Optimization of LiNO₃-Mg(OH)₂ composites as thermo-chemical energy storage materials

To reduce the emission of greenhouse gases, the substitution of fossil fuel by renewable energy sources is increasingly important. Matching energy supply and demand is however required, even more so if intermittent renewable energy sources are employed. Thermal energy storage then offers significant advantages. Thermo-chemical energy storage systems, using reversible reactions, have a high reaction enthalpy that exceeds the storage capacities of sensible and latent heat modes. Magnesium hydroxide is a candidate TCES material for such a system at temperature around 300 °C, and adaptable when doping Mg(OH)₂ with metal salts. Both pure Mg(OH)₂ and its composites with 1, 3, 6 and 10 wt% LiNO₃ are studied. The present work validates this TCES process and develops reaction rate equations needed for its design. The LiNO_3-doping significantly reduces the onset temperature of dehydration. For pure Mg(OH)₂, the temperature is 325 °C. It is reduced to 289 °C when 1 wt% LiNO₃ is present, and further reduced to 269 °C at a dosage of 10 wt% LiNO₃. Whereas the dehydration of pure Mg(OH)₂ is slow, with a rate constant k of 1.72 10^-5 s^-1 at 300 °C, adding increasing amounts of LiNO₃ progressively increases the reaction rate constant to ~10^-2 s^-1 at 300 °C when 10 wt% LiNO₃ is present. The kinetic expressions enable to predict the conversion yield and amount of heat stored or released for any desired temperature and selected duration of the heat-induced dehydration. LiNO_3- doped Mg(OH)₂ have a high potential in TCES applications when the heat source is available at temperatures between 250 and 400 °C, since the equilibrium temperature and the extent of de-hydration Mg(OH)₂ can be tuned to the required temperature range by adding different wt% of LiNO₃.