Concentrating Photovoltaics (CPV) |
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PrincipleIn Concentrating Photovoltaics (CPV), a large area of sunlight is focused onto the solar cell with the help of an optical device. By concentrating sunlight onto a small area, this technology has three competitive advantages:
Concentrating light, however, requires direct sunlight rather than diffuse light, limiting this technology to clear, sunny locations. It also means that, in most instances, tracking is required. Despite having been researched since the 1970s, it has only now entered the solar electricity sector as a viable alternative. Being a young technology, there is no single dominant design. The most common classification of CPV- modules is by the degree of concentration, which is expressed in number of "suns". E.g. "3x" means that the intensity of the light that hits the photovoltaic material is 3 times than it would be without concentration.
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ConcentrationHere are some examples of concentrator technologies and examples for both line and point concentrators. Although there might be differences in execution or materials used, most designs will follow one of those concepts.. Fresnel LensA Fresnel lens, named after the French physicist, comprises several sections with different angles, thus reducing weight and thickness in comparison to a standard lens. With a Fresnel lens, it is possible to achieve short focal lenght and large aperture while keeping the lens leight. Fresnel lenses can be constructed
With the high concentration ratio in a Fresnel point lens, it is possible to use a multi-junction photovoltaic cell with maximum efficiency. In a line concentrator, it is more common to use high efficiency silicon. |
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Parabolic MirrorsHere, all incoming parallel light is reflected by the collector (the first mirror) through a focal point onto a second mirror. This second mirror, which is much smaller, is also a parabolic mirror with the same focal point. It reflects the light beams to the middle of the first parabolic mirror where it hits the solar cell. The advantage of this configuration is that it does not require any optical lenses. However, losses will occur in both mirrors. SolFocus has achieved a concentration ratio of 500 in point concentrator- shape with dual axis- tracking. |
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ReflectorsLow concentration photovoltaic modules use mirrors to concentrate sunlight onto a solar cell. Often, these mirrors are manufactured with silicone-covered metal. This technique lowers the reflection losses by effectively providing a second internal mirror. Low concentration cells are usually made from monocrystalline silicon. No cooling is required. The largest low-concentration photovoltaic plant in the world is Sevilla PV with modules from three companies: Artesa, Isofoton and Solartec. |
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Luminescent ConcentratorsIn a luminescent concentrator, light is refracted in a luminescent film, and then being channelled towards the photovoltaic material. This is a very promising technology, as it does not require optical lenses or mirrors. Moreover, it also works with diffuse light and hence does not need tracking. The concentration factor is around 3. There are various developments going on. For instance, Covalent are using an organic material for the film, whilst Prism Solar use holographic film. Furthermore, this concentrator does not need any cooling, as the film could be constructed such that wavelenghts that can not be converted by the solar cell would just pass thru. Hence, unwanted wavelenghts would be removed. |
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CoolingMost concentrating pv systems require cooling. Passive Cooling: Here, the cell is placed on a cladded cermaic substrate with high thermal conductivity. The ceramic also provides electrical isolation. Active Cooling: Typically, liquid metal is used as a cooling fluid, capable of cooling from 1,700°C to 100°C. |
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Εxamples
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