Thermochemical storage (TCM) systems, are a promising alternative for harnessing solar energy, allowing energy availability (summer) to be decoupled from its demand in the context of home heating (winter). During the winter season, peak heat demand is a major disruption to the power system, especially at a time when much of the heat demand will be met by electrically powered systems such as heat pumps. Seasonal thermal storage would enable their more effective management, bringing benefits in terms of optimization of the generation fleet, reduction of fossil sources and less criticality of the power grid.

One of the most widely used TCM materials is 13 X zeolites, a material that can adsorb water vapor and release heat. Zeolites are an economical material with good performance, both in terms of temperature rise during the release phase and in terms of energy density (120 kWh/m3). Energy density is a key factor in the process as it affects the reactor volume. Higher energy densities can be achieved by using hydrated salts, such as MgSO4.7H2O (MSH), as TCM material, which can provide densities up to 780 kWh/m3. TCM systems based on MSH exploit the hydration/dehydration reaction, which is endothermic in one direction (charging) and exothermic in the opposite direction (discharging).

Because of deliquiescence problems, the hydrated salts must be deposited in a porous matrix. The support must have an optimized porosity, providing a trade-off between small pores and high pore volume that maximize energy density, and larger pores that provide high heat and mass transfer.

In the context outlined above, a first line of activity involved the development of MSH-based materials suitably supported by a matrix with optimized porosity. For this purpose, different types of support will be considered and the porosity will be controlled, creating a hierarchical pore distribution by varying the precursors and porosity generators.

A laboratory-scale screening facility has been designed to test magnesium sulfate materials: the facility involves a packed-bed reactor containing 10 to 40 g of pellets. The reactor can be supplied with air, humidified, in the absorption phase or with dry air in the regeneration phase. The reactor is placed inside an electrically heated furnace capable of bringing the material to the required operating temperature (25-40°C in absorption, 230-280°C in regeneration).

The plant is properly instrumented so that the temperature rise during the discharge phase and the amount of steam absorbed can be evaluated. The plant is operated by PLC and controlled by the user through a PC interface (HMI).

The Report is available on the Italian site