Unlike traditional photovoltaic systems, which directly convert the sun’s energy to electricity by liberating electrons, these new plants use “concentrated solar power,” or CSP.
Every afternoon during the summer, millions of people across the American Southwest come home from work and switch on their air conditioners, straining the power grid in states like Arizona. Traditional solar power—although perfectly suited to the sunny climes of this region—can’t meet this demand since the surge in use peaks just as the day’s sun is disappearing.
That’s why most power suppliers diversify, using electricity from different sources to meet local needs. Solar power is abundant in the middle of sunny, clear days, but energy from other sources—coal, nuclear, or hydroelectric power for example—is necessary at night or when the weather is bad.
But increasingly efficient technology is allowing solar plants to contribute for a longer period of time each day and produce energy even in cloudy conditions. The key is a design that allows them to store the sun’s energy to be used later. And new facilities, such as the Solana power plant that recently came online in Gila Bend, Arizona, are increasing solar energy’s niche by producing electricity several hours after the sun sets.
Decked out in a too-large hard hat, neon yellow vest, and some very trendy safety goggles, Ars recently had the opportunity to visit Solana and find out precisely how these plants power cities after dark.
Unlike traditional photovoltaic systems, which directly convert the sun’s energy to electricity by liberating electrons, these new plants use “concentrated solar power,” or CSP. As the name suggests, CSP relies on either mirrors or lenses to collect and focus the sun’s heat, which is then used to generate power.
CSP isn’t exactly novel. According to legend, Archimedes used giant hexagonal mirrors to create a “death ray” to set fire to Roman ships, saving the city of Syracuse from invasion more than 2,000 years ago. In a more recent era, I remember watching my brother perform small-scale mass murder with a magnifying glass, concentrating the sun’s rays onto the delicate bodies of insects.
But in recent years, CSP technology has blossomed. Its first large-scale test was the Department of Energy-funded Solar One, built in California in 1982. Solar One was a “power tower” plant, in which mirrors are arranged in a circular pattern around a central tower. The mirrors concentrate and direct energy onto a receiver at the top of the tower, which heats synthetic oil inside. The hot fluid is then used to boil water in a traditional steam turbine.
A huge solar facility called Ivanpah recently went online in California’s Mojave Desert. It consists of three separate plants with massive power towers—each 46 stories tall—with a combined capacity of nearly 400 megawatts. It’s currently the largest solar thermal facility in the world.
Today, CSP’s popularity is exploding. Nearly 1.3 gigawatts of CSP power came online in 2012 or will debut this year; that’s enough to power about a million homes. Huge new plants are currently under construction in South Africa, Chile, China, Morocco, Israel, and elsewhere.
The major advantage of CSP is the potential for storage. Since the sun’s energy is already being converted to heat, it’s relatively straightforward to store some of this heat to create electricity later. A variety of storage media are used, including oil and beds of packed rock, but the most common is molten salt. These salts are highly effective at retaining thermal energy and can be heated to very high temperatures (over 1000 degrees Fahrenheit), making storage extremely efficient.
With thermal storage, these plants can either produce at maximum efficiency during the day or store some of the energy as heat to convert later when the sun isn’t shining. This adds not only flexibility to the power system—since the production curve can be tailored to match the demand curve—but also stability in case of cloudy or stormy conditions.
The view from the top of Solana’s cooling towers is impressive: row upon row of shiny mirrors stretch out in all directions, tilted toward the sun like thousands of soldiers standing at attention. It’s clear from this vantage point that Solana is in a league of its own.
When this CSP plant went online in October of 2013, it became the largest working parabolic trough plant in the world (although Spanish parent company Abengoa is currently building an even bigger version in the Mojave Desert). Solana covers more than 1,900 acres in southern Arizona—that’s the equivalent of more than 1,400 football fields packed with mirrors and power equipment.
Abstractly, the concept behind a parabolic trough plant is the same as that in a power tower plant: concentrate the sun’s energy to heat fluid, which then boils water to create steam. However, the details differ pretty significantly.
In a parabolic trough plant like Solana, the mirrors are curved inward, with a glass tube running along the deepest point, or trough, of each mirror. The tube is full of synthetic oil (also known as heat transfer fluid, or HTF). The concave mirrors concentrate light onto this HTF, heating it to 740 degrees Fahrenheit. The system is extremely efficient in collecting heat and concentrating it to a blistering level; when I asked what would happen if I touched the tube, the reply was a curt "Trust me, you definitely don’t want to do that."
Once the oil is up to temperature, about 270 miles of pipe transport it to the power block, where the HTF takes one of two pathways, depending on Solana’s current needs.
In the most direct route, the HTF is simply sent to the steam generator to boil water and create steam to drive two 140-megawatt turbines, just like a traditional power plant. This is the more efficient of the two pathways, since virtually no energy is lost between collection and conversion.
Alternatively, to store this energy for later production, the HTF can instead be sent to one of 12 giant salt tanks at Solana, where hot oil heats molten salt. Each tank can hold 12,000 tons of salt (the salt mixture used at Solana is 40 percent sodium nitrate and 60 percent potassium nitrate). The tanks function just like huge thermoses, holding the heat from the HTF for up to six hours. When electricity is needed, the hot salt is transferred into another holding tank, passing through a heat exchanger on the way. Cold HTF then passes through the heat exchanger in the opposite direction, picks up the heat, and travels to the steam generator in the power block to generate electricity.
Arizona Public Service, the state’s largest electric utility, has agreed to purchase all the power generated by Solana for 30 years. On a daily basis, APS determines how Solana should produce and store its energy in order to best meet local demand. Abengoa estimates that under optimal conditions, Solana can produce enough electricity to power 70,000 homes.
One traditional measure of a power plant’s utility is its capacity factor. A facility’s capacity factor is the ratio of the energy it produces over a certain amount of time compared to the potential energy the plant could produce if it could operate at full capacity the entire time. Plants converting renewable energy generally have low capacity factors because resources like wind and sunlight aren’t always available. But thanks to thermal storage, facilities like Solana have capacity factors of more than forty percent—that’s twice as good as plants using photovoltaic technology.
In many ways, Solana’s system is highly automated. Each mirror assembly is outfitted with temperature and pressure sensors, as well as a hydraulic sun-tracking system to maximize the heat captured.
Maintaining this giant solar field, however, is complicated. The mirrors need to be kept virtually spotless, because the cleaner they are, the more quickly the HTF is heated to the optimal temperature. To keep all 3,200 mirrored collectors bright and shiny, the company has a fleet of trucks that spray and scrub each mirror either on a bi-weekly basis or as needed. Breakage is minimal, since the mirrors are tempered glass—just like car windshields—but Solana officials estimate that about one percent of mirrors require replacement each year.
Despite the good press and promising numbers, CSP and solar thermal storage facilities have some significant limitations.
One major drawback of plants like Solana is their water consumption. Because a significant amount of water is needed for cooling these systems, water use in these facilities is much higher than that in natural gas or coal-fired power plants. In fact, “wet-cooled” parabolic trough plants consume more water than any other type of power plant. Compounding this problem, solar plants are usually located in hot, dry areas where water is scarce.
However, Solana officials are careful to point out that the land the plant is located on was previously used for alfalfa farming, an agricultural practice known for its high water consumption. Solana uses about 3,000 acre-feet of water from underground aquifers each year, or about 10 percent of the water previously used to grow alfalfa on the same land.
Beyond water, the environmental impact of thermal storage plants is generally considered to be relatively low. The salts used for storage are the same ones used in common fertilizers, and they pose no known environmental danger (but just like fertilizer, they do set off explosive detectors at the airport; we were given the opportunity to touch the salt mixture during the tour, but strongly advised not to if we were boarding a flight home later that day).
But there is one ecological caveat: before construction on Solana started, Abengoa had to relocate an undisclosed number of western burrowing owls, a species protected under the Migratory Bird Treaty Act and listed as a National Bird of Conservation Concern by the US Fish and Wildlife Service. The huge new Ivanpah facility in California faces similar criticism, as its construction displaced or killed more than 100 desert tortoises, a threatened species.
On a technological level, some experts believe that thermal storage plants that use molten salt are at a major risk for corrosion. In general, salts are extremely corrosive, and the metal pipes and storage tanks used in facilities like Solana could be prone to problems. Although Abengoa maintains that corrosion is not a concern, it is possible that molten salt storage facilities may face problems over the long term.
Because cost is one of the largest drawbacks of solar thermal systems, the Department of Energy is investing heavily in research that will reduce the construction and operating expense of these facilities. The program, called the Sunshot Initiative, aims to reduce the overall price tag of solar thermal systems to less than $15 per kilowatt-hour; the current cost can exceed $30 per kilowatt-hour.
Dr. Yogi Goswami, a mechanical engineer at the University of Florida, is working on new technology to meet the Department of Energy’s goals. His idea is to harness and store energy via the phase change of salt. Goswami’s concept starts out in a familiar way: traditional solar technology concentrates energy and heats salt. But here, the salt is packed into small balls encapsulated in a ceramic and metal coating. As the salt is heated, it changes from solid to liquid—storing thermal energy—then releases the energy later when it reverts back to a solid state.
By taking advantage of phase change, Goswami can get 12 hours of thermal storage while also reducing costs, since the highly efficient system requires less salt and smaller storage tanks. An added benefit of Goswami’s concept is that the coating on the balls is corrosion-resistant, eliminating concerns about salt-related corrosion.
Abengoa is going in a different direction: improving storage time by simply increasing storage volume. The company is currently planning a solar plant in Chile that can store energy for more than 17 hours thanks to storage towers with a capacity of about 50,000 tons of molten salt.
But companies like Abengoa are also investigating alternatives to current technology to improve efficiency and reduce cost. One way to improve upon Solana’s system would be to eliminate the need for synthetic oil altogether, instead using water as the heat transfer fluid. Water can tolerate higher temperatures than oil, and this would eliminate the need for components such as heat exchangers. That switch alone could reduce overall costs by as much as 10 percent.
So while current solar thermal storage technology is far from the perfect solution to global energy issues, the concept is certainly one that can transform our power system. And once costs come down, it’s likely that more of our power will come from the sun—no matter if it’s above or below the horizon.