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Fig. 5: Module processing procedure in Freiberg
© Sunicon

Fig. 6: Yield from the recycling process
© Sunicon
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Reprocessing

From the pilot system…

The researchers developed a sustainable recirculation system for photovoltaic products. After small-scale testing (2002 – 2005), the separation process was transferred to the Freiberg pilot system. In the current project, solar silicon reprocessing methods which are as environmentally friendly as possible are being developed. Here, one aim is to reduce the toxicity and environmental impact of etching solutions, thus also reducing the costs of disposing of the residual solutions. On the other hand, improved etching processes enable a higher light yield and thus higher efficiency for new solar cells. The original recycling objective of recovering undamaged cells was abandoned because today's low solar cell thicknesses (< 180 μm) and modules' heavy prior damage after dismantling and transport mean that it is no longer profitable. Therefore, broken solar cells are obtained and cleaned, so that pure Si is produced. This is re-melted into polycrystalline ingots, from which new wafers are then produced. This avoids the laborious extraction of Si from highpurity quartz. This, the world's first pilot system for the recycling of all standard solar modules, has been running since 2002 and processes a diverse range of solar cells which use different technologies and come from different manufacturers. This system has already recycled the oldest German PV system and the oldest Belgian PV system. The new process is mainly used to recycle standard modules with solar cells made of monocrystalline, polycrystalline or amorphous silicon. Solar cells from different manufacturers require treatments specifically adapted to suit their layer systems. The system separates the modules' components in two process steps. In the first, thermal step, the module's plastic parts are removed, and glass and frame parts are disassembled, sorted, then recycled (see Fig. 5). At the beginning of the second step, not only broken solar cells, but also whole solar cells are recovered. The solar cell structure is removed for wafer recovery by means of an etching process. Recoering solar silicon from broken solar cells has proven more economical. Running the pilot system still uses considerable amounts of energy and raw materials. The manual separation and the low throughput also entail relatively high costs.

… to automated recycling

With energy-optimised automated systems, the environmental impact caused by the thermal process and material separation should be reduced to one third. The yield and purity achieved when reprocessing an old Belgian system (“Chevetogne”) in the pilot system are shown in Fig. 6. Developers hope to achieve similar yield and purity on a large scale. It is expected that with higherquality end products, higher throughputs and higher recycling rates, it is possible to operate in such a way as to cover costs, or even to make a profit. The broken Si obtained in the automatic systemis subsequently melted down and made into modern solar cells. Compared to old modules from the 1980s, the efficiency of the new modules can be as much as 50% higher. In comparison with the manufacture of modules from new wafers, production from recycled wafers saves enormous amounts of energy.

Another method for the recycling of thin-film modules

Another method, specifically for the recycling of CdTe thin-film solar modules, has been used by the US photovoltaic company First Solar since the end of the 1990s. Here, whole and broken modules and production waste are all processed in a single procedure. This results in recovery of 90% of the glass and 95% of the semiconductor material. This method is used at various locations in the USA, as well as in Frankfurt an der Oder, Germany. The modules are roughly broken down in a shredder, then ground in a hammer mill to form pieces which are 4 – 5 mm in size. In this process, the lamination seal is broken. In a stainless-steel drum, the semiconductor layers are removed with acid. Subsequently, solid materials (glass and larger pieces of lamination film) are separated off. The metal-bearing liquid is purified and concentrated in a threestage precipitation process with increasing pH values. Semiconductor material for new modules can be obtained from the resulting filter cake.

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Addresses

Coordination
Sunicon AG

Project partner
TU Freiberg, AOC