Dual-timeframe optimisation of hybrid salt-based battery–supercapacitor systems — A techno-economic sizing framework

The integration of renewable energy sources into power grids presents significant challenges due to their inherent intermittency, particularly in islanded and weak-grid systems where high variability leads to curtailment and continued reliance on diesel generation. This study proposes and evaluates a hybrid energy storage system (HESS) combining a sodium–nickel chloride (NaNiCl2 ) battery and a supercapacitor (SC), supported by a dualtimeframe optimisation and sizing framework explicitly designed to coordinate long-duration energy shifting with fast power-balancing services. The framework integrates electro-thermal component models within a twotier, multi-period optimisation strategy, enabling consistent assessment of short-term operational feasibility and long-term techno-economic performance. Using real-world operational data from the Graciosa Island (Azores) microgrid, the results demonstrate that the proposed HESS effectively manages short-term power fluctuations and multi-hour energy shifting, reducing annual diesel consumption by up to 3.7% and renewable energy curtailment by up to 9.7%, with substantially higher benefits during high-renewable periods. The SC plays a critical role in enhancing grid stability by buffering fast transients and alleviating battery power and thermal limitations. A comprehensive techno-economic analysis is performed across multiple scenarios, spanning high and neutral renewable production levels and different CO2 valuation assumptions. While most configurations remain economically challenging under current cost conditions over full-year operation, best-case scenarios with high renewable availability, elevated CO2 valuation, and reduced storage capital costs yield positive net present values. The main contributions of this study are threefold: (i) the development of a unified dual-timeframe optimisation framework that simultaneously addresses fast power dynamics and long-duration energy shifting; (ii) the explicit integration of electro-thermal feasibility into long-term techno-economic HESS assessment; and (iii) a quantitative evaluation of seasonal renewable variability and its implications for HESS operation and economic viability using real-world microgrid data. Overall, the study demonstrates the technical viability of the proposed HESS and identifies the conditions under which it enhances renewable integration and resilience in cost-sensitive systems. The proposed framework is technology-agnostic and transferable to both islanded and grid-connected systems