Mirrors and lenses properly shaped can concentrate the benign rays of the sun into a powerful flame, something people have known for at least 2,500 years. In the 1500s, for example, in one of his notebooks Leonardo da Vinci suggested boiling water for a dye factory with the heat generated by a curved mirror four miles in diameter! Though the mirror was never built, Leonardo’s mentor, Andrea del Verrocchio, used a much smaller one for soldering.
In the 1800s, concern grew over steam engines’ prodigious appetite for fuel. Once they consumed all the coal in Europe, as some people expected, what would industry do? “Reap the rays of the sun,” replied one man of science, who went out to build a dish-shaped mirror that focused sunlight onto a boiler to help resolve what he believed was the impending demise of fossil fuels.
By 1914, Scientific American reported the first economical solar-powered engine operating in Egypt, which had at the time little in the way of accessible fuel but plenty of sun. A few years later, this same region was tapping its plentiful oil, as was North America. No one worried too much about energy from then on, and interest in solar power faded.
The Need Returns
With oil now at record prices and the knowledge of the harm burning fossil fuels can cause, solar-powered engines once again generate a lot of interest.
Advocates for the technology — “solar thermal energy” — have calculated that they would need less than 1 percent of all the world’s deserts to power the entire globe. They have quite a task ahead of them to achieve this goal, although the world’s only long-term operating solar thermal plant, built 22 years ago in California’s Mojave Desert, points the way.
The plant consists of long rows of curved trough-shaped reflectors that follow the sun throughout the day. They focus sunlight onto an absorber — pipe surrounded by glass — located above each trough. As a fluid, usually oil, passes through, it heats to around 300 degrees Celsius. The extremely hot oil collects at the power plant, where it heats liquid water into steam that then drives a turbine. The plant so far has produced 11,000 gigawatt-hours of electricity, worth almost $2 billion, and is expected to continue producing electricity until 2035.
It’s eerily similar to an Egyptian solar plant built 94 years ago. Trough reflectors’ long-standing record of reliable performance has allowed confidence in the technology for new entrants, one of which, Acciona’s Nevada Solar One, has just come on line south of Las Vegas. The plant, the largest built in the world in the last 17 years, is rated at more than 60 megawatts and is now the third largest in the world.
Ironically, the founders of the plant — actually a series of plants — in the Mojave Desert say experience has taught them trough technology’s limitations. They now embrace a different approach — “power towers” — to produce electricity from the concentrated rays of the sun.
More than 250 years ago, Comte de Buffon, an 18th-century French experimentalist, focused 124 flat mirrors on a model ship in Paris and burned it, anticipating power tower technology while demonstrating that Archimedes indeed could have burnt the Roman fleet, as some ancient writers claimed (but modern scholars disproved).
A modern power tower consists of a tall tower surrounded on all sides by rows of flat mirrors. On top of the tower sits a central receiver, through which molten salt moves. The flat mirrors follow the sun’s path to always focus sunlight onto the receiver. In this way, the molten salt gets much hotter than the oil heated by trough concentrators. The super-heated fluid creates steam, again to run a turbine. Excess molten salt goes to a storage tank which, according to plans, would allow the plant to operate continuously, something no other solar-electric plant can do.
Advocates for power towers argue that they can produce electricity cheaper than the trough technology for five reasons, some based on costs (flat glass used for the mirrors is simpler to build than curved mirrors required by troughs, and less concrete and steel is used) and some on mechanics (less heat is lost during operation as there is less piping, higher temperatures allow for the use of more efficient steam turbines, and the molten salt storage allows for 24/7 operations).
Trough advocates retort that no commercial power towers of any size — a power tower built by Spain’s Solucar outside Seville produces about 11 megawatts — have yet been built. A joint Spain-Dubai venture plans to spend more than $1 billion to build three commercial power towers expected to go on line by 2012.
The least developed of solar thermal power devices, the solar-fired Stirling engine, also has its roots in the past. In 1872, the Swedish-American technologist John Ericsson built a prototype: A curved mirror focused the heat of the sun onto an exposed cylinder, causing the air inside to expand to push down a piston; inrushing cold air pushed the piston up. The piston continued to move as long as the heat of the sun continued to bear down on the cylinder.
Today’s proposed device relies on hydrogen as the medium whose change in pressure due to heating and cooling cycles drives the pistons inside the engine placed at the focus of a dishlike reflector. Its modularity and higher efficiency give it an advantage over competing technologies, but unlike troughs and power towers, the Stirling cannot store solar energy and requires specially built engines.
Lots of Juice, Couple of Issues
Solar thermal power plants, unlike solar water heaters and photovoltaics, require direct sunlight to operate. Such conditions consign them to arid regions usually distant from population centers. Transmission lines specifically dedicated to such plants must be built, at a current price of $1,000 per megawatt per mile. Required cooling towers call for large amounts of water at times scarce in environments otherwise ideal for the technology. (Air can be used but is more costly.)
The key advantage to solar thermal plants, Stirling engines excepted, is that they use the same electrical-generation equipment as do most other power plants. The only difference is the fuel. Instead of fossil fuels or nuclear, solar thermal plants’ heat source is the sun — free, readily available and clean burning. Compatibility with other thermal generators allows for hybridization with other fuels so the plants can run continuously, eliminating downtime when the sun is not shining.
Plans exist for the deployment of at least five gigiwatts of solar thermal plants worldwide by 2010. Under the right economic conditions, experts estimate that, in the American Southwest alone by 2030, enough solar thermal plants could be built with the capacity of 80 gigawatts, greater than all the current conventional electrical generation plants that serve California.
