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Title:Modeling, fabrication and optimization of optically transparent - thermally insulating silica aerogels for solar thermal applications
Author(s):Gunay, Ahmet Alperen
Advisor(s):Miljkovic, Nenad
Department / Program:Mechanical Sci & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Solar Energy
Optically Transparent - Thermally Insulating
Silica Aerogels
Abstract:Solar thermal energy conversion has the potential to reduce greenhouse gas emissions via the offset of fossil fuel burning power generation methods. By capturing the suns energy and using it to heat steam as part of a Rankine cycle, electrical energy can be renewably produced. Furthermore, solar thermal collectors have high potential for domestic heating when deployed at the rooftop scale, reducing fossil fuel consumption used for home heating needs. The efficiency of these solar applications is highly dependent on the ability of the collecting device to absorb the incoming solar energy, and minimize thermal losses to the environment. Current techniques utilize vacuum tubes to eliminate convective losses, in combination with selective surfaces (high absorptivity in the solar spectrum, and low emissivity in the infra-red (IR)) to minimize thermal re-radiation. Here, we present an alternate approach that operates at atmospheric pressures with simple, black, absorbing surfaces. An Optically Transparent Thermally Insulating (OTTI) layer was assumed to be coated on the back side of the black, broadband absorber. This absorber was assumed to have perfect transparency and opacity in the solar spectrum and Infra-Red (IR), respectively. In order to provide a deeper understanding of the link between the optimum OTTI layer material properties and the overall solar thermal efficiency, we developed a coupled radiative-conduction heat transfer (HT) model used to predict how the investigated OTTI layers will behave when they are used as solar thermal absorbers. The optimum properties that were obtain were then incorporated into the HT model to study the thermal performance under various optical concentrations (1 – 20 suns), solar thermal absorber temperatures (20 – 200°C), and external heat transfer coefficients (10 – 100 W/m2K). The results showed potential solar thermal conversion efficiencies of ≈90% can be attained by utilizing OTTI layers as insulators in the solar thermal absorbers. To check if a material to have the assumed material properties can be fabricated, silica aerogels were procured, synthesized and characterized. Due to their naturally high and low transmissivities in the solar and IR spectrums, respectively, silica based aerogels coated on the back with highly absorbing (black) surfaces offer a potential solution to create simple and inexpensive solar thermal absorbers. To test our hypothesis, we fabricated tetramethyl orthosilicate (TMOS) and tetraethyl orthosilicate (TEOS) based silica aerogels. The formed gels were aged for 3 weeks and dried using a carbon dioxide supercritical point dryer. The obtained aerogel optical properties were characterized using ultraviolet-visible (UV-Vis) and Fourier Transform Infrared (FTIR) spectroscopy, showing spectrally averaged (0 < λ < 2.7 µm) transmissions of ≈87% for a sample thickness of 4mm. To minimize the effect the O-H group transmission reduction occurring in the solar spectrum, we modified the baseline aerogels to be hydrophobic with a silane treatment to shield the exposed hydroxyl groups on the surface. Hydrophobic modification resulted in an increase of the spectrally averaged transmissions to ≈ 94%. This study sheds light on the applicability of silica aerogels on black coatings as ideal solar thermal absorbers and offers insights into new avenues for performance improvement of solar thermal energy systems.
Issue Date:2016-04-27
Rights Information:Copyright 2016 Ahmet Alperen Gunay
Date Available in IDEALS:2016-07-07
Date Deposited:2016-05

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