Normalized charging exergy performance of stratified sensible thermal stores

• Published in: Solar energy Vol. 136; pp. 487 - 498
• Main Authors: Njoku, H.O., Ekechukwu, O.V., Onyegegbu, S.O.
• Format: Journal Article
• Language: English
• Published: New York Elsevier Ltd 15.10.2016; Pergamon Press Inc
• Subjects: CFD analysis; Energy efficiency; Exergy performance analysis; Heat transfer; Normalized exergy efficiency; Solar energy; Stratified thermal storage; Thermal energy; Normalized exergy efficiency; Stratified thermal storage; Exergy performance analysis; CFD analysis
• ISSN: 0038-092X; 1471-1257
• DOI: 10.1016/j.solener.2016.07.032

•Charging SSTES performances are assessed with the normalized exergy efficiency η‾x.•η‾x compares SSTES with perfectly stratified and fully mixed stores based on exergy.•An unsteady axisymmetric model of the SSTES was implemented using finite volume CFD.•At low Ri, PeD and AR values, η‾x improves as these dimensionless parameters increase.•Rational results with η‾x are obtainable only for charging before one tank volume change. 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 (PeD) varying from 5×103 to 100×103, Richardson number (Ri) varying from 10 to 104, and overall heat loss coefficients (U) varying from 0 to 100Wm−2K−1. η‾x increases with PeD, Ri and AR, with the most significant increases occurring at low values of these parameters, and appreciable increases are no longer obtained beyond PeD≈30×103, Ri≈103 and AR≈3. η‾x also falls monotonically as the U values increase.