Comparison of energy, exergy and entropy generation-based criteria for evaluating stratified thermal store performances

摘要

It is well established that stratification improves the performance of stratified sensible thermal energy stores (SSTES). A detailed review is presented of studies in which the contribution of stratification to the performance SSTES is assessed on the basis of energy and exergy analyses, and lately, entropy generation analysis. To obtain clear and useful distinctions between stratified and non-stratified stores, and for stores at different states of stratification, analyses that incorporate second law considerations (i.e. exergy/entropy generation) are required. Also, based on outcomes of computational fluid dynamics (CFD) simulations of SSTES, we present results of energy efficiency, eta(e), normalized exergy efficiency, Fix, and entropy generation number, N-s, assessments of SSTES, corresponding to three possible approaches. Energy efficiencies, are shown to be limited in their ability to quantifying these improvements, while (eta) over bar (x) is introduced as an improved exergy-based performance measure. Comparing (eta) over bar (x) with N-s it is found that (eta) over bar (x) is effective for assessing store performances prior to the exit of the thermocline from the store, while N-s offers useful assessments for the full duration of store operation. The parametric dependence of (eta) over bar (x) and N-s on some significant dimensionless variables (Peclet number, Pe(D), Richardson number, Ri, and aspect ratio, AR) is such that as Pe(D), Ri and AR increase (eta) over bar (x), increases. N-s decreases with increasing Pe(D) and AR but the effect of Ri on it depends on the value of Ri. (C) 2016 Elsevier B.V. All rights reserved.