Development of Nanosalt and Heat Transfer Optimization for Solar Energy Storage

This thesis is concerned with solar energy storage systems in terms of storage materials and storage systems for high temperature applications. The main focus has been given to either improve the thermophysical properties of the storage medium or to improve the design of the storage medium by optimizing the solar energy storage system. Nitrate salts have been chosen as the phase change material with nanoparticles as additive materials. Different types or concentrations of nanoparticles and different types of nitrate salt have been selected and studied. There are different objectives, starting from a few grams (up to 5 g) to kilograms (up to 3 kg), that have been considered through this research. In the first objective, nanosalts, which are nanoparticles seeded in the nitrate salts, have been prepared by two different methods, either the 2-step method or the 1-step method. Nanosalt prepared by the 2-step method showed higher specific heat capacity (cp) than the base salt by 10.5% with higher thermal conductivity (k) values up to 60%. Up to 6% increments in total thermal energy storage have been observed for nanosalt (iron oxide (Fe2O3) nanoparticles and binary nitrate salt). Additionally, the 1-step method was used to prepare copper oxide (CuO) nanoparticles directly inside the nitrate salt, which showed improvements in cp in comparison to base nitrate salt. In the second objective, alongside with the thermophysical properties measurements, material characterizations have been considered using different devices to show the morphology of the surface area of salts and nanosalts samples. In the third objective, an experimental rig has been designed and built to study heat transfer of salts and nanosalts. Temperature measurements have been made at different axial, radial and azimuthal locations. Both charging and cooling of the salt (or nanosalt) were studied, showing that improvements in the charging process are related to the type or concentration of the nanoparticles material. Overall, heat transfer is improved in the case of nanosalt compared to salt alone. Two different types of salt were tested in this experiment rig which was single salt (sodium nitrate) and binary solar salt (sodium nitrate: potassium nitrate by 60:40 molar ratio) with different additives materials such as CuO (by 0.5 wt. %) and Fe2O3 (by 0.1 wt. %, 0.5 wt. % and 1 wt. %). In the fourth objective, Computational Fluid Dynamics software has been used to solve the charging process of salt and nanosalt. A validation for the ANSYS CFX code (version 17.0) is conducted by comparing the experimental data with the simulation data. A good agreement is obtained for both cases of salt or nanosalt. In the fifth objective, an optimization for the solar energy storage system has been conducted. Different designs for the storage system employing finned structures were studied. The new combination effect of both nanosalt and fins system has been studied using the validated CFX code, showing a promising improvement in the charging process in comparison to salt alone, nanosalt alone or salt-fins system alone. As a result, our overall aim is to improve the thermal properties of nitrate salts and optimize the thermal energy storage system.