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Date of Award

2022

Document Type

Campus Access Dissertation

Degree Name

Doctor of Philosophy in Mechanical Engineering-Engineering Mechanics (PhD)

Administrative Home Department

Department of Mechanical Engineering-Engineering Mechanics

Advisor 1

Sajjad Bigham

Committee Member 1

Kazuya Tajiri

Committee Member 2

Ana Dyreson

Committee Member 3

Joshua Pearce

Abstract

Next-generation high-flux concentrated solar receivers are deemed to operate at temperatures exceeding 1000ºC to enable advanced power cycles with high thermal-to-electric conversion efficiencies. The high temperature of a solar receiver, however, increases radiation losses (∝ T4), thereby degrading the solar-to-thermal conversion efficiency. Additionally, state-of-the-art 3D-printed metal and supper-alloy-based solar receivers fail to properly operate at high receiver temperatures due to excessive oxidation and creep experienced. To overcome the trade-off between the solar-to-thermal and thermal-to-electric conversion efficiency at high temperatures, and meet material requirements, transformational innovations in both the design and material of solar receivers should be made.

This research proposes 3D-printed hierarchically-ordered and nature-inspired ceramic volumetric receivers for high-flux high-temperature solar thermal collectors. Particularly, a new class of high-flux volumetric solar receivers inspired by the stone lotus plant is introduced and examined. The conceptualized lotus-inspired solar receiver offers a low optical thickness and excellent light-trapping effects combined with strong convective effects. As a result, the lotus-inspired receiver remarkably improves both optical and thermal characteristics, thereby augmenting the solar-to-thermal conversion efficiency at high temperatures. These characteristics enable substantially increasing the outlet temperature of the receiver working fluid (i.e., air), thereby maximizing the so-called “volumetric effect” (i.e., a higher outlet fluid temperature than the receiver’s aperture temperature).

To withstand high temperatures of future high-flux solar receivers and realize intricate topologies of the lotus-inspired class of solar receivers, here, 3D-printed ceramic volumetric receivers are adapted for high-flux solar receivers. Ceramics demonstrate some highly desirable thermal, mechanical, and corrosion resistance properties, including strength at temperatures exceeding 1000°C. However, ceramics are brittle and hard to be machined or shaped to a desired solar receiver architecture. Therefore, the proposed work matures ceramic 3D printing for advanced high-flux solar collectors.

To better understand and thus design such advanced high-flux solar receivers, a detailed understanding of key parameters affecting opto-thermal performance of high-flux receivers is necessary. Therefore, a 3D pore-level Monte Carlo ray-tracing model was developed to examine the conjugate radiative, convective, and conductive heat transfer mechanisms of 3D-printed ceramic solar collectors. The model was first employed for lattice receivers with uniform and non-uniform porous structures. This allowed to systematically understand coupled optical and thermal characteristics of solar receivers at the pore level. It was found that the solar-to-thermal conversion efficiency might be thermally or optically limited depending on the thermo-hydraulic characteristics of the working fluid. The understanding obtained from the pore-level Monte Carlo ray-tracing modeling paved the way for the design of the lotus-inspired high-flux volumetric solar receivers. The initial lotus-inspired solar receiver concept employed curved-petal layered surfaces. The ray tracing study visualizing the interaction of incoming rays with idealized mirror surfaces indicated a significant portion of the incoming rays to the curved-petal receiver are reflected back to the environment. Subsequently, two new straight and needle-petal solar receivers inspired by the layered dissipative thermal shield design of the James Web Telescope were conceptualized. The ceramic 3D-printing method was then employed to fabricate ceramic receivers and investigate their performance experimentally with an in-house high-heat-flux solar simulator.

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