Parabolic troughs (PT) are the most mature of the concentrating solar power technologies and they are commercially proven. The first systems were installed in 1912 near Cairo in Egypt to generate steam for a pump which delivered water for irrigation. At the time, this plant was competitive with coal-fired installations in regions where coal was expensive.
In the trough system, sunlight is concentrated by about 70–100 times on absorber tubes, achieving operating temperatures of 350oC to 550oC. A heat transfer fluid pumped through the absorber tube transfers the thermal energy to a conventional steam turbine power cycle. Most plants use synthetic thermal oil to transfer heat. The hot thermal oil is used to produce slightly superheated steam at high pressure, to feed a steam turbine connected to a generator to produce electricity. Thermal oil has a top temperature of about 400oC, which limits the conversion efficiency of the turbine cycle. Researchers and the industry have developed alternatives. One example is the direct generation of steam in the absorber tubes, another using molten salt as the HTF. Prototype plants of both types are currently being built.
Currently, parabolic troughs are the most widely used technology around the world, particularly in Spain and the United States where the installed capacity of operating plants is over 2,370 MW and 1,836 MW, respectively. Plants range in size from 5 to 280 MW. Parabolic troughs are considered a “mature” technology. For example, in terms of supply chain, a number of manufacturers currently fabricate this technology. What’s more, there is good experience in engineering procurement and construction (EPC) and 20 years of operating experience allows for good confidence on the operation. Therefore, projects using parabolic trough technology can be considered low-risk.
A new generation of parabolic trough plants aims to reach a higher HTF temperature, allowing the full integration of the solar field and the storage system. This “second generation” should provide significant improvements in the average conversion efficiency and further reduction of costs. Although a demonstration plant has already been built, adequate operating experience is still needed and components with enhanced performance and durability are being studied and developed.