Graduation Semester and Year

2014

Language

English

Document Type

Thesis

Degree Name

Master of Science in Aerospace Engineering

Department

Mechanical and Aerospace Engineering

First Advisor

Donghyun Shin

Abstract

Meeting the continuously increasing demand of `clean' energy is the biggest challenge of the energy research field. Amongst the many renewable sources of energy, energy from the Sun is one of the biggest sources in the form of light and heat. Photovoltaic cells and concentrated solar power are few of the solar energy harnessing technologies. Photovoltaic cells use semiconductor materials to directly convert energy from sun into electricity. CSP uses a combination of solar receivers (mirrors or lenses) to concentrate the solar thermal energy which is further converted into electricity using the common thermodynamic cycle. One of the Photovoltaic (PV) technology's disadvantages over CSP is that it delivers power only in direct sunlight and it cannot store excess amounts of produced energy for later use. This is overcome by CSP. Thermal energy storage (TES) devices are used in CSP which store large amount of thermal energy for later use. TES use organic materials like paraffin wax, synthetic oils etc. as their heat transfer fluids (HTF).One of the drawbacks of CSP technology is the limiting operating temperatures of HTF (up to 400 ?C), which affects the theoretical thermal efficiency. Increasing this operating temperature up to 560 ?C, which is the creeping temperature of stainless steel, can enhance the efficiency (from 54% to 63%). Hence, the uses of molten salts, which are thermally stable up to 600 ?C, have been proposed to use as TES. With advantages of high operating temperature, low cost and environmentally safe these salts have disadvantage of poor thermo-physical properties like low specific heat capacity and thermal conductivity. A lot of experimental results of enhancements in the thermo-physical properties of molten-salt embedded with nanofluids have been reported. Nanofluids are solvents doped with nanoparticles. These reports suggest formation of nanostructures with liquid layer separations in the base salts as possible cause of enhancements. But there has been very limited computational analysis study to support these findings. Tiznobaik et al saw nanostructure formed near nanoparticles and concluded it to be the primary cause for the enhanced specific heat capacity in carbonate nanofluids, (Li2CO3-K2CO3/SiO2). The formation of nanostructure was reasoned with dense layers and concentration gradient seen within the surrounding molten salt mixture. In this study an attempt has been made to elucidate and support this finding using computational analysis. Molecular Dynamic simulations have been performed using LAMMPS to analyze the cause of nanostructure formations. For the simulation, a periodic box of Li2CO3-K2CO3 (62:38) and a SiO2 nanocluster was made in Material Studio. After lot of initializations, a stable system was achieved and analysis showed concentration gradient around the nanocluster. Same analysis will help to prove the theory of concentration gradient in other combination of salts and nanomaterials. This will also act as a base for finding the thermo-physical properties like heat capacity, thermal conductivity, density etc. of such salts to further validate the experimental results showing their enhancements.

Disciplines

Aerospace Engineering | Engineering | Mechanical Engineering

Comments

Degree granted by The University of Texas at Arlington

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