A variable cross-section annular fins type metal hydride reactor for improving the phenomenon of inhomogeneous reaction in the thermal energy storage processes

Abstract

Metal hydride (MH) has been of great interest as one of the potential thermochemical heat storage materials. Previous studies have revealed that with the progress of the reaction, the inhomogeneous reaction will gradually appear in the metal hydride heat storage reactor (MHHSR), which will lead to the decrease of the heat output capacity of the reactor. In this study, an innovative MHHSR with a variable cross-section annular fin (VCSAF) structure is proposed. The thermal coupling model between the powder bed and VCSAF is established. It is found that the VCSAF can effectively resolve the inhomogeneous reaction phenomenon in the reaction process by adjusting the inclination angle between outer profile of VCSAF and outer edge of bed (θfin). At the same time, the influence of different fin structures parameters is analyzed on the uniformity of the reactor based on sensitivity analysis. The width of the largest fin (Lfin,max) is the main factor affecting the uniformity in comparison with fin thickness (hfin), fin spacing (Δh). With the increase of Lfin,max, Δh and hfin, exergy output (Ex,out) changes from 266.12 kJ to 249.65 kJ, 222.64 kJ to 282.99 kJ, and 264.01 kJ to 255.82 kJ, respectively, gravimetric exergy-output power of reactor (GEOPR) changes from 41.96 W kg−1 to 48.22 W kg−1, 49.88 W kg−1 to 42.41 W kg−1, and 43.11 W kg−1 to 47.40 W kg−1, respectively, and gravimetric efficient exergy-output (GEEO) remains unchanged. Besides, the simulation results of four different designs: no fin (NF), same cross-section fin (SCSF), longitudinal fin (LF) and VCSAF are compared to assess their performance. Compared to NF, SCSF and LF, VCSAF has a more uniform heat transfer effect and better performance. Finally, the performance improvement of the enlarged VCSAF reactor is analyzed. With the increase of length (L), the platform end time difference between the SCSF and VCSAF and between LF and VCSAF (Δt1 and Δt2) increases from 800 s to 2800 s and 2250 s to 4800 s, respectively. Compared with NF, the growth rates of Ex,out, exergy efficiency (ηEx), GEEO and GEOPR for VCSAF change from 4.04% to 16.07%, 6.79% to 14.42%, 10.60% to 22.22%, and 31.74% to 6.81%, respectively. Compared with SCSF, the growth rates of Ex,out, ηEx, GEEO and GEOPR change from 12.41% to 26.23%, 9.74% to 17.97%, 12.54% to 26.01%, and 5.00% to 2.18%, respectively. Compared with LF, the growth rates of Ex,out, ηEx, GEEO and GEOPR change from 14.52% to 34.02%, 10.12% to 20.18%, 13.53% to 27.99%, and 7.70% to 5.56%, respectively. The VCSAF structure has an excellent performance in eliminating inhomogeneous reaction. With the enlargement of the reactor, the heat output capacity has been improved obviously, which has an important guiding significance for the engineering application of the MH high-temperature heat storage technology.