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Feasibility Study on a New Energy Harvesting Electromagnetic Device Using Aerodynamic Instability Hyung-Jo Jung, Seung-Woo Lee, and Dong-Doo Jang Civil and Environmental Engineering Department, KAIST, Daejeon 305-701, South Korea Energy harvesting systems convert ambient energy from environment such as vibration, sunlight, wind, temperature gradient, etc. into electrical energy. Among several ambient energy sources, wind energy can be considered as one of the most promising sources because of its attractive features such as efficiency and economic merit. However, if an ordinary type of wind turbine is used for providing the electricity to low-power equipments (e.g., light poles, wireless sensors for structural health monitoring, etc.), it might be too inefficient and too costly. Recently, on the other hand, alternative (or innovative) approaches for wind power systems have been investigated by focusing on the aerodynamic instability phenomena such as galloping, flutter and vortex shedding. This paper first proposes a new energy harvesting system using wake galloping. To this end, the energy harvesting system based on wake galloping is designed and manufactured. And then, a series of wind tunnel tests are carried out in order to validate the efficiency and effectiveness of the proposed energy harvesting device. From these tests, the applicability of the proposed energy harvesting system using aerodynamic instability (i.e., wake galloping) is experimentally verified. Therefore, it can be an efficient energy harvesting system. Moreover, it can be used as an alternative energy source for low-power equipment, resulting in much simpler structural health monitoring systems without batteries for wireless sensors. Index Terms—Aerodynamic instability, electromagnetic induction, energy harvesting, wake galloping, wind energy.
I. INTRODUCTION
E
NERGY harvesting systems, which have recently received much interest due to energy crisis and concerns on environmental issues, convert ambient energy from environment such as vibration, sunlight, wind, temperature gradient, etc. into electrical energy [1]–[3]. Among several ambient energy sources, wind energy can be considered as one of the most promising sources because of its attractive features such as efficiency and economic merit compared to solar energy. Especially, small wind power systems can be used to run low-power equipments (e.g., light poles, wireless sensors for structural health monitoring, etc.,), resulting in stand-alone systems (no need for connection to electric distribution system or grid). However, if an ordinary type of wind turbine is used for this purpose, it might be too inefficient and too costly. Instead of using conventional geared, rotating airfoils to pull energy from the wind, alternative (or innovative) approaches for wind power systems have been investigated by focusing on the aerodynamic instability phenomena such as galloping, flutter and vortex induced vibration. Matsumoto et al. [4] and Frayne [5], [6] independently developed power generation systems using the divergent oscillations such as flutter, galloping, and coupling flutter which have such an energy that they can destroy a bridge (e.g., Tacoma Narrow Bridge failure in 1940). The aerodynamic instability phenomena should be controlled and driven to a predictable motion to provide mechanical power supply for running machinery and electricity generators. However, it is not easy to control divergent oscillations because their amplitudes may increase drastically according to an increase of Manuscript received March 07, 2009; revised April 23, 2009. Current version published September 18, 2009. Corresponding author: H.-J. Jung (e-mail:
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2009.2024769
wind speed. Therefore, harvesting energy steadily in the condition of variable wind speed using divergent oscillations is too difficult. On the other hand, vortex induced vibration may occur at relatively low wind speed. However, it occurs only within a very narrow range of wind speed along with relatively small amplitude. Therefore, an energy harvesting system using vortex induced vibration may be inefficient. According to previous researches [7], [8] and our preliminary wind tunnel tests, a different type of aerodynamic instability phenomenon, that is, wake galloping, which is caused by the wake interference between two circular cylinders, may be more appropriate than other phenomena such as vortex induced vibration, galloping and flutter. Wake galloping has a wide range of wind speed with large but finite amplitude. Therefore, it can be more easily controlled compared to flutter and galloping and its effectiveness may be much better than that of vortex induced vibration. This paper first proposes an energy harvesting system using wake galloping phenomenon of structures under wind load. To this end, the energy harvesting system based on wake galloping is designed and manufactured. And then, a series of the wind tunnel tests are carried out in order to experimentally verify the efficiency and effectiveness of the proposed energy harvesting device. II. CHARACTERISTICS OF WAKE GALLOPING When a bluff body is set up in a flow, the flow field around body generates the flow-induced forces and these forces excite the flow-induced vibrations. Moreover, this vibration of the body can change the flow field, and then modified forces are generated. These modified forces can change the body response again. Finally, stationary response is produced [9]. The various aerodynamic phenomena are briefly listed in Table I. In this study, wake galloping is considered to be more appropriate than other phenomena such as vortex induced vibration, galloping and flutter for energy harvesting system. According to preceding researches [7], [11] and our preliminary wind tunnel
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TABLE I AERODYNAMIC PHENOMENA OF BLUFF BODIES [10]
tests, the characteristics of wake galloping phenomenon are as follows. 1) Wake galloping may occur at critical wind speed propor: mass per tional to Scruton number length, : critical damping ratio, : air density, : cylinder diameter), and it remains within wide range of wind speed, but it may disappear at high wind speed due to change of : wind speed, : visReynolds number cosity of air). 2) The amplitude of wake galloping may vary depending on such conditions as structural properties, flow properties, wind directions, etc. It generally becomes larger as winds get faster, but it is considered that the maximum amplitude of wake galloping cannot exceed three times of a cylinder diameter because the instability caused by the wake interference decreases when the relative angle for wind of the parallel cylinder increases. 3) The spacing of the cylinders is one of the most important parameters which govern the characteristics of wake galloping and it can be said that wake galloping occurs remarkably between 2-D to 6D spacing. In the case of about 3-D spacing of the cylinders, a drastic increase of vibration may occur. In the case of about 5D spacing, on the other hand, the amplitude of vibration increases gradually.
Fig. 1. Experimental setup.
Fig. 2. Equivalent circuit and energy transformation process.
TABLE II MODEL DATA
III. NEW ENERGY HARVESTING ELECTROMAGNETIC DEVICE A. Design and Composition of Device Based on the characteristics of wake galloping investigated in the previous chapter, a new energy harvesting system shown in Fig. 1 is first designed and manufactured. And then, a series of the wind tunnel tests are carried out in order to experimentally validate the efficiency and effectiveness of the proposed energy harvesting system. Fig. 2 describes its equivalent circuit and the energy transformation process. As seen from the figures, a permanent magnet on the end of the vibration-cylinder is working as a linear rotor in a solenoid coil to generate the electricity. Table II demonstrates the experimental setup for the test and the detailed data of the model used. The main specifications of wind tunnel used for the test are as follows. • Test section: 1 m(width) 1.5 m(height) 6 m(length) • Range of test wind speed: 0.3–22.5 m/s The spacing of the model is selected as 5D in order that the amplitude of vibration may grow gradually. The mass of the model and the damping ratio of the system is set up as low as possible. Permanent magnets are attached at both ends of the model (Neodymium, 10 mm 25 mm 3 EA) as a part of the electromagnetic energy harvesting device.
Fig. 3. Displacement response of the proposed device (without solenoid coil).
Fig. 3 represents the experimental test results. As seen from the figure, wake galloping occurs under wind speed of 0.5 m/s and the peak displacement of vibration is 8 cm (i.e., 1.6D) at wind speed of 10 m/s. The proposed system has the following properties as intended: (1) a very low onset wind speed of vibration and (2) a wide range of wind speed with large but a finite amplitude. Therefore, it can be more easily controlled compared to flutter and galloping. Also, its effectiveness may be much better than that of vortex induced vibration. These properties enable this device to convert wind energy to electrical energy stably. B. Effect of Solenoid Coils In order to convert wind energy to electrical energy using the device manufactured at the previous section, solenoid coils should be equipped around permanent magnets. The detailed
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TABLE III SOLENOID COIL DATA
Fig. 6. Generated power response according to solenoid coils.
Fig. 4. Displacement response of the proposed device.
Fig. 7. Displacement response according to natural frequencies of the device.
is sensitive (see Fig. 5). Therefore, the configuration of the solenoid coil (e.g., coil diameter, number of turns, etc.) is not one of the key parameters to maximize the efficiency of the proposed energy harvesting device. Fig. 5. Induced voltage response according to solenoid coils.
data of three different solenoid coils used in this test are shown in Table III. A series of the wind tunnel tests are carried out in order to investigate the characteristics of vibration, induced voltage, and generated electric power with respect to various solenoid coils. Experimental test results are shown in Figs. 4 to 6. It is demonstrated from Fig. 4 that an increase of the damping properties is not noticeable, so that there are little differences in the displacement responses before and after the attachment of solenoid coils. According to Faraday’s law, the induced electromotive force in a coil is equal to the negative of the rate of change of magnetic flux times the number of turns in the coil. Therefore, the RMS value of induced voltage is proportional to the number of turns in the solenoid coil as seen from Fig. 5. The generated electric power is calculated from the relations /Z, P: generated among voltage, resistance, and power ( electric power, V: induced electromotive force, Z: impedance of solenoid coil). However, the inductance of the coil and natural frequencies of the magnets are very small, so consequently the reactance is also negligible relative to the resistance. According to Fig. 6, the electric power generated from the device is not sensitive to types of solenoid coils, whereas the induced voltage
C. Effect of Natural Frequencies of Device In order to investigate the effect of natural frequencies of the device on the efficiency of the energy harvesting, the additional wind tunnel tests are conducted. The same device in the previous test (i.e., the coil with coil diameter of 0.8 mm) is considered. The three different cases are tested as follows: (1) 1.95 Hz, (2) 2.95 Hz and (3) 4.80 Hz natural frequencies. The frequencies were altered by using different springs in Fig. 1. Test results are demonstrated in Figs. 7 to 9. As shown in Fig. 7, the onset speed of wake galloping in each case is different according to the natural frequency of the device. It is reasonable because the onset speed of wind-induced vibration is inherently proportional to the natural frequency of the system. Although the onset speed is different, the amplitudes of vibration are almost the same after wake galloping occurs as shown the figure. Figs. 8 and 9 represent the induced voltage and the generated electric power responses according to natural frequencies of the device, respectively. The induced voltage is linearly increased with respect to the natural frequency of the device based on the relations among natural frequency, rate of change of magnetic flux, and induced electromotive force. Since the inductance of coil is very small, its impedance can be assumed to be constant and the generated electric power shows a square increase with respect to its natural frequency. Therefore, the proposed system
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TABLE IV COMPARISON OF EFFICIENCY AMONG ENERGY HARVESTING SYSTEMS USING AERODYNAMIC INSTABILITY PHENOMENA
Fig. 8. Induced voltage response according to natural frequencies of the device.
Fig. 9. Generated power response according to natural frequencies of the device.
A series of wind tunnel tests are carried out in order to verify the efficiency and effectiveness of the proposed energy harvesting device. As seen from the test results, one can obtain the average generated power of 0.3–1.13 W under wind speed with 1.8–5.6 m/s. In the proposed system, the electromagnetic induction can be easily increased. That is, much larger generated power can be expected. It is, therefore, clearly mentioned that the applicability of the proposed energy harvesting system using aerodynamic instability (i.e., wake galloping) is experimentally verified, and it is an efficient energy harvesting system. Moreover, it can be used as an alternative energy source for low-power equipment, resulting in much simpler structural health monitoring systems without batteries for wireless sensors. ACKNOWLEDGMENT
with the natural frequency of 4.8 Hz shows the best efficiency in power generating (see Fig. 9).
This work was supported in part by the KAIST Institutes funded by the Korea Ministry of Education, Science and Technology.
D. Feasibility of the Proposed System The proposed system with natural frequency of 4.8 Hz can generate the average power of 0.3–1.13 W under wind speed with 1.8–5.6 m/s as seen from Fig. 10. Also, the electromagnetic induction in the system can be easily increased up to four in the same test model case. In other words, it is expected that much larger electric power can be generated from the proposed energy harvesting device. This level of the generated power is supposed to be sufficient for the low-power equipment such as wireless sensors. Moreover, the efficiency of the proposed system can be significantly improved if the optimization technique is introduced for the device design. The relevant research is underway. For reference, the efficiencies of energy harvesting systems using aerodynamic instability phenomena are shown in Table IV. As seen from the table, the proposed energy harvesting system using wake galloping shows better efficiency than other energy harvesting systems using divergent oscillation such as flutter and galloping under wind speed of 6 m/s. IV. CONCLUSION In this study, a new energy harvesting system using wake galloping phenomenon of structures is proposed for the first time.
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