Local entropy generation analysis of transient processes – an innovative approach for the design improvement of a Thermal Energy Storage with Integrated Steam Generator

The application of second-law methodologies for the analysis of unsteady processes, such as TES charging and discharging, is complicated by the fact that the optimization ofthe design involves the search for an optimal time history, i.e. the one that minimizes the global entropy produced during a finite time interval. Most of the works done in the field consists of lumped parameters studies, which do not provide by any mean local information. On the other hand, entropy generation studies through the use of continuum theory have demonstrated to be a very powerful tool. However, a fundamental question arises on which is the most suitable time to stop the operation and analyse the process. In the present work, we aim at bridging this literature gap by proposing a modified local Entropy Generation Analysis (EGA) that well suits the study of unsteady processes. Firstly, the transient performance of a CSP cogeneration plant with a thermocline thermal energy storage (TES) tank and a submerged steam generator are analysed to identify uncommon temporal behaviours of the components. Thereafter, the modified EGA methodology is used to improve the design of the TES-integrated steam generator. Thanks to the introduction of three novel local indicators that record valuable information on the evolution of the system, namely the cumulated local entropy generation, its characteristic time and lifespan, the main criticalities are highlighted and well localized in both time and space and thus the identification of possible design improvements is simplified. The modifications proposed, i.e. the installation of a gate valve and an impeller on the molten salts side of the steam generator, bring an 11.2 % reduction of the irreversibilities generated in the steam generator, a 7.7 % improvements of the thermodynamic performance of the integrated system and a 4.7 % increase of the exergy yield of the entire plant.