Ocean wave energy harvesting with high energy density and self-powered monitoring system

The paper addresses the challenge of reliably powering widely distributed ocean sensors for the Ocean Internet of Things (OIoT). Conventional wired power is impractical and battery replacement is difficult, especially in harsh marine environments. Ocean wave energy offers high availability and energy density compared to wind and solar. However, many existing wave energy harvesters are bulky, costly, and inefficient at irregular low frequencies typical of ocean waves. Triboelectric nanogenerators (TENGs) provide an alternative but are susceptible to water ingress and coating degradation. This study proposes an electromagnetic energy harvesting approach leveraging point-defect metamaterials to localize and concentrate elastic wave energy at ultra-low frequencies (≈0.9–2 Hz) to efficiently power a self-sustained ocean monitoring node capable of wireless data transmission.

Prior large-scale wave energy converters include attenuators, oscillating converters, pressure-differential and point absorbers, oscillating water columns, and bulge head converters, but they face maintenance and efficiency challenges for irregular low-frequency waves. TENG-based approaches (liquid–solid contact, fully enclosed, biomimetic, and networked TENGs) have shown promise for ocean blue energy but suffer durability issues (water ingress, coating damage) in severe weather. Metamaterials, especially those with point defects, can localize acoustic/elastic energy at defect sites and have been explored for enhanced vibration and acoustic energy harvesting and band-gap engineering. Studies show maximal energy extraction near defect-band frequencies. Building on this, the authors target in-plane elastic wave modulation and concentration at a defect to harvest ocean wave energy efficiently while avoiding phase inconsistencies from distributed multi-harvester layouts.

- Device design: A floatable 4×4-cell metamaterial plate (0.3 m × 0.3 m, ABS substrate) with 15 passive rolling-ball resonant cells and one active electromagnetic energy harvesting cell at a point defect. Resonant cells tune elastic wave propagation; the defect forms a localized resonance concentrating energy. The active cell comprises an outer shell, an inner shell, a rolling NdFeB magnetic ball, and five series-connected induction coils attached to the inner shell. - Circuit and power management: AC output is bridge-rectified to DC, then fed to a series/parallel conversion module controlling multiple 0.1 F–0.22 F capacitors. During charging, capacitors are paralleled (via ADG719-EP and SGM7222 chips) to accumulate charge at low voltage; during discharge, they are switched in series to provide 3.3 V DC to the sensing and wireless modules. Energy storage and charging/discharging characteristics were measured. - Finite element analysis (FEA): 2D periodic rolling-ball resonator cell analyzed over the Brillouin zone to obtain band structures. A point defect (active cell) creates a defect band within a local resonance band gap (0.9–2 Hz) and a higher-frequency Bragg gap (6.7–9.6 Hz). Displacement fields under vertical surface excitation (a = 1 g) demonstrate energy localization at the defect near 1.9 Hz. Parametric studies varied substrate thickness, cell size, shell size, resonant mass, and plate material to tune the defect-band frequency range. - Theoretical modeling: A nonlinear ball-pendulum electromagnetic energy harvester model using Lagrangian mechanics and Kirchhoff’s law; kinetic (translational/rotational) and potential energy terms defined; coupled electromechanical equations derived with damping and electromagnetic coefficients to describe ball motion and induced current. - Fabrication: 3D-printed components for shells and ABS plate; assembly of passive resonant cells; modification of one cell into the active harvester by adding inner shell, magnetic ball, and coils. - Experimental setups: - Simulated wave environment: Water flume mounted on tri-axial shakers (ETS Solutions M437A) driven by ECON PREMAX, amplified by MPA407; accelerations up to 2 g with x, y, z excitations to emulate stable and harsh seas; PCB Piezotronics sensors used; closed-loop control to maintain constant excitation; signals filtered. - Characterization: Oscilloscope measurements of harvester voltage; rectified charging/discharging of external capacitor (C=0.22 F) with optimal load R1=6.2 Ω; frequency sweeps to measure RMS voltage/current, power, and power density across 0.7–3.1 Hz. - Field tests: Day and night deployments (wind speeds 4.5 m/s and 4.2 m/s); FFT of measured wave height to determine ambient wave spectra; real-time monitoring of turbidity, conductivity (salinity proxy), pH, and temperature; wireless transmission to a receiver/cloud.

- Band structure and localization: FEA shows a local resonance band gap at 0.9–2 Hz with a defect band around 1.9 Hz where displacement localizes at the defect, creating a high-energy-density region. Bragg gap observed at 6.7–9.6 Hz. - Geometry effects: Thinner substrates, larger cells, larger shell radius, and heavier resonant masses broaden/shift the local resonance band to better match ocean wave frequencies; material choice modestly affects bandwidth; ABS selected for buoyancy/strength. - Output in simulated waves: - Stable small waves (a=0.1–0.5 g, f=2 Hz): Peak induced voltages up to 0.17 V; cycles correspond to ball motion states; under 0.1 g, peaks ≈0.07 V. - Harsh waves including vertical and multi-axis excitation (a=1.5–2.0 g): Instantaneous voltages up to 0.27–0.30 V; device remains functional even when flipped; in flipped state peak ≈0.23 V. - Frequency response and power density: - At f=2 Hz (within defect/local resonance band): RMS voltage ≈0.13 V; RMS power ≈2.72 mW; power density ≈81.1 W/m³. - At f=4 Hz (outside band): RMS voltage ≈0.01 V; RMS power ≈0.01 mW; power density ≈1.02 W/m³. - In-band vs out-of-band gains: Voltage increased ≈1300%; power increased by ~three orders of magnitude; power density increased by ~80× in band. - Energy storage: With rectification and C=0.22 F, capacitor charged to ≈0.156 V in 282 s and discharged to near 0 V over 512 s (total cycle ≈794 s) through R1=6.2 Ω, demonstrating usable energy harvesting and delivery. - Field deployment: - Ambient wave spectra concentrated in 0.5–1 Hz and 1.5–1.9 Hz; peak output power near defect band with maxima ≈21.1 mW and 22.8 mW in two time periods. - Continuous operation: After pre-discharge, 40 min charging enabled the monitoring system; system provided real-time turbidity, conductivity, pH, and temperature over day and night; stable 24 h operation claimed across weather conditions. - Comparative performance (literature table): The proposed electromagnetic point-defect metamaterial harvester exhibits very low internal resistance (optimal load ≈6.2 Ω) and high current (≈23.23 mA), with reported power density up to ≈99.81 W/m (noting unit conventions) compared to piezoelectric/triboelectric devices, and advantages of simple structure and no chemical coating.

The study demonstrates that introducing a point defect into a periodic metamaterial plate concentrates low-frequency elastic wave energy at the defect, enabling efficient electromagnetic harvesting from ultra-low-frequency ocean waves typical of real seas. This concentrated energy extraction avoids phase mismatch issues that arise in distributed multi-harvester arrays and yields substantial gains in voltage, power, and power density within the defect-band frequency range. Experimental results in simulated and field conditions confirm robust operation across stable and harsh environments, including multi-axis excitation and device flipping, while the tailored power management (series/parallel capacitor switching) overcomes the low-voltage nature of electromagnetic harvesters to sustain a 3.3 V monitoring subsystem. The approach directly addresses the OIoT power challenge by enabling a self-powered sensing node capable of real-time environmental measurements and wireless data transmission, supporting scalable, cable-free ocean monitoring networks.

The authors present a floatable, corrosion-resistant, point-defect metamaterial-based electromagnetic energy harvester integrated with bespoke rectification and series/parallel energy storage to power an ocean monitoring node. The device achieves strong energy localization in the 0.9–2 Hz band, high instantaneous voltages under harsh excitations, and markedly higher power and power density within the defect band. Field tests verify practical self-powered sensing (turbidity, conductivity, pH, temperature) with wireless transmission and continuous operation. This work advances compact, durable, low-frequency wave energy harvesting for the Ocean IoT. Future work could focus on tuning and adaptive control to track time-varying sea states, scaling arrays while maintaining phase coherence, ruggedizing against extreme flips and impacts, improving energy storage and conversion efficiency, and long-term marine durability validation.

- The combination of vertical and lateral excitations can break symmetry and mistune the defect localization, potentially reducing energy concentration efficiency. - Electromagnetic harvesters inherently produce low voltages; although mitigated by series/parallel capacitor switching, the approach may limit direct powering of higher-voltage loads without additional conversion. - Under flipping or extreme conditions, output decreases relative to optimal orientation (e.g., peak ~0.23 V in flip state) although functionality remains. - Simulations and experiments capped at 2 g to avoid structural damage; extreme sea states beyond this limit were not tested. - Field tests demonstrate day–night feasibility but long-term reliability, biofouling resistance, and maintenance over months/years were not reported.