Design of nitrate-based molten salt/nanoporous aluminum membrane composite structures with improved thermophysical properties for use in concentrated solar termal plants

Due to the development of environmental awareness and optimization of existing energy technologies, the use of salt melts as a heat transfer agent (HTA) and thermal energy storage (TES) fluid has been brought up again for a wide variety of applications due to the unique physicochemical properties. Because, salt solutions and their mixtures have all the desired properties for an industrial fluid due to their important advantages such as low viscosity, low vapor pressure, low cost, high chemical stability and environmental friendliness. Today, these melts are widely used as working fluids in many different fields such as nuclear power plants1, chemical industry2, oil refineries3 and concentrated solar power (CSP) plants4. In all these applications, the salt mixture type and proportions are selected according to their usage areas. Choosing a suitable HTA and TES material is important to minimize the cost of solar receiver, thermal storage tanks and heat exchangers and achieve high thermodynamic efficiency. Existing molten salt ITAs have high melting points (> 200 ° C) and begin to decompose above 600°C. The main problem with this application is that salt melts can freeze easily in the evening or in the winter, and in this case, it obstructs the working conditions by blocking the pipeline. Therefore, the development of inexpensive salt melt compositions with a lower melting point and a higher thermal stability is important in both improving the thermodynamic efficiency of CSP plants and improving operating conditions. In this thesis, a new nanostructured salt solution was designed to achieve low melting point (<200°C) and high thermal conductivity. Thermal and structural of potassium nitrate (KNO3), sodium nitrate (NaNO3) and their eutectic mixtures (Solar Salt, 60% by weight NaNO3 - 40% KNO3) placed in anodic alumina oxide (AAO) with different pore sizes (between 25 and 400 nm). features are examined. Thermal characterization of the prepared AAO / salt composite structure was investigated by differential scanning calorimetry (DSC) and thermal properties analyzer (TPS), and its structural characterization was examined by scanning electron microscopy (SEM) and X-ray diffraction (XRD). In characterization with DSC, a sudden drop in the melting temperatures of KNO3, NaNO3 and eutectic mixture compressed into AAO pores was observed. According to DSC results, no systematic decrease in pore diameter was observed in AAO / KNO3 and AAO / Solar Salt composite structure, but it was observed that melting temperature decreased more as pore diameter increased in AAO/NaNO3 composite structure. It has been observed that the maximum melting temperature decrease is 173°C in AAO / KNO3 composite structure with pore diameter of 400nm. According to the TPS measurements, a significant increase in thermal conductivity is observed in all composite structures. It was observed that this increase was not systematic in AAO / KNO3 and AAO / Solar Salt composite structures, but it was systematic in AAO / NaNO3 composite structure. The highest thermal conductivity increase is seen in AAO / KNO3 composite structure with pore diameter of 35nm. XRD analysis showed that KNO3 forms a single crystal structure directed parallel to the longitudinal axis of the pores.