In the high desert of southern Spain, not far from Granada, the Mediterranean sun bounces off large arrays of precisely curved mirrors that cover an area as large as 70 soccer fields. These parabolic troughs follow the arc of the sun as it moves across the sky, concentrating the sun’s rays onto pipes filled with a synthetic oil that can be heated to 400C. That super-heated oil is used to boil water to power steam turbines, or to pump excess heat into vats of salts, turning them a molten, lava-like consistency.
The salts are just fertilisers - a mix of sodium and potassium nitrate - but they represent a significant advance in the decades-old technology of solar thermal power production, which has traditionally used mirrors to heat water or oil to generate electricity-producing steam. Now, engineers can use the molten salts to store the heat from solar radiation many hours after the sun goes down and then release it at will to drive turbines. That means solar thermal power can be used to generate electricity nearly round-the-clock.
The plant in southern Spain, known as Andasol 1, began operating last November and now provides 50 megawatts of power, enough electricity to supply 50,000 to 60,000 homes year-round. Andasol 2 will come online later this summer, with Andasol 3 already under construction. When the entire Andasol complex is completed in 2011, it is expected to generate enough electricity to power 150,000 households - about 600,000 people.
In the face of mounting concern about climate change, developing alternatives to coal and natural gas combustion has taken on a new urgency, and the construction of utility-scale solar thermal power plants in deserts and arid areas is looking like an increasingly promising option. In the United States alone, solar thermal power projects are now being built near fast-growing centres of electricity consumption, such as Las Vegas, Los Angeles, and Phoenix. The first major solar thermal plant to be completed in decades, dubbed Nevada Solar One, started providing 64 megawatts of power to the neon lights of Las Vegas in 2007, although it lacks the latest molten-salt technology. Across the globe, utilities are currently building or planning solar thermal projects in North Africa, Spain, and Australia, among other regions.
Some of the recent claims for solar thermal power have been stunning. Researchers at the German Aerospace Center have estimated that 16,000 square kilometres of solar thermal power plants in North Africa - paired with a new infrastructure of high-voltage, direct-current transmission lines - could provide enough electricity for all of Europe. And scientists have estimated that constructing solar thermal power plants on less than 1 per cent of the world’s deserts - an area roughly the size of Austria - could meet the entire world’s energy needs.
Of course, solar thermal has been here before, experiencing a boom in the late 1970s and early 1980s. Its progress then was stalled by collapsing fossil fuel prices, as well as a lack of government support. Today, some critics of the technology fault it for taking up acreage in fragile deserts.
The case for solar thermal power hinges on economics. The sun bathes the Earth with an average of 6 kilowatt-hours of power per square metre over the course of a day, and a concentrated solar power plant like Andasol is the cheapest way to harvest a portion of that. Photovoltaics - semiconductor panels that convert sunlight to electricity - deliver power at roughly 40 cents per kilowatt-hour, while conventional solar thermal power plants can do so for around 13 cents per kilowatt hour, according to the US National Renewable Energy Laboratory. This is only marginally more expensive than the average US price for coal-generated electricity in 2008 of 11 cents per kilowatt hour. The cutting-edge technology of using molten salts to store solar-generated heat is considerably more expensive, but experts expect that price to fall steadily as the technology improves and is mass-produced.
Roughly 612,000 megawatt-hours of electricity from the sun were produced in 2007, according to the Energy Information Administration (EIA), and solar thermal collectors sufficient to cover more than 15 million square feet were shipped and ready for installation that year - more than double the amount in 1998.
In the United States, some 3,100 megawatts of solar thermal power are planned by 2012, and capacity worldwide is expected to reach 6,400 megawatts within three years - roughly 14 times the current amount. Still, electricity from the sun contributes just 1 per cent of the renewable energy generated in the US, and all renewables taken together only provide 7 per cent of US energy needs.
Traditionally, solar thermal power plants have been built two ways - using trough-like mirrors to focus the sun’s heat on water or oil in nearby pipes, or using mirrors to focus solar radiation on a central spot, such as a liquid-filled “power tower”.
Since 1984, vast arrays of curved mirrors have concentrated the sun’s rays on regimented lines of pipe filled with a synthetic oil at the Solar Energy Generating Systems (SEGS) power plant in the Mojave Desert of California. The SEGS plant was part of a brief boom in alternative energy projects in the late 1970s and early 1980s, when enthusiasm for the technology ran high in the wake of the first energy crisis. But the Reagan administration phased out research and development funding for solar thermal technology, as well as delaying tax credits that had driven the creation of projects like SEGS. Coupled with cheap fossil fuel prices in the late 1980s and 1990s, solar thermal power could not compete - although it still grew by roughly 4 per cent a year from 1986 to 2000, according to the EIA.
Given its long operation record, developers are now copying SEGS and its parabolic trough collectors. In the US alone, nearly 1,800 megawatts of such power plants are scheduled to be completed - mainly in the desert Southwest - by 2011, according to investment research from Friedman, Billings, Ramsey & Co.