Concentrating Solar Power Technologies

Main articles: Solar thermal energy, Parabolic trough, and Solar power tower

A 16th century engraving shows early interest in using concentrated sunlight.

Concentrated sunlight has been used to perform useful tasks from the time of ancient China. A legend claims Archimedes used polished shields to concentrate sunlight on the invading Roman fleet and repel them from Syracuse in 212 BC. Leonardo Da Vinci conceived using large scale solar concentrators to weld copper in the 15th century. In 1866, Auguste Mouchout successfully powered a steam engine with sunlight, the first known example of a concentrating solar-powered mechanical device. Over the following 50 years, inventors such as John Ericsson, and Frank Shuman developed solar-powered devices for irrigation, refrigeration and locomotion. The progeny of these early developments are the concentrating solar thermal power plants of today.^[64]

Concentrating Solar Thermal (CST) systems use lenses or mirrors and tracking systems to focus a large area of sunlight into a small beam. This is then used to generate electricity. Moreover, the high temperatures produced by CST systems can be used to provide process heat and steam for a variety of secondary commercial applications (cogeneration). However, CST technologies require direct insolation to function and are of limited use in locations with significant cloud cover. The main methods for producing a concentrated beam are the solar trough, solar power tower and parabolic dish; the solar bowl is more rarely used. Each concentration method is capable of producing high temperatures and high efficiencies, but they vary in the way they track the sun and focus light.

Parabolic troughs are a more cost-effective alternative to parabolic dishes for use in concentrating solar thermal projects.

A solar trough consists of a linear parabolic reflector that concentrates light onto a receiver positioned along the reflector's focal line. The reflector is made to follow the sun during the daylight hours by tracking along a single axis. A working fluid (oil, water) flows through the receiver and is heated to 500 °C before transferring its heat to a distillation or power generation system.^[65] Trough systems are the most developed CST technology. The Solar Electric Generating System (SEGS) plants in California and Plataforma Solar de Almería's SSPS-DCS plant in Spain are representatives of this technology.^[65]

A parabolic dish or dish/engine system consists of a stand-alone parabolic reflector that concentrates light onto a receiver positioned at the reflector's focal point. The reflector tracks the sun along two axes. A working fluid (hydrogen, helium, air or water) flows through the receiver where it is heated to 1000 °C before transferring its heat to a Stirling engine for power generation. Parabolic dish systems display the highest solar-to-electric efficiency among CST technologies and their modular nature offers scalability. The Stirling Energy Systems (SES) and Science Applications International Corporation (SAIC) dishes at UNLV and the Big Dish in Canberra, Australia, are representatives of this technology.

The PS10 solar power tower near Seville concentrates sunlight from a field of heliostats on a central tower.

A solar power tower consists of an array of flat reflectors (heliostats) that concentrate light on a central receiver atop a tower. Focusing is critical and the reflectors track the sun through the day and the year on two axes. A working fluid (air, water, molten salt) flows through the receiver where it is heated up to 1500 °C before transferring its heat to a power generation or energy storage system. Power towers are less advanced than trough systems but they offer higher efficiency and better energy storage capability. The Solar Two in Daggett, California and the Planta Solar 10 (PS10) in Sanlucar la Mayor, Spain are representatives of this technology.

A solar bowl consists of a fixed parabolic reflector that concentrates light onto a receiver which tracks the focus of light as the sun moves across the sky. One has been constructed in Marseilles, France and another in Auroville, India.

Concentrating Photovoltaic (CPV) systems convert concentrated light into electricity by PV rather than heat engines. They also use tracking systems, mirrors, and lenses to achieve high concentration ratios and are able to reach efficiencies above 40%. ^[66] A solar power station planned for Victoria, Australia will use heliostat concentrating PV technology similar to the power tower concept. ^[67]

Concentrating Photovoltaics (CPV) is a term used when sunlight is concentrated onto photovoltaic surfaces for the purpose of electrical power production. Solar concentrators of all varieties may be used for this, often mounted on a solar tracker in order to keep the focal point upon the cell as the sun moves across the sky.

Compared to conventional flat panel solar cells, CPV is advantageous because the solar collector is less expensive than an equivalent area of solar cells. CPV system hardware is typically priced around 3 USD/Watt, whereas silicon flat panels are commonly 5 USD/Watt (not including any associated power systems or installation charges). Semiconductor properties allow solar cells to operate more efficiently in concentrated light, as long as the cell junction temperature is kept cool by a suitable heat sinks. CPV operates most effectively in sunny weather, since clouds and overcast conditions create diffuse light which essentially can not be concentrated.

Low Concentration CPV

Low concentration CPV are systems with a solar concentration 2-10 suns. For economic reasons, conventional silicon solar cells are typically used, and at these concentrations, the heat flux is low enough that the cells do not need to be actively cooled. The laws of optics dictate that a solar collector with a low concentration ratio can have a high acceptance angle, and thus does not require active solar tracking.

Medium Concentration CPV

From concentrations of 10 to 100, the CPV systems require solar tracking and cooling, making them more complex.

High Concentration CPV

These systems have point-focus systems using dish reflectors or (fresnel) lenses that concentrates sunlight in the range of 100 to 1000 suns or more. The solar cells require high capacity heat sinks to avoid thermal destruction, and to manage temperature related performance losses. Solar cells based on Gallium arsenide (GaAs) are favored over Silicon, as they tolerate operating temperatures above 200°C, and performance is less affected by high temperatures. GaAs solar cells designed for non-concentrating space-based satellites are considered unsuitable for terrestrial use, due to the high current density encountered in CPV (up to 8 A/cm²), which requires the cell to be design with a specialized conductor track layout.

Much of the original research into multijunction photovoltaics was sponsored by governments and the astronautics industry. More recently, the technical research and product development of CPV systems has grown due to investment in terrestrial electric generating systems. Recent technological advances in triple-junction solar cells have yielded 40.7% conversion efficiency[1]. Commercial vendor Concentrix has published a module efficiency of 23.5%. Commercial vendor SolFocus has published a module efficiency of over 22%.

See also

• Spectrolab, a manufacturer of CPV cells.

References

• C-Rating project
• Understanding Solar Concentrators
• Soliant Energy
• Solfocus
• Concentrix Solar
• Smart Electric
• Green and Gold Energy
• Pyron Solar
• Arontis Solar Concentrator

Fresnel Lens displayed in the Musée national de la marine in Paris, France

A Fresnel lens (pronounced [freɪ'nel]) is a type of lens invented by French physicist Augustin-Jean Fresnel. Originally developed for lighthouses, the design enables the construction of lenses of large aperture and short focal length without the weight and volume of material which would be required in conventional lens design. Compared to earlier lenses, the Fresnel lens is much thinner, thus passing more light and allowing lighthouses to be visible over much longer distances.

Development

The idea of creating a thinner, lighter lens by making it with separate sections mounted in a frame is often attributed to Georges-Louis Leclerc, Comte de Buffon.^[1] However, it is difficult to find any other sources that link Buffon to work with optics. French physicist and engineer Augustin-Jean Fresnel is most often given credit for the development of this lens for use in lighthouses. According to Smithsonian, the first Fresnel lens was used in 1822 in a lighthouse on the Gironde River in France, Cardovan Tower; its light could be seen from more than 20 miles out.^[2] Scottish physicist Sir David Brewster is credited with convincing The United Kingdom to use these lenses in their lighthouses.^[3][4]

Detailed information

The Fresnel lens reduces the amount of material required compared to a conventional spherical lens by breaking the lens into a set of concentric annular sections known as Fresnel zones.

In the first (and largest) variations of the lens, each of these zones was a different prism. Though a lens might look like a single piece of glass, closer examination reveals that it is many small pieces. It was not until modern computer-controlled milling equipment (CNC) could turn out large complex pieces that these lenses were single pieces of glass.

For each of these zones, the overall thickness of the lens is decreased, effectively chopping the continuous surface of a standard lens into a set of surfaces of the same curvature, with discontinuities between them. This allows a substantial reduction in thickness (and thus weight and volume of material) of the lens, at the expense of reducing the imaging quality of the lens.

Graphic examples

1: Cross section of a Fresnel lens 2: Cross section of a conventional plano-convex lens of equivalent power

Close-up of a lighthouse lens

Uses

Cape Meares Lighthouse (Oregon, USA) first-order Fresnel lens

For the reasons given above, Fresnel lenses tend to be used in applications where imaging quality is not critical, or where the bulk of a solid lens would be prohibitive. Cheap Fresnel lenses can be stamped or moulded out of transparent plastic and are used in overhead projectors, projection televisions, and hand-held sheet magnifying glasses. Fresnel lenses have been used to increase the visual size of CRT displays in pocket televisions, notably the Sinclair TV80. Fresnel lenses are also used in traffic lights and solar forges.

Fresnel lenses can concentrate much more sunlight than normal convex lenses, and melt certain materials and instantly ignite others. Commercial Fresnel lenses are often available from scientific supply stores and are made of bendable plastic. They can be employed in homemade solar cookers and solar collectors to heat water for domestic use.

Perhaps the most widespread use of Fresnel lenses was in automobile headlamps, where they allow the roughly-parallel beam from the parabolic reflector to be shaped to meet requirements for dipped and main beam patterns, often both in the same headlamp unit (such as the European H4 design). For reasons of cost, weight and impact resistance, newer cars have dispensed with glass Fresnel lenses, using multi-faceted reflectors with plain polycarbonate lenses. However, Fresnel lenses continue to be widely used in automobile tail, marker and backup lights.

High-quality glass Fresnel lenses were used in lighthouses; most are now retired from service. Lighthouse Fresnel lens systems typically include extra annular prismatic elements, arrayed in faceted domes above and below the central planar Fresnel, in order to catch all light emitted from the light source. The light path through these elements can include an internal reflection, rather than the simple refraction in the planar Fresnel element.

Glass Fresnel lenses also are used in lighting instruments for theater and motion pictures (see Fresnel lantern); such instruments are often called simply Fresnels. The entire instrument consists of a metal housing, reflector, lamp assembly, and Fresnel lens. A holder in front of the lens can hold a colored plastic film (gel) to tint the light or wire screens or frosted plastic to diffuse it. Many Fresnel instruments allow the lamp to be moved relative to the lens focal point, to increase or decrease the size of the light beam. The Fresnel lens is useful in the making of motion pictures not only because of its ability to focus the beam brighter than a typical lens, but also because the light is a relatively consistent intensity across the entire width of the beam of light.

Aircraft carriers typically use Fresnel lenses in their optical landing system. The "meatball" light aids the pilot in lining up for the landing. In the center are amber and red lights composed of Fresnel lenses. Although the lights are always on, the angle of the lens from the pilot's point of view determines the color and position of the visible light. If the lights appear above the green horizontal bar, the pilot is too high. If it is below, the pilot is too low, and if the lights are red, the pilot is very low.

New applications have appeared in solar energy, where Fresnel lenses are used to concentrate sunlight (with a ratio of almost 500) onto solar cells. Thus the active solar cell surface can be reduced to a fraction compared to conventional solar modules. This offers a considerable cost-saving potential by low material consumption, and it is possible to use high-quality and expensive solar cells, which achieve a very high efficiency under concentration due to thermodynamic effects.^[5]

Fresnel reflectors are also currently being incorporated into next-generation solar thermal energy systems. See solar power for more information. The Polaroid SX-70 camera used a Fresnel reflector as part of its viewing system.

Multi-focal Fresnel lens are also used as a part of retina identification camera, where they provide multiple in- and out-of-focus images of a fixation target inside the camera. For virtually all users, at least one of the images will be in focus, thus allowing correct eye alignment.

Fresnel lens has seen applications in to enhancing passenger reading lights on Airbus aircraft. In a dark cabin, the focused beam of light does not dazzle neighboring passengers.

Fresnel lenses have also been used in the field of popular entertainment. The British rock artist Peter Gabriel made use of them in his early solo live performances to magnify the size of his head, in contrast to the rest of his body, for dramatic and comic effect. In the Terry Gilliam film Brazil, plastic Fresnel screens appear ostensibly as magnifiers for the small CRT monitors used throughout the offices of the Ministry of Information. However, they occasionally appear between the actors and the camera, distorting the scale and composition of the scene to humorous effect.

Lens of Loschen-lighthouse, Bremerhaven

Lens of a lighthouse in Rozewie, Poland

Sizes of lighthouse lenses

Fresnel's lighthouse lenses fell into six orders based on their focal length. The largest (first order) lens has a focal length of 920 mm (36 in), and an optical area 2590 mm (8.5 ft) high. The complete assembly is about 3.7 m (12 ft) tall and 1.8 m (6 ft) wide. The smallest (sixth order) has a focal length of 150 mm (5.9 in) and an optical area 433 mm (17 in) high.^[6][7]

Subsequent development extended this to seventh and eighth orders, an intermediate three-and-one-half order, and two orders even larger than first: mesoradial and hyperradial.

Projection uses

Fresnel lenses of different focal lengths (one collimator, and one collector) are used in commercial and DIY projection. The collimator lens has the lower focal length, and is placed closer to the light source, and the collector lens, which focuses the light into the triplet lens, is placed after the projection image (an active matrix LCD panel in LCD projectors).

Generating solar power

Fresnel reflectors are used in Concentrated Solar Power (CSP) plants to produce energy from the sun.

References

External links

• Lighthouse Getaway: Fresnel lens (contains photographs.)
• Random Destructive Acts via Focused Solar Radiation.