Thermal (that is, infrared) imaging has been demonstrated to be an effective technique for the diagnosis of operating fuel cells and fuel-cell stacks. Thermal imaging can be used to identify a variety of phenomena, described below, that are associated with nonuniform generation of heat. Hence, thermal imaging is expected to be widely used in fuel-cell research, development, and manufacturing.
The performances of fuel-cell stacks (especially micro-fuel-cell stacks) are commonly adversely affected by nonuniform distributions of fuel, high resistances of electrical interconnections, cell-to-cell variations, and other phenomena associated with nonuniform generation of heat. The analysis of such phenomena by means of individual cell measurements (e.g., point-probe electric-potential measurements) can be quite tedious, even for a short stack; in the case of flat-pack micro-fuel cells, such measurements are almost impossible. Under these circumstances, one is often left guessing as to the causes of reduced performance.
The generation of heat always accompanies the operation of a fuel cell. The generation of heat is due to inefficiency of the basic fuel-cell electrochemical reaction, crossover (residual diffusion through the fuel-cell solid-electrolyte membrane) of fuel (usually, the fuel is methanol), and electrical heating of interconnection resistances. Temperature differences occur if any of these heat-generating processes occur differently in different parts of a fuel-cell stack. For example:
- Nonuniform distribution of fuel across the surfaces of electrodes leads to nonuniform distribution of electric current and hence temperature differences;
- High-resistance interconnections in a stack distinguish themselves by producing more heat than the others do; and
- Variations among cells within a stack, arising from variables in fabrication, can cause one or more cells to be more efficient than the others are, leading to thermal gradients.
These examples illustrate how mapping the surface temperature of a fuel-cell stack during operation can yield useful information about the processes occurring in the stack.
Infrared cameras equipped with quantum-well infrared photodetectors (QWIPs) can detect temperature differences as small as 0.005 K. Such a camera has been found to be particularly useful for monitoring processes in fuel cells. For example, the figure shows the temperature variations on the cathode side of a six-fuel-cell flat pack, both in the open-circuit condition and with an electrical load connected. The cell marked 4 exhibits a temperature greater than do the others, even in the open-circuit condition: this is attributed to a high methanol-crossover rate in the particular cell. With the load connected, the interconnections also exhibit differences in temperature, and some of them can be identified to be substantially more resistive than others are.
This work was done by Sekharipuram Narayanan and Thomas Valdez of Caltech for NASA's Jet Propulsion Laboratory.
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Thermal Imaging for Diagnosing Fuel Cells
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Overview
The document titled "Thermal Imaging for Diagnosing Fuel Cells" discusses the application of thermal (infrared) imaging as an effective diagnostic tool for fuel cells and fuel-cell stacks. It highlights the challenges faced in fuel-cell performance due to nonuniform heat generation, which can arise from various factors such as uneven fuel distribution, high electrical interconnection resistances, and variations among individual cells within a stack.
Thermal imaging is particularly valuable in identifying these issues, as it allows for the mapping of surface temperatures across fuel-cell stacks during operation. This technique can reveal critical information about the underlying processes affecting performance, which is especially important for micro-fuel-cell stacks where traditional measurement methods can be cumbersome or impractical.
The document notes that the generation of heat in fuel cells is a byproduct of inefficiencies in the electrochemical reactions, fuel crossover through the solid-electrolyte membrane, and electrical heating due to resistance in interconnections. These heat-generating processes can lead to temperature differences across the stack, which thermal imaging can effectively visualize. For instance, nonuniform fuel distribution can cause variations in electric current and temperature, while high-resistance interconnections can produce excess heat.
The use of infrared cameras equipped with quantum-well infrared photodetectors (QWIPs) is emphasized, as they can detect minute temperature differences as small as 0.005 K. This sensitivity allows for detailed monitoring of fuel-cell processes. The document provides an example of thermal imaging results from a six-fuel-cell flat pack, illustrating how temperature variations can indicate issues such as high methanol crossover rates in specific cells and differences in interconnection resistances when an electrical load is applied.
The work presented in the document was conducted by Sekharipuram Narayanan and Thomas Valdez at Caltech for NASA’s Jet Propulsion Laboratory. The findings underscore the potential of thermal imaging to enhance the understanding and performance of fuel cells, paving the way for advancements in fuel-cell research, development, and manufacturing.
In summary, the document highlights the significance of thermal imaging in diagnosing fuel-cell performance issues, providing a non-invasive and efficient method to analyze and improve fuel-cell technology.
