Sep. 6--The three power plants proposed for California Valley represent a range of solar power technologies.

Two will use photovoltaic panels. The other will generate electricity in a tried-and-true fashion by boiling water and passing the steam through turbine generators. It will use the sun's heat to boil the water, called solar thermal -- a technology used only at a few other locations nationwide.

Each of these technologies will bring its own efficiencies and benefits. Each will also carry its own costs and impact on the environment.

Ausra Inc. of Mountain View plans to build a 177-megawatt plant adjacent to the site of a defunct Arco solar plant of the 1980s and '90s and less than a mile north of the Carrisa Plains School.

The facility would cover 640 acres -- one square mile -- and contain 195 solar steam generators.

Each generator would consist of 10 1,200-foot-long mirror assemblies that concentrate sunlight on a water-filled pipe 50 feet above. This arrangement magnifies the sun by a factor of 50 and heats the water beyond the boiling point.

This superheated steam will pass through two turbine generators and go onto a series of fan-cooled condensers.

The steam generators will stand 56 feet tall and will cover almost the entire square-mile facility. The condensers will be 115 feet tall, roughly the size of a 10-story building, making them some of the tallest buildings in the county.

Because the plant is solar thermal and produces 177 megawatts, which exceeds the state's permitting threshold, the California Energy Commission is the permitting authority. Ausra applied for its permit in October 2007. The company expects to get its permit early next year.

Construction is expected to take three years, and Ausra will employ from 300 to 350 workers. About 75 long-term employees will be needed to run the plant.

Advantages: It's a compact facility using proven and efficient steam generator technology. It can continue to generate power during 15 to 20 minutes of cloud cover.

Disadvantages: It's an industrialized facility with tall buildings. Steam condensers and other equipment are so noisy employees working in the plant may have to wear hearing protection. The facility will use 21 acre-feet of water a year, taken from underground aquifers, in a region that receives scant rainfall.

Covering 4,200 acres or more than 6.5 square miles and producing 550 megawatts of power, this project is the largest. The project was first proposed in July 2008 by OptiSolar but was taken over by First Solar of Tempe, Ariz., in April.