An international research team has proposed to use deep eutectic solvents (DESs) in a new PV module recycling process intended to separate ethylene vinyl acetate (EVA) adhesive films from end-of-life solar panels.

“DESs are green, biodegradable solvents with a price advantage and the ability to be reused multiple times,” the research's lead author Shaoyuan Li, told pv magazine. , noting that the separation of EVA is a prerequisite for the recovery of valuable metals from end-of-life modules. “Conventional pyrolytic separation can lead to the release of hazardous elements like fluorine from the backsheet. Commonly used wet reagents, such as toluene, trichloroethylene, and tetrahydrofuran, pose issues due to their high toxicity and the challenges of waste disposal. DES solvents have garnered significant attention in the environmental field.”

A DES is usually composed of two or more components that form hydrogen-bonded complexes, consisting of hydrogen bond acceptors (HBA) and hydrogen bond donors (HBD).

The DESs used in the research were obtained by stirring chloromethylene (ChCl) and Oxa powder at a molar ratio of 1:1 at 70 C for 2 hours.

The scientists investigated, in particular, the separation parameters and the leaching mechanism of the proposed mechanism and conducted a series of tests on PERC solar modules manufacturer by China-based Longi. The cells were mechanically dismantled from the modules and then cut into 20 mm x 20 mm fragments. The fragments were then soaked in a hydrothermal reactor with the DES solvent at 175 C for 10 hours to dissolve the EVA encapsulant.

Under optimized conditions, the EVA separation rate reached 100%, accompanied by an aluminum (Al) removal efficiency of 98.4%.

“The separation mechanism indicates that when the temperature exceeds 175 C, the significant difference in thermal shrinkage rates between EVA and TPT leads to a warping effect,” Li explained. “Under high temperatures and the influence of ChCl-Oxa, the ester groups on the EVA side chains are converted into hydroxyl groups. Compared to traditional reagents toluene, the DES separation system demonstrates superior dissolution efficiency, significantly enhancing the integrity of the silicon wafers.”

The research group also found that, after 10 cycles of DES solvent use, EVA can still achieve a 100% separation rate. “The low cost and recyclability of DES solvent present greater economic potential for industrial production,” Li said.

The new process was described in the paper “Green recycling of end-of-life photovoltaic modules via Deep-Eutectic solvents,” published in the Chemical Engineering Journal. “To sum up, this research puts forward a new type of environmental protection technology that can separate EVA from end-of-life PV modules with high efficiency,” Li said.

The research team comprised scientists from Australia's University of New South Wales (UNSW) and the Kunming University of Science and Technology in China.