Laboratory and Finite Element Simulation of PZT

Laboratory and Finite Element Simulation of PZT Transducers for Potential Energy Harvesting from Roadway Pavements Hossein Roshani1, S.M. ASCE, Samer Dessouky2, M. ASCE, Arturo Montoya3 and A.T. Papagiannakis4, F. ASCE 1

Graduate Research Assistant, 2Associate Professor, 3Assistant Professor, 3 Professor Department of Civil and Environmental Engineering, University of Texas at San Antonio, One UTSA Circle, San Antonio, TX 78249

ABSTRACT The United States transportation system consists of about 6 million kilometers of highway pavements exposed to energy-potential resources from vehicle vibrations and traffic loading strains. Energy harvesting is a process that captures unused ambient energy such as heat, vibration, stress or movement that would otherwise be lost. Piezoelectric transducers (PZT) are considered ideal candidates for harvesting energy in pavement structures as they convert mechanical strain into low voltage. For this purpose, two types of piezoelectric geometry were evaluated; single layer cylindrical disks and shells, suitable for compression and bending state of stresses, respectively. A prototype surrounding the disks with copper plated was evaluated within asphalt mixture molds to study the effect of layer thickness, and loading speed and magnitude on output voltage. Finite element analysis was conducted to simulate prototype response and different geometry under loading. Results suggested that voltage is highly dependent on loading magnitude and speed and independent from the layer thickness at ambient conditions. Different PZT geometry have presented similar voltage output under similar loading conditions. KEYWORDS: Energy Harvesting, Piezoelectric, Highway, Asphalt Mixture, Finite Element

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INTRODUCTION Vehicle vibrations, traffic loading strains and thermal gradients are energy resources that can be potentially converted into usable electric power. Capturing the unused energy is the major and challenging aspect in harvesting process. Successful energy harvesting from highway pavements can lead to sustainable transportation infrastructure systems. The ability to capture this energy cheaply along with great efficiency and clean energy production is considered an area of great interest. The concept of energy conservation and development of alternative energy resources becomes urgently needed due to the high cost and environmental impact associated with fossil fuels. Consequently, the search for environmental-friendly, low-cost energy resources becomes more and more necessary (Sodano et al. 2004). One of the main sources of the unused ambient energy is the vibration and stresses caused by vehicles movements along the roadways. This energy can be harvested using piezoelectric transducers (Hill et al. 2014; Ali et al. 2011). The piezoelectric transducers (PZT) are capable of generating electric voltage due to the application of loading stresses and vibrations. The PZT is activated when subjected to load oscillation. As the magnitude of load change with time, the PZT polarizes to form two distinct surface charges forming the electric voltage (Phillips 2008). This voltage can then be used in charging batteries or capacitors or directly connected to the grid. Considering cycle traffic loading on pavement at any given point, the PZT can be used as potential candidate materials for energy harvesting. The PZT are available in several shapes and forms including single crystal (e.g. quartz), piezoceramic, thin film (e.g. sputtered zinc oxide), screen printable thick-films based upon piezoceramic powders and polymeric materials such as polyvinylidene fluoride (Beebly et al. 2006). Accordingly, they could be widely used to harvest vibration and strain energy caused by vehicles on roadways (Hill et al. 2014). Several attempts have recently been conducted to evaluate the possibility of using highways to generate useable energy. Kang-Won et al. (2010) focused on capturing the thermal energy from the temperature gradient across the pavement layers. They investigated the feasibility of solar cell or photovoltaic technologies as a harvesting solar energy system in roadway pavements. Current challenges in solar system are their susceptibility to sustain harsh conditions in pavement from excessive traffic loading and environmental condition changes. First field trial was attempted by Innowattech® in Israel roadways. Researchers embedded a series of PZT harvester devices under wearing surface layer (Baldwin et al. 2011; Edery 2010). Findings on the generated power was not documented. Recently Xiong (2014) conducted a field study to evaluate several energy harvester prototypes using PZT. The study demonstrated the ability to harvest 3.1 mWatt per passing vehicle. The study also remarked that the total output energy significantly depends on the total axles of vehicle. Studies concerning the application of PZT in highways to harvest energy have not been well researched yet. Various aspects such as the effect of traffic speed, traffic volume and vehicle type are still unknown. Therefore, there is a press need to evaluate the feasibility of using PZT as energy harvesters for roadway infrastructures.

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MATERIALS AND METHODS In this study, a prototype was developed using PZT to evaluate the effect of field-simulated variables such as traffic load and speed in generating electric voltage. The energy harvester’s components included two highly-conductive copper plates surrounding the PZT disks. Two 2mm thick nonconductive polystyrene sheets mounted to the plates were used to support the disks from slippage and align them in a uniform symmetrical pattern. The copper plates are 6.35 mm in thickness and 152 mm in diameter and act as electrodes to collect the electrical potential generated from the PZT disks. The plates are connected to an oscilloscope device through electric wire mounted to each copper plate. The oscilloscope monitors the voltage generated from the compression on the PZT. For maximum voltage output, single layer PZT of 8 mm in length and diameter were used. The PZT has a density of 7.6 gm/cm3 and Young's Modulus of 6.8e10 N/m2. To overcome surface irregularity, each PZT disk was glued at both ends with electrically conductive epoxy to ensure even and full contact with the copper plate. The two copper plates are housed in-between two asphalt cores as shown in Figure 1. A dense-graded asphalt mixture was molded using PG 70-22 and locally sourced limestone aggregates. The mixtures were compacted to 6 inches in diameter and sawed to meet specific thickness for the testing program. The laboratory setup can be seen in Figure 1.

Figure 1. The piezoelectric disks assembly and the prototype housed in the hydraulic loading system.

EXPERIMENTAL SETUP AND PROCEDURES The laboratory testing includes investigating the effect of pavement thickness, speed, traffic volume and loading in the prototype voltage output. The mix thicknesses above the prototype were 1.5 and 5 inches (38 and 127 mm) representing the minimum and maximum layer coverage for higher voltage output. The bottom thickness was remained at 5 inch representing supporting layers in existing structures. The loading duration is a function of size of vehicle speed. The increase in loading width is associated with slower vehicle speed. For different road classifications three loading widths 50, 100, and 150 milliseconds were considered. To account for traffic volume, each loading cycle was represented using the time between two consecutive tires. To find the duration of one cycle of loading in different traffic volumes, the traffic volume for interstate 3

highway, arterial road, and local road with average daily traffic (ADT) of 270,000, 100,000 and 10,000 respectively were considered. Traffic loading was determined for two types of vehicles: passenger car and eighteen-wheeler truck with tire pressure of 30 and 100 psi (207 and 690 kPa), respectively. The tire pressure and the prototype cross section were used to calculate the applied vertical peak load in the laboratory testing. All loading modes were represented by a sinusoidal wave shape, which is proven to be the most accurate shape to existing field loading pattern (Papagiannakis et al. 2008). To assess the time-dependency characteristic of the energy harvesting prototype, the total time duration of each test, containing numerous cycles, was also examined to evaluate the consistency and variability of the output voltage over time. The uniaxial compression test was performed on the laboratory prototype using the Universal Testing Machine to simulate the aforementioned field variables. Furthermore, the output voltage generated by the energy harvesting prototype was measured and monitored by an Oscilloscope. All tests were performed in ambient room temperature of 25° C.

RESULTS AND DISCUSSIONS Figure 2 shows the output voltage generated by 8 PZT disks for four different loads at three different loading durations. As expected, increasing the peak load and decreasing load duration resulting in increasing the output voltage. This suggests that increasing the rate of loading induces a larger output voltage. Since the prototype is going to be embedded in pavement, the evaluation of layer thickness above the prototype is an important criterion to study. The results in Figure 2 suggest that the top layer has negligible effect on the output voltage. This is expected due to the similarity of state of stresses imposed on the PZT in both cases. However, it is expected that changing layer thickness will alter the voltage response at elevated mix temperature. Moreover, the application of confinement (lateral) pressure in tri-axial testing setup will also exert the layer thickness effect. The simulated full load cycle duration of 270,000, 100,000 and 10,000 ADT were set at 0.67, 1.2 and 6 sec, respectively. This represent a full loading cycle between two consecutive axles of two vehicles. Increasing traffic volume shortens length of cycle (loading and unloading time). Figure 3 suggests that ADT has no effect on the instantaneous output voltage. However, the major advantage of higher traffic volume is increasing the voltage charge accumulation that can be stored in batteries. The figure suggests that the voltage is dependent on the loading cycle only and no effect is noticed from the full cycle length including the unloading (rest) duration.

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Figure 2. Output voltage due to different load magnitude and duration using a) 1.5 in and b) 5 inch top thickness

Load Width 50 ms

Load Width 100 ms

Load Width 150 ms

40 Output Voltage (Volts)

35 30 25 20 15 10 5 0 0

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2

3 4 5 Whole Load Cycle (sec)

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Figure. 3. Effect of traffic speed on output voltage for three different ADT.

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FINITE ELEMENT MODELING The laboratory testing was supplemented by a three-dimensional finite element simulation to provide a detailed insight of the vertical displacement and stress distribution induced on the PZT prototype. A quasi-static Finite Element Analysis (FEA) was conducted using ABAQUS. The geometric and material properties of the assembled prototype described previously were used to generate the sandwich-like model of the testing setup. The assembly parts were modeled with linear elastic isotropic material properties obtained from literature. The assemblage was discretized into a total of 145,344 eight-node (brick) elements. A 1 kN concentrated force was applied at the center of the top aluminum disk. The assembled prototype was free to deform radially, while roller boundary conditions were imposed to the bottom surface to replicate experimental setup conditions. The analysis results show that the load path mechanism is analogous to that of a slab-column structure. At the interface between the copper plates and the piezoelectric disks, the load is distributed equally among the eight piezoelectric disks due to their even radial arrangement. This behavior is supported by the uniform displacement field at the top surface of the PZT disks in Figure 4(a), where the top section of the assembly has been removed from the figure for clarity purposes. As the load is transferred from the piezo-devices to the bottom copper plate, the vertical displacement decreases in the radial direction. Figure 4(b) shows that the vertical-induced stress of the piezoelectric disks is also even on the top surface. Stress variations are observed along the length of the piezoelectric devices due to bulging effects as shown in the magnified column in Figure 4(b). Nonetheless, it is concluded that the mechanical behavior of the eight disks is equivalent and will produce comparable voltage.

Figure 4. Vertical (a) displacement (meter) and (b) and stress fields (Pa) on the bottom section of the assembled prototype.

EFFECT OF PZT GEOMETRY To simulate the effect of flexural bending of asphalt layer under tire loading, A PZT shell can be used. The shell is supported at the ends and allowed to bend under concentrated loading/displacement as shown in Figure 5. To investigate the effect of shape and loading a PZT 6

shell with 150 mm length, 30 mm width, and 8 mm depth was simulated under concentrated loading. This dimensions represent commercially available PZT shell for energy harvesting application. The shell under bending condition was modeled by applying a linear displacement in the mid-span. The displacement was considered to be 1.48E-6 m representing a 350 microstrain deformation from 18 wheeler truck loading condition using common material propertied of asphalt pavement in literate. As shown in Figure 5, the shell was polarized along the longitudinal length with estimated voltage of 10.7 volts determined by the absolute difference of mid- and end-span of the shell. In uniaxial loading conditions the PZT molds were generated around 80 volts with an average 10 volts/disk. It is suggested that the disk and shell will lead to comparable voltage output under similar loading conditions. Therefore, for maximizing energy harvesting a combination of the two geometries can be utilized. Further laboratory testing on PZT shell is ongoing to validate the finite element simulation.

Figure 5. The electric voltage generated by PZT shell under flexural loading

CONCLUSIONS To investigate the best performance of an energy harvesting system, a laboratory prototype was developed based on the mechanism of PZT materials. Several field-simulated variables such as pavement thickness, type of vehicle and traffic speed and volume, were examined. The study suggest that maximizing output voltage can be achieved through increased tire loading and speed. This will maximize the loading pressure and minimizing loading time. This suggest that interstate highway are great candidate for implementation of these devices. Traffic volume represented in the duration of unloading time between consecutive tires has no effect in instantaneous voltage measurements but it contributes to improving energy storage. Pavement thickness on top of the prototype had negligible effect in voltage output. However integrating temperature-dependent analysis and lateral confinement could better validate the role of layer thicknesses more precisely. Finally, the PZT geometry has led to similar voltage output under similar filed loading conditions.

REFERENCES 1-Sodano, H. A., Inman, D. J., and Park, G. A. (2004). “Review of Power Harvesting from Vibration using Piezoelectric Materials.” The Shock and Vibration Digest, 36(3), 197 – 205. 7

2-Hill, D., Agarwal, A., and Tong, N. (2014). “Assessment of Piezoelectric Materials for Roadway Energy Harvesting.” Energy Research and Development Division Final Project Report, DNV KEMA Energy & Sustainability, Oakland, CA. 3-Ali, S. F., Friswell, M. I., and Adhikari, S. (2011). “Analysis of Energy Harvesters for Highway Bridges.” Journal of Intelligent Material Systems and Structures, 22(16), 1929‐ 1938. 4-Beeby, S. P., Tudor, M. J., and White, N. M. (2006). “Energy harvesting vibration sources for microsystems applications.” Journal of Measurement Science & Technology, Vol. 17(12), 175–195. 5-Kang-Won, W., and Correia, A. J. (2010). “A Pilot Study for Investigation of Novel Methods to Harvest Solar Energy from Asphalt Pavements.” Final report for Korea Institute of Construction Technology (KICT), Kingston, RI. 6-Baldwin, J. D., Roswurm, S., Nolan, J., and Holliday, L. (2011). “Energy Harvesting on Highway Bridges.” Final Report FHWA-OK-11-01. Oklahoma Department of Transportation, 2011. 7-Edery, L. A. (2010). “Innowattech: Harvesting Energy and Data; A Standalone Technology.” Israel National Roads Company, Ltd. First International Symposium. The Highway to Innovation, Tel Aviv, Israel, 64-81. 8- Xiong, H. (2014). “Piezoelectric Energy Harvesting for Public Roadways,” Ph.D. Dissertation, Virginia Polytechnic Institute and State University, Department of Civil Engineering, VA. 9- Phillips, J. R. (2008), Piezoelectric Technology Primer. CTS Electronics Corporation, Albuquerque. 10- Papagiannakis, A.T., and Masad, E. (2008). Pavement Design and Materials. John Wiley and Sons Inc. Newark, NJ. 11- APC International, LTD. Physical and piezoelectric properties of APC materials, https://www.americanpiezo.com/apc-materials/physical-piezoelectric-properties.html (July 08, 2015). 12- ABAQUS 6.12 [Computer software], Dassault Systèmes Simulia Corp, Providence, RI.

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