Second Law Optimization of a Sensible Heat Thermal Energy Storage System With a Distributed Storage Element—Part II: Presentation and Interpretation of Results

Abstract

This study explores the behavior of a flat-slab, sensible heat thermal energy storage system, the physical design and operation of which have been optimized to minimize the production of entropy by thermodynamic irreversibilities. The analytical model is developed in Part I of this work. This part includes a description of the numerical model and the presentation and interpretation of the results of a system optimization study. The major results of this study show that: 1) any Second Law model of a thermal energy storage system must include a distributed storage element in order to make realistic estimates the thermodynamic performance of the system; 2) unconstrained optimization of the design of a thermal energy storage system tends to yield a system that is undesirably large, but by constraining the number of transfer units (NTU), it is possible to design systems of a realistic size without seriously degrading the thermodynamic performance; 3) counterflow systems operated without a dwell period are the most efficient type of system; and 4) the use of a dwell period with a counterflow system, or the operation of a system in parallel flow instead of counterflow, degrades the thermodynamic performance of the system and increases the required system size (NTU) in comparison to a counterflow system operated without a dwell period.