23 November 2010
Solar radiation arriving on the earth’s surface has an average power density of about 1000 watt per square meter (or 833 watt per square yard). For converting this energy to electrical power, photo-voltaic- and heat-collector technologies are filling up the empty areas in our world, many of which use special mirrors and optics for collecting the solar energy.
History of solar energy
Pirelióforo at the 1904 Universal Expo
Concentrated solar power is not a new idea. During the siege of Syracuse (214 - 212 B.C.) Archimedes possibly used mirrors to destroy the Roman fleet focusing sunlight onto approaching ships and causing them to catch fire. Also Leonardo da Vinci considered using the Sun designing his Solar Concentrator in 1515. The inventor of the first large scale solar device was a Portuguese scientist/priest, Manuel António Gomes, or Padre Himalaya. His biggest achievement was the Pirelióforo, a device in which thousands of mirrors over a surface of 80 square meters concentrated solar energy up to a temperature of 3500 degrees Celsius: enough to melt most metals and stones. The huge installment was a major attraction during the 1904 Universal Expo of St. Louis.
Solar fields for global energy consumption
Mirrors are a considerable cost factor in solar plants because thousands are used to concentrate the light.
The first solar mirror fields were built in the beginning of the 20th century. In 1911 and 1912, solar pioneer Frank Schuman constructed two solar plants, the first in Tacony, Philadelphia (USA) with 20 kW of total output. Next, in Maadi (Egypt), he designed as 5 rows of 62 meters long parabolic mirrors for a total output of 88 kW. He developed plans for building 52.600 km² of mirror collectors in the Sahara to produce 198 MW, which was equivalent to the world’s energy consumption in 1909.
Frank Shuman's Maadi plant in Egypt
The first solar plants in California
During the period 1980-1990, construction took place of the largest solar energy generating facility in the world at the time, Solar Energy Generating Systems (SEGS). It consists of nine concentrating solar power (CSP) in California's Mojave Desert, where insolation is among the best available in the US. The first SEGS plants use collector mirrors made by Flabeg GmbH, a former subsidiary of the Pilkington Group, and are capable of producing 354 MW of power.
SEGS power plant in California
Optical technologies for solar plants
Large scale solar energy systems require optics for concentrating the sunlight. The most mature technologies use parabolic troughs with mirrors that heat a liquid and transports the energy to a storage tank. For more on CSP see “Concentrating Solar Thermal: Sun power even in the dark".
Parabolic trough solar power plant. (Graphic courtesy of Saint-Gobain Thermosolar)
The special mirrors use materials including glass, plastic films or aluminum. Only low-iron glass has sufficient durability to survive environmentally harsh conditions at typical solar plant locations such as deserts. It also offers the highest reflectivity, with the rear face silver coated and covered with copper and other subsequent protection layers.
Next Page: Optical technologies for solar plants (continued)