2008

Heat Transfer Analysis for Optimizing Solar Cell Casting Equipment

Finite element analysis was used to develop a miniature furnace to cast the solar cell wafers.

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.