Simultaneous impedance measurement of 46 cells in 1 kw SOFC stack: Evaluation of the fuel flow rate distribution among the cells

An ac impedance measuring system which enables the simultaneous impedance measurement of 47 cells has been developed and the measurements were carried out under several operating conditions using 1 kW class SOFC module. Two semi-circles were observed in the complex impedance plot for each cell, one at the high frequency region and the other at the low frequency region; they were assumed to be attributable to the electrochemical reaction and the gas conversion impedance (GCI), respectively. The observed GCI has been confirmed to have its original nature, namely, U_f and flow rate dependencies. Although relatively large temperature distribution among the cells was found in the stack, the cell voltage distribution was comparatively even. It was presumed from the distribution of the observed GCIs that the temperature dependent gaseous flow rate is attributable to such even distribution of the cell voltage, i.e. the lower the cell temperature, the more fuel is supplied to the cell because of the difference of gas density and of viscosity. The evaluation of the gaseous flow rate distribution was successfully conducted using the developed system analyzing the low frequency arc impedance.