An incredible vista unfolds from the castle walls of La Calahorra. The snow-covered peaks of the Sierra Nevada line the horizon while an expanse of fields stretches for kilometres into the distance beneath the old citadel. Above all, however, no other vantage point offers a better view of a technological wonder that is unique not only here in southern Spain: in the midst of the brown and green of the surrounding landscape, six-metre-high parabolic mirrors glisten in the sunlight. Standing in rows several hundred metres long, they cover an enormous area: 510,000 square metres – a space large enough for more than 70 soccer fields. The mirrors collect the energy for the world’s largest solar power plant, Andasol1, whose construction began in the Granada region in June 2006. Before the end of this year the parabolic trough plant will be generating 50 megawatts of power and supplying 200,000 people with electricity – solely from the power of the sun. Construction work on Andasol2 began a year ago and planning has already started for Andasol3. The pioneer of this clean power plant technology is a small firm from southern Germany. Erlangen-based Solar Millennium AG developed Andasol1 and built the plant with Spanish partners.
The special thing about the parabolic trough power plant is the technology. In contrast to solar cells, it does not transform sunlight directly into electricity. With an accuracy of 98%, the mirrors reflect the incoming rays of light onto an absorber tube, also known as a receiver, that runs through the focal region of the collector. The receiver contains a heat transfer fluid that is heated to a temperature of roughly 400 degrees Celsius by the concentrated sunlight. The fluid transports the solar heat to a heat exchanger where steam is produced to drive turbines that in turn generate electricity. In the case of the technology from Germany, this even functions at night: salt-based storage units integrated in the array of collectors release the necessary heat after the sun has gone down. The receiver technology is especially finicky. It consists of a metal tube and an outer glass tube. There is a vacuum between the two tubes to insulate the metal tube and keep heat loss to an absolute minimum. Because as much solar radiation as possible is meant to pass through the outer glass covering without being reflected and then reach the metal tube where it is absorbed, special materials are required whose composition is a closely guarded secret. Worldwide only two companies have ventured into the production of these receivers. One of them is the Mainz-based glass and solar cell specialist Schott, whose technology is also collecting the sun’s energy in Spain.
Efficiency is one of the reasons for the favourable forecasts of Henner Gladden. The Solar Millennium board member responsible for technological development sees enormous opportunities for parabolic trough power plants: “In Spain alone, the potential generating capacity adds up to 10,000 to 15,000 megawatts. That corresponds to 10 to 15 conventional large-scale power stations.” These figures are also confirmed by the German Aerospace Center (DLR). In 15 years, power plants of this kind could supply the world with clean electricity from northern Africa and southern Europe. Plans are already lying in the drawer at Solar Millennium waiting to be put into practice. The same applies to another technology of the future: solar updraft. The engineers in Erlangen want to build kilometre-wide expanses of glass roofing around a roughly one-kilometre-tall chimney. The air under the glass will be heated by the sun and rise through the chimney – this wind energy then drives turbines at the foot of the enormous tower to generate electricity. Nobody will have to climb to the top of a castle to admire these installations.
