Triboelectric Nanogenerator Harvests Ocean-Wave Energy

What you’ll learn:

The challenges faced by energy harvesting powered by ocean waves.
How a new design overcomes the existing limitations.
The tested performance of the new approach.

There seem to be no limits to the ingenious ways that designers are devising to harvest energy or take existing approaches and exploit and enhance them. It makes sense, as harvested energy is not only “free,” but also long-lasting. And, in many cases, it’s the most practical way to power a small or modest remote sensor-driven data-acquisition load.

Thus, using the widely available ocean-wave energy seems like a “no brainer.” However, the reality is that many practical issues must be overcome.

Now, a research team at the Pacific Northwest National Laboratory (PNNL) has developed a contact-separation mode triboelectric nanogenerator (TENG) with a simple structure for harvesting wave energy and powering marine sensors and transmitters. Although this isn’t the first cylindrical TENG (C-TENG)—several models are already in use—the PNNL team maintains that this design overcomes weaknesses of those existing ones.

Achieving Higher Frequency

The key is a new mechanism for wave-driven energy-harvesting TENGs that can convert the low-amplitude, low-frequency ocean waves into high-frequency mechanical motion for more effective power generation (Fig. 1). This new TENG must be able to operate and be triggered by any wave conditions, even in the middle of the ocean where waves have uniform or random low amplitude and frequency.

1. This simple graphical abstract shows the basic operating scenario and associated frequency multiplication of the pendulum swing mode.

Water wave-energy harvesting with pendulum-structured TENGs fall into two types, using either contact-separation or free-standing layer modes. The contact-separation mode generates electricity as the triboelectric-induced charge transfer occurs when two layers of opposite charges come into contact periodically in the presence of an external mechanical force.

A free-standing layer mode TENG consists of dielectric materials and electrodes. It generates electricity when the dielectric material slides over the electrodes, which induces the triboelectric charge transfer across the electrodes.

This novel C-TENG is designed to increase the operating frequency of the TENG using low-frequency, uniform water waves to increase the power generation. The new frequency-multiplied cylindrical TENG (FMC-TENG) converts the low-frequency wave energy into potential energy using a proof mass held by the repulsive force of two magnets.

Whenever the restoring force (weight) of the proof mass exceeds the magnetic force, the potential energy of the proof mass is transformed into kinetic energy via a high-frequency swing motion for generating high output power.

TENG Construction

The FMC-TENG includes two main components: an internal rotor and an external stator (Fig. 2). The cylindrical stator is manufactured from a UV-curing resin, using a stereolithography 3D printer. The internal rotor has an outer diameter (OD) of 43 mm, while the external stator has an inner diameter (ID) of 44 mm and a thickness of 1.72 mm.

2. Device structure and working mechanism of the FMC-TENG: (a) Schematic of the FMC-TENG, (b) photograph of the internal rotor (left) and external stator (right) of the FMC-TENG, and (c) assembled FMC-TENG. (d) Illustration of the working principle of FMC-TENG in the ocean, and (e) electricity generation process.

Aluminum electrodes are attached to its inner wall and a neodymium magnet is secured to the bottom of the stator. To enhance the output performance, a soft-contact mechanism between fluorinated ethylene propylene (FEP) and rabbit fur is attached in the FMC-TENG. The outer surface of the internal rotor contains six extruded rectangular ridges (70 × 10 × 1 mm), each covered with FEP film (0.05 mm thick).

Work on the project began with semi-analytical modeling of the FMC-TENG to theoretically verify the effectiveness of the FMC-TENG (Fig. 3). Then, the electrical characteristic of the C-TENG was evaluated by comparing it with a conventional C-TENG using a wave-motion simulator that mimics the wave motion in the ocean.

3. Results of analytical modeling and simulation: (a) Conventional and FMC-TENG. (b) Electric potential distribution of the TENG in COMSOL simulation. MATLAB simulation results: (c) velocity and (d) displacement of the conventional and FMC-TENG at 0.33 Hz. (e) Simulation results of the semi-analytical modeling (COMSOL+MATLAB): output power vs. resistive load, and (f) maximum output power vs. time of the conventional and FMC-TENG at 0.33 Hz.

Test Results

Final optimization of the FMC-TENG was performed in a 12-meter-long water tank with adjustable wave height and frequency (Fig. 4). The peak power density of the FMC-TENG reached 6.67 W/m³ at a wave frequency of 0.33 Hz.

4. Electrical output of the FMC-TENG in the water tank: (a) Wave maker in 12-meter water tank. (b) Photograph of three types of FMC-TENG (30°, 40°, and 60°). (c) Open-circuit voltage vs. time and (d) short-circuit current vs. time for three types of FMC-TENG at 0.33 Hz. (e) Peak open-circuit voltage vs. water wave frequency and (f) peak short-circuit current vs. wave frequency for three types of FMC-TENG. (g) RMS current and average power vs. resistive load. (h) Power density vs. resistive load and (i) peak power density vs. resistive load for 60° type TENG system at 0.33 Hz.

The device sustainably powered up an array of 27 LEDs and was able to charge up a capacitor up to 1.8 V for driving an acoustic transmitter (Fig. 5). They also tested and compared the new design to existing C-TENG implementations.

5. Demonstration of powering an acoustic transmitter: (a) Photograph of TENG array system and the acoustic transmitter. (b) Demonstration of powering acoustic transmitter and receiving the acoustic signal (transmitter identification). (c) Charging and discharging of the 1000-µF storage capacitor for powering the acoustic transmitter.

This patent-pending design is detailed in their 16-page paper “Frequency-multiplied cylindrical triboelectric nanogenerator for harvesting low frequency wave energy to power ocean observation system” published in Nano Energy. While it’s behind a paywall, just click on the button “View Open Manuscript” at the top to see the entire paper.