Self-powered and speed-adjustable sensor for abyssal ocean current measurements based on triboelectric nanogenerators

Deep-ocean exploration requires accurate, long-duration measurements of ocean currents, especially in the abyssal zone (4000–6000 m). Existing electromagnetic, acoustic, and mechanical current meters face issues including sensitivity to tiny flows (centimeters per second), interference and environmental constraints, high cost, limited pressure resistance, narrow measurement ranges, and reliance on battery power that causes under-sampling and poor spatiotemporal resolution. The study addresses the need for a self-powered, high-pressure-resistant, and highly sensitive sensor capable of capturing subtle abyssal currents. The authors propose a triboelectric nanogenerator (TENG)-based deep-sea current sensor (DS-TENG) employing magnetic coupling and a variable-spacing mechanism to overcome pressure limits and extend measurement range, aiming to enable distributed, long-term ocean observations. They report laboratory validation, coastal calibration, and abyssal field deployment to 4531 m demonstrating linear sensing performance and robust operation.

Conventional current meters: Electromagnetic devices suffer reduced accuracy under external magnetic field interference; acoustic devices, while precise and suitable for profiling, struggle in turbid or ultra-clean waters and near reflective bottoms and are costly; mechanical meters are simple and low-cost but have limited pressure resistance, narrow measurement range, and still require external power for transduction. Triboelectric nanogenerators (TENGs) have emerged as lightweight, cost-effective, and efficient harvesters of low-frequency mechanical energy and as self-powered sensors across domains such as speed, vibration, and biomedicine. TENGs have been explored for fluid dynamics sensing (wind speed, pipeline flow, raindrops) and marine energy harvesting, but deep-sea current sensing remains underexplored. Challenges include designing a highly sensitive, durable structure that withstands high pressure and corrosion while detecting subtle currents. The proposed DS-TENG leverages advances in TENG materials and structures to fill this gap.

System design and working principle: The DS-TENG integrates a pressure-resistant sealed tank, PCB for data acquisition/processing/communication, a flexible TENG, a magnetic coupling with variable spacing, and a six-cup vertical-axis rotor for omnidirectional energy harvesting. Ocean current-induced differential pressure on the rotor cups generates rotation, transferred to the TENG via a non-contact magnetic coupling (active/passive magnets) that improves sealing and pressure resistance. Variable-spacing magnetic coupling: Two selectable transmission clearances (state 1: smaller gap; state 2: larger gap) are implemented via a convex-chute mechanism to tune starting torque and measurement range. Smaller gaps favor high-velocity coupling and stability; larger gaps reduce starting torque for low-velocity sensitivity. TENG structure and mechanism: A highly flexible contact-mode TENG uses rabbit fur (positive tribomaterial) against FEP film (negative tribomaterial) on Cu electrodes, minimizing friction and maximizing sensitivity. Triboelectrification and electrostatic induction produce alternating voltages as fur slides across patterned Cu electrodes beneath the FEP. Theoretical relations for transferred charge, open-circuit voltage, and short-circuit current are cited; COMSOL simulations (±70 µC/m² surface charge; 1 mm gap) verify potential distributions across operation states. Material and structural optimization: Rabbit fur is enzyme-treated and plasma-etched to roughen the surface for greater contact area. Fur length optimization at 100 rpm shows a maximum VOC of 121.6 V at 16 mm; shorter lengths reduce contact, longer lengths cause electrode cross-contact and add friction. Negative tribomaterial tests (PI, PTFE, FEP) select FEP-Cu for best output; thinner FEP increases output, but 0.02 mm is fragile, so 0.05 mm is chosen. Increasing electrode pairs slightly reduces voltage due to smaller electrode area and potential simultaneous contact. Long-term stability shows <6% induced charge reduction after 30 days; SEM indicates negligible wear. Magnetic coupling characterization: Transmission clearance of 0–24.5 mm was tested. Increasing clearance reduces maximum transmitted rpm. With 12.5 mm clearance, stable operation up to ~1000 rpm; at 15.5, 18.5, and 21.5 mm, the passive magnet maximum speeds before slip are ~600, 160, and 100 rpm, respectively. Beyond 24.5 mm, coupling fails. Response delay increases with clearance (e.g., 0.48 s at 12.5 mm to 0.96 s at 18.5 mm). Laboratory electrical characterization: With 12.5 mm clearance, DS-TENG outputs increase slightly with rpm due to improved contact: at 1000 rpm, Vpp 157.61 V, Ipp 14.35 µA, Q 74.17 nC; at 2000 rpm, Vpp 161.69 V, Ipp 19.25 µA, Q 74.47 nC. Output frequency f_vac is linearly correlated with motor rpm (R² = 0.99, 0–1000 rpm). At 120 rpm, DS-TENG charges 6 µF to 8.1 V in 100 s. Load tests show decreasing current and increasing voltage with resistance; peak power 34.86 µW at ~2 × 10^9 Ω. Directional tests show >95% retention of f_vac across horizontal directions. Pressure cycling to 49.5 MPa confirms deep-sea survivability (~4500 m equivalent). Coastal calibration (still-water tower method): DS-TENG and a commercial meter are co-mounted 1 m deep ahead of a speedboat. Two transmission states characterized: state 1 (small clearance) covers high velocities, producing stable voltages from 0.22–6.69 m/s with linear f–velocity relation (R² = 0.99). State 2 (large clearance) is sensitive to low flow with 0.02 m/s start-up and linearity up to 0.26 m/s (R² = 0.99). Combined range: 0.02–6.69 m/s. Deep-sea integration and data pipeline: DS-TENG mounted on an ROV; signal conditioning filters and amplifies voltages, then ADC converts and transmits via RS-232 over an optical tether to a shipboard platform for processing and GUI visualization. Field trials (May 2–4, 2023; TAN SUO ER HAO): ROV stops at set depths (100–1000 m) for 10 s sampling; in abyssal waters (~4000 m), runs at 0.5–2 kn; performs acceleration/deceleration maneuvers against current. Data acquisition at 200 Hz, telemetry at 20 Hz. Maximum test depth: 4531 m. DS-TENG outputs compared to onboard DVL for validation.

- The DS-TENG is fully self-powered, requiring no external power for sensing, and remains operational under deep-sea hydrostatic pressures up to 49.5 MPa. - Measurement range spans 0.02–6.69 m/s using a variable-spacing magnetic coupling, 67% wider than traditional vertical-axis current sensors. - Low-velocity sensitivity: startup at 0.02 m/s (state 2). High-velocity robustness: stable sensing up to 6.69 m/s (state 1). - Linear sensing: Laboratory f_vac–rpm linearity R² = 0.99 (0–1000 rpm). Coastal calibration shows f–velocity R² = 0.99 in both states. Deep-sea ROV trials show f–velocity R² = 0.97 over 0.16–0.76 m/s. - Directional robustness: >95% consistency of output frequency across horizontal directions at the same speed. - Electrical performance: At 1000 rpm, Vpp ≈ 157.61 V, Ipp ≈ 14.35 µA, Q ≈ 74.17 nC; at 2000 rpm, Vpp ≈ 161.69 V, Ipp ≈ 19.25 µA, Q ≈ 74.47 nC. Peak output power ≈ 34.86 µW at ~2 × 10^9 Ω (120 rpm). Charges 6 µF to 8.1 V in 100 s at 120 rpm. - Magnetic coupling characterization: Maximum transmittable rpm decreases with clearance; beyond ~24.5 mm gap, coupling fails. Response delay increases from ~0.48 s (12.5 mm) to ~0.96 s (18.5 mm). - Longevity: <6% performance reduction over 30 days; rabbit fur shows no appreciable wear (SEM). - Field validation: Successful continuous operation for ~6 h at a depth of 4531 m; DS-TENG velocity closely matches DVL, with prolonged-operation accuracy >88.5%.

The DS-TENG addresses key challenges of abyssal ocean current sensing by combining self-powered triboelectric transduction with a pressure-resilient, sealed design and contactless magnetic coupling. The variable-spacing mechanism allows tuning between low-starting-torque sensitivity for subtle flows and strong coupling for high-velocity regimes, yielding a continuous measurement range from 0.02 to 6.69 m/s. Linear relationships between output frequency and stimulus (rotational speed or flow velocity) in laboratory, coastal, and deep-sea conditions confirm that the frequency signal is a robust proxy for velocity. Pressure testing and field deployment to 4531 m demonstrate structural reliability and functional stability under extreme hydrostatic pressure. Agreement with an onboard DVL validates measurement accuracy and shows potential utility for both ambient current monitoring and vehicle speed estimation. Collectively, these results substantiate the feasibility of distributed, long-duration, high spatiotemporal resolution observations without reliance on battery-powered transduction in harsh deep-sea environments.

This work introduces a fully integrated, self-powered deep-sea current sensor (DS-TENG) that leverages a flexible TENG, contactless magnetic coupling, and a variable-spacing mechanism to deliver wide-range (0.02–6.69 m/s), highly sensitive, and pressure-resistant flow measurements. Laboratory and coastal tests establish linear sensing performance, and abyssal deployment to 4531 m confirms operational robustness and strong agreement with a commercial DVL during prolonged operation. The DS-TENG eliminates the need for external power for signal transduction and expands the effective measurement range beyond traditional rotating-shaft designs, providing a promising path toward scalable, distributed ocean observation networks with improved spatiotemporal coverage. The system’s ability to infer submersible speed further broadens its application scope. Future work suggested by the study includes deployment within stereoscopic ocean observation networks and broader integration on mobile platforms for comprehensive ocean monitoring.

- Magnetic coupling constraints: As the transmission clearance increases, maximum transmittable speed decreases, response delay increases, and beyond ~24.5 mm the coupling fails. Selecting clearance involves a trade-off between low-speed sensitivity and high-speed transmission stability. - Material trade-offs: Thinner FEP improves output but is fragile (0.02 mm unsuitable for long-term use), necessitating a 0.05 mm compromise. Slight performance attenuation (~<6% over 30 days) due to fur wear/deformation may occur. - Response dynamics: Nonzero response delay (≈0.48–0.96 s) between active/passive magnets may limit capturing very rapid transients, though deemed acceptable for typical ocean flows. - Calibration context: Coastal calibration used a speedboat (still-water tower method) and deep-sea validation relied on relative velocity from an ROV and DVL; absolute current measurements in fixed moorings or diverse ocean conditions were not reported within this study.