Arc Furnace Delivers Low Cost Silicon For Solar Arrays

July 26, 2011

Technology Editor Bill Wong takes a close look at Renewable Silicon International's ChemArc process that is poised to significantly cut the cost of photovoltaic panels.

William G. Wong

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RSI ChemArc 6N silicon ingot

ChemArc ion exchange system

ChemArc mix and feed system

RSI electric arc furnace top

RSI electric arc furnace tap

I recently had a chance to visit Renewable Silicon International (RSI) to see their new production like for creating low cost silicon for photovoltaic (PV) solar cells. RSI's technology is called ChemArc. It delivers 6N+ pure silicon (Fig. 1) for solar cell manufacture at a price significantly lower than the alternatives that are typically targeting semiconductor manufacturers. Crystalline silicon (c-Si) is a major cost component in the creation of solar panels and RSI's ChemArc technology could significantly reduce this cost thereby making the payback of solar power lower and more competitive with other power generation technologies.

You can check out my visit to RSI on Engineering TV where I shot the following videos.

ChemArc Process

7N (99.99999%) pure silicon is required for semiconductors. It can be used for solar cells but the efficiency is similar if the purity is 6N instead. There are a number of ways to get this purity and a number of companies like RSI are going after this part of the market. In terms of volume, solar arrays can generate a significantly larger demand for silicon than the semiconductor industry. Luckily silicon is the most abundant element on earch after oxygen. The trick is to get it into a form that is usable.

At a high level, RSI simply takes water glass made sand plus soda ash and adds sugar in an electric arc furnace. The melt that comes out of the bottom of the furnace is 6N bulk crystalline silicon. Further refinement can occur when the block of silicon is processed by the panel vendor.

The approach has many advantages. It is scalable, safe, and emits carbon dioxide but no caustic effluent. Arc furnces can be tiny or large enough to generate giant buckets of steel. In theory it would even be possible to run the electric furnace from PV panels. The initial production ChemArc system that RSI is building takes up a tiny part of a large building. This means a ChemArc system could place near sources of materials or near panel production facilities.

The process is very efficient. It only requires 25 kWh/kg.

The trick that RSI pulls is starting with high purity feedstocks. It then improves this quality using an ion exchange system (Fig. 2) on the liquid components. It turns out that removing the impurities that plague PV silicon are more easily removed at this stage. The silicon and carbon sources are processed separately. These high purity components are then mixed in dry form (Fig. 3) and fed into the arc furnace (Fig. 4). A tap at the bottom of the furnace (Fig. 5) lets liquid silicon flow into ingot molds.

RSI does have a couple secret sauce items so this presentation is just a high level view.

PV Competition

RSI will have to compete with other silicon technologies like silicon thin-film cells based on amorphous silicon (a-Si) or nanocrystalline silicon (nc-Si). These are built using chemical vapor deposition (CVD). The typically methodology is plasma-enhanced CVD (PE-CVD) using silane and hydrogen gas. Amorphous silicon has a higher bandgap (1.7 eV) than crystalline silicon (1.1 eV) allowing the former to more readily absorb light especially infrared.

Anwell Technologies built a 40 MW a-Si thin-film PV solar panel manufacturing facility in Henan, China. Xunlight has created a 25 MW wide-web, roll-to-roll photovoltaic manufacturing system specifically designed for thin-film PV solar panel.