3.1. Structural Design and Working Principle
The CCR-TENG uses a structure in which the lift-type external blade and the drag-type internal blade are counter-rotating coaxially.
Figure 1a shows the force analysis of a cross-section of CCR-TENG, in which the external blade wind wheel axis is located on the radial line of the blade circle, and the blade moves along the wing path to a position on the upwind side. Assuming that the wind enters from the left side, the pink vector
represents the lift force acting on the blade, the black vector
represents the drag force acting on the blade, and the red vector
represents the resultant force of the lift force
and the drag force
. The resultant force generates a torque on the wind turbine axis, which causes the external blade to rotate counterclockwise and the internal blade to rotate clockwise. According to the theoretical analysis, the forces on different cross sections of the internal and external blades are different, but always produce sufficient moment to realize coaxial counter-rotation. Video S1 shows CCR-TENG continuously powering two 14 W fluorescent lamps at a fixed wind speed of 4 m/s and records the coaxial counter-rotation effect of CCR-TENG at 0.5 times recording speed. The low relative rotational speed of CCR-TENG at this wind speed and the significant rotation of both the internal and external blades make it easy to observe the coaxial counter-rotating effect. The internal and external blades are designed as streamlined rotation types to prevent the blades from becoming stuck and unable to start at certain angles of attack in
Figure 1b.
In this work, the coaxial counter-rotating structure is proposed to enhance the relative rotational speed of wind turbine blades during low-speed incoming wind. The sum of the rotational speeds of the internal and external blades determines the relative rotation speed of CCR-TENG. As shown in Equation (1), the resistance force
experienced by the blade rotation exhibits an exponential relationship with the linear velocity
, indicating that the lower the rotational speed, the smaller the resistance force. The coaxial counter-rotating structure allows CCR-TENG to achieve a higher relative rotation speed while experiencing a significantly lower combined resistance force on the internal and external blades than the resistance force generated by a single blade rotating at the same relative speed. Thus, when facing low-speed wind, CCR-TENG encounters less resistance, freeing up more energy to increase the rotational speed of the wind turbine blades and resulting in a higher relative speed. Compared to a single-blade design, the coaxial counter-rotating structure reduces the overall resistance of the device during rotation under the same wind conditions, leading to a higher relative speed. The above are the key advantages of using the coaxial counter-rotating structure.
where the drag force acting on the blades is represented by
,
denotes the dimensionless drag coefficient, the contact area between the air and the blades is represented by
, the density of the fluid (air) is represented by
, and
represents the linear velocity of the blade tip.
In this design, the CCR-TENG can be classified as in non-contact mode (NC-Mode) and partially soft-contact mode (SC-Mode). The friction materials and the conductive copper film are initially uncharged in NC-Mode. Moreover, during the rotation of NC-TENG under wind conditions, there is no frictional charging between the friction materials. Before the experiment, the friction materials are “pre-charged” by friction with an artificial polyester fur strip to achieve a saturated surface charge. However, the NC-Mode CCR-TENG experiences continuous charge decay during operation. To address this issue, the CCR-TENG employs a ternary dielectric soft-contact friction mechanism. As demonstrated in
Figure 1c, the upper friction interface consists of PTFE film and nylon film. At the same time, a soft brush serves as the triboelectric medium layer on the lower copper electrode, utilizing polyester fur material. As depicted in
Figure 1d, the NC-Mode CCR-TENG’s working principle involves charge generation, a transfer state, charge transfer and a stable state. When the polyester fur transitions from the initial state (
) to nylon, it comes into contact with and frictionally charges the nylon film. Due to the higher electronegativity of the polyester fur compared to the nylon film [
31], the friction between them causes the separation of positive and negative charges, resulting in the nylon film carrying positive charges. Still, the polyester fur has negative charges (
). As the turntable rotates, the polyester fur comes into frictional contact with the more electronegative PTFE film, generating new frictional charges (
), which, along with the existing electrons on the polyester fur, are transferred to the PTFE film. Until the next piece of nylon film contacts the polyester fur, this will cause the PTFE film to acquire more electrons (
), which is a crucial factor enabling the CCR-TENG to output high voltage [
32]. With the turntable’s rotation, the next piece of nylon film frictionally contacts the polyester fur and acquires positive charges through charge transfer (
). According to the electrostatic induction effect, the charges on the electrodes redistribute between the two sets of electrodes via an external load to balance the potential difference. After cycling (ii–v), the charges on the PTFE and nylon film surfaces reach saturation (
).
Figure 1.
Structural design of the CCR-TENG. (
a) Force analysis of a cross-section of the CCR-TENG; (
b) Structural diagram of the CCR-TENG; (
c) Schematic of SC-Mode of the CCR-TENG: (
i) structural diagram, (
ii) practical diagram; (
d) Charge transfer diagram in SC-Mode of the CCR-TENG (The “+” and “−” in the picture indicate positive and negative charge); Comparison of (
e) Startup wind speeds [
22,
33,
34,
35,
36,
37,
38,
39,
40]; (
f) Relative rotational speeds (at 5 m/s) [
22,
35,
37,
38,
39,
40]; and (
g) Peak power density per unit wind speed between the CCR-TENG and other wind harvesting TENGs [
22,
35,
36,
37,
38,
39,
40].
Figure 1.
Structural design of the CCR-TENG. (
a) Force analysis of a cross-section of the CCR-TENG; (
b) Structural diagram of the CCR-TENG; (
c) Schematic of SC-Mode of the CCR-TENG: (
i) structural diagram, (
ii) practical diagram; (
d) Charge transfer diagram in SC-Mode of the CCR-TENG (The “+” and “−” in the picture indicate positive and negative charge); Comparison of (
e) Startup wind speeds [
22,
33,
34,
35,
36,
37,
38,
39,
40]; (
f) Relative rotational speeds (at 5 m/s) [
22,
35,
37,
38,
39,
40]; and (
g) Peak power density per unit wind speed between the CCR-TENG and other wind harvesting TENGs [
22,
35,
36,
37,
38,
39,
40].
After frictional charging of the dielectric surface, air breakdown may occur between the two dielectrics due to the air gap. The maximum charge density is then present at the surface of the electrode [
33]:
According to the capacitance model of the freestanding triboelectric-layer mode of the TENG [
34], the output short-circuit current of the CCR-TENG can be expressed as:
Here, represents the potential difference between the dielectrics, is the maximum charge density on the electrode surface, denotes the vacuum permittivity constant and denotes the electrode total area, is the air gap, fixed at 3 mm. and represent the effective contact area and surface charge density.
When charge transfer reaches saturation, an increase in wind speed boosts the turbine’s rotation speed and shortens the duration of each charge transfer cycle while the contact area remains unchanged. It implies that CCR-TENG achieves higher short-circuit current with increasing rotation speed at charge transfer saturation, but voltage and charge are unaffected by frequency. The unique design of the coaxial counter-rotating structure enhances the TENG’s low-speed startup performance, rotating performance and electrical output. To better exhibit the performance of the CCR-TENG, the startup wind speed, rotational speed, and power density per wind speed unit are compared with related designs in
Figure 1e–g. The CCR-TENG is superior in all three comparisons, and the detailed parameters are recorded in
Table S1. In
Figure 1e, startup wind speeds in references range from 1 to 4.5 m/s, and the startup wind speed for the CCR-TENG with NC-Mode is only 1 m/s [
22,
35,
36,
37,
38,
39,
40]. As depicted in
Figure 1f, at a wind speed of 5 m/s, the CCR-TENG achieves a rotational speed of 420 rpm with SC-Mode, while none of the references exceed 250 rpm [
22,
35,
37,
38,
39,
40]. In
Figure 1g, normalized power density at different wind speeds is compared with previous works, showing that the power density per wind speed of the CCR-TENG is 746 mW/m
3·s/m, which is more than three times the highest value in the references [
22,
35,
36,
37,
38,
39,
40]. In the comparison, the CCR-TENG has the best performance of startup wind speed, rotation speed, and electrical output performance.
3.2. Blade Selection and Structural Optimization
For better startup and rotation performance of the TENG, a combination of S1046 airfoil-type external blades and traditional Savonius drag-type internal blades was chosen to fabricate the CCR-TENG [
41,
42]. Considering manufacturing cost and wind energy utilization efficiency, a three-blade structure was chosen for the airfoil-type knives. At the same time, the traditional two-blade design was selected for the drag-type blades. The blade selection and structural optimization process of the wind turbine of the CCR-TENG are investigated and shown in
Figure 2.
Figure 2a analyzes the rotational speed variation after combining the internal and external blades of the CCR-TENG. The relative rotational speed of the coaxial counter-rotating blades is higher than that of the individual internal and external blades but significantly lower than the sum of their individual rotational speeds. The results demonstrate a significant interaction between the internal and external blades during operation in the coaxial counter-rotating mode. This interaction needs to be reduced to optimize performance, as it is not feasible to arbitrarily combine lift and drag blades. Further experimental research is needed to explore the structural optimization of the CCR-TENG to leverage its advantages fully.
Figure S1a further demonstrates the rotational speed of the internal and external blades during coaxial counter-rotating. After combination, the internal blades exhibit better low-speed startup performance, achieving low-speed start at 1 m/s wind velocity, while the external blades startup and achieve coaxial counter-rotating only after 3 m/s. At wind velocities exceeding 6 m/s, the rotational speed of the external blades surpasses that of the internal blades, indicating superior rotational performance of the external blades under higher wind velocities.
For single-blade TENG, the blade’s chord length is considered a crucial factor in enhancing the startup performance of the external blade. An increase in the chord length increases the blade thickness and static moment coefficient, making it easier for the wind turbine to achieve low-speed startup [
43]. External blades with chord lengths of 8 cm, 9 cm, and 10 cm were separately produced and combined with internal blades to form CCR-TENGs of different specifications. The relative rotation speeds of the CCR-TENGs were measured, and the experimental results in
Figure 2b indicate that the CCR-TENG achieves the highest relative rotation speed when the external blade chord length is 9 cm.
Figure S1b,c show the rotation speeds of the external and internal blades of CCR-TENGs with different chord lengths under various wind conditions. Increasing the chord length of the external blade reduces the startup wind speed and increases its rotation speed. Still, it may also reduce the direct wind-receiving area of the internal blade, significantly decreasing its rotation speed. When the external blade chord length is 9 cm, the CCR-TENG achieves the highest relative rotation speed.
Figure 2c illustrates the relative rotational speeds of the CCR-TENG with an external blade chord length of 9 cm and the distribution of speeds between the internal and external blades. The internal blade can start rotating at 1 m/s and achieves a speed of 33 rpm, while the external blade starts spinning at 3 m/s and reaches a speed of 45 rpm. Beyond 4 m/s, the rotational speed of the external blade exceeds that of the internal blade. The internal blades are optimized to mitigate the mutual influence of the internal and external blades during coaxial counter-rotating and increase the relative rotation speed.
As shown in
Figure 2d–f, the optimized internal blade and the 9 cm chord length external blade formed CCR-TENGs of different specifications. There are 16 sets of CCR-TENGs with different structures. The CCR-TENG, where the external blade chord length is 9 cm and the internal blade placement is not optimized, is defined as the original CCR-TENG, corresponding to the cases of
Figure 2d–f, where the change in the horizontal coordinate is zero. As shown in
Figure S1d, the internal blades are made with external reduction.
Figure 2d demonstrates the relationship between the relative rotation speed of the CCR-TENGs with different external ratio of reduction and wind speed. The relative rotation speed with external ratio reduction shows a pattern of initial ascent followed by a decline, with the highest relative rotation speed at an external reduction ratio of 0.22. At 9 m/s, the rotation speed increased from 1440 rpm (no reduction) to 1650 rpm. External reduction reduces the moment of inertia of the internal blade, resulting in lower resistance under the same wind conditions and demonstrating higher rotation speed. At the same time, the mutual influence between the internal and external blades is reduced. As shown in
Figure S1e, the external blade rotational speed of the CCR-TENG after the external reduction is improved compared with that of the original CCR-TENG external blade and increases with the increase of the retraction ratio.
As shown in
Figure S1f, the internal blade rotational speed of the CCR-TENG increases and then decreases with the rise of the retraction ratio after the external reduction. At the time of retraction 0.28, the external blade rotational speed is lower than that of the original CCR-TENG external blade, mainly due to the external blade’s rapidly increasing rotation speed affecting the internal blade’s rotation. Researchers generally consider a blade overlap ratio of 0.15–0.20 optimal for the static startup and power performance of Savonius blades [
44,
45].
Figure S1g shows the demonstration of internal blade overlap treatment.
Figure S1h,i show the variation in the rotation speed of the internal and external blades of the CCR-TENG under different wind conditions when an overlap ratio exists. The rotation speed of the internal blade shows a pattern of initial ascent followed by a decline with the increase in the overlap ratio, with the most significant high at 0.18. However, the rotation speed of the external blade decreases with an increasing overlap ratio.
Figure 2e demonstrates the relative rotation speed of the CCR-TENGs with different overlap ratio as a function of wind speed. The CCR-TENG exhibits the optimal startup and rotation performance when the overlap ratio is 0.18. Under a wind speed of 1 m/s, CCR-TENG can start and further increase its rotational speed by 17 rpm. Moreover, it enhances the relative rotation speed to reach a maximum speed of 1500 rpm at a wind speed of 9 m/s.
Figure 2f investigates the effect of external reduction and internal overlap of the same size on the rotation speed of the CCR-TENG. Under a wind condition of 9 m/s, the relative rotation speed of the CCR-TENG with 2 cm external reduction and internal overlap is 1580 rpm, higher than the relative rotation speed of the original CCR-TENG (1440 rpm) but lower than that of CCR-TENG with an external reduction ratio of 0.22 (1650 rpm).
Among the four structures, external reduction, internal overlap, external reduction with internal overlap, and the original CCR-TENG, the external reduction ratio of 0.22 exhibits the highest relative rotation speed, as shown in
Figure 2g.
Figure 2h demonstrates the relative rotation speed of the CCR-TENG with an external reduction ratio of 0.22 and the ratio of the rotation speeds of the internal and external blades during coaxial counter-rotating as a function of wind speed. The internal blade can start at 1 m/s wind speed, while the external blade starts rotating at 3 m/s and achieves coaxial counter-rotating. After 3 m/s, the rotation speed of the external blade exceeds that of the internal blade. Since the results are obtained in NC-Mode, the non-connected CCR-TENG with an external reduction ratio of 0.22 is called the NC-Mode CCR-TENG, and this will be used for subsequent electrical energy testing in NC-Mode. It transitions into SC-Mode by attaching one piece of polyester fur strip to the rotating turntable of the NC-Mode CCR-TENG.
Figure 2i shows soft-contact friction structure selection for the above 16 CCR-TENGs at 8 m/s wind speed. Among the different CCR-TENGs, the performance of the CCR-TENG with an external reduction ratio of 0.22 is optimal. Therefore, this CCR-TENG, which undergoes triboelectric friction with a ternary dielectric and has an external reduction ratio of 0.22, is called the soft-contact mode (SC-Mode) CCR-TENG. The structure will be used for subsequent electrical energy testing in SC-Mode. The original TENG consists of the non-optimized internal blade and the external turntable of an unassembled S1046 airfoil-type external blade, and this structure will be used in the comparison to derive the NC-mode performance improvement ratio of the CCR-TENG.
3.3. Electrical Output of the NC-Mode CCR-TENG
The impact of the coaxial counter-rotating structure was assessed by the electrical output of the CCR-TENG in NC-Mode within a low-speed wind tunnel.
Figure 3a shows the structure of the NC-Mode CCR-TENG with an air gap of 3 mm.
Figure 3b presents an electrical potential simulation of the NC-Mode CCR-TENG, indicating an estimated potential difference of approximately 2.2 × 10
3 V between adjacent electrodes.
Figure S2 shows the simulation of charge density for the NC-Mode CCR-TENG. The simulation results suggest that the charge density is approximately 13.5 × 10
−6 C/m
2. Both of the above results are simulation results based on experimental conditions.
Figure 3c–e illustrate the relationships between
,
,
, and wind speed for the NC-Mode CCR-TENG.
increases with increasing wind speed.
and
exhibit similar trends, initially improving and stabilizing with higher wind speeds, and reaching stability at 3 m/s inlet wind speed. The maximum induced charge density and the corresponding maximum potential difference occur at 3 m/s,
, stabilizing at 2.2 kV, in
Figure 3d, and
at 0.28 µC in
Figure 3e. The contact area remains constant, while increasing the wind speed shortens each charge transfer cycle. As a result,
continues to grow with higher wind speeds, reaching 50 µA at 9 m/s for the CCR-TENG, as shown in
Figure 3c.
Figure 3f compares short-circuit current output of the NC-Mode original TENG and the NC-Mode CCR-TENG at different wind speeds. At 9 m/s wind speed, the NC-Mode CCR-TENG achieves an
of 50 µA, while the NC-Mode original TENG only reaches 35 µA, the coaxial counter-rotating structure of the CCR-TENG enhances short-circuit current output by 42.9%. It is found that short-circuit current of the CCR-TENG only became noticeably higher than the original TENG when the wind speed exceeded 3 m/s. This conclusion is because, at wind speeds below 3 m/s, both devices only rotate at low speeds, resulting in a relatively small short-circuit current. Even though the coaxial reverse structure can enhance the relative rotation speed of the TENG, its effect is limited when the wind speed is below 3 m/s.
Figure 3g compares short-circuit current output before and after the parallel connection of two TENGs in the CCR-TENG. Due to device fabrication limitations, the parallel CCR-TENG exhibits a slightly lower
than the twice short-circuit current of the single-blade CCR-TENG, with a maximum of 0.098 mA.
Figure 3h shows the voltage changes with wind speed before and after the parallel connection of two TENGs in the CCR-TENG, with no change in voltage values. It can be concluded that the NC-Mode CCR-TENG exhibits good startup and rotation performance, especially as it can start at an ultra-low wind speed of 1 m/s. However, during the electrical output experiments, it was observed that the friction charges of the NC-Mode CCR-TENG tend to decay easily, resulting in an unstable electrical output.
3.4. Electrical Output of SC-Mode CCR-TENG
During experiments, changes in electrical performance are observed for the NC-Mode CCR-TENG due to charge decay. In contrast, the SC-Mode CCR-TENG is expected to have a constant electrical performance when exposed to a fixed wind speed. Experiments were conducted in a low-speed wind tunnel to attain the electrical output of the SC-Mode CCR-TENG.
Figure 4 investigates the electrical output performance of CCR-TENG with different influencing factors.
Figure 4a indicates that CCR-TENG outfitted with one polyester fur strip exhibits improved startup and rotational performance.
Figure S3a,b further demonstrate that an increment in the number of polyester fur strips significantly impedes the ability of both the internal and external blades to rapidly increase their rotational speed within the wind velocity range of 3–5 m/s, suggesting that a larger quantity of polyester fur strips diminishes the CCR-TENG’s rotational efficacy and impedes the startup. The evidence from both figures consistently supports the superior startup and rotational behavior of the CCR-TENG when one polyester fur strip strand is employed.
Figure S3c compares the variations in electrical short-circuit current output of the CCR-TENGs with varying numbers of polyester fur strips under different wind speeds. One polyester fur strip generates higher short-circuit current output, up to 56 µA. The SC-Mode CCR-TENG with polyester fur strip has subsequently been called the optimal soft-contact mode (OS-Mode) CCR-TENG. The OS-Mode CCR-TENG will be used to conduct the subsequent electrical performance experiments.
Figure 4b–d depict the relationships of
,
, and
with wind speed for the OS-Mode CCR-TENG.
increases with wind speed, while
and
increase up to a wind speed of 4 m/s and remain essentially constant thereafter. At the maximum experimental wind speed of 9 m/s, the OS-Mode CCR-TENG achieves
of 56 µA,
of 4.9 kV, and
of 0.33 µC. Adding a polyester fur strip to the original TENG and employing ternary dielectric soft-contact friction is called the original SC-Mode TENG.
Figure 4e shows the variation of short-circuit current output with wind speed for the OS-Mode CCR-TENG versus the original SC-Mode TENG. At a wind speed of 9 m/s, the
of the original SC-Mode TENG is 39 µA, while the
of the OS-Mode CCR-TENG is 56 µA, which is an improvement of 41%. The OS-Mode CCR-TENG has a higher short-circuit current output but lower startup performance than the original SC-Mode TENG below 4 m/s wind speed. Still, due to the addition of polyester fur, it can only be initiated at wind speeds of 3 m/s and above.
Figure 4f compares short-circuit current variation with wind speed before and after paralleling the two TENGs at both ends of the OS-Mode CCR-TENG. The
of the CCR-TENG after the paralleling was connected almost doubles compared with the
of the single-side OS-Mode CCR-TENG, which can be up to 0.11 mA.
Figure 4g shows the comparison of the transferred charge before and after the paralleling of the TENG at both ends of the OS-Mode CCR-TENG, and the
of the CCR-TENG after paralleling can reach 0.64 µC.
To systematically investigate the power output of the SC-Mode CCR-TENG, an external high-voltage resistor bar is used as a load resistance to analyze and compare the power output of the OS-Mode CCR-TENG. The peak power of the SC-Mode CCR-TENG at a wind speed of 9 m/s is obtained. The voltage and short-circuit current measured under variable external resistance (10 MΩ to 475 MΩ) are shown in
Figure S4. The short-circuit current decreases with increasing resistance, while the voltage shows the opposite trend. The power increases in the low-resistance stage and falls in the high-resistance stage. At around 40 MΩ, the maximum power is 256 mW, the peak surface power density is 6.2 W/m
2. The peak volumetric power density per unit wind speed is 746 mW/m
3·s/m, as shown in
Table S1. At the same time, plotting the correlation between root mean square current density (J
rms) current density and power is necessary to prevent the effect of peak current on the CCR-TENG power output plot [
46]. The J
rms and power of the OS-Mode CCR-TENG with different load resistances (2 MΩ to 225 MΩ) are shown in
Figure 4h at 9 m/s wind speed. The variation of short-circuit current with wind speed for the NC-Mode CCR-TENG and the OS-Mode CCR-TENG is shown in
Figure 4i. Incorporating polyester fur increases the rotational resistance of the CCR-TENG, resulting in the OS-Mode CCR-TENG being able to startup when the wind speed reaches 3 m/s. Additionally, in the wind speed range of 3–6 m/s, it does not exhibit a higher electrical short-circuit current output than the NC-Mode CCR-TENG. With the increase in wind speed, the OS-Mode CCR-TENG gains more energy to counteract the friction from the polyester fur, progressively showing a higher electrical short-circuit current output than its NC-Mode counterpart when wind speed reaches 6 m/s. It means that each of the two modes of the CCR-TENG has advantages, the NC-Mode CCR-TENG is more suitable for collecting energy from ultra-low wind speeds than the SC-Mode CCR-TENG. Stable output is a requirement for the CCR-TENG to realize the in-situ energy supply of distributed WSNs, and a long-term durability experiment can better evaluate the durability of the TENG [
47]. As shown in
Figure S5, the durability test of the OS-Mode CCR-TENG is conducted under 4 m/s wind speed, and the output remains unchanged after 750 k rotation cycles.