| With the progress of society and development of industrial economy, human beings’ demand for energy is increasing. However, non-renewable resources such as coal, oil and natural gas are far from enough to meet the need of sustainable development of society and economy, what’s worse, they lead to green house, environmental pollution as well as ecological destruction, seriously affecting human’s living condition.Therefore, countries of the world are constantly developing recycled energy to replace fossil resources such as coal,oil and natural gas. Among all the reusable resources, solar energy has more advantages, for instance, zero pollution and universal availability. Accordingly, technology related to photovoltaic power generation is attached increasingly greater importance in countries of the world, and most governments have made policies and laws or taken other measures to offer subsidies and policy support to the industry of solar photovoltaic power-generation.At present, solar cells such as thin-film solar cells, silicon solar cells and solar cells based on semi-conductor materials are widely used. Among silicon solar cells,multi-crystalline silicon cells, despite their lower light-to-electricity conversion efficiency in comparison with monocrystalline silicon solar cells, take up 60% market shares in photovoltaic industry due to their merits of low production costs, low energy consumption and low attenuation; besides, the technology of multi-crystalline silicon ingots is mature and the purity demand for raw material is comparatively low.The bottleneck of solar power-generation industry lies in lower light-to-electricity conversion efficiency and higher cost compared with conventional energy,so it becomes the focus of countries of the world how to improve the conversion efficiency ofsolar cells and mean while lower the production cost effectively. In the early stage, to improve conversion efficiency, scholars at home and abroad employed directional solidification method to grow quasi-single crystal silicon, whose conversion efficiency, with lower production cost, is almost the same as that of single crystalline silicon, but mono-like silicon ingots have not been promoted universally due to inconsistency, instability andlow utilization rate. In the respect of production cost reduction, a multi-crystalline silicon furnace with 4ingots was developed by a domestic enterprise. By using the ingot furnace, the amount of ingot from single furnace was increased and production cost was reduced. However, owing to immature technology, such furnaces were only used in a few factories for purification of silicon material. In recent years, with the over supply of photovoltaic capacity, the production risk is much higher, so such furnace is less applied or promoted now.This paper systematically studies ways to reduce energy loss in the process of multi-crystalline silicon ingot formation, guarantee stable production and improve the quality of multi-crystalline silicon with an aim to improve photoelectric conversion efficiency of solar cells, reduce energy consumption and lower costs for photovoltaic power-generation,thereby speeding up the application of photovoltaic products. The main contents include:1. Study on Thermal Field Visualization.Aimed at resolving the problem of low thermal efficiency of existing equipment,thermodynamic mathematical model sconcerned with the formation of multi-crystalline silicon ingot were built and heat-transfer equations of base material were established through analytical study of multi-crystalline silicon ingot in the phase of silicon material melting,crystal growth and ingot annealing. Based on actual physical structure of equipment,three-dimensional visual models of physical field were made; then, with the application of simulation software, thermodynamic simulation analysis of the physical field was performed so as to know the rule of temperature distribution in heating chamber. Next, through the analysis and comparison of simulation results and experiment data, both simulation models and boundary conditions were tested and corrected to improve the accuracy of established physical model. In addition, based on simulation result, two modification schemes were put forward to improve effective working space of heating chamber. In the end, numericalcalculation for optimized model of thermal field was made and simulation results were used to guide equipment modification. The experiment result indicated that after reconstruction guided by the second scheme, charge weight of the improved equipment increased as high as70%, Energy consumption for per kilogram of silicon ingot decreased by about 30%, when all the performance indicators of silicon ingots meet the technical requirements.2. Study on Characteristics of Temperature ControlStep response method was applied to identify parameters of temperature control model in ingot production. Since crystalline growth processis time-dependent and non-linear with pure delay, biginertial, strong coupling and multiple variables, the paper creatively proposes to add fuzzy controller to control central point temperature at the bottom of the crucible in a closed-loop system, forming cascade control system with PID control of temperature in heating belt. The control precision was improved by estimating and accurate controlling of central point temperature at the bottom of the crucible. MATLAB software packages were used for simulation design of the proposed fuzzy control algorithm, and ultimately the reasonable and right control strategies were drawn. The results showed that: when parameters of controlled object are uncertain, adaptive fuzzy control is better than conventional PID control in terms of anticipation and anti-interference capability. Further more, by estimating the tendency of temperature change in multi-crystalline silicon production, adaptive fuzzy control eliminates the effect of random destabilization on system and the defects of manual intervention. Thereby the capability of self-adaption and anti-interference of the control system was improved. With a stable control system, crystalline quality showed more uniformity.3. Study on optimization of processing parametersBased on crystal growth kinetics and equipment characteristics, key processing parameters of crystalline growth were analyzed; in addition, theoretical analyses and comparison of whole melting processing, monocrystalline processing and semi-molten processing were made and the performance of silicon ingots and silicon wafers made under three different technologies and conditions were tested. According to experiments performed with semi-molten and whole melting parameters, after modification of thermal field andoptimization of temperature control system, grains grown in semi-molten technology showed more uniformity, and the silicon ingot has a significant advantage in utilization rate,minority carrier lifetime and conversion efficiency.The numerical calculation of thermal field and simulation of fuzzy control in this paper can offer theoretical guidance for improvement of existing multi-crystalline silicon ingot furnace. The semi-molten process proposed in this paper was put into actual use and after 1year’sindustrial-scale production; the results showed that performance of multi-crystalline silicon ingots can meet the technical requirements with yield rate being more than 90%.Furthermore, the production capacity and economic benefit were significantly enhanced. The study should also provide reference to the design and application of similar ingot furnaces. |
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