Development of a turbine-based ocean thermal energy harvesting mechanism applied to the underwater glider

Student thesis: Doctoral Thesis

Abstract

Underwater gliders (UGs) are gaining popularity in ocean exploration. Extending the range and duration of UGs is becoming a current research focus; however, one of the primary obstacles to achieving this is the limited energy supply. This study developed a turbine-based energy harvesting mechanism (EHM) by researching the operational mechanism and design method of its turbine. The turbine-based EHM originated from the energy harvesting mechanism proposed by NASA. In NASA’s original idea, when the turbine-based EHM is in operational mode, a thermal buoyancy engine exploits the ocean's temperature differences to change the glider's buoyancy by phase change material (PCM), driving it to move in the water; these manoeuvres power a turbine mounted behind the glider to harvest energy. Firstly, before delving into the turbine study, the conceptual design of NASA’s turbine-based EHM is refined. To enhance the feasibility of NASA’s turbine-based EHM, its original energy harvesting mode is modified according to the consideration of engineering constraints. In the new mode of the turbine-based EHM, the moving trajectory of UGs has been changed; the thermal buoyancy engine generates ballast force to propel UGs vertically through the ocean and simultaneously drives the turbine of the hull to rotate, thereby extracting energy from the fluid. Moreover, the original method of converting thermal energy into hydraulic energy of EHM is modified. This new energy harvesting mode of the turbine-based EHM will be adopted in subsequent research related to turbines. Secondly, focusing on the turbine, to efficiently optimize it for harvesting the kinetic energy provided by the thermal buoyancy engines, a mathematical model based on blade element momentum theory (BEMT) is established to conduct extensive analysis. An enhanced BEMT model is integrated. Through numerous analyses using the mathematical model, a comprehensive understanding of the turbine's design philosophy is derived, including the optimal combination of Cp and Ct, the choice of the design tip speed ratio (TSR), the selection of the radius, and more. The model reveals a significant deviation from the traditional turbine design philosophies. Furthermore, the mathematical model optimizes the turbine mounted behind UG hull and provides a preliminary estimate of the turbine-based EHM's capabilities. Thirdly, as the understanding of the turbine's operating mechanisms in this study deepens, the Actuator Disk Theory is expanded. The current BEMT-based turbine design method assumes achieving the Betz limit when the axial induction factor (a) reaches 1/3. However, this only applies to turbines driven by a constant velocity, i.e., the velocity-driven turbine. The turbine in EHM, driven by ballast force, is essentially a force-to-velocity turbine, meaning it operates under a constant force and is powered by the velocity generated by this force. Consequently, this study extends the actuator disk theory for force-to-velocity turbines and identifies the relationship between the axial induction factor, power, and energy yield of the force-to-velocity turbine. Based on this relationship, a new BEMT-based design method is proposed for the preliminary design of force-to-velocity turbines, adapting to the unique working principles of the turbine in this study. A case study demonstrates and verifies the developed method, showing that the new method can quickly and effectively identify the optimal design for force-to-velocity turbines. Additionally, during the expansion of the actuator disc theory, some design philosophies of the turbine in this study, related to the optimal combination of Cp and Ct, are mathematically elucidated. The effectiveness and limitations of these philosophies, previously summarized, are discussed in light of these equations. Lastly, to validate the energy harvesting capability of the turbine-based EHM, high-fidelity computational fluid dynamics (CFD) simulations were conducted using an optimally designed turbine based on previous findings. Using a typical UG hull and a turbine designed based on the prior design philosophy and the enhanced BEMT method with the expanded actuator theory, DFBI simulation of EHM was carried out with CFD software. CFD results suggest that a theoretically self-sustainable UG with unlimited endurance might be achievable, ignoring the lifespan of the components in the system. They also indicate that the turbine-based EHM might have the potential to compete with other OTEC-PCM systems in the future.
Date of Award19 Dec 2023
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsUniversity of Strathclyde
SupervisorLaibing Jia (Supervisor) & Mehmet Atlar (Supervisor)

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