Dynamic Analysis to Enhance the Performance of a Rotating-Disk

Jun 19, 2019 - The horizontal plane has a 200 μm thick layer of PTFE on the bottom to .... simulation tool: (d) perpendicular planes and (e) horizont...
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Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 25170−25178

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Dynamic Analysis to Enhance the Performance of a Rotating-DiskBased Triboelectric Nanogenerator by Injected Gas Hyeonhee Roh, Jinsoo Yu, Inkyum Kim, Yunseok Chae, and Daewon Kim* Department of Electronic Engineering, Institute for Wearable Convergence Electronics, Kyung Hee University, 1732 Deogyeong-daero, Giheung-gu, Yongin 17104, Republic of Korea

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ABSTRACT: A rotating-disk-based triboelectric nanogenerator (TENG) generating electrical energy from wind usually includes a propeller. TENGs are widely used because their high frequency of rotation allows them to generate a relatively high output current. Deep analysis of the gas flow in a TENG is essential to improve its energy conversion efficiency. However, previous studies have isolated the propeller and the TENG as separate entities that harvest wind energy and generate electrical energy, respectively. Most studies focused on each entity because considering both the dynamics and the TENG operation together is an intricate process. This paper introduces a dynamic analysis of the gas flow by dividing it into four vertical and horizontal directions and carrying out a COMSOL simulation to verify the pressure on the propeller and the flow of the gas. The electrical outputs are measured while varying the height and angle of the inlet and the number of wings on the propeller. After optimization, the P-TENG generated a high output power density of 283.95 mW/m2, which can light up 205 light-emitting diodes and drive a commercial small electronic appliance. In addition, optimizing the P-TENG through a variety of analyses allowed it to provide sustainable power to a self-powered wireless sensor system. KEYWORDS: triboelectric nanogenerator, air dynamics, propeller, flow, simulation constantly varying gas flow, which can increase or decrease the rotational force. To collect the mechanical energy from wind effectively, it is essential to understand how the gas flow fluctuates with the movement of the structure of the TENG. However, only a few papers have addressed both the operation of TENG and dynamics simultaneously because considering them together is extremely complicated and difficult.32,33,35 In this paper, we present a dynamic analysis of the influence of gas flow on a propeller-TENG (P-TENG). The P-TENG was specially designed to demonstrate the relationship between TENG operation and dynamics (mechanical and gas dynamics). The P-TENG is fabricated using a three-dimensional (3D) printer, which provides the advantages of simple and lightweight fabrication. In addition, the aluminum foil and acrylic are used as the electrode and substrate, respectively. The P-TENG adopts the operating mode of a rotating-disk-based type and works in the noncontact mode to prevent abrasion of the surface and loss of rotation force by friction. Triboelectric and electrostatic effects occur between polytetrafluoroethylene (PTFE) on the propeller and the aluminum (Al) electrode and cause a flow of current at the external loading resistor.

1. INTRODUCTION In recent years, triboelectric nanogenerators (TENGs) have been widely used to harvest ambient mechanical power and turn it into electrical energy.1−10 A TENG can generate electrical energy using two major principles: contact electrification and electrostatic induction.11−14 TENGs have a lot of advantages, such as simple fabrication, low cost, a wide choice of materials, and environmental friendliness. Also, TENGs can harvest various types of energies by hybridizing with other concepts of energy-harvesting generators, such as electromagnetic, thermoelectric and peizoelectric nanogenerators, or by being combined other kind of TENGs to collect different types of mechanical energies at once.35−39 As a result, TENGs have attracted attention as a next-generation power source. TENGs can produce high output voltage, but the value of the output current from them is relatively low.15,16 However, TENGs that scavenge wind energy can produce a high output current because of their high frequency.34 Also, a rotating-diskbased TENG that can generate a high output current has been studied extensively.16−23 A rotating-disk-based TENG usually gathers mechanical energy produced from its rotational force by a motor or wind. In particular, the wind-driven TENG usually consists of a propeller connected to a rotator to convert the fluidic movement into the rotational energy.24−31 The interaction between the wind and the propeller causes a © 2019 American Chemical Society

Received: April 4, 2019 Accepted: June 19, 2019 Published: June 19, 2019 25170

DOI: 10.1021/acsami.9b05915 ACS Appl. Mater. Interfaces 2019, 11, 25170−25178

Research Article

ACS Applied Materials & Interfaces

Figure 1. Schematics of the propeller-TENG (P-TENG). (a) Structure of the P-TENG. (b) Gas flow and influence on the forward and following wings. (c) Working principle of the P-TENG. separated (electrode 1, electrode 2) by a 2 mm gap and covered with an Al foil. The main bodies consist of the exterior frame and axial pillar. The exterior frame (a diameter of 8.8 cm and a height of 3 cm) has an inlet hole and an outlet on the side and top, respectively. Thus, gas can be injected into the inlet and escaped to the upper surface of the P-TENG. The ABS axial pillar is inserted in the center of the propeller and serves as its central axis for constant rotational motion. The total length of the axial pillar was 28 mm, and three types of the axial pillars were produced to allow the distance between the stator and rotator to be adjusted from 1 to 3 mm. 2.2. Operation of the Propeller-TENG and Electrical Measurements. To operate the P-TENG, N2 gas was injected into the inlet at constant pressures of 0.1, 0.15, 0.2, and 0.25 MPa using a regulator and the wind speeds under the corresponding pressures were 6, 8.5, 11.5, and 13 m/s, respectively. An electrometer (Keithley 6514) was used to measure the open-circuit voltage and short-circuit current generated by the rotating P-TENG.

We conducted experiments in various conditions, varying the height and angle of the inlet and the number of wings on the propeller. The different electrical outputs from the various conditions were compared to find the optimal structure of the P-TENG. We also used a COMSOL simulation to calculate the pressure and stress loaded onto the propeller by the injected gas. The gas flow can be divided into four vertical and horizontal directions, and the rotational motion of the propeller can be accelerated or decelerated by each flow. The P-TENG optimized according to those analyses delivered a maximum open-circuit voltage of about 62 V and a maximum short-circuit current of 8.3 μA. The P-TENG thus generated a high output power density of 283.95 mW/m2, which allowed it to power 205 light-emitting diodes (LEDs) in real time or a commercial timer without any other power source. Also, the optimized P-TENG provided sustainable electrical energy to a self-powered wireless sensor system. This stored electrical energy generated by P-TENG enabled Bluetooth transmission and reception and drove the sensing, processing, and data transmission of a commercial temperature and humidity sensor. The investigation of P-TENG provides an in-depth and comprehensive analysis of the gas flow and maximizes energy conversion efficiency by optimizing the structure of the device.

3. RESULTS AND DISCUSSION As illustrated in Figure 1a, the P-TENG consists of an exterior frame, a propeller, an axial pillar, a polytetrafluoroethylene (PTFE) film, an electrode made of aluminum (Al), and an acrylic substrate. The exterior frame, propeller, and axial pillar of the P-TENG were 3D-printed acrylonitrile butadiene styrene (ABS) copolymer. A photograph of the P-TENG is provided in Figure S1. The P-TENG is composed of three main parts: a rotator, a stator, and a main body. The propeller, which is shaped similar to the letter “L” in the cross-sectional view, is the rotator. There are two planes that compose the propeller, one is perpendicular to the ground and the other is horizontal. The horizontal plane has a 200 μm thick layer of PTFE on the bottom to serve as a triboelectric layer. The stator consists of two electrodes (electrode 1, electrode 2) fully separated by a 2 mm gap. The main body is composed of the exterior frame and axial pillar. The inlet and outlet are on the side and top of the exterior frame, respectively, so that gas can be injected into the P-TENG via the inlet and easily escape through the outlet. The axial pillar plays the role of maintaining

2. EXPERIMENTAL SECTION 2.1. Fabrication of the Propeller-TENG. The P-TENG consists of three parts: a rotator, a stator, and a main body. The rotator and main body parts of P-TENG were 3D-printed acrylonitrile butadiene styrene (ABS) copolymer. The propeller (rotator), with a diameter of 7.8 cm, contains a plane extending vertically. This perpendicular plane, with a width of 3.2 cm and a height of 2 cm, converts the gas flow into rotational motion. At the bottom of the perpendicular plane on the propeller, there is a horizontal plane with a 200 μm thick layer of PTFE underneath. The propeller is fabricated using 4, 8, or 16 sectors, which have 2, 4, and 8 perpendicular planes (wings), respectively. The stator has two acrylic substrates with a thickness of 200 μm and a diameter of 8.8 cm. The two substrates are completely 25171

DOI: 10.1021/acsami.9b05915 ACS Appl. Mater. Interfaces 2019, 11, 25170−25178

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Figure 2. Electrical measurements of the P-TENG. (a) Short-circuit current (Isc), (b) open-circuit voltage (Voc), and (c) frequency of Isc under different gas injection pressures. (d) Isc, (e) Voc, and (f) frequency of Isc with different distances between the rotator and stator. (g) Isc, (h) Voc, and (i) frequency of Isc with different numbers of sectors.

flows from electrode 1 to electrode 2 until it reaches the electrical equilibrium state (Figure 1c-ii). Thereafter, the electrical potential difference between electrode 1 and electrode 2 reaches its maximum value when the PTFE is located exactly on electrode 2 (Figure 1c-iii). As the propeller keeps rotating, the overlapped area between the PTFE and electrode 2 decreases, and the following propeller wing begins to overlap with electrode 1. As a result, the positive charges move from electrode 2 to electrode 1, and the electrical potential of the Al electrodes decreases (Figure 1c-iv). To determine the condition for harvesting the maximum electrical output from the P-TENG, we changed the gas pressure at the inlet, the distance between the stator and the rotator, the number of sectors (propeller wings), and the height and angle of the inlet. First, we conducted pre-experiments to verify the effects of various kinds of gases on the electrical output, as shown in Figure S2. We injected three kinds of gases, oxygen (O2), nitrogen (N2), and carbon dioxide (CO2), into the circleshaped inlet of the P-TENG under constant pressure (from 0.1 to 0.25 MPa). When the injected gas collided with the propeller, the gas spread out in all directions, so the circular shaped inlet was chosen to increase the accuracy and to simplify the analysis. Also, the reason for using pure gases is that these kinds of gases occupy about 98% of air. (The composition of air in percent by volume, at sea level at 15 °C and 101 325 Pa, is as follows: nitrogen 78%, oxygen 20%, and carbon dioxide: 0.03%.) Regardless of the kind of gas, the P-

the distance between the propeller (rotator) and the Al electrode (stator) and acts as the central axis for the propeller, promoting its regular rotation. As shown in Figure 1b, the gas was injected through the inlet directly and collided with the plane of the forward propeller wing. Some of the gas escapes through the opened outlet, but the rest of the gas remains inside the P-TENG and indirectly influences the following propeller wing in the opposite direction. The propeller can rotate when the gas effect on the forward wing is larger than that on the following wing. For an accurate analysis, we divided the gas flow into four directions: up, down, left, and right (the inset of Figure 1b). The detailed working principle of the P-TENG is schematically depicted in Figure 1c. For simplicity, the process is demonstrated in two dimensions. The electrical energy generation mechanism of the P-TENG can be interpreted as two major processes: a precharging process between the PTFE and the Al electrode and an electrostatic induction process. The precharging process was performed by friction between the rotator and the stator for 30 s of each measurement. This initial contact electrification generates opposite charges on the surfaces of the PTFE and the Al electrode, as depicted in Figure 1c-i. We define the first electrode, at a position that fully overlaps with the PTFE, as electrode 1 and the electrode at the next position as electrode 2. As the propeller is rotated by the injected gas, the PTFE moves toward electrode 2, and the overlapping area between the PTFE and electrode 1 gradually decreases, causing electrostatic induction. The current thus 25172

DOI: 10.1021/acsami.9b05915 ACS Appl. Mater. Interfaces 2019, 11, 25170−25178

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ACS Applied Materials & Interfaces TENG generated the same amount of short-circuit current and open-circuit voltage. Thus, we confirmed that the electrical output of the P-TENG is independent of the type of gas because the P-TENG harvests energy only from the physical rotational force, not from a chemical reaction. Therefore, we conducted all of the following experiments with N2 gas because it has the lowest reactivity. The short-circuit current (Isc), the open-circuit voltage (Voc), and the frequency of the P-TENG were measured, as shown in Figure 2. As the pressure of the injected N2 gas increased from 0.1 to 0.25 MPa, Isc gradually increased from 1.8 to 8.3 μA, and Voc remained nearly constant at 62 V (Figure 2a,b). Also, the frequency of Isc increased to 222 Hz as the injection pressure increased, as shown in Figure 2c. Higher pressures caused higher rotational speeds, which increased the current according to the following equation,24 where ΔQsc is the quantity of the charge transferred between the PTFE and the Al foil. I=

ΔQ sc Δt

(1)The relationship between the distance of vertical separation and the electrical output was analyzed using the different lengths of axial pillars to maintain a distance of 1−3 mm. As shown in Figure 2d,e, Isc and Voc decreased by 4.8 μA and 34 V, respectively, as the vertical separation increased, decreasing more rapidly as the separation became larger. The frequency of the P-TENG is independent of changes in the vertical separation distance, but the amplitude of the electrical output was greatly reduced, as displayed in Figure 2f. An electrode with eight segments was paired with a propeller with four blades (wings), and we defined this propeller as the 8-sector propeller. The number of propeller blades is half the number of electrode segments (for example, the 16-sector propeller has 16 segments of the electrode, 8 segments of the propeller, and 8 blades as shown in Figure 1a). As shown in Figure 2g, the 4-, 8-, and 16-sector propellers generated the maximum Isc of 3.8, 5, and 8.3 μA, respectively. As the number of sectors increased, the rotating cycle time decreased and more current flow was produced. The measured maximum Voc values of the 4-, 8-, and 16-sector propellers were 61, 62.4, and 62 V, respectively. The frequency of the 16-sector propeller was 3.2 times higher than that of the 4-sector propeller, and the Isc waveform was similar to the sine waveform, as shown in Figure 2i. In contrast, a small vibration caused by unstable rotational motion was detected in the waveform of the 4-sector propeller. Because the 4-sector propeller has the lightest weight (7.25 g) and the largest empty area between the segments among the propellers (8-sector propeller: 10.85 g, 16-sector propeller: 16.87 g), it generates unstable rotational motion. As shown in Figure 3, we measured the electrical outputs from the 8-sector P-TENG to analyze the differences caused by varying the height of the inlet position, and COMSOL MULTIPHYSICS 5.0 software simulation was performed to provide further interpretation. As illustrated in Figure 3a, we set the inlet of the P-TENG at low, middle, and high positions in 1 mm intervals. The Isc was influenced by the height of the inlet position, but Voc remained constant at about 62 V (Figure 3b,c). When gas was injected in the low position at a pressure of 0.25 MPa, the Isc reached its greatest value of 5.5 μA. Isc decreased with the height of the inlet in relatively constant intervals. The propeller did not rotate when the inlet position was high and gas was injected at a pressure of 0.1 MPa. In the

Figure 3. Comprehensive analysis of the gas flow as the height of the inlet in the P-TENG increases. (a) Schematic illustrations of the gas flow with the inlet in the (i) low, (ii) middle, and (iii) high positions. (b) Isc and (c) Voc with the various inlet heights. The color profiles of the pressure applied to the forward wing of the propeller as determined using the COMSOL simulation tool: (d) perpendicular planes and (e) horizontal plane of an 8-sector P-TENG with the inlet in the (i) low, (ii) middle, and (iii) high positions.

low and middle positions, the injected gas can generate sufficient pressure on the propeller, but not in the high position. It will be demonstrated in more detail later in this paper with simulation results. The pressure applied to the perpendicular and horizontal planes by the injected gas was verified using COMSOL (Figure 3d,e). The model was constructed similar to the P-TENG, and the speed of the injected gas at the inlet was assumed to be 100 m/s and the outlet pressure was set to zero. The fluid properties of air were used as a setting by software (density ρ = 1.225 kg/m3, dynamic viscosity η = 1.983 × 10−5 P as under temperature T = 293.15 K). The laminar flow was applied as physics condition, and the model solved with the Navier−Stokes equations. Figure 3d shows the perpendicular planes of the forward wing, which receive different applied pressures depending on the height of the inlet. As the height of the inlet increases, the position of the red point also rises, indicating that the position of the highest pressure is rising. Because the injected gas collides with the perpendicular plane and diffuses into the periphery, an elliptical isoelectric line is observed. The 8-sector propeller has four wings, which means that the P-TENG can be divided into four zones based on the propeller blades, similar to the top view in Figure 4i (pink). Figure 3e shows the top view of the zone in which the gas is injected, i.e., the horizontal plane of the 8-sector P-TENG. The inlet is located in the middle of the circumference (the bottom left in 25173

DOI: 10.1021/acsami.9b05915 ACS Appl. Mater. Interfaces 2019, 11, 25170−25178

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3e-i, and the high pressure caused by the injection port is no longer observed. As shown in Table 1, when the inlet is positioned in the middle, the greatest forces of 291.59 and 104.28 μN are applied to the perpendicular and horizontal planes, respectively, but Isc is smaller than when the inlet is in the low position. This result can be explained by dividing the gas flow into two directions. The propeller of the P-TENG has an L shape in the crosssectional view. When the gas is injected through the inlet, it flows along the L-shaped propeller from the horizontal plane to the perpendicular plane. For example, with a low-position injection, the gas is sufficiently converted into rotational force because it all moves in the same direction toward the open outlet at the top, as shown in Figure 3a-i. However, the gas flow from the middle and high positions can be divided into two flows, one of which flows upward toward the outlet (Fu) and the other of which flows downward (Fd) after it hits the perpendicular plane of the forward wing. Because the downward flow cannot immediately escape from the PTENG, it gathers at the bottom and applies force to the following wing in a direction that disturbs the rotational motion. In the middle position, the injection produces similar amounts of upward and downward flows, and the two thrusts cancel each other (Figure 3a-ii). Thus, the rotation speed from a middle-position injection cannot be faster than that from a low-position injection. The high-position injection is shown in Figure 3a-iii. The injected gas immediately escapes through the outlet, so only a small amount of force is applied to the propeller. As a result, with the low and middle-position injections, the sum of effective flow of gas creates a sufficient amount of pressure on the propeller, but with a high-position injection, the propeller cannot convert wind to rotational energy. The injected gas must have at least 0.15 MPa pressure to rotate the propeller with the high-position injection. Because the P-TENG harvests the rotational energy generated by the propeller, it is important to investigate how the injected gas affects the propeller. Constantly injected gas exerts a constant pressure at a particular point on the propeller, generating a torque force that rotates the propeller. Figure 4 interprets the relationship between the electrical outputs of the P-TENG and the inlet angles using a theoretical analysis of the dynamics and a COMSOL simulation. Figure 4a shows the parameters used to analyze the relationship between the P-TENG and the inlet angle. The injected gas applies pressure to the perpendicular plane to generate torque force (τ), a physical quantity that specifies the effect of rotating an object and is the product of the force and the distance to the fulcrum, as shown below.

Figure 4. Comprehensive analysis of the gas flow as the inlet angles of the P-TENG vary. (a) Schematic illustration of the parameters used to analyze the relationship between the P-TENG and the inlet angle; the inset shows the parameter used to calculate the radius according to the rotation angle. (b) Top view of the P-TENG with various inlet angles. Isc of the (c) 4-sector, (d) 8-sector, and (e) 16-sector PTENGs with various inlet angles. The calculated radius according to the rotation angle of the (f) 4-sector, (g) 8-sector, and (h) 16-sector cases. (i) Color profiles of the pressure applied to the perpendicular planes of the propeller as determined using the COMSOL simulation tool: the forward wing, following wing, and top view of the P-TENG with a 45° inlet angle.

Figure 3e), and the forward wing and the following wing are located on the top and right sides, respectively. In Figure 3e-i, high pressure is shown at the circumference of the circle because the inlet is located near the horizontal plane. Except for that area, the pressure is exerted only in the upper region, close to where the forward wing is located. As shown Figure 3e-ii, when the inlet is positioned at the middle height, the pressure is applied to a larger area than in Figure

(2)

τ = r × F = Fr sin α

Here, F is the applied force, r is the distance from the center of a circle to the point of the applied force, and α is the degree of inclination of the inlet with respect to the line crossing the PTENG in the plane view. F is determined by the injection

Table 1. Results of the COMSOL Simulation: Different Applied Pressures and Forces Depending on the Height of the Inlet perpendicular plane minimum value of applied pressure (cPa) maximum value of applied pressure (cPa) integral of applied pressure (μN)

horizontal plane

low

middle

high

low

middle

high

4.873 35.855 220.56

6.384 52.434 291.59

3.594 45.118 157.14

4.26 × 10−6 50.434 126.7

6.66 × 10−6 24.764 104.28

2.46 × 10−6 5.485 43.45

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Figure 5. (a) Charging curves of a 0.1 μF capacitor by P-TENGs with 4, 8, and 16 sectors, with the angle chosen to generate the maximum electrical output from each sector; the inset shows the diagram of the rectifying circuit. (b) Load resistance dependency of the power density of the P-TENG and (c) increased power density after reactive ion etching (RIE); the inset shows the nanostructure on the PTFE surface. Digital snapshots of the P-TENG powering (d) 205 green LEDs and (e) a commercial timer. (f) Commercial temperature and humidity sensor and a smartphone in a self-powered wireless sensor system that transmitted data via Bluetooth connection.

pressure, and r changes constantly as the propeller rotates. For an ideal interpretation, the height of the inlet is fixed to the middle, and r is assumed to be the point on the plane that receives the greatest pressure. We conducted experiments with angles from 0 to 60° in PTENGs with 4-, 8-, and 16-sector propellers (Figure 4b). Figure 4c−e shows the measured Isc. The maximum output per angle varies depending on the number of sectors, but none of the propellers rotated when the inlet angle was zero. As shown in Figure 4c, when the inlet angle was 45°, the P-TENG generated the largest Isc. The Isc of the 4-sector propeller was slightly different from the others and saturated at 3.7 μA because as the injected pressure increased, both the torque force and the friction force between the propeller and axial pillar also increased. As shown in Figure 4e, the 16-sector propeller generated the largest Isc at an inlet angle of 60°, unlike the 4-sector and 8-sector propellers (Figure 4c,d). To analyze this phenomenon, a theoretical analysis and COMSOL simulation were performed. As explained earlier, the rotation of the P-TENG depends on various factors beyond the inlet angle. We focused on three of those factors: torque force, the number of propeller wings, and the gas flow.

The maximum current with a 16-sector propeller was generated when the inlet angle was 60°, whereas the maximum current with 4- and 8-sector propellers was generated at 45°. To analyze the reason why the optimized angle in the 16-sector case is different from 4- and 8-sector cases, the change in rotational force was theoretically calculated according to the angle of the inlet and the number of sectors. Because the propeller rotates continuously, the value of r changes in real time and it can be calculated using the parameters in Figure 4a. r′ =

r sin α (0° ≤ β ≤ 90° − α) sin(α + β)

r′ =

r sin α (90° − α ≤ β ≤ 90°, γ = π − α − β) sin(γ )

(3)

Here, r′ is the value of r changing with rotation and β is the rotated angle of the propeller. As shown in Figure 4f−h, r decreases until it reaches a certain rotated angle and then increases again. When the rotated angle is 0°, the radius varies according to the angle of the inlet, as shown in the COMSOL simulation result in Figure S3. The right side of the rectangular plane is the center axis of the P-TENG, and the left side is the boundary with the outside. As the inlet angle increases, the red 25175

DOI: 10.1021/acsami.9b05915 ACS Appl. Mater. Interfaces 2019, 11, 25170−25178

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ACS Applied Materials & Interfaces

shown in the inset of Figure 5a, and was charged by 4-, 8-, and 16-sector P-TENGs under 0.25 MPa pressure conditions. The charging characteristics of each P-TENG (with different numbers of sectors and angles) are shown in Figure S4: the 4-sector P-TENG had a maximum charging voltage of about 90 V, which was independent of the inlet angle. In the 4-sector P-TENG, the inlet angle affected only the time required to reach the maximum voltage; however, in the 8-sector and 16sector P-TENGs, the maximum charging voltage varied with the inlet angle. The 8-sector P-TENG had its maximum charging voltage at 45°, and the 16-sector P-TENG had its maximum charging voltage at 60° (Figure S4b,c). Figure 5a shows the overall ability of the P-TENGs to charge a 0.1 μF capacitor when the inlet angle was optimized to the number of sectors. As the number of sectors increased, the charging rate also increased, with the voltage of the capacitor reaching 103 V in 2.8 s with the 16-sector P-TENG, 102 V in 38.8 s with the 8sector P-TENG, and 97 V in 119.14 s with the 4-sector PTENG. Figure 5b presents the relationship between the output power density and load resistance. The output voltage of the PTENG with the 16-sector electrode and propeller and a working pressure of 0.25 MPa increased along with the load

area, which indicates the highest pressure, moves to the left. For an ideal interpretation, r is assumed to be the point indicated in red and is calculated by the following equation. r=4×

α 90°

(4)

The value of r′ calculated by eq 3 exceeds r (4 cm) when the angle of rotation is about 140, 114, or 84° or more when the inlet angle is 30, 45, or 60°. Therefore, when the propeller rotates beyond a certain angle, the injected gas cannot apply pressure on the propeller and hits only the exterior frame. In the case of the 4-sector propeller, there is a range of r′ that exceeds 4 cm at all inlet angles, and the case of the 8-sector propeller has a range that is unaffected by the injected gas when the angle of the inlet is 60°. However, as the number of sectors increases, the number of blades increases and the angle between the blades decreases. Thus, the angle between the forward wing and the following wing decreases from 180 to 90 to 45° in 4-, 8-, and 16-sector propellers, respectively. So, with the 16-sector propeller, in which the blades exist very close to each other, the following wing enters the range of influence before the forward wing has moved outside it, even when the inlet angle is 60°. The 16-sector propeller thus has a range in which r′ is always less than 4 cm at all inlet angles. Therefore, the r′ of the 16-sector propeller is repeated in a short section (from 0 to 45°). According to eq 2, the value of r directly affects the magnitude of the torque force. This is why the 16sector propeller produced the maximum current when the inlet angle was 60°, whereas the 4- and 8-sector propellers generated the maximum current when the inlet angle was 45°. We also analyzed the electrical output characteristics of the P-TENG using the gas flow, as shown in Figures 4i and S3. In the P-TENG, a small gap exists between the propeller and the exterior frame to maintain the frictionless rotation of the propeller. As shown in Figure S3, as the red area approaches the left side, some of the gas escapes through that gap. In this case, the flow of the injected gas can be divided into two phases in the horizontal direction: the direct effect on the forward wing (Fl) and the indirect effect on the following wing (Fr) as shown in Figure 4i (top view). First, the injected gas flows naturally to the center of the propeller after it collides with the forward wing, but then it leads to pressure on the following wing in a direction that disturbs the original rotation. Figure 4i shows the pressure applied to the forward and following wings and the top view with the 30° inlet angle. As the number of sectors increases, the volume of each divided zone (pink in the top view) decreases because the number of zones within the propeller blades increases. As a result, the 16sector propeller has 4 times less volume in each divided zone than the 4-sector propeller, so the adjacent wings of the propeller have more influence on each other. Therefore, the magnitude of the pressure delivered to the following wings increases along with the number of sectors. Consequently, a large inlet angle causes a greater torque to be applied to the propeller, but it also generates an undesirable escape flow through the gap between the propeller and the exterior frame (Fl). Also, a larger number of wings generates frequent rotation and increases the effect of the disturbing force between adjacent wings (Fr). The Isc trends thus vary depending on the inlet angle and the number of wings. As shown in Figure 5, the electrical performance of the PTENG with an optimized structure was investigated. The 0.1 μF capacitor received current through a rectifying circuit, as

V2

resistance. The output power was calculated as P = R , where V is the output voltage and R is the load resistance. The power density was calculated by dividing the maximum output power by the dimension of the electrode of the 16-sector propeller (4141.541 mm2). The largest instantaneous output power density of the P-TENG was 175.297 mW/m2 with an external load resistance of 1.5 MΩ. The output power density can be enhanced by forming inconstant nanostructures on the PTFE surface through reactive ion etching (RIE). In etching the PTFE, we used O2 gas with a flow ratio of 20 standard cubic centimeters per minute. The plasma was accelerated at a power of 20 W, and the total etching time was 180 s. The inset of Figure 5c shows a scanning electron microscope picture of the nanostructured PTFE surface after RIE, which increased the output power density to 283.95 mW/m2, a 61.98% increase. However, the maximum output power density after RIE remained 1.5 MΩ, suggesting that the modification of the surface structure did not affect the internal resistance of the PTENG. The nanostructures were not easily damaged by friction because the P-TENG operates in the noncontact mode. As shown in Figure S5, a scanning electron microscopy image was taken to compare the surface structures of the PTENG operating in the noncontact mode and contact mode. The surface durability experiment was conducted for 3000 cycles in each mode. In the noncontact mode, the treated PTFE surface was maintained over 3000 cycles, but in the contact mode, the surface nanostructures became crushed because of the increasing number of operating cycles. With its enhanced power output, the optimized P-TENG consecutively powered light-emitting diode (LED) bulbs and a commercial stopwatch in a demonstration of its energyharvesting ability. As shown in Figure 5d, the output power of the P-TENG could light 205 LEDs in real time without any storage component. Due to its high operating frequency of 222 Hz in the 16-sector P-TENG, the green LEDs were simultaneously and continuously illuminated (Video S1). Video S2 shows that 40 LEDs were lit up brightly by blowing through the P-TENG from a human mouth. Thus, the PTENG is well driven even at a small scale of flow energy. 25176

DOI: 10.1021/acsami.9b05915 ACS Appl. Mater. Interfaces 2019, 11, 25170−25178

Research Article

ACS Applied Materials & Interfaces As illustrated in Figure 5e, the P-TENG successfully powered a commercial timer. The P-TENG was connected in parallel with a power management circuit. After about 6.5 s, the voltage of the capacitor was charged to about 1.3 V, allowing the commercial timer to be driven in real time, as shown in Video S3 and Figure S6. Furthermore, to demonstrate that the P-TENG can effectively scavenge wind energy, a temperature and humidity sensor was connected to a self-powered wireless sensor system, as shown in Figure 5f. The P-TENG was connected in parallel with a power management circuit that included a rectifier, a resistor, a capacitor, a commercial temperature and humidity sensor, and an electrometer to measure the output voltage of the capacitor. The power generated by the P-TENG could be stored in the capacitor through the rectifying circuit. The capacitor was connected to the wireless sensor node to supply the power required for sensing, processing, and transmitting data. The temperature and humidity values acquired by the commercial sensor were processed by the signal processing unit, and then the processed signal was sent to a smartphone via a Bluetooth module (Video S4).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Homepage: http://ned.khu.ac. kr/.

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ORCID

CONCLUSIONS In summary, the deep gas flow analysis has demonstrated ways to improve the conversion efficiency of a rotating-disk-based PTENG. Experiments were divided into two parts: those to determine the electrical characteristics of the disk-based TENG and those to examine the gas flow within the P-TENG. We changed the pressure of the injected gas, the distance between the stator and rotator, the number of propeller wings, and the height and angle of the inlet. The experimental results show that gas flow directly increases the rotational force and indirectly decreases the rotational force on the propeller. We also used COMSOL simulations to analyze the pressure applied to the plane of the propeller, which increased the reliability of our study. Based on our analyses, the optimized structure of the P-TENG has a 16-sector propeller, a low inlet height, a 60° inlet angle, and PTFE with a nanostructured surface. The P-TENG with that optimized structure produced a high power density of 283.95 mW/m2, a 61.98% improvement from the nonoptimized version. The optimized P-TENG powered 205 LEDs in real time because of its high frequency of 222 Hz. It was separately able to activate a commercial timer without any other power source. Thus, our dynamic analysis of injected gas flow in a rotating-disk-based TENG allowed us to design and build a P-TENG that can be used as a highfrequency power source. In the final experiment, we showed that the optimized P-TENG can scavenge wind energy effectively enough to supply the electrical energy needed by a self-powered wireless sensor system. The harvested energy generated by the P-TENG was stored in a capacitor and used to drive a commercial temperature and humidity sensor and transmit the data to a smartphone via a Bluetooth connection.

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with different inlet angles; charging curves of a capacitor by the P-TENG with different inlet angles; scanning electron microscopy images of the PTFE surface of the P-TENG in the noncontact and contact modes; and output voltage of a capacitor used to drive a commercial timer (PDF) Simultaneous lighting of 205 green LEDs by P-TENG (Video S1) (AVI) Lighting of 40 green LEDs by a human mouth (Video S2) (AVI) Operating a commercial timer in real time (Video S3) (AVI) Acquiring and transmitting data from a commercial temperature sensor to a smartphone via a Bluetooth module (Video S4) (AVI)

Daewon Kim: 0000-0003-1246-5035 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2018R1A6A1A03025708). This work was also supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP; Ministry of Science, ICT & Future Planning) (No. NRF-2018R1C1B5045747).

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b05915. Photograph of the propeller triboelectric nanogenerator (P-TENG); electrical outputs of P-TENG operated using different kinds of gases; COMSOL simulation result: the pressure applied to the perpendicular planes 25177

DOI: 10.1021/acsami.9b05915 ACS Appl. Mater. Interfaces 2019, 11, 25170−25178

Research Article

ACS Applied Materials & Interfaces

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