Triboelectric–Electromagnetic Hybrid Generator for Harvesting Blue Energy
KeywordsTriboelectric nanogenerator Electromagnetic generator Hybrid generator Water flow Power source
A hybrid generator including contact–separation-mode triboelectric nanogenerators (CS-TENGs) and rotary freestanding-mode electromagnetic generators (RF-EMGs) with the potential to harvest water flow-based blue energy from the environment was designed.
The magnet pairs that produce attraction were used to achieve packaging of the CS-TENGs part, protecting it from being affected by the ambient environment.
In addition to powering light-emitting diodes, the generator can charge commercial capacitors and use the stored energy to power an electronic water thermometer.
Much effort has been made to meet the huge energy demand of modern society while minimizing environmental cost . Widely distributed water kinetic energy is an abundant source for large-scale applications and is much less dependent on seasonality, day–night, weather, and temperature variations [2, 3, 4]. Especially in the form of water flow, it contains a gigantic reserve of kinetic energy, but is hardly utilized in an effective way [5, 6, 7]. Recently, triboelectric nanogenerators (TENGs) have emerged as a powerful technology for harvesting low-frequency mechanical energy with characteristics including lightweight, low cost, and wide selection of materials [8, 9, 10, 11, 12, 13]. More improvements have been made in the use of TENGs to achieve a human–machine interface [14, 15]. Essentially, TENGs demonstrate much better output performance than that of traditional electromagnetic generators (EMGs) at low frequency (typically 0.1–3 Hz), which confirms the possible application of TENGs for harvesting irregular and low-frequency motion energy such as that from water flow [16, 17]. How to use this novel technology to achieve energy collection and conversion attracts much attention.
The original idea of using TENGs for water wave energy was proposed by the liquid–solid electrification-enabled process. During the submerging and surfacing process due to traveling water waves, current flows between the electrodes to screen the charges on the triboelectric layer of the TENG, thereby producing electric power [18, 19, 20]. However, the output performance decreases dramatically to almost zero at a high ion concentration in a real water environment owing to the streaming potential theory [21, 22, 23]. An additional strategy was put forward involving a freestanding, fully enclosed TENG that packs a rolling ball in its interior to form a rocking spherical shell [24, 25, 26, 27]. Later on, various designs based on the hybridization of TENGs and EMGs were developed [28, 29, 30, 31]. The magnet pairs of EMGs produce the noncontact attractive force that enables the fully enclosed packaging of the TENG part, protecting it from the ambient environment. Meanwhile, the complementary outputs can be hybridized and maximized in a broad frequency range. Nevertheless, these hybrid generators are still in the development stage for water flow energy collection, and more research is highly desired to optimize their structure and improve their performance toward practical applications.
In this work, we present the design of a hybrid generator based on contact–separation-mode TENGs (CS-TENGs) in conjunction with rotary freestanding-mode EMGs (RF-EMGs). Five CS-TENGs were initially fixed in an enclosed cylinder to isolate the impact of water. Relying on the attraction force between the magnets, two triboelectric layers of the CS-TENG contact and separate periodically during the rotation process. The device durability is greatly enhanced with respect to that of TENGs based on the sliding mode. This ingenious design combines the output feature of both CS-TENGs and RF-EMGs at different rotation speeds. Remarkably, compared with other structures, the cylinder-like structure is easier to be driven by water flow. Water flow impacts the impeller, allowing the device to rotate at a steady rate. Furthermore, the device was installed in a turbulent place to directly power LEDs and clearly demonstrated a higher output from the CS-TENGs at low frequency and from the RF-EMGs at high frequency. As a demo, it can also charge commercial capacitors and use the stored energy to power an electronic water thermometer.
3 Experimental Section
3.1 Fabrication of Nanowire Array on Polytetrafluoroethylene Surface
The nanowire array was created on a polytetrafluoroethylene (PTFE) surface by a one-step plasma reactive ion etching process reported previously. The PTFE films were cleaned with alcohol, isopropyl alcohol, and deionized water successively and then dried in an oven at 50 °C. A thin layer of Cu film was deposited on the cleaned PTFE surface by sputtering. Then, inductively coupled plasma (ICP) etching was utilized to produce aligned nanowire-like structures on the surface. Specifically, Ar, O2, and CF4 gases were added in the ICP chamber with flow ratios of 15.0, 10.0, and 30.0 sccm, respectively. A power of 400 W was used for plasma generation, and a power of 100 W was used for accelerating the plasma ions. The PTFE film was etched for 6 min to obtain the nanowire-like structures.
3.2 Assembly of the Hybrid Generator
First, two acrylic sheets with a size of 40 × 40 mm2 were shaped by a laser cutter as the substrates and a thin Al foil (40 × 40 mm2) was then attached to the top substrate as the top electrode, and a thin Al foil of the same size was attached to the bottom substrate. The surface of the Al foil was covered by the PTFE film. The nanostructures were fabricated on the surface of the PTFE film by ICP etching. In an RF-EMG unit, a coil was inserted between two magnets. Two acrylic cylinders with the same width but different diameters were sheathed and fixed together on two acrylic disks to form a closed space. Five coils were arranged on the outer surface of the smaller acrylic cylinder at equal spacings. Next, a CS-TENG was fixed on each coil. Then, a magnet was fixed at the top of each CS-TENG. The closed space contained the CS-TENG part and the stator part of the RF-EMG. A thin acrylic tube passed through the center of the closed cavity and was connected to it via two bearings. Five magnets were equally spaced and arranged in the middle of the tube, and two impellers were distributed at both ends of the tube.
3.3 Electrical Measurement
The surface morphology of the PTFE thin film was characterized by scanning electron microscopy (SEM, FEI Co., model Quanta-200). The output voltage signal and the output current signal were acquired via a programmable electrometer (Keithley model 6514). The software platform was constructed using LabVIEW and was capable of realizing real-time data collection and analysis. A rotating motor (86BYG250D, MA860H) was applied to drive the device to rotate.
4 Results and Discussion
In summary, we have designed and demonstrated a hybrid generator including CS-TENGs and RF-EMGs with the potential to harvest water flow-based blue energy from the environment. The output performance of the CS-TENGs and RF-EMGs was measured under the regular action of the rotary motor, and the key concept and design of our device are to combine the two generators together. Thus, the CS-TENG can harvest low-frequency energy, whereas the RF-EMG produces larger output in a high-frequency range. The generated output from the RF-EMGs can reach a peak voltage of 0.59 V and a peak current of 1.78 mA at 100 rpm. For the CS-TENGs, an output voltage and current of 315.8 V and 44.6 μA, respectively, were achieved at 100 rpm, demonstrating the applicability of the generator in a real environment. Moreover, although the CS-TENGs can directly drive a series of LEDs at low or high rates, the RF-EMGs can only light up all the LEDs at high rates. The magnet pairs that produce the attraction force are used to achieve packaging of the CS-TENG part, protecting it from the external environment. The rectified outputs have also been demonstrated to charge commercial capacitors, whose stored energy can power an electronic thermometer in a self-powered water-temperature sensing system.
The work was funded by Natural Science Foundation of China (NSFC) (Grant No. U1432249), the National Key R&D Program of China (Grant 2017YFA0205002), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). This is also a project supported by Collaborative Innovation Center of Suzhou Nano Science & Technology. Dr. Z. Wen thanks the support from China Postdoctoral Science Foundation (2017M610346), Natural Science Foundation of Jiangsu Province of China (BK20170343), and Nantong Municipal Science and Technology Program. Dr. Yina Liu thanks the support from Jiangsu University National Science Research Program (16KJB110021).
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