| The development of sustainable and clean energy sources is imminent due to the energy deficit and environment pollution.Solar energy is one of the most renewable energy sources,and the effective utilization of it has become vitally important.As the photovoltaic(PV)installations have increased dramatically,the photovoltaic technologies have got more and more attention in the world.Many processes exist for the production of polysilicon,which is the most dominant materials for PV industry,such as the Siemens process,metallurgical process and silane process.The Siemens process is the main technology for polysilicon production due to its high quality and mature technology.But the high cost and energy consumption are the main challenges presently.Increasing the maximum deposition radius of the silicon rod can reduce the energy consumption of the polysilicon production.However,due to the presence of the temperature gradient inside the silicon rod,silicon melting occurs and the rod is broken under the action of thermal stress.Increasing the number of silicon rods and optimizing the arrangement of silicon rods is an effective solution to reduce the energy consumption of polysilicon production,and is also one of the effective ways to increase the production capacity of polysilicon.Therefore,the heat transfer phenomenon in different Siemens reduction furnaces(12 pairs of rods and 24 pairs of rods)is systematically studied.And the method of distributed simulation coupling silicon rod surface heat loss and electric heating model of silicon rods has been proposed.Then the coupled thermal stress model was used to establish the electro-magnetic-thermal-stress coupling model of the silicon rod.Finally,the energy consumption was analyzed and a scheme for reducing energy consumption was obtained.The main contents and results are given as follows:A dimensionless analysis of the relationship between dimensionless parameters and convective heat transfer coefficients describing the Siemens reduction furnace configuration(number of silicon rods,diameter of silicon rod,length of silicon rod)and operating conditions(temperature,air intake)A dimensionless X-H(defined as n?)describing the number of silicon rods and the space area within the Siemens reactor was proposed.It was found that convective heat transfer increased significantly when Ra was greater than 1.4×107,d/L was greater than0.04,and X-H was greater than 3.0.And through dimensionless analysis,the correlation of the convective heat transfer described by the Siemens reduction furnace is obtained:Nu/Ra0.584(d/L)-0.366=0.00003(X-H)(10)0.00039,and verify the validity of the correlation.The results show that the average relative errors of predicted and calculated results are 1.4%and3.2%,respectively,indicating that empirical correlation can effectively predict convective heat transfer.(2)The DO radiation model of Siemens reduction furnace was established.The model was validated by 12 pairs of Siemens reduction furnace industrial data.The relative error was 7.1%,and the maximum relative error was 8.3%,indicating that the model was accurate and reliable.The effect of the deposition radius of the silicon rod on the radiative heat loss was studied using a model.The results show that for an external silicon rod(outer ring silicon rod of12 pairs of rods and 24 pairs of rods Siemens reduction furnace),as the radius of the silicon rod deposition increases,radiation heat loss gradually increases;for internal silicon rods(inner silicon ring rods of 12 pairs of rod Siemens reduction furnaces,24 pairs of rods of Siemens reduction furnace inner ring and central silicon rods),the radiation heat loss of rods increases with the deposition radius of silicon rods.The first increases and then decreases,there is an extreme value.As the radiation rate of the reactor wall is increasing,the radiant heat loss in the Siemens reduction furnace has been gradually increased,and Siemens reduces the inner ring of silicon rods(12 pairs of rods and 24 pairs of rods)and/or the central silicon rod(24 pairs of rods).The radius of the silicon rod gradually increases when the radiant heat loss occurs at a maximum value.(3)The electric heating(DC and AC)model of the silicon rod was established,and the validation of the model was verified by the current curve of industrial operation.For 12 pairs of rod Siemens reduction furnaces,the relative error of the inner ring is 4.89%and the relative error of the outer ring is 4.76%;for the 24 pairs of rod Siemens reduction furnace,the relative error of the inner ring silicon rod is 7.55%,and the relative error of the silicon ring of the ring is5.55%.The relative error of the outer ring silicon rod is 7.43%,indicating that the model calculation result is reliable.The electric heating model of the silicon rod was used to study the thermoelectric behavior(including temperature distribution,current density distribution,thermal stress distribution,current and voltage operating curves,etc.)of 12 pairs of rods and 24 pairs of rods in a Siemens reduction furnace.The results show that:When the silicon rods is heating by current,there is a temperature gradient inside the silicon rod.As the frequency of the alternating current increases,the center temperature of the silicon rod becomes lower.When the silicon rod is heated by direct current,the current density inside the silicon rod is almost the same;when the silicon rod is heated by alternating current a high density of current density is formed on the silicon rod surface inside the silicon rod;Current-voltage operation curves of the silicon rods on different rings are obtained;When the silicon rod radius is small,the current supplied to the silicon rod is almost the same at different alternating current frequencies;As the radius of the silicon rod increases,the external silicon rod needs to supply more current than the internal silicon rod.(4)A novel polycrystalline silicon reduction furnace“DC-AC hybrid current heating”heating method is proposed.When the silicon rod radius is less than 7cm,direct current heating is used.When the silicon rod radius is greater than 7cm,alternating current heating is used.When the dc power supply is switched to ac power to continue heating of the inner ring silicon rod,the current intensity to be provided is lower than the current intensity provided by the DC power source,and the lower the current intensity to be supplied is,as the frequency of the AC power increases.However,the required voltage is increased,and as the frequency of the AC power supplying the power increases,both the self-inductance resistance and the mutual inductance increase,and the required voltage increases.For Siemens reduction furnaces,the current strength and voltage of the outer ring silicon rod are greater than the inner silicon rods(12 pairs of rod inner rings,24 pairs of rod inner rings and middle rings);and for 24 pairs of rod Siemens reduction furnaces,the current strength of the central silicon rods and the voltage is greater than the inner silicon rod.(5)When the silicon rod is heating by direct current(maximum deposition radius of 7 cm),the average specific energy consumption of the polysilicon in the Siemens reduction furnace for the 12 pairs of rods is 55 kWh/kg-Si,and for the 24 Siemens reduction furnace is 49 kWh/kg-Si;The average unit energy consumption of polysilicon in Siemens reduction furnaces for 12 pairs of rods and 24 pairs of rods is 40 kWh/kg-Si and 38 kWh/kg-Si,respectively,when the silicon rods is heating by mixed current(maximum deposition radius is 10 cm).With the increase of the maximum deposition radius of silicon rods,the average unit energy consumption of polysilicon gradually decreases.The average energy consumption of the 24 rod Siemens reduction furnace is lower than 12 pairs of Siemens reduction furnaces.Through the above analysis,we can provide two different methods for saving energy and reducing consumption of polysilicon:increasing the maximum growth radius of polysilicon and increase the number of silicon rods in the polysilicon reduction furnace. |
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