A novel design for faster hydrogen storage: A combined semi-cylindrical and central return tube heat exchanger

Abstract

Metal hydrides (MH) have recently attracted significant interest for hydrogen storage as they provide large storage capacity and a high degree of safety. The main disadvantage, however, is that storage speed is compromised by their low rate of hydrogen absorption. One possible way to accelerate the absorption reaction, and thus improve storage performance regarding incoming hydrogen transfer speed, is to increase the heat transfer rate inside the storage system. Among internal heat exchangers, using helical coil and semi-cylindrical coil heat exchangers significantly improves the heat transfer performance inside the storage system because of the secondary circulation. However, the central area of the storage system still has a lower heat transfer rate as this area is far away from the heat transfer fluid. For this purpose, the development of a heat exchanger structure is considered for the new achievement of this study. A semi-cylindrical coil heat exchanger incorporating a central return tube (SCHE-CR) is first developed from a semi-cylindrical coil heat exchanger (SCHE) to improve heat exchange rate at the MH central area. The tube size's effect on absorption is analysed. Further, a combination of internal and external heat exchangers is considered for further improvement of MH storage performance. The operating parameters of the heat transfer fluid for various heat exchanger configurations are investigated to determine optimal values. Results from numerical simulations indicate that absorption duration reduces by 30 % in the SCHE-CR case compared to the SCHE. Increasing the tube diameter of SCHE-CR results in a 40 % faster absorption reaction. Using SCHE-CR with a cooling jacket reduces the absorption duration by 51 % compared to the SCHE-CR. Among other operating conditions, the operating temperature of the cooling fluid is found to significantly affect the hydrogen absorption reaction with up to a 36 % enhancement of the absorption rate. However, the heat transfer coefficient between cooling fluid and MH, is not found to have a significant effect, as the improvement of the absorption rate is only 8 %. The new MH reactor configuration would be beneficial to improve heat exchange in MH storage applications.