Second law optimization of a sensible heat thermal energy storage system with a distributed storage element

This numerical study defined the behavior of a sensible heat thermal energy storage system whose physical design and operation had been optimized to minimize the production of thermodynamic irreversibilities. Unlike previous studies, it included the effects of transient conduction within the storage material.

A dimensionless set of governing equations was defined for a com plete storage-removal cycle that included the effects of entropy generation due to convection and viscous effects in the flowing fluid, two-dimensional transient conduction within the storage material, and to convection due to the discharged hot fluid coming to equilibrium with the environment during the storage period. A computer program was written to solve this equation set and this program was in turn con trolled by a sophisticated optimization routine to determine a dimensionless storage time, flow channel half-height, and heat transfer coefficient that resulted in a minimum amount of availability destruction.

The results of this analysis showed that entropy generation within the storage material due to transient conduction was a major contributor to the total thermal irreversibilities associated with the operation of a sensible heat thermal energy storage system. For the counterflow con figuration and over the range of design variables examined, material entropy generation accounted for between 26.% and 60.% of the total thermal availability destruction that occurred during a complete storage-removal cycle. It was also shown that the storage material aspect ratio (the ratio of a section's half-thickness to its length) had a significant impact on the optimum design of a storage system. Its influence was second only to the fluid mass velocity.

Other significant results of this study were:

a. The thermodynamic efficiencies for the storage systems were extremely poor in that it destroyed from 20.% to 82.% of the entering thermal and pressure availability.

b. A counterflow configuration without a dwell period was shown to operate more efficiently than a parallel flow configuration with or without a dwell period. Depending on the value of the dimensionless mass velocity, the parallel flow configurations increased the total thermal entropy generated, over the corresponding counterflow design, from 12.% to 67.%.

c. Dwell periods were shown to be impractical because of their extreme length; dimensionless times on the order of 6500.0 were required. These are much greater than the optimum storage period times defined for the counterflow configurations without dwell periods, which ranged from 0.5 to 6.0.