Experimental study on the energy harvesting performance of semi-active oscillating hydrofoils

Highlights

• Reduced frequency, pitch angle, aspect ratio and damping are tested for a semi-active hydrofoil to identify optima.
• A segment-wise correction for non-uniform inflow improves energy harvesting efficiency estimates, raising peak by 9.26%.
• A coupled framework links structure, dynamics and power output, revealing interaction mechanisms between heave and pitch.

Abstract

This study designed a semi-active oscillating hydrofoil tidal energy harvesting device, and conducted systematic hydrodynamic experiments to investigate the impact of reduced frequency (f∗), pitch angle (θ₀), aspect ratio (AR), damping coefficient (C_f​) on energy conversion efficiency. Lift, pitching moment, and heave kinematics were measured to compute power coefficients and overall efficiency. A sectional correction was applied to account for vertical flow non-uniformity in the test section. Within the explored ranges, an efficiency peak was observed near f∗ ≈ 0.12–0.14 and θ₀ ≈ 70°; after correction, higher AR (e.g., AR = 8) tended to be associated with larger efficiencies, and an intermediate damping level (C_f ≈ 1.15) coincided with comparatively higher values. This study provides experimental evidence and theoretical guidance for optimizing the design of oscillating hydrofoil tidal energy systems.

Introduction

With the continuous growth of global energy demand, the excessive use of fossil fuels has led to increasingly severe environmental problems. Therefore, the development of stable, efficient, and clean renewable energy sources has become urgent. Among these, tidal current energy has emerged as a promising direction in marine renewable energy due to its high power density, predictability, and stable resource availability [[1], [2], [3]]. Tidal energy is widely distributed along coastlines globally and is particularly well-suited for coastal islands and remote areas, demonstrating considerable development potential [4,5].

Traditional rotary-type tidal current turbines, although technically mature, suffer from several drawbacks, such as high flow velocity requirements, poor startup performance at low speeds, and risks of harming marine organisms [6]. In recent years, oscillating hydrofoil-based tidal energy harvesting technologies have attracted increasing attention. Compared with traditional turbines, they offer significant advantages:① Excellent low-speed startup capability, enabling wider application in various flow conditions; ② The maximum blade speed of the oscillating hydrofoil is much lower than that of conventional turbines under optimal operation, reducing ecological impacts on aquatic life; ③ Oscillating hydrofoils do not experience centrifugal forces during operation, improving structural robustness [7]; ④ The oscillating hydrofoil can cover a larger cross-sectional area in shallow waters, offering better deployment potential in such environments [8].

The oscillating hydrofoils can be classified into three types: fully active [9], fully passive [10], and semi-active [11]. Fully active oscillating hydrofoils use external actuators (e.g., servo motors) to simultaneously control both pitching and heaving motions. Simpson [12] experimentally investigated fully active oscillating wings and confirmed their energy conversion capability. Kinsey and Dumas [13] numerically explored the optimal combination of frequency, amplitude, and phase across various Reynolds numbers, confirming that coordinated optimization can significantly improve efficiency. Picard-Deland et al. [14] proposed a large-amplitude oscillating wing turbine that achieved 44–49 % efficiency at a Reynolds number of 500,000, but noted the difficulty of controlling pitch motion. He et al.[15,16] developed a high-Reynolds-number CFD model in OpenFOAM and emphasized the importance of synchronizing the trajectory with the effective angle of attack to maximize energy extraction. Mo et al. [[17], [18], [19]] conducted a series of studies on oscillating hydrofoils with WIG effects and 3D vortex dynamics. Their results revealed that ground effects and tip vortex control can enhance efficiency by 7–13 % in multi-foil setups, with potential efficiencies exceeding 40 %. Yang et al. [20] studied the influence of blockage and ground effects on power output in confined channel flows, further enriching the understanding of fully active systems. Zhao et al. [21] conducted a systematic numerical study on the fluid–structure interaction and energy-harvesting performance of single and tandem NACA0015 oscillating foils under lateral wall-confinement conditions. Despite its high controllability, fully active motion is not practically feasible and therefore has little potential for real-world application in tidal energy harvesting.

Fully passive oscillating hydrofoils rely solely on fluid-structure interaction for energy harvesting without any external actuation, greatly simplifying system structure and reducing maintenance. Peng and Zhu [22] numerically identified self-excited oscillations in passive hydrofoils. Young [23] investigated the relationships between structural parameters proper damping and stiffness, large-amplitude stable oscillations and high efficiency. Boudreau et al. [24] experimentally achieved a 31 % power extraction efficiency and a power coefficient of 0.86 at Re = 21,000. Duarte et al. [25] conducted systematic experiments at Re = 6 × 10⁴, identifying optimal pitch axis locations (between 31 % and 39 % of chord length) and confirming the role of structural parameters in achieving stable oscillations. Recent studies further expanded the understanding of passive systems. Kim et al. [26] demonstrated that a dual-hydrofoil system with chain-spring coupling could self-start. Using closed-flume experiments and quasi-steady analysis, Zhao et al. [27,28] developed a fully passive oscillating-foil harvester that self-oscillates via mechanical end stops, and showed that, in a tandem configuration, wake coupling can significantly boost the downstream power and overall efficiency. Wang et al. [29] evaluated passive hydrofoils under wave conditions, confirming that spring-loaded hydrofoils can sustain large responses even in regular wave environments. While fully passive systems are advantageous in simplicity and maintenance, they require precise matching of structural damping, inertia, and flow conditions. This results in performance variability and challenges in parameter optimization.

Semi-active oscillating hydrofoils combine the advantages of both fully active and fully passive systems. Typically, the pitching motion is actively controlled, while the heaving response is generated passively through fluid-structure interaction. Deng et al. [30] numerically revealed the significant influence of inertia on the efficiency of semi-active oscillating wings. Teng et al. [31] analyzed the impact of non-sinusoidal pitching motions on semi-active systems and found that while small-amplitude non-sinusoidal motions improved performance, large amplitudes reduced efficiency. Zhan et al. [32] studied semi-active oscillating wings in gusty flows and showed that adjusting the phase difference between gusts and pitch motions to 180° significantly increased efficiency. Ma et al. [33] used high Reynolds number simulations to examine the effects of spring stiffness, damping, pitch amplitude, and frequency on semi-active wings. Liu et al. [34] investigated coupled pitching semi-active systems and found that increasing pitch amplitude enhanced leading-edge vortex strength and energy output. He et al. [35] studied semi-passive hydrofoils under wing-in-ground (WIG) effects, achieving peak efficiencies of around 39.9 %. Furthermore, using two-dimensional simulations, Zhao et al. [28] studied wake interactions of tandem semi-active oscillating foils at different spacings and phase differences, and showed that, for suitable parameters, favorable wake–foil coupling can reduce the downstream pitching power and increase its net output by about 15 % compared with a single foil. Liu et al. [36] performed water tunnel experiments on series-coupled semi-active wings and reported that proper arrangement can improve efficiency.

Several comprehensive reviews on energy harvesting using oscillating hydrofoils have been conducted by researchers worldwide. Wu et al. [37] reviewed the fluid dynamics of flapping wings, summarizing relevant experimental techniques and numerical methods, and analyzing how key parameters (Reynolds number, reduced frequency, flapping amplitude, aspect ratio, etc.) affect the wake structure and aerodynamic/hydrodynamic performance. Similarly, Liu et al. [38] presented an extensive review of flapping-hydrofoil energy-harvesting devices: they provided detailed descriptions of motion combinations, control strategies, equations of motion, and performance metrics, and categorized the influencing factors into kinematic, hydrodynamic, and structural groups. Rostami and Armandei et al. [39] surveyed renewable-energy devices driven by vortex-induced motions, classified the associated vibration phenomena (flutter, galloping, vortex-induced vibration, etc.), and reviewed the corresponding energy-harvesting techniques and their performance. Despite significant progress in numerical studies, experimental research on oscillating hydrofoil tidal energy devices remains relatively limited, especially for semi-active.

This study designed and constructed a semi-active oscillating hydrofoil physical prototype, in which the pitching motion is actively controlled while the heaving motion responds passively. A comprehensive series of hydrodynamic experiments was conducted to evaluate the effects of key parameters—including oscillation frequency, pitch angle, aspect ratio, and damping coefficient—on the energy conversion efficiency of the hydrofoil. These results offer valuable reference for the design and development of tidal energy harvesting devices. Compared with existing studies, this research presents the following three key innovations.

• An experimental investigation of the influence of multiple parameters on the energy harvesting efficiency of a semi-active flapping hydrofoil was systematically conducted, identifying the optimal combination of reduced frequency and pitch angle;
• A layered flow velocity correction method was proposed and applied to quantitatively evaluate the error in energy performance caused by aspect ratio;
• A coupled analysis framework linking structural parameters, dynamic response, and energy output was established, revealing the interaction mechanism between pitching energy consumption and heaving energy harvesting.