Exergy effeciency and local heat production in solid oxide fuel cells

Abstract

The electric work method has been applied to a unit cell of the solid oxide fuel cell. A new equation for the cell power is derived, which takes into account temperature gradients of the system. Local heat productions and consumptions in the cell have been calculated using new data on the transported entropy of oxygen ions. Exergy efficiency calculations are carried out for the unit cell at 1000°C indicating the relative importance of losses due to overpotentials, ohmic resistance and cracks in the electrolyte, incomplete reactions and temperature gradients. Energy economy is obtained for direct electrochemical conversion of methane in the unit cell when the overpotential at the fuel electrode is less than 0.21 V for an electric current density j = 1 A cm −2. Ohmic resistance of the electrolyte plays a minor role. A natural temperature gradient of 10 K across the cell reduces the work from the cell by 0.6%. The heat production in the cell is asymmetrical. A 3% gain in exergy efficiency is obtained by changing the pressure from 1 to 4 bar. The results will have a bearing on cell design and material development.