Thermochemical storage (TCM), and more broadly high-density energy storage (including both TCM and phase change materials, PCM), represents a very promising technology for energy demand management, especially in the context of utilizing renewable energy sources, which are variable and unpredictable over both short and long terms (seasons). In the previous three-year period, a screening of materials for thermochemical storage was conducted, identifying magnesium sulfate heptahydrate as one of the most promising materials for hydration/dehydration cycles. However, using the pure salt presents several challenges, including high pressure losses, deliquescence issues, and instability during cycles, most notably the formation of a hydrated salt layer on the surface. This layer can prevent water from diffusing into the unreacted salt, thus hindering the completion of the reaction. To address these issues, the salt was deposited on various types of supports. The comparison between the different supported materials developed revealed that the material’s performance during charge/discharge cycles is highly dependent on the type of support used. Even for the best-tested material, the operational conditions employed in the tests did not allow full exploitation of the material (slow heat release dynamics, incomplete hydration).

Building on the activities of the previous three-year period, the current work focused on developing high-energy-density materials in collaboration with the Catalysis and Catalytic Processes group at the Department of Energy, Politecnico di Milano. The collaboration aimed to develop magnesium sulfate-based materials supported on alumina, with optimized porosity to ensure both high salt loading (high energy density) and good diffusion of the gaseous reactant (high reaction/diffusion rates). For this purpose, Politecnico developed an innovative preparation procedure that allowed the creation of supports with controlled porosity while maintaining good mechanical properties. The materials prepared by Politecnico were tested through absorption and regeneration cycles to evaluate their performance in terms of heat release due to the reaction.

During this activity, a new test bench, designed in 2019, was built and tested. This bench was used for the experimental testing campaign, starting with the first tests on zeolite 13X-based materials. This material was chosen to initiate the experimental campaign because it is a commercially available material, available in large quantities, and therefore does not present issues related to deactivation or supply shortages in the event of vapor condensation problems during the initial tests. Such problems would be particularly challenging if magnesium sulfate-based materials were used, given magnesium sulfate’s high solubility in water and the complex preparation procedure for these materials, which results in limited available quantities. Additionally, zeolites are among the most extensively studied materials for seasonal heat storage applications due to their good activity and high stability, making them an appropriate reference for comparing the performance of developed materials. Several heat storage reactor prototypes based on zeolite (MONOSORP, STAID, E-HUB/ECN) have been developed, achieving energy densities of up to 120 kWh/m³. Vapor absorption/heat release and vapor desorption/heat storage tests were conducted on zeolite 13X using two types of reactors: a screening reactor (20 g) and an intermediate reactor (approximately 700 g).

Finally, a further development of this activity involved constructing a mathematical model of the test reactor for storage materials. This development was carried out in collaboration with the Air Lab group at the Department of Energy, Politecnico di Milano. The mathematical model, developed in the Matlab environment, is designed to describe the reactor’s behavior during storage/regeneration tests, both for zeolite-based materials and magnesium sulfate/alumina, and will be validated based on the results of the experimental campaign conducted at the RSE test bench.