Efficient waste heat recovery from molten carbonate fuel cells through graphene-collector thermionic generators

Abstract

The waste heat released by molten carbonate fuel cells (MCFCs) contains considerable energy that can be captured for electric power generation. Nonetheless, the current high-grade heat harvesting devices for the MCFC cogeneration of electrical power are still constrained by low power output or energy conversion. To address this challenge, we present a novel hybrid system that combines an MCFC and a graphene-collector thermionic generator (GCTG), in which the GCTG can efficiently harness the exhaust heat released from the MCFC and generate additional electricity. The results show that the hybrid system’s maximum power density reaches 0.298 W/cm2, which is 1.6 times that of the sole MCFC at 923 K, demonstrating that the hybrid system delivers a significant boost in output performance. Furthermore, finite-time thermodynamics estimates the hybrid system’s optimal operating regions and key parameter designs. The optimal performance of the hybrid system can be further improved by selecting the optimal area ratio, raising the MCFC operating temperature, strengthening the heat transfer coefficient, lowering the thermal emissivity of the thermionic emitter, and manufacturing the perfect optical reflector. The MCFC-GCTG outperforms other MCFC-based hybrid systems regarding output performance, demonstrating that GCTG can more effectively exploit the waste heat released from MCFCs than other heat recovery devices. This study offers valuable theoretical guidance for the strategic optimization of the MCFC-GCTG hybrid system, thereby paving a constructive strategy towards realizing advanced, high-performance MCFC cogeneration systems.