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The first step of our value chain, is the crystallisation of feedstock, where Eco-Solar sees two major opportunities to significantly reduce the carbon footprint: reuse of argon purge gas and reusable crucibles.
Argon gas recovery during crystallisation
During this process step, pure argon gas, used to remove contaminants, is currently being vented into the air. The effluent argon gas is typically composed of argon and a few contaminants at the ppm level, chiefly composed of carbon monoxide, hydrogen, methane and other hydrocarbons.
Though perhaps argon is abundantly available in the earth’s atmosphere (approximately 1% of mass), there are still (ecological and financial) costs involved in the production of pure gas. For instance, NorSun’s wafer facility with an equivalent output of ~350 MWp of wafers per year, requires ~1.4 million Nm3 argon, resulting in a typical bill of 0.7-1.4 million EUR per year, accounting for 3-5% of the wafer cost.
Though in other industries, argon gas recycling has become state of the art, in PV-production this was not yet the case. However, GR2L had 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 re-generable 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.
Savings of over 95% could be realised if the waste argon is purified and reused as will be assessed by bifa in an LCA.
Reusable crucibles to save resources during crystallisation
During the crystallisation step, crucibles are the main containers for the (molten) silicon feedstock. As silicon at high temperature reacts easily with almost everything, there are only a few crucible materials that will meet purity requirements for manufactures. One of the main drawbacks of using silica, as is current standard practice, is the (ecologic and financial) cost due to single use. Silica crucibles contribute up to ~30% of the conversion cost from Si-feedstock to the as-grown ingot.
STEULER has developed a concept for reusable silicon nitride crucibles, for both crystallization processes, DS and Cz.
In Eco-Solar the technical potential of these crucibles for crystallisation of multicrystalline silicon has been investigated at SINTEF. Currently, one silicon nitride crucible has been used for 5 subsequent crystallisation runs. The obtained silicon material quality is similar to material crystallised from standard silica crucibles. It is aimed for at least 10 times reuse. The near future plan is to scale it up to larger silicon ingots and then to manufacture solar cells and modules out of the silicon grown in this material in order to demonstrate that this technique is suitable for PV applications.
Savings of more than 85% can be realised with this novel technology.