Portable/mobile hydrogen energy storage system with an electrolyzer, metal hydrides, and a fuel cell

Abstract

A hydrogen energy storage system for portable/mobile applications with an unprecedentedly high energy density was developed in this work. The developed system does not require an infrastructure to supply hydrogen over 100 bar or liquefied hydrogen and can be charged in various environments without replacing the hydrogen storage medium. In particular, an application-oriented design and system integration strategy were developed to maximize the energy density while incorporating conventional technologies for the electrolyzer (Ely), the metal hydride (MH), and the polymer electrolyte membrane fuel cell (PEMFC). To improve both the energy density and the usability of the system, we divided the charging component and the discharging component in a conventional regenerative fuel cell system composed of a closed cycle of "electricity supply-hydrogen production-hydrogen storage-electricity production." The charging component supplies hydrogen to the discharging component and consists of the Ely and the MH cooling system as one system. The discharging component supplies electricity to the application and is composed of the MH, the PEMFC, and the power conditioning system (PCS) as one system. To utilize an MH as an energy storage medium in the portable/mobile field, we studied how to store and release hydrogen with high efficiency and maximize the electric energy density. The hydrogen absorption/desorption characteristics and durability of MHs were investigated to select a suitable MH for the system. In particular, to solve the heat transfer problem, which has been a bottleneck of existing MH technology, a quantitative thermal management method was applied. As a result, the MH charging time was reduced to 1/4 of the conventional charging time. Finite element method (FEM) analysis and engineering scale-up process development were conducted to increase the system capacity. A total of four versions of MH tanks for prototype systems were manufactured, and the design reproducibility was confirmed. The energy conversion system was developed to efficiently convert the stored hydrogen energy into electrical energy. By introducing the fuel utilization control method of the self-humidifying PEMFC stack, the FC efficiency was improved by 4.6%p. Hybrid operation in which the PEMFC takes the baseload under maximum efficiency conditions and the battery takes the variable load was used. The load-following characteristics under various load conditions were confirmed while maintaining maximum discharge efficiency. A prototype of the charging and discharging components was developed. The charging component can supply hydrogen at a pressure of 30 bar at 99.999% of purity with a flow rate of 0.5 $Nm^3/hr$. The discharging component provides a nominal power output of $31.5 W_e$ at 12 $V_{DC}$ for 38 hours, with one recharge. We find it significant that the discharging component shows an energy density of $410 W_eh/L$, which is twice that of the world's best Li-battery module at the 2.9-L level. Based on these results, the proposed system was confirmed to have potential for use in mobile/portable applications.