Development and assessment of a novel hydrogen storage unit combined with compressed air energy storage

Abstract

• This study develops a novel compressed-gas hydrogen storage system. • CAES is integrated to keep the tank at the storage pressure. • Power generation via CAES covers energy demands of hydrogen compression and cooling. • The overall energy efficiency is varied from 99.1% to 35.0%. • The overall exergy efficiency is ranged from 96.97% to 34.38%. The present study concerns the development and performance assessment of a novel hydrogen storage system which is operated at a constant pressure where it is also integrated with a compressed air storage system to supply the necessary pressure needs. The uniqueness of the system is that there is a two-chamber storage system where the air is stored in one chamber while hydrogen is stored in the other one. These two chambers work in a synchronized manner where one is compressed while the other one is expanded. For example, the air is compressed in the air chamber to expand hydrogen for releasing to the fuel cell and vice versa. Integration of the compressed air storage into the present system helps keep the hydrogen storage chamber at the desired storage pressure. For this purpose, the air is compressed into the chamber during the hydrogen discharging period, while air is released from the chamber during the hydrogen charging period. In order to exploit the additional benefit of the compressed air, an ammonia-fueled Brayton cycle is incorporated into the current system. Furthermore, this newly developed system is first analyzed thermodynamically by using both energy and exergy approaches to confirm its conceptually correct functionality and write the balance equations for system analysis and secondly assessed for its performance through energy and exergy efficiencies. Moreover, the results indicate that the compressed air as a part of the Brayton cycle covers the total energy demands of hydrogen compression and cooling. In terms of storage efficiencies, the energy and exergy efficiencies for the charging period are found to be 72.65% and 71.52%, while they become 35.3% and 35.24% for discharging period, respectively. The overall system energy and exergy efficiencies are calculated to be 35.00% and 34.38% for a period of 12 hour charging and a period of 6 hour discharging.