Ceramic nanoparticles enhancement of latent heat thermal energy storage properties for LiNO₃/NaCl: Evaluation from material to system level

Focusing on the development of the next generation latent heat thermal energy storage (TES), molten salt is one of the most promising candidates, while it suffers from small thermal charging rate and low energy storage density. Adding ceramic nanoparticles (NPs) with high thermal conductivity and high chemical stabilities has been proposed to alleviate above problems. However, it's very challenging to enhance both thermal conductivity and specific heat capacity (C_p) simultaneously. Besides, induced viscosity increment by NPs will instead reduce the thermal charging rate of TES system, due to suppressed natural convection of molten salts. Herein, concurrent enhancement in solidus thermal conductivity and C_p is demonstrated by doping MgO NPs into LiNO₃/NaCl, which are improved by 63.5 % and 32.3 % at 4 wt%, respectively. The underlying mechanism is attributed to very low interfacial thermal resistance (R_b = 2.424 × 10⁻⁹ K·m²·W⁻¹) between MgO and LiNO₃/NaCl. Benefiting from enhanced C_p, the total energy storage density increases from 662.9 J·g⁻¹ to 671.7 J·g⁻¹ for temperature range of 50–300 °C despite decreased phase change enthalpy. The viscosity has a sharp increase from 4.2 cP to 22.4 cP when NPs concentration rises from 0 wt% to 10 wt% at 360 °C. When being applied in TES system, the optimal concentration of MgO NPs is found to be 4 wt%. The thermal charging rate of TES system is suppressed when NPs concentration is too low or too high due to limited thermal conduction and inhibited natural convection, respectively. Viscosity is verified as important as thermal conductivity in system level evaluation. This work helps to guide the design of high-performance molten salts in TES system, so as to achieve both faster thermal charging rate and higher energy storage density.