Category Solar Collectors and Panels, Theory and Applications

Market & prospective

Large installations of CPV are not yet common. Until the end of 2009, about 21MW of CPV systems have been reported as set on Sun (Kurtz, 2009; Extance & Marquez, 2010); large part of them (13 MW) are from one HCPV technology of modules based on Fresnel lenses concentrator developed by Amonix Inc. in the last twenty years; the fraction of operational systems with low concentration level (LCPV) is less then 1MW, mainly of Entech Solar products installed in the 90’s. Although at the end of 2009 around 70 vendors of photovoltaic concentrator systems have been found (EPRI, 2009), the CPV is a small niche of the photovoltaic market; indeed, at the end of 2009 already 7 GWp of PV modules have been installed and grid connected around the world...

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Systems

The CPV system is composed of many parts which must cooperate efficiently; generally, the modules or assemblies must follow the Sun in its apparent motion, to ensure the collection of the direct irradiation from the cells, through the optics. The possibility of the concentrators to catch only the direct portion of the sunlight, with an additional circumsolar light dependent on the acceptance angle of the optics, is an important limitation for the CPV respect to standard photovoltaics. Diversely, the necessity to follow the Sun is not generally a limitation; indeed tracking installations are already in fields for standard, flat plate modules too...

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Solar cells assemblies

In general, the cells for concentration are assembled on supporting substrates, treated similarly to bare dies in electronic technology. So, the process is completely different to that for standard PV assembling, but can take advantages by the huge progresses, standardizations and experiences collected during the last decades by the electronic devices industry.

Depending on the cells nature (materials, sizes and manufacturing technologies) and on the operative working conditions, different mounting technologies are used. Generally, the surface mounting technologies (SMT) directly derived from power electronics are applied. Even in this particular subset of components there’s plenty of different solutions...

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Solar cells of III-V materials

The highest conversion efficiency for solar cells has been obtained with the multijunctions approach. Through epitaxial growth the deposition of crystalline layers of compound semiconductors is possible whenever specific requirements on the lattice parameter are satisfied (Yamaguchi, 2002). Many layers of different semiconductors are stacked in order to create a structure where the first layers appear transparent at the light absorbed by the semiconductors at their bottom. This is obtained decreasing the band gaps of the compound semiconductors, from the frontal surface to the rear. The Germanium is often used as substrate material, both for its lattice parameter as well as for its band gap adapt for the bottom cell function...

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Silicon solar cells

High efficiency silicon solar cells have been manufactured since the 80’s (Green, 1987). These cells were manufactured in labs with microelectronic technology steps and with ultrapure crystals, in order to allow for the maximal performances; efficiency in the order of 27% have been achieved for back contact solar cells under around 100x and in the order of 25% under around 250x for cells produced by Amonix Inc. (Yoon et al., 1994). However, the fabrication processes required for these cells is expensive, and the ultimate device cost is comparable to that for multijunction solar cells on III-V semiconductors. Mainly for this reason the back contact technology is no longer used for CPV under the mentioned value of concentration; Sunpower Corp...

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Solar cells

The solar cells used in CPV are made with many different technologies, depending on the kind of used concentrator. In general, for low and medium concentration level, up to about

300 Suns, cells made of Silicon are still used; for higher concentrations, cells based on III-V semiconductors are usually employed; these latter cells allow for efficiency in the order of 40% and find their natural application under high concentration. Due to the high cost of the base materials and processes, these ultra-high efficiency cells found application for space satellites and for terrestrial concentrators...

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Luminescent concentrators

The aforementioned solutions and methods to concentrate the light are not the only developed for photovoltaic applications. One important limitation of these designs is the necessity to use tracking structure to follow the sun. This constrain must not be considered always a limitation; indeed, especially for utility scale installations, tracking structure are used for standard flat plate modules too, in order to improve the energy harvesting, being always on the plane perpendicular to the sunrays...

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Dichroic concentrators

The idea to split the sun spectrum in light beams and to drive theem toward different cells of selected material is not new. As well as the idea of concentrating the light, it can be realized in a number of different configurations; the constrains for its implementations are mainly related to the costs of these assemblies, considering that additional complexities are introduced; indeed, to split the solar spectrum, two physical ways are possible: dispersion through a transparent prism or reflection/transmission through dichroic filter working for light interference. The light is concentrated too, in order to reduce the costs of the cells dedicated to defined wavelengths...

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