According to the U.S. Department of Energy, photovoltaic power will be competitive in price with traditional sources of electricity within 10 years. 1 Contributing to this trend, Solar Power Industries, Inc. (SPI) of Belle Vernon, PA, uses finite element analysis (FEA) software from Pittsburgh-based ALGOR Inc., as a tool for developing faster, more efficient and less expensive ways to manufacture solar cells.
SPI used the software to analyze the process of casting silicon ingots, which are used to manufacture the photovoltaic cells, inside a directional solidification furnace. Said Chenlei Wang, Ph.D., senior engineer with SPI, “ALGOR multiphysics software helped us optimize the furnace’s hot-zone design, which was the key factor for the project.”
Developed by Solar Power Industries, Inc. a rooftop solar array (shown at bottom), converts sunlight into electricity and feeds directly into the Carnegie Mellon University main power supply.
“Our current annual production capacity for processing polycrystalline silicon feedstock into completed solar cells has grown to 40 megawatts,” said Wang. “We plan on significantly increasing capacity to reach 250 megawatts over the next several years.”
Chenlei Wang, Ph.D., Senior Engineer, stands in front of SPI’s minicaster. The furnace produces silicon ingots that are used for research into making solar cells.
SPI’s solar cell manufacturing process consists of three main steps:
1) Ingot and wafer production – High-quality silicon feedstock (containing specific quantities of dopants such as boron 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.
2) 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.
3) 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. For example, 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% of the building’s electricity needs.2 That’s enough energy to support more than 80 computers while reducing the need for fossil fuel-based electricity. 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, the first step was to develop a miniature version of a directional solidification furnace (called a “minicaster”) to efficiently cast small ingots for research. “The minicaster would allow us to evaluate candidate feedstock sources and growth techniques on less material and with faster turnaround times,” said Wang. “We can conduct research on variables such as silicon feedstock, doping and solidification cycles.”
Two different types of results are displayed from the ALGOR steady coupled fluid flow and thermal analysis of the melting phase: on the top, fluid velocity vectors within the silicon melt; and, on the bottom, temperature contours throughout the assembly.
Part of the PEDA funding was used to purchase Algor multiphysics software, which was used for the analysis of the minicaster design.
Inside the minicaster, silicon feedstock is loaded in a vitreous quartz crucible. Graphite plates surround the crucible for 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 to achieve the desired cooling rate and directional solidification growth.
Wang explained, “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.”
“We created a 3-D model of the minicaster in Autodesk Inventor,” said Wang. “Then, we modeled the cross-sectional geometry in ALGOR.”
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,” said Wang. “The strong velocity field inside the silicon melt cannot be neglected. Therefore, we used multiphysics analysis to couple the calculation of the silicon melt flow field and temperature field, which accounted for the effect of natural convection.”
The first silicon ingot produced by the minicaster (top) exhibited regions at the top where solidification proceeded erratically. After the hot zone was modified based on ALGOR analysis results, higher-quality ingots were produced (bottom).
Wang performed a steady coupled fluid flow and thermal analysis 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. The software’s 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 some concerns. “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,” explained Wang. “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.“
ALGOR transient heat transfer analysis results show the temperature distribution with optimal
insulation lift, heater position and heater power level.
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%, surface solidification was prevented during the growth process,” said Wang. “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.”
“The PEDA-funded research project was finished in 2006,” said Wang, “but research using the minicaster is still ongoing. Additionally, I am using ALGOR to simulate other crystal growth furnaces.”
ALGOR Inc.
www.algor.com
Sources:
1. “Solar History Timeline: The Future” (http://www.eere.energy.gov/solar/solar_time_future.html), U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Solar Energy Technologies Program, January 5, 2006.
2. “Solar Energy Helps Power Campus Facility” (http://www.cmu.edu/cmnews/extra/050718_solar.html), LeeAnn Baronett, Carnegie Mellon Today, July 18, 2005.
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