. "Appendix D: Environmental Considerations for Photovoltaics." The Power of Renewables: Opportunities and Challenges for China and the United States. Washington, DC: The National Academies Press, 2010.
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The Power of Renewables: Opportunities and Challenges for China and the United States
TABLE D-1 Economic Factors for an Annual Production of 1,000 Tons of Polysilicon in China
Yield of polysilicon
1,000 tons per annum
U.S.$ 0.17 billion
U.S.$ 70–80/kg (China)
U.S.$ 25/kg (Europe and U.S.)
0.6 billion kWH per annum
Power produced by solar cells
200 MW per annum
Power regeneration ratio (over 20 years)
Power generation cost
7–12 times biomass power generation, 6–10 times wind power generation, 11–18 times traditional coal power generation.
8,000 tons per annum
In the above steps, there are two pollution problems that need to be solved: the capture of by-product CO2 in the preparation of industrial silicon, and the recycling of the tail gas in the purification of SiHCl3 from the gas mixture.
Reduction of SiHCl3
High purity polysilicon is obtained from the reduction of SiHCl3. The reactions are as follows.
Main reaction: SiHCl3 + H2 = Si + 3HCl↑,
Secondary reaction: 4SiHCl3 = Si + 3SiCl4 + 2H2↑.
Si obtained from reactions (1) and (2) is deposited to form high purity polysilicon. In this process, about 25 percent of trichlorosilane is converted into polysilicon, and the remainder into tail gas. A large amount of tail gas containing many useful materials such as H2, HCl, SiHCl3, and SiCl4 is emitted from the reduction furnace. Moreover, the tail gas includes many erosive and toxic substances. The tail gas must be recycled, or else it will cause serious pollution and also increase costs.
Silicon tetrachloride (SiCl4) is the main component in the tail gas. It is a highly corrosive and toxic liquid. About 8,000 tons of by-product SiCl4 are produced in the production of 1,000 tons of polysilicon. Due to the expensive treatment cost of SiCl4, most Chinese companies do not have such treatment equipments, which then makes tail gas treatment become the bottleneck in the manufacture of polysilicon.