Significance of cost-effective Si recovery from diamond wire sawing in PV production value chain

Within the foreseeable future, Si PV will remain as the dominant solar energy technology worldwide (reference PV Status Report 2016, European commission, October 2016, EUR 28159 EN). At the end of 2016, cumulative global installed PV has surpassed 310 gigawatts (GW), compared to just 40 GW at the end of 2010. This high speed in the growth of PV installation will continue in the next decades. And, the future of large-scale PV on our planet is determined by the deployment of Si-based technology. In the report of Solarbuzz, it is stated that up to 2018, more than 90% of the solar PV modules produced are forecast to be Si types.

In the Si-based PV production value chain as shown in the figure 1, the upstream part usually refers to five steps: (1) feedstock Si, (2) Ingot, (3) Wafers, (4) Solar cells and (5) Solar modules.

Figure 1: Si-based PV production value chain.

For the dominant Si based PV technology, Si wafers are mostly produced by multi-wire sawing process. The wires perform cutting action against the Si ingot in a continuous loop or a to-and-fro motion, as shown in figure 2. The PV-grade Si removed by the cutting-away action of the wires, ends up as a dust from the ingot and is suspended in the liquid producing a mud-like sludge, the so-called “kerf-loss”, which is unavoidable at present. This lost Si accounts for 40-50% of the ingot’s total mass and constitutes the most important economic loss in the PV production value chain today.

Figure 2: PV wafer production process by wire sawing (from publications of CRS-reprocessing).

Compared to traditional slurry based wafering technology, diamond wire sawing is taking more and more ground, because of its potential to have lower cost-of-ownership and higher quality wafers. Figure 3 shows the market trend of diamond wire sawing compared to slurry sawing. It indicates clearly that diamond wire sawing has strong growth momentum and will replace slurry sawing to be the dominant technology on the market in the coming 5-10 years.

Figure 3: Evolution of kerf from traditional abrasive-based slurry sawing and diamond wire sawing for PV wafer production.