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Piezoelectric Wind-Energy-Harvesting Device with Reed and Resonant Cavity Jun Ji, Fanrang Kong, Liangguo He, Qingchun Guan, and Zhihua Feng

Jpn. J. Appl. Phys. 49 (2010) 050204

# 2010 The Japan Society of Applied Physics

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Japanese Journal of Applied Physics 49 (2010) 050204

RAPID COMMUNICATION

Piezoelectric Wind-Energy-Harvesting Device with Reed and Resonant Cavity Jun Ji, Fanrang Kong, Liangguo He, Qingchun Guan, and Zhihua Feng Department of Precision Machinery and Precision Instrumentation, University of Science and Technology of China, Hefei, Anhui 230026, China Received October 9, 2009; accepted February 12, 2010; published online May 6, 2010 A wind-energy-harvesting device utilizing the principle of a harmonica was created. A reed in a resonant cavity vibrated efficiently with the blowing wind, and a piezoelectric element stuck on the reed generated electricity. The dimensions of the wind inlet were approximately 30 20 mm2 . The device was investigated with a wind speed ranging from 2.8 to 10 m/s. An output power of 0.5 – 4.5 mW was obtained with a matching load of 0.46 M. The energy conversion efficiency of the device could reach up to 2.4%. # 2010 The Japan Society of Applied Physics DOI: 10.1143/JJAP.49.050204

P

ortable and wireless devices are widely used nowadays. However, most of them are powered by traditional batteries, which have a limited lifespan. It is also troublesome to replace or recharge the batteries in some applications, such as sensors in remote or inaccessible places. Due to the newly developed technologies in lowpower electronics, wireless sensor nodes now require power consumption of only a few hundreds of microwatts or less. Thus, it is now possible to power portable and wireless devices by scavenging ambient power. Many research efforts have been focused on the possibility of harvesting ambient power, especially the conversion of the mechanical vibration energy around us to electrical energy.1–3) Recently, research works have also been focused on the possibility of converting wind energy to electricity.4–9) Priya and coworkers attempted to use windmills to harvest wind energy in 2005 and 2007.4,5) Both experiments utilized conventional fans whose shafts were connected to cam systems to make piezoelectric bimorphs vibrate, thus generating electricity. Weimer et al. developed an anemometer-based remote-area power-harvesting system, which utilized a windmill and an electromagnetic generator.6) Kamoji et al. studied a system using helical Savonius rotors with a high efficiency.7) While all the abovementioned studies utilized the rotation mechanism, other research studies used of only one reed-like device to scavenge wind energy.8) The reed-like device is simple, however, it appears inefficient. In this paper, we report a newly developed wind-energyharvesting device whose working principle is different from those of the devices utilizing the rotation mechanism introduced above. It is based on a reed-like device but with some auxiliary parts included. The outline of the device is shown in Fig. 1. The main body is a rectangular cavity made of structured steel resembling a part of a harmonica. A reed with a piezoelectric element was precisely enclosed by a rectangular aperture on one side of the rectangular cavity. One end of the reed was fixed to the cavity, while the other end could move up and down freely. The reed was placed on the inner surface of the cavity, and thus a wind valve was formed between the reed and the aperture (outlet). A small gap between the free end of the reed and the side steel sheet was preset. The width of the gap affects the characteristics of the wind-energy-harvesting device greatly, such as cut-in and cut-out wind speeds. The piezoelectric element was attached near the fastened end of the reed with

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Fig. 1. device.

(Color online) Outline drawing of the wind-energy-harvesting

epoxy adhesive DP460 (3M). When the wind blows towards the inlet, the reed vibrates, and the piezoelectric element is forced to generate alternative electricity. The dimensions of the reed and the piezoelectric element were optimized to maximize the amount of generated electricity.10,11) The process governing the formation of self-sustained oscillations is surprisingly complex.12,13) The resonator is the air column inside the cavity and is typically characterized by the linear wave equation. At the aperture where the reed is located, the relationship between airflow and pressure is simply nonlinear, which is a prerequisite for the formation of a self-sustained oscillation from a continuous supply of air. In this newly developed wind-energy-harvesting device, the only moving part is the reed, which does not cause any stick-slip friction. Thus, it is intended to have a long lifespan. Because of the resonant cavity, the reed can be excited easily and efficiently. This has been shown by various experiments, which will be introduced in the following paragraphs. The resonant vibration frequency of the reed could be determined mainly by the structure and mechanical parameters of the reed, when the cavity was heavy and sufficiently hard. The dimensions of the reed were 200 15 0:8 mm3 (L W H), and those of the piezoelectric element were 20 15 0:2 mm3 (L W H). The reed of the prototype device had a resonant frequency of about 24.5 Hz, which was designed to allow the device to work in a normal wind speed range. A lower frequency is suitable for a weaker wind, while a higher frequency is suitable for a stronger wind. The wind was generated by an electrical fan. The wind speed at the inlet of the cavity was controlled by adjusting the distance between the fan and the cavity. The wind speed was measured using an anemometer AZ-8918. The voltages

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Fig. 2. Equivalent circuit of the piezoelectric wind-energy-harvesting system.

generated by the piezoelectric element were monitored using Keithley 4200-SCS. As the impedance of the piezoelectric element was about 10 M, which is not negligible, a special method was adopted to minimize the effect of the probe linking the piezoelectric element to the 4200-SCS. The performance of the system was investigated by connecting a resistive load to the piezoelectric element. The equivalent circuit of the system is presented in Fig. 2.3) The voltage across the load can be expressed as 1 k Rp j!cp Vload ¼ Iq Rload ; 1 k R k R p load j!c p where Iq is the current source of the vibrating piezoelectric element [Iq ¼ dqðtÞ=dt], Cp the parallel capacitance of the vibrating piezoelectric element, Rp the parallel resistance of the vibrating piezoelectric element, and Rload the resistance of the load (including the input resistance of the probe linked to the 4200SCS). The following equivalent circuit parameters were obtained using a precision LCR meter (GW LCR-8101): Cp ¼ 14 nF and Rp ¼ 20:0 G at a frequency of 24.5 Hz, which is the resonant frequency of the reed. The average power delivered to the load can be calculated using the expression: P¼

2 Vload

2Rload

:

According to the equivalent circuit shown in Fig. 2, the power reaches the maximum value when the load is as follows: Rp opt : Rload ¼ 1 þ j!C R p p

Ropt load

Thus, the optimum can be determined as 0.46 M. This value is in good agreement with the experimental results shown in Fig. 3, which gives the relationships among the output power, load resistance, and wind speed. Figure 4 shows output power and efficiency as functions of wind speed, with an optimal load of 0.46 M. From the power-wind speed curve, the power almost increases with the wind speed. The cut-in wind speed (threshold of the wind speed required to start the reed to vibrate) is found to be 2.8 m/s, and the cut-out wind speed (the maximum wind speed above which the operation can harm the structure) is found to be 10 m/s. An output power of 0.5 – 4.0 mW can be expected when the wind speed is around 4.5 – 9 m/s. If the wind speed is very high, the structure of the device not only will be adversely affected, but also the output power will decrease.

Fig. 3. (Color online) The relationships between output power, the load resistance, and the wind speed.

Fig. 4. Output power and the energy conversion efficiency with a resistive load R ¼ 0:46 M.

To calculate the energy conversion efficiency, the input wind energy is determined by Pin ¼

1 Av3 ; 2

where v is the wind speed, the air density, and A the crosssectional area of the inlet. The input power is proportional to the cross-sectional area A and the cube of the wind speed v. The conversion efficiency () from wind energy to electric energy can easily be calculated. Theoretically, the maximum efficiency from a windmill mechanism is about 59%. The conversion efficiencies with a certain load (0.46 M) at various wind speeds were measured and are shown in Fig. 4. The conversion efficiency was high in a large wind speed range, and the maximum conversion efficiency could reach up to 2.4%, which is much larger than those in most small scale wind-energy-harvesting devices. In comparison, the small-scale windmill conversion efficiency was approximately 0.4%, which is deduced from the parameters indicated in the previously published paper.4) In conclusion, this study demonstrates a simple methodology for scavenging power from freely available wind on a small scale. The device utilizes a resonant cavity to increase the vibration amplitude of a reed with a piezoelectric element. As the device has no sliding parts, a long life-span can be expected. A small prototype setup can scavenge hundreds of microwatts from the surrounding environment to power portable and wireless devices, with an efficiency of 0.8 – 2.4%, which is much higher than that of a small windmill.

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