Development of a modular cradle to cradle process ...

33rd European Photovoltaic Solar Energy Conference and Exhibition

Development of a modular cradle to cradle process-chain for c-Si-PV panel recycling J. Glatthaar1, , E. Kamdje2, J. B. Barnickel1, M. Dax3, V. Schaub4, H. G. Stevens5, B. Jehle6, U. Ricklefs2, E. A. Stadlbauer1, H.Weigand1 1 Kompetenzzentrum ZEuUS,Technische Hochschule Mittelhessen (THM), Wiesenstr. 14, D-35390 Giessen; +49-641309-2548; [email protected] 2 Kompetenzzentrum NanoP,Technische Hochschule Mittelhessen (THM), Wiesenstr. 14, D-35390 Giessen 3 Ruehl Solar, Konrad Becker Str.1, D-35102 Lohra-Kirchvers 4 Abfallwirtschaft Lahn-Dill (AWLD), Am grauen Stein, D-35614 Asslar 5 SM InnoTech GmbH&Co.KG, Vennweg 18, D-46395 Bocholt 6 ZME Elektronik Recycling GmbH, Auf dem langen Furt 17, D-35452 Heuchelheim

ABSTRACT: End-of-life photovoltaic modules are part of the electric and electronic equipment addressed by the WEEE-directive. Thus, defined recycling rates need to be fulfilled for this particular waste stream. As opposed to largely destructive recovery technologies available on the market this project aims at developing a comprehensive cSi-PV-panel recycling concept capable of conserving technological intelligence inherent to end-of-life PV modules. The approach is characterized by a tailor-made repair/recycling process for the individual modules based on a reliable failure analysis. Therefore, visual inspections of the end-of-life modules are complemented by electroluminescence measurements and the determination of current-voltage characteristics. The most appropriate recycling procedure is assigned to each module. Ideally refurbishment can be achieved by eliminating panel defects in single repair steps (e.g. exchange of broken glass or wafers or defect connection boxes), which restore full function. Photovoltaic modules with irreparable damages are successively dismantled into components. All intact parts are recovered for the assembly of 2 nd life PV modules. Broken and destroyed parts are fed into industrial raw material repair/recycling process chains. This cascaded recycling approach is expected to be more environmentally beneficial compared to existing raw material recycling in terms of energy demand and resource consumption. Keywords: PV modules, Silicon Solar Cells, Recycling, , Encapsulation

1

INTRODUCTION

to exchange of damaged connecting cables and junction boxes or reframing of panels to restore water-tightness. Recently, the photovoltaics industry has started to consider recyclability in the latest design of photovoltaic panels [4]. Combining these concepts, in this report we present an integral approach fathoming the feasibility to widely conserve the chemical and technological intelligence inherent in spent PV modules by a stepwise repair/ recycling process (Figure 1).

Since 2012 photovoltaic panels (PV) have been included in the EU WEEE Directive [1]. Therefore, collection and recycling of discarded end-of-life PVmodules to an extent of 80% is mandatory. On the other hand, the photovoltaics industry has started voluntary efforts on PV recycling, particularly regarding the organization of collection, transport and treatment (recycling) of out-of-spec, damaged and End-of-Lifephotovoltaic panels by PV CYLCLE. A typical photovoltaic panel consists of a laminate stabilized by an aluminum frame joint by silicone-based edge sealant or double-sided adhesive tapes preventing ingress of moisture. In 1st generation modules the laminate consists of the Si-cells embedded in layers of polymer encapsulant, typically an ethylene vinyl acetate copolymer (EVA) which bonds to a front glass and a polymer back sheet. Current technologies mainly recycle the bulk materials glass, aluminium and copper by destructive methods completely disuniting the module compound structure [2]. Obviously. there is a need for a recycling technology that conserves function and value of module components. Moreover, there is an increasing demand for fully operative, refurbished or reproduced older models of photovoltaic panels at the growing second hand market for photovoltaic components. The first integral approach was shown by Wambach et.al. who introduced a thermal dismantling technique [3]. Later on the applied pyrolysis techniques were improved and combined with dissolution of the laminate in organic solvents or strong acids by other research groups. However, afore methods are energy-consuming, especially if working at elevated temperatures. Therefore, successful business cases are rather scarce. The refurbishment in many cases is limited

Figure 1 The PV-Rec recycling concept Based on the experimental failure analysis, each module is assigned the most appropriate recycling procedure. In accordance with the EU-WEEE requirements the ideal case is refurbishment: PV modules

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33rd European Photovoltaic Solar Energy Conference and Exhibition

with minor damages end up in tailor-made repair procedures to be specified below, which restore full function and result in 2nd life modules in a most valueconserving manner. Modules with severe or irreparable damages are successively dismantled into components used in the assembly of the 2nd life PV modules. Broken and destroyed parts are fed into industrial raw material recycling process chains. The variety of module design (size, silicon cell types, frame, glass, back sheets, sealants and encapsulants) is the main challenge both for the dismantling and the repair. Essential steps of this modular cradle to cradle process-chain in c-Si-PV panel recycling are: i. Failure analysis ii. Chemo-mechanical removal of the aluminum frame iii. Chemo-thermal removal of the glass and back sheet iv. Dissolution of the encapsulant, mainly ethylene vinyl acetate EVA

2

Special, catalytically active solvent mixtures reduce the adhesion of the silicon sealant or are able to dissolve it. Suitable mixtures were identified through immersion experiments [9]. 2.3 Chemo-thermal removal of the glass and back sheet The temperature dependence and the influence of selected chemicals on the stiffness of the encapsulant were tested using rectangular specimen (length:160mm, width:10mm, thickness:0,5mm) prepared from samples of ethylene vinyl acetate copolymer foil PHOTOCAP 15585P HLT (Specialized Technology Resources), laminated under industrial standard conditions at 140 °C and having a gel content of 85%) [10,11]. The tensile strength tests were conducted on a standard universal testing machine. A heating unit was applied which consists of a stainless steal tube with an inner diameter of 13mm, equipped with a tailor-made heating collar (Horst GmbH), power regulator and PT 100 thermocouple. In a first test series separation techniques were applied to small test samples (30mm × 30mm) with PET type back sheet (BP Solar) or with PVF type back sheet (Hyundai) cut from spent mono crystalline Si solar panels. The tests were expanded to whole intact wafers and to whole solar panels with different defects (e.g. broken front glas, intact front glas and broken wafers).

EXPERIMENTAL

2.1 Failure Analysis A prerequisite to this approach is the availability of a reliable failure detection and analysis. Different types of defects are known in spent photovoltaic panels ranging from problems in insulation, open circuits, contacts, glass and wafer fracture to partial delamination [5,6]. In a first step, these defects are localized and classified mainly by visual inspection while still being installed. This preliminary assessment is complemented by testing both the performance of the entire module as well as single cells in a c-Si panel. Measurements applied are: (a) image analysis concerning the geometry of conductors and single cells, (b) powered by an external current source modules and single cells show heating and electroluminescence. These effects are analyzed by image analysis (c) current-voltage characteristics with and without of illumination, (d) illumination with modulated high power light sources on single cells as well as the whole module

2.4 Dissolution of the encapsulant,mainly ethylene vinyl acetate EVA Swelling and polymer dissolution properties of selected suitable chemicals were tested in immersion experiments [9]. In a first test series small test samples (30mm × 30mm) with PET type back sheet (BP Solar) or with PVF type back sheet (Hyundai) were cut from spent mono crystalline Si solar panels as well as from almost intact modules. Chemical cocktails were tested in terms of dissolution rates. In a second series of tests, intact wafers (125mm × 125mm) were immersed into these chemical cocktails.

2.2 Chemo-mechanical removal of the aluminum frame Starting point for the development of a nondestructive mechanical removal of the aluminum frames is a force analysis of the sealant joint. Tensile strength tests, plain strain tests and tensile shear strength tests were conducted on an Inspect table BLUE 5k 222 EDC testing machine. The specimen were prepared from HelioBond® PVA 205 (Koemmerling), a typical 1k silicone sealant (Figure 2) [7, 8]. The joint surfaces were cleaned and pretreated with Koerabond HG 83 (Koemmerling). Mechanical or chemical pre-treatment routes were derived from the results of the force analysis and tested with pressed or bolt Aluminum frames.

3

RESULTS AND DISCUSSION

3.1 Failure Analysis Especially mechanical defects like cracks and scratches in glass and sometimes on conductors, broken cells etc. could easily be detected and identified by image analysis. (Figure 3).

Figure 3 Image analysis detects totally broken glass although the single cells show full efficiency

Figure 2 Examples of specimen used in different experiments; Left: Lap shear strength Right: Specimen modeling the u shape joint

At externally powered modules images were taken in the NIR (~ 1 µm) spectral range. These images show

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heated areas. In these areas the cells work well. Dark areas do not contribute (Figure 4) . Shorts show strong spots. From these images defect cells and shorts can be localized.

(Figure 5). At the head a mixed adhesion cohesion break is observed. This phenomenon is object of further studies and simulations.

Figure 4 Externally powered modules Left: Defect string (marked) and defect single cells, black areas reduce the energy captured by that cell; Right: Marked defect single cells To capture these images three CMOS-cameras were tested. The “Solar Cam for NIR” from MBJ gave best results. On the other hand this camera is expensive (~ 4T€). The “UI-3240LE-NIR-SL” of IDS and the “Raspberry PI 2-V2” (~ 30 €) show a very low sensitivity in the NIR. The images of the modules are captured in a dark room at maximum time of exposure (20 ms). Then the mean is taken on ten from these images. In most cases the resulting noisy mean images would be sufficient to detect black areas too but with a very cheap setup. Under the test conditions used none of these cameras was able to detect electroluminescence at a wavelength of 1.3 µm. A single cell in a module illuminated by a modulated light source (frequency 80 ° C the curves drop and the tensile strength is strongly reduced from 15 N/mm2 down to 0.5 N/mm2. At the same time the maximum strain is reduced from over 500% at room temperature to 100% at 120°C. This suggests that a series of new repair options can be performed in a temperature window from 80 to 120°C. Options include: Exchange of broken Si cells Nondestructive removal of back sheets Separation of front glass and of intact Si cells Exchange of damaged EVA encapsulant layers Similar effects as observed for the temperature dependence of EVA were also found for several of the tested chemical compounds. This provides a case-by-case alternative to the thermal treatment of the encapsulant.

Figure 6

Figure 8

3.4 Dissolution of the EVA encapsulant Reported dissolution capabilities of chemicals for cross-linked EVA foils with a gel-content of about 80 to 85 % are limited to pure solvents like Trichloroethylene, Toluene or mixtures of Toluene and Dichlorobenzene [13]. The dissolution is accelerated at elevated temperatures of up to 100 °C and with simultaneous ultrasound treatment. However, under these treatment conditions the EVA swells faster than it dissolves. The strain generated thereby causes a breakdown of the Si cells. Clamping the laminate in between two glass plates during the solvent treatment prevented the destruction of Si cells [14]. The use of mixtures of ether derivatives of phenol and benzyl alcohol and even more complex mixtures with special chemicals like tetradecene and other long chained ester and polyether derivatives is reported [15]. Many of the applied chemicals are hazardous therefore requiring safety measures when used in the disassembly of PV-modules. Ongoing work is related to the identification of chemical mixtures capable to cause an adhesion rupture at the glass or Si cell interface rather than dissolution of the encapsulant. We were able to find chemicals with comparable swelling tendencies, comparable dissolution abilities but less hazardous properties compared to toluene. Mixtures of these chemicals with basic solvents were found to remove PET-based back sheets within 1 to 2 hours of treatment at 80°C and PVF based back sheets within 5 to 8 hours at 80°C. Additionally, in case of multi-layered back sheets, these mixtures are able to separate the PET or PVF layers from the EVA layers within the back sheets. These mixtures are able to liberate intact Si cells from the EVA encapsulant residues. In case of broken front glass 20 to 50% of the broken glass particles were removed without extra effort, but needed very long treatment times >24h. Since we have found a better way of removal of broken glass (see 3.3) we focus onto the chemical removal of encapsulant and back sheets. The tests are now extended step-by-step towards the immersion of complete sets of PV panels.

Stress-strain behavior of EVA at varied treatment temperature

When the front glass of the modules is broken neither the above mentioned nor established wire saw cutting can be applied because the glass-encapsulant-Si-cell interface is too uneven. For this case, we have identified a new method to separate broken front glass directly from the laminate (Figures 7, and 8). Therewith, the glass particles could be removed from a single Si cell within less than 60 s without the aid of chemicals (see 3.4). The method is currently under optimization. After chemical removal of encapsulant residues and back sheet material this will provide a high-purity glass fraction suited for recycling.

4 Figure 7

Laminate after treatment

CONCLUSIONS

We have investigated new approaches for a functionconserving recycling. The image analysis is a powerful tool for the detection and localization of cells, circuits and main defects in the visible spectral range. In the NIR spectral range it can be applied to detect shorts and areas of considerable function losses in the cells.

Laminate with broken front glass before treatment (single Si cell)

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In most cases image acquisition and analysis can be done at low cost and the setup can be used both for the VIS and NIR. An easy non-destructive removal of pressed or bolt aluminum frames is accomplished by mechanical or combined mechanical and chemical pretreatment routines, derived from tensile strength studies of the silicone sealant. Suitable mixtures of silicone-dissolving catalysts and solvent were identified and successfully tested. After the frame removal, the remaining laminate becomes quite flexible and needs extra stabilization. Two new routines, (i) the thermo-mechanical treatment and (ii) the chemical treatment, have been developed for the failure-diagnosis-based stepwise dismantling of the laminate. Both routines need only moderately elevated temperatures. (i) After warm-up to temperatures between 80°C to 140°C the back sheet is easily removed from the laminate. Broken Si cells, identified in the failure diagnosis, can be removed in a next step accomplishing a final replacement with an intact Si cell of the same type. Intact front glass can be separated e. g. through wire saw cutting. A new technique allows the separation of broken front glass. (ii) Back sheets can chemically removed in dissent time from the laminate. The chemical mixtures identified from immersion tests, accomplish the liberation of Si cells from the EVA residues as well as the separation of the different layers (EVA/PET/PVF) within multi-layered back sheets. Broken front glass parts can be removed only partially by chemical treatment. 5

[7] [8]

[9]

[10]

[11] [12] [13] [14] [15]

ACKNOWLEDGEMENT

This project (HA project no. 497/16-09) is funded in the framework of Hessen ModellProjekte, financed with funds of LOEWE – Landes-Offensive zur Entwicklung Wissenschaftlich-ökonomischer Exzellenz, Förderlinie 3: KMU-Verbundvorhaben (State Offensive for the Development of Scientific and Economic Excellence).

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[2] [3]

[4]

[5]

[6]

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