Phase change material (PCM)-based ocean thermal energy harvesting is a relatively new method, which extracts the thermal energy from the temperature gradient in the ocean thermocline. Its basic idea is to utilize the temperature variation along the ocean water depth to cyclically freeze and melt a specific kind of PCM. The volume expansion, which happens in the melting process, is used to do useful work (e.g., drive a turbine generator), thereby converting a fraction of the absorbed thermal energy into mechanical energy or electrical energy. Compared to other ocean energy technologies (e.g., wave energy converters, tidal current turbines, and ocean thermal energy conversion), the proposed PCM-based approach can be easily implemented at a small scale with a relatively simple structural system, which makes it a promising method to extend the range and service life of battery-powered devices, e.g, autonomous underwater vehicles (AUVs). This dissertation presents a combined theoretical and experimental study of the PCM-based ocean thermal energy harvesting approach, which aims at demonstrating the feasibility of the proposed approach and investigating possible methods to improve the overall performance of prototypical systems. First, a solid/liquid phase change thermodynamic model is developed, based on which a specific upperbound of the thermal efficiency is derived for the PCM-based approach. Next, a prototypical PCM-based ocean thermal energy harvesting system is designed, fabricated, and tested. To predict the performance of specific systems, a thermo-mechanical model, which couples the thermodynamic behaviors of the fluid materials and the elastic behavior of the structural system, is developed and validated based on the comparison with the experimental measurement. For the purpose of design optimization, the validated thermo-mechanical model is employed to conduct a parametric study. Based on the results of the parametric study, a new scalable and portable PCM-based ocean thermal energy harvesting system is developed and tested. In addition, the thermo-mechanical model is modified to account for the design changes. However, a combined analysis of the results from both the prototypical system and the model reveals that achieving a good performance requires maintaining a high internal pressure, which will complicate the structural design. To mitigate this issue, the idea of using a hydraulic accumulator to regulate the internal pressure is proposed, and experimentally and theoretically examined. Finally, a spatial-varying Robin transmission condition for fluid-structure coupled problems with strong added-mass effect is proposed and investigated using fluid structure interaction (FSI) model problems. This can be a potential method for the future research on the fluid-structure coupled numerical analysis of AUVs, which are integrated with and powered by the PCM-based thermal energy harvesting devices. / Doctor of Philosophy / The global ocean, which covers about 71% of the Earth’s surface, absorbs a great amount of heat from the sunshine everyday, making it a reliable and renewable source of thermal energy. Also, the temperature of the ocean water varies with depth, which provides a necessary condition (i.e, a temperature gradient) to extract the thermal energy. If harvested and converted into electrical energy using small scale portable devices, the ocean thermal energy can be a potential energy resource to provide power for autonomous underwater vehicles (AUVs), which are conventionally powered by on-board rechargeable batteries. To this end, this dissertation presents a study of using solid/liquid phase change materials (PCMs) to extract thermal energy from the temperature gradient in the ocean. The basic idea is to use the warm surface water and deep cold water to melt and freeze the PCM cyclically. In the meantime, the volume of PCM will expand and contract accordingly. Therefore, a turbine generator can be driven by the volume expansion in the melting process, thereby converting a fraction of the absorbed thermal energy into electrical energy. This study includes four key aspects. First, to evaluate the theoretical full potential of the PCM-based approach, a solid/liquid phase change thermodynamic model – which represents an idealized energy harvester – is developed. Based on the thermodynamic model, an upperbound of the thermal efficiency is derived. Secondly, two prototypical systems, as well as a thermo-mechanical model which can predict the performance of specific designs, are developed. Third, for the purposes of performance improvement and pressure regulation, the latter of which is associated with the structural safety, a hydraulic accumulator is added to the existing system and its effects are examined using both experimental and theoretical methods. Finally, a computational method is proposed and demonstrated, which can be a potential tool for the design of PCM-based ocean thermal energy harvesting systems when they are integrated with exiting AUVs.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/100989 |
| Date | 10 June 2019 |
| Creators | Wang, Guangyao |
| Contributors | Aerospace and Ocean Engineering, Wang, Kevin Guanyuan, Chao, Yi, Ha, Dong S., Philen, Michael Keith, Gilbert, Christine Marie |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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