| dc.description.abstract | Concentrating Solar Power (CSP) plants are progressively gaining commercial success around the world. As of 2016, CSP plants account for a total installed capacity of 4.8GW. A significant cost advantage for CSP plants is enabled by technologies that have afforded cheap Thermal Energy Storage (TES) platforms that have significantly enhanced the reliability of CSP plants for their integration into the power grids. TES platforms provide a means for matching the phase lag between the peaks in energy demand and the diurnal peaks in insolation (solar energy availability and solar power production) thereby balancing the demand and supply of power as well as for mitigating the load fluctuations on the electricity grid. Typically, molten salt eutectics are employed in commercial CSP plants as both TES medium and as Heat Transfer Fluid (HTF) due to their stability at high temperatures and low vapor pressures (thus obviating the need for costly and bulky pressure vessels, that are typically required for other working fluids, such as steam) as well as for their minimal environmental impact (molten salts are environmentally benign materials). Molten salts however suffer from poor thermo-physical properties (e.g., low specific heat capacity and thermal conductivity values) and are highly corrosive, especially at elevated temperatures. Stable suspension of nanoparticles in solvents are termed as nanofluids. Literature reports have shown anomalous enhancements in the thermo-physical properties of nanofluids. Hence, doping molten salts with nanoparticles (“molten salt nanofluids”) can enable significant enhancement in the specific heat capacity and thermal conductivity (over that of the neat solvent).
However, the high cost of nanoparticles (~ $1000/ kg) pose a significant barrier to their commercial implementation for molten salts (which cost less than $0.50/kg). Hence, even at 1% mass concentration, nanoparticles can triple the cost of the molten salt nanofluids (i.e., neglecting other issues – such as materials handling costs). In this study, a one-step synthesis procedure was explored for generation of nanoparticles in-situ in the molten salt eutectic (solvent) from cheap additives for realizing a cheap method that is amenable for industrial scale production of molten salt nanofluids. Additives that were explored in this study include: unstable salts – such as aluminum nitrate nonahydrate (which undergo thermal decomposition when the solid mixture of molten salt and the additive material is heated to form a melt pool) resulting in the in-situ formation of ceramic nanoparticles (e.g., alumina). The solvents (molten salts) explored in this study include: (a) binary nitrate eutectic composed of NaNO3-KNO3 (60:40 by mass fraction); and (b) A ternary nitrate eutectic composed of LiNO3-NaNO3-KNO3 (38:15:47 by molar ratio). The enhancement in the specific heat capacity and thermal conductivity values of the molten salt nanofluids samples were measured in this study. The specific heat capacity values were measured using a transient Temperature-History (T-History) technique that was adapted and modified in this study for the molten salt samples. The thermal conductivity values were measured using a custom-designed concentric cylinder experimental apparatus (which enabled the data acquisition of the circumferential distribution of the steady state temperature profiles and the estimation of the corresponding 1-D radial temperature gradients in molten salt samples that were confined in the cylindrical annulus). These measurement strategies were adapted from prior reports in the literature and were modified in the current study - in order to obviate the effects of nanoparticle precipitation on the instrument error and minimize the measurement uncertainties. Experiments were also performed to study forced convection subcooled boiling heat transfer of de-ionized water (DI Water) and aqueous nanofluids in a circular pipe. Heat removal rates from the heated pipe are measured for various mass concentrations of nanofluids, wall temperatures and coolant flow rates. Anomalous thermal behavior was observed in boiling heat transfer coefficient measurements resulting from the precipitation of the nanoparticle on the heater surface i.e. pipe surface. The precipitation of nanoparticles (resulting in formation of “nanofins” leading to the enhancement of the surface area) resulted in increase in net convective heat transport. Hence, this study highlights the importance of surface morphology in determining the efficacy of nanofluids as coolants. Furthermore, flow visualization was performed to study the flow boiling regimes for various working fluids explored in this study. | en |
