Exergetic evaluation of operation results of 5-kW-class SOFC-HCCI engine hybrid power generation system

A solid oxide fuel cell (SOFC)-based hybrid power generation system using homogeneous charge compression ignition (HCCI) was previously proposed, based on which a proof-of-concept test has already been conducted by the Korea Institute of Machinery and Materials. In this study, energy and exergy analyses were performed to evaluate the experimental data obtained from the proof-of-concept test of the SOFC-HCCI engine hybrid system. In the analyses, particular attention is paid to the SOFC stack, HCCI engine, balance of plant (BOP) hot-box subsections and, accordingly, the energy and exergy flows within and between the above subsections. For the BOP hot-box, the energy and exergy analyses reveal different results. The energy analysis demonstrated that 75.1% of the transferred heat was recovered by the BOP components and pipes; only 25% of the heat, i.e., 638 W, was lost. However, the exergy analysis identified that only 35.1% of the transferred exergy was recovered; thus, 64.9% of the exergy was destroyed. Based on the energy and exergy analysis results, it is observed that the amount of exergy destruction exceeds the heat loss as the latter contributes to the former; this is in addition to the exergy destruction due to the irreversibility of the heat transfer between the components located in the BOP hot box. Based on the analyses of the entire system, the various heat losses and exergy destruction are quantitatively identified. In particular, significant heat loss and exergy destruction occur in the engine, additional burner, and BOP components. To improve the efficiency of the system, the insulation of the system can be reinforced, thereby reducing heat loss and exergy destruction and burner usage can be minimized. This suggests that the heat losses from the system components, mostly in the HCCI engine, should be reduced or reused to improve the overall efficiency of the system.