Maximum power density analyses of a novel hybrid system based upon solid oxide fuel cells,vacuum thermionic generators and thermoelectric generators

Apart from electricity, solid oxide fuel cell (SOFC) generates a great deal of high-grade exhaust heat, which must be immediately removed to guarantee SOFC's normal operation. To harvest the exhaust heat and improve the overall energy conversion efficiency, a new hybrid system model based upon a SOFC, a vacuum thermionic generator (VTIG) and a thermoelectric generator (TEG) is first proposed. Considering the main thermodynamic-electrochemical irreversible effects, the performance indicators assessing the whole system performance are mathematically derived. In comparison with the performance of sole SOFC, the effectiveness and feasibility of the presented system are verified. Numerical calculation examples illustrate that maximum achievable power density (MAPD) and its corresponding efficiency, exergetic efficiency and exergy destruction rate are, respectively, 26.8%, 9.8%, 9.8% and 8.8% larger than that of the stand-alone SOFC. Exhaustive sensitivity analyses are further conducted to investigate the impacts of various parameters on the tri-generation system performance. Results indicate that the grain size and average pore diameter of electrodes in SOFC and the thermoelectric element number in TEG can be optimized to maximize the hybrid system power density.