The development of the Ocean Internet of Things (OIoTs) necessitates a robust and sustainable ocean environment monitoring system. Currently, powering and transmitting data from widely distributed sensors across vast ocean areas poses a significant challenge. Traditional methods using cables or batteries are impractical due to the sheer number of sensors, their remote locations in harsh environments, and the difficulty of maintenance. This paper addresses this challenge by proposing a novel energy harvesting approach that leverages the potential of ocean wave energy, a readily available and abundant resource. Ocean wave energy presents several advantages over other renewable energy sources in this context. It offers the highest energy density among renewable options, avoids chemical pollution, and exhibits a high availability rate (up to 90% of the time), surpassing solar and wind energy. Existing wave energy harvesting technologies often involve large, complex, and costly systems, such as attenuators, oscillating wave surface converters, and point absorbers. While these technologies are improving, they often face challenges in harvesting irregular, low-frequency wave energy efficiently, and issues with maintenance and construction costs. Triboelectric nanogenerators (TENGs) have emerged as a promising alternative, offering a relatively simple and potentially cost-effective solution. However, traditional TENGs are susceptible to water ingress and material degradation in harsh environments like typhoons and thunderstorms. This necessitates a more robust and reliable solution. This research proposes a metamaterial-based energy harvesting system designed to overcome these limitations.

Previous research has explored various methods for wave energy harvesting, including the use of piezoelectric and triboelectric mechanisms. Mutsuda et al. developed a self-powered buoy utilizing ocean wave energy, but the scalability and robustness of such systems remain a challenge. Kazemi et al. presented a waterproof piezoelectric harvester, but its power output was relatively low. Zhang et al.'s work on triboelectric nanogenerators showed promise but highlighted the issue of durability in harsh conditions. The application of metamaterials to vibration energy harvesting has shown great potential in concentrating energy at defect locations, significantly improving the efficiency of conversion. Studies have demonstrated the use of point defects in metamaterials to enhance acoustic and elastic wave energy harvesting, offering inspiration for the current research.

This study proposes a novel approach using point-defect metamaterials to enhance the efficiency of ocean wave energy harvesting. The system consists of a metamaterial plate with a periodic array of rolling-ball resonators. An electromagnetic energy harvesting cell, acting as a point defect, is strategically placed within the array. This configuration is designed to concentrate the wave energy at the defect location, maximizing energy extraction. **Theoretical Modeling:** Finite element analysis was employed to study the energy concentration characteristics of the point-defect metamaterials. The analysis focused on the response of the metamaterial plate at the defect location under various wave excitation frequencies. The model accounts for the dynamic interaction between the rolling magnetic ball and the electromagnetic coils, capturing the conversion of kinetic energy into electrical energy. Lagrange's equation and Kirchhoff's law are used to derive the dynamical equations governing the system. **Experimental Setup:** Experiments were conducted in a simulated ocean wave environment using a tri-axial shaker to generate waves of varying amplitudes and frequencies. The motion of the rolling magnetic ball within the energy harvesting cell was monitored, and the output voltage was measured. Experiments were performed under various conditions to simulate different wave states ranging from stable to harsh, including scenarios with multi-directional excitations and even plate overturning to assess the robustness of the proposed system. **Self-Powered Monitoring System:** A prototype self-powered ocean environment monitoring system was constructed, integrating the metamaterial energy harvesting device with sensors for temperature, pH, and salinity. The system included a rectifier circuit to convert the AC output of the energy harvester into DC power for charging a capacitor. Charge/discharge tests were performed to evaluate the system's energy storage capacity and power delivery capabilities. **Field Application Tests:** Finally, a field test was conducted to assess the performance of the self-powered ocean environment monitoring system in a real-world setting. The system's energy harvesting capability and environmental sensing effectiveness were evaluated under realistic ocean conditions. Wave height data were analyzed to determine the dominant frequencies of ocean waves and compare these frequencies with the frequencies of maximum power output from the metamaterial energy harvester.

The key findings of this study include: 1. **High Energy Density:** The metamaterial-based energy harvesting device achieved a high energy density of 99 W/m³, significantly surpassing previously reported results for similar systems. This superior performance is attributed to the efficient energy concentration effect of point-defect metamaterials. 2. **Effective Energy Harvesting Across Various Wave Conditions:** The device demonstrated effective energy harvesting across various simulated ocean wave conditions, ranging from low-amplitude stable waves to high-amplitude harsh waves, including conditions leading to metamaterial plate flipping. Even under extreme wave conditions (up to 2g acceleration), the device maintained a substantial power output. 3. **Robust System Performance:** The self-powered ocean environment monitoring system reliably functioned in both laboratory and field settings. The energy harvesting device demonstrated sufficient energy conversion to power the sensor array and transmit data wirelessly under various real-world ocean conditions for a 24-hour period. 4. **Optimized Design Parameters:** Finite element analysis and experimental results guided the optimization of design parameters such as plate thickness, cell size, outer shell size, and rolling ball size, to maximize the energy harvesting efficiency within the relevant frequency range of ocean waves. 5. **Frequency-Dependent Energy Harvesting:** The device exhibits maximum energy harvesting within a specific frequency range corresponding to the local resonance band gap of the metamaterial, showcasing the effectiveness of metamaterial-based energy concentration. Experimental results showed a significant increase (1300%) in output voltage at 2 Hz (within the band gap) compared to 4 Hz (outside the band gap). 6. **Practical Application and Data Transmission:** Field tests validated the effectiveness of the self-powered ocean environment monitoring system in collecting real-time data on various ocean parameters such as temperature, pH, and salinity, demonstrating its practical applicability for OIoT deployments.

The results demonstrate the effectiveness of leveraging the energy concentration properties of point-defect metamaterials to achieve highly efficient ocean wave energy harvesting. The system successfully addresses several limitations of previous technologies, including low energy density and vulnerability to harsh environmental conditions. The high energy density achieved in this study (99 W/m³) significantly surpasses that of previously reported piezoelectric and triboelectric wave energy harvesters. The system’s ability to operate reliably under various wave conditions, including those simulating extreme weather events, highlights its practical value for long-term, self-sustaining deployments in challenging oceanic environments. The successful integration with environmental sensors demonstrates the feasibility of building a self-powered, wireless OIoT node capable of continuously monitoring key parameters. The frequency-dependent nature of energy harvesting further supports the effectiveness of the metamaterial design in directing energy towards the harvesting device. The findings suggest a substantial advancement in the field of wave energy harvesting and open up new possibilities for developing more sustainable and robust ocean monitoring networks.

This research successfully developed a high-density wave energy harvesting device utilizing the unique properties of point-defect metamaterials. The device's capacity to concentrate ocean wave energy at a specific location, coupled with its robust design and high energy density, significantly advances the field of wave energy harvesting for low-power applications. The integrated self-powered ocean environment monitoring system demonstrates its practical potential for implementing sustainable and long-term ocean monitoring within the context of the growing OIoT. Future research could explore further optimization of the metamaterial design, exploration of alternative materials with enhanced durability, and integration with more sophisticated sensor networks and data analytics capabilities.

While the study demonstrates significant advancements, some limitations warrant consideration. The experimental setup utilized a simulated ocean wave environment; further field testing under diverse and prolonged real-world conditions is needed to fully validate the system’s long-term reliability. The current design focuses on a specific frequency range; future studies could explore broader bandwidth energy harvesting capabilities. Additionally, the scalability of the metamaterial design for larger-scale deployments requires further investigation.