Multi vs. mono Part 2: DW Slicing, the game changer

Industry Insights

Back to middle 2000s when solar energy just started to rise in light of renewable, PV industry was filled with tremendous possibilities about turning sunlight into electricity.

Si-c panel was just squarely on a list of PV products, multi and mono having their own fields. Or more accurately, mono was more ready to move forward. Before multi casting (an entirely U.S. or EU reliant tech back then) got on stream, there had been large number of growing factories serving 3inch or 4 single crystal ingots for dioxide components. Turning to PV grade growing was an easy and happy job for them as solar growing was considered as lower tech in many prospects. When Germany’s genius FiT stimulated PV plan came along in 2015, the fledgling PV people just woke up finding themselves in middle of festivity over one night. Everything went shortage. Every price went soaring. Only problem was to seize supply. Seeing many folds higher profits, dioxide factories quickly turned into solar ingot growing business. Soon equipment makers ensued. For several years China PV trading market was flooded with mono products, which was the only thing PV people could get hold of.  

This was until 2008 when LDK China finished building a multi wafer factory with GW scale. Casting furnace was five times more productive than mono growing and extremely easy to operate. Chinese investors, from whatever remote background, were flying around the world, huge wads of cash in hand, trying to get on any cast furnaces they could find. The wild growth went for several years, until the catastrophic collapse in 2011 and almost everything was shattered. When people managed to get back on feet, they found themselves in middle of huge overcapacity and cost become survival rule. Multi technique suddenly became a good fit for cost perspective. Factories across the industry started to shutdown mono lines or convert them to multi. Hundreds of small growing factories closed, along with thousands of growers going wreckage, except small portion drifting back to dioxide growth. Mono proportion started to decline from 2011. The fall accelerated, until hit 15% the all time low in 2015.

By 2013, there were four to five independent mono wafer factories left in the market. In fact they represented almost entire mono capacity. As all in-house verticals have totally shutdown their own growing machines, only keeping a few mono cell and module lines with wafers outsourced. There were some focused on mono panels in global market, like SunPower of N type, LGE Korea, SunEdision, and some scattering in TW and Japan. Orders were scanty and mono wafer makers, even though few left,  were starving.

What happened then, for mono to make this remarkable comeback?

When natural growth of demand doesn't show up, people just go on to hatch new demands. Or, pillage neighbors, if it's what it takes. No offense, if this page doesn't explain this tone, next will. After all, they had to turn interest back to mono, factory lines back to mono, before it was too late. It might be the first time in Si-c world that one camp turned hostile to the other, as it had to. And the best weapon is price. Before they really got price tool in hand, they gone through hell of countless experiments, failures, and investment gone in smoke. In the end, they made it. A glance at some notable achievements.

Mono is 30% higher cost than multi at growing process, while slicing is the same. Over the last years, mono guys filled up growing gap substantially. And more amazingly, they made out huge advantage at slicing part.

For growing, mono increased loading size by 200%, from lower than 100kg per charge to near 300kg of late 2015. Hotzone , the core part to hold and melt polysilicon, made big jump from 22inch of mainstream to 26inch. This might not be able to beat casting process, which went over 400% of loading increase in same time frame. But thanks to scanty number of mono growers, mono factories were able to move all growers up to north part of China, where electricity price cut by half. This became more significant a cost saving, given the fact that electricity took over 40% of growing cost. Before 2013, one growing process only generated one piece of ingot. Then it came along 2 pieces in one recharging process, then 3-4 pieces in one continuous pulling process. Productivity got life up to 40% or 50%, meaningful push of cost down.

Pulling speed. High speed means more ingot grown per certain time, and more cost cut. Pulling speed increased by 100% from 0.6mm/min to 1.2mm/min, over last four years.

Application of advanced materials. Main materials of pulling machine were graphite based, until 2014 when they were largely replaced by carbon fiber composite (CFC) materials. CFC provides 2-3 times longer lifetime, stronger and more thermal feasible, than graphite. It also played important role in hotzone size increase, helping hotzone parts made to be thinner, flexible, and preventing energy lose.

Finally, all these look like crumbs on the way of cost down, in front of diamond wire slicing. We list some comparison data between DW slicing and the conventional slurry based. Productivity increased by whopping six times (cutting time from 8 hours per cut of slurry down to 1.2hrs of DW ). DW wire size (70-80um) significantly thinner than slurry type (100-110um), raising cutting yield by 25% (47pcs – 58pcs). DW’s cutting feature of real cut through, rather than squeeze motion of slurry wire, would largely reduce silicon kurf lose. It totally skips usage of SiC powder, which is No. 2 highest cost item in slurry slicing process right after wires. More importantly, wafer will be the only cleaning material without SiC fluid waste. A big saving on emission treatment too, and environment friendly.  

None of these has happened in small number of phases, nor in short time, nor with small funds. In the end, cost went down by 60% at slicing part, and 30% at growing. There is a saying, crystal-related efficiency different between multi and mono wafers is 0.4%, and cost gap $0.10/pc. That said, within that range, people would turn to buy mono products.

This is what happened to TW cell makers in late 2015, and Chinese makers in late 2016 (of course combined with force of FRP projects). People are not only building up new mono lines, but only shutting down multi lines and cutting multi orders.

Multi is in trouble now.

But doesn’t this look familiar to what mono faced to back to 2014-15? That hard time propelled mono almost reborn and become stronger. That was a body less than 5GW back then. Now multi camp is a giant of over 50GW (China alone), not to mention at least 10GW on the way within 2017. Multi has one hundred times less reasons to lose this.

Author:Xiaodong(Bond) Wang Former Director Int'l Business Development

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