Patent ID: 12209568

DETAILED DESCRIPTION

Embodiments described herein relate to a hybrid wave energy converter (WEC) and marine buoy system for wave energy harvesting and marine sensing, and particularly to a triboelectric-electromagnetic hybrid nanogenerator (TEHG) that includes a soft ball-based triboelectric nanogenerator (SB-TENG) and an electromagnetic generator (EMG) for harvesting low-frequency wave energy from arbitrary directions in a body of water, such as an ocean, sea, or lake. The SB-TENG can include triboelectric soft balls that are freely moveable between separated electrodes to provide a moving triboelectric layer. The soft balls can significantly increase contact area with electrodes of the SB-TENG compared to hard triboelectric materials to generate substantially more triboelectric charges. The EMG can include a magnet that is freely moveable relative to copper coils to induce an electromagnetic current.

Under the agitation of water waves, the soft balls and the magnet can simultaneously move back and forth to convert the water wave motions into electricity. For example, the shell can be configured to move due to exterior fluid movement, such as wave agitation, to move the soft balls between the separated electrodes and the magnet across the lower divider to generate, respectively, triboelectric and electromagnetic charging. In some embodiments, under an operating frequency of 1 Hz, the SB-TENG and the EMG can reach a maximum output peak power of 0.5 mW and 8.5 mW, respectively. In other embodiments, the output power of the TEHG can be, for example, about 10 W/m3to about 50 Wm3or about 100 mW/kg to about 1,000 mW/kg. The TEHG has been demonstrated to power dozens of light-emitting diodes and drives a digital temperature sensor to monitor the water temperature for an extended period. The TEHG improves the output performance of TENGs and presents an improved self-powered water-sensing system driven by low-frequency water waves.

FIGS.1A and2Aillustrate respectively a schematic drawing and cross-sectional view of a TEHG10that can be used for energy harvesting ocean or lake water waves in accordance with an embodiment. The TEHG10includes a buoyant, waterproof and enclosed shell12and a plurality of stacked cavities14,16,18,20within the shell12. The shell12can have a substantially spherical shape although polygonal, ovular, curved, or other enclosed shapes can be employed. The shell12can include an upper portion22, middle portion24, and lower portion26. The upper portion22, middle portion24, and lower portion26can extend or be arranged along z-axis of an x-y-z coordinate system. The shell12can be formed from a substantially rigid waterproof material, such as an acrylic and have a diameter of about 100 mm to about 10 m, for example about 100 mm to about 1 m. Optionally, the shell12can be waterproofed with a waterproof sealant, coating, or glue to avoid potential water leakage, which may affect the generator's energy harvesting performance.

The plurality of cavities14,16,18,20within the shell12are defined by a plurality of substantially parallel circular or disc shaped dividers30,32,34that are spaced from one another along the z-axis and extend radially from the z axis to an inner surface36of the shell12and substantially parallel to an x-y plane of the x-y-z coordinate system. Each cavity14,16,18,20can be defined by adjacent parallel dividers in the shell. The dividers can be formed from, for example, circular or disc shaped polylactic (PLA) sheets. The circular PLA sheets can be made as substrates by a 3D printer with the same thickness but different diameters.

The parallel dividers30,32,34include an upper divider30in the upper portion22that defines the upper cavity14, a lower divider34in the lower portion26that defines lower cavities18,20, and a plurality of middle dividers32in the middle portion24that defines middle cavities16. AlthoughFIGS.1A and2Aillustrate a shell12that includes three middle cavities16, it will be appreciated that the shell12can include more or less middle cavities16depending on the number and spacing of the middle dividers32.

Each middle divider32includes an upper surface40, a substantially parallel lower surface42and a set of spaced apart electrodes44,46affixed to the upper surfaces40of the middle dividers32. As illustrated inFIG.2B, which is a top-plan schematic view of a middle divider31, the spaced apart electrodes44,46can have semi-circular shapes that are separated from one another by a gap48that extends between the electrodes44,46. However, it is alternately envisioned that each middle divider32may instead contain multiple alternating pairs of electrodes, such as four or six that are separated from one another on the upper surfaces40of the middle dividers32. The electrodes44,46can be formed by affixing or depositing a conductive material on a substrate50that forms the middle divider32. By way of example, the spaced apart electrodes44,46can be formed from two flat copper or copper alloy conductive tapes that are bonded onto the surface of a PLA sheet as fixed triboelectric layers and conductive electrodes44,46. The two spaced apart electrodes44,46can be separated with a gap48of, for example, about 1 mm.

Each middle cavity16defined by the middle dividers32includes a plurality of freely moveable chargers60, such as freely moveable triboelectric soft balls60, located on the upper surface40of each middle divider32. The soft balls60can freely move or roll back and forth on top of electrodes44,46and over gap48on the middle dividers32driven by small wave agitations. The chargers or soft balls60are negatively charged and the electrodes44,46are positively charged and the movement of the chargers60between separated electrodes44,46upon movement of the shell result in the generation of an alternating current. These freely moveable triboelectric soft balls60and stationary electrodes44,46form the SB-TENG, which has the function of wave energy harvesting.

The freely moveable soft balls60are elastic and flexible to enhance contact with the electrodes44,46. The SB-TENG described herein includes multiple soft balls60within each middle cavity16. For example, at least three and more preferably at least six soft balls16are provided in each middle cavity. The multiple balls60for each middle cavity can have superior output performance as compared to a single ball of the same size. In some embodiments, the number of soft balls provided on each middle divider32or arranged in each middle cavity16can vary depending on the size of the balls or cavity typically at least 2, 3, 4, 5, 6, 7, 8, or more balls are provided in each middle cavity16.

Each of the moveable soft balls60can include an outer shell and a diameter that allows the soft balls to roll back and forth along a middle divider in a respective middle cavity between the electrodes. For example, the moveable soft balls60can have a diameter of, for example, about 10 mm to about 20 mm, or about 12 mm. The shell can be formed of a negative triboelectric material, such as a silicone material, e.g., Ecoflex. For example, the soft balls can be made of a silicone rubber (e.g., Ecoflex-30) and can deform when pressed with a small force (FIG.1c).

Advantageously, the shell thickness can be minimized to enhance the SB-TENG output voltage. For the shell of the soft balls can have a thickness of about 100 μm to about 10 mm depending on the size of the soft ball. It was found that the output voltage of the SB-TENG decreases with the thickness of the silicone rubber shell. This difference should be due to the smaller contact area between the soft ball with a thicker shell and the electrodes. Although the thickness of the soft shell can be further reduced, its robustness will decrease, and the damping force will increase significantly.

The outer shell of soft balls can define an internal cavity that can be filled with a liquid. Filling the soft ball with a liquid can allow the soft ball to generate more considerable deformation and contact area to improve the electricity generation of the TENG. In some embodiments, the liquid used to fill the soft ball can include a liquid polymer without free ions, such as Ecoflex or lubricating oil. Advantageously, a liquid polymer without free ions does not adversely affect the generation of contact electrification charges on the electrode.

In some embodiments, each of the moveable soft balls can be coated with a polytetrafluoroethylene (PTFE) powder on an outer surface of the soft balls to reduce the damping force but simultaneously enhance contact electrification. As shown inFIG.3a, the soft balls with PTFE powders are less sticky and can roll off fast on an inclined Cu surface. In contrast, the soft balls without PTFE powders are very sticky, and the large damping force prevents them from rolling off the inclined Cu surface. In some embodiments, the PTFE powders are adsorbed on the surface of the soft balls when highly sticky silicone rubber is used to form the soft balls.

A conductive coil70, such as a conductive copper coil with a circular pattern, is positioned in the lower cavity20below the lower divider34on a bottom interior surface72of the lower portion26of the shell12. A circular magnet74is positioned on an upper surface76of the lower divider34above the conductive coil70in the other lower cavity18of the lower portion26of the shell12. The circular magnet74is freely moveable on the upper surface76of the lower divider34. Upon movement of the shell12, the circular magnet74slides relative to the coil70so that the coil70cuts magnetic induction lines, inducing an electromagnetic current. This coil-magnet structure forms an EMG, which has the function of wave energy harvesting and lowers the TEHG's center of gravity to prevent possible overturning.

FIG.1dshows the working principle of the SB-TENG unit, which is based on the conjugation of triboelectrification and electrostatic induction. When actuated by small wave agitations, the soft balls in the cavities of the SB-TENG will freely move or roll back and forth between the two electrodes, resulting in an alternating current through an external circuit. In brief, at the initial state (FIG.1D<II>), when the soft balls are in contact with the left-hand electrode, the soft balls are negatively charged. In contrast, the electrode has the same number of positive charges in the saturated state due to its different abilities to attract electrons. At this moment, the tribo-charges will remain on the surfaces of the two triboelectric layers for an extended period. When the soft balls roll to the right-hand electrode under the wave agitations, the equilibration of the electric field will be changed so that the free electrons flow from the right-hand electrode to the left-hand electrode, resulting in a forward current in the external load (FIG.1D<II>). When the soft balls leave the left-hand electrode and contact the right-hand electrode, all the positive charges on the left-hand electrode will be driven to the right-hand electrode (FIG.1D<III>). After that, the soft balls will move back to their original positions, and the free electrons flow back to the right-hand electrode, generating a reverse current in the external load (FIG.1D<IV>).FIG.1eillustrates the working mechanism of the EMG unit, which is based on electromagnetic induction. Under the wave agitation, the magnet slides right-left with the coil so that the coils cut magnetic induction lines, inducting an electromagnetic current. With the cyclic motions of the magnet, alternating electricity will be generated continuously in the coil.

The TEHG harvests mechanical energy to charge energy storage unit and power electronics, forming a self-powered electronics system. The energy harvesting performance of TENGs is dependent on the external resistor loading. As shown inFIGS.5A and5B, the output voltages of the SB-TENG and the EMG increase with the increasing resistance loads while the output currents exhibit a reverse trend. It was found that when the external resistance is 700 MΩ, the SB-TENG reaches a peak power of 0.5 mW, while the EMG has a peak power of 8.6 mW at a load resistance of 180Ω. The above results indicate that SB-TENG can be regarded as a current source with a high impedance, whereas the EMG is equal to a voltage source with a low impedance.

Furthermore, the output performance of the TEHG can be further enhanced by optimizing the structural design and proper material selection.FIG.5Cexhibits the charging voltage curves of a capacitor (10 μF) using an SB-TENG, an EMG, and a TEHG at a fixed charging time of 60 s and a working frequency of 1.0 Hz. Due to the relatively low output power and current, the charging rate of the SB-TENG is lower than that of the EMG. As for the EMG, the charging voltage quickly reaches about 2.1 V, and after that, the charging voltage increases slowly because of the low output voltage of EMG. By combining the advantages of the two parts, TEHG shows a much faster charging rate and a higher charging voltage than the individual energy-harvesting unit SB-TENG or EMG.FIG.5Dshows the charging voltage curves of a 10 μF capacitor by a TEHG under different frequencies. With the working frequency increasing, more energy can be harvested, and a higher charging voltage can be achieved. Moreover, as shown inFIG.5E, a TEHG can charge a smaller capacitor to reach a higher voltage at a faster charging rate during the same charging time.

FIG.5Gshow schematic diagrams for circuit100of a self-powered WEC system, which includes a TENG102, EMG104, conductors106, a resister108, a first rectifier110, a second rectifier112, a storage capacitor114, and two switches116and118. Conductors106may be insulated wires, stamped metallic conductors, printed circuit traces, or the like. The first rectifier110and second rectifier112are used to convert alternating current (AC) electricity to direct current (DC) electricity. The working mechanism for this circuit is: first, when switch116is on and118is off, storage capacitor114is charged by the TENG102and EMG104and their voltage is monitored by a voltmeter120. Then, when the voltage is charged to a specific value, switch118is turned on and the stored energy is discharged to drive a connected electronic device.

It is envisioned that for charging voltages of a capacitor as a function of the charging time under different frequencies, the charging rate is expected to increase with the frequency, and more energy should be harvested with a higher frequency. The smaller the capacitor, the higher the charging voltage and the faster the charging speed.

In some embodiments, the TEHG can include or be electrically connected to at least one electrical component. The electrical component can include, for example, a sensor, a light, a timer, a display, a power storage unit, or a transmission unit that is part of or electrically connected to the TEHG. For example, a plurality of TEHGs can be included in a wave energy converter (WEC) system wherein each of the plurality of TEHGs are electrically connected to an electrical component at least one of a sensor, a light, a timer, display, a power storage unit, or a transmission unit. The TEHGs of the WEC system can float on top of and/or are partially or fully submerged in the water, and are moored to a floor by cables and an anchor block. Electrical lines can connect adjacent TEHGs and to a power storage or transmission unit.

The invention is further illustrated by the following example, which is not intended to limit the scope of the claims.

EXAMPLE

In this example, we describe a triboelectric-electromagnetic hybrid nanogenerator (TEHG) based on soft balls to convert mechanical energy into electricity. As a proof-of-concept, three layers of soft ball-based triboelectric nanogenerator (SB-TENG) units and an electromagnetic generator (EMG) unit are integrated into a spherical acrylic shell (FIG.1). For the TENG part, liquid/silicon soft balls were used to replace the traditional hard triboelectric material as the moving triboelectric layer that can significantly increase the contact area to generate much more triboelectric charges. The EMG part composed of a magnet and copper coils is placed at the bottom of the device. Under the agitation of water waves, the soft balls and the magnet can simultaneously move back and forth in the shell. Thus, the water wave motions are converted into electricity. The key parameters that affect the energy harvesting performance of the TEHG were investigated, including the type of the liquid core materials, the thickness of the silicone rubber shell, and the number of layers of the soft balls. The output performance of the SB-TENG and EMG were studied under various mechanical triggering conditions. We finally demonstrate that the new TEHG can light dozens of light-emitting diodes (LED) and power a digital temperature meter for environment monitoring.

Experimental Section

Fabrication of the Soft Balls

The silicone rubber (Exoflex 00-30, Smooth-On, Inc.) was prepared by mixing the silicone base and precursor (Parts A and B) with a weight ratio of 1:1. As shown inFIG.8, a PTFE ball with a diameter of 12 mm was utilized as a mold and a fine needle was inserted into the PTFE ball. We dipped the PTFE ball into liquid silicone rubber and then placed it in an oven at 80° C. for 2 hours. After that, the silicone shell was peeled off from the PTFE ball.

Finally, we injected liquid materials into the soft silicone shell through a small hole and sealed it with mixed silicone rubber. The sealed soft balls were placed in the oven for another 1 h for further use.

Fabrication of the TEHG

The circular PLA sheets are made as substrates by a 3D printer with the same thickness (1.5 mm). Two flat Cu conductive tapes are bonded onto the surface of the PLA sheets with a gap of ˜1 mm and connected with lead wires. Finally, the EMG, the soft ball, and the PLA sheets are integrated into an acrylic spherical shell with a diameter of 12 cm, as shown inFIG.1A.

Electrical Measurements of the TEHG

The open-circuit output voltage, short-circuit current and transferred charges of the SB-TENGs, the EMGs, and the TEHGs were measured by a current preamplifier (Keithley 6514 System Electrometer). A linear motor (LinMot MBT-37-120) was applied to drive the TEHG in the air and generate the water wave in a water tank. The software LabVIEW was programmed to acquire real-time control and data. All measured data were processed with MATLAB and Origin.

Results

Design and Working Principle of the TEHG

FIG.1Aillustrates the structural design of the TEHG, which consists of two major parts: multilayered soft ball-based TENG and EMG. As a proof-of-concept, we fabricate a three-layers TENG unit by filling different numbers of soft balls (FIG.1B) into the cavity spaces between the adjacent circular polylactic acid (PLA) sheets. The circular PLA sheets are made as substrates by a 3D printer with the same thickness but different diameters. Two flat Cu conductive tapes are bonded onto the surface of the PLA sheets as the fixed triboelectric layers and conductive electrodes. The two electrodes are separated with a gap of ˜1 mm (FIG.1A). The soft balls made of silicone rubber (Ecoflex-30) have a diameter of ˜12 mm, and they can deform when pressed with a small force (FIG.1C). The soft ball has an internal cavity for filling liquid in so that the soft ball can generate more considerable deformation and contact area to improve the electricity generation of the TENG. The Ecoflex shell will be a negative tribo-material to form a pair with the copper layer. The detailed fabrication process of the soft ball can be found in the Experimental Section. To better observe the deformation and motion of the balls, we add a small amount of blue dye to the silicone rubber. Additionally, a copper coil with a circular pattern is embedded at the bottom of the acrylic shell, which has a dimension of Ø45 mm×5 mm. A circular magnet with a diameter of 20 mm is placed on top of the bottom PLA sheet. This coil-magnet structure forms an EMG, which has the function of wave energy harvesting and lowers the device's center of gravity to prevent possible overturning. An acrylic sphere with a diameter of 120 mm is used as a shell and sealed well with waterproof glue to avoid potential water leakage, which may affect the generator's energy harvesting performance.

FIG.1Dshows the working principle of the TENG unit, which is based on the conjugation of triboelectrification and electrostatic induction. When actuated by small wave agitations, the soft balls in the cavities of the device will freely move or roll back and forth between the two Cu electrodes, resulting in an alternating current in the external circuit. In brief, at the initial state (FIG.1D<II>), when the soft balls are in contact with the left-hand Cu electrode, the soft balls are negatively charged. In contrast, the Cu electrode has the same number of positive charges in the saturated state due to its different abilities to attract electrons. At this moment, the tribo-charges will remain on the surfaces of the two triboelectric layers for an extended period. When the soft balls roll to the right-hand Cu electrode under the wave agitations, the equilibration of the electric field will be changed so that the free electrons flow from the right-hand electrode to the left-hand electrode, resulting in a forward current in the external load (FIG.1D<II>). When the soft balls leave the left-hand electrode and contact the right-hand electrode, all the positive charges on the left-hand electrode will be driven to the right-hand electrode (FIG.1D<III>). After that, the soft balls will move back to their original positions, and the free electrons flow back to the right-hand electrode, generating a reverse current in the external load (FIG.1D<IV>).FIG.1Eillustrates the working mechanism of the EMG unit, which is based on electromagnetic induction. Under the wave agitation, the magnet slides right-left with the coil so that the coils cut magnetic induction lines, inducting an electromagnetic current. With the cyclic motions of the magnet, alternating electricity will be generated continuously in the coil.

Effect of the Materials and Structural Design on the Energy Harvesting Performance of SB-TENG

To make the soft balls highly elastic and flexible, we use silicone rubber to fabricate the ball's shells. The soft balls are expected to have a larger effective contact area with the electrodes to enhance the output performance of the TENG. However, it is found that the soft balls made of silicone rubber become sticky and generate a relatively large damping force at the interface, thus dramatically hindering the motions of the soft balls under the small agitations. Therefore, we stick polytetrafluoroethylene (PTFE) powders on the surface of the soft balls to reduce the damping force but simultaneously enhance contact electrification. As shown inFIG.3A, the soft balls with PTFE powders are less sticky and can roll off fast on an inclined Cu surface.

In contrast, the soft balls without PTFE powders are very sticky, and the large damping force prevents it roll off from the inclined Cu surface. It should be noted that the PTFE powders are adsorbed on the surface of the soft balls relying on the highly sticky silicone rubber. We also added PTFE powders to the silicon rubber solution to obtain the composite material. However, it is found that this method has only a minimal effect on reducing the damping force and the higher content of PTFE powders decreases the flexibility of the soft ball. It is well known that PTFE is one of the most widely used triboelectric materials because of its high negativity in the triboelectric series.FIG.3Bdemonstrates that the TENG made of soft balls with PTFE powders can generate better output performance than that without PTFE powders under the same external agitations, attributing to the high tribo-negativity of PTFE.

Additionally, we examine the effect of liquid materials on the output performance of the SB-TENG. We inject a few kinds of liquids: tap water, deionized (DI) water, lubricating oil, and Ecoflex B, into the soft shell to compare the TENG's performance. The output performance of a TENG made of rigid PTFE balls is also studied for comparison.FIGS.3C and3Dpresent the influences of liquid type on the open-circuit voltage and short-circuit transferred charge of the SB-TENGs. It is obvious that the SB-TENGs filled with lubricating oil and Ecoflex B exhibit much higher output performance than those made of rigid PTFE balls. The SB-TENGs with Ecoflex B can generate a peak value of 70 V and 22 nC, respectively.

On the other hand, the SB-TENGs with tap water and DI water have a significantly lower output performance, about 30% or less of those made with Ecoflex B. The lower performance may be due to the screen effect of ions or charges in the water. As shown inFIG.6, the positive ions in the water can attract negative charges on the Ecoflex, which can interfere with the generation of contact electrification charges on the Cu electrode. Unlike water, Ecoflex B and lubricating oil are liquid polymers without free ions so that they wouldn't affect the contact electrification charge amount on the Cu electrode. To check the stability of the device, we test the short-circuit transferred charges of SB-TENGs made with different liquids at different times (0 h and five days) after fabrication. The test was conducted under the same testing conditions, and the number of samples increased to six. It can be seen fromFIG.3Ethat the SB-TENGs made with lubricating oil have a considerable degradation of the output performance compared to those made with DI water, tap water, and Ecoflex B. The samples with Ecoflex B demonstrated little change after five days. This phenomenon may be attributed to the silicone rubber's low oil resistance and the oil's resultant leakage, especially when the thickness of the silicone rubber shell is skinny. Thus, in the following experiments, we will choose Ecoflex B as the filled liquid for new SB-TENGs.

In addition, we also investigate the influence of the shell thickness of the soft balls. As shown inFIG.3F, five different kinds of soft balls are made with different thicknesses by increasing the coating-curing times. It is observed that the output voltage of the SB-TENG decreases with the thickness of the silicone rubber shell. This difference should be due to the smaller contact area between the soft ball with a thicker shell and the electrodes. Although the thickness of the soft shell can be further reduced by adjusting the mixed ratio of the Ecoflex 30, its robustness will decrease, and the damping force will increase significantly.

We further compare the output performance of the SB-TENGs made of soft balls and PTFE balls. As shown inFIG.3G, the output performances of two kinds of TENGs have a similar increasing trend with the number of moving balls in the cavities. However, the increasing rate becomes smaller with more moving balls in the cavities. This change is attributed to the larger damping force and the smaller moving space in the device when having more balls. It can be found that the output voltage of the SB-TENG has been significantly improved compared to that of the rigid PTFE ball-based TENG.FIG.3Hpresents the output voltages of SB-TENGs with a different number of layers. Three layers can be integrated into the limited space of the SB-TENG unit. The measured output voltage of an SB-TENG with a single layer is around 180 V, and it can be enhanced to 430 V when having three layers in the device. This result indicates that a multilayered design can fully use the limited space in the device and significantly improve the output performance of the TENG.

Effect of the Excitation Mode and Conditions on the Energy Harvesting Performance of TEHG

Water waves in the ocean are very complex and may constantly change due to the uncertainty associated with many factors, such as wind, gravity, and planetary motion. To characterize the energy harvesting performance of the TEHG under different wave excitation conditions, we have categorized the motions into two basic movement modes, namely the translational mode and the swing mode (FIGS.4A and4F). A linear motor is used to simulate the two movement modes with assigned acceleration, speed, and displacement amplitude. The TEHG device is attached to a platform fixed on the linear motor for the translational mode to apply the vibration. With the excitation frequency varies from 0.5 Hz to 2 Hz, the peak values of the output voltage and current generated by the SB-TENG increase to 450 V and 2 μA, respectively (FIG.4B). The output voltage and current of the EMG have a similar variation trend, in which the voltage monotonically increases from 0.8 V to 3 V and the current rises from 4 mA to 15 mA as the frequency increases from 0.5 Hz to 2 Hz (FIG.4C). These results are in consistence with the Faraday's law, i.e., the voltage and current of the EMG are positively correlated with the moving velocity of the magnet.FIGS.4D and4Eshow the effect of translational amplitude of the excitation on the output performance of TEHG. The output voltages and currents of the SB-TENG and the EMG are proportional to the translational amplitude at a frequency of 1.5 Hz. This variation should originate from the higher inertial acceleration of the soft balls and the magnet induced by the larger translational amplitude. Remarkably, less kinetic energy can be transferred to the soft balls and the magnet for a smaller translational amplitude and a lower frequency, thereby generating a much smaller electric output.

For the swing mode, as shown inFIG.4G, the peak output voltage and current of the SB-TENG at an orientation angle of 45° increase rapidly with the increasing swing frequency, from 285 V and 0.35 μA at 0.25 Hz to 410 V and 1.3 μA at 1.0 Hz, and then decrease at higher frequencies (>1.0 Hz). The initial increasing stage is attributed to the increased moving speed of the soft balls. However, when the swing frequency is higher than 1.0 Hz, the soft balls oscillate unstably to generate more collisions among the moving balls, resulting in uncompleted travels between the two Cu electrodes. On the other hand, the output performance of the EMG monotonically increases with the swing frequency (FIG.3H) due to the larger movement space and lower frictional force.

FIGS.4I and4Jshow the TEHG's output performance variation with the orientation angle. Five different orientation angles, from 15° to 55°, were controlled under a constant swing frequency of 0.5 Hz. The output voltage and current of the SB-TENG gradually increase with the orientation angle, which is attributed to the more significant moving displacement of the soft balls under a larger orientation angle. The output voltage and current of the EMG first increase with the orientation angle increasing from 15° to 35° because the larger orientation angle generates a more significant moving displacement and a faster-moving speed of the magnet. However, when the orientation angle increases from 35° to 55°, the output performance of the EMG is kept nearly as constant due to the constraint of the shell on the continuous increase of the moving displacement and the magnet speed.

Evaluation of the New TEHG and Self-Powered Systems

The energy harvesting performance of TENGs is dependent on the external resistor loading. To optimize the energy harvesting performance, we have to investigate the impedance of the SB-TENG and the EMG. As shown inFIGS.5A and5B, the output voltages of the SB-TENG and the EMG increase with the increasing resistance loads while the output currents exhibit a reverse trend. It is found that when the external resistance is 700 MΩ, the SB-TENG reaches a peak power of 0.5 mW, while the EMG has a peak power of 8.6 mW at a load resistance of 180Ω. The above results indicate that SB-TENG can be regarded as a current source with a high impedance, whereas the EMG is equal to a voltage source with a low impedance. According to the total volume of the device, the volumetric power density of the TEHG was calculated to be 10.1 Wm−3. As shown inFIG.7, we list the volumetric power density of several triboelectric-electromagnetic hybrid nanogenerators from previous studies, and the TEHG exhibits the highest volumetric charge density.

Furthermore, the output performance of our device could be further enhanced by optimizing the structural design and proper material selection.FIG.5Cexhibits the charging voltage curves of a capacitor (10 μF) using an SB-TENG, an EMG, and a TEHG at a fixed charging time of 60 s and a working frequency of 1.0 Hz. Due to the relatively low output power and current, the charging rate of the SB-TENG is lower than that of the EMG. As for the EMG, the charging voltage quickly reaches about 2.1 V, and after that, the charging voltage increases slowly because of the low output voltage of EMG. By combining the advantages of the two parts. TEHG shows a much faster charging rate and a higher charging voltage than the individual energy-harvesting unit SB-TENG or EMG.FIG.5Dshows the charging voltage curves of a 10 μF capacitor by a TEHG under different frequencies. With the working frequency increasing, more energy can be harvested, and a higher charging voltage can be achieved. We also test the TEHG to charge different capacitors at the frequency of 1.0 Hz. As shown inFIG.5E, a TEHG can charge a smaller capacitor to reach a higher voltage at a faster charging rate during the same charging time.

We place the TEHG device into a water tank to demonstrate its great potential in harvesting water wave energy (FIG.5F). Under the excitation of water waves, the TEHG can lighten dozens of light-emitting diodes (LEDs). We can integrate the TEHG with a rectifier, a storage capacitor, and two switches to form a self-powered electronics system, which can be used to power a sensor for long-term service (FIG.5G). The TEHG can charge a capacitor (100 μF) in less than 10 s by harvesting water wave energy (FIG.5H). When the charging voltage reaches ˜1.5 V, the thermometer powered by the capacitor is activated to measure the water temperature. After discharging, the TENG can continue to charge it to the working voltage of 1.5 V to power the sensor to work again.

We have proposed a new triboelectric-electromagnetic hybrid nanogenerator for water wave energy harvesting. The soft balls were designed as the moving triboelectric layer to increase the contact area. At the same time, a multilayered structure was adopted to fill more soft balls to utilize the device space fully. The energy harvesting performances of the SB-TENG and the EMG were investigated under different mechanical excitation conditions and configurations. With the optimal design, the SB-TENG and the EMG achieved a maximum output peak power of 0.5 mW and 8.5 mW, respectively, under an operating frequency of 1.0 Hz. Finally, we demonstrated a TEHG-based self-powered electronic system to drive a digital temperature sensor to measure the water temperature. This new design provides an innovative and promising approach to effectively harvesting low-frequency water wave energy in the ocean for marine monitoring and electricity generation.

From the above description of the invention, those skilled in the art will perceive improvements, changes and modifications. Such improvements, changes and modifications within the skill of the art are intended to be covered by the appended claims. All references, publications, and patents cited in the present application are herein incorporated by reference in their entirety.