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General comment Thermal upcycling that aims to fully recover componentsincluding high-purity Si wafers. Additional thermal and chemical processes are involved, referred as “thermal recycling”. Thermal delamination separates the module by thermally decomposing the encapsulation layer between glass and solar cells. The polymeric encapsulation layer, mostly EVA, can be either pyrolyzedinto acetic acid, propane, propene, ethane, methane, and other combustible oils and gases under an inert gas environment (T1) or burned off under an oxygen environment (T2). In method T1, SolarCells Inc. first proposed recovery of Si wafers and functioning solar cells from old modules by pyrolytically decomposing EVA in an inert atmosphere in a tube furnace at about 500 °C. Prior to EVA vaporisation, the backsheet was manually peeled off. This pyrolytic delamination was also demonstrated using a fluidised bed reactor and a mufflefurnace with combustible gases recovered as heat or electricity. A peak temperature of 450–600 °C with a holding time of30 minutes to 1 hour was enough for full decomposition.Deutsche Solar and Solarworld demonstrated the thermal delaminationrecycling process for decommissioned modules in a 300 kW PV plants in Germany and Belgium. The delaminated solar cells were remanufactured into new modules and served a second life. Since 2005,the thickness of Si solar cells has been reduced to less than 200µm,making wafers more likely to break or crack during the thermal process. Cracking has been attributed to the mechanical stress caused bydecomposing gas behind the glass and the thermal deformation of the EVA at the rear. The breakage issue can be overcome either by toolmodification (e.g. additional fixed jig with grooves apart) orsimple pre-treatments (e.g. front glass cracking and rear EVA patterning) to quickly release stress generated from the decomposed gas. An optimised process could recover nearly 100%w of the tempered glass and high-purity solar grade Si. In method T2, instead of pyrolysis, EVA was combusted in an oxygen environment to provideenergy for the heating furnace. |
