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Research Paper

Analysis of Residual and Operational Thermal Stresses in a Planar SOFC

[+] Author and Article Information
Henning Severson

Department of Mechanical and Structural,
Engineering and Materials Science,
University of Stavanger,
Stavanger 4036, Norway
e-mail: h.severson@jkn.no

Mohsen Assadi

Department of Petroleum Engineering,
University of Stavanger,
Stavanger 4036, Norway

1Corresponding author.

Contributed by the Advanced Energy Systems Division of ASME for publication in the JOURNAL OF FUEL CELL SCIENCE AND TECHNOLOGY. Manuscript received December 31, 2010; final manuscript received July 11, 2013; published online September 13, 2013. Assoc. Editor: Abel Hernandez.

J. Fuel Cell Sci. Technol 10(6), 061001 (Sep 13, 2013) (14 pages) Paper No: FC-10-1142; doi: 10.1115/1.4025051 History: Received December 31, 2010; Revised July 11, 2013

A structural model has been developed for analysis of residual stresses for anode- and electrolyte-supported planar solid oxide fuel cells (SOFC). This model was also used for analysis of thermally induced stresses during operation for three different case studies with the electrolyte-supported geometry. Temperature distribution in the solid parts of the cell was modeled by means of an in-house electrochemical model, and the results were exported to the structural model. In the case studies, the impact of air and fuel inlet temperatures, steam reforming, and operation voltages on thermal stresses were studied. Weibull statistics were used for the prediction of failure probabilities and design considerations. Base case geometry for the electrolyte-supported cell was 50, 150, and 50 μm for anode, electrolyte, and cathode thicknesses, respectively, and for the anode-supported cell 1000, 20, and 50 μm, respectively. Analysis of residual stresses showed that, compared with the anode-supported cell, the electrolyte-supported cell experienced considerably higher stress levels in the anode and cathode due to the thick electrolyte, while the stress levels in the electrolyte were lower. For the anode-supported cell, maximum stress levels were 57, −12, and −678 MPa in the anode, cathode, and electrolyte, respectively, with negative values indicating compressive and positive values, tensile stresses. For the electrolyte-supported geometry, the corresponding levels were 282, 100, and −308 MPa, respectively. With a failure probability of 1E-6 and an electrolyte thickness of 10 μm, the minimum allowable anode thickness was estimated to be 1000 μm. For an electrolyte-supported cell, optimal thicknesses of electrolyte and anode were considered to be 100 and 100 μm, respectively, while the thickness of the cathode showed low impact. During operation, the stress levels were reduced considerably, since high operating temperatures reduce the temperature difference to the sintering temperature (1250 °C). Concerning the presence of methane in the fuel and the effect of steam reforming, small amounts of methane—as low as 10% of molar mass—were found to induce a cooling effect with correspondingly high gradients. With 45% methane in the fuel, the tensile stress level in the anode was about 130 MPa; the impact of thermal gradients was considered to be 40 MPa and the cooling effect also 40 MPa.

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References

Figures

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Fig. 1

Schematic of modeling approach

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Fig. 2

Anode-supported cell geometry, base case

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Fig. 3

Electrolyte-supported cell geometry, base case

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Fig. 4

Principal stresses in anode, electrolyte, and cathode for various (a) anode, (b) cathode, and (c) electrolyte thicknesses. Thickness of layers not varied are tanode = 1000 μm, tcathode = 50 μm, and telectrolyte = 20 μm.

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Fig. 5

Second principal stresses in anode, electrolyte, and cathode for base case, marked with A, E, and C, anode-support

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Fig. 6

Failure probability of anode at different anode and electrolyte thicknesses with 1% of anode volume exposed

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Fig. 7

Principal stresses in anode, electrolyte, and cathode for various (a) anode, (b) cathode, and (c) electrolyte thicknesses. Thickness of layers not varied are tanode = 50 μm, tcathode = 50 μm, and telectrolyte = 150 μm.

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Fig. 8

First and second principal stresses in anode, electrolyte, and cathode for base case marked with A, E, and C, electrolyte-support

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Fig. 9

Cell temperature distribution for inlet temperatures of fuel and air of 600 °C, 700 °C, 800 °C, and 900 °C [66]

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Fig. 10

Principal stresses in anode, cathode, and electrolyte at operation temperatures of 600–1000 °C

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Fig. 11

First principal stress distribution in the (a) anode, (b) cathode, and (c) electrolyte at operation temperature of 900 °C (base case)

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Fig. 12

Second principal stress distribution in the electrolyte near the anode at operation temperature of 900 °C (base case)

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Fig. 13

Failure probability of cathode and electrolyte at operation temperatures 600–1000 °C with 1% of cathode and electrolyte volume exposed

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Fig. 14

Solid temperature distribution for H2/CH4 ratios of 90/0, 80/10, 40/30, and 10/45 [66]

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Fig. 15

Principal stresses in anode, cathode, and electrolyte with CH4 in fuel varying from 0% to 45%

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Fig. 16

First principal stress distribution in the anode near the electrolyte with 45% CH4, 45% H2O, and 10% H2 in the fuel

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Fig. 17

Failure probability of cathode and electrolyte with CH4 in the fuel varying from 0% to 45% with 1% of cathode and electrolyte volume exposed

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Fig. 18

Solid temperature distribution ( °C) at 0.6, 0.7, 0.8, and 0.9 V [66]

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Fig. 19

Maximum first principal stresses in anode, electrolyte, and cathode at 0.6–0.9 V

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Fig. 20

First principal stress distribution in the anode near the electrolyte at 0.6 V

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Fig. 21

Failure probability of cathode and electrolyte at 0.6–0.9 V with 1% of cathode and electrolyte volume exposed

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