Southern California Public Power Authority (SCPPA) recently took an option to purchase a 200 megawatt EnviroMission solar updraft tower being built in La Paz, Ariz. The utility will buy the power produced by the tower and could buy the tower in the 10th year of its power-purchase agreement. It also has the option to buy the power produced by a second 200 megawatt tower that will be built at the site.

SCPPA singed a 30-year power purchase agreement for the first tower system, said Chris Davey, EnviroMission’s business development manager. SCPPA will have the right to purchase the first tower facility at La Paz in either the tenth or thirtieth year of its operation, he said. The utility also took an option on a second power-purchase agreement to buy the power generated from a second 200 megawatt La Paz Solar Tower development. “It’s a game-changer for EnvironMission. We’re moving from being a developer of an idea to a deliverer of a project,” he said.

Normally, when people talk about solar concentrating systems with solar towers, they’re talking about a central tower surrounded by rings of heliotropes that focus sunlight on a collector that absorbs the sunlight as heat, but this type of solar tower is more like a chimney. It uses the sun to heat air and convects that heated air to a chimney filled with turbines. As the heated air moves through the chimney as wind, it pushes the turbines, generating electricity. Construction on the project, which is being designed by Arup, is slated to begin in 2012.

At 200 megawatts each, the towers aren’t small. In fact, at roughly 2,500 feet high, the only structures larger are the world’s tallest skyscrapers. The chimneys will be surrounded by a greenhouse of sorts that’s a mile in diameter. The greenhouse will heat the air and move it toward the chimney. The chimney will house 32, 6.25 megawatt, turbines to produce electricity.

The air will move at a rate of about 35 miles an hour and be heated to roughly 160 degrees Fahrenheit before it reaches the chimney, according to Davey. “It will be very hot, but in terms of power production we’re a very, very low temperture,” he said. In the middle of the structure it will be too hot, but in the outer parts of the greenhouse there will be an opportunity to grow some low-lying agriculture.

While still new, the first solar updraft tower was built in the 1980’s to demonstrate the technology. This will be the first utility-scale project using the towers. “The technology had been demonstrated in two locations. This will be a larger deployment of that same design,” said Dave Walden, energy systems manager at SCPPA.

“It’s part of an ongoing acquisition of renewable energy from many projects,” Walden said. The utility is purchasing the power as it works to meet California’s renewable portfolio law, which requires all utilities in the state to source 33 percent of their power from renewable sources such as solar or wind.

Image courtesy of SCPPA