Desing of fly ash based nanocomposites for improvement thermal energy storage capacity of the nitrate salts

In recent years, the demand for sustainability and energy efficiency in the energy sector has been rapidly increasing. Renewable and clean energy sources are needed to meet the world's growing energy demand. Concentrated solar power (CPS) plants are seen as an environmentally friendly and efficient source of electricity production among alternative energy sources. In CPS plants, the development of fluids with high performance and low cost that can be used as both thermal energy storage (TES) fluids and heat transfer agents (HTA) is crucial. Today, due to their physicochemical and thermodynamic properties, salt melts and various mixtures are used in CSP plants and many applications. The fact that salt melts are environmentally friendly materials with high chemical stability, low viscosity, low vapor pressure, high energy density, and low cost make them an ideal fluid to be used in the energy sector. However, when molten nitrate salts are used alone, their thermal stability is low, limiting the storage time. Also, due to the high melting points (>200 °C) and low decomposition temperatures (at temperatures above 600 °C) of molten salt fluids, they can freeze in cold weather conditions and darkness, causing pipes to clog. Therefore, forming nanocomposites by mixing with a material with high thermal conductivity, such as fly ash, contributes to improving the thermodynamic properties of molten salts. Thus, low-cost HTA and TES materials with low melting points, high storage capacity, and high thermal stability will be obtained in CSP plants. In this thesis study, the aim was to design cheap molten salt and fly ash nanostructures with high thermal conductivity, low melting temperature, and high decomposition temperature. The structural and thermal properties of fly ash-potassium nitrate (KNO3) mixtures with different weight percentages of fly ash (0.3%, 0.5%, 1%, 2%, and 5%) were experimentally investigated. The structural properties of the prepared nanofluids were examined with scanning electron microscopy (SEM) and X-ray diffraction (XRD); thermal characterization was investigated using thermal properties analyzer (TPS), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA) methods. The results of the DSC analysis revealed that the highest increase in melting temperature was observed at a rate of 0.2% in the sample containing 1% fly ash-KNO3 (within the experimental error range). Upon examination of the heat capacity values, the highest increase was detected at 29.69% in the sample containing 0.5% fly ash-KNO3, indicating a positive effect of the binary crystallization procedure on the increase in heat capacity. Contrary to expectations, TGA characterization demonstrated a decrease in decomposition temperatures as the percentage of fly ash increased, and it also indicated a decrease in weight loss percentages with an increase in temperature. Thermal conductivity measurements revealed a linear increase in thermal conductivity values with the addition of fly ash, with the greatest increase achieved at 5% addition of fly ash. The crystallographic structure of the materials was examined using XRD, demonstrating that the addition of fly ash to KNO3 altered the sizes and quantities of KNO3 crystals while preserving their ferroelectric property. SEM images showed that the fly ash did not agglomerate on the KNO3 crystals but dispersed uniformly.