In the production of silicon ingots, argon is used as a purge gas to remove contaminants during ingot crystallisation. Gaseous impurities, if not removed, can react with the molten silicon leading to particle precipitation and contamination of the material. The argon purity, suggested for PV-manufactures, has to meet the Semi standard (PV6-1110), i.e. 99.999%. For a process cycle which last ~50 hours, typically 20-40 Nl/min argon gas is consumed and after passing through the furnace the argon is vented to the atmosphere. It is estimated that 3-5 million m3of argon are used per 1GWp of silicon wafer output. High purity argon supply is shortening and argon prices have been increasing over the last years. Cryogenic processes used to liquefy argon demand large amounts of electricity and financial cost and account for most of the CO2 emissions. This represents a significant cost to the manufactures who typically vent the gas to the atmosphere after passing it through the production line.
NorSun’s wafer facility with an equivalent output of ~350 MWp of wafers per year, will require ~1.4 million Nm3. Resulting in a typical argon bill of 0.7-1.4 million EUR per year, accounting for 3-5% of the wafer cost. The effluent argon gas is typically composed of argon and a few contaminants at the ~5000 ppm level, chiefly composed of carbon monoxide, hydrogen, methane and other hydrocarbons. Large savings could be realised if the waste argon is purified and reused. There are a number of purification technologies available on the market, which are developed for the microelectronic industry to purify process gases from ppm to ppb level, but for the PV industry to refined and costly.
GR2L has developed a relatively simply and cost efficient technique for argon-gas-recycling, providing a recovered gas stream of around 99.9999 purity. This method is based on a chemical looping combustion process to convert combustible species in the exhaust gas stream to carbon dioxide and water, followed by efficient of carbon dioxide and water in regenerable reactor beds. Such a recycling system has been integrated in NorSun’s factory collecting and recycling exhaust gas from 8 crystal pullers. First results from crystals grown with argon recycling show similar material performance as grown by standard process without recycling. In the near future solar cells and modules will be manufactured from this material to confirm the technological potential and qualify for customers.
GR2L argon recycling system connected to 8 crystal pullers at NorSun’s manufacturing side in Årdal Norway.