Comparative study of solid oxide fuel cell-combined heat and power system designs for optimal thermal integration

A comparative study on various solid oxide fuel cell (SOFC)-combined heat and power system layout designs is conducted to suggest its optimized structure. Thermodynamic models of a SOFC stack and balance of plant components are developed by using empirical correlations dependent on their design variables. 14 potential system layout designs are categorized by thermal integration methods through rigorous literature review and evaluated for their thermodynamic feasibility and performance by T-Q diagram analysis. Results show that preemptive air heating prior to combustion of stack exhaust gases or fuel heating is not thermodynamically feasible due to substantial heat transfer during air heating. Independent heat recuperation of the anode exhaust gas from a SOFC stack also shows thermodynamic non-viability given its low heat capacity rate. 4 effective layouts are selected and further investigated by proceeding detailed thermodynamic analysis. The system layout employing direct combustion just after the SOFC stack and branching of the hot gas stream (combustion gas) results in the pressure drop of 23.5 kPa and parasitic power of 1.04 kW (less than 5% of gross power). These are 2-4 times smaller than those of other layouts. Accordingly, the proposed layout provides the highest electrical efficiency of 55.5% and exergetic efficiency of 53.5%.