Piezoelectric Energy Harvesting - Coursework Example

Piezoelectric Energy Harvesting Introduction This study focuses on how power can be harvested when vehicles travel on highways with the energy generated being used in powering of road lighting, signage and sensors. Cost effective harvesting of energy from vehicles moving along highways there may be a big step towards realization of infrastructure sustainability. A considerably high percentage of the greenhouse gas menace originate from the products of combustion of coal in the generation of electricity, and due to the limitation of transmitting power though grid lines, there is substantial losses when power is being used in areas that are far from their point of generation (Hooker, 1998). Thus having the lighting, signage and sensor systems being powered by the power harvested from the highway may be a good avenue of ensuring that there is reduced pollution and a limit put on the use fossil fuel. Objective The objective of the project is employing piezoelectric technology so as to generate amount of electrical power from the compressive loads of traffic along the highways. Concept The piezoelectric effect technology is a well known technology that can be employed to have mechanical energy directly converted to electrical form of energy. Piezoelectric effect is experienced naturally in polycrystalline structures without a centre of symmetry where polar axes of dipoles lack common direction as can be seen in figure 1. Figure 1: Crystal Dipoles On application of compressive stress to the crystal, the polar axes which in their initial states had random orientation polar axes will become aligned and thus producing an electric charge on the surface of the crystal as can be seen in Figure 2. Figure 2: Crystal Charge Production Under Mechanical Stress Direct piezoelectric effect can be represented by a matrix equation as In this equation D gives the electric displacement vector, T gives the stress vector, the dielectric permittivity matrix at constant mechanical stress is given by ε T , d represents the piezoelectric constant matrix while E gives the vector of the electric field. Fig.3: Road embedded with piezoelectric transducers A vehicle passing over a road will cause vertical deflection in the road, with the deflection being proportional the vehicle’s weight and the asphalt stiffness. The source of the electric energy which is harvested is a portion of mechanical energy that is linked to vertical deformation in asphalt which is a fraction of the energy generated by the vehicle through combustion of fuel. The vertical load from the wheels of the vehicle produces compressive stress that tends to reduce with depth. The ideal point where the piezoelectric generators are embedded is about 5cm from the road surface as this is a point of maximal compression stress. The load from the wheels deforms both the asphalt layer and the generators beneath, this portraying a similar typical deformation that is exhibited in piezoelectric column that is under axial loading. The scenario is as shown in figure 4 Fig.4. Production of electricity from roads Piezoelectric Energy Harvesting Coupling Modes The coupling modes that find application in piezoelectricity are the 21-mode and the 33-mode. For 31-mode, there is generation of energy by the piezoelectric material as a result of transverse displacement. Vibration is induced into the piezoelectric material through the excitation caused by vertical deflection in the pavement. In the course of the vibration, there will be maximization of both the amplitude of excitation and the transverse displacement when the vibration caused by the wheels has a frequency that is in resonance with the material with the power output being at its peak at the mean time. For the case of 33-mode there is a linear increase of power with increase in pavement deflection or the stress experienced along the poling direction of the piezoelectric material. Figure 4 illustrates the two modes of coupling courtesy of information gathered in Anton and Sodano (2007) review. For cases involving small forces, 31-mode is found to be more efficient in comparison to 33-mode Baker et al. (2005). However, in a situation where large forces are involved, the 33-mode of configuration exhibit higher efficiency and durability. According to Inman (2010), the deflection induced in pavement by a passenger car can be expressed as Where V is the velocity of the car in km/h. From the equations it is observed that for 31-mode the frequency of excitation and the power output are determined by velocity of car. With the power output being much lower at off-resonance and the vehicles velocity exhibiting randomness, the 33-mode becomes more suitable as opposed to the 31-mode in this application. Fig.5. The illustration of 33- (Left) and 31- (Right) Coupling Mode of Piezoelectric Energy Harvesting Selection of Materials Piezoceramics crystalline materials and polymers are commonly used as piezoelectric materials. Researchers and engineers prefer piezoceramic over the other materials because of high performance even though it as a shortcoming of being more brittle than the other materials. Lead-zirconate-titanate piezoceramic which has many variations with varied ratios of chemical composition has dominated the market owing to its cost effectiveness. In addition to the configuration, the properties of the piezoelectric material also play an important role in conversion efficiency. It has been noted in many studies that the level of transduction achieved is determined by piezoelectric charge constant, d, and piezoelectric voltage constant, g. when used in 33-mode with the constants being denoted by d33 and g33. With respect to test findings of Erturk (2009), the conclusion is that high piezoelectric coefficients would result to more efficient power output. Smaller elastic compliance was also proved to have contribution to good power performance. The findings by CD Richards indicated quality factor, Q is important in ensuring that the energy system is operating at its optimum where lower Q would translate to piezoelectric material having less damping and this could result into energy loss incurred in heat transfer processes (Richards et al., 2004). When materials with high quality factor are used, there would be minimal loss of energy and more energy would be available for conversion to electricity. Thus great quality factors are necessary for building an efficient energy harvesting system. Sensitivity of is an indicator of piezoelectric materials to transform pressure in electric energy. The voltage which is generated by the transducer approximates to Where V stands for induced voltage, E stands for induced electric field; s gives the spacing of electrodes. Also d is the piezoelectric coefficient, ε0 the permittivity of vacuum and ε being the relative permittivity of the piezoelectric material, T is the stress component. During material selection process, apparently, higher coefficient d33 should be considered. For a brittle material, there should be a limit to the strain level of PZT. In order to cope with high strain levels, a Poly Vinylidene Fluoride (PVDF) film coated with poly (3,4-ethylenedioxy-thiophene) / poly (4-styrenesulfonate) [PEDOT/PSS] electrodes has been developed by Lee et al. (2005). Rectification and Storage of piezoelectric power With excitation that results from traveling vehicles being random, it is expected that the power produced would exhibit randomness too and the random power is not compatible with the target end use which is micro-scale electronics. The voltage output generated by piezoelectric materials need to be rectified before storage or being put into use. The incorporation of artificial circuits is normally the solution to providing usable power from energy harvesting. The proposition put forward by Badel et al. is that there are two variations of the SSHI, Parallel SSHI (PSSHI) and Series SSHI (SSSHI) (Badel, 2006). There is parallel connection of the voltage processing for the case of PSSHI while for SSSHI the device is connected in series as can be seen in figure 6. The figure gives a case where the output power rectification consists of 4 diodes and a capacitor. In order to develop the highest DC current the forward voltage drop in diodes need to be as small as possible. Fig.6. The Layout of the SSSHI (a) and PSSHI (b) Interfacial Circuits. There is a scheme of the whole system shown in Fig. 7. Fig.7. The Scheme of the Interfacial Circuit for Energy Harvesting System. In Module 1 there is rectification so that it can be put into a form that is usable while in Module 2 there is optimization of the power flow coming from the rectified power output in Module 1. Module 3 is an optional module and this may serve to boost the power output that is generated in the energy harvesting system in some circumstance. By use of the bridge rectifier in module 1 all the negative voltage can be converted to positive. A combination of a loading resistor and a super capacitor would represent module 2. Addition of a certain loading resistor would help in maximization of the voltage on the charging capacitor or battery and this would ensure that the power from energy harvest is optimal and this would represent module 3. With the external excitation from vehicles tires on each piezoelectric material not being synchronized, there is a chance of production of opposite voltage interface by the piezoelectric material. In order to realize power output optimization in such a scenario there is need for addition of diode rectifier on each part of piezoelectric material in the direction of travel of the vehicle as illustrated in figure 8. Fig.8. The layout of the Candidate Interfacial Circuit. Conclusion This paper has given the fundamentals of design and evaluation of energy harvesting system through use of piezoelectric materials. It has been seen that piezoelectric energy harvesting can provide stable and long lasting clean power out put. The electrical energy produced, can be suitable for powering transportation infrastructure facilities such as sensors, small- and micro- scale healthy monitoring electronic. Reference Anton, S. and Sodano, H., (2007). A Review of Power Harvesting Using Piezoelectric Materials, Smart Materials and Structures, 16(3), pp. R1-R21. Badel, A., Benayad, A., Lefeuvre, E., Leburn, L., Richard, C. and Guyomar, D. (2006). Single Crystals and Nonlinear Process for Outstanding Vibration-Powered Electrical Generators, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, 53(4), pp. 673-683. Baker, J., Roundy, S. and Wright, P. (2005). Alternative Geometries for Increasing Power Density in Vibration Energy Scavenging for Wireless Sensor Networks, Proc. 3rd Int. Energy Conversion Engineering Conf., pp. 1-12, San Francisco, California, USA. Erturk, A. (2009). Electromechanical Modeling of Piezoelectric Energy Harvesters, Ph.D. thesis, Engineering Science and Mechanics, Virginia Tech, Blacksburg, VA, USA. Hooker, Matthew W. (1998). “Properties of PZT-Based Piezoelectric Ceramics between -150 and 250˚C.” National Aeronautics and Space Administration. Langley Research Center. Hampton, VA.. Inman, D. (2010). Response to Harmonic Excitation, Engineering Vibration, 3rd edition, pp. 136-138, Prentice Hall, Upper Saddle River, NJ, USA. Richards, C.D., Anderson, M.J., Bahr, D.F. and Richards, R.F. (2004). Efficiency of Energy Conversion for Devices Containing a Piezoelectric Component, Journal of Micromechanics and Microengineering, 14(5), pp. 717. Lee, C.S., Joo J., Han, S., Lee, J.H. and Koh, S.K. (2005). Poly (Vinylidene Fluoride) Transducers with Highly Conducting Poly (3,4-ethylenedioxythiophene) Electrodes, Proc. Int. Conf. on Science and Technology of Synthetic Metals, Vol. 152, pp. 49􀂱52. Read More

For 31-mode, there is generation of energy by the piezoelectric material as a result of transverse displacement. Vibration is induced into the piezoelectric material through the excitation caused by vertical deflection in the pavement. In the course of the vibration, there will be maximization of both the amplitude of excitation and the transverse displacement when the vibration caused by the wheels has a frequency that is in resonance with the material with the power output being at its peak at the mean time.

For the case of 33-mode there is a linear increase of power with increase in pavement deflection or the stress experienced along the poling direction of the piezoelectric material. Figure 4 illustrates the two modes of coupling courtesy of information gathered in Anton and Sodano (2007) review. For cases involving small forces, 31-mode is found to be more efficient in comparison to 33-mode Baker et al. (2005). However, in a situation where large forces are involved, the 33-mode of configuration exhibit higher efficiency and durability.

According to Inman (2010), the deflection induced in pavement by a passenger car can be expressed as Where V is the velocity of the car in km/h. From the equations it is observed that for 31-mode the frequency of excitation and the power output are determined by velocity of car. With the power output being much lower at off-resonance and the vehicles velocity exhibiting randomness, the 33-mode becomes more suitable as opposed to the 31-mode in this application. Fig.5. The illustration of 33- (Left) and 31- (Right) Coupling Mode of Piezoelectric Energy Harvesting Selection of Materials Piezoceramics crystalline materials and polymers are commonly used as piezoelectric materials.

Researchers and engineers prefer piezoceramic over the other materials because of high performance even though it as a shortcoming of being more brittle than the other materials. Lead-zirconate-titanate piezoceramic which has many variations with varied ratios of chemical composition has dominated the market owing to its cost effectiveness. In addition to the configuration, the properties of the piezoelectric material also play an important role in conversion efficiency. It has been noted in many studies that the level of transduction achieved is determined by piezoelectric charge constant, d, and piezoelectric voltage constant, g.

when used in 33-mode with the constants being denoted by d33 and g33. With respect to test findings of Erturk (2009), the conclusion is that high piezoelectric coefficients would result to more efficient power output. Smaller elastic compliance was also proved to have contribution to good power performance. The findings by CD Richards indicated quality factor, Q is important in ensuring that the energy system is operating at its optimum where lower Q would translate to piezoelectric material having less damping and this could result into energy loss incurred in heat transfer processes (Richards et al., 2004). When materials with high quality factor are used, there would be minimal loss of energy and more energy would be available for conversion to electricity.

Thus great quality factors are necessary for building an efficient energy harvesting system. Sensitivity of is an indicator of piezoelectric materials to transform pressure in electric energy. The voltage which is generated by the transducer approximates to Where V stands for induced voltage, E stands for induced electric field; s gives the spacing of electrodes. Also d is the piezoelectric coefficient, ε0 the permittivity of vacuum and ε being the relative permittivity of the piezoelectric material, T is the stress component.

During material selection process, apparently, higher coefficient d33 should be considered. For a brittle material, there should be a limit to the strain level of PZT. In order to cope with high strain levels, a Poly Vinylidene Fluoride (PVDF) film coated with poly (3,4-ethylenedioxy-thiophene) / poly (4-styrenesulfonate) [PEDOT/PSS] electrodes has been developed by Lee et al. (2005).

Read More