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A parabolic trough is a type of
solar thermal collector A solar thermal collector collects heat by absorbing sunlight. The term "solar collector" commonly refers to a device for solar hot water heating, but may refer to large power generating installations such as solar parabolic troughs and s ...
that is straight in one dimension and curved as a
parabola In mathematics, a parabola is a plane curve which is mirror-symmetrical and is approximately U-shaped. It fits several superficially different mathematical descriptions, which can all be proved to define exactly the same curves. One descri ...
in the other two, lined with a polished metal
mirror A mirror or looking glass is an object that reflects an image. Light that bounces off a mirror will show an image of whatever is in front of it, when focused through the lens of the eye or a camera. Mirrors reverse the direction of the im ...
. The
sunlight Sunlight is a portion of the electromagnetic radiation given off by the Sun, in particular infrared, visible, and ultraviolet light. On Earth, sunlight is scattered and filtered through Earth's atmosphere, and is obvious as daylight when ...
which enters the mirror parallel to its plane of symmetry is focused along the focal line, where objects are positioned that are intended to be heated. In a solar cooker, for example, food is placed at the focal line of a trough, which is cooked when the trough is aimed so the Sun is in its plane of symmetry. For other purposes, a tube containing a fluid runs the length of the trough at its focal line. The sunlight is concentrated on the tube and the fluid heated to a high temperature by the energy of the sunlight. The hot fluid can be piped to a
heat engine In thermodynamics and engineering, a heat engine is a system that converts heat to mechanical energy, which can then be used to do mechanical work. It does this by bringing a working substance from a higher state temperature to a lower stat ...
, which uses the heat energy to drive machinery, or to generate electricity. This solar energy collector is the most common and best known type of parabolic trough. When heat transfer fluid is used to heat steam to drive a standard turbine generator,
thermal efficiency In thermodynamics, the thermal efficiency (\eta_) is a dimensionless performance measure of a device that uses thermal energy, such as an internal combustion engine, steam turbine, steam engine, boiler, furnace, refrigerator, ACs etc. For a ...
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 photovoltaic cells but less than
Stirling Stirling (; sco, Stirlin; gd, Sruighlea ) is a city in central Scotland, northeast of Glasgow and north-west of Edinburgh. The market town, surrounded by rich farmland, grew up connecting the royal citadel, the medieval old town with its me ...
dish concentrators. Large-scale solar thermal power plants need a method for storing the energy, such as a thermocline tank, which uses a mixture of silica sand and quartzite rock to displace a significant portion of the volume in the tank. It is then filled with the heat transfer fluid, typically a molten nitrate salt. As of 2014, the largest solar thermal power systems using parabolic trough technology include the 354 MW SEGS plants in California, the 280 MW
Solana Generating Station The Solana Generating Station is a solar power plant near Gila Bend, Arizona, about southwest of Phoenix, completed in 2013. When commissioned it was the largest parabolic trough plant in the world and the first U.S. solar plant with molten s ...
with
molten salt heat storage Thermal energy storage (TES) is achieved with widely different technologies. Depending on the specific technology, it allows excess thermal energy to be stored and used hours, days, months later, at scales ranging from the individual process, ...
, the 250 MW
Genesis Solar Energy Project The Genesis Solar Energy Project is a concentrated solar power station located in the Mojave Desert on of Bureau of Land Management land, in eastern Riverside County, California. The plant is owned/managed by Genesis Solar, LLC, a subsidiary of ...
, the Spanish 200 MW Solaben Solar Power Station, and the Andasol 1 solar power station.NREL.go
Concentrating Solar Power Projects in the United States
17 February 2014
NREL-go
Concentrating Solar Power Projects in Spain
17 February 2014


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 the sunlight striking the collectors at an angle but only requires the trough to be aligned with the change in
season A season is a division of the year based on changes in weather, ecology, and the number of daylight hours in a given region. On Earth, seasons are the result of the axial parallelism of Earth's tilted orbit around the Sun. In temperate and ...
s, avoiding the need for tracking motors. This tracking method approaches theoretical efficiencies at the spring and fall
equinox A solar equinox is a moment in time when the Sun crosses the Earth's equator, which is to say, appears directly above the equator, rather than north or south of the equator. On the day of the equinox, the Sun appears to rise "due east" and se ...
es with less accurate focusing of the light at other times during the year. The daily motion of the sun across the sky also introduces errors, greatest at the sunrise and sunset and smallest at noon. Due to these sources of error, seasonally adjusted parabolic troughs are generally designed with a lower concentration acceptance product. 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 of the system to all kinds of errors, including those referenced above. The theoretical maximum is better achieved with more elaborate concentrators based on primary-secondary designs using
nonimaging optics Nonimaging optics (also called anidolic optics)Roland Winston et al., ''Nonimaging Optics'', Academic Press, 2004 R. John Koshel (Editor), ''Illumination Engineering: Design with Nonimaging Optics'', Wiley, 2013 is the branch of optics concerned wi ...
which may nearly double the concentration of conventional parabolic troughsDiogo Canavarro et al., ''New second-stage concentrators (XX SMS) for parabolic primaries; Comparison with conventional parabolic trough concentrators'', Solar Energy 92 (2013) 98–105 and are used to improve practical designs such as those with fixed receivers.Diogo Canavarro et al., ''Infinitesimal etendue and Simultaneous Multiple Surface (SMS) concentrators for fixed receiver troughs'', Solar Energy 97 (2013) 493–504 Heat transfer fluid (usually
thermal oil An oil is any nonpolar chemical substance that is composed primarily of hydrocarbons and is hydrophobic (does not mix with water) & lipophilic (mixes with other oils). Oils are usually flammable and surface active. Most oils are unsaturated ...
) runs through the tube to absorb 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 (; sco, Stirlin; gd, Sruighlea ) is a city in central Scotland, northeast of Glasgow and north-west of Edinburgh. The market town, surrounded by rich farmland, grew up connecting the royal citadel, the medieval old town with its me ...
dish concentrators.


Design

A parabolic trough is made of a number of solar collector modules (SCM) fixed together to move as one solar collector assembly (SCA). A SCM could have a length up to or more. About a dozen or more of SCM make each SCA up to length. Each SCA is an independently-tracking parabolic trough. A SCM may be made as a single-piece parabolic mirror or assembled with a number of smaller mirrors in parallel rows. Smaller modular mirrors require smaller machines to build the mirror, reducing cost. Cost is also reduced in case of the need of replacing a damaged mirror. Such damage may occur due to being hit by an object during bad weather. In addition, V-type troughs exist which are made from 2 mirrors and placed at an angle towards each other. In 2009, scientists at the
National Renewable Energy Laboratory The National Renewable Energy Laboratory (NREL) in the US specializes in the research and development of renewable energy, energy efficiency, energy systems integration, and sustainable transportation. NREL is a federally funded research an ...
(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 Concentrated solar power (CSP, also known as concentrating solar power, concentrated solar thermal) systems generate solar power by using mirrors or lenses to concentrate a large area of sunlight into a receiver. Electricity is generated when ...
by replacing glass-based models with a
silver Silver is a chemical element with the symbol Ag (from the Latin ', derived from the Proto-Indo-European ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, white, lustrous transition metal, it exhibits the highest electrical ...
polymer sheet that has the same performance as the heavy glass mirrors, but at a much lower cost and weight. It also is much easier to move and install. The glossy film uses several layers of polymers, with an inner layer of pure silver. 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.


Enclosed trough

The enclosed trough architecture encapsulates the solar thermal system within a greenhouse-like glasshouse. The glasshouse creates a protected environment to withstand the elements that can reduce the reliability and efficiency of the solar thermal system.Deloitte Touche Tohmatsu Ltd
"Energy & Resources Predictions 2012"
2 November 2011
Lightweight curved solar-reflecting mirrors are suspended within the glasshouse. A single-axis tracking system positions the mirrors to track the sun and focus its light onto a network of stationary steel pipes, also suspended from the glasshouse structure. Steam is generated directly using oil field-quality water, as water flows along the length of the pipes, without heat exchangers or intermediate working fluids. The steam produced is then fed directly to the field's existing steam distribution network, where the steam is continuously injected deep into the oil reservoir. Sheltering the mirrors from the wind allows them to achieve higher temperatures and prevents dust from building up as a result from exposure to humidity.
GlassPoint Solar GlassPoint is a private company founded in 2009 that designs and manufactures solar steam generators which use solar thermal technology to generate steam for industrial processes. Overview GlassPoint's enclosed trough technology enclos ...
, the company that created the Enclosed Trough design, states its technology can produce heat for EOR for about $5 per million British thermal units in sunny regions, compared to between $10 and $12 for other conventional solar thermal technologies.Goossens, Ehren
"Chevron Uses Solar-Thermal Steam to Extract Oil in California"
"Bloomberg", October 3, 2011
Enclosed troughs are currently being used at the
Miraah Miraah is a solar thermal energy plant that is under construction in Oman for the production of steam for solar thermal enhanced oil recovery. In July 2015, Petroleum Development Oman and GlassPoint Solar announced that they signed a $600 mill ...
solar facility in
Oman Oman ( ; ar, عُمَان ' ), officially the Sultanate of Oman ( ar, سلْطنةُ عُمان ), is an Arabian country located in southwestern Asia. It is situated on the southeastern coast of the Arabian Peninsula, and spans the mouth of ...
. In November 2017, GlassPoint announced a partnership with
Aera Energy Aera Energy LLC (or simply Aera) is a natural gas, oil exploration and production company started as a joint venture between Shell plc (through Shell USA) and Mobil (which later merged to form ExxonMobil). Headquartered in Bakersfield, Californ ...
that would bring parabolic troughs to the
South Belridge Oil Field The South Belridge Oil Field is a large oil field in northwestern Kern County, San Joaquin Valley, California, about forty miles west of Bakersfield. Discovered in 1911, and having a cumulative production of over of oil at the end of 2008, i ...
, near
Bakersfield Bakersfield is a city in Kern County, California, United States. It is the county seat and largest city of Kern County. The city covers about near the southern end of the San Joaquin Valley and the Central Valley region. Bakersfield's populat ...
, California.


Early commercial adoption

In 1897,
Frank Shuman Frank Shuman (; January 23, 1862 – April 28, 1918) was an American inventor, engineer and solar energy pioneer known for his work on solar engines, especially those that used solar energy to heat water that would produce steam. Career In 1892 ...
, a U.S. inventor, engineer and solar energy pioneer built a small demonstration solar engine that worked by reflecting solar energy onto square boxes filled with ether, which has a lower boiling point than water, and were fitted internally with black pipes which in turn powered a steam engine. In 1908 Shuman formed the Sun Power Company with the intent of building larger solar power plants. He, along with his technical advisor A.S.E. Ackermann and British physicist Sir
Charles Vernon Boys Sir Charles Vernon Boys, FRS (15 March 1855 – 30 March 1944) was a British physicist, known for his careful and innovative experimental work in the fields of thermodynamics and high-speed photography, and as a popular science communicator th ...
, developed an improved system using mirrors to reflect solar energy upon collector boxes, increasing heating capacity to the extent that water could now be used instead of ether. Shuman then constructed a full-scale steam engine powered by low-pressure water, enabling him to patent the entire solar engine system by 1912. Shuman built the world's first solar thermal power station in
Maadi Maadi ( ar, المعادي / transliterated:   ) is a leafy suburban district south of Cairo, Egypt, on the east bank of the Nile about upriver from downtown Cairo. The Nile at Maadi is parallelled by the Corniche, a waterfront promenade a ...
,
Egypt Egypt ( ar, مصر , ), officially the Arab Republic of Egypt, is a List of transcontinental countries, transcontinental country spanning the North Africa, northeast corner of Africa and Western Asia, southwest corner of Asia via a land bridg ...
between 1912 and 1913. Shuman's plant used parabolic troughs to power a 45-52 kilowatt (60-70  hp) engine that pumped more than 22,000 litres of water per minute from the
Nile River The Nile, , Bohairic , lg, Kiira , Nobiin: Áman Dawū is a major north-flowing river in northeastern Africa. It flows into the Mediterranean Sea. The Nile is the longest river in Africa and has historically been considered the longest riv ...
to adjacent cotton fields. Although the outbreak of World War I and the discovery of cheap oil in the 1930s discouraged the advancement of solar energy, Shuman's vision and basic design were resurrected in the 1970s with a new wave of interest in solar thermal energy. In 1916 Shuman was quoted in the media advocating solar energy's utilization, saying:


Commercial plants

Commercial plants using parabolic troughs may use thermal storage at night while some are hybrids and support
natural gas Natural gas (also called fossil gas or simply gas) is a naturally occurring mixture of gaseous hydrocarbons consisting primarily of methane in addition to various smaller amounts of other higher alkanes. Low levels of trace gases like carbon d ...
as a secondary fuel source. In the United States the amount of fossil fuel used in order for the plant to qualify as a renewable energy source is limited to a maximum 27% of electricity production. Because they include cooling stations, condensers, accumulators and other things besides the actual solar collectors, the power generated per square meter of area varies enormously. As of 2014, the largest solar thermal power systems using parabolic trough technology include the 354 MW SEGS plants in California, the 280 MW
Solana Generating Station The Solana Generating Station is a solar power plant near Gila Bend, Arizona, about southwest of Phoenix, completed in 2013. When commissioned it was the largest parabolic trough plant in the world and the first U.S. solar plant with molten s ...
with
molten salt heat storage Thermal energy storage (TES) is achieved with widely different technologies. Depending on the specific technology, it allows excess thermal energy to be stored and used hours, days, months later, at scales ranging from the individual process, ...
, the 250 MW
Genesis Solar Energy Project The Genesis Solar Energy Project is a concentrated solar power station located in the Mojave Desert on of Bureau of Land Management land, in eastern Riverside County, California. The plant is owned/managed by Genesis Solar, LLC, a subsidiary of ...
, the Spanish 200 MW Solaben Solar Power Station, and the Andasol 1 solar power station.


See also

*
Parabola In mathematics, a parabola is a plane curve which is mirror-symmetrical and is approximately U-shaped. It fits several superficially different mathematical descriptions, which can all be proved to define exactly the same curves. One descri ...
*
Parabolic reflector A parabolic (or paraboloid or paraboloidal) reflector (or dish or mirror) is a reflective surface used to collect or project energy such as light, sound, or radio waves. Its shape is part of a circular paraboloid, that is, the surface genera ...
* Solar cooker *
List of solar thermal power stations This is a list of the largest facilities generating electricity through the use of solar thermal power, specifically concentrated solar power. Operational Under construction Announced Cancelled Decommissioned * Eurelios ...


References


Bibliography

* *


External links


Kramer Junction Solar Power Plants
satellite image, Google Map.

– Tool for calculating the shape of a parabolic trough {{Authority control Solar thermal energy