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Energy harvesting based on acoustically oscillating liquid droplets
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Energy harvesting based on acoustically oscillating liquid droplets Young Rang Lee, Jae Hun Shin, Il Song Park, Kyehan Rhee, Sang Kug Chung ∗ Department of Mechanical Engineering, Myongji University, Yongin, South Korea
a r t i c l e
i n f o
Article history: Received 16 April 2014 Accepted 5 March 2015 Available online xxx Keywords: Energy scavenging Piezoelectric effect Acoustics Drop oscillation
a b s t r a c t This paper presents a novel actuator for harvesting energy from ambient acoustic noise using acoustically oscillating droplets. When a liquid droplet sitting on a piezocantilever is excited by an acoustic wave around its natural frequency, its oscillation simultaneously bends the piezocantilever, which generates electric power owing to the piezoelectric effect. The oscillation amplitudes of water droplets with three different sizes (2, 4, and 6 l) hanging from a solid substrate were first investigated using high-speed images. The results showed that the droplet oscillation amplitude was strongly dependent on the applied frequency and was proportional to the droplet size. The maximum droplet oscillation amplitude occurred at the natural frequency of the droplets. Energy harvesting based on acoustically oscillating droplets was separately tested using a commercial piezocantilever. The oscillation behaviors of water droplets hanging from a flexible piezocantilever were also studied using high-speed images. The bending displacement and generated voltage of the piezocantilever by the acoustically oscillating water droplets were measured with a high-speed camera and digital oscilloscope, respectively, for different droplet sizes and distances between the droplet and piezoactuator. Both the bending displacement and generated voltage were strongly affected by the applied frequency and proportional to the droplet size but were inversely proportional to the distance. The force generated from the acoustically oscillating droplets was measured by using a load cell. The maximum force generated from the acoustically oscillating droplet (4 l) was about 123.9 N at the maximum bending displacement (about 1.5 mm). The output voltage and power generated from the piezocantilever actuated by the acoustically oscillating droplets were measured with a custom-made electric circuit (mainly consisting of a voltage rectifier and load) for different droplet sizes. The maximum generated power for the load (10 ) was measured to be about 80 W. As proof of concept, storage capacitor charging tests were conducted for 0.1 and 1 F capacitors using the acoustically oscillating droplets in three different sizes. © 2015 Elsevier B.V. All rights reserved.
1. Introduction With the growing interest in portable wireless devices, the development of micro energy harvesting technology has become an important task [1–3]. Energy harvesting, which is also called power harvesting or energy scavenging, is defined as capturing energy from one or more surrounding energy sources, collecting it, and storing it for later use. Although portable wireless devices offer several advantages, such as flexibility and the ability to facilitate the placement of sensors in previously inaccessible locations, the use of batteries as the power source limits their potential because of the short battery life [2–6]. Hence, various micro energy harvesting technologies, mainly powered by four energy sources—light, radio-frequency
∗ Corresponding author. Tel.: +82 31 330 6346. E-mail address: [email protected] (S.K. Chung).
electromagnetic radiation, thermal gradients, and mechanical motion—have been investigated and developed as potential alternatives to batteries [2,7]. Recently, Krupenkin and Taylor investigated a new type of high-power energy harvesting system based on the reverse electrowetting-on-dielectric principle that generates electrical energy through the interaction of arrays of moving tiny liquid droplets with a multilayer thin film [8]. They developed an in-shoe system that can harvest energy generated by walking and use it to recharge portable electronic devices later; their system received substantial attention from the mass media. Among the various types of energy harvesting technologies, piezoelectric energy harvesting based on mechanical vibration is the most popular owing to its simple structure [9–12]. The piezoelectric effect converts mechanical strain into electric voltage and current. This paper presents a novel actuator for harvesting energy from ambient acoustic noise using acoustically oscillating droplets. When a liquid droplet attached to the tip of a piezocantilever is excited by an acoustic wave around its natural frequency, its
http://dx.doi.org/10.1016/j.sna.2015.03.009 0924-4247/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: Y.R. Lee, et al., Energy harvesting based on acoustically oscillating liquid droplets, Sens. Actuators A: Phys. (2015), http://dx.doi.org/10.1016/j.sna.2015.03.009
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Fig. 1. Acoustically oscillating droplet-induced motion-powered energy harvester: (a) a water droplet oscillates when it is acoustically excited by a piezoactuator around its natural frequency. (b) when an acoustically oscillating droplet is placed at the end-tip of a piezocantilever, the oscillating motion of the droplet induces continuous bending of the piezocantilever as a reaction, which generates electric power.
oscillation simultaneously bends the piezocantilever, which generates electric power owing to the piezoelectric effect, as shown in Fig. 1 [13,14]. The envisioned energy harvesting system can extract mechanical power from acoustic noise over a wide range of frequencies using liquid droplets with different sizes and natural frequencies and convert the mechanical power to electrical power for wireless electronic devices. This new type of actuation technique is a simple but useful tool not only for energy harvesting systems but also for potential acoustic wave sensors and actuators in the future. Note that a preliminary report on this work was presented at the International Conference on Micro Electro Mechanical Systems held in San Francisco, USA [15].
2. Theoretical background The behavior of a liquid droplet exposed to external disturbances is not only of scientific interest but is also an important research topic for various industrial applications such as power plants and heating, ventilation, and air-conditioning systems [16,17]. Liquid droplets generated from condensation processes attach to the surface of heat exchangers and reduce their efficiency because they provide thermal resistance. Hence, to efficiently remove liquid droplets from a surface, various methods such as surface treatments and vibration techniques have been developed along with the oscillation analysis of liquid droplets under external disturbances [17–20]. Rayleigh and Kelvin first studied the free oscillation behavior of spherical liquid droplets [21,22]. Later, Lamb extended the investigation to the oscillation behavior and frequency of a liquid sphere surrounded by an outer fluid with a different density [23,24]. Strani and Sabetta analyzed the oscillation of a liquid droplet in partial contact with a concave solid substrate by combining the Green function method with the Legendre series expansion [24]. Although their analysis was developed for a liquid droplet sitting on a concave solid substrate, it can be applied to predicting the oscillation behavior of a non-wetting droplet sitting on a plain solid substrate [25]. A pendant-shaped liquid droplet attached to the bottom tip of a piezocantilever is affected by gravity; however, it can adhere to the
substrate owing to surface tension, as shown in Fig. 1(b). The force balance between gravity and surface tension is as follows [17]: mg = d sin
(1)
where m is the mass of the liquid droplet, g is the gravitational acceleration, d is the diameter of the contact area, is the surface tension, and is the contact angle. When a liquid droplet is excited by an acoustic wave around its natural frequency, its oscillation motion simultaneously induces the bending of the piezocantilever to generate electric power. However, the oscillation motion of the liquid droplet is weak and negligible at different frequencies because it only responds around its natural frequency [26,27]. Hence, estimating the natural frequency is important to current energy harvesting systems. The most easily accessible theory for the natural frequency is for the oscillation of a free liquid droplet. The natural frequency of the nth mode oscillation of a free droplet is given by [17]
fn =
1 n(n − 1)(n + 2) 3 2 R
12 .
(2)
where is the surface tension of the droplet, is the density of the droplet, R is the radius of the droplet, and the mode number n denotes the number of nodes occurring in the oscillation. The oscillation of a water droplet (2, 4, and 6 l) was observed with a high-speed camera (Phantom Miro eX4, Vision Research, Inc.). When a water droplet hanging from a solid substrate coated with a Teflon layer was acoustically excited by a cylindrical piezoactuator (PIC151, Physik Instrumente, Inc.) placed 4 mm away around its natural frequency, it continuously deformed (oscillated) with respect to the applied frequency, as shown in the inset of Fig. 2. This figure plots the oscillation amplitudes of the droplet actuated by acoustic waves generated from the piezoactuator in a wide range of frequencies as measured from high-speed images. The droplet oscillation amplitude was found to be strongly dependent on the applied frequency and proportional to the droplet size. The maximum droplet oscillation amplitude occurred at the droplet’s natural frequency, which is a function of the droplet size [20,26,28]. The natural frequencies for the droplets (124, 96, and 63.3 Hz for 2, 4, and 6 l, respectively) in the experiment deviated from the theoretical values (1.79%, 20%, and 27.6% for 2, 4, and 6 l, respectively).
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Fig. 2. Measurement of oscillation amplitude of acoustically oscillating droplet hanging from solid substrate at different frequencies and fixed distance (4 mm) using high-speed camera.
This may be because the theoretical natural frequencies were derived for free inviscid liquid droplets. 3. Experiment results and discussions First, the dynamic behavior of an acoustically excited water droplet attached to a flexible piezocantilever (LDT2-028K/L w/rivets, Measurement Specialties, Inc.) was investigated by using high-speed images. Fig. 3 shows the schematic diagram of the experimental setups. The setups mainly consisted of electrical and optical systems. A cylindrical piezoactuator (PIC151, Physik Instrumente, Inc.) was used with a function generator (33210A, Agilent Co.) and voltage amplifier (PZD700, Trek Co.) to actuate a liquid droplet. Test images were obtained by using a charge-coupled device camera (EO-1312C, Edmund Optics) or high-speed camera (Phantom Miro eX4, Vision Research Inc.) integrated with a zoom lens (VZMTM 450i eo, Edmund Optics) and saved on a personal computer. A small water droplet (2, 4, and 6 l) was injected with a microsyringe (600 Series MICROLITERTM Syringes model 62, Hamilton Co.) and attached to the bottom end of the piezocantilever (73 (L) × 1.5 (W) × 0.2 (T) mm3 ). When the droplet was acoustically excited with a cylindrical piezoactuator (diameter: 30 mm, height: 20 mm) placed 4 mm away around its natural frequency, the shape of the deformed droplet was captured, as shown in the inset of Fig. 4. The oscillation amplitudes of the droplets were measured over a wide range of frequencies from high-speed images; the results are
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Fig. 4. Measurement of oscillation amplitude of acoustically oscillating droplet hanging from flexible piezocantilever at different frequencies and fixed distance (4 mm) using high-speed camera.
plotted in Fig. 4. The droplet oscillation amplitude depended on the applied frequency and increased with the droplet size. However, the oscillation amplitude for the droplet on the flexible piezocantilever was much larger than when the droplet was on the solid substrate, and the natural frequency of the droplet on the flexible piezocantilever was much lower than when it was on the solid substrate because of the use of the flexible piezocantilever for energy harvesting. The energy harvesting induced by acoustically oscillating water droplets with three different sizes (2, 4, and 6 l) was tested with the same piezocantilever. When a water droplet injected by a microsyringe was acoustically excited by a piezoactuator at its natural frequency, its oscillation simultaneously made the piezocantilever bend up and down, as shown in Fig. 5. When the piezoactuator was turned off, the droplet stopped oscillating, and the piezocantilever also stopped its vibration. The piezocantilever’s bending displacement and generated voltage from the acoustically oscillating water droplets were measured for three different droplet volumes (2, 4, and 6 l) with a high-speed camera and digital oscilloscope (TDS3012, Tektronix, Inc.), respectively. Both the bending displacement and generated voltage were strongly affected by the applied frequency and were proportional to the droplet size, as shown in Fig. 6. The maximum bending displacement and generated voltage occurred at the natural frequency. The effect of the distance between the droplet and piezoactuator was investigated separately. Fig. 7 plots the bending displacement and voltage of the piezocantilever from the acoustically oscillating
Fig. 3. Schematic diagram of experimental setups mainly consisting of electrical and optical systems.
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Fig. 5. Snapshots of continuous bending of piezocantilever by oscillation of droplet attached to end-tip of piezocantilever.
Fig. 6. Measurement of bending displacement and generated voltage of piezocantilever actuated by acoustically oscillating droplet at different frequencies: (a) displacement vs. frequency; (b) voltage vs. frequency.
water droplets with three different sizes (2, 4, and 6 l) at different distances from the piezoactuator. Both the bending displacement and generated voltage were inversely proportional to the distance, which clearly shows that the envisioned actuator was powered by acoustic energy sources. The force generated from an acoustically oscillating droplet was measured using a load cell (GSO-100, Transducer Techniques, Inc.),
as shown in Fig. 8. When the same piezocantilever used for the energy harvesting induced by the acoustically oscillating droplet was tested (pushed and bent) by a load cell attached with a precise traverse system, the bending force was measured for each bending displacement and compared with the bending displacement induced by the oscillating droplet, as shown in Fig. 8. The maximum force generated from the acoustically oscillating droplet
Fig. 7. Measurement of bending displacement and generated voltage of piezocantilever actuated by acoustically oscillating droplet at different distances from piezoactuator: (a) displacement vs. distance; (b) voltage vs. distance.
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Fig. 8. Measurement of force induced by acoustically oscillating droplet using load cell.
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(4 l) was about 123.9 N at the maximum bending displacement (about 1.5 mm). The output voltage and power generated from the piezocantilever actuated by acoustically oscillating droplets with three different sizes (2, 4, and 6 l) were measured by using a custommade electric circuit mainly consisting of a voltage rectifier and load, as shown in Fig. 9. To rectify the alternating current (AC) signal generated from the piezocantilever, a full wave-bridge type rectifying circuit consisting of four diodes was used. Fig. 9(a) shows the used circuit block diagram [29]. The results showed that the output voltage increased with the electrical load, whereas the output power decreased with increasing load; the maximum power for the load (10 ) was measured to be about 80 W for the oscillating droplet (6 l). As proof of concept, storage capacitor charging tests were conducted for 0.1 and 1 F capacitors using acoustically oscillating droplets with three different sizes (2, 4, and 6 l), as shown in Fig. 10. The results showed that the voltage for the 0.1 F capacitor was lower than that for the 1 F capacitor; on the other hand, the saturation time (about 60 s) for the 0.1 F capacitor was shorter than that (about 75 s) for the 1 F capacitor. The voltage on
Fig. 9. Output voltage and power generated from piezocantilever actuated by acoustically oscillating droplet using custom-made electrical circuit mainly consisting of voltage rectifier and load.
Fig. 10. Storage capacitor charging curves: (a) 0.1 F capacitor; (b) 1 F capacitor.
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the charging capacitor was proportional to the droplet size. Note that the maximum voltages on the 0.1 and 1 F capacitors were 143 and 176 mV, respectively, for the oscillating droplet (6 l). 4. Conclusion A novel actuator for harvesting energy from ambient acoustic noise using acoustically oscillating droplets was developed. The dynamic behavior of water droplets with three different sizes (2, 4, and 6 l) actuated by acoustic excitation at different frequencies was observed with a high-speed camera. The droplet oscillation amplitude was found to be strongly dependent on the applied frequency and proportional to the droplet size. The maximum droplet oscillation amplitude occurred at the droplet natural frequency. The energy harvesting was tested separately using a commercial piezocantilever actuated by acoustically oscillating droplets. The piezocantilever’s bending displacement and generated voltage by the acoustically oscillating water droplets were measured with a high-speed camera and digital oscilloscope, respectively. Both the bending displacement and generated voltage were found to be strongly affected by the applied frequency and proportional to the droplet size. The effect of the distance between the droplet and piezoactuator was also investigated. Both the bending displacement and generated voltage were found to be inversely proportional to the distance, which clearly shows that the envisioned actuator is powered by acoustic energy sources. The force generated from the acoustically oscillating droplets was then measured with a load cell. The maximum force generated from the acoustically oscillating droplet (4 l) was about 123.9 N at the maximum bending displacement (about 1.5 mm). The output voltage and power generated from the piezocantilever actuated by the acoustically oscillating droplets were measured using a custommade full wave-bridge type rectifying circuit consisting of four diodes. The maximum generated power for the load (10 ) was measured to be about 80 W for the oscillating droplet (6 l). Finally, as proof of concept, storage capacitor charging tests were conducted for 0.1 and 1 F capacitors using the acoustically oscillating droplets. This new type of actuation technique is a simple but useful tool not only for energy harvesting systems but also for potential acoustic wave sensors and actuators.
[10] H.-U. Kim, W.-H. Lee, H.V. Rasika Dias, S. Priya, Piezoelectric microgeneratorscurrent status and challenges, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 56 (2009) 1555–1568. [11] R. Amirtharajah, A.P. Chandrakasan, Self-powered signal processing using vibration-based power generation, IEEE J. Solid-State Circuits 33 (1998) 687–695. [12] S.P. Beeby, M.J. Tudor, N.M. White, Energy harvesting vibration sources for microsystems applications, Meas. Sci. Technol. 17 (2006) R175. [13] X. Noblin, A. Buguin, F. Brochard-Wyart, Vibrations of sessile drops, Eur. Phys. J.-Spec. Top. 166 (2009) 7–10. [14] H.A. Sodano, D.J. Inman, G. Park, Comparison of piezoelectric energy harvesting devices for recharging batteries, J. Intell. Mater. Syst. Struct. 16 (2005) 799–807. [15] Y.R. Lee, J.H. Shin, I.S. Park, S.K. Chung, A novel actuator for energy harvesting using an acoustically oscillating liquid droplet, in: The 27th International Conference on Micro Electro Mechanical Systems, San Francisco, California, USA, 2014, pp. 897–900. [16] J.H. Moon, B.H. Kang, H.-Y. Kim, The lowest oscillation mode of a pendant drop, Phys. Fluids 18 (2006) 021702. [17] H.-Y. Kim, Drop fall-off from the vibrating ceiling, Phys. Fluids 16 (2004) 474. [18] S. Dash, N. Kumari, S.V. Garimella, Frequency-dependent transient response of an oscillating electrically actuated droplet, J. Micromech. Microeng. 22 (2012) 075004. [19] J.M. Oh, S.H. Ko, K.H. Kang, Shape oscillation of a drop in ac electrowetting, Langmuir 24 (2008) 8379–8386. [20] E.D. Wilkes, O.A. Basaran, Forced oscillations of pendant (sessile) drops, Phys. Fluids 9 (1997) 1512–1528. [21] L. Rayleigh, The Theory of Sound, Macmillan, 1894. [22] T.G. Leighton, The Acoustic Bubble, Academic Press, 1997. [23] H. Lamb, Hydrodynamics, Cambridge University Press, 1932. [24] M. Strani, F. Sabetta, Free vibrations of a drop in partial contact with a solid support, J. Fluid Mech. 141 (1984) 233–247. [25] R.W. Smithwick III, J.A.M. Boulet, Vibrations of microscopic mercury droplets on glass, J. Colloid Interface Sci. 130 (1989) 588–596. [26] O.A. Basaran, D.W. DePaoli, Nonlinear oscillations of pendant drops, Phys. Fluids 6 (1994) 2923. [27] J.O. Kwon, J.S. Yang, S.J. Lee, K. Rhee, S.K. Chung, Electromagnetically actuated micromanipulator using an acoustically oscillating bubble, J. Micromech. Microeng. 21 (2011) 115023. [28] J.M. Oh, D. Legendre, F. Mugele, Shaken not stirred—on internal flow patterns in oscillating sessile drops, EPL 98 (2012) 34003. [29] G.K. Ottman, H.F. Hofmann, A.C. Bhatt, G.A. Lesieutre, Adaptive piezoelectric energy harvesting circuit for wireless remote power supply, IEEE Trans. Power Electron. 17 (2002) 669–676.
Biographies
Young Rang Lee received the Bachelor’s degree of mechanical engineering from Myongji University in 2013. He currently is a graduate student in Myongji University and his research interests lie on the development of energy harvesting systems.
Acknowledgements This work was supported by the 2014 research fund of Myongji University in Korea. References [1] S.R. Anton, H.A. Sodano, A review of power harvesting using piezoelectric materials (2003–2006), Smart Mater. Struct. 16 (2007) R1. [2] P.D. Mitcheson, E.M. Yeatman, G.K. Rao, A.S. Holmes, T.C. Green, Energy harvesting from human and machine motion for wireless electronic devices, Proc. IEEE 96 (2008) 1457–1486. [3] H.S. Kim, J.-H. Kim, J. Kim, A review of piezoelectric energy harvesting based on vibration, Int. J. Precis. Eng. Manuf. 12 (2011) 1129–1141. [4] K.A. Cook-Chennault, N. Thambi, A.M. Sastry, Powering MEMS portable devices—a review of non-regenerative and regenerative power supply systems with special emphasis on piezoelectric energy harvesting systems, Smart Mater. Struct. 17 (2008) 043001. [5] Y.C. Shu, I.C. Lien, Analysis of power output for piezoelectric energy harvesting systems, Smart Mater. Struct. 15 (2006) 1499–1512. [6] M. Philipose, J.R. Smith, B. Jiang, A. Mamishev, S. Roy, K. Sundara-Rajan, Batteryfree wireless identification and sensing, IEEE Pervasive Comput. 4 (2005) 37–45. [7] S. Roundy, P.K. Wright, J. Rabaey, A study of low level vibrations as a power source for wireless sensor nodes, Comput. Commun. 26 (2003) 1131–1144. [8] T. Krupenkin, J.A. Taylor, Reverse electrowetting as a new approach to highpower energy harvesting, Nat. Commun. 2 (2011) 448. [9] S. Priya, Advances in energy harvesting using low profile piezoelectric transducers, J. Electroceram. 19 (2007) 167–184.
Jae Hun Shin received the Bachelor’s degree of mechanical engineering from Myongji University in 2013. He currently is a graduate student in Myongji University and his research interests lie on MEMS and Microfluidics applications.
Il Song Park received the Bachelor’s degree of mechanical engineering from Myongji University in 2013. He currently is a graduate student in Myongji University and his research interests lie on the development of a magnetically driven microrobot for on-chip micromanipulation.
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Kyehan Rhee is a professor of the department of mechanical engineering at the Myongji University in Korea. He received his Ph.D. degree from the University of Minnesota, Minneapolis, USA and worked as a Post-Doctoral Fellow in the Pennsylvania State University, State College, U.S.A. His research interests include hemodynamics, microfluidics, polymer smart material actuators and their application in biomedical devices. He serves as an editor of the International Journal of Precision Engineering and Manufacturing and the Journal of Biomechanical Science and Engineering.
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Sang Kug Chung is an associate professor of the department of mechanical engineering at the Myongji University in Korea. He received the Ph.D. degree in Mechanical Engineering and Materials Science from the University of Pittsburgh in 2009 along with the Graduate Research Excellence Award. He received the M.S. degree from Pohang University of Science and Technology (POSTECH) and B.S. from Myongji University. He had worked for the development of the world first Liquid Lens at Samsung Electro-Mechanics from 2003 to 2009. Upon joining the faculty at Myongji University in 2009, he has directed the Microsystems Laboratory. And he has also served as a principal investigator in the Advanced Microfluids Engineering Research Laboratory (AMERL) since 2013. His research is in microfluidics and MEMS, including design and fabrication of micro/nano actuators and systems.
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Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Energy harvesting based on acoustically oscillating liquid droplets Young Rang Lee, Jae Hun Shin, Il Song Park, Kyehan Rhee, Sang Kug Chung ∗ Department of Mechanical Engineering, Myongji University, Yongin, South Korea
a r t i c l e
i n f o
Article history: Received 16 April 2014 Accepted 5 March 2015 Available online xxx Keywords: Energy scavenging Piezoelectric effect Acoustics Drop oscillation
a b s t r a c t This paper presents a novel actuator for harvesting energy from ambient acoustic noise using acoustically oscillating droplets. When a liquid droplet sitting on a piezocantilever is excited by an acoustic wave around its natural frequency, its oscillation simultaneously bends the piezocantilever, which generates electric power owing to the piezoelectric effect. The oscillation amplitudes of water droplets with three different sizes (2, 4, and 6 l) hanging from a solid substrate were first investigated using high-speed images. The results showed that the droplet oscillation amplitude was strongly dependent on the applied frequency and was proportional to the droplet size. The maximum droplet oscillation amplitude occurred at the natural frequency of the droplets. Energy harvesting based on acoustically oscillating droplets was separately tested using a commercial piezocantilever. The oscillation behaviors of water droplets hanging from a flexible piezocantilever were also studied using high-speed images. The bending displacement and generated voltage of the piezocantilever by the acoustically oscillating water droplets were measured with a high-speed camera and digital oscilloscope, respectively, for different droplet sizes and distances between the droplet and piezoactuator. Both the bending displacement and generated voltage were strongly affected by the applied frequency and proportional to the droplet size but were inversely proportional to the distance. The force generated from the acoustically oscillating droplets was measured by using a load cell. The maximum force generated from the acoustically oscillating droplet (4 l) was about 123.9 N at the maximum bending displacement (about 1.5 mm). The output voltage and power generated from the piezocantilever actuated by the acoustically oscillating droplets were measured with a custom-made electric circuit (mainly consisting of a voltage rectifier and load) for different droplet sizes. The maximum generated power for the load (10 ) was measured to be about 80 W. As proof of concept, storage capacitor charging tests were conducted for 0.1 and 1 F capacitors using the acoustically oscillating droplets in three different sizes. © 2015 Elsevier B.V. All rights reserved.
1. Introduction With the growing interest in portable wireless devices, the development of micro energy harvesting technology has become an important task [1–3]. Energy harvesting, which is also called power harvesting or energy scavenging, is defined as capturing energy from one or more surrounding energy sources, collecting it, and storing it for later use. Although portable wireless devices offer several advantages, such as flexibility and the ability to facilitate the placement of sensors in previously inaccessible locations, the use of batteries as the power source limits their potential because of the short battery life [2–6]. Hence, various micro energy harvesting technologies, mainly powered by four energy sources—light, radio-frequency
∗ Corresponding author. Tel.: +82 31 330 6346. E-mail address: [email protected] (S.K. Chung).
electromagnetic radiation, thermal gradients, and mechanical motion—have been investigated and developed as potential alternatives to batteries [2,7]. Recently, Krupenkin and Taylor investigated a new type of high-power energy harvesting system based on the reverse electrowetting-on-dielectric principle that generates electrical energy through the interaction of arrays of moving tiny liquid droplets with a multilayer thin film [8]. They developed an in-shoe system that can harvest energy generated by walking and use it to recharge portable electronic devices later; their system received substantial attention from the mass media. Among the various types of energy harvesting technologies, piezoelectric energy harvesting based on mechanical vibration is the most popular owing to its simple structure [9–12]. The piezoelectric effect converts mechanical strain into electric voltage and current. This paper presents a novel actuator for harvesting energy from ambient acoustic noise using acoustically oscillating droplets. When a liquid droplet attached to the tip of a piezocantilever is excited by an acoustic wave around its natural frequency, its
http://dx.doi.org/10.1016/j.sna.2015.03.009 0924-4247/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: Y.R. Lee, et al., Energy harvesting based on acoustically oscillating liquid droplets, Sens. Actuators A: Phys. (2015), http://dx.doi.org/10.1016/j.sna.2015.03.009
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Fig. 1. Acoustically oscillating droplet-induced motion-powered energy harvester: (a) a water droplet oscillates when it is acoustically excited by a piezoactuator around its natural frequency. (b) when an acoustically oscillating droplet is placed at the end-tip of a piezocantilever, the oscillating motion of the droplet induces continuous bending of the piezocantilever as a reaction, which generates electric power.
oscillation simultaneously bends the piezocantilever, which generates electric power owing to the piezoelectric effect, as shown in Fig. 1 [13,14]. The envisioned energy harvesting system can extract mechanical power from acoustic noise over a wide range of frequencies using liquid droplets with different sizes and natural frequencies and convert the mechanical power to electrical power for wireless electronic devices. This new type of actuation technique is a simple but useful tool not only for energy harvesting systems but also for potential acoustic wave sensors and actuators in the future. Note that a preliminary report on this work was presented at the International Conference on Micro Electro Mechanical Systems held in San Francisco, USA [15].
2. Theoretical background The behavior of a liquid droplet exposed to external disturbances is not only of scientific interest but is also an important research topic for various industrial applications such as power plants and heating, ventilation, and air-conditioning systems [16,17]. Liquid droplets generated from condensation processes attach to the surface of heat exchangers and reduce their efficiency because they provide thermal resistance. Hence, to efficiently remove liquid droplets from a surface, various methods such as surface treatments and vibration techniques have been developed along with the oscillation analysis of liquid droplets under external disturbances [17–20]. Rayleigh and Kelvin first studied the free oscillation behavior of spherical liquid droplets [21,22]. Later, Lamb extended the investigation to the oscillation behavior and frequency of a liquid sphere surrounded by an outer fluid with a different density [23,24]. Strani and Sabetta analyzed the oscillation of a liquid droplet in partial contact with a concave solid substrate by combining the Green function method with the Legendre series expansion [24]. Although their analysis was developed for a liquid droplet sitting on a concave solid substrate, it can be applied to predicting the oscillation behavior of a non-wetting droplet sitting on a plain solid substrate [25]. A pendant-shaped liquid droplet attached to the bottom tip of a piezocantilever is affected by gravity; however, it can adhere to the
substrate owing to surface tension, as shown in Fig. 1(b). The force balance between gravity and surface tension is as follows [17]: mg = d sin
(1)
where m is the mass of the liquid droplet, g is the gravitational acceleration, d is the diameter of the contact area, is the surface tension, and is the contact angle. When a liquid droplet is excited by an acoustic wave around its natural frequency, its oscillation motion simultaneously induces the bending of the piezocantilever to generate electric power. However, the oscillation motion of the liquid droplet is weak and negligible at different frequencies because it only responds around its natural frequency [26,27]. Hence, estimating the natural frequency is important to current energy harvesting systems. The most easily accessible theory for the natural frequency is for the oscillation of a free liquid droplet. The natural frequency of the nth mode oscillation of a free droplet is given by [17]
fn =
1 n(n − 1)(n + 2) 3 2 R
12 .
(2)
where is the surface tension of the droplet, is the density of the droplet, R is the radius of the droplet, and the mode number n denotes the number of nodes occurring in the oscillation. The oscillation of a water droplet (2, 4, and 6 l) was observed with a high-speed camera (Phantom Miro eX4, Vision Research, Inc.). When a water droplet hanging from a solid substrate coated with a Teflon layer was acoustically excited by a cylindrical piezoactuator (PIC151, Physik Instrumente, Inc.) placed 4 mm away around its natural frequency, it continuously deformed (oscillated) with respect to the applied frequency, as shown in the inset of Fig. 2. This figure plots the oscillation amplitudes of the droplet actuated by acoustic waves generated from the piezoactuator in a wide range of frequencies as measured from high-speed images. The droplet oscillation amplitude was found to be strongly dependent on the applied frequency and proportional to the droplet size. The maximum droplet oscillation amplitude occurred at the droplet’s natural frequency, which is a function of the droplet size [20,26,28]. The natural frequencies for the droplets (124, 96, and 63.3 Hz for 2, 4, and 6 l, respectively) in the experiment deviated from the theoretical values (1.79%, 20%, and 27.6% for 2, 4, and 6 l, respectively).
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Fig. 2. Measurement of oscillation amplitude of acoustically oscillating droplet hanging from solid substrate at different frequencies and fixed distance (4 mm) using high-speed camera.
This may be because the theoretical natural frequencies were derived for free inviscid liquid droplets. 3. Experiment results and discussions First, the dynamic behavior of an acoustically excited water droplet attached to a flexible piezocantilever (LDT2-028K/L w/rivets, Measurement Specialties, Inc.) was investigated by using high-speed images. Fig. 3 shows the schematic diagram of the experimental setups. The setups mainly consisted of electrical and optical systems. A cylindrical piezoactuator (PIC151, Physik Instrumente, Inc.) was used with a function generator (33210A, Agilent Co.) and voltage amplifier (PZD700, Trek Co.) to actuate a liquid droplet. Test images were obtained by using a charge-coupled device camera (EO-1312C, Edmund Optics) or high-speed camera (Phantom Miro eX4, Vision Research Inc.) integrated with a zoom lens (VZMTM 450i eo, Edmund Optics) and saved on a personal computer. A small water droplet (2, 4, and 6 l) was injected with a microsyringe (600 Series MICROLITERTM Syringes model 62, Hamilton Co.) and attached to the bottom end of the piezocantilever (73 (L) × 1.5 (W) × 0.2 (T) mm3 ). When the droplet was acoustically excited with a cylindrical piezoactuator (diameter: 30 mm, height: 20 mm) placed 4 mm away around its natural frequency, the shape of the deformed droplet was captured, as shown in the inset of Fig. 4. The oscillation amplitudes of the droplets were measured over a wide range of frequencies from high-speed images; the results are
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Fig. 4. Measurement of oscillation amplitude of acoustically oscillating droplet hanging from flexible piezocantilever at different frequencies and fixed distance (4 mm) using high-speed camera.
plotted in Fig. 4. The droplet oscillation amplitude depended on the applied frequency and increased with the droplet size. However, the oscillation amplitude for the droplet on the flexible piezocantilever was much larger than when the droplet was on the solid substrate, and the natural frequency of the droplet on the flexible piezocantilever was much lower than when it was on the solid substrate because of the use of the flexible piezocantilever for energy harvesting. The energy harvesting induced by acoustically oscillating water droplets with three different sizes (2, 4, and 6 l) was tested with the same piezocantilever. When a water droplet injected by a microsyringe was acoustically excited by a piezoactuator at its natural frequency, its oscillation simultaneously made the piezocantilever bend up and down, as shown in Fig. 5. When the piezoactuator was turned off, the droplet stopped oscillating, and the piezocantilever also stopped its vibration. The piezocantilever’s bending displacement and generated voltage from the acoustically oscillating water droplets were measured for three different droplet volumes (2, 4, and 6 l) with a high-speed camera and digital oscilloscope (TDS3012, Tektronix, Inc.), respectively. Both the bending displacement and generated voltage were strongly affected by the applied frequency and were proportional to the droplet size, as shown in Fig. 6. The maximum bending displacement and generated voltage occurred at the natural frequency. The effect of the distance between the droplet and piezoactuator was investigated separately. Fig. 7 plots the bending displacement and voltage of the piezocantilever from the acoustically oscillating
Fig. 3. Schematic diagram of experimental setups mainly consisting of electrical and optical systems.
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Fig. 5. Snapshots of continuous bending of piezocantilever by oscillation of droplet attached to end-tip of piezocantilever.
Fig. 6. Measurement of bending displacement and generated voltage of piezocantilever actuated by acoustically oscillating droplet at different frequencies: (a) displacement vs. frequency; (b) voltage vs. frequency.
water droplets with three different sizes (2, 4, and 6 l) at different distances from the piezoactuator. Both the bending displacement and generated voltage were inversely proportional to the distance, which clearly shows that the envisioned actuator was powered by acoustic energy sources. The force generated from an acoustically oscillating droplet was measured using a load cell (GSO-100, Transducer Techniques, Inc.),
as shown in Fig. 8. When the same piezocantilever used for the energy harvesting induced by the acoustically oscillating droplet was tested (pushed and bent) by a load cell attached with a precise traverse system, the bending force was measured for each bending displacement and compared with the bending displacement induced by the oscillating droplet, as shown in Fig. 8. The maximum force generated from the acoustically oscillating droplet
Fig. 7. Measurement of bending displacement and generated voltage of piezocantilever actuated by acoustically oscillating droplet at different distances from piezoactuator: (a) displacement vs. distance; (b) voltage vs. distance.
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Fig. 8. Measurement of force induced by acoustically oscillating droplet using load cell.
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(4 l) was about 123.9 N at the maximum bending displacement (about 1.5 mm). The output voltage and power generated from the piezocantilever actuated by acoustically oscillating droplets with three different sizes (2, 4, and 6 l) were measured by using a custommade electric circuit mainly consisting of a voltage rectifier and load, as shown in Fig. 9. To rectify the alternating current (AC) signal generated from the piezocantilever, a full wave-bridge type rectifying circuit consisting of four diodes was used. Fig. 9(a) shows the used circuit block diagram [29]. The results showed that the output voltage increased with the electrical load, whereas the output power decreased with increasing load; the maximum power for the load (10 ) was measured to be about 80 W for the oscillating droplet (6 l). As proof of concept, storage capacitor charging tests were conducted for 0.1 and 1 F capacitors using acoustically oscillating droplets with three different sizes (2, 4, and 6 l), as shown in Fig. 10. The results showed that the voltage for the 0.1 F capacitor was lower than that for the 1 F capacitor; on the other hand, the saturation time (about 60 s) for the 0.1 F capacitor was shorter than that (about 75 s) for the 1 F capacitor. The voltage on
Fig. 9. Output voltage and power generated from piezocantilever actuated by acoustically oscillating droplet using custom-made electrical circuit mainly consisting of voltage rectifier and load.
Fig. 10. Storage capacitor charging curves: (a) 0.1 F capacitor; (b) 1 F capacitor.
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the charging capacitor was proportional to the droplet size. Note that the maximum voltages on the 0.1 and 1 F capacitors were 143 and 176 mV, respectively, for the oscillating droplet (6 l). 4. Conclusion A novel actuator for harvesting energy from ambient acoustic noise using acoustically oscillating droplets was developed. The dynamic behavior of water droplets with three different sizes (2, 4, and 6 l) actuated by acoustic excitation at different frequencies was observed with a high-speed camera. The droplet oscillation amplitude was found to be strongly dependent on the applied frequency and proportional to the droplet size. The maximum droplet oscillation amplitude occurred at the droplet natural frequency. The energy harvesting was tested separately using a commercial piezocantilever actuated by acoustically oscillating droplets. The piezocantilever’s bending displacement and generated voltage by the acoustically oscillating water droplets were measured with a high-speed camera and digital oscilloscope, respectively. Both the bending displacement and generated voltage were found to be strongly affected by the applied frequency and proportional to the droplet size. The effect of the distance between the droplet and piezoactuator was also investigated. Both the bending displacement and generated voltage were found to be inversely proportional to the distance, which clearly shows that the envisioned actuator is powered by acoustic energy sources. The force generated from the acoustically oscillating droplets was then measured with a load cell. The maximum force generated from the acoustically oscillating droplet (4 l) was about 123.9 N at the maximum bending displacement (about 1.5 mm). The output voltage and power generated from the piezocantilever actuated by the acoustically oscillating droplets were measured using a custommade full wave-bridge type rectifying circuit consisting of four diodes. The maximum generated power for the load (10 ) was measured to be about 80 W for the oscillating droplet (6 l). Finally, as proof of concept, storage capacitor charging tests were conducted for 0.1 and 1 F capacitors using the acoustically oscillating droplets. This new type of actuation technique is a simple but useful tool not only for energy harvesting systems but also for potential acoustic wave sensors and actuators.
[10] H.-U. Kim, W.-H. Lee, H.V. Rasika Dias, S. Priya, Piezoelectric microgeneratorscurrent status and challenges, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 56 (2009) 1555–1568. [11] R. Amirtharajah, A.P. Chandrakasan, Self-powered signal processing using vibration-based power generation, IEEE J. Solid-State Circuits 33 (1998) 687–695. [12] S.P. Beeby, M.J. Tudor, N.M. White, Energy harvesting vibration sources for microsystems applications, Meas. Sci. Technol. 17 (2006) R175. [13] X. Noblin, A. Buguin, F. Brochard-Wyart, Vibrations of sessile drops, Eur. Phys. J.-Spec. Top. 166 (2009) 7–10. [14] H.A. Sodano, D.J. Inman, G. Park, Comparison of piezoelectric energy harvesting devices for recharging batteries, J. Intell. Mater. Syst. Struct. 16 (2005) 799–807. [15] Y.R. Lee, J.H. Shin, I.S. Park, S.K. Chung, A novel actuator for energy harvesting using an acoustically oscillating liquid droplet, in: The 27th International Conference on Micro Electro Mechanical Systems, San Francisco, California, USA, 2014, pp. 897–900. [16] J.H. Moon, B.H. Kang, H.-Y. Kim, The lowest oscillation mode of a pendant drop, Phys. Fluids 18 (2006) 021702. [17] H.-Y. Kim, Drop fall-off from the vibrating ceiling, Phys. Fluids 16 (2004) 474. [18] S. Dash, N. Kumari, S.V. Garimella, Frequency-dependent transient response of an oscillating electrically actuated droplet, J. Micromech. Microeng. 22 (2012) 075004. [19] J.M. Oh, S.H. Ko, K.H. Kang, Shape oscillation of a drop in ac electrowetting, Langmuir 24 (2008) 8379–8386. [20] E.D. Wilkes, O.A. Basaran, Forced oscillations of pendant (sessile) drops, Phys. Fluids 9 (1997) 1512–1528. [21] L. Rayleigh, The Theory of Sound, Macmillan, 1894. [22] T.G. Leighton, The Acoustic Bubble, Academic Press, 1997. [23] H. Lamb, Hydrodynamics, Cambridge University Press, 1932. [24] M. Strani, F. Sabetta, Free vibrations of a drop in partial contact with a solid support, J. Fluid Mech. 141 (1984) 233–247. [25] R.W. Smithwick III, J.A.M. Boulet, Vibrations of microscopic mercury droplets on glass, J. Colloid Interface Sci. 130 (1989) 588–596. [26] O.A. Basaran, D.W. DePaoli, Nonlinear oscillations of pendant drops, Phys. Fluids 6 (1994) 2923. [27] J.O. Kwon, J.S. Yang, S.J. Lee, K. Rhee, S.K. Chung, Electromagnetically actuated micromanipulator using an acoustically oscillating bubble, J. Micromech. Microeng. 21 (2011) 115023. [28] J.M. Oh, D. Legendre, F. Mugele, Shaken not stirred—on internal flow patterns in oscillating sessile drops, EPL 98 (2012) 34003. [29] G.K. Ottman, H.F. Hofmann, A.C. Bhatt, G.A. Lesieutre, Adaptive piezoelectric energy harvesting circuit for wireless remote power supply, IEEE Trans. Power Electron. 17 (2002) 669–676.
Biographies
Young Rang Lee received the Bachelor’s degree of mechanical engineering from Myongji University in 2013. He currently is a graduate student in Myongji University and his research interests lie on the development of energy harvesting systems.
Acknowledgements This work was supported by the 2014 research fund of Myongji University in Korea. References [1] S.R. Anton, H.A. Sodano, A review of power harvesting using piezoelectric materials (2003–2006), Smart Mater. Struct. 16 (2007) R1. [2] P.D. Mitcheson, E.M. Yeatman, G.K. Rao, A.S. Holmes, T.C. Green, Energy harvesting from human and machine motion for wireless electronic devices, Proc. IEEE 96 (2008) 1457–1486. [3] H.S. Kim, J.-H. Kim, J. Kim, A review of piezoelectric energy harvesting based on vibration, Int. J. Precis. Eng. Manuf. 12 (2011) 1129–1141. [4] K.A. Cook-Chennault, N. Thambi, A.M. Sastry, Powering MEMS portable devices—a review of non-regenerative and regenerative power supply systems with special emphasis on piezoelectric energy harvesting systems, Smart Mater. Struct. 17 (2008) 043001. [5] Y.C. Shu, I.C. Lien, Analysis of power output for piezoelectric energy harvesting systems, Smart Mater. Struct. 15 (2006) 1499–1512. [6] M. Philipose, J.R. Smith, B. Jiang, A. Mamishev, S. Roy, K. Sundara-Rajan, Batteryfree wireless identification and sensing, IEEE Pervasive Comput. 4 (2005) 37–45. [7] S. Roundy, P.K. Wright, J. Rabaey, A study of low level vibrations as a power source for wireless sensor nodes, Comput. Commun. 26 (2003) 1131–1144. [8] T. Krupenkin, J.A. Taylor, Reverse electrowetting as a new approach to highpower energy harvesting, Nat. Commun. 2 (2011) 448. [9] S. Priya, Advances in energy harvesting using low profile piezoelectric transducers, J. Electroceram. 19 (2007) 167–184.
Jae Hun Shin received the Bachelor’s degree of mechanical engineering from Myongji University in 2013. He currently is a graduate student in Myongji University and his research interests lie on MEMS and Microfluidics applications.
Il Song Park received the Bachelor’s degree of mechanical engineering from Myongji University in 2013. He currently is a graduate student in Myongji University and his research interests lie on the development of a magnetically driven microrobot for on-chip micromanipulation.
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Kyehan Rhee is a professor of the department of mechanical engineering at the Myongji University in Korea. He received his Ph.D. degree from the University of Minnesota, Minneapolis, USA and worked as a Post-Doctoral Fellow in the Pennsylvania State University, State College, U.S.A. His research interests include hemodynamics, microfluidics, polymer smart material actuators and their application in biomedical devices. He serves as an editor of the International Journal of Precision Engineering and Manufacturing and the Journal of Biomechanical Science and Engineering.
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Sang Kug Chung is an associate professor of the department of mechanical engineering at the Myongji University in Korea. He received the Ph.D. degree in Mechanical Engineering and Materials Science from the University of Pittsburgh in 2009 along with the Graduate Research Excellence Award. He received the M.S. degree from Pohang University of Science and Technology (POSTECH) and B.S. from Myongji University. He had worked for the development of the world first Liquid Lens at Samsung Electro-Mechanics from 2003 to 2009. Upon joining the faculty at Myongji University in 2009, he has directed the Microsystems Laboratory. And he has also served as a principal investigator in the Advanced Microfluids Engineering Research Laboratory (AMERL) since 2013. His research is in microfluidics and MEMS, including design and fabrication of micro/nano actuators and systems.
Please cite this article in press as: Y.R. Lee, et al., Energy harvesting based on acoustically oscillating liquid droplets, Sens. Actuators A: Phys. (2015), http://dx.doi.org/10.1016/j.sna.2015.03.009