Solar Power Industries' (SPI) current annual production capacity for processing polycrystalline silicon feedstock into completed solar cells has grown to 40 megawatts, with plans to increase capacity to 250 megawatts over the next several years. SPI's solar cell manufacturing process consists of three main steps:
- Ingot and Wafer Production—High-quality silicon feedstock (containing specific quantities of dopants such as boron in order to alter electrical properties) is melted and solidified inside a directional solidification furnace to cast polycrystalline silicon ingots. The ingots are cut into rectangular blocks with a square cross-section, and then the blocks are sawed into thin multicrystalline wafers.
- Cell Production — The wafers are etched to remove surface damage caused by sawing. The wafers are then processed in a series of steps to produce photovoltaic cells.
- Module Assembly — Individual cells are connected by soldering to flat wires. Strings of cells are then joined to parallel connector wires and laminated to produce a solar module.
Modules can be installed in a solar energy system to convert captured sunlight into usable electricity. SPI installed a rooftop array of 120 solar panels at a building on the Carnegie Mellon University campus, which feeds directly into the main power supply, providing approximately 10 percent of the building's electricity needs. The system also reduces the output of greenhouse gases by more than 31,600 pounds per year.
SPI received funding from the Pennsylvania Energy Development Authority (PEDA) for a research program aimed at expanding the supply of silicon feedstock for producing ingots by the directional solidification technique. Since casting of commercial-size ingots is expensive and time-consuming, there was a need to develop a miniature version of a directional solidification furnace (called a "minicaster") to efficiently cast small ingots for research. The smaller size of the minicaster would allow for the evaluation of candidate feedstock sources and growth techniques on less material and with faster turnaround times.
Inside the minicaster, silicon feedstock is loaded in a vitreous quartz crucible. Graphite plates surround the crucible, providing mechanical support. Surrounding the crucible is a bank of resistive heaters that uniformly heats the charge. A movable insulation cage serves as the primary means by which the desired cooling rate and directional solidification growth is achieved. In order to assess the design of the minicaster "hot zone" prior to fabricating the components, finite element modeling and analysis was first carried out for the melting phase and then the solidification phase.
A 3D model of the minicaster was created in Autodesk Inventor software, and the cross-sectional geometry was modeled in ALGOR finite-element analysis software. Custom-defined, temperature- dependent, orthotropic material properties were specified for the silicon feedstock, quartz crucible, graphite heaters, and insulation. Thermal loads were defined for internal heat generation, surface radiation at the outside surfaces, and body-to-body radiation between exposed internal surfaces. Fluid velocities were specified for surfaces that surrounded the silicon.
Natural convection due to buoyancy plays an important role for transport phenomena inside the silicon melt. The strong velocity field inside the silicon melt cannot be neglected. The SPI team used multiphysics analysis to couple the calculation of the silicon melt flow field and temperature field, which accounted for the effect of natural convection. Steady coupled fluid flow and thermal analysis were performed to obtain the convective fluid flow and temperature results for the melting phase.
For the solidification phase, a lower internal heat generation value was used to simulate lower temperatures while cooling. Transient heat transfer analysis results allowed SPI to better understand the minicaster's solidification process. Upon examining the first silicon ingot produced by the minicaster, SPI noticed that most of the ingot's surface was flat and smooth, but there were some regions at the top of the ingot where the solidification proceeded erratically. This was thought to be associated with an undesired solidification at the top of the melt, which initiated while solidification was occurring from the bottom upward. Such solidification was predicted by the thermal finite- element model of the growth.
In order to maximize ingot quality, multiple transient heat transfer analyses of the solidification phase were conducted to determine the best placement and output power for the minicaster's heaters. By adjusting the heater position and increasing the heater power level by 25 percent, surface solidification was prevented during the growth process. Another effective way to modify the thermal environment was by adjusting the insulation lift distance. The resulting solidification interface was flat and slightly convex to the silicon melt, which is beneficial for high-quality silicon crystal growth.
This work was done by Dr. Chenlei Wang, Senior Engineer for Casting Technology, at Solar Power Industries, Belle Vernon, PA, using software from ALGOR, Inc., Pittsburgh, PA. For more information, click here .