Graduation Semester and Year

2016

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Mechanical Engineering

Department

Mechanical and Aerospace Engineering

First Advisor

Donghyun Shin

Abstract

Concentrated solar power uses general thermodynamic cycles (such as Rankine or Gas turbine cycle) to produce electricity and thus its efficiency primarily relies on the operating temperature of thermal energy storage. Current thermal energy storage medium is organic material such as synthetic oil or fatty acid. However, these materials are not stable at high temperatures due to their thermal decomposition. Using molten salts as thermal energy storage medium is a very attractive option since they are thermally stable at high temperatures (over 600 °C). They also have very lower vapor pressure (for reducing mechanical stress on structure materials), less reactive, and abundant in nature in comparison with the conventional thermal energy storage materials. However, their low specific heat hinders the use of molten salts as thermal energy storage materials. The low specific heat of molten salts can be enhanced by doping with nanoparticles. Solvents doped with nanoparticles (termed as nanofluids) are well known for their large enhancement of thermal conductivity. In this study, the low specific heat of molten salts were enhanced by doping with nanoparticles. SiO2 nanoparticles were dispersed in a mixture of Li2CO3-K2CO3 at 1% concentration by weight showed 25% enhanced specific heat. From the subsequent material characterization study, a large amount of special fractal-like nanostructures was observed all over the nanofluids. Four different sizes of nanoparticles were tested to prepare nanofluids to verify the effect of nanoparticles on specific heat and the result showed almost no variation in specific heat with nanoparticle size. This implies that nanoparticles may not have direct effect on the enhanced specific heat but may help the formation of the fractal-like nanostructures. The fractal-like nanostructures are formed by molten salt molecules electrostatically interacting with nearby nanoparticles and may be responsible for the enhanced specific heat of nanofluids. To verify this, two batches of nanofluids were prepared and one was treated with a very small amount of hydroxide to interrupt the proposed electrostatic interaction. The result showed that the one treated with hydroxide did not form the fractal-like nanostructures and no specific heat enhancement was observed, while the other nanofluid showed constant 25 % enhanced specific heat. In order to have a better understanding of the effects of the formed nanostructures in nanofluid sample, the rheological properties of the pure and nanofluid samples were studied. The results shows increase not only in the average value of viscosity on the nanofluid sample, but also changing in the behavior of the fluids. That is, the nanofluid samples shows high amount of non-Newtonian behavior compare to the one of pure samples which shows Newtonian behavior. Based on the experimental results, and applying proposed theories to have a better understanding of thermophysical properties of nanostructures, this study proposes a new specific heat mechanism theories of molten salt. A comparison study was conducted using the proposed mechanism to explain the difference between conventional nanofluids whose specific heat decreases and molten salt-based nanofluids whose specific heat increases. The result of this study is expected to not only help to design advanced thermal energy storage for concentrated solar power applications but also help to answer unsolved questions in the field of nanofluids.

Keywords

Thermal energy storage, Nanomaterials, Specific heat capacity, Viscosity

Disciplines

Aerospace Engineering | Engineering | Mechanical Engineering

Comments

Degree granted by The University of Texas at Arlington

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