Parabolic trough

Parabolic trough

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For parabolic trough reflectors used for solar cooking, see Solar cooker.

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Array of parabolic troughs.

A parabolic trough is shaped as a parabola in the x-y plane, but is linear in the z direction

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 collector that is straight in one dimension and curved as a parabola in the other two, lined with a polished metal mirror. A tube, frequently a Dewar tube, runs the length of the trough at its focal line. The mirror is oriented so that sunlight which it reflects is concentrated on the tube, which contains a fluid which is heated to a high temperature by the energy of the sunlight. The hot fluid is piped to equipment, such a heat engine, which uses its energy for some purpose, such as generating electricity.

This solar energy collector is the most common and best known type of parabolic trough. The paragraphs below therefore concentrate on this type.

Contents
[hide] 1 Efficiency
2 Design
3 Variations 3.1 Enclosed trough

4 Usage by commercial plants
5 See also
6 References
7 Bibliography
8 External links

[edit] Efficiency

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 due to cosine loss but only requires the trough to be aligned with the change in seasons, avoiding the need for tracking motors. This tracking method works correctly at the spring and fall equinoxes with errors in the focusing of the light at other times during the year (the magnitude of this error varies throughout the day, taking a minimum value at solar noon). There is also an error introduced due to the daily motion of the sun across the sky, this error also reaches a minimum at solar noon. Due to these sources of error, seasonally adjusted parabolic troughs are generally designed with a lower solar concentration ratio.

Parabolic trough concentrators have a simple geometry, but their concentration is about 1/3 of the theoretical maximum for the same acceptance angle, that is, for the same overall tolerances for the system. Approaching the theoretical maximum may be achieved by using more elaborate concentrators based on primary-secondary designs using nonimaging optics.[1][2]

[edit] Design

Heat transfer fluid (usually oil) runs through the tube to absorb the concentrated sunlight. This increases the temperature of the fluid to some 400°C.[3] 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) but less than Stirling dish concentrators.[4]

Most mirrors used are parabolic and single-piece. In addition, V-type parabolic troughs exist which are made from 2 mirrors and placed at an angle towards each other.[5]

In 2009, scientists at the National Renewable Energy Laboratory (NREL) and SkyFuel teamed to develop large curved sheets of metal that have the potential to be 30% less expensive than today’s best collectors of concentrated solar power by replacing glass-based models with a silver polymer sheet that has the same performance as the heavy glass mirrors, but at a much lower cost and much lower weight. It also is much easier to deploy and install. The glossy film uses several layers of polymers, with an inner layer of pure silver.[6]

As this renewable source of energy is inconsistent 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, typically a molten nitrate salt.

[edit] Variations

[edit] Enclosed trough

Enclosed trough systems are used to produce process heat. The design encapsulates the solar thermal system within a greenhouse-like glasshouse. The glasshouse creates a protected environment to withstand the elements that can negatively impact reliability and efficiency of the solar thermal system.[7] Lightweight curved solar-reflecting mirrors are suspended from the ceiling of the glasshouse by wires. A single-axis tracking system positions the mirrors to retrieve the optimal amount of sunlight. The mirrors concentrate the sunlight and focus it on a network of stationary steel pipes, also suspended from the glasshouse structure.[8] Water is carried throughout the length of the pipe, which is boiled to generate steam when intense sun radiation is applied. Sheltering the mirrors from the wind allows them to achieve higher temperature rates and prevents dust from building up on the mirrors.[7]

[edit] Usage by commercial plants

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 area varies enormously.

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.[9]

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 went online in November 2008[10] and has a nominal output of 49.9 MW.

[edit] See also
Parabola
Parabolic reflector
Solar cooker

[edit] References

1.^ Julio Chaves, Introduction to Nonimaging Optics, CRC Press, 2008 ISBN 978-1-4200-5429-3
2.^ Roland Winston et al.,, Nonimaging Optics, Academic Press, 2004 ISBN 978-0-12-759751-5
3.^ Absorber tube temperature
4.^ Patel99 Ch.9
5.^ V-type parabolic troughs
6.^ “Award-Winning Solar Reflectors Will Cut Production Costs”. www.energyboom.com. Retrieved 2009-11-25.
7.^ a b Deloitte Touche Tohmatsu Ltd, “Energy & Resources Predictions 2012”, 2 November 2011
8.^ Helman, Christopher, “Oil from the sun”, “Forbes”, April 25, 2011
9.^ “Kramer Junction SEGS III, IV, V, VI,VII.”. Solel.
10.^ “The Construction of the Andasol Power Plants”. Solar Millennium AG.