Transient local entropy generation analysis for the design improvement of a thermocline thermal energy storage

Second-law methodologies for the design improvement of thermal energy storage (TES) systems are complicated by the fact that TES charging and discharging are highly dynamic (i.e. unsteady) in nature and most of the works done in the field consist of black-box parametric studies, where no information at the local level can be obtained by the analyst. In the present paper, we aim at filling this gap by the introduction of three novel indicators, namely the cumulated local exergy destruction, the characteristic time and the lifespan of entropy generation. These figures of merit provide the designer with a powerful tool that facilitates the identification of the temporal and spatial location of criticalities and the development of design improvements. To test the effectiveness of this innovative methodology, we focused on the design improvement of a molten salts thermocline TES tank for concentrated solar power applications. We found that analyzing the cumulated exergy destruction only is a blind way to proceed because very limited insights can be obtained on the physical phenomenon. In this case, the total irreversibilities can be reduced only by 7% and 12% with the help of a porous and a solid baffle respectively. On the other hand, the simultaneous examination of the three parameters allows to generate a far better design by using a compounded baffle able to reduce the total entropy generation of more than 60% compared to the initial configuration.