The Ivanpah Valley of the Mojave Desert in California is home to spiky yucca trees, long-nosed leopard lizards, loggerhead shrikes, and a rare species of tortoise—and soon, the largest solar thermal energy plant in the world.

More than six years in the making, the Ivanpah plant is now slated to begin generating power before summer's end. It was designed by BrightSource Energy to use more than 170,000 mirrors to focus sunlight onto boilers positioned atop three towers, which reach nearly 500 feet (150 meters) into the dry desert air. The reflected sunlight heats water in the boilers to make steam, which turns turbines to generate electricity—enough to power more than 140,000 homes. (See related quiz, "What You Don't Know About Solar Power.")

Scaling Up Solar

At 377 megawatts (MW), Ivanpah's capacity is more than double that of the Andusol, Solnava, or Extresol power stations in southern Spain, which previously were the largest in the world (150 MW each). (See related: "Pictures: Spanish Solar Energy.") The 1980s-era SEGS, or Solar Energy Generating System, also in the Mojave, about 100 miles southwest of Ivanpah, has a 354-MW capacity, but it is a collection of nine plants.

Viewed from above, the mirrors seem to angle their faces like enormous silvery blooms craning to the sun. At ground level, the facility stands on a 3,500-acre swath of federal land inhabited by the threatened desert tortoise (Gopherus agassizii). Once found across deserts of the American West, the species now inhabits parts of California, Utah, Arizona, and Nevada. But its numbers have dwindled: Scientists estimate some populations have declined by as much as 90 percent.

Although initial surveys indicated fewer than 20 desert tortoises occupied the Ivanpah project area, more than 150 individuals ended up being found. Biologists working for BrightSource cleared the area, carefully moving tortoises to "nursery pens" adjacent to the site before releasing them to nearby habitat. [See the video below for the story of the process and how they've fared.]

These slow-moving desert reptiles are able to survive a year or more without water and live for as long as 80 years, burrowing underground to keep cool and feasting on wildflowers in the spring. Yet they have proven vulnerable to encroaching human development. "Here's an animal that's been around 200 million years that may be disappearing," said Ed LaRue, a biologist with the Desert Tortoise Council, a nonprofit dedicated to conservation of the desert tortoise in the Southwestern United States and Mexico. "Solar, especially at the level that it's being proposed in the Mojave Desert, is a new threat."

BrightSource and its partners, NRG and Google, received a $1.6 billion federal loan guarantee in April 2011 in support of the project. And it may be only the first of many such developments on public lands in the Golden State. Although several large-scale solar projects in California have sputtered (including two from BrightSource) and technical challenges are considerable, the U.S. Department of the Interior's Bureau of Land Management (BLM) announced a move in early July to prioritize more than 300,000 acres of public lands in six Western states for use by utility-scale solar plants, with nearly half of that acreage in California. (See related: "Desert Storm: Battle Brews Over Obama Renewable Energy Plan.")

Acting on the heels of President Obama's recent call for federal approval of 10,000 megawatts of renewable energy projects on public lands by 2020, the BLM prohibited mining claims on the 300,000 acres for the next 20 years. (See related story: "Obama Pledges U.S. Action on Climate Change, With or Without Congress.") Some environmentalists have raised concerns about how fragile desert ecosystems will be altered in the renewable energy drive, but developers and advocates of large-scale clean power say the ultimate goal is to reduce dependence on fossil fuel that is far more harmful to both land and atmosphere. (See related story: "Monterey Shale Shakes Up California's Energy Future.")

"We're combining innovative technology with traditional power-block technology to produce carbon-free, reliable renewable power," said Joseph Desmond, senior vice president of marketing for BrightSource. (A power-block facility includes a steam heat exchanger, steam-turbine generator, and the electrical equipment in a substation.) "When you're talking about fossil fuels, you have to factor in the land used for exploration, extraction, processing, and then transportation," he said. "People sometimes forget this is actually a very efficient utilization of a sustainable energy resource." (See related pictures: "Oil Potential and Animal Habitat in the Monterey Shale.")

Ivanpah's developers also addressed concerns about the typically high consumption by solar thermal plants by deploying an air-cooling system that they say reduces water use 90 percent compared to conventional technology. (See related story: "Water Demand for Energy to Double by 2035.")

First of Many?

BrightSource's desert plant is one of the largest projects in California's ambitious push for renewable energy. (See related story: "California Tackles Climate Change, But Will Others Follow?") The state aims to generate 33 percent of its electricity from renewable sources like wind, solar, and geothermal by 2020. Two other solar projects designed for the Ivanpah Valley are now working their way through the approval process: The proposed Stateline and Silver State South projects, both from the company First Solar, would generate 300 megawatts and 350 megawatts, respectively. Further north, a solar farm proposed for construction across 3,000 acres (1,214 hectares) of the Panoche Valley, would generate nearly 400 megawatts—if it can survive legal challenges from environmental groups and clear other hurdles, like signing on a utility to buy the electricity and obtaining federal permits.

"There's a trade-off," said Larry LaPre, a biologist with BLM. "If there were no push toward renewable energy, animals like the desert tortoise and plants like the Joshua tree could be impacted quite a bit [by climate change]." Indeed, the U.S. Fish and Wildlife Service (FWS) cites global climate change and drought as "potentially important long-term considerations" for the desert tortoise's recovery. (See related story: "IEA Outlook: Time Running Out on Climate Change.") Rising temperatures and reduced rainfall expected to result from climate change could ravage the species' food supply. At the same time, FWS recognizes that it has not evaluated the potential long-term effects of big renewable energy projects that fragment or isolate desert tortoise conservation areas, possibly "cutting off gene flow between these areas."

And so in the Mojave, the question that divides renewable energy supporters and wildlife advocates is this: Is it a good trade? "There's so much land out here where the biological resources have been compromised," such as old agricultural land and areas on the urban fringe, said LaRue. "I think it would be a great resource if it was just put in the right place."

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