Exergy analysis of packed and fluidized bed thermal energy storage systems
J.A. Almendros-Ibáñez, M. Díaz-Heras, J.F. Belmonte, C. Sobrino
Abstract
This work presents a detailed exergy analysis that compares two different thermal energy storage (TES) systems that use particles as energy storage media and air as the heat transfer fluid: packed and fluidized beds. The study permits the optimization of both TES systems in terms of exergy storage capacity and quantifies the irreversibilities that appear in both cases. In addition, the influence of air flow rate and particle size has been quantified. The results indicate that when focusing solely on the storage process, there is an optimal charging time that minimizes the exergy destruction in both cases: topt∼mscp,s/ṁacp,a. Although the exergy destruction of the fluidized bed during the beginning of the charging process is higher than that in the packed bed, at the end of the process, the total exergy destruction in both beds is similar. However, in the full storage-recovery process the total exergy destruction is doubled in the fluidized case, with an entropy generation number of approximately 0.6 in comparison with a value of 0.3 for the packed bed. Besides, a sensitivity analysis varying the particle size and the air flow rate indicates that the particle size has a much higher influence on the fluidized bed case due to the change in the minimum fluidization velocity and its influence on the heat transfer rate. Finally, a case study is presented using the storage bed for building heating, and an exergy analysis concludes that the fluidized bed generates 534 W/K of entropy, more than three times the entropy generated in the packed bed (161 W/K), due to the higher pressure drop needed to fluidize the particles.