ESTIMATION OF ACOUSTIC ENERGY HARVESTED FROM SOUND USING ELECTROMAGNETIC TRANSDUCER

15 th International Conference on Experimental Mechanics PAPER REF: 3174 ESTIMATION OF ACOUSTIC ENERGY HARVESTED FROM SOUND USING ELECTROMAGNETIC TRANSDUCER Nicolae Filip 1(*), Gabriel Fodor 2, Lucian Candale 1 1 Dep. Automotive Eng. and Transportation, Tech. University of Cluj-Napoca, Faculty of Mechanics, Romania 2 Dep. of Mechanical Engi., Tech. University of Cluj-Napoca, Faculty of Machine Buildings, Romania (*) Email: nicolae.filip@aert.utcluj.ro ABSTRACT This paper presents the results of the work carried out to estimate the electricity harvested from sound in laboratory conditions. Using a laboratory stand set up in this purpose and a data acquisition system designed to collect the experimental results, the tests were carried out to measure the electricity obtained from pure sound waves. The experimental results showed the best conversion around the transducer resonant frequency, at 30 Hz. Maximum electric values of 0,03105 A electric current and voltage of 0,304914 V were obtained for 81,7 db(a) sound pressure level of generated wave. The mathematical evaluation of the results offers information to establish the distribution law from to acoustic into electric conversion. Keywords: electromagnetic, sound, wave, conversion, voltage, electric current, exponential, coefficient. INTRODUCTION The environmental impact of the noise guides the researchers in the field of noise reduction and noise harvest. Providing electric energy from so-called waste energy (noise), offers more possibilities to assure new renewable energy for hand devices or for different applications. The researches developed in the last decades lead to three conversion ways of the acoustic energy: electromagnetic using the electromagnetic induction effect, electrostatic or capacitive and piezoelectric. Tiruthani (2008) and Yldiz (2009) described the mentioned noise harvesting ways presenting their advantages and disadvantages. The capability of electromagnetic devices to harvest the energy was explored and tested by a few authors. J Cho (2005) shows the fact that the electromechanical coupling coefficient is crucial for the performance of piezoelectric energy conversion devices. Richard (2008) proposed a new electro-magnetic transducer for converting ambient kinetic energy into useful electrical power. The piezoelectric way of conversion was the most explored. Sodano (2004) develops conversion techniques using MEMS (Micro-Electro-Mechanical-Systems). Horowitz (2005) demonstrates the capability of the vibrating energy conversion into electrical energy, starting from the same MEMS. Stewart (2008) develops the physical acoustics of energy harvesting for piezoelectric transducers. ICEM15 1

Porto/Portugal, 22-27 July 2012 Tenghsien (2008) proposed the design, fabrication, and characterization of an acoustic wave actuated microgenerator for power system applications in mobile phones. Jae-yun Lee (2009) using a resonator having a piezoelectric membrane wall, obtained up to 14V c.c. for elementary waves generated in the 100 Hz to 1400 Hz range. Chon and Lee obtained 0,05 1,65 V from environmental sound using acoustical cones for transforming the spherical waves into plane ones. Acoustical energy was also used as an intermediary step in converting thermal energy into electrical one. This is the case of Synko (2007) and Loh (2010). In this research an electromagnetic transducer was used in order to evaluate the harvesting efficiency for pure generated waves. EXPERIMENTAL STAND AND TEST SETUP The experimental stand (fig. 1) consists of: generating waves device (loudspeaker) (1), closed acoustic pipe (320 mm diameter and 1480 mm length) (2); N121 Environmental Noise Analyzer (3), memory card (4), microphone NOR 1225 (5) placed in front of the electromagnetic transducer SAL 3050 (6), Data Acquisition System (DAQ) (7), PC (8), booster (9) and HM507 oscilloscope (10). Fig 1. Experimental stand. 1 tone generator (loudspeaker), 2 acoustic pipe; 3 N121 noise analyzer; 4 flash card; 5 - microphone; 6 electromagnetic transducer; 7 data acquisition device; 8 PC; 9 booster; 10 oscilloscope. The input pure sound wave generated by a trial NGH Tone Generator, amplified via booster was furnished to the loudspeaker (1). The acoustic pipe ensures a plane wave displacement. The output electrical signal is discharged to the DAQ system having as input impedance a 10 Ώ resistor in parallel with a 470 µf capacitor (Filip and all 2001),. The acquisition frequency is software controlled in steps of: 10, 51, 124 and 249 Hz. The main characteristics of the electromagnetic transducer are: resonance frequency 29 Hz, power 120 W, electrical impedance 8 Ω, and maximum SPL 92 db. 2

15 th International Conference on Experimental Mechanics For the acoustic pipe were calculated: the resonance frequency, the acoustic and mechanical impedance. To calculate the pipe resonance frequency the equation for plane waves is used [14]: c n f r = (1) 4 l + 0,4 d ( ) where: c is the sound velocity 343, 26 m s-1 for the 20 0 C ambient temperature and corresponding to 1,2041 kg m -3 air density; n - natural odd number; l length of the pipe; d - the pipe diameter. In laboratory conditions (20 0 C) the calculated resonance frequency was: 52,7 Hz for corresponding sound speed. The calculated acoustic impedance and the mechanical impedance of the pipe were also found to be: 413,006 N s m-3 and 38,162 N s m-1. The pure sound waves are generated in the frequency range of 25 Hz up to 80 Hz using a 5 Hz step. The reason of the range of the generated wave setup was to cover both resonance frequencies (of the transducer and of the pipe) within an acceptable area. Increasing the amplitude of the wave, the voltage and electric current were measured for each frequency step. The sound pressure level (SPL) was also registered by an N121 analyzer. EXPERIMENTAL RESULTS The measurements carried out are detailed in voltage and electric current values for generated waves depending upon the sound pressure level. Each measurement consists of an acquired sample of about 2500 values with maximum acquisition frequency of 249 Hz. The registered data were statistically analyzed, figures 2 and 3 showing the calculated mean values. Electric current [A] 0.030 0.025 0.020 0.015 0.010 25 Hz 30 Hz 35 Hz 40 Hz 45 Hz 50 Hz 55 Hz 60 Hz 65 Hz 70 Hz 75 Hz 80 Hz 0.005 0.000 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94 96 98 100 SPL [db(a)] Fig. 2. The voltage harvested for different sound waves frequencies. ICEM15 3

Porto/Portugal, 22-27 July 2012 Voltage [V] 0.30 0.25 0.20 0.15 0.10 25 Hz 30 Hz 35 Hz 40 Hz 45 Hz 50 Hz 55 Hz 60 Hz 65 Hz 70 Hz 75 Hz 80 Hz 0.05 0.00 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94 96 98 100 SPL [db(a)] Fig. 3. The electric current harvested for different sound waves frequencies. The voltage and electric current distribution have the highest values measured for low frequencies around to the resonance frequency of the transducer. The influence of the pipe resonance frequency consists of maintaining high values of the sound conversion also for the 50 Hz and 55 Hz generated wave. Maximum mean values were 0,03105 A corresponding to 0,304914 V for 30 Hz wave frequency and 81,7 db(a) sound pressure level (SPL). The generated sound pressure level was limited by the transducer (inverted loudspeaker) characteristics. All measurements show the existence of an inferior limit of the sound pressure from which the energy conversion begins. Increasing the sound pressure level, we can observe an exponential distribution of the electrical signal harvested both in voltage and in electric current. THE EXPERIMENTAL RESULTS EVALUATION The experimental results evaluation consists of identifying a conversion law to precisely describe the energy recovered in terms of the harvested electrical energy values (in voltage and electric current). Having found a similar pattern of the acoustical into electrical energy conversion process for all the tests carried out, the identification of a probability law was needed. Following the regression of the experimental results, presented in table 1, we found that the best 4

15 th International Conference on Experimental Mechanics approximating distribution law for the tested electromagnetic transducer is of exponential growth type, having the equation (fig. 4): x t y = y 0 + A e (2) where: y 0 is the initial value (the origin); A and t the experimental regression coefficients and x the generated sound pressure level. Fig. 4. The exponential growth regression for the experimental results obtained for harvested electric current and voltage. For all fitted regressions the R square test results were up 0,9477 to 0,9895 and that fact showing an acceptable conformity of the results: approximately 94% of the variations in the response variable can be explained by the exponential growth distribution law (Nagelkerke 1991 and Balakrishnan 2011). The remaining 6% can be explained by unknown, lurking variables or inherent variability. Table 1. The exponential growth regression coefficients resulted for voltage harvested. Frequency [Hz] A coefficient t coefficient R square 25 5.60 10-10 3.90994 0.99049 30 5.96 10-9 4.65371 0.99126 35 5.26 10-10 4.19229 0.99504 40 4.38 10-11 3.87122 0.98062 45 8.6110-12 3.75171 0.97317 50 4.07 10-12 3.75604 0.97412 55 5.55 10-11 4.33263 0.96524 60 9.06 10-13 3.75927 0.96322 65 4.65 10-14 3.44742 0.95397 70 2.23 10-14 3.4334 0.94148 75 1.32 10-17 2.79337 0.962128 80 4.80 10-20 2.45399 0.95408 The next step of the results evaluation consists of explaining the constant terms of the equation (1). Considering the A coefficient variation depending on the frequency of the ICEM15 5

Porto/Portugal, 22-27 July 2012 generated wave a normal (Gauss) distribution resulted, with the 29,68 mean value near to the resonant frequency of the transducer (29 Hz) (fig. 5). Fig. 5. The A coefficient Gauss regression fit for experimental results. Analyzing in the same way the t coefficient it was appreciated that it represents the rate of growth of the harvested signal in voltage or electric current, depending on the generated wave frequency (fig. 6). In the case of t coefficient, a second peak was observed around the pipe resonant frequency too. Fig. 6. The t coefficient depending on the wave frequency and its polynomial regression. 6

15 th International Conference on Experimental Mechanics CONCLUSIONS The tests carried out show that the conversion of sound energy into electricity is possible even by using common loudspeakers as electromagnetic transducers. The existence of a lower pressure sound limit of the conversion yields from the experimental results. The regression of the experimental results enabled the identification of the distribution law which states the predictability of the conversion process as an exponential growth type, sustained also by R square test results values (in all cases over 94%). Regarding the coefficients of the identified distribution law, these explain the conversion conditions: around the transducer resonance frequency the harvesting efficiency is most favorable, corresponding to an optimum rate of conversion. ACKNOWLEDGEMENT This work was supported by CNCSIS UEFISCDI, project number PNII IDEI code 2531/2008. REFERENCES Balakrishnan, N., Rao, C.R. Handbook of Statistics, Elsevier, Vol. 20. 2001, pp.711. Bertsche, B. Reliability in Automotive and Mechanical Engineering. VDI-Buch, Springer- Verlag, Berlin Heidelberg 2008, pp 40 42. Blauert, J., Xiang, N. Acoustics for Engineers. Springer, 2008. pp 96-97. Cho, J. Anderson, M., Richards, R., Bahr, D., Richards, C. Optimization of electromechanical coupling for a thin-film PZT membrane: I. Modeling. Journal Of Micromechanics And Microengineering, 2005, pp. 1797 1803. Chon, A., Lee, J. Converting Ambient Sound into Electric Energy by Utilizing a Dynamic Cone Speaker. California State Science Fair. Project no S0703. Filip, N., Chiriciuc, M., Candale, L. About the noise energy conversion from road transportation. IN-TECH Proceedings, Bratislava, 2011, pp: 399-401. Horowitz, S. B., Sheplak, M., Cattafesta, L. N., Nishida, T. MEMS Acoustic Energy Harvester. PopwerMEMS, 2005, pp 13 16. Lai, T., Huang, C., Tsou, C. Design and Fabrication of Acoustic Wave Actuated Microgenerator for Portable Electronic Devices. DTIP of MEMS and MOMS, 2008. Lee, J-y,Kim, S., Kim, K., Kim, J., Choi, B. An Experimental study on the energy harvesting system using piezoelectric elements (PVDF). PowerMEMS 2009, Washington DC, USA, December 1-4, 2009, pp 324 327. Nagelkerke, N. A Note on a General Definition of the Coefficient of Determination, Biometrika, vol. 78, no. 3, pp. 691 692, 1991. ICEM15 7

Porto/Portugal, 22-27 July 2012 Richard, L. W. Brad, C. Hugo, J. Mark, F. Development of an Electro-Magnetic Transducer for Energy Harvesting of Kinetic Energy and its Applicability to a MEMS-scale Device. Nanopower Forum Costa Mesa, California 2008 Sodano, H. A., Park, G., Inman, D. J. Estimation of Electric Charge Output for Piezoelectric Energy Harvesting. Strain 2004, 40, pp 49 58. Stewart, S. The Physical Acoustics of Energy Harvesting. IEEE International Ultrasonics Symposium Proceedings, 2008, pp 1046 / 1055. Yildiz, F. Potential Ambient Energy-Harvesting Sources and Techniques, Teh Journal of Technology Studies, volume XXXV, Number 1, Fall 2009, pp 40-48. 8