STUDY ON STRUCTURE OF PZT PILES BASED TRANSDUCER FOR HARVESTING ENERGY FROM ASPHALT PAVEMENT

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1 STUDY ON STRUCTURE OF PZT PILES BASED TRANSDUCER FOR HARVESTING ENERGY FROM ASPHALT PAVEMENT Hongduo Zhao * Professor, Key Laboratory of Road and Traffic Engineering of Ministry of Education, Tongji University, China * 4800 Coan Highway, Shanghai , China hdzhao@tongji.edu.cn Luyao Qin Master candidate, Key Laboratory of Road and Traffic Engineering of Ministry of Education, Tongji University, China Yujie Tao Master, Key Laboratory of Road and Traffic Engineering of Ministry of Education, Tongji University, China Jianming Ling Professor, Key Laboratory of Road and Traffic Engineering of Ministry of Education, Tongji University, China ABSTRACT: The purpose of this paper is to design a structure of PZT (Lead zirconate titanate) piles based transducer for harvesting energy from an asphalt pavement. Through finite element analysis and laboratory test, the PZT piles with various sizes were discussed and the performance of the transducer was evaluated. The results show that the PZT pile with round shape has less compressive and shear stress. The displacement difference at the surface of transducer decreases with the increasing of PZT piles number. Considering the displacement and fabrication costs, PZT piles number of 16 is suggested to the transducer. There is a minimum vertical displacement difference between pavement and PZT piles of transducer existing when the total area of PZT piles is 8cm 2, height is 2-9.5cm and thickness of cap is 5mm and 10mm. After two hundred thousand wheel loads, a lager deformation appeared and part of PZT piles were damaged in the transducer with 2mm of cap thickness. Before the transducer broken, it kept generating about 700V electric potential. KEY WORDS: Asphalt pavement, PZT, transducer, harvesting energy, piezoelectricity, finite element analysis 1. INTRODUCTION Energy harvesting is a process converting ambient energy into electric energy. One of the popular methods is utilizing the piezoelectric effect of transducer to generate electric energy from external stress. If the piezoelectric transducers are embedded in the asphalt pavement, the part of energies generated by the work of vehicle load and gravity can be harvested. PZT(Lead zirconate titanate) is the most popular piezoelectric material due to its high cost-effectiveness. Most of typical piezoelectric transducers are composed of PZT, such as Multilayer [1,2], Moonie [3], Cymbal [3], Bimorph [4], RAINBOW (Reduced and Internally Biased Oxide Wafer) [5], THUNDER (Thin Layer Unimorph Ferroelectric Driver and Sensor) [6] and MFC (Macro-Fiber Composite) [7]. However, none of them is designed for the asphalt pavement environment. Through finite element analysis (FEA), the performance of transducers Copyright 2013 IJPC International Journal of Pavements Conference, São Paulo, Brazil Page 1

2 can be evaluated by the electromechanical coupling factor k and the energy transmission coefficient λ max. A comparison of results is listed in Table 1 [8]. This results show that the PZT pile and Multilayer have the highest k and λ max, which indicates that they have high ability to transfer and output the energy. Cymbal and Bridge are also suitable for their k, λ max and medium stiffness. However, the energy level of those transducers is very low. They could be improved for further application. Table 1. Comparing results of transducers [8] Transducer k λ max U E (mj) Stiffness PZT pile High Multilayer High THUNDER Low Bridge Medium Cymbal Medium MFC Very low Moonie Medium The aim of this paper is discussing the performance of PZT piles with various structure parameters for harvesting energy from an asphalt pavement. Through finite element analysis, the relation between the displacement difference and size of PZT piles is presented and the coupling effect between transducer and pavement are also discussed. On this basis, the performance of transducer with suggested structure is tested. 2. PROTYPE OF TRANDUCER STRUCTURE In the design of prototype of transducer structure, the energy conversion efficiency and compression strength should be taken into account. The stress is mainly from the vertical vehicle load in the pavement, so it is considered as the major external stress for the PZT piles. Assuming the 3 rd axle is the vertical poling direction for PZT, the energy produced by external stress can be calculated by Equation 1 [9]. U E = 1 2 d 33g 33 T 3 2 At (1) Where: A is the area of PZT; d 33 is piezoelectric strain constant; g 33 is piezoelectric voltage constant; t is the thickness of PZT; T 3 is the stress on the top of PZT; U E is energy harvested from the pavement for single PZT. From Equation 1, it can be found that the U E is increase with the d 33, g 33 and T 3. Thus, the PZT 5H [10] with higher (d 33 g 33 ) value is selected as the piezoelectric material in this paper. In order to magnify the stress on the PZT, a structure shown as Figure 1 is designed. The stress on the PZT piles can be calculated by Equation 2. T 3 = σa c na (2) Copyright 2013 IJPC International Journal of Pavements Conference, São Paulo, Brazil Page 2

3 Where: A c is the area of the steel cap; n is the number of PZT piles used in one transducer; σ is the vertical stress in the pavement. σ A c T 3 A Steel PZT pile Figure 1. Structure of pavement transducer prototype As one type of PZT materials, PZT 5H was tested by the compression experiment in laboratory. Experimental results show that the compression strength of PZT 5H is about 262MPa [11]. Assuming the tire pressure σ=0.7mpa (25kN for one tire), A c =0.04m 2 and 1.3 of vehicle impact coefficient, it can be calculated that the total area of PZT piles should be more than 1.4cm 2. Otherwise, the PZT piles would be broken by the dynamic vehicle load. 3. STRUCTURE EVALUATION FACTORS A successful transducer should have the ability to convert as more as possible mechanical energy and maintain its deformation within limits. The displacement difference at surface of transducer can be used to evaluate the deformation. At the same time, the transducer should not reduce the performance of the pavement and increase the fuel consumption of the vehicle. It requires the transducer has good coupling effect with pavement, which means that the surface minimum displacement of pavement with transducer is almost same or even less than the displacement without transducer. Summarizing from above, the displacement difference and minimum displacement are used as the evaluation factors in the following analysis. 4. STRUCTURE DESIGN PARAMETER ANALYSIS ABAQUS is used in this paper to perform the finite element analysis of PZT plies based transducer. 4.1 Shape of PZT Piles Three shapes, which are square, hexagon and round (Figure 2 ~ Figure 4), of PZT piles are compared through finite element analysis. The area and boundary condition of these PZT are same in FE models. The maximum stress in PZT is listed in Table 2. It shows that the PZT with round shape has less compressive and shear stress, which means that this shape is suggested to PZT piles of transducer. Copyright 2013 IJPC International Journal of Pavements Conference, São Paulo, Brazil Page 3

4 Figure 2. Square shape Figure 3. Hexagon shape Figure 4. Round shape Table 2. Maximum stress in PZT with various shapes Maximum Stress Square shape Hexagon shape Round shape Compressive stress (MPa) Shear stress (MPa) Number of PZT Piles The displacement difference at surface of transducer with various number of PZT piles are analyzed through FEA. Different number of PZT piles corresponds to different layout. In order to reduce the displacement difference, PZT piles are symmetrical arranged in each layout. The layouts with different number of PZT piles are showed in Figure 5. Assuming the thickness of roof (h c ) is 5mm, the height of PZT piles (h t ) is 10cm and the total area of PZT piles (A t ) is 8cm 2, the maximum and minimum of displacement on roof can be obtained. The FEA results are showed as Table 3. a) 1 piles b) 4 piles c) 9 piles d) 12 piles e) 16 piles Figure 5. FE Model with various PZT piles Copyright 2013 IJPC International Journal of Pavements Conference, São Paulo, Brazil Page 4

5 Table 3. Displacement difference of PZT piles with various numbers Number of PZT piles Minimum displacement (mm) Displacement difference (mm) From the Table 3, it can be found that the minimum displacement and displacement difference decrease with the increasing of PZT piles number. The similar relation is explored for displacement shown in Figure 6. However, the more PZT piles lead to less single PZT piles area, which is easy to lose stability under the dynamic vehicle load. Considering the stress, displacement and fabrication costs, PZT piles number of 16 is suggested to transducer. Figure 6. Displacement of 16 PZT piles in FEA 4.3 Total Area and Height of PZT Piles and Thickness of Cap The deformation of the transducer is close related to h r, h t and A t. Thus, these design parameters should be discussed together in FEA. For purpose of compare the surface minimum displacement between pavement with and without transducer, the thickness of pavement should be the same as the height of PZT piles. The parameters of pavement structure are showed as Table 4. If the minimum displacement of pavement and transducer are almost same, it indicates that the transducer has good coupling with pavement. Table 4. Parameters of pavement structure Structure Resilient modulus (MPa) Poisson s ratio Density (g/cm 3 ) Wearing course Binder course The PZT piles of transducer with various h t and A t are analyzed in FE models with different h r. The comparing analysis results are showed as Figure 7~Figure 9. Copyright 2013 IJPC International Journal of Pavements Conference, São Paulo, Brazil Page 5

6 Figure 7. Minimum displacement vary in h r =2mm Figure 8. Minimum displacement vary in h r =5mm Figure 9. Minimum displacement vary in h r =10mm From those Figures, it can be found that the minimum displacement increase with the increasing h t and decreasing A t. The vertical displacement difference between pavement and PZT piles of transducer is less than 5%, when the A t =8cm 2, h t =2-9.5cm and h r =5mm and 10mm. As for the h r =2mm, the minimum displacement and vertical displacement difference are greater than others. It means that the transducer with h r =5mm and 10mm has Copyright 2013 IJPC International Journal of Pavements Conference, São Paulo, Brazil Page 6

7 better coupling effect than the one with h r =2mm. However, the more thickness of roof lead to less height of PZT piles, which would weaken the ability of transducer to energy conversion. Considering the vertical displacement difference and energy conversion efficiency, PZT piles of transducer with A t =8cm 2, h r =5mm and h t =2-9.5cm is decided as the optimum size. 5. PERFORMANCE TEST Based on the above analysis, PZT piles based transducers with round shape, 16 of piles number, A t =10.1cm 2, h t =2.1cm and h r =2mm and 5mm was made to test. Each PZT pile is compose of three small PZT cylinders which height is 7mm and diameter is 9mm. And the cap of transducer is a 20cm 20cm square sheet steel. The performance of this transducer was tested by a 1/3 Model Mobile Load Simulator (MMLS3). MMLS3 is a pavement testing frame with four groups of tire imitating the tire motion of motor vehicles. It could apply cyclic loading on the PZT piles of transducer. The maximum velocity and applied load of the tires are 2.5m/s and 0.75MPa. The picture of MMSL3 is showed as Figure 10. Before the test, the PZT piles of transducer would be placed on the same level with pavement, which is showed as Figure 14. In addition, Tektronix oscilloscope was used to collect and obtain the electric signal and voltage. Figure 10. MMLS3 instrument Figure 11. Placement of PZT piles During the experimental period, the PZT piles of transducer generated about 700V electric potential between the surfaces of small PZT pile. The output electric energy of the whole transducer is estimated about 3.21mJ. If the frequency of vehicle load is at 20Hz [12], then about =64.2mW electric energy will be harvested by each designed PZT piles of transducer from the asphalt pavement. The output electric energy can be increasing with the increasing the height of PZT piles. After two hundred thousand wheel loads of MMLS3, the central zone of steel cap with h r =2mm appeared lager deformation and part of PZT piles were damaged (see Figure 12). By contrast, the PZT piles of transducer with h r =5mm has no obvious deformation and damaged (see Figure 13). It indicated that the deformation resisting capability of transducer with h r =5mm is stronger than the transducer with h r =2mm, which confirm the validity of the FEA conclusion. At the same time, other structure parameters have proved to be reasonable. Copyright 2013 IJPC International Journal of Pavements Conference, São Paulo, Brazil Page 7

8 Figure 12. Deformation with h r =2mm Figure 13. Deformation with h r =5mm 6. CONCLUSIONS This paper discussed the performance of PZT piles based transducer for harvesting energy from an asphalt pavement. Through finite element analysis, the vertical displacement of PZT piles and displacement difference between pavement with and without transducer were analyzed to examine the suitability of transducer for pavement. At the same time, the output energy and deformation were obtained in the laboratory experiment. From the study presented in this paper, significant conclusions are made as follows: (1) In order to magnify the stress applying on the PZT piles to harvest more energy, a prototype structure of PZT piles based transducer was designed. Based on the compression strength of PZT 5H, the A t 1.4cm 2 is suggested for transducer to sustain tire pressure of 0.7MPa or more. (2) Considering the compressive and shear stress, round shape is recommended for the PZT pile section. The FEA results show that the displacement difference at surface is decrease with the increasing number of PZT piles. However, the more PZT piles lead to less single PZT piles area, which is easy to lose stability and increase the fabrication costs. Thus, 16 PZT piles is suggested to transducer. (3) The minimum displacement of transducer increases with the increasing h t and decreasing A t. When the A t =8cm 2, h t =2-9.5cm and h r =5mm and 10mm, the vertical displacement difference between pavement and PZT piles of transducer is less than 5%. It means that the transducer with h r =5mm and 10mm has better coupling effect with pavement. Considering the vertical displacement difference and energy conversion efficiency, PZT piles of transducer with A t =8cm 2, h r =5mm and h t =2-9.5cm is decided as the optimum size. (4) The transducers with 16 PZT piles, round shape, A t =10.1cm 2, h t =2.1cm and h r =2mm and 5mm were designed to test. In the performance test, the transducers generated about 700V electric potential between the surfaces of small PZT pile and outputted almost 3.21mJ of electric energy. (5) After two hundred thousand wheel loads, a lager deformation appeared and part of PZT piles were damaged in the transducer with h r =2mm. By contrast, the PZT piles of transducer with h r =5mm has no obvious deformation and damaged, which confirm the validity of the FEA conclusion. ACKNOWLEDGEMENT: This paper is supported by the National High Technology Research and Development Program (863 program) of China under Grant 2012AA112505, Science and Technology Copyright 2013 IJPC International Journal of Pavements Conference, São Paulo, Brazil Page 8

9 Commission of Shanghai Municipality Project under Grant and Chinese National Natural Science Foundation Project (No ). REFERENCES: [1] Heinzmann, A., Hennig, E., Kolle, B., Kopsch, D., Richter, S., Schwotzer, H. and Wehrsdorfer, E. ACTUATOR 2002, 8th Int.Conf, New Actuators, Bremen, Germany, [2] Uchino, K. Ferroelectric devices, CRC press, New York, [3] Dogan, A. Flextensional Moonie and cymbal actuators, Ph.D. Dissertation, the Pennsylvania State University, 1994, pp [4] Roundy, S.J. Energy scavenging for wireless sensor nodes with a focus on vibration to electricity conversion, Ph.D. Dissertation, Department of Mechanical Engineering, University of California, Berkeley, California, [5] Haertling, G. H. Electromechanical properties of Rainbow device, Applications of Ferroelectrics, Proceeding of the Ninth IEEE International Symposium on. August, University Park, PA, 1994, pp [6] Mossi, K. M., Selby, G. V. and Bryan, R. G. Thin-layer composite unimorph ferroelectric driver and sensor properties. Mater Lett, 35, 1998, pp [7] Smart Material Corp What is a macro fiber composite? [8] Hongduo, Z., Jianming, L. and Jian, Y. A comparative analysis of piezoelectric transducers for harvesting energy from asphalt pavement. Journal of Ceramic Society, Japan, 120, 2012, pp [9] Hongduo, Z., Jian, Y. and Jianming, L. Finite element analysis of Cymbal piezoelectric transducers for harvesting energy from asphalt pavement. Journal of Ceramic Society, Japan, 118, 2010, pp [10] Song, D. and Xiao, M., Piezoelectric effects and application, Popular science press, Beijing, China, [11] Yujie, T. Structure and performance of piezoelectric transducer for asphalt pavement energy harvesting. Master dissertation, Tongji University, Shanghai, China, [12] Lombaert, G. and Degrande, G. The experimental validation of a numerical model for the prediction of the vibrations in the free field produced by road traffic. Journal of Sound and Vibration, 262, 2003, pp Copyright 2013 IJPC International Journal of Pavements Conference, São Paulo, Brazil Page 9