Solar Power » Technologies

Concentrated Solar Thermal: Collecting heat from the sun

Current technologies for large-scale thermal power plants can be distinguished in the way they collect and concentrate solar radiation. As diffuse light can not be focused, only locations with a high proportion of direct light will be suitable for this technology. Target locations include Southern Spain and many parts of the U.S. where annual energy yield is above 2,000kWh/m².


Parabolic Trough

Several parallel rows of collectors, 20 - 400m in length, are made of parabolic reflectors. They focus the light onto a specially coated absorber tube that runs along a caustic line. Solar radiation heats up a heat-transfer fluid, which is subsequently used in a heat exchange.

The troughs track the sun over the course of the day along the central axis as the sun travels from East to West.

This is the most mature CST technology with a concentration ratio of around 100x.

An example is the solar thermal power plant, Andasol, in Andalusia, Spain (2008 - 2011) with a collector surface of 512,000 m².


Solar Dish

The solar dish is a parabolic reflector that can turn on two axes to track the sun light. It reflects the direct light onto a thermal receiver positioned at the focal point of the mirror. Temperatures can rise up to 1,000°C with concentration ratio up to 1,000x. The electrical output of individual reflectors ranges from 10 - 50kW.

Examples can be found at the Plataforma Solar in Almeria, Spain.

In connection with a stirling engine, these systems could be used stand-alone.


Solar Tower

Solar Power Tower

In solar tower plants, the solar radiation is concentrated onto a central heat absorber by hundreds of reflectors on the ground. As with the other concentrating devices, the reflectors track the angle of the sun and positions themselves automatically (dual axis tracking required). Temperatures can reach up to 1,300°C, which is much higher than in the other configurations. The heat generated inside the collector is used to generate electricity, a process that is more efficient the higher the temperature.

Europe's first commercial solar power plant stands in Seville, Spain (2006). It achieves an output of 11MW. Additions are planned.


Linear Fresnel Reflector

Linear Fresnel Reflector

In this latest technology, reflector stripes are tilted in a way that is similar to a Fresnel lens such that all incoming beams are reflected into collector that is situated around 3m above the mirrors. The idea here is similar to the parabolic trough, except that it does not require a huge parabolically shaped mirror, which is expensive to manufacture. Instead, the reflector stripes can be flat.

In 2009, Novatech-Biosol constructed a Fresnel power plant in Southern Spain with a mirror surface space of 18,000m², producing 1.4MW of electricity.

Parabolic Trough Linear Fresnel Solar Tower Parabolic Dish
Linear Systems: 500x at 400°C  Point Systems: 1,000x at 1,000°C - 1,300°C
Parabolic Trough System(credit: Schott Solar) Linear Fresnel CST System(credit: Ausra) Solar Tower System in Spain(credit: Abengoa Solar) Parabolic Stirling Dish(credit: Stirling Energy Systems)
The most mature technology, it has a solar-to-net efficiency of 14% annually. This technology is a new entrant to the market. It has less expensive mirrors than parabolic trough. High temperatures result in high thermal efficiency. 25% solar efficiency (annually) if connected to a combined-cycle power plant. Stand-alone or in clusters. Have Stirling engine with the receiver. Achieves peak efficiency solar - to - electric of 31% according to Stirling Energy Systems. No water cooling required.
  Typical size: 30 - 150MW Typical size: 30 - 160MW

Disdavantage: thermal storage only via battery.

Note: Stirling Energy Systems filed for bankruptcy in 2011. Technology has been discontinued.

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