The massive solar plant nearing completion in the California’s Mojave desert doesn’t look like the solar plants you might be used to seeing. It has no solar panels, for one thing. Instead, it has mirrors—300,000 of them—all arrayed in rings around three giant towers. The mirrors reflect sunlight onto vats of water sitting on top of the towers, heating them to 500 degrees and powering a steam turbine, providing enough energy for 140,000 homes. When it goes online at the end of the year, it will be one of the biggest solar plants in the world. But the technology at its heart is relatively simple: mirrors, water boilers, and steam turbines.
The plant, called Ivanpah, is funded by Google, NRG, and BrightSource, a company that specializes in what’s called concentrated solar power, or CSP, a method of using focused sunlight to turn a steam generator. The technology isn’t new: a small test plant that uses mirrored troughs to heat oil-filled tubes has been running in California for 20 years. Going back further, you could point to the French inventor Agustin Mouchot, who experimented with solar powered steam engines in the 19th century, thinking we were about to run out of coal. The current batch of plants are huge—thousands of acres—and use computer-controlled mirrors to heat boilers that sit on top of towers. (You can take a virtual tour of Ivanpah's triple-tower array here.) Three giant CSP plants are scheduled to go online in the next few months, and the companies that have spent years and billions of dollars building them hope they’ll provide a valuable new source of renewable energy.
The main advantage of this type of solar plant is that it’s easy to store energy from it and save it for later use, something that will become more of an issue as we get more energy from windmills and solar panels. Energy from windmills and solar panels is cheaper, but it’s not always there when it’s needed: demand might be high on a windless or cloudy day, or low on a windy sunny one. A smarter power grid that can redirect power from renewable sources to where it’s needed can help cushion some of this streakiness, but we’re always going to need some reliable, always-available—the industry term is “dispatchable”—source of power. Right now that’s coal, gas, nuclear, and hydroelectric. The CSP industry is hoping solar can join that list, even providing power at night.
A National Renewable Energy Laboratory report from earlier this year found that CSP with energy storage could help California meet its goal of getting a third of its energy from renewable sources by 2020. Mark Mehos, an author of the study, said that CSP’s value will go up as California gets more of its energy from photovoltaic and wind, the ability to provide storage will become more important.
Ivanpah doesn’t have storage, but Brightsource says it’s looking into storage for future plants. Two other large plants currently under construction are being built with energy storage. A 280 megawatt plant under construction in Arizona stores excess heat in vats of molten salt, allowing it to provide energy six hours into the evening, when air conditioning use is highest. A 110 megawatt plant in Nevada promises 10 hours of storage.
It’s far cheaper to store energy in the form of heat than in a battery. A frequently cited comparison in the industry is that a thermos of coffee stores about the same amount of energy as a laptop battery, but one is $5 and the other is $150. With a CSP plant, when demand for energy is low, excess heat can be siphoned off and stored, then used to power steam generators when demand is high.
The fate of this kind of solar plant is far from guaranteed. The reason all these plants are coming online at about the same time is that they were conceived at around the same time: 2009, when stimulus-package loan guarantees for clean energy were plentiful, before photovoltaic panel prices dropped, and before the price of natural gas plummeted. In fact, most of the newer CSP projects are happening abroad, in places such as Saudi Arabia, South Africa, and China, where natural gas is still expensive. Ironically, if the technology doesn’t catch on as a power source, it might have a future in oil extraction: Chevron is currently using a Brightsource tower to create steam to inject into oil wells.
Another drawback: These plants are huge, which can cause all sorts of problems. They require acres and acres of sun drenched land. Ivanpah, which is 3,500 acres, ran into trouble a while ago when endangered desert gopher tortoises were found on the site. Brightsource ended up spending 22 million to find 150 tortoises and move them to pens elsewhere. More recently, Brightsource and Abengoa’s 500 megawatt plant in Riverside County, California, ran into trouble when the California Energy Commission said it would have “significant” environmental and visual impacts.
Whether or not CSP is economically viable in the near future, it’s an appealingly simple principle. As that guy whose Jaguar got melted by the curved glass of the Walkie Talkie building in London can attest, focused solar rays have a lot of power. Hot liquids stay hot for a long time. It makes sense that we could put those two facts to use.