The theme of this article is devoted to the analysis of development of optical concentrators – of important components, which allow to increase energy effectiveness of the thermal and photovoltaic solar power systems and to lower the cost of production of the thermal and electric energy from the energy of sun radiation. During two decades already the concentrator technology are found in the stage of speed-up development and not anymore need the special proofs for confirmation of rights to the existence. A question is in that, whether we will be able rationally to take advantage of the possibilities, which are in the basis of these technologies and in that time does cause not the harm to the surrounding and not prang the tailings of shaky equilibrium between human activity and nature.

Opening of theme is conducted in the following sections:

-                Sun radiation and solar concentrator technologies;

-                Determination and classification of optical concentrators;

-                Refractive and mirror concentrators;

-                Diffraction (holographic) concentrators;

-                Waveguide concentrators;

-                Luminescent concentrators;

-                Nanostructure concentrators.

Sun radiation and sun power technologies. The solar radiation is energy of electromagnetic radiation of wide range of spectrum that is formed as a result of nuclear reactions, which flow in the body of the Sun and convert one type of particles into other. On Earth we deal with the some changed types of sun radiation. Direct radiation is a sun energy that acts on a body without the change of direction. Diffuse radiation is a sun energy that acts on a body after the change of direction as a result of reflection and dispersion by an atmosphere. Absorbability is a part of sun energy that is taken by a surface, on which it falls. Transmittance is a part of sun energy that is skipped by a body. Reflectance is the part of sun energy that is reflected by a body. 

The sun radiation is mainly concentrated in the range of wavelengths of 0,28…3,0 µm. His spectrum consists of ultraviolet radiation, concentrated in the range of wavelengths of 0,28.0,38 µm (~ 2 %), visible radiation, concentrated in the range of wavelengths of 0,38…0,78 µm (~ 49 %) and infra-red radiation; concentrated in the range of wavelengths of 0,78.3,0 µm (~ 49 %). Sun energy is characterized by a solar constant (~ 1,365…1,367 kW/m²) – the amount of energy of the Sun that would be incident during 1 s on a plane (1 m²) perpendicular to the rays, at a distance of one astronomical unit.

The sun radiates approximately 1,1x10²⁰ kWh in a second. The external layers of atmosphere intercept approximately one millionth part of this energy, or 1,5 x 10¹⁸ kWh annually, providing by 10000 times more energy, than is today consumed in a whole world.

Sun energy can be transformed into thermal and electric forms of energy by means of the known technologies, the most applied among them thermal and photonic. Countries - the leaders of introduction of sun energy conduct the research and developments in three basic directions: photovoltaic technologies and equipment (PV), concentrating photovoltaic technologies and equipment (CPV) and concentrating thermo-voltaic technologies and equipment (Concentrating Solar Power - CSP).

The highest achievement of solar power engineering became the creation of Solar Power Plant (SPP), which convert energy of sun radiation into the thermal and electric one by means of receivers of solar energy. Photovoltaic Solar Power Plant, which provides a direct transformation of sun energy into electric energy, is the typical example of industrial sun technologies. As the receivers of sun energy, in particular, photovoltaic solar cells on the basis of photoeffect are used. Considerable lacks of Photovoltaic Solar Power Plant – low energy effectiveness and high cost of production of electric energy - displaced the attention of developers and investors onto the concentrating technologies of production of electric energy.

In the last decades are realized or are in the stage of realization a few projects of the Concentrating Photovoltaic Solar Power Plants and Thermodynamic Solar Power Plants, in which the energy of sun radiation is used as a source of heat in the thermodynamics cycle of transformation of thermal energy into mechanical by means of Solar Steam Generator and then into electric energy. The Thermochemical Cycle of Solar Energy Conversion consists of the successive reverse endothermic and exothermic reactions, in which on the first stage the sun energy is expended in the endothermic reactions, and. on the second stage, the energy, emitted at the exothermic reactions, is passed to the user.

The progress of introduction of sun technologies is largely determined by the use of optical concentrators in general architecture of construction of the solar power systems.

Determination and classification of optical concentrators. The optical solar concentrator is the structural element intended for concentration of energy of sun radiation onto the receiver of solar energy. One or the aggregate of concentrators forms the optical concentrating system.

The optical concentrators allow to collect the maximal amount of sun radiation and to direct it at the receiver of sun energy. Thus there is possibility to decrease the mass and sizes of receiver of sun energy and to use though a high-expensive, but more effective photovoltaic transformer. Additionally, the greater density of sun radiation multiplies an energetic efficiency of transformation of sun radiation into electric energy.

Optical concentrators can be classified in accordance with many signs, that lie in the basis of construction and work of these elements: in accordance with physical principle of inclination of optical rays; in accordance with spectral composition of sun radiation, that is concentrated on the receiver of sun energy; in accordance with angular aperture of the optical system of «collection» of direct and inclined rays; depending on the form of working surface of optical element and physical features of forming of outcoming wave front that are used for the correction of the optical beam; depending on the form of the focused optical spot and on the form of receiver of sun energy; in accordance with the amount of the parallelly located focusing elements in the optical concentrating system; according to the amount of the focusing elements in the optical concentrating system, located consistently on the way of the motion of optical rays; in accordance with physical principles, that lie in the basis of origin of eventual radiation; in accordance with the features of distribution of wave front; depending on chemical composition of materials that are used for making of optical concentrator; depending on the correlation of wavelengths and geometrical sizes of elementary structural formations, accountable for the inclination of optical beam in concentrators.  

In accordance with physical principles, that lie in the basis of inclination of optical rays, optical concentrators can be classified into refractive, reflective (mirror) and diffractive (holographic) elements.

Relating to a spectral composition of sun radiation, that is going on the receiver of sun energy, sun concentrators are divided into the device of ultraviolet, visible, infra-red, multispectral and monochromatic radiations.

Depending on the angular aperture of the optical system and possibility of collection of direct and inclined optical beams, in this number, dissipated sunbeams, the optical concentrators can be classified onto the constructions of narrow and wide apertures.

In accordance with the form of working surface that are used for the correction of aberrations of optical beam, the concentrators elements are divided into spherical and aspheric (cylinder, parabolic, hyperbolic et al).

Depending on the form of the focused optical beam and of the form of receiver of sun energy the concentrators are divided into the point and linear elements.

In accordance with the amount of the parallelly located focusing elements in the optical concentrating system the concentrators are divided into single-element and multiple elements.

Depending on the amount of the focusing elements in the optical concentrating system, located consistently in the direction of motion of optical rays, the concentrators are divided into single-cascaded and double-cascaded.

Relating to the physical principles that lie in the basis of origin of resulting radiation, concentrators are classified into the elements of the primary and secondary radiations. The concentrators of the secondary radiation are yet named luminescent.

In accordance with the features of distribution of wave front during concentration of sun radiation the optical concentrators are divided into wave-guided and volume elements.

In accordance with the chemical composition of materials that are used for making of optical concentrator, the sun concentrators can be divided into organic and inorganic (polymeric).   

Depending on the correlation of wavelength of optical radiation and geometrical sizes of elementary structural formations accountable for the inclination of optical beam, the sun concentrators can be divided into macroscopic and nanostructured.

It is necessary to mark that the division of sun concentrators onto certain groups is conditional and does not represent in the complete measure of their variety.

The prospects of development and introduction of concentrators. During a few decades the optical concentrators are developed in the direction of improvement of classic technologies and of the creation of principally new concentrating devices, in that number, based on integration in one sample of possibilities of a few technologies with the purpose of reduction of prices and increasing of their availability for users. The developments of optical concentrators are based on the last achievements of physics of solid, applied optics, holography and quantum electronics.

The most used nowadays are sun concentrators of the first generation – refractive on the basis of glass or polymeric lenses and mirror concentrators. To the refractive concentrators such failings are peculiar: a presence of chromatic aberrations, large mass, complication of making and accordingly high cost, therefore they did not find of wide application. In the mirror concentrators the chromatic aberrations are absent. A correction of other curvatures of wave front, that behave to aberrations of 3-th and higher orders, is achieved by the use of aspheric (parabolic, hyperbolical and other) surfaces. Multielements mirror dish concentrators and aspheric linear (cylinder, parabolic-cylinder, hyperbolic-cylinder) concentrators are the most widespread along with concentrators on the basis of flat mirrors (heliostats), that are used for collection of radiation in solar tower power plants.

The second generation of sun concentrators is related with the use of diffraction holographic elements. To these diffraction holographic concentrators the holographic focusing elements (Fresnel lenses) and linear diffraction gratings belong, which are used for coupling of radiation with the optical waveguides. Holographic concentrators are made by holographic method, by means of the photographic record of interference picture that arises up at interaction of objective and plane wave front laser beams on a photographic plate, with a next photochemistry treatment, and by means of method of computer synthesis of holograms with the subsequent mechanical making. The main advantages of holographic concentrators are small thickness and low cost. The limitations of holographic concentrators are low diffraction efficiency in the wide spectral range of sun radiation and also the presence of chromatic aberrations of sun beams.

Subsequent development of optical concentrators is related with the combination of holographic and waveguide technologies of concentration of optical radiation, with the creation of luminescent waveguide concentrators. Luminescent waveguide concentrators are based on the absorption of sun radiation by luminescent atoms with the next emission of the secondary radiation and directing him onto the small photovoltaic sun element located on the end of waveguide plate.

At creation of high effective and inexpensive optical concentrators the specialists however lay most hopes on the use of achievements of nanotechnologies. Today on the stage of researches there is a plenty of nanostructure elements (quantum points, nanowires, nanorods, nanotubes) and new materials on their basis, which possess by new useful properties (electrical, magnetically, photovoltaic), not inherent for the ordinary materials. These nanostructures can be used for creation of the high-effective integrated photovoltaic sun devices with built-in nanostructured elements, with possibilities of concentrator and absorber.

Vasil Sidorov on May 10, 2010 from Technopark QUELTA

in Queltanews. sidorovvasil@gmail.com