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A parabolic trough collector (PTC) 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. The sunlight 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 (e.g. ORC or water/steam Rankine cycle), 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 ranges from 50 to 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 engine 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 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 with molten salt heat storage, the 250 MW Genesis Solar Energy Project, the Spanish 200 MW Solaben Solar Power Station, and the Andasol 1 solar power station.NREL.gov Concentrating Solar Power Projects in the United States, 17 February 2014NREL-gov Concentrating Solar Power Projects in Spain, 17 February 2014
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 opticsRoland Winston et al.,, Nonimaging Optics, Academic Press, 2004 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) 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 to 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 engine dish concentrators.Patel99 Ch.9
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 (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 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 salt.
Lightweight curved solar-reflecting mirrors are suspended within the glasshouse. A Solar tracker positions the mirrors to track the sun and focus its light onto a network of stationary steel pipes, also suspended from the glasshouse structure.Helman, Christopher, "Oil from the sun", "Forbes", April 25, 2011 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, 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 solar facility in Oman. In November 2017, GlassPoint announced a partnership with Aera Energy that would bring parabolic troughs to the South Belridge Oil Field, near Bakersfield, California.
Shuman built the world's first solar thermal power station in Maadi, Egypt between 1912 and 1913. Shuman's plant used parabolic troughs to power a 45–52 kilowatt (60–70 Horsepower) engine that pumped more than 22,000 litres of water per minute from the Nile River 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:
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 with molten salt heat storage, the 250 MW Genesis Solar Energy Project, the Spanish 200 MW Solaben Solar Power Station, and the Andasol 1 solar power station.
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