BrightSource Energy's Ivanpah solar thermal plant — soon to be the largest concentrating solar power plant in the world — is expected to produce enough power for 140,000 homes. (Isaac Brekken/For The Washington Post)

Tower One glows white, so bright against the pale blue sky that even at mid-afternoon in the Mojave Desert it would be easy to conclude that it is designed to illuminate the valley floor below.

In fact, hundreds of thousands of glittering mirrors, carefully arranged across a broad swath of desert, reflect sunlight upward onto the tower and two others like it, heating them to 1,000 degrees Fahrenheit and causing the glow. Water in big pipes atop the towers turns to steam. The steam spins turbines, which generate electricity.

If all goes well, the Ivanpah Solar Electric Generating System will send that power across the Golden State early this year, becoming the largest solar plant in the world to concentrate the sun’s rays to produce electricity.

Such utility-size solar plants are beginning to appear across the United States, with 232 under construction, in testing or granted permits, many in the Southwest and California, according to the Edison Electric Institute, which represents utilities.

The scale of the largest plants is difficult to imagine in the eastern part of the country, where a relative lack of available open land and unobstructed sunlight have limited solar facilities to perhaps a tenth the size of the West’s plants. But in the West, ample sun, wide-open spaces, financial incentives, falling costs and state mandates have made big solar plants possible.

View Graphic Tower of solar power

“Right now you’re seeing the gold rush of renewable [energy] projects coming on line,” said Fong Wan, senior vice president for energy procurement at Pacific Gas and Electric, the big Northern California utility that has purchased about two-thirds of the electricity the Ivanpah plant will produce.

Smaller-scale future?

But even as the largest plants are helping utilities meet state requirements for renewable energy, the appetite for them may be waning, according to experts. The next phase of solar development — especially in the East — may feature smaller projects located closer to cities. Environmental groups want regulators to look at sites such as landfills and industrial zones before allowing construction in largely undisturbed environments such as deserts.

“Part of the beauty is that solar is scalable, literally from the back of a cellphone all the way to a million panels in the desert,” said Rhone Resch, president and chief executive of the Solar Energy Industries Association. “The market is still trying to determine what is the optimal size.”

The very largest plants, like BrightSource Energy’s $2.5 billion Ivanpah system and the Topaz Solar Farm, which will produce current with 9 million photo­voltaic panels, can generate as much electricity as a coal- or natural-gas-fired power plant.

But there is still a long way to go. In 2012, coal and natural gas plants produced 37 percent and 30 percent of U.S. electricity, respectively, according to the U.S. Energy Information Administration, while wind generated 3.5 percent and solar just 0.1 percent.

Environmental challenges

And the road to big solar energy’s development has been difficult. Lawsuits against the large plants accuse developers and the federal government of spoiling the fragile desert environment and the habitats of wildlife there. On Dec. 13, the California Energy Commission tentatively refused to permit another BrightSource project because of its concerns that super-heated plumes of air from the towers and mirrors might harm birds. A small number of singed dead birds have turned up at Ivanpah, according to media reports.

Ivanpah is a “concentrating solar” thermal plant. The better-known variety — like the flat solar panels on homes — convert sunlight directly into electricity via photovoltaic cells. The price of those panels has dropped so low that those plants are much cheaper to build than facilities that use the sun’s heat to turn water to steam.

Thermal plants such as Ivan­pah have advantages — they are more reliable — but their future may depend on finding some way to store heat so power is available whenever needed.

“The benefit of a thermal solar plant like Ivanpah is it’s not subject to the wild swings in production that a [photovoltaic] plant is,” said Randy Hickok, senior vice president of NRG Solar, which holds a majority stake in the project. Another major investor is Google.

Environmental groups, for their part, have sometimes found themselves in the awkward position of choosing between their dual goals of protecting desert species and promoting clean, renewable energy.

The powerful Sierra Club, for example, chose not to side with other, smaller groups that sued the Interior Department and its Bureau of Land Management to block Ivanpah over the damage they said it would do to the threatened desert tortoise’s habitat on federal land. The Sierra Club was not happy about Ivanpah’s impact, but it took no position, said Bruce Nilles, director of its Beyond Coal campaign.

“I think they were very misguided,” said Michael Connor, California director of the Western Watersheds Project, which lost a bid to halt Ivanpah in federal court but has appealed the decision. “It’s all about ‘we’ve got to do something, we’ve got to get something going here’ . . . instead of working out strategies [and] alternatives.”

Ivanpah is undergoing testing, its three 46-story towers rising out of the vast desert like Bugsy Siegel’s Flamingo Hotel in nearby Las Vegas did almost 70 years ago. Motorists regularly pull off Interstate 15 at the California-

Nevada border to get a better look at the arrays of mirrors on 3,500 acres around the three towers, and to ask: What exactly is going on here?

Driving the boom

The boom was set in motion in 2002, when California told its big electric utilities they would have to generate 20 percent of the state’s electricity from renewable sources such as sun and wind by 2010. In 2011, the state toughened its “renewable portfolio standard” to 33 percent by 2020. (Thirty states, including Maryland, and the District of Columbia have adopted such requirements. Virginia is among a handful of states that have set “goals” for the use of renewable energy.)

Companies began proposing to build large plants, many of them on federal land in California’s huge, sparsely populated deserts. In 2008, they were aided by the eight-year extension of a federal investment tax credit available for renewable energy projects and later by Energy Department loan guarantees and incentives in the federal economic stimulus package. BrightSource received a $1.6 billion loan guarantee that was critical to the project, according to Joseph Desmond, senior vice president for marketing.

The result is the growth in solar power that is plainly visible in parts of the state as well as in Arizona, Nevada and elsewhere. Pacific Gas and Electric, for example, will provide about 11 percent of its power from solar by 2020, up from zero a decade earlier, Wan said.

In the early days of the rush, the Bureau of Land Management reviewed plant proposals on an ad hoc basis as developers brought them forward. That resulted in some siting decisions, including Ivanpah’s, that environmentalists and conservationists have criticized.

In 2012, the agency created 17 solar zones covering 285,000 acres of federal land in six Southwestern states, an attempt to steer projects toward areas where environmental review showed that the least amount of damage would be done. There are now 19 zones and more than 300,000 acres of federal land in the program, according to BLM officials.

“We are in a lot better place,” said the Sierra Club’s Nilles. “There’s a more orderly process in place.”

Easing the impact

At the Ivanpah plant, an initial survey showed that construction would displace only a small number of desert tortoises, but as work began, it became clear that many more were living there. The company has spent $56 million to build fences and raise tortoises in its “Head Start” pen, where 55 have been born in captivity and will be fitted with devices that allow biologists to follow them when they are returned to the desert. Though two hatchlings were lost to fire ants, Desmond said that the ancient species’ survival rate is much higher under BrightSource’s care than it is in the wild.

Responded Connor: “That’s like arguing it’s okay to pave the desert over because we can move all the animals to a zoo.”

To mitigate its impact, Ivan­pah’s owners spent $11.4 million to purchase and manage 7,000 acres of habitat for tortoises and other wildlife in other parts of the state.

There is little argument that the project has brought advanced technology to an area of rock and scrub that is home to a golf course, three casinos, some fast-food restaurants and a few stores. Computers guide 173,500 sets of paired mirrors, or “heliostats,” so they can follow the sun for as long as possible each day and generate the maximum amount of heat on the boiler tubes. Eventually, Ivanpah will supply electricity to 140,000 homes.

Robotic devices, controlled by a single person, traverse the rows of heliostats, cleaning the mirrors every couple of months, usually at night, Desmond said. BrightSource, which created the machines, won’t show them publicly.

The plant uses air to cool the water that flows through the boiler tubes. As a result, Desmond said, Ivanpah’s annual water use is the same as just two holes of the nearby golf course.