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

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 www.ecn.nl

Content • Why fuel flow distribution measurements ? • The stack • EIS configuration • The method • The measurements • Fuel flow distribution • Validation • Conclusions 2 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 3 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 4 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% H 2 /N 2 (dry) • Performance: 810 mV @ 10A (U f = 53%) • Voltage measured per block of 3 cells 5 June 24 2009, Trondheim

EIS configuration Gamry FC350 Kikusui PLZ1004W I V Staxera Stack Mk-100 6 June 24 2009, Trondheim

EIS as function of the fuel flow 2,0 2,0 2,0 2,0 2,0 Electron and ionic Electron and ionic Electron and ionic Electrodes Electrodes Electrodes Gas conversion Gas conversion Gas conversion 0.1 Hz 0.1 Hz dc=10 A 1,5 1,5 1,5 1,5 1,5 0.1 Hz 0.1 Hz ac=0.5A 1,0 1,0 1,0 1,0 1,0 1 Hz 1 Hz 1 Hz 1 Hz 100kHz – 0.01Hz 10 Hz 10 Hz 10 Hz 10 Hz 0,5 0,5 0,5 0,5 0,5 -Z" (ohm.cm²) -Z" (ohm.cm²) -Z" (ohm.cm²) -Z" (ohm.cm²) -Z" (ohm.cm²) 100 Hz 100 Hz 100 Hz 100 Hz 0.01 Hz 0.01 Hz 0.01 Hz 0.01 Hz Fuel: 40%H 2 /N 2 (dry) 10 kHz 10 kHz 10 kHz 10 kHz 0,0 0,0 0,0 0,0 0,0 U f = 53% (nominal) 0 1 2 3 4 5 6 7 0 0 0 0 1 1 1 1 2 2 2 2 3 3 3 3 4 4 4 4 5 5 5 5 6 6 6 6 7 7 7 7 1 kHz 1 kHz 1 kHz 1 kHz -0,5 -0,5 -0,5 -0,5 -0,5 120% of 90% of 90% of the fuel flow the fuel flow the fuel flow -1,0 -1,0 -1,0 -1,0 -1,0 -1,5 -1,5 -1,5 -1,5 -1,5 -2,0 -2,0 -2,0 -2,0 -2,0 Z' (ohm.cm²) per block of 3 cells Z' (ohm.cm²) per block of 3 cells Z' (ohm.cm²) per block of 3 cells Z' (ohm.cm²) per block of 3 cells Z' (ohm.cm²) per block of 3 cells • Gas Conversion Impedance (GCI): measure for the fuel utilisation • Basis for the determination of the fuel flow distribution 7 June 24 2009, Trondheim

The method GCI of one cell block GCI of other cell at 90-120% of the blocks at 100% of anode stack flow the anode stack flow U f of cell blocks U f = f(GCI) no U f = U f stack ? normalization blocks yes Flow distribution 8 June 24 2009, Trondheim

GCI of one cell block GCI of other cell at 90-120% of the blocks at 100% of anode stack flow the anode stack flow EIS as function of the fuel flow (1680 hrs) U f = f(GCI) U f of cell blocks no 2,0 U f blocks = U f stack ? normalization Electron and ionic Electrodes Gas conversion 0.1 Hz yes 1,5 Flow distribution 1,0 1 Hz 10 Hz 0,5 -Z" (ohm.cm²) 100 Hz 0.01 Hz 10 kHz 0,0 0 1 2 3 4 5 6 7 1 kHz -0,5 120 - 110 - 100 - 95 - 90% -1,0 (% of the nominal fuel flow) -1,5 Cell block 4 -2,0 Z' (ohm.cm²) per block of 3 cells → Fit of EIS-spectra for the determination of the GCI values 9 June 24 2009, Trondheim

GCI of one cell block GCI of other cell at 90-120% of the blocks at 100% of anode stack flow the anode stack flow Fit of the EIS spectrum U f = f(GCI) U f of cell blocks no U f blocks = U f stack ? normalization 0,02 yes R 1 R 2 Flow distribution 0,02 R o 0,01 CPE 1 CPE 2 0,01  cm 2 R o 2,2 -Z" (ohm)  cm 2 R 1 1,2 0,00 CPE 1 7,0 S 0,00 0,01 0,02 0,03 0,04 0,05 0,06 0,07 0,08 n 1 0,58 - -0,01  cm 2 R 2 2,4 -0,01 CPE 2 37 S n 2 0,97 - -0,02 -0,02 Z' (ohm) per block of 3 cells  = measurement, line = fit → Fuel utilisation as function of the Gas Conversion Impedance (GCI) 10 June 24 2009, Trondheim

GCI of one cell block GCI of other cell at 90-120% of the blocks at 100% of anode stack flow the anode stack flow Fuel utilisation as function of the GCI U f = f(GCI) U f of cell blocks no U f blocks = U f stack ? normalization 0,70 yes 0,65 Flow distribution 0,60 90% Fuel utilisation (%) 0,55 95% 100% 0,50 110% 0,45 120% 0,40 0,35 Cell block 4 0,30 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) → U f = f(GCI), second order power equation 11 June 24 2009, Trondheim

GCI of one cell block GCI of other cell at 90-120% of the blocks at 100% of anode stack flow the anode stack flow EIS of cell blocks at nominal fuel flow U f = f(GCI) U f of cell blocks no 2,0 U f blocks = U f stack ? normalization yes 1,5 Flow distribution 1,0 0,5 -Z" (ohm.cm²) 0,0 0 1 2 3 4 5 6 7 -0,5 Cell block 6 - 9 - 5 -1,0 -1,5 -2,0 Z' (ohm.cm²) per block of 3 cells → Fit of spectra for determination of the GCI values of cell block 1-10 12 June 24 2009, Trondheim

GCI of one cell block GCI of one cell block GCI of other cell GCI of other cell at 90-120% of the at 90-120% of the blocks at 100% of blocks at 100% of anode stack flow the anode stack flow anode stack flow the anode stack flow Fuel utilisation as function of the GCI U f = f(GCI) U f = f(GCI) U f of cell blocks U f of cell blocks no no normalization U f blocks = U f stack ? normalization U f blocks = U f stack ? 0,70 yes yes 0,65 Flow distribution Flow distribution 0,60 90% Fuel utilisation (%) 0,55 95% 100% 0,50 110% 0,45 120% 0,40 0,35 0,30 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) U f = 51% blocks normalization U f stack = 53% 13 June 24 2009, Trondheim

GCI of one cell block GCI of one cell block GCI of other cell GCI of other cell at 90-120% of the at 90-120% of the blocks at 100% of blocks at 100% of anode stack flow anode stack flow the anode stack flow the anode stack flow Normalization U f = f(GCI) U f = f(GCI) U f of cell blocks U f of cell blocks no no U f blocks = U f stack ? normalization normalization U f blocks = U f stack ? 0,70 yes yes 0,65 Flow distribution Flow distribution 0,60 90% Fuel utilisation (%) 0,55 95% 100% 0,50 110% 0,45 120% 0,40 0,35 0,30 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) U f = 53% blocks Fuel flow distribution U f stack = 53% 14 June 24 2009, Trondheim

GCI of one cell block GCI of other cell at 90-120% of the blocks at 100% of anode stack flow the anode stack flow The fuel flow distribution U f = f(GCI) U f of cell blocks no normalization U f blocks = U f stack ? 120% 120% yes Flow distribution 115% 115% 110% 110% Flow distribution (%) Flow distribution (%) hours hours 105% 105% 1680 1680 100% 100% 2330 95% 95% 90% 90% 85% 85% 80% 80% 1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 9 9 10 10 Cell block Cell block Results hold for this Staxera Mk100 stack; current stacks have improved flow distribution !! 15 June 24 2009, Trondheim

Validation by lowering the fuel flow (10A, 2350 hrs) 2,45 2,45 Voltage of block of 3 cells (V) . Voltage of block of 3 cells (V) . 2,40 2,40 Cell block 2,35 2,35 6 2,30 2,30 8 2 2,25 2,25 9 10 2,20 2,20 3 1 2,15 2,15 5 7 2,10 2,10 4 2,05 2,05 40% 50% 60% 70% 80% 90% 100% 40% 50% 60% 70% 80% 90% 100% Fuel utilisation of the cell (%) Fuel utilisation of the stack (%) 120% 115% Fuel flow distribution 110% Flow distribution (%) 105% hours 1680 100% 2330 95% 90% 85% 80% 1 2 3 4 5 6 7 8 9 10 Cell block 16 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 Acknowledgement 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. 17 June 24 2009, Trondheim