Ma, Binjian (2017-08). Experimental and Numerical Investigation of Molten Salt Nanomaterials for Enhanced Thermal Energy Storage (TES) and Heat Transfer Fluid (HTF). Doctoral Dissertation.
Thesis
Concentrating solar power (CSP) plants have been widely commercialized internationally for generating electricity from solar energy. Thermal energy storage (TES) systems are typically used in CSP plants to balance the fluctuations in demand with the intermittency of supply. In various CSP plants, molten salts are used as both the primary heat transfer fluid (HTF) and as TES medium. However, molten salts suffer from poor thermo-physical properties, e.g., specific heat capacity is typically less than 2 J/(g.K) and thermal conductivity is typically less than ~1 W/(m.K). Doping molten salts with minute quantities of nanoparticles has been shown to enhance their thermo-physical properties (also known as molten salt nanofluids). Stable dispersion of nanoparticles realized in different solvents (i.e., nanofluids) has been demonstrated to cause anomalous enhancement in the resulting thermo-physical property values. Traditional approaches employed for mixing nanoparticles in solvents often results in agglomeration and precipitation (fouling). This results in compromised reliability and not being cost-effective for industrial applications, such as in CSP plants. In this study, an innovative one-step synthesis protocol was developed and the techno-economic feasibility of using the molten salt nanofluids was explored for CSP applications. Modulated Differential Scanning Calorimetry (MDSC) and TemperatureHistory (T-History) method were used to measure the specific heat capacity of the nanomaterial samples at high temperatures (~500 ?C). In addition, the thermal conductivity of the nanofluid samples were also measured using a customized concentric cylinder test apparatus. Solar salt (NaNO3-KNO3) was used as the neat solvent (base fluid) material. Various nanoparticles (SiO2, Al2O3, MgO, ZnO) were either procured directly from commercial suppliers or generated in-situ from chemical reactions. Different parameters were explored in the synthesis: nanoparticle type, concentration, synthesis temperature, synthesis time, dispersing agents, etc. Numerical models were developed to elucidate the mechanism of specific heat capacity enhancement of the synthesized nanomaterials and to explore the thermal-hydraulic performance of molten salt nanofluid samples in a flow loop. Molecular dynamics (MD) simulations were performed to elucidate the morphology of the compressed layer formed due to adsorption of the solvent molecules on the surface of a nanoparticle surface. Chemical kinetics simulations were performed to predict the nucleation and growth rate of ensembles of nanoparticles during one-step synthesis. CFD simulations were performed to predict the heat transfer coefficient of the molten salt nanofluids in a flow loop. The results from the experimental and numerical investigation demonstrated that the one-step synthesis protocol for nanofluids involving generation of nanoparticles in-situ from cheap additives is a cheap and cost-effective approach for industrial applications (e.g., CSP) for enhancing the energy storage capacity and power rating as well as for extending the life-cycle of equipment (e.g., heat exchangers).
