By Randall Gemmen and William Rogers,
National Energy Technology Laboratory, and
Michael Prinkey, Fluent, Inc.
The art and science of computer simulation are just beginning to make important contributions to the development of lower-cost, higher-efficiency, solid-state fuel cell systems, and those contributions are expected to grow rapidly as more accurate and robust models are developed. Building on previous work with one-dimensional codes, researchers at the U.S. Department of Energy’s (DOE) National Energy Technology Laboratory (NETL) are moving to full 3-D analysis of complex solid-state fuel cell geometries. Unlike the simpler analyses, 3-D analyses provide the fuel cell designer with not only performance predictions, but also thermal stress distributions and the ability to explore complex design options in a rapid, cost-effective manner.
Shown are the contours of current density on electrolyte. Illustration courtesy of Fluent, Inc.
Solid-state fuel cells, as the name implies, are comprised of solid-state components that can be fabricated using automated manufacturing technologies developed largely in the semi-conductor industry. A DOE coordinated national research initiative, the Solid State Energy Conversion Alliance (www.seca.doe.gov), is supporting the design and development of solid-state systems based on a 5-kW solid-state module that can be mass-produced and used in residential or auxiliary power unit applications, or applied to large fuel cell systems.
Although recent breakthroughs including advances in thin-film capabilities, anode-supported cells, compact fuel-processing technology, and improvements in power electronics at the device level will assist in the commercialization of these modules, there are still significant design issues to overcome.
One of the tools being developed to address these issues is an accurate computational fluid dynamics (CFD) method for simulating the performance of fuel cells so that design advances can proceed rapidly without the need for time-consuming build-and-test efforts. While available one-dimensional models can adequately predict performance, they cannot address other critical issues such as thermal stress distribution, an important consideration related to cell life. By coupling the CFD-predicted fluid flow behavior with the details of electrochemistry occurring in fuel cells, improved predictions are possible, which in turn could lead to improved fuel cells.
Work at NETL has already provided initial verification of the new CFD tool through comparisons with predictions by other one-dimensional codes. Based on the successful initial validation, NETL researchers are performing more sophisticated, three-dimensional simulations using FLUENT, a CFD product from Fluent Inc. One configuration studied was tubular solid oxide fuel cell (SOFC) technology, including a support tube on the cathode side of the cell. Six chemical species were tracked in the simulation: H2, CO2, CO, O2, H2O, and N2. Fluid dynamics, heat transfer, electrochemistry, and the potential field in electrode and interconnect regions were all simulated. Voltage losses due to chemical kinetics, ohmic conduction, and diffusion are accounted for in the model.
NETL’s CFD research to date demonstrates two major points. First, CFD analysis provides results that agree well with existing one-dimensional codes that have been validated experimentally. Second, CFD can provide the detailed temperature and chemical species information needed to develop improved fuel cell designs. Additionally, the output of the FLUENT-based fuel cell model has been coupled to finite element-based stress analysis software to model thermal stresses in the porous and solid regions of the cell. These steps should make it possible, in the near future, to create a model that will accurately predict the performance of cells and stacks so that critical design information, such as the distribution of cell and stack stresses, can be provided to the fuel cell design engineer.