# Research Progress in Fluid Energy Collection Based on Friction Nanogenerators
## 2 Recent Developments in Triboelectric Nanogenerator-Based Fluid Energy Harvesting Systems
### Wind Energy Collection
#### 2.1.1 Rotating Structure AC Output Device  
The conventional meteorological wind speed sensor assembly achieves high flexibility by using several revolving wind cups and rotational sensor architectures. Utilizing high-efficiency impellers or wind cups for the rotational drive, nanogenerator energy harvesting devices are often used in low-wind situations. This conventional rotating construction could deliver outstanding energy production. [PERSON] et al. proposed a high-performance triboelectric-electromagnetic hybrid nanogenerator (RS-HG) with a rotating structure [31]. The application setup is shown in Figure 3a, the structure is shown in Figure 3b, and the RS-HG structure consists of transverse TENG (L-TENG), top TENG (T-TENG), and bottom electromagnetic generator (EMG). Increasing the bandwidth of the collected wind velocity increases the application of TENG. As shown in Figure 3c, [PERSON] et al. reported a coaxial structure with a dual-axis D-TENG. The D-TENG has two cups with different shapes and arm lengths, and the two different cups are mounted on a coaxial shaft to complement each other's rotational characteristics and harvest energy at ultra-wideband wind speeds of 2.2-16 m/s [32].  
[PERSON] et al. reported a wind energy harvesting device that combines electromagnetic induction generator EMG and sliding friction nanogenerator TENG with the structure shown in Figure 3d [33]. When the wind speed is 9 m/s, the maximum average power outputs that can be achieved are 0.33 and 32.87 mW, respectively. Overhead power lines are often disturbed by wind direction, posing a serious potential risk factor. As shown in Figure 3e, [PERSON] et al. proposed a non-contact WM-TENG, which consists of T-TENG and B-TENG. The T-TENG is driven by a turbine, while the B-TENG located at the bottom is driven by a wind vane [34]. The wind sensor on the hot air balloon is an important part of the hot air balloon safety system. Traditional wind sensors on hot-air balloons are powered by lithium batteries, which have unsatisfactory reliability. [PERSON] et al. proposed a self-powered positioning system for hot air balloons based on a multi-module triboelectric nanogenerator (MM-TENG) as shown in Figure 3f. The structure is cylindrical, with a double-layer cylindrical soft-contact triboelectric nanogenerator (DLCSC-TENG) on the side of the cylinder (Figure 3g) and a disc-shaped triboelectric nanogenerator (DS-TENG) on the underside of the cylinder (Figure 3h). The wind speed threshold of DLCSC-TENG is reduced to 0-6 m/s, and eight regular directions can be monitored through the output voltage waveform [35]. The main theme of these studies is focused on using wind cups or blades with sufficient strength and moment of inertia, and a certain direction of incident airflow to drive the rotor of energy harvesting equipment, driving the friction interface layer and generating AC current output of traditional TENGs. Table 1 summarizes the performance of some wind energy collection AC output devices in recent years.