The necessity of creation of powerful solar plants needs in the use of large beams of sun radiation that are caught by large optical focusing elements – sun concentrators. 

The sun concentrators of the first generation - the refractive concentrators on the basis of glass or polymeric lenses and the mirror concentrators are today used most often and possess good quality. The first experience of creation of refractive and mirror elements was accumulated during developments of large telescopes, of astronomic devices that are used outside an earth atmosphere, and also of systems of the remote sensing of atmosphere. 

Refractive solar concentrator represents by itself a lens or group of lenses – the optical details made from transparent material limited by two surfaces, the index of refraction of which differs from the index of refraction of environment. From the points of view of physical optics the action of optical surface is determined in the change of direction and form of wave front, or in the change of direction of motion of sunbeams in notions of geometrical optics. An optical surface is a staple accountable for concentration of radiation, therefore large attention of producers is applied on the quality of the optical surface (accordance of optical surface to the theoretical form). For high-quality surfaces a possible deviation of form of surface from theoretical is determined by a few hundredths of wave-length of the solar radiation or by a few hundredths of micrometer.

During last years the spherical and flat surfaces prevailed in the imaging and concentrating optical systems. It did not stop the development of the optical systems, but directed the researches at that way, on which for many optical systems the near theoretically possible values of parameters were attained.

It is necessary to mark, the possibilities of the use of spherical surfaces in the concentrating systems are large, but not boundless. The intricate problems of creation of sun concentrators can be decided only subject to the condition of the use of the other types of surfaces – aspheric, in other words, no spherical. The aspheric surfaces allow to improve optical parameters, to decrease the amount of optical surfaces in a general chart and, thus, to simplify construction of the concentrating system, to decrease mass and overall sizes.

A number of firms and universities works on the development of the high effective multielement concentrators on the basis of glass (Soliant Energy, Stellaris Corporation, Pacific Solar Tech, University of Illinois, Wakonda Technologies, and Practical Instruments) and polymeric (Sunengy) microlenses. It is reported (Jeffrey M. Gordon) about achievement of concentration of radiation, equivalent to the 5000 «suns» at the use of multielement concentrators on the basis of glass microlenses in combination with energy effective heterostructure photovoltaic sun elements with an area near 1 mm². 

The concentrators formed from Fresnel surfaces are the special type of refractive concentrators. A Fresnel lens can be examined as one of not traditional elements of aspheric optics. Possibility of its creation practically on any surface, in that number, on a plane, is the positive sign of Fresnel lens. This advantage allows considerably to decrease its thickness at large diameters and, accordingly, to decrease mass in comparison with the ordinary refractive lenses of similar sizes. In the case of the concentrating system some decline of requirements to exactness of Fresnel surfaces is possible. It allows their making from organic glass by the highly technological methods of pressing or casting and, thus, to extend an application domain, in particular, at creation of concentrating solar devices. To this time the wide use of Fresnel lenses was found only in the optical systems of lighting devices, in particular, in lighthouses, traffic-lights and some condenser devices. The limited application of Fresnel lenses in the imaging systems can be accounted for by the limited possibilities of correction of aberrations, after the exception of spherical one. 

Enterprises intensively work on the development of multielement concentrators on the basis of Fresnel glass (Concentrix Solar GmbH) and polymeric (Amonix, Inc., Entech Solar) microlenses. Similar microlenses provide collection of radiation at level of 500 «suns» and integration with compact photovoltaic sun elements with energy effectiveness of transformation at level of 38%. Sharp Electronics represented a sun panel, which consists of multielement concentrator on the basis of 270 Fresnel lenses and photovoltaic solar cells on the basis of gallium arsenide. Electric power of panel makes 2,9 kW.

To the refractive concentrators peculiar such failing, as inhomogeneity of optical material, presence of air bubbles and optical indignations, that show up in dispersion of optical radiation. At large sizes and mass of details the removal of these failing becomes a large technological problem. Except for that, making of refractive elements is linked with the use of complicated technological processes, beginning from the receipt of glass purveyance, mechanical polishing and concluding by the interference process of quality control. Taking into account the necessity of overcoming of influencing of chromatic aberrations, which are peculiar for refractive elements, it is possible to understand the reason of that, why refractive concentrators did not find wide application in the industrial samples of the solar power systems. The multielement systems, made from a plenty of the small lenses located in one plane and concentrating a sun beam on the separate receivers of sun energy, possibly, are the exception from the rules. 

Mirror sun concentrators in a greater measure correspond to the requirements of the solar power systems.

As a mirror concentrator one or, on occasion, two mirrors are used.

A single concave mirror concentrator is most widespread. The advantage of mirror concentrator is the absence of chromatic aberrations. A correction of other curvatures of wave front caused by aberrations of 3rd and higher orders is achieved by the use of aspheric (parabolic, hyperbolical and other) surfaces, the form of which is created by the rotation of the curved lines of the second order around their axes. It is needed to mark that only a concave parabolic mirror creates the stigmatic image of a remote point that is found on his optical axis. Therefore in practice in sun concentrators the parabolic mirrors are always used. But a parabolic mirror revolts the image of point of object, which does not lay on an optical axis, under action of aberrations such as comma and astigmatism. This factor demonstrates the real difficulties of concentration of optical rays of no point-type irradiative object, which the Sun is.

At creation of especially exact optical concentrators the principles of adaptive optics based on the synthesis of classic methods of optics and control theory can be used. Application of such principles allows to achieve high quality of concentration of radiation in the conditions of action of external factors, for example, thermal deformations of optical surfaces, indignation of air environment, by the use of phase adapters.

In a single mirror concentrator the receiver of sun energy is disposed in a focal plane, resulting in screening of central part of mirror. Certainly, the size of receiver is insignificant in comparison with the diameter of mirror; therefore the central screening is unimportant.   

In accordance with the type of parabolic mirror the sun concentrators are divided into Parabolic Dish Concentrators, the form of which is created by the rotation of parabola around their axis, and Parabolic Trough Concentrators, the form of which is created by a parabola that is displaced parallel to itself.

The manufacturing of Parabolic Dish Concentrators of large sizes is related with difficulties, therefore in practice the Mirror Faceted Concentrator that consists of separate glass mirrors of flat or curvilinear form is used. As a reflecting coverage the silver or aluminum are usually used.

Mirror Faceted Dish Concentrators collect the sun radiation in a focal plane into a point spot. Parabolic Trough Concentrators collect the sun radiation in a focal plane into a line; therefore they often are yet called linear concentrators.

The double mirror concentrators are less widespread. The rays of sunlight fall on the primary mirror that is called main. The main mirror reflects the rays on the secondary mirror. The common focal distance of double element concentrator is called an equivalent focal distance. Each mirror is not flat and changes wave front and aberration of optical beam. The primary mirror forms the image of irradiative object (the Sun) in the point of the main focus, and the second mirror carries him into the point of the second focus.

Cassegrain concentrator is double elements construction. This optical system provides a comfortable disposure of receiver of sun energy beside of a main mirror. The general length of the system is considerably less than equivalent focal distance of the system. For passing of light to the receiver of sun energy in a main mirror the central hole is executed. The system made from two mirrors provides small losses of sun radiation at reflections.

Parabolic Dish Concentrators, in particular, are used in thermal power solar plant on the basis of small Stirling engines-generators (Stirling Energy Systems, Infinia). The batteries of parabolic mirrors focus sunbeams on the receivers located in the point of focus of every dish. A liquid in a receiver is heated to the temperature of 1000 ^oC and directly used for production of electric power with the help of the generator, connected with a receiver. The working equipment assures electrical power of 7.25 kW. High power efficiency (29%) and small initial charges do the Stirling systems today the most effective among all solar technologies.

Combining of Parabolic Dish Concentrators and photovoltaic sun elements with the purpose of creation of the Concentrating photovoltaic module and, in an ideal, concentrator photovoltaic solar power plant (Solar Systems, Australia; Instituto de Concentracion Fotovoltaica, Spain) is considered yet more perspective. The concentrators of such systems multiply by 500…1500 times the density of sun radiation on the high-effective geterostructure semiconductor receiver of sun energy. The concentrating photovoltaic systems have considerably less sizes and are more energy effective (26%) in relation with the ordinary photovoltaic systems. They also are considerably cheaper and need less investment on the stage of production. For surveillance of concentrators and receivers after the Sun the two-axis heliostats are usually used. 

Another well-known optical concentrator - Parabolic Trough is a main part of thermal-electric sun power plants (Flagsol, Germany). Parabolic-cylinder mirror cuvettes concentrate light onto receivers-tubes filled by liquid. This liquid is heated till about 400 ^oC and pumped through the row of heater-exchangers; the overheated steam is here produced that rotates a turbine - electric generator. With the purpose to decrease the losses the receiving tube can be surrounded by an additional glass tube. Parabolic-cylinder concentrators usually include at itself a one-axis or two-coordinate system of surveillance after the Sun (Solargenix Energy). SkyFuel represented the samples of parabolic-cylinder concentrators on the basis of plastic substrates with the special mirror coverage, which provide considerable reduction of general mass of equipment.

Multibars linear Fresnel concentrators that appeared in last time at the market (Ausra) are the special type of mirror parabolic-cylinder concentrators. In a linear Fresnel system the large diameter half-pipe is replaced with a series of long narrow strips of mirrors side-by-side on the ground, which concentrate the light on the receiver line. As the Sun moves from east to west, the individual mirror rows adjust their tilt to concentrate the sunlight onto the static receiver, which does not move - unlike in a parabolic trough system, where the receiver moves with the mirrors. Multielement concentrators on the basis of flat mirrors (heliostats) are the separate group of concentrators that are used for collection of radiation in Solar Tower Power Plants (SolarReserve).

Large sizes and mass of refractive and mirror sun concentrators of the first generation is the substantial failing, which force developers to conduct the search of other concentrating technologies on the basis of thin films and nanostructure materials compatible with integral technologies of making of semiconductor sun elements.

Vasil Sidorov on May 17, 2010 from Technopark QUELTA

in Queltanews. sidorovvasil@gmail.com