Deep ocean exploration is vital for understanding Earth's history, finding marine minerals, and studying deep-sea ecosystems. Ocean currents are key oceanographic parameters influencing seabed geology, global climate, and biological communities. However, abyssal ocean exploration (4000–6000 m depth) is challenging due to the extreme environment and the need for ultra-sensitive sensors to measure typically small currents (centimeters per second). Current methods, including electromagnetic, acoustic, and mechanical current meters, have limitations. Electromagnetic meters suffer from magnetic field interference. Acoustic meters, while precise, are unsuitable for certain water conditions and are costly. Mechanical meters are simple and affordable but have limited pressure resistance and narrow measurement ranges. Critically, all these require external power supplies (batteries), limiting long-term, large-scale deployments with high spatiotemporal resolution. Triboelectric nanogenerators (TENGs) offer a solution. They are cost-effective, lightweight, and efficient at harvesting low-frequency mechanical energy, suitable for self-powered sensors. While TENGs have been applied in various sensing fields, their use in deep-sea ocean current sensing remains largely unexplored. This study addresses this gap by developing a highly sensitive, pressure-resistant TENG-based system for deep-sea current monitoring.

The existing literature extensively documents the challenges of measuring ocean currents in the abyssal zone. Studies highlight the limitations of electromagnetic current meters due to susceptibility to external magnetic fields and the high cost and environmental limitations of acoustic Doppler current profilers (ADCPs). Mechanical current meters, while simpler and cheaper, often lack sufficient pressure resistance for deep-sea deployments and have limited measurement ranges. The reliance on batteries in all these technologies significantly restricts the duration and scope of deep-sea current monitoring, hindering the acquisition of high-resolution data across extended time periods. Recent research on triboelectric nanogenerators (TENGs) has shown their potential for energy harvesting and self-powered sensing in various applications, including wind speed, pipeline velocity, and raindrop sensing. However, the application of TENGs for deep-sea ocean current sensing is relatively novel and underexplored. This research aims to fill this gap by developing and testing a TENG-based system capable of reliable and long-term current measurements in the challenging abyssal environment.

The researchers designed a fully integrated, self-powered deep-sea current monitoring system (DS-TENG) employing a flexible TENG for energy harvesting and sensing. The DS-TENG consists of a pressure-resistant waterproof sealing and tank, a printed circuit board for data acquisition and communication, a TENG and magnetically coupled variable-spacing structure, and a rotating cup structure for energy harvesting and current sensing. The rotating cup structure features six conical cups around a central rotating disk. Ocean currents create differential water pressure, rotating the cups and transferring energy to the TENG via a contactless magnetic coupling. This design is crucial for pressure resistance in the deep-sea environment. A variable-spacing mechanism adjusts the magnetic coupling distance, expanding the measurement range. The TENG uses flexible fluorinated ethylene propylene (FEP) and fur as tribomaterials, minimizing friction resistance and improving sensitivity and durability. The TENG's working principle is based on triboelectrification and electrostatic induction. The fur and FEP film generate electrostatic charge when rubbing against each other. This charge is transferred to copper electrodes, producing a flow-velocity-dependent electrical signal. The researchers optimized the DS-TENG's materials and structure, including fur length, tribomaterials, electrode number, and magnetic coupling, to maximize sensitivity, measurement range, and output stability. The optimization involved experimentation with different fur lengths, tribomaterials (PI, PTFE, FEP), FEP thicknesses, and electrode numbers, analyzing the effects on the open-circuit voltage (Voc), short-circuit current (Isc), and transferred charge (Q). The magnetic coupling's transmission clearance was also optimized to balance starting torque and maximum velocity measurement range. The DS-TENG's performance was characterized using a stepper motor, analyzing output voltage, current, charge, and power under different rotational speeds and load resistances. The effect of flow direction was also investigated. Pressure resistance was tested up to 49.5 MPa. Field experiments were conducted using a still water tower method with a speedboat for calibration and a deep-sea validation at 4531m in the South China Sea using a remotely operated vehicle (ROV). The DS-TENG was integrated with the ROV and a signal processing system for real-time data transmission. Data were compared against a commercial Doppler velocity log (DVL).

The DS-TENG demonstrated excellent performance in both laboratory and field tests. The optimized design achieved a maximum open-circuit voltage (Voc) of 157.61 V and a peak-to-peak current (Ip-p) of 14.35 µA at 1000 rpm. The variable-spacing mechanism effectively broadened the measurement range to 0.02–6.69 m/s, which is 67% wider than traditional vertical-axis current sensors. A strong linear correlation (R² = 0.99) was observed between the output frequency and the rotational speed in laboratory tests, indicating excellent linearity for velocity sensing. In field tests, the DS-TENG accurately measured ocean currents, showing a linear correlation (R² = 0.97) between flow velocity and output frequency in the range of 0.16–0.76 m/s at 4531m depth. The DS-TENG successfully operated continuously for 6 hours at abyssal depths, demonstrating its long-term stability and reliability. The system withstood hydrostatic pressure exceeding 45 MPa, proving its suitability for abyssal ocean deployment. The results from the DS-TENG closely matched those from the DVL, validating its accuracy. The high linear correlation (R² = 0.97) between the DS-TENG's output frequency and the flow velocity validated its effective performance in capturing subtle changes in flow velocity during the deep-sea experiment. The consistent accuracy (exceeding 88.5%) of both the DS-TENG and the DVL over a 6-hour period highlighted the sensor's exceptional stability and reliability under abyssal ocean conditions. The successful operation at a record depth of 4531 m in the South China Sea underscores the DS-TENG's capabilities in extreme environments.

The successful development and deployment of the DS-TENG represents a significant advancement in deep-sea ocean current monitoring. The self-powered nature of the system eliminates the limitations imposed by battery life, enabling extended deployment periods for high-resolution data acquisition. The wide measurement range and high sensitivity of the DS-TENG allow for the accurate measurement of a broader spectrum of current speeds, capturing both subtle flows and stronger currents. The ability to withstand extreme pressure conditions makes it a highly suitable technology for abyssal ocean exploration. The close correlation between the DS-TENG measurements and the DVL data validates its accuracy and reliability. This technology has the potential to revolutionize oceanographic research by enabling cost-effective, long-term, and large-scale deployments of current monitoring systems, leading to a better understanding of ocean dynamics and their influence on climate and ecosystems. The dual-range measurement capability is particularly noteworthy. The ability to switch between high and low sensitivity modes allows for more efficient data collection, optimizing the system's performance across a wider range of flow conditions.

This study successfully demonstrated a self-powered, highly sensitive, and pressure-resistant ocean current monitoring system (DS-TENG) based on a triboelectric nanogenerator. The DS-TENG achieved a wide measurement range (0.02–6.69 m/s), high sensitivity, and robust pressure resistance, enabling successful operation at a record depth of 4531 m in the South China Sea. The system's self-powered nature and high accuracy offer a promising solution for sustainable, high-resolution ocean current monitoring. Future research could focus on further miniaturization, integration of multiple sensors for comprehensive oceanographic data collection, and development of wireless communication strategies for wider deployment and easier data access.

While the DS-TENG demonstrated impressive performance, some limitations exist. The current prototype's size and weight might still need optimization for easier deployment in certain deep-sea applications. The long-term durability of the biological fur material under constant abrasion requires further investigation. Although the system demonstrated excellent linearity over a wide velocity range, further calibration and validation across diverse oceanic conditions would enhance its reliability and accuracy. Finally, while the contactless magnetic coupling provides high pressure resistance, the transmission efficiency could be further optimized to enhance power generation, particularly at very low flow velocities.