Arrays of parabolic troughs

A diagram of a parabolic trough solar farm (top), and an end view of how a parabolic collector focuses sunlight onto its focal point.

A parabolic trough is a type of solar thermal energy collector. It is constructed as a long parabolic mirror (usually coated silver or polished aluminum) with a Dewar tube running its length at the focal point. Sunlight is reflected by the mirror and concentrated on the Dewar tube. The trough is usually aligned on a north-south axis, and rotated to track the sun as it moves across the sky each day. Alternatively the trough can be aligned on an east-west axis, this reduces the overall efficiency of the collector but only requires the trough to be aligned with the change in seasons, avoiding the need for tracking motors. Heat transfer fluid (usually oil) runs through the tube to absorb the concentrated sunlight. The heat transfer fluid is then used to heat steam in a standard turbine generator. The process is economical and, for heating the pipe, thermal efficiency ranges from 60-80%. The overall efficiency from collector to grid, i.e. (Electrical Output Power)/(Total Impinging Solar Power) is about 15%, similar to PV(Photovoltaic Cells) and less than Stirling dish concentrators.^[1]

Current commercial plants utilizing parabolic troughs are hybrids; fossil fuels are used during night hours, but the amount of fossil fuel used is limited to a maximum 27% of electricity production, allowing the plant to qualify as a renewable energy source. Because they are hybrids and include cooling stations, condensers, accumulators and other things besides the actual solar collectors, the power generated per square meter of space ranges enormously.

Energy storage

As this renewable source of energy is inconstant by nature, methods for energy storage have been studied, for instance the single-tank (thermocline) storage technology for large-scale solar thermal power plants. The thermocline tank approach uses a mixture of silica sand and quartzite rock to displace a significant portion of the volume in the tank. Then it is filled with the heat transfer fluid, a molten nitrate salt.

Existing plants

The largest operational solar power system at present is one of the SEGS plants and is located at Kramer Junction in California, USA, with five fields of 33 MW generation capacity each.^[2]

The 64 MW Nevada Solar One also uses this technology. In the new Spanish plant, Andasol 1 solar power station, the 'Eurotrough'-collector is used. This plant is scheduled to go online in summer 2008^[3] and has a nominal output of 49.9 MW.

Popular Culture

A parabolic trough solar plant appears in the movie Gattaca.

Notes and References

Bibliography

Duffie, John; Williams Beckman (1991). Solar Engineering of Thermal Processes, Second Edition (in English), New York: John Wiley & Sons, Inc.. ISBN 0471510564. 

Patel., Mukund (1999). Wind and solar power systems. Boca Raton London New York Washington, D.C.: CRC Press. ISBN 0-8493-1605-7. 

External links

• Kramer Junction Solar Power Plants - satellite image, Google Map.
• Picture Perfect Parabolic Solar Collector Systems