Fuel utilization and its effect on solid oxide fuel cell performance

Rustam, S S (2014) Fuel utilization and its effect on solid oxide fuel cell performance. Masters thesis, Indian Institute of Technology, Hyderabad.

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Abstract

Solid oxide fuel cells (SOFC) are energy conversion devices which have better efficiency and performance than other devices. A SOFC can operate with low noise, less harmful emissions and with higher output energy for a given amount of fuel. Fuel cells are devices which converts chemical energy directly into electrical energy. A detailed model is presented which involves all physics and examines the fuel utilization during cell operation. During operation we introduce the fuel into the cell side and unreacted fuel comes out with exhaust gases thus going waste. The fraction of fuel that is used is called fuel utilization (Uf) and higher Uf is desirable. To increase fuel utilization we can vary the inlet fuel velocity, temperature, and other operating parameters. Microstructural properties such as electronic conductivity, ionic conductivity, triple phase boundary (TPB) length, porosity affect the cell performance directly and are themselves dependent upon the manufacturing process. We need to optimise among the properties to get desirable behaviour during cell operation since changing one parameter effects the effective electrode properties differently. Cell geometry and dimensions are also important factors in deciding performance including fuel utilization. The effect of varying various parameters for the electrodes, specifically in the anode, was investigated to suggest improved performance of the overall cell. Thickness of the anode and cathode do not affect the cell performance. The ratio of the partial pressure of water vapour to hydrogen increases with increasing utilization. This can lead to the oxidation of the nickel in the anode which is detrimental to the durability of the cell. This work shows that at 800oC, for a fuel velocity that gives Uf =80% at 0.8 V, lowering the cell voltage below 0.7 V drives Uf above 99%. It is only at these very severe conditions that Ni oxidation becomes favourable towards the fuel exit.

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