Parabolic trough
Overview
 
A parabolic trough is a type of solar thermal energy
Solar thermal energy
Solar thermal energy is a technology for harnessing solar energy for thermal energy . Solar thermal collectors are classified by the United States Energy Information Administration as low-, medium-, or high-temperature collectors. Low-temperature collectors are flat plates generally used to heat...

 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
Focus (optics)
In geometrical optics, a focus, also called an image point, is the point where light rays originating from a point on the object converge. Although the focus is conceptually a point, physically the focus has a spatial extent, called the blur circle. This non-ideal focusing may be caused by...

. Sunlight
Sunlight
Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.When the direct solar radiation is not blocked...

 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, due to cosine loss, but only requires the trough to be aligned with the change in season
Season
A season is a division of the year, marked by changes in weather, ecology, and hours of daylight.Seasons result from the yearly revolution of the Earth around the Sun and the tilt of the Earth's axis relative to the plane of revolution...

s, avoiding the need for tracking motors.
Encyclopedia
A parabolic trough is a type of solar thermal energy
Solar thermal energy
Solar thermal energy is a technology for harnessing solar energy for thermal energy . Solar thermal collectors are classified by the United States Energy Information Administration as low-, medium-, or high-temperature collectors. Low-temperature collectors are flat plates generally used to heat...

 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
Focus (optics)
In geometrical optics, a focus, also called an image point, is the point where light rays originating from a point on the object converge. Although the focus is conceptually a point, physically the focus has a spatial extent, called the blur circle. This non-ideal focusing may be caused by...

. Sunlight
Sunlight
Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.When the direct solar radiation is not blocked...

 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, due to cosine loss, but only requires the trough to be aligned with the change in season
Season
A season is a division of the year, marked by changes in weather, ecology, and hours of daylight.Seasons result from the yearly revolution of the Earth around the Sun and the tilt of the Earth's axis relative to the plane of revolution...

s, avoiding the need for tracking motors. This tracking method works correctly at the spring and fall equinox
Equinox
An equinox occurs twice a year, when the tilt of the Earth's axis is inclined neither away from nor towards the Sun, the center of the Sun being in the same plane as the Earth's equator...

es 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
Acceptance angle (solar concentrator)
Acceptance angle is the maximum angle at which incoming sunlight can be captured by a solar concentrator. Its value depends on the concentration of the optic and the refractive index in which the receiver is immersed...

, 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
Nonimaging optics
Nonimaging optics is the branch of optics concerned with the optimal transfer of light radiation between a source and a target...

.

Heat transfer fluid (usually oil) runs through the tube to absorb
Absorption (electromagnetic radiation)
In physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy for example, to heat. The absorption of light during wave propagation is...

 the concentrated sunlight. This increases the temperature of the fluid to some 400°C. 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
Stirling engine
A Stirling engine is a heat engine operating by cyclic compression and expansion of air or other gas, the working fluid, at different temperature levels such that there is a net conversion of heat energy to mechanical work....

 dish concentrators.

Current commercial plants utilizing parabolic troughs are hybrids; fossil fuel
Fossil fuel
Fossil fuels are fuels formed by natural processes such as anaerobic decomposition of buried dead organisms. The age of the organisms and their resulting fossil fuels is typically millions of years, and sometimes exceeds 650 million years...

s 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, condenser
Condenser (steam turbine)
A surface condenser is a commonly used term for a water-cooled shell and tube heat exchanger installed on the exhaust steam from a steam turbine in thermal power stations. These condensers are heat exchangers which convert steam from its gaseous to its liquid state at a pressure below atmospheric...

s, accumulators
Accumulator (energy)
An accumulator is an apparatus by means of which energy can be stored, such as a rechargeable battery or a hydraulic accumulator. Such devices may be electrical, fluidic or mechanical and are sometimes used to convert a small continuous power source into a short surge of energy or vice versa...

 and other things besides the actual solar collectors, the power generated per square meter of area varies enormously.

Types of mirrors

Usually, mirrors are used which are parabolic and are of a single piece.
In addition, V-type parabolic troughs exist which are made from 2 mirrors and placed at an angle towards each other.

Mirror coatings

In 2009, scientists at the National Renewable Energy Laboratory
National Renewable Energy Laboratory
The National Renewable Energy Laboratory , located in Golden, Colorado, is the United States' primary laboratory for renewable energy and energy efficiency research and development. The National Renewable Energy Laboratory is a government-owned, contractor-operated facility; it is funded through...

 (NREL) and SkyFuel
SkyFuel
SkyFuel is a designer and provider of thermal concentrating solar power equipment for utility scale power generation and industrial applications of steam. Energy from thermal CSP plants can be stored for later use and deliver electricity matched to the load profile of the utility company...

 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
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...

 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.

Energy storage

As this renewable source of energy is inconsistent by nature, methods for energy storage have been studied, for instance the single-tank (thermocline
Thermocline
A thermocline is a thin but distinct layer in a large body of fluid , in which temperature changes more rapidly with depth than it does in the layers above or below...

) 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
Molten salt
Molten salt refers to a salt that is in the liquid phase that is normally a solid at standard temperature and pressure . A salt which is normally liquid at STP is usually called a room temperature ionic liquid, although technically molten salts are a class of ionic liquids.-Uses:Molten salts have...

.

Existing plants

The largest operational solar power system at present is one of the SEGS
Solar Energy Generating Systems
Solar Energy Generating Systems is the largest solar energy generating facility in the world. It consists of nine solar power plants in California's Mojave Desert, where insolation is among the best available in the United States...

 plants and is located at Kramer Junction
Kramer Junction, California
Kramer Junction is a small community in the Mojave Desert at the intersection of U.S. Route 395 and State Route 58.Solar Energy Generating Systems sites SEGS III-VII are located less than a mile to the northwest....

 in California
California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...

, USA, with five fields of 33 MW generation capacity each.

The 64 MW Nevada Solar One
Nevada Solar One
Nevada Solar One is a concentrated solar power plant, with a nominal capacity of 64 MW and maximum capacity of 75 MW spread over an area of 400 Acres. The projected CO2 emissions avoided is equivalent to taking approximately 20,000 cars off the road annually. The project required an investment of...

 also uses this technology. In the new Spanish plant, Andasol 1 solar power station
Andasol 1 solar power station
The Andasol solar power station is Europe's first commercial parabolic trough solar thermal power plant, located near Guadix in Andalusia, Spain. Its name is a combination of Andalusia and Sol .- Description :...

, the 'Eurotrough'-collector is used. This plant went online in November 2008 and has a nominal output of 49.9 MW.

Popular culture

A parabolic trough solar plant appears in the movie Gattaca
Gattaca
Gattaca is a 1997 science fiction film written and directed by Andrew Niccol. It stars Ethan Hawke, Uma Thurman and Jude Law with supporting roles played by Loren Dean, Ernest Borgnine, Gore Vidal and Alan Arkin....

 as well as in the CD insert for the album Mellon Collie and the Infinite Sadness
Mellon Collie and the Infinite Sadness
Mellon Collie and the Infinite Sadness is the third album by American alternative rock band The Smashing Pumpkins, released October 24, 1995 on Virgin Records. Produced by frontman Billy Corgan with Flood and Alan Moulder, the twenty-eight-track album was released as a two-disc CD and triple LP...


External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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