多层结构中聚合物对二氧化碳的吸收：光伏组件中聚（乙烯-醋酸乙烯酯）的情况

CO₂ absorption into a polymer within a multilayer structure: The case of poly(ethylene-co-vinyl acetate) in photovoltaic modules

Axel Briand ^{a b}, Antoine Leybros ^a, Claire Audoin b, Jean Christophe Ruiz ^a, Fabrice Lamadie ^a, Agnès Grandjean ^a
aCEA, DES, ISEC, DMRC, Univ Montpellier, Marcoule, France
^bUniv. Grenoble Alpes, CEA, Liten, Campus Ines, 73375 Le Bourget du Lac, France

Abstract
The delamination of photovoltaic modules with supercritical CO2 is an interesting approach to integrate photovoltaics into a circular economy. This article describes the first phase of this process: the absorption of CO2 into poly(ethylene-co-vinyl acetate) (EVA-28), one of the layers in a photovoltaic module. The swelling of the polymer, determined by an on-line coupling between high pressure cell and an optical method using digital camera, under 60–200 bar at 60, 75 and 90 °C, was well reproduced by a modified Sanchez-Lacombe equation of state. The diffusion coefficient of CO2 into EVA-28 was determined using the same approach and was found to range, at 130 bar, from 1.1 × 10−9 m2·s−1 at 60 °C to 1.8 × 10−9 m2·s−1 at 90 °C. A preferential CO2 diffusion at the rear side of the cell, due to a high porosity, and at the “backsheet” interface was highlighted.

超临界二氧化碳光伏组件的分层是将光伏技术整合到循环经济中的一种有趣的方法。本文描述了这一过程的第一阶段：将CO2吸收到聚乙烯-醋酸乙烯酯（EVA-28）中，这是光伏模块中的一层。聚合物的溶胀是通过高压电池和使用数码相机的光学方法之间的在线耦合确定的，在60–200巴、60、75和90°C下，通过修改的Sanchez-Lacombe状态方程很好地再现。使用相同的方法测定了CO2向EVA-28中的扩散系数，发现在130巴下，60°C时的扩散系数为1.1×10−9 m2·s−1，90°C时为1.8×10−9m2·s−1。由于高孔隙率，CO2在电池背面和“底片”界面的优先扩散被强调。

Introduction
Since the early 2000s, increased worldwide demand and the transition to renewable energy have driven a sharp increase in solar photovoltaic (PV) energy production. This growth is expected to continue, with an estimated PV capacity of between 7.6 and 22 TWp in 2050 [1]. First generation PV panels are now reaching the end of their lifetimes. The cumulative mass of end-of-life PV panels worldwide is expected to reach between 2 and 8 million tons by 2030 [2]. In this context of growing demand for solar-generated electricity and with the aim of recovering valuable materials, there is considerable demand for cost-effective and eco-friendly recycling processes. Existing PV panels mostly contain crystalline silicon PV cells: until 2016, more than 80% of installed PV panels were based on the Si-Al-BSF technology [3]. However, since cell technologies have and will continue to evolve rapidly, recycling techniques must be adaptable to new developments.

A crystalline silicon-based PV panel consists of a PV module stiffened by an aluminum frame and electrically controlled by a junction box. The PV module itself is a set of electrically connected crystalline silicon cells sandwiched between two layers of encapsulating polymer, a glass front face and a polymer “backsheet” (Fig. 1). The encapsulating polymer used in more than 90% of PV panels as of 2019 was poly(ethylene-co-vinyl acetate) (EVA) [1]. The “backsheet” is a multilayer assembly of polymers, most often a polyethylene terephthalate (PET) layer sandwiched between two layers of a fluoropolymer such as poly(vinyl fluoride) (PVF).

Recycling these modules involves several stages [4]. First a pre-treatment to remove the aluminum frame and the junction box; second, thermal [5], [6], chemical [7], [8], [9] and/or mechanical [10] treatments to delaminate the multilayer structure (silicon cell/glass/backsheet/EVA); and finally, a specific chemical treatment to extract the valuable metals (Ag, In, Si…) from the silicon cells [11], [12], [13], [14] or to recycle the silicon [15].

The chemical processes used for the delamination step typically involve organic solvents to dissolve the EVA (mainly by heating or with ultrasound or mechanical stirring). The reaction is slow however (> 1 h for 3 cm thick modules), and the solvents themselves are difficult to recycle [7]. Thermal treatments at temperatures above 500 °C to decompose the EVA and separate the glass from the cell are also lengthy (> 3 h for 5 × 5 cm2 pieces) [6]. The polymers also release toxic gases (HF from the backsheet and CO) as they decompose, requiring complex post-treatment of the gaseous effluents [5], substantially increasing the environmental burden of the recycling process [4]. Finally, existing mechanical treatments are generally poorly selective and lead to the loss of valuable materials such as silver and indium [10]. To integrate PV modules into a circular economy and reduce their environmental impact, we have therefore been investigating the use of supercritical CO2 (SC-CO2) as a greener means to recycle end-of-life PV panels.

In SC-CO2, the different layers of a PV module are separated, layer by layer, without breaking the glass and contaminating any of the valuable materials. The critical stage of PV delamination in a SC-CO2 medium is the separation of the EVA from the various components it encapsulates. The polymer absorbs SC-CO2 before rapid depressurization (at 1–300 bar·s−1) leads to foaming, which as observed for other polymers [16], [17], [18], can lead to loss of adhesion at the interfaces [19] and even delamination [20].

This paper focuses on the first stage of SC-CO2-induced delamination: the absorption of CO2 into EVA in PV modules. The absorption of CO2 into polymers has been well studied [21], [22], [23], [24]. In particular, Shieh and Lin [25] have investigated how different grades (i.e. with different weight percentages of vinyl acetate: EVA-16, EVA-18, EVA-25 and EVA-28) absorb CO2 at temperatures ranging from 25 °C to 52 °C and pressures of up to 340 bar, and Jacobs et al. [26] have studied CO2 absorption by EVA-40 at between 50 °C and 75 °C from atmospheric pressure up to 250 bar. However, the absorption of CO2 into a polymer in a multilayer structure has never been described. The absorption of a gas into a polymer depends mainly on two properties of the system: the phase equilibrium of the gas and the polymer (the solubility of the former in the latter) and the absorption kinetics (diffusion coefficient) of the gas in the polymer. In this study, we also consider how the absorption of CO2 into EVA is affected by the interfaces in the PV module.

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