Interconnect geometric effect on performance of solid oxide fuel cells (SOFCs) running on alternative fuels

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Interconnect geometric effect on performance of solid oxide fuel cells (SOFCs) running on alternative fuels

 

Author: Sun, Qiong
Title: Interconnect geometric effect on performance of solid oxide fuel cells (SOFCs) running on alternative fuels
Degree: Ph.D.
Year: 2017
Subject: Solid oxide fuel cells.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Building and Real Estate
Pages: xxvi, 135 pages : color illustrations
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2950034
URI: http://theses.lib.polyu.edu.hk/handle/200/8867
Abstract: SOFC is attracting intensive attention in the area of clean energy conversion due to its high efficiency and environment-friendliness. The development of SOFCs stack is the key to commercialize SOFCs technology since single SOFCs must be connected into a stack to achieve a practical output voltage and high energy density for real applications. In a traditional planar type SOFC stack, the interconnect is an important component as it serves for several crucial functions, including collecting current, gas sealing and structurally forming flow channels for uniform gas distribution. The ribs of interconnect are defined as the part that separate any two neighboring fuel channels (or air channels at the cathode side). Due to the existence of interconnect ribs, the gas diffusion path and electron conducting path are strongly dependent on the interconnect rib width. As a result, the stack performance is sensitive to the interconnect rib width as the concentration loss and ohmic loss largely depend on the gas diffusion path and electron conducting path, respectively. However, there is dilemma in designing the value of interconnect rib width for the sake that concentration loss and ohmic loss are oppositely correlated to the width value: a smaller rib width is desired to reduce the concentration loss by shortening the gas diffusion path, while a larger rib width is preferred to decrease the ohmic loss by shortening the electron conducting path and increasing the contact area of interconnect rib and electrode. Therefore, there should be an optimum interconnect rib width value for the overall performance of SOFC stack. By reviewing literatures on geometrical optimization of planar SOFC stack by modeling, heat transfer is mostly neglected which could lead to inaccurate modeling results and misleading optimization suggestions, especially in the case of H2 fed planar SOFC stack, of which the maximum temperature difference can reach 100K along the gas channels. The stack performance is vulnerable to such large temperature difference in terms of overwhelming thermal stress, material compatibility and durability. Thus, it is necessary to incorporate the heat transfer into the modeling of SOFC stack. Based on the previous models, a comprehensive three-dimensional (3D) model of a unit cell of planar SOFC stack is further developed, considering electrons and ions conducting, electrochemical reactions, chemical reactions, gas transport and heat transfer. This 3D model is used to investigate the optimum interconnect rib width by series of parametric studies. A dimensionless variable, Ra, defined as the ratio of interconnect rib width to the width of the whole unit cell, is proposed to characterize the rib geometry, which is an easy-to-use parameter for practical stack geometric design. In addition, to generalize the results of optimum Ra, the SOFC stack fed with three different usual fuels (hydrogen, syngas and methane) are also modeled. Model validation is firstly conducted on the single cell level, and further validated in H2 fed stack case, by the comparison of optimum Ra between the current model and other researcher's simulation works under the isothermal assumption.
For the planar SOFC stack fed by three different fuels, the relations between the optimum Ra and main factors (pitch width, cathode porosity, ASR) are found to be similar. Considering the large difference between fuel diffusion coefficient and oxygen diffusion coefficient in H2 fed planar SOFC stack, an asymmetric design for the interconnect rib is proposed and the optimum anode Ra (Raaasym) and optimum anode Ra (Racasym) are accordingly obtained. The simulated stack performance indicates that the SOFC stack benefits significantly from this novel asymmetric design. This asymmetric design is also applied in syngas or methane fed SOFC stack. However, there is only slight increase in stack performance when optimum asymmetric design is adopted in SOFC fed by syngas or by methane with internal reforming. Followed by parametric studies, empirical equations are derived to predict the optimum Ra of planar stack, which is a constructive guideline for practical geometric design of SOFC stack. It is confirmed by experiments that CO electrochemical oxidation also occurs in H₂- H₂O-CO-CO₂ gas mixture system, therefore, it is unclear whether SOFC with proton ions conducting electrolyte (H-SOFC) is better than SOFC with oxygen ion conducting electrolyte (O-SOFC) in terms of maximum thermodynamic efficiency since CO electrochemical oxidation occurs in anode side of O-SOFC. In the later part of this thesis, a thermal dynamic study is conducted to investigate the effects of CO electrochemical oxidation on cell performance. It is shown that the maximum efficiency of H-SOFC is higher than that of O-SOFC when CO electrochemical reaction in O-SOFC is neglected, which is identical to the results in the previous literature. However, when CO electrochemical reaction in O-SOFC is involved, O-SOFC has higher maximum efficiency than H-SOFC.

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