Around 95% of the current production of photovoltaic (PV) modules is based on silicon wafers, which need to be extremely pure so that high efficiencies can be achieved when processed as solar cells. Silicon PV technology begins with the carbothermic reduction of quartz in an arc furnace to produce 99%-pure metallurgical silicon. This purity level is not enough, and a subsequent ultra-purification process converts the metallurgical silicon to trichlorosilane, which is distilled and converted back to silicon in a Siemens-type reactor.
A huge investment in electricity and cooling is needed for ultra-purification, with energy demand in the range of 50-75 kWh/kg, which translates in the largest fraction of the carbon footprint in the manufacturing of a PV module. This is why alternative purification routes that avoid the transformation to chlorosilanes by applying metallurgical-like methods are under research. The silicon feedstock produced this way, the so-called Upgraded Metallurgical Grade Silicon (UMG), requires less energy for its manufacture, in the range of 30 kWh/kg. This clear environmental benefit is accompanied by a reduction in the manufacturing costs. The challenge for UMG is to demonstrate that it does not come with a reduction in the solar cell performance, so that the beneficial impact “per kg” is also maintained as “per watt”.
At the Solar Energy Institute we have been involved in the sustained R&D effort carried out by the Spanish joint venture Ferrosolar for the development of a UMG technology, resulting in industrial-type multicrystalline PERC solar cells fabricated on the solar silicon purified by Ferrosolar that have surpassed the 20% efficiency target, plus the demonstration of similar energy yield of a PV system with UMG-based modules to that of a neighboring reference with conventional modules.
So, it has been demonstrated that UMG silicon can be the basis of a low-cost environmental-friendly supply of solar silicon.
REFERENCE:
Míguez Novoa JM, Hoffmann V, Forniés E, Mendez L, Tojeiro M, Ruiz F, Funes M, del Cañizo C, Fuertes Marrón D, Dasilva Villanueva N, Caballero LJ, Arıkan B, Turan R, Canar HH and Sánchez Plaza G (2024), Production of upgraded metallurgical-grade silicon for a low-cost, high-efficiency, and reliable PV technology. Front. Photonics 5:1331030. doi: 10.3389/fphot.2024.1331030.
