ON THE TOP The 60,000-sq-ft rooftop solar array on Moscone Center is part of San Francisco’s program to use only pollution-free power. (Photo courtesy of Power Light)

Long a small-scale darling of green-energy advocates, solar power recently has been bulking up with announcements of several high-profile, large-scale projects. While solar still appears to be years away from competing with conventional generation, innovation and steadily falling prices are speeding up the timetable.

"The cleanliness of the resource" is one of solar’s appeals, says Peter Johnston, manager of technology development at Arizona Public Service Co., Phoenix, the state’s largest electric utility. APS announced plans last month to build a 1-MW solar trough plant, the first of its kind in the U.S. since 1988. Raleigh, N.C.-based SolarGenics LLC will install the equipment at APS’ Saguaro powerplant in Red Rock, Ariz., about 30 miles north of Tucson. The plant is scheduled to come on line by April 2005 at a cost of $5 million to $6 million.

At the Saguaro plant, parabolic mirrors will concentrate sunlight and heat mineral oil to between 250° and 500°F. Flowing through a heat exchanger, the heated oil will vaporize pentane, a hydrocarbon, which in turn will drive the turbine. The vapor then will condense back into liquid form and the cycle will repeat.

This approach represents a departure from existing solar thermal plants, which turn turbines with superheated steam. "If you’re working with superheated steam, you need a big plant," requiring a $150-million to $200-million investment, says Johnston. Instead, APS will use a 1-MW turbine similar to a geothermal plant’s.

Widespread use is "more a question of investor confidence than further technological development," says Terry Peterson, manager of solar and green power marketing at the Electric Power Research Institute, Palo Alto, Calif. A commercial-size plant needs investors with deep pockets "and pretty cold blood and they haven’t been stepping forward," he says.

As solar thermal awaits further investment, photovoltaic panels are drawing interest. PowerLight Corp., Oakland, Calif., recently completed a 675-kW, $7.4-million installation and energy-efficiency upgrade at San Francisco’s Moscone Center. In March, Alameda County, Calif., supervisors gave PowerLight the go-ahead for a 1.1-MW, $4.8-million program spread among six county buildings. And Sun Power & Geothermal Energy Inc., San Francisco, is installing an $8.4-million, 1-MW solar array for Butte County to power two jails and the county administration building. Interest also continues among smaller commercial and residential installations, industry officials say.

Like other renewables, photovoltaic systems need government subsidies to be attractive to customers. "If the incentives weren’t there, then savings wouldn’t be enough to pay for the project," says Matt Muniz, energy program manager for Alameda County. Yet prices have dropped significantly, from about $4.50 to $3.83 per Watt installed, including incentives, says Muniz.

Costs should fall by another 50% over the next half-dozen years, says Dan Shugar, PowerLight’s president. "This market, by the end of the decade, will need very little by way of incentives to make this technology happen," he predicts. Shugar says initial capital costs for large solar photovoltaic systems run between $6,000 and $8,000 per kW. The Moscone project costs exceed that because an energy efficiency upgrade is included in them.

One potential record-setter faces opposition. Southern California Edison announced plans last December for a 5-MW plant near Barstow, Calif., that it claims would be the world’s largest central solar photovoltaic station. Critics argue that SCE’s procurement, which resulted in the selection of Pasadena-based TrueSolar Solutions LLC, was not competitive and that the project would drain $60 million from the state fund that provides financial incentives for renewable technology, says Matt Freedman, an attorney for The Utility Reform Network, a San Francisco-based consumer group. Freedman believes the central station concept is outmoded and that solar photovoltaics are best suited "for distributed generation, not for covering the desert."