Nanoconfined water properties for measuring the state-of-charge of sorption thermal energy storage

The ongoing increase in atmospheric carbon dioxide (CO₂)—the main anthropogenic greenhouse gas—is one of the most critical indicators of the planet’s energy imbalance. Emissions from fossil fuel combustion are the primary driver of global warming, with direct consequences for the climate and the stability of ecosystems. Therefore, the decarbonization of the energy system has become a global priority, requiring a steady transition toward renewable sources and high-efficiency, low-emission processes. However, the intrinsic intermittency of major renewable sources such as solar and wind energy demands the development of sustainable storage technologies capable of managing energy surpluses and reducing dependence on fossil fuels. In this context, sorptive thermal systems offer a promising solution: the reversible adsorption–desorption process of sorptive materials allows the storage of thermal energy in the form of chemical bonds. These systems exhibit high energy density, strong compatibility with low-temperature heat sources (e.g., waste heat from solar thermal collectors or low-grade industrial streams), and long-term stability, as they operate without thermal losses. Consequently, they can act as a technological bridge between renewable energy generation and efficient thermal management, being applicable in solar cooling, dehumidification, and seasonal storage cycles. Among sorptive materials, Zeolite 13X stands out as a benchmark for water adsorption, chemical heat storage, desalination, and humidity control, thanks to its highly ordered microporous structure and strong hydrophilicity. The first objective of this work is to experimentally characterize the dielectric behavior of Zeolite 13X at different hydration levels by means of Broadband Dielectric Spectroscopy (BDS) measurements performed at INRiM (Istituto Nazionale di Ricerca Metrologica, Italy). The second part focuses on reproducing and interpreting the dielectric phenomena through Molecular Dynamics (MD) simulations performed with GROMACS, aiming to correlate the macroscopic dielectric response with the microscopic statistical behavior and structure of confined water. The experimental spectra were processed using Havriliak–Negami (HN) fitting models to separate low- and high-frequency relaxation contributions and study their correlation with the water content, providing quantitative insight into the polarization mechanisms. The integration of experimental and computational results enabled the formulation of a predictive model capable of estimating the hydration level of the zeolite from the measured dielectric parameters. The outcomes of this work demonstrate that the dielectric response of confined water in adsorbent materials can serve as an effective indicator of the adsorption state. This approach could enable, in future applications, the development of integrated dielectric sensors for zeolite-based systems, facilitating process control, enhancing overall cycle efficiency, and allowing direct, non-gravimetric assessment of water content.