In-situ Determination of the Anode Flow Distribution in a SOFC Stack - - PowerPoint PPT Presentation

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In-situ Determination of the Anode Flow Distribution in a SOFC Stack under Nominal Operating Conditions by EIS

Diagnostic tools, 23-24 June 2009, Trondheim, Norway Nico Dekker, Hans van Wees, Bert Rietveld

2 June 24 2009, Trondheim

Content

• Why fuel flow distribution measurements ?
• The stack
• EIS configuration
• The method
• The measurements
• Fuel flow distribution
• Validation
• Conclusions

3 June 24 2009, Trondheim

Why fuel flow distribution measurements?

• Fuel flow distribution over the cells in a stack
• High electrical efficiency requires high fuel utilisation (≥80%)
• High fuel utilisation: severe demands on flow distribution
• Differences in cell voltage
• Local depletion of the fuel
• Degradation

4 June 24 2009, Trondheim

Why fuel flow distribution measurements?

Important information for stack design

• Manifold channels
• Active area

Operation of stack

• Creep & corrosion
• Fuel composition
• Temperature distribution

5 June 24 2009, Trondheim

Stack and experimental conditions

Staxera Mk-100 stack

• 30 cells (ESC)
• Active area: 81 cm²
• Inlet temperature: 750°C
• Outlet temperature: 820°C
• Fuel supply: internal manifolding
• Fuel 40% H2

/N2 (dry)

• Performance: 810 mV @ 10A (Uf

= 53%)

• Voltage measured per block of 3 cells

6 June 24 2009, Trondheim

EIS configuration

Kikusui PLZ1004W Gamry FC350 Staxera Stack Mk-100 I V

7 June 24 2009, Trondheim

• 2,0
• 1,5
• 1,0
• 0,5

0,0 0,5 1,0 1,5 2,0 1 2 3 4 5 6 7 Z' (ohm.cm²) per block of 3 cells

• Z" (ohm.cm²)
• Gas Conversion Impedance (GCI): measure for the fuel utilisation
• Basis for the determination of the fuel flow distribution
• 2,0
• 1,5
• 1,0
• 0,5

0,0 0,5 1,0 1,5 2,0 1 2 3 4 5 6 7 Z' (ohm.cm²) per block of 3 cells

• Z" (ohm.cm²)

0.01 Hz 0.1 Hz 1 Hz 10 Hz 100 Hz 1 kHz 10 kHz

• 2,0
• 1,5
• 1,0
• 0,5

0,0 0,5 1,0 1,5 2,0 1 2 3 4 5 6 7 Z' (ohm.cm²) per block of 3 cells

• Z" (ohm.cm²)

0.01 Hz 0.1 Hz 1 Hz 10 Hz 100 Hz 1 kHz 10 kHz Electron and ionic Electrodes Gas conversion

• 2,0
• 1,5
• 1,0
• 0,5

0,0 0,5 1,0 1,5 2,0 1 2 3 4 5 6 7 Z' (ohm.cm²) per block of 3 cells

• Z" (ohm.cm²)

0.01 Hz 0.1 Hz 1 Hz 10 Hz 100 Hz 1 kHz 10 kHz 90% of the fuel flow Electron and ionic Electrodes Gas conversion

• 2,0
• 1,5
• 1,0
• 0,5

0,0 0,5 1,0 1,5 2,0 1 2 3 4 5 6 7 Z' (ohm.cm²) per block of 3 cells

• Z" (ohm.cm²)

0.01 Hz 0.1 Hz 1 Hz 10 Hz 100 Hz 1 kHz 10 kHz 90% of the fuel flow Electron and ionic Electrodes Gas conversion 120% of the fuel flow

EIS as function of the fuel flow

dc=10 A ac=0.5A 100kHz – 0.01Hz Fuel: 40%H2 /N2 (dry) Uf = 53% (nominal)

8 June 24 2009, Trondheim

The method

GCI of one cell block at 90-120% of the anode stack flow GCI of other cell blocks at 100% of the anode stack flow Uf = f(GCI) Uf

• f cell

blocks Uf

blocks

= Uf stack ? Flow distribution normalization no yes

9 June 24 2009, Trondheim

• 2,0
• 1,5
• 1,0
• 0,5

0,0 0,5 1,0 1,5 2,0 1 2 3 4 5 6 7 Z' (ohm.cm²) per block of 3 cells

• Z" (ohm.cm²)

0.01 Hz 0.1 Hz 1 Hz 10 Hz 100 Hz 1 kHz 10 kHz

120 - 110 - 100 - 95 - 90% (% of the nominal fuel flow) Electron and ionic Electrodes Gas conversion

GCI of one cell block at 90-120% of the anode stack flow GCI of other cell blocks at 100% of the anode stack flow Uf = f(GCI) Uf of cell blocks Uf blocks = Uf stack? Flow distribution normalization no yes

EIS as function of the fuel flow (1680 hrs)

Cell block 4

→ Fit of EIS-spectra for the determination of the GCI values

10 June 24 2009, Trondheim

Fit of the EIS spectrum

GCI of one cell block at 90-120% of the anode stack flow GCI of other cell blocks at 100% of the anode stack flow Uf = f(GCI) Uf of cell blocks Uf blocks = Uf stack? Flow distribution normalization no yes

• 0,02
• 0,02
• 0,01
• 0,01

0,00 0,01 0,01 0,02 0,02 0,00 0,01 0,02 0,03 0,04 0,05 0,06 0,07 0,08 Z' (ohm) per block of 3 cells

• Z" (ohm)

Ro 2,2  cm2 R1 1,2  cm2 CPE1 7,0 S n1 0,58

• R2

2,4  cm2 CPE2 37 S n2 0,97

• Ro

R1 R2 CPE1 CPE2

 = measurement, line = fit

→ Fuel utilisation as function of the Gas Conversion Impedance (GCI)

11 June 24 2009, Trondheim

Fuel utilisation as function of the GCI

GCI of one cell block at 90-120% of the anode stack flow GCI of other cell blocks at 100% of the anode stack flow Uf = f(GCI) Uf of cell blocks Uf blocks = Uf stack? Flow distribution normalization no yes

0,30 0,35 0,40 0,45 0,50 0,55 0,60 0,65 0,70 0,4 0,5 0,6 0,7 0,8 0,9 1,0 1,1 1,2 1,3 Gas Conversion Impedance (ohm cm²) Fuel utilisation (%) 120% 110% 100% 95% 90%

→ Uf = f(GCI), second order power equation Cell block 4

12 June 24 2009, Trondheim

GCI of one cell block at 90-120% of the anode stack flow GCI of other cell blocks at 100% of the anode stack flow Uf = f(GCI) Uf of cell blocks Uf blocks = Uf stack? Flow distribution normalization no yes

• 2,0
• 1,5
• 1,0
• 0,5

0,0 0,5 1,0 1,5 2,0 1 2 3 4 5 6 7 Z' (ohm.cm²) per block of 3 cells

• Z" (ohm.cm²)

Cell block 6 - 9 - 5

EIS of cell blocks at nominal fuel flow

→ Fit of spectra for determination of the GCI values of cell block 1-10

13 June 24 2009, Trondheim

Fuel utilisation as function of the GCI

GCI of one cell block at 90-120% of the anode stack flow GCI of other cell blocks at 100% of the anode stack flow Uf = f(GCI) Uf of cell blocks Uf blocks = Uf stack? Flow distribution normalization no yes

0,30 0,35 0,40 0,45 0,50 0,55 0,60 0,65 0,70 0,4 0,5 0,6 0,7 0,8 0,9 1,0 1,1 1,2 1,3 Gas Conversion Impedance (ohm cm²) Fuel utilisation (%) 120% 110% 100% 95% 90%

Uf

blocks

= 51% Uf stack = 53% normalization

GCI of one cell block at 90-120% of the anode stack flow GCI of other cell blocks at 100% of the anode stack flow Uf = f(GCI) Uf of cell blocks Uf blocks = Uf stack? Flow distribution normalization no yes

14 June 24 2009, Trondheim

Normalization

0,30 0,35 0,40 0,45 0,50 0,55 0,60 0,65 0,70 0,4 0,5 0,6 0,7 0,8 0,9 1,0 1,1 1,2 1,3 Gas Conversion Impedance (ohm cm²) Fuel utilisation (%) 120% 110% 100% 95% 90%

GCI of one cell block at 90-120% of the anode stack flow GCI of other cell blocks at 100% of the anode stack flow Uf = f(GCI) Uf of cell blocks Uf blocks = Uf stack? Flow distribution normalization no yes

Uf

blocks

= 53% Uf stack = 53% Fuel flow distribution

GCI of one cell block at 90-120% of the anode stack flow GCI of other cell blocks at 100% of the anode stack flow Uf = f(GCI) Uf of cell blocks Uf blocks = Uf stack? Flow distribution no yes normalization

15 June 24 2009, Trondheim

80% 85% 90% 95% 100% 105% 110% 115% 120% 1 2 3 4 5 6 7 8 9 10 Cell block Flow distribution (%) 1680 hours 80% 85% 90% 95% 100% 105% 110% 115% 120% 1 2 3 4 5 6 7 8 9 10 Cell block Flow distribution (%) 1680 2330 hours

The fuel flow distribution

GCI of one cell block at 90-120% of the anode stack flow GCI of other cell blocks at 100% of the anode stack flow Uf = f(GCI) Uf of cell blocks Uf blocks = Uf stack? Flow distribution normalization no yes

Results hold for this Staxera Mk100 stack; current stacks have improved flow distribution !!

16 June 24 2009, Trondheim

2,05 2,10 2,15 2,20 2,25 2,30 2,35 2,40 2,45 40% 50% 60% 70% 80% 90% 100%

Fuel utilisation of the stack (%) Voltage of block of 3 cells (V) .

6 8 2 9 10 3 1 5 7 4 Cell block 2,05 2,10 2,15 2,20 2,25 2,30 2,35 2,40 2,45 40% 50% 60% 70% 80% 90% 100%

Fuel utilisation of the cell (%) Voltage of block of 3 cells (V) .

Validation by lowering the fuel flow (10A, 2350 hrs)

Fuel flow distribution

17 June 24 2009, Trondheim

Conclusions

• Easy-to-use method for the determination of the fuel flow

distribution in a SOFC stack by EIS at nominal operating conditions:

• GCI of one cell as function of the fuel stack flow
• GCI of all cells at the nominal flow → flow distribution
• Validity shown by increasing the fuel utilization
• Simple tool for characterisation of a stack initially and in time

without disturbing the nominal operating condition This work was carried out as part of the EU project

• GreenFuelCell. The authors would like to thank the European

Commission for their financial support of this work.

Acknowledgement