Europe’s greener photovoltaic value chain
The EU’s mission is to make energy production more sustainable through exploitation of clean and renewable energy sources. Despite the greater foothold of solar energy systems, the modus operandi still relies on a ‘take-make-dispose’ logic. Extracting resources, transforming them into products and simply discarding them at end of life is highly resource intensive and wastes valuable materials. The EU-funded project Eco-Solar aimed to maximise resource efficiency and integrate circular economy thinking. “Eco-Solar grasps the whole picture one has to look at the full value chain – ingot crystallisation, wafer slicing, cell production and module assembly,” notes Martin Bellmann who has been in charge of the project. Improvements to the crystallisation process Eco-Solar introduced a simple and cost-efficient technique for recovering over 95 % of the argon gas used for silicon ingot crystallisation. The method is based on chemical looping combustion that converts combustible species in the exhaust gas stream into carbon dioxide and water, leaving high-purity argon, which can directly be reused. For crystallising silicon ingots, researchers proposed crucibles based on silicon nitride ceramics that can be reused. These crucibles not only allow easy removal of ingots after production cycle completion, but also prevent impurities from entering silicon and reduce stresses produced during crystallisation. Improvements in cutting and wafering Silicon wafers are produced by slicing large silicon blocks. Eco-Solar improved the sawing process by using thinner diamond wires of approximately 60 μm. The project also investigated the recycling of the silicon dust produced when forming the silicon wafers. “The industrial sawing process is wasteful, with as much as 45 % of the valuable original material lost as fine silicon powder and ending up in landfills,” notes Bellmann. Eco-Solar’s new technique refines waste so that it can be reused as secondary feedstock for silicon ingot crystallisation, in silicon nitride crucible manufacturing and in lithium ion batteries. Minimising silver content and water use Diffusing phosphorus into the silicon wafer is one of the most critical steps in solar cell manufacturing. Project partners successfully combined several wet chemical etching and cleaning steps after phosphorus diffusion into a single process that reduces waste and carbon footprint of solar cell manufacturing. What’s more, they investigated the potential of certain industrial systems that can save more than 90 % of water wasted after wafer processing. Project partners also experimented with different solar cell architectures to minimise silver use. An improved interconnection scheme that eliminates the need to solder a contact tape onto the rear side of silver pads and the front side of busbars reduces silver content by 35 %. Solar cell repair and recycling Another project achievement was the development of an automated system for recognising small defects in solar cells. This ‘cell doctor’, which cuts or isolates non-defect cell areas thanks to laser processing can reduce the amount of scrapped solar cells by 50 %. Project partners also demonstrated the ability to re-open modules at end of life and disassemble them into their major components. Automated module disassembly equipment was designed along with diagnosis tools for screening module component quality. Eco-Solar’s forward-looking concepts promote resource efficiency, re-use and recycling along the entire value chain. This will be key in achieving sustainable solar power in Europe.
Keywords
Eco-Solar, solar cell, crystallisation, module, value chain, recycling, resource efficiency, argon, phosphorus, photovoltaic, silicon nitride ceramics
