Thermal characterization of MOFs for thermal energy storage

Nowadays the integration of thermal energy storage (TES) into HVAC systems seems to play a key role in determining a decrease in heating and cooling energy needs in the buildings sector, which will keep growing. Indeed, TES systems allow to decouple generation and demand for heat, power and cooling and to store renewable solar energy, dealing with its intermittency, improving energy efficiency systems and thus leading to a reduction in fossil fuel energy consumption and GHG emissions. The current state of the art of energy storage technologies requires the design of novel adsorbents to achieve relevant improvements in this field. Metal-Organic Frameworks (MOFs) are a new class of porous crystalline materials that are organized by the coordination between inorganic metal units and organic ligands. They are particularly attractive because of their tunable amphiphilic character, pore volume, pore size and specific surface area, by changing the chemical constituents or adding function-alized linkers. Moreover, they show low desorption temperature and faster kinetics than other physical or chemical adsorbents. In this work, a series of water stable MOFs have been investigated in order to choose which to synthesize and test. Firstly, screening their energy storage capacity and desorption temperature, compared to those of other adsorbent materials, MOFs and MOF-based composites were found to exhibit the highest energy density at the lowest desorption temperature. Then, since experimental data from literature were found at closed system testing conditions, it was decided to test the most promising ones under open system conditions. The work was focused on the first exploitation of pristine MIL-125-NH2 and two composites, MIL-125-NH2+ (45%)CaCl2 and MIL-125-NH2+ (45%)SrBr2. After synthesis, it was performed X-ray powder diffraction to verify the actual chemical composition of materials, nitrogen adsorption isotherms to assess pore volume, pore size and specific surface area and TG-DSC tests, i.e. simultaneous termogravimetric and differential scanning calorimetric tests, to both calculate the energy storage capacity and carry out hydrothermal stability and kinetics analyses. On one hand, results highlight the strong impact that specific surface area has on adsorption properties of MOFs. A decrease of this parameter can badly affect the water uptake and energy storage capacity, as in the case of MIL-125-NH2. On the other hand, MOF-based composites confirm their outstanding adsorption properties, showing higher water uptake and energy density values, even if at the expense of kinetics, with respect to the pristine MOF and still keeping lower desorption temperatures than the ones for inorganic salts. All the materials tested, as expected from literature data, show high hydrothermal stability, which makes them interestingly suitable for thermal energy storage applications.