Second Law Performance Analysis of a Large Thermal Energy Storage Vessel using CFD

Abstract

This work is an example of a CFD-assisted design and characterization process for thermal energy storage vessels. A general modeling technique for future works is also proposed. The Short-Term Thermal Storage (STTS) tanks at the Drake Landing Solar Community (DLSC) were used as the principal case study. The performance characterization of the STTS tanks and the evaluation of other tank designs were made under solar charging conditions and for the STTS “Hot Tank” only. Three sets of simulations were undertaken for each tank design, each representing a different state of inlet conditions reflected in the DLSC’s operational manual. Characterization of the STTS tanks was done mainly by applying a set of 2nd Law characterization indices, both existing and new, using exergy as the primary Figure of Merit. It was evident that significant mixing occurs in the current STTS tanks due to the ineffective placement of the inlet ports and the lack of an appropriate flow diffuser to prevent mixing. For example, at the end of the simulations exhibiting constant inlet temperature and flow rate, the total exergy in the original STTS tank was only 68% of a perfectly-stratified vessel. A modified design of the STTS tanks, which only shifted the position of the inlet port and center baffle, significantly improved this value to over 90%. Additional analysis also indicated that the STTS tanks would benefit from a simple flow distributor or inlet manifold that would address stratification issues inherent to variable temperature inlet conditions. However, further analysis on this particular design configuration is needed. The characterization methods employed in this work represent an effective means to differentiate between the stratification effectiveness of various thermal storage vessel designs. This work would further benefit from a future study that compares changes to the STTS tanks’ stratification efficiency with changes to the DLSC’s overall performance, including a cost-benefit analysis.