Creation of large-scale manufacture of polycrystalline silicon for PV converters demands not only uses of more powerful sources of solar radiation and application of more powerful concentrating systems.

There is an imperative need of search of the new technical decisions providing an effective supply of a concentrated sunlight to a silicon for melting. The important factor is also a necessity of maintenance of maximum active interaction between volume of melt and gas — liquid border which provides active formation of volatile impurities contained substances and their intensive removal into a gas phase.

It is possible to apply for these purposes the big solar furnaces (BSF) adapted to the technological aims, such as, for example, BSF of Weizman Institute ( Rehovot, Israel) [5], BSF of the National Centre of Scientific Researches of 1000 kW (Odejo, France) [6] and BSF with 1000 kW thermal capacity of the Academy of Sciences of Republic of Uzbekistan (Parkent, Uzbekistan) [7].

In all specified installations technological operations are carried out in a special technological tower. A furnace in Odejo and a furnace in Parkent represent a design executed with a horizontal optical axis. It consists of parabolloid mirrors and heliostates systems..

One of primary goals of the BSF in Uzbekistan, in particular, is the possibility of performance of basic researches of physical and chemical processes at heat treatment of various objects by the concentrated light flow in a spectrum of a sunlight, i. e. studying of the mechanism of
influence of photons of a spectrum of a sunlight (Е = 0,4-5,0 eV, Solar radiation up to 1,7.103 Wt/cm2) on various objects.

At the same time, the structure of the available equipment provides an opportunity of manufacture of large portions (up to tens tons) materials as powders, granules or billets. It allows to use such installations both for industrial production of silicon, and for researches. Transmission of a solar energy along horizontal optical axis of BSF does not allow in this case to create a stable bath of melt at an end of an ingot as it was described in [2]. In this case also it is difficult to provide stable removing of the fused zone from focus of the solar furnace for crystallization of silicon. Another than described in [2] technical decisions for increase of the area of contact of melt volume with its surface with the purpose of an intensification of process of silicon purification by removal of impurities into a gas phase are required.

In this case the following scheme of technological process performance can be represented (Fig.2).

Fusion of silicon is carried out in the rotating drum-type furnace-crucible. The supply of a solar energy is carried out through a special aperture in the end face of a drum.

Silicon melted in the rotated furnace-crucible under solar energy influence may form shapes presented at fig.3 [8].

The most active mixing and liquid volume-surfase interraction take place for condition «b”. But in this case direct concentrated sunlight can reach open surface of internal furnace isolation and destroy it. So the case «c» is cosidered as the most appropriate for practical use [8].

The fused silicon from the rotating furnace after proccesing goes to a crystallizer.

In a crystallizer with special screens the gradients of temperature providing formation of silicon billets with columnar structure are created. Seeds at the bottom of a crystallizer provide given crystallograf orientation of big monocrystalline grains.

It is possible to expect, that the material obtained by these means will meet SOG-Si requirements.

c

Fig.3. Possible shapes of melted silicon within furnace-crucible (section across it axis) during rotation.

a — n> (K/2 n)(Vg/R); b — n<(K/2 n)(Vg/R); c- n&(K/2 n(Vg/R), where n — speed of rotation; K-coefficient; g-gravity acceleration coefficient; R-radius of internal surface of melted silicon.

The specified process equipment can be applied as well to obtaining of silicon directly from quartz by carbothermic reduction with the subsequent purification before casting in a crystallizer.

Work in this direction requires participation of the organizations having at their disposal necessary solar installations (BSF), possessing experience in creations and operation of the process equipment on silicon manufacture and on fabrication of solar cells on its basis. Realization of research projects possible on a multilateral basis within the framework of the international programmes would allow to bring in the significant contribution to development of a solar industry.