A novel approach for the recycling of thin film photovoltaic modules
Introduction
Photovoltaic (PV) energy production is one of the most promising technologies of the renewable energy sources. By the end of 2007, the cumulative installed capacity of solar PV systems around the world had reached more than 9200 MW (EPIA, 2008).
The thin film technology using cadmium telluride (CdTe) and copper indium disulphide/diselenide (CIS) gains in importance due to its low production costs and the low energy and materials demands during production. The energy pay-back time for such module is as low around 1 year for CdTe- (Fthenakis and Alsema, 2006, Fthenakis et al., 2008) and 2.8 years for CIS-systems (Raugei et al., 2007). However, based on the average prices presented in Fig. 1 (USGS, 2008), the prices for the rare materials indium and tellurium may continue to increase.
The estimated lifetime of a thin film module is about 25–35 years of power generation. Regarding the growing number of installed PV systems, the end-of-life management will become increasingly important. Thin film panels are destined to make a big impact in the future, but when they reach their end-of-life, hazardous substances used in the panels may harm the environment if they are not recycled or disposed of properly. For example, heavy metals present in them can be toxic as well as carcinogenic or teratogenic. Processing methods must take these facts into account.
An evaluation of photovoltaic recycling strategies showed that currently only two processes in the market are operated in an industrial scale. The company Deutsche Solar (Solar World) applies a treatment to recycle crystalline silicon modules (Wambach, 2001). CdTe thin film modules are recycled using a combination of mechanical and chemical process steps. This technology is established by the company First Solar (Fthenakis and Wang, 2006). Processes for other technologies are under development, mainly still in laboratory scale.
This paper presents results of the EU-LIFE project RESOLVED (REcovery of SOLar Valuable materials, Enrichment and Decontamination), coordinated by the Federal Institute for Materials Research and Testing in order to demonstrate the feasibility of a sustainable photovoltaic thin film module recycling by means of (wet-)mechanical processes (RESOLVED, 2008).
Section snippets
Background
The rapid development and growth of the photovoltaic market and the estimated quantities of photovoltaic waste in the near future, as well as the scarcity of semiconductor materials, demand a sustainable recycling of PV modules. Already in the 1990s first recycling strategies for PV thin film modules were being developed (Bohland et al., 1997, Bruton et al., 1994, Eberspacher et al., 1994, Fthenakis et al., 1996, Menezes, 1996, Wambach, 1998). Due to the increased production of photovoltaic
Materials
Within the RESOLVED project the recycling of thin film modules was performed using CdTe and CIS modules. The investigated CdTe PV modules are composed of four layers:
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a Transparent Conducting Oxide (TCO) layer consisting of SnO2 which acts as a front contact on glass substrate.
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a cadmium sulphide (CdS) film which serves as a n-type layer.
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a CdTe film which is an absorber layer.
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a back contact consisting of a copper layer.
The structuring of the layers into cells takes place between the various
Recycling strategies
The RESOLVED project aims to demonstrate sustainable recycling strategies for photovoltaic thin film modules based on (wet-) mechanical processes to reduce the amount of chemicals used for conventional recycling and the amount of waste.
Two strategies were developed to accomplish the recycling of both end-of-life modules with firstly intact carrier glass (RS-1) and secondly broken modules (RS-2). The main steps in both strategies are:
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Destruction of the laminate.
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Separation of the semiconductor
Thermal dismantling
The stepwise heating of the PV modules to a temperature of 500 °C results in the dismantling of the laminate compound. The carrier glass with the semiconductor materials on its surface was further treated in the blasting process. The clean protection glass can be recycled completely.
Particle size reduction
The particle size reduction is an essential process step to provide suitable material for the subsequent steps. The grain size has to be small enough for the attrition but too much fine glass particles could cause
Discussion
The feasibility of wet mechanical processing of end-of-life PV modules was demonstrated by using attrition and flotation as well as by the application of dry mechanical (vacuum blasting) and wet mechanical methods. The processes are also applicable for residual waste from thin film module production. Attrition, subsequent rinsing and sieving at 150 μm, as well as flotation resulted in enrichment of the valuables, i.e. CdTe and CIS, see Fig. 6.
Indeed the enrichment was more than tenfold for the
Acknowledgments
The work was funded within the EU-LIFE programme (project RESOLVED, LIFE04 ENV/D/000047). Part of the experimental work has been performed by Ms. Gudrun Sapich (formerly BAM).
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