(617es) Three-Dimensional Fluid Dynamic Analysis of a Newly Designed 100 cm2 Single Cell with Internal Flow Channels for Molten Carbonate Fuel Cells

Molten carbonate fuel cells (MCFCs) were developed to produce clean and efficient power conversion. MCFCs are one of the promising sources of renewable energy for the high-efficiency cogeneration of electricity and heat with minimal environmental impact [1]. MCFCs are operated at a high temperature of approximately 650 Ë?C. Because of the high temperature, precious metal catalysts are not required during the operation. MCFCs employ an electrolyte of molten carbonate salt mixtures. Carbonate ions act as an electric charge carrier.

The current collector lies between electrodes and cell frames. The current collector forms gas flow channels. The matrix and electrolyte separate anode gases (H₂, CO₂ and H₂O) and cathode gases (Air and CO₂). In the anode side, H₂ reacts with Carbonate ions from electrolytes to produce water (H₂O), carbon dioxide (CO₂). Current generated by electro chemical reaction of the MCFC flows through the contact area between the current collector and the electrodes. The geometry of the current collector and gas flow channel affects the flow characteristics which have a large effect on the performance of fuel cells. Also, the arrangements of the shielded slot plate influence gas flow characteristics [2].

In this work, a newly designed 100 cm² cell frames with internal flow channels for molten carbonate fuel cells (MCFCs) was investigated using three-dimensional fluid dynamic analysis. Also, the effect of the current collector structure on the performance of molten carbonate fuel cells (MCFCs) was investigated. From the simulation, pressure drop, flow field, and gas mole fractions inside the cell frame were studied.

In the operation of the single cell, a Ni - 5wt% Al anode, a lithiated NiO cathode, and γ-LiAlO₂ matrices were used. For the electrolyte, Li₂CO₃ and K₂CO₃ (62:38) were used. The single cell was operated at 620Ë?C. The gas utilizations for the anode side (H₂) and the cathode side (CO₂, O₂) were fixed to 0.4 at 150mA/cm².

Cell frames with inner flow channels were utilized. The cell frame with internal flow channels has advantages in controlling the temperature of the cell frame and components. In addition, the gas uniformity of the gas flow channel increases by separating the gas flow line in the cell frame. The gas flow line outside the cell frame acts as a pre-heater. The size of the cell frame was 130 mm (length), 130 mm (width), and 30 mm (height). The active area of the single cell was 100cm². In order to make close contacts between current collector sand electrodes, a sealing pressure of 0.2 MPa was applied to the cell frame using a hydraulic pressure.

To investigate the effect of the current collector on the performance of MCFCs. Different types of current collectors were employed in experiments and simulation. In this work, three types of current collectors such as a sheet with sheared protrusions (shielded slot plate) and perforated sheets were employed. In case 1 and case 2, the shielded slot plates were used as a current collector. The difference between case 1 and case 2 is the stacking direction of the shielded slot plate. In case 3, the perforated sheet was used as a current collector.

In the simulation, the commercial CFD code (COMSOL multi-physics, COMSOL Inc.) was employed in order to calculate the steady state such as the energy balance, species balance, momentum and continuity equations. In the simulation, the polarization model by Yuh and Selman [ref] was adopted. In this polarization model, total polarizations were the sum of anode polarization resistance (R_a), cathode polarization resistance (R_c) and ohmic resistance (R_ohm). Each polarization resistance was correlated using linear multiple regression in various gas conditions [3].

In previous studies, the gas flow channels were considered to be a porous media [4]. The flow characteristics in a porous media were modeled with Darcyâ??s law. In the simulation model with Darcyâ??s law, detailed flow characteristics of flow channels were not considered. In addition, the heat transfer between the current collector and gases was not properly considered in the simulation model. Instead of using porous media approach, the flow channels were three-dimensionally modeled.

Three types of heat transfer models such as natural convection between the heat chamber and the cell frame, forced convection between the gas and the gas flow channel and conduction between the cell frame and the MCFC components were considered in the simulation. Thermal properties of the mixture gases of the anode and cathode gas were calculated with respect to the temperature, gas compositions, and electro-chemical reactions.

For the verification of the simulation model, experimental results were compared with the simulation results. The performance of the cell can be predicted precisely. The distributions of the gas concentration, current density, and polarization components were investigated using the simulation results. The cell frame with internal flow channels showed improved thermal characteristics. Cell frames with internal flow channels were operated without any hot-spots. From the simulation results and experimental results, it was found that the current collector structure, which makes large gas open area to electrodes, results in effective gas supply to electrodes, and as a result, the performance of the fuel cells increases.

References

[1] R. O'Hyare, S. W. Cha, W. Collela, F. B. Prinz. Fuel cells - Fundamentals. New York: John Wiley & Sons; 2006

[2] Li X, Sabir I. Review of bipolar plates in PEM fuel cells: Flow-field designs. International Journal of Hydrogen Energy. 2005;30:359-71.

[3] Yuh C, Selman J. The Polarization of Molten Carbonate Fuel Cell Electrodes I. Analysis of Steady�State Polarization Data. Journal of the Electrochemical Society. 1991;138:3642-8.

[4] Kim H, Bae J, Choi D. An analysis for a molten carbonate fuel cell of complex geometry using three-dimensional transport equations with electrochemical reactions. International Journal of Hydrogen Energy. 2013;38:4782-91.