(112c) Efficient Polycrystalline Silicon Production

Solar energy is an unlimited source that can satisfy mankind energy needs, generating an environmental impact comparatively lower than conventional sources of energy. In particular, photovoltaic solar (PV) energy has gained support over the years. It relies on transforming the solar radiation into power by means of semiconductor materials. Polycrystalline silicon wafers are used as starting material for capturing PV solar energy. Nowadays silicon is the most important material for the photovoltaic industry. The challenge currently faced by the PV industry is to reduce the production costs of polycrystalline silicon. More than 60% of the costs of solar cells are due to the production process of polycrystalline silicon [1]. There are two main possibilities to achieve cost reduction. One is the development of new production processes of polycrystalline silicon at a fair cost, and the other is to optimize existing processes achieving maximum energy conversion efficiency, and thus obtaining the lowest cost of polycrystalline silicon.

In this work, both strategies are used. A new optimized polycrystalline silicon production route is proposed based on the screening of different alternatives and process integration [2], and the operating conditions were optimized to evaluate the tradeoff within the process. The process is divided into four main sections. The first section is the carboreduction of SiO₂ using C to obtain metallurgical silicon. The modeling of the carboreduction reactor was developed by means of a subrogated model. The information on the distribution of the products in the C/SiO₂ carboreduction reactor was obtained from the work of Wai and Hutchison [3]. The second section is the reactor of the hydrogenation of silicon tetrachloride in the presence of metallurgical silicon. A two-stage strategy is used to model this unit using the experimental data in Ding et al. [4]. First, a Gibbs free energy minimization model is developed. In a second stage, a surrogate model is developed to be included in the flowsheet optimization; the third section consists in the purification of the chlorosilanes obtained from the previous reactor. Previous work optimized the distillation columns in ASPEN using a stochastic optimization approach [2]. Surrogate models are developed as a function of the feed rate and operating conditions of the columns. Finally, the fourth section is the conversion of trichlorosilane into polysilicon in a Siemens deposition reactor. The chemical vapor deposition of polysilicon from trichlorosilane and hydrogen is modelled based on the work of Del Coso et al. [5]. The entire process is modeled in GAMS as an NLP model for the optimization of the operating conditions.

The optimization of the process allowed to solve the tradeoff between yield and energy consumption along the process. For a production facility of 2000 t/y of polycrystalline silicon, the investment cost added up to promising values. The investment required for the process is 9.98 M$, which is mainly concentrated in the three main reactors, the two distillation columns, and auxiliary equipment such as compressors, tanks, and heat exchangers. The optimization shows that to maximize the profit of the process, an operating cost of 6.46 M$/y is required. Profits after operating expenses, and considering the sale of polycrystalline silicon and byproducts of the process (SiC, SiH2Cl₂, SiCl₄, HCl, and H₂), are 10 M$/y, presenting a competitive price of polycrystalline silicon of 8.93 $/kg, below the commercial price estimated at 11 $/kg [6].

References

[1] Müller, A., Ghosh, M., Sonnenschein, R., & Woditsch, P. (2006). Silicon for photovoltaic applications. Materials Science and Engineering: B, 134(2-3), 257-262.

[2] Ramírez-Márquez, C., Contreras-Zarazua, G., Martín, M., & Segovia-Hernández, J. G. (2019). Safety, Economic and Environmental Optimization Applied to Three Processes for the production of solar grade silicon. ACS Sustainable Chemistry & Engineering.

[3] Wai, C. M., & Hutchison, S. G. (1989). Free energy minimization calculation of complex chemical equilibria: Reduction of silicon dioxide with carbon at high temperature. Journal of Chemical Education, 66(7), 546.

[4] Ding, W. J., Yan, J. M., & Xiao, W. D. (2014). Hydrogenation of silicon tetrachloride in the presence of silicon: thermodynamic and experimental investigation. Industrial & Engineering Chemistry Research, 53(27), 10943-10953.

[5] Del Coso, G., Del Canizo, C., & Luque, A. (2008). Chemical vapor deposition model of polysilicon in a trichlorosilane and hydrogen system. Journal of the Electrochemical Society, 155(6), D485-D491.

[6] PVinsigths Grid the Word (2019). http://pvinsights.com/index.php