Solar Energy in Spain

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Technologies
The most common technology so far, and the one in use at Andasol 1, is based on a series of parabolic troughs, huge curved mirrors about 18 feet wide that collect the sun’s energy and focus it at a point in the middle of the trough. Glass tubes filled with oil stream through that focus point along a long loop of troughs. The mirrors slowly track the sun from east to west during daytime hours, and the oil reaches about 400 degrees Celsius (about 750 degrees F).

The heat transfer fluid then travels to the steam generator, where the heat from the oil is transferred to water, immediately turning the water into steam. That steam powers a turbine, the same technology used in conventional power plants.

The tower technology works on the same principle as the troughs—the sun’s heat—but uses curved mirrors called heliostats, mounted on trackers that shift position with a slight mechanical groan every few seconds, that direct the sun’s light to a central receiver at the top of the tower. Testing towers exist in Spain, the US, and Israel, but the Solúcar PS10 site is the first commercial application of the technology.

At PS10, 624 heliostats, 120 square meters each (nearly 1300 square feet), concentrate solar radiation at the top of a 115 meter tower (about 377 feet). A receiver at the top transfers the heat directly to water, and the pressurized steam reaches 250 degrees Celsius.

The engineering aspects of building such a plant take into account both the need to heat up the receiver—and also to moderate the energy directed at that receiver. “At this plant, we’re working with the potential of about 3000 suns, but the absorption panels can only handle 600 suns. We have to control the aiming to protect the solar panels. So it has to be very well designed and operated to provide the best results,” says Valerio Fernández, head of engineering and commissioning for Solúcar.

Fernández says that so far the facility is operating as intended, but that improvements have been incorporated into future towers. “This isn’t the best temperature for the highest efficiency,” says Fernández “but we wanted to test the safety and security of the design for this first installation.

We’ll do the remaining research necessary in order to use higher temperatures in future plants.” He explains that the cooling system for the boiler is more complicated as temperatures increase, but that once those changes are implemented, the tower’s efficiency could improve by 20 percent.

The tower is also supported by a small amount of natural gas, used when a stretch of rainy or overcast days prevent the plant’s full operation and the stored energy cannot stretch far enough through the end of the rainy phase. “It’s good to be able to maintain stability, not be stopping and starting up the turbines more than once a day, as they’re designed to do,” says Fernández.

When completed in 2012, the entire Solúcar facility called the Sanlúcar La Mayor Solar Platform will house more than 300 MW of solar power, utilizing tower and trough technologies along with PV installations. Abengoa, owner of Solúcar, has also signed plans to build combined cycle power plants in Algeria and Morocco using parabolic troughs in conjunction with natural gas power plants.

One of the main advantages of solar thermal power, in addition to the cost benefit, is the potential for power storage. The Solúcar tower uses a system of heat storage system based on pressurized water. SENER’s Andasol site is using a more advanced storage taking advantage of the specific properties of molten salts, the first time this technology has been implemented commercially.

Located about an hour outside Granda, home to the world-famous Alhambra, Andasol 1 will provide power well into the evening hours. SENER, constructing the plant with a company called COBRA, has built extra troughs that will direct their heated oil to 28,000 tons of molten salt (the salts are imported from Chile). The salts must reach a high enough temperature to liquefy—and then they must be maintained in a liquid stage to prevent them from causing blockages. Tubes with heated oil flow into the molten salts, raising the temperature even higher, and the salts retain the heat energy. As evening falls, the thermal energy is transferred back to heating oil, which will continue on to the heat exchanger and power the steam turbine.

One of SENER’s innovations in this field was the development of new simulation software, called SENSOL, that takes into account all the parameters necessary to build a solar plant, determining the production costs and the appropriate dimension for that plant. This technology has also been used outside the country; the Japanese Institute of Technology purchased SENER’s services to determine the best dimension of a solar plant they wanted to develop.

Andasol is SENER’s first solar thermal site, though they’ve already broken ground on another site nearby, and a third is in the planning process for a location in the northern part of country.

The company has run into hurdles in building this facility, the first major parabolic trough system in Spain. “There have been a lot of challenges,” says Nora Castañeda, engineer in charge of the site’s construction, laughing. “We can begin with the design itself. It was difficult to find the right manufacturers, because there are so few suppliers of the parts. We had to learn how to assemble a solar field like this in a short time. Once we solved one problem, another appeared.”

But as quickly as problems have appeared, she says, the staff worked hard to find solutions. They built an assembly plant on-site and worked with Spanish construction companies to create appropriate jigs with laser trackers for building the extremely precise structure for the parabolic mirrors and then transporting the system to the field without altering the precision. Castañeda says she expects that the lessons learned from Andasol 1 will help drive down the cost of constructing future systems.

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