Abstract:The thermal behaviour for the DTA, DSC and TGA measurements have been carried on the solid phase transformation for the binary eutectic mixture of 60 wt% sodium nitrate (NaNO 3 ) and 40 wt% potassium nitrate (KNO 3 ). Thermal energy storage materials are important for the technology that is applied to reduce cost solar thermal power generation. The NaNO 3 -KNO 3 system is a binary inorganic salt system and it is one of the most promising thermal storage materials. The methods are based on the principle that a change in the physical state of a material is accompanied by the liberation or absorption of heat. The various techniques of thermal analysis are designed for the determination of the enthalpy accompanying the changes in the physical properties of the material. The thermal measurements showed a reversible phase transition at ~114°C during heating process and at ~108°C during cooling process. It has been shown also the presence of thermal hysteresis during this transformation with a magnitude of the hysteresis temperature ~8°C. The thermogravimetery analysis (TGA) indicated that the eutectic system (Na 0.6 K 0.4 )NO 3 is thermally stable up to the melting point at ≅225°C. This means that the sample under study is structurally stable. DTA measurements were also carried out for the sample at different heating rates of (2, 5, 10, 15 and 20°C/min). Some thermal parameters such as the transition point, enthalpy and the activation energy for the transformation process were estimated at each heating rate. It has been also shown that these parameters are affected by the heating rate. The noticeable effect of heating rate on the thermal parameters means that the heating rate is a main factor to change the thermal interaction potential of the Na and K atoms around the nitrate group (NO 3 -) during the phase conversion for the eutectic (Na 0.6 K 0.4 )NO 3 system.

Here, we investigate the effect of adding nano-silica particles on the thermo-physical properties of the (Na0.6K0.4)NO3 based thermal energy storage systems. Five different systems tagged as M00, M01, M02, M03 and M04, with different nano-silica percentage of 0, 1, 2, 3, and 4 wt%, respectively, were prepared. Various experimental techniques were employed to study the thermo-physical properties of the systems during (solid-solid) phase P1, (solid-liquid) phase P2 and (liquid-solid) phase P3, and to clarify the effect of nano-silica on the thermal energy storage efficiency during both charging and discharging processes. According to the Differential Scanning Calorimeter (DSC) thermal analysis, it was found that the system M02 whose nano-silica addition rate of 2 wt%, has the most favorable thermal characteristics (i.e., highest specific heat and lowest enthalpy change). Moreover, the addition of 2 wt% represents the optimum distribution of nano-silica inside the principal base system M00. This leads to an improvement in the porosity of the system due to the degree of homogeneity caused by the thermophoresis effect distribution, the high surface area of the nano-silica with the activity of the M00 matrix alongside the degree of the alkalinity of nano-silica. Besides, the electric conductivity measurements showed that the 2wt% percentage is the optimum one for thermal energy storage systems.