Normalized charging exergy performance of stratified sensible thermal stores

Abstract This paper presents the performance assessment of stratified sensible thermal energy stores (SSTES) on the basis of the normalized exergy efficiency, η ‾ x . Assessments based on η ‾ x provide comparisons with performances of both the perfectly stratified and the fully mixed stores, which offer the best and worst performances, respectively. This is in contrast with energy and exergy efficiencies, which compare SSTES with only the perfectly stratified store. A dimensionless unsteady axisymmetric model of vertical cylindrical SSTES was implemented using a finite volume numerical scheme. The effect of some significant parameters on SSTES performance were considered by performing computations for aspect ratios (AR) between 1 and 4, Peclet number ( Pe D ) varying from 5 × 10 3 to 100 × 10 3 , Richardson number (Ri) varying from 10 to 10 4 , and overall heat loss coefficients ( U ) varying from 0 to 100 W m −2  K −1 . η ‾ x increases with Pe D , Ri and AR, with the most significant increases occurring at low values of these parameters, and appreciable increases are no longer obtained beyond Pe D ≈ 30 × 10 3 , Ri ≈ 10 3 and AR ≈ 3 . η ‾ x also falls monotonically as the U values increase.