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 (NaNO3) and 40 wt% potassium nitrate (KNO3). Thermal energy storage materials are important for the technology that is applied to reduce cost solar thermal power generation. The NaNO3-KNO3 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 (Na0.6K0.4)NO3 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 (NO3-) during the phase conversion for the eutectic (Na0.6K0.4)NO3 system.
Highlights
Thermal energy storage systems are a key technology for reduced cost solar thermal power generation [1, 2]
Differential Scanning calorimetry (DSC) technique is used in a wide range for the thermal investigation
The Differential Scanning Calorimetry (DSC) measurements showed a thermal hysteresis during this transformation with a magnitude of the hysteresis temperature ~8°C
Summary
Thermal energy storage systems are a key technology for reduced cost solar thermal power generation [1, 2]. The phase diagram of NaNO3-KNO3 is determined by DSC and high-temperature x-ray diffractometry [11] Thermal and mechanical properties of three representative salts for use in thermal storage systems have been evaluated as a function of temperature, thermal conductivity, specific heat, and the apparent heat of fusion were obtained using a differential scanning calorimeter [9]. Using additives such as graphite to NaNO3-KNO3 mixture leads to insignificant thermal conductivity improvement in overall application and the thermo physical properties for energy storage will be improved [10]. The thermal measurements for the eutectic (Na0.6K0.4)NO3 system were carried out by using (DSC) during heating and cooling at rate of 10°C/min, (TGA) and Differential Thermal Analysis (DTA) at heating rates of 2, 5, 10, 15 and 20°C/min
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