The possibilities of production of electric power with the help of only electronic photovoltaic technologies are very attractive. But it is necessary to admit that this fantastically large attractiveness can not be in a complete measure realized in tearing off from optical methods and optical devices. Optical methods and optical elements when integrating with photovoltaic elements allow to create sun power technologies, in which parity between energy effectiveness and cost is achieved.

On today the wide distribution in the systems of sun energy was got only by optical concentrators of the first generation, in particular, refractive and mirror optical devices. They allow to attain the high level of concentration of sun radiation and to use a small quantity of high effective photovoltaic material. Unfortunately, concentrators of the first generation have large sizes and mass and need the use of labor intensive methods of tooling at making and are not compatible with the integral technologies, which are used today in production of semiconductor photovoltaic receivers of sun energy.

Thin film diffraction holographic concentrators are the real alternative to macroscopic refractive and mirror concentrators. But they are characterized by considerable chromatic aberrations and relatively low diffraction efficiency and for entering to the power market they need in conducting of additional researches. 

The new possibilities in creation of the concentrating photovoltaic systems appear at combining of linear diffraction holographic structures with optically transparent thin film optical waveguide concentrators. In this direction, in particular, researchers and specialists of Nizhyn Laboratories of Scanning Devices are working.

Linear diffraction grating is the regular diffraction structure, which represents by itself an interchange of the transparent and opaque lines with a permanent step between them. The step of diffraction grating (distance between diffraction lines) determines the angle of diffraction of nth order. The wider is the step of diffraction grating, the smaller is the angle of diffraction of sun radiation that falls on the grating surface at right angles (Fig. 1). A regular diffraction grating is more technological in comparison with focusing diffraction elements and diffraction grates with a variable step between diffraction lines and can be made, for example, by a holographic method. With the purpose of minimization of sizes of high-cost crystalline photovoltaic receivers such linear diffraction grating is deposited on the upper surface of thin film optical waveguide concentrator, from the side of falling of sun radiation (Fig. 2).

Excitation of optical wave in the volume of optical waveguide takes place due to diffraction of sun radiation that has flat wave front on the diffraction structure of grating. The step of grating is the distance between neighboring transparent or dark lines gets out such, that wave, excited, for example into he +1st order of diffraction, goes out from a diffraction grating under the angle of φ_(-1) › 90⁰ – β_TIR, where β_TIR  - the angle of total internal reflection (TIR), a constant value for the given waveguide and wave-lengths. It is necessary to take into account that maximum of intensity of radiation that is pumped over in +1st order is determined by the angle of incidence of flat wave front on a diffraction structure. The waves excited thus spread in a thin film optical waveguide in a direction, perpendicular to the direction of linear diffraction structure and are concentrated on the edge ends of waveguide.

The certain advantages in transporting of optical waves are inherent to the three-pellicle optical waveguide (Fig. 3,a) and gradient optical waveguide (Fig. 3,b).

The three-pellicle flat optical waveguide (Fig. 3,a) consist of three layers 1, 2 and 3 of very thin optically transparent material. It is characterized by the stepped change of index of refraction on the interface of central layer (core of waveguide) and upper and lower layers. Thus the index of refraction of n₂ of core layer 2 is higher than an index of refraction n₁ of upper 1 and lower 3 layers. The propagation of optical rays in waveguide is accompanied by the stepped change of the direction with a next exit from a waveguide (ray 1¹), by the reflection on the interface «layer 1 – air» (ray 2¹) and by reflection on the interface «layer 2 – layer 1» (ray 3¹).

The three-pellicle optical waveguide (Fig. 3,a) with the stepped change of index of refraction on the interfaces is idealizing models, from which the real waveguide differ strongly. Even, in the case, when initial materials used for the core and shell are homogeneous, at heating and drawing out of tapes there is diffusion of material that washes out boundaries between the core and shell layers. At the certain terms a gradient waveguide own the best properties for concentration of radiation, than waveguide tapes with the stepped change of index of refraction.

Photovoltaic cells set on the edge ends of similar waveguide take in the solar radiation and convert it into electric energy. The described construction of the photovoltaic module with flat waveguide concentrator (Fig. 4) is very effective and allows decreasing the use of photovoltaic crystal material, for example, silicon. 

Another architecture of the concentrating sun module is offered by Prism Solar Technologies, Inc. (Fig. 5). The sun module developed by a firm consists of two glass plates, between which the rows of thin holograms are located. An upper plate executes the role of optical waveguide, and a lower plate serves in quality of a substrate. On the lower side of substrate, in the gaps between holograms, silicon solar PV cells are located. The hologram of the module is created by the record of interference pattern that arises up at interference of two monochromatic beams of laser radiation. In a concentrator element it executes the roles of spectral filter and concentrating diffractive element. A hologram reflects the sun radiation of certain wave-length onto the internal surface of waveguide. Later the sun radiation spreads in waveguide due to action of phenomenon of the total internal reflection. At the entering in an optically transparent window between holograms the sun radiation falls on a photovoltaic silicon element.

The researchers of University California, San Diego offered the combined chart of waveguide concentrator, in which the sun radiation is entered into the waveguide by means of a plenty of microlenses (Fig. 6). Microlenses are located in one plane and convert the flat wave front of sun radiation into a plenty of small spherical wave fronts. The lower surface of waveguide is structured by prismatic elements so that in focus of every microlens there is a mirror microprizm with an angle at a top of 120⁰. The rays deflected from mirrors are sent onto the edge end of waveguide. Calculations testify that at the use of microlenses by a diameter 2,5 mm and focal ratio F/2.45 (f:) at length of waveguide of 200 mm the efficiency of concentration of radiation arrives of 89%. At a length of 600 mm the efficiency of concentration of radiation goes down to 81,9 %. The decrease of efficiency of concentration at lengthening of waveguide is explained by the losses of radiation at second reflection from the structured (by prisms) lower surface of optical waveguide. It is foreseen, that at the use of polymeric lenses an optimum correlation will be provided between a cost and an energy effectiveness of this combined concentrator and accordingly of the sun photovoltaic module (January 2010 issue journal Optics Express).

Without regard to the variety of architectural decisions, the concentrating photovoltaic sun modules on the basis of waveguide structures are to own the following functions: maximal «collection» of direct and dissipated sun radiation of wide range of light-spectrum in a small spatial volume; transporting of the concentrated sun radiation to the photovoltaic cell with the smallest losses; effective transformation of energy of the concentrated sun radiation into an electric energy.

For embodiment of the named functions the diagram of such concentrating photovoltaic solar module, as a rule, consists of three basic parts: concentrator, waveguide and photovoltaic receiver. On occasion the functions of concentrator, waveguide and photovoltaic receiver are not spatially parted and integrated in one segment.

Vasil Sidorov on May 18, 2010 from Technopark QUELTA

in Queltanews. sidorovvasil@gmail.com