Stack impedance

Yuan XZ, Sun C, Blanco M, Wang H, Zhang J, Wilkinson D (2006) AC impedance diagnosis of a 500 W PEM fuel cell stack part I stack impedance. J Power Sources 161 920-8... 

Figure 6.35 shows the stack impedance (Figure 6.35C) calculated from Zn (Figure 6.35A) and Zw (Figure 6.35B). Similarly, through measurements of Ha, Hb, Hc, and Hd. the values of ZY, Zn, Zm, and ZiV can be calculated. The calculation procedure is shown below.

Andreasen et al. introduced a stack model that is suitable for prediction and analysis using EIS [45]. The typical output of such a measurement is a Nyquist plot, which shows the imaginary and real parts of the impedance of the measured system. The fuU stack impedance depends on the impedance of each of the single cells of the stack. Equivalent circuit models for each single cell can be used to predict the stack impedance at different temperature profiles of the stack. The results showed that a simple equivalent circuit model can be used to simulate the stack behavior. It was concluded that a more thorough characterization is required to predict the voltage dynamics under all operating conditions. 

Pure metallic cobalt has a soHd-state transition from cph (lower temperatures) to fee (higher temperatures) at approximately 417°C. However, when certain elements such as Ni, Mn, or Ti are added, the fee phase is stabilized. On the other hand, adding Cr, Mo, Si, or W stabilizes the cph phase. Upon fcc-phase stabilization, the energy of crystallographic stacking faults, ie, single-unit cph inclusions that impede mechanical sHp within the fee matrix, is high. 

Answer Use the plant s PSA to determine the risk of accidents that include containment failure from overpressurization. Then make a preliminary design of a vented containment that has sufficiently low impedance to the gas at the pressure predicted for the most severe accident sequences such that the containment is not damaged. This containment bypass will include iodine and HEPA filters as well as scrubbers and a discharge through a stack. Estimate the dose that the population would get using this bypass for comparison with the PSA result for ruptured containment sequences. 

The tight stacking when the racks are closed impedes air flow around the pallets, so this system is not suitable where some cooling of the product may be required. 

Rain caps at the end of the pipe are intended to keep rain from entering the system. Builders use various cap designs for this purpose. The use of rain caps can cause a loss of airflow in a system, which may lessen the effectiveness of the system. It is advisable to use a rain cap that is designed in such a way as to not seriously impede airflow. For more information on rain caps and stack design, see the ASHRAE Fundamentals Handbook.19... 

Other types and aspects of polymer-electrolyte fuel cells have also been modeled. In this section, those models are quickly reviewed. This section is written more to inform than to analyze the various models. The outline of this section in terms of models is stack models, impedance models, direct-methanol fuel-cell models, and miscellaneous models. 

When fuel cells work under heavy load conditions, it becomes more difficult to carry out the same measurements of AC impedance. One limitation on the application of EIS to large fuel cell stacks is that most commercially available load banks operating at higher currents do not have good frequency responses (ca. 10... 

Numerous studies have been conducted on AC impedance in single cells. From a practical point of view, stack experiments provide more interesting data because a single cell is rarely used in real-world applications. With AC impedance it is possible to separate the impedances of each cell of the stack, to understand the gas distribution within the stack, and to detect a failure in the stack. 

Diard et al. [37] studied the impedance of a commercial 10 W four-cell stack with individual cell geometric areas of 24 cm2 (8x3 cm2). The stack was operated on H2/02 at 80°C, and the experiments were performed with equal flow rates of 20 mL/min and pressures of Pu = 0.5 bar and P0 = 0.2 bar at the stack inlet. The... 

Figure 6.34. A Connections used for the EIS study of each element of a fuel cell stack running on a load. WE working electrode CE counter electrode RE reference electrode. B Two-electrode arrangement used for the measurement of Zw [37], (Reproduced by permission of ECS—The Electrochemical Society, and the authors, from Diard JP, Glandut N, Le-Gorrec B, Montella C. Impedance measurement of each cell of a 10 W PEMFC stack under load.)...

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