Parabolic Trough

A parabolic trough collector, or PTC, is a type of solar thermal collector that is straight in one dimension and curved as a parabola in the other two, lined with a polished metal mirror. Sunlight entering the mirror parallel to its plane of symmetry is focused along a focal line, where a tube containing a fluid runs the length of the trough; the sunlight concentrated on the tube heats the fluid to a high temperature, and the hot fluid can then be piped to a heat engine, such as an organic Rankine cycle or a water and steam Rankine cycle, to drive machinery or generate electricity. When heat transfer fluid is used to heat steam to drive a standard turbine generator, thermal efficiency ranges from 50 to 80 percent, while overall efficiency from collector to grid is about 15 percent, similar to photovoltaic cells but less than Stirling dish concentrators; large-scale solar thermal power plants using this technology need a method for storing energy, such as a thermocline tank using a mixture of silica sand and quartzite rock along with a heat transfer fluid, typically a molten nitrate salt.