Silicon cell production is usually done in silos, each of the step done independently of the other. In each step Silicon is heated and reheated to 1800 degrees centigrade.
Quartz to Panels
For mining and preparation of quartz
A site location is determined by Geospatial Mapping, a test for suitability of quartz is extracted and certified for production. One is checking for a minimum of ferrous impurities. A site extraction plan is developed.
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This first step, conversion to metallurgical grade silicon, combines in proportion Quartz, Coke and Woodchips for a carbothermal reaction in a submerged arc furnace. The process yields 99.5% (2N) Silicon, carbon dioxide and silica fumes. It consumes energy up to 12 MWh per ton of metallurgical grade Silicon. Depending upon raw material quality this process may limit impurities as in table below:
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Fluidized Bed Reactor (FBR) silicon refining process, a next-generation method of refining silicon. With the FBR process, solar-grade silicon at a lower cost, while using 80-90 percent less energy than the traditional Siemens method by converting silane gas to high purity silicon. Or a metallurgical process uses molten aluminum as a solvent to remove boron, phosphorus and other impurities from MG-Si. The impurities have an affinity to stay in the melt, from which silicon grows in form of pure flake-like crystals. This combination of aluminum and silicon also reduces the melting point of the alloy, enabling lower-temperature operations. This process must limit impurities as in table below:
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To make the sheets, two high-temperature strings, or filaments, rise through a shallow crucible of molten silicon. A thin film of silicon spans the space between them, supported at the edges by the strings. “It’s like pulling a wand out of a soap solution,” Sachs said, “and the edge of the wand holds the soap film in place.” But in String Ribbon the film instantly turns solid, forming an 8-centimeter-wide silicon ribbon, just 200 microns thick. The process is continuous. Chunks of silicon enter the furnace and melt, the strings unwind from spools, and the emerging ribbon is cut to 2-meter lengths without stopping. Each ribbon is then laser-cut into wafers, which go directly onto a belt for the next step in becoming solar cells and ultimately high-efficiency solar panels.
Stauffer, Nancy MITEI, Feb 2007, Building solar cells from ribbons.
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Light weight wafers can be shipped anywhere for panel assembly or fabrication.
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By combining first three or four steps under a single roof reduces electricity usage, a major cost component in the manufacture of silicon wafers. Steps we are taking in achieving this major breakthrough:
1. Production of Silicon for semiconductor fabrication by tapping and achieving 9N purity required
A major use of pure silicon is now in solar energy application. Some silicon may be tapped, upon achieving purification to solar grade Silicon, for semiconductor fabrication to achieve 9N purification of Silicon.
2. Demonstration of benefits by integrating production of silicon to cost of solar energy:
Reducing Carbon Footprint, cost of electric production per watt and potential savings due to manufacturing integration
3. Challenges in achieving integration in measuring quality of silicon purity at high temperatures above 1200-1800 degrees centigrade
High temp chromatic testing
high temperature metallurgy when a sample is withdrawn from a high temperature process it melts a vessel or pipe used
high temperature tapping
4. Computer Simulation of both the current and integrated process
Simulation software to manage the integrated manufacturing process and confirm savings and reduced carbon footprint.
Silicon cell from Quartzite
The manufacturing process is in four stages. Each stage done in isolation requires transportation, heating and cooling silicon multiple times. Stages are identified in the diagram as follows:
Q Quartzite mining
S Conversion to MG – Silicon
N Conversion of MG silicon to Solar Grade Silicon
W Conversion of Solar Grade Silicon to Wafers
5. Setting up of a pilot plant to proceed from current to integrated process
For each of the steps in Solar cell manufacturing, the facility will use
S – mini SAF furnace for conversion of Quartz to MG Grade Silicon,
N – an induction furnace for conversion to 5N silicon,
W – furnace for string ribbon manufacturing.