasingly cheap to harvest, thanks to continuing technical breakthroughs.

Solar is here today, but at about three times the cost of conventionally generated electricity (18 to 22 cents per kilowatt, compared with 5 to 10 cents/ kW for conventional). However, thanks to advances including the use of "plastic" solar cells to replace the more expensive silicon versions, the U.S. Department of Energy believes the cost of solar will be on par with that of conventional electricity within 10 years. By that time, two other contenders--hydrogen fuel cells and nanoscale electric generators--will be at about the same stage of development as solar cells are today.

Photovoltaic solar cells work by absorbing units of light, or photons, in a semiconductor, thereby energizing its electrons enough to drive circuitry. "There is of course a huge interest in solar cells worldwide," said Jim Tully, vice president and chief of semiconductor research at Gartner Dataquest. "The Japanese government, for example, expects that 50 percent of power for homes there will be from solar sources by 2030." Japan holds just 20 percent of the $3 billion to $4 billion world solar cell market today, according to Solarbuzz LLC (San Francisco). Germany has the greatest number of solar installations, accounting for 57 percent of the total in 2005, with the United States at only 7 percent, the rest of Europe at 6 percent and the rest of the world at 10 percent.

Of the various solar cells available, efficiencies range from about 6 percent for the least expensive amorphous-silicon models on glass or plastic substrates to as high as 30 percent for multijunction gallium arsenide cells on monocrystalline wafers, which cost up to 100 times more. Monocrystalline and polycrystalline silicon solar cells, the most popular types, have efficiencies ranging from about 10 to 18 percent. Ready-to-install modules sell for about $4 per watt.

"We believe that monocyrstalline and polycrystalline silicon systems both have pathways open to substantially reduce costs, potentially through techniques such as reducing the thickness of their wafers," said Craig Cornelius, program manager for the Solar America Initiative, a new alternative-energy effort at the Department of Energy (see May 22, page 40). "But we are technology-agnostic, because we recognize that there are many other technologies being developed, such as thin-film device makers who have a number of very promising approaches to achieve very low manufacturing costs." Devices with lower conversion efficiency than mono- or polycrystalline silicon cells include amorphous silicon, cadmium telluride, copper indium diselenide and other, similar alloys.

"Also, the very high efficiencies of concentrating photovoltaic systems and multijunction solar cells make them attractive too--that is, if ways can be found to bring down the cost of their cells," Cornelius said.

Monocrystalline and polycrystalline silicon solar cells hold 93 percent of the worldwide market today, according to Solarbuzz, and offer electricity for about 18 to 22 cents/kW. One sign of their promise: The world's largest vendor of chip-fabrication equipment, Applied Materials Inc., has expanded its product portfolio to include thin-film solar cells with its recent acquisition of Applied Films (Longmont, Colo.) for a record $464 million in cash.