Design Considerations for Piezoelectric Energy Harvesting Systems

Wahied G. Ali Electrical Engineering Department, College of

Engineering, King Saud University, P.O. Box 800, Riyadh 11421, KSA Wahied@ksu.edu.sa

Gihan Nagib Information Technology Department, College of Computer and Information Sciences, King Saud

University, Riyadh, KSA Gihan@ksu.edu.sa

Abstract—Ultra low power microelectronic devices such as wireless sensor nodes can be self powered using piezoelectric energy harvesting systems. Piezoelectric energy harvesting is the technology to convert wasted vibration energy to an electrical energy output. This paper highlights the design considerations for piezoelectric energy harvesting (PEH) systems such as: material selection, resonant frequency, geometrical shape, loading factor, electrical connections, AC-DC converters, DC-DCconverters, and energy storage media. Morphological analysis is developed to investigate the best alternative solution for each design parameter. The evaluation criterion is the maximum power transfer from input vibration energy to the electrical output waveform. The standards for PEH are also presented to enhance the system performance. The published results in the literature are discussed to affirm the concluding remarks in this paper.

Keywords—piezoelectric materials, energy harvesting, vibration, morphological analysis, design.

I. INTRODUCTION

The recent development of ultra low power microelectronic devices had led to the design of self power devices using energy harvesting techniques. Energy harvesting is a method to generate electrical power from natural (green) energy sources such as:

� Solar, wind, wave energy, and hydro-electricity for high power generation in megawatts

� Vibration, geothermal, light, and RF for low power generation in milliwatts

Mechanical vibration is a wasted energy which presents around most machines and the motion of biological systems. The idea of vibration-to-electricity conversion was proposed by Williams and Yates [1].

TABEL 1. Number of PEH publications Year sciencedirect ieeexplore2011 223 105

2010 128 104

2009 91 66

2008 78 58

2007 30 27

In general, there are three mechanisms to harvest the electrical energy from the vibration energy: electrostatic,electromagnetic, and piezoelectric [2]. The most of the experimental research on all transduction mechanisms are published in [3-4]. Table 1 shows the increasing attention in research field of piezoelectric energy harvesting during the last five years [4].

Piezoelectric materials are perfectly used for energy harvesting from ambient vibration sources, because they can efficiently convert mechanical strain to an electrical charge without any additional power and have a simple mechanical structure [4]. They also have many advantages over other alternative techniques such as: large power density, ease of applications, and capability to be fabricated at different scales: macro, micro, and nano-scale. The basic components in a generic piezoelectric energy harvesting system are shown in Figure 1 [5]. The functional description of basic blocks in this diagram is given as follows:

Vibration Source: Variable frequency and amplitude source of energy.

Energy Harvesting Transducer: Exchanges energy with the vibration source. Real power should be extracted from the source and converted to an electrical form. Reactive power is exchanged between source and generator as energy passes between the mass and spring. The frequency tuning may exist to allow the transducer to adapt its structure to maximize the harvested energy (optional for smart design).

Accelerometer: It may be necessary to have an independent measurement of the vibration source so that the target resonant frequency and damping can be calculated.

Figure 1. Piezoelectric energy harvesting system [5].

978-1-4673-4810-2/12/$31.00 ©2012 IEEE

Frequency Tuning Actuator: This is an optional block as it may be possible to tune the generator using reactive power exchange with the interface circuit. This block aims to match the resonant frequency of the harvester with the vibration source to extract the maximum energy from mechanical to electrical output.

Power Processing Interface: A circuit which connects to the transducer to enable maximum energy extraction from the transducer (i.e. maximum power point operation). The circuit configuration is highly dependent on transducer type. Real power will flow from transducer to the interface circuit. Controlling this real power will change the damping of the generator, which is a key to adaptability of the generator. In addition, a reactive power exchange between the interface circuit and the transducer could allow the resonant frequency of the generator to be modified.

Electrical Energy Storage: Required to cope with intermittency of generation and consumption. Probably, a super-capacitor or a rechargeable battery or combination of both can be used.

Voltage Regulation: This is required for two reasons: the voltage on the storage element may change depending on the rate of power generation and usage (and may change a lot if the storage element is a capacitor). In addition, the electronics load may request a particular voltage to be supplied in order to minimize its power consumption.

Computational Load: The part of the system powered by the energy harvesting system. This is the part of the load that does the work of the wireless sensor and has nothing to do with power generation. It will be energy aware and efficient, but it does not control the energy harvesting system at all. It can send a demand signal back to the harvester control for performance scheduling through output voltage of the voltage regulator.

Harvester Control: Computational circuits responsible for calculating the required damping and resonant frequency (or converting this into reluctance, or real and reactive power) and sending these demand signals to the frequency tuning component and the power processing interface. This module requires measurements of power/energy/voltage at different parts of the system to achieve its tasks effectively.

In the literature, several recent books are published in this research field [6-9]. Several review papers are also published in all different aspects concerning energy harvesting technologies [10-25]. In this paper, the selection of design parameters will be highlighted. The paper is organized as follows: Section II presents the morphological analysis for PEH systems. Section III highlights the standards for the PEH design parameters. While: section IV concludes the paper and presents the future research directions.

II. MORPHOLOGICAL ANALYSIS Morphological analysis is an effective creative thinking

technique in the design process. Its procedure involves analyzing the problem to identify the primary parameters which are involved and then to consider alternative solutions to achieve the required results. The primary parameters and

alternative solutions may then be listed in a matrix to be assessed and the best solution then will be picked up. The assessment criterion for piezoelectric energy harvesting system aims to maximize the energy conversion from mechanical vibration to electrical output. In the literature, the harvester resonant frequency has to match the frequency of the applied vibration source signal. This necessary condition increases the output electrical power to its maximum. Furthermore, design primary parameters to be investigated involve the following items:

� Material selection � Geometry and Structure of different layers � Loading modes d31 and d33 � Electrical connection, parallel and series � Fixation of piezoelectric cantilever � AC-DC Converters � DC-DC Converters � Storage media

A. Material Selection Sodano et al. developed a model of the piezoelectric energy

harvesting device and estimated the electric charge output for piezoelectric energy harvesting. In 2004, they reviewed piezoelectric power harvesting from vibration and discussed the future goals that must be achieved such as improving power output and energy conversion efficiency. They compared different cantilever piezoelectric energy harvesting devices, including MIDE Quick Pack, PZT-5A, PZT-5H, and MFC. The output power from either Quick Pack or MFC harvester was lower than extracted from PZT. PZT material was more effective to recharge capacitive loads. The low power outputs of Quick Pack and MFC resulted from their low capacitance characteristics [24].

In 2005, Sodano and Daniel compared between piezoelectric energy harvesting devices for rechargeable batteries. They compared the effectiveness of the MFC, Quick Pack and PZT for the use as energy harvesting devices. This was done by determining the efficiency of each device used in the experiments. They proved that the Quick Pack had a very high efficiency at resonance but not when excited at other frequencies. However, all of the efficiencies were fairly low because of the excitation method used. It was shown that as the capacity of the battery increases the Quick Pack begins to become less effective than the PZT. The PZT was proven to be more effective in the random vibration. Whatever, the PZT is brittle and can be easily broken in harsh vibration environments. Therefore, some of researches prefer to use Quick Pack for its robustness [26].

B. Geometry and Structure Selection In the literature, the best geometry to increase the output

power was the tapered shape; while the most commercial samples are rectangular in shape for ease of fabrication. Examples of piezoelectric structures are: unimorphs, bi-morphs, multilayered stacks, rainbows, s-morphs, moonie and cymbal [4]. A bimorph structure is the most commonly used in PEH applications. A bimorph is a cantilever consists of a thin metallic layer sandwiched between two thin piezoelectric layers.

a) Bimorph

b) Unimorph

Figure 2. Bimorph and unimorph energy harvesters.

A piezoelectric unimorph has one active (i.e. piezoelectric)

layer and one inactive (i.e. non-piezoelectric) layer (see Figure 2). Bimorph cantilever consists of a center shim laminated between two piezoelectric layers. It can improve the mechanical strength, although reducing motion, which improves the safety. Bimorph cantilever produces more output voltage than unimorph structure and is commonly used.

C. Loading Modes

The loading modes are illustrated in Figures 3-a and 3-b, wherein x, y, and z are labeled 1, 2 and 3, respectively. In 33-mode, the voltage and stress act in the same direction and in 31-mode the voltage acts in the 3 direction, while the mechanical stress acts in the 1 direction. For structures with a rectangular cross section and surface area, the poling direction is denoted as the ‘3’ direction, and 33 loading refers to the collection of charge on the electrode surface perpendicular to the polarization direction when tensile or compressive mechanical forces are applied along the polarization axis.

Figure 3. Loading modes of piezoelectric structures [4].

Figure 4. Bimorph cantilever electrical connections [4].

When a material experiences ‘31’ loading, the charge is collected on the electrode surface perpendicular to the polarization direction, when force is applied perpendicular to the axis of polarization. Since the coupling factor of the 33-mode is higher than 31-mode, the 33-mode can achieve higher energy conversion. However, for a very low vibration source and limited device size, the 31-mode conversion is more suitable for energy harvesting, since larger strains can be produced [4].

D. Electrical Connections

Depending on the polarization and wiring configuration of the layers, there are two electrical connections, series and parallel. Series connection refers to the case where voltage source is required to increase the output voltage. A two-layer device wired for series connection uses only two wires (one attached to each outside electrode), as shown in Figure 4-a. Parallel connection refers to the case where the current source is required to increase the output current. A two-layer bending element wired for parallel connection requires three wires (one attached to each outside electrode and one attached to the center shim), as shown in Figure 4-b. For the same motion, a two-layer element poled for parallel operation needs only half the voltage required for series operation. The two layers offer the opportunity to reduce output current by half and to increase output voltage twice when configured for series operation, which is commonly used in most applications.

E. Mechanical Fixation

Figure 5 shows two methods of mechanical fixation: a) cantilever beam and (b) simple beam. The case of cantilever beam has a power input at one end and the other end is free to move, this method provides maximum compliance.

a) Cantilever beam

b) Simple beam

Figure 5. Fixation methods for energy harvester.

The case of simple beam has two clamped ends and one input power at the centre. The cantilever case is more suitable to get the maximum power from the piezoelectric harvester.

F. AC-DC Converters

Different types of rectifiers can be used to convert the AC transducer voltage to DC voltage such as half wave bridge rectifier, full wave bridge rectifier and voltage doublers, as shown in Figure 6 respectively. Power efficiency is an important factor while choosing the suitable rectifier. The full wave diode bridge rectifier and the centre tap voltage doubler have a relatively higher efficiency than the half wave rectifier, if the load is resistive. The problems with the rectifier are the minimum voltage requirements due to the forward voltage drop of the two diodes in series and the leakage current when the diodes are reverse biased. So, it is an important to choose diodes with a low leakage and low turn-on voltage. Voltage doubler is an electric circuit with an AC input and a DC output of roughly twice the peak input voltage. A full-wave version of the voltage doubler has the advantage of lower peak diode currents, improved ripple and better load regulation but requires a centre-tap to the transformer as well as more components like center tap transformer. Full wave bridge rectifier is commercially used due to its simplicity and effective performance.

Recently, synchronous rectifiers can be used instead of Schottky diodes to improve the rectifier efficiency as shown in Figure 7. Here, the body diode of a MOSFET is used instead of a discrete pn junction diode. The transistor is turned on when the body diode begins conduction. The voltage drop across the body diode during conduction is negligible, consequently yielding very low power loss and high efficiency.

Figure 6. AC-DC Rectifier circuits.

Figure 7. Synchronous rectifier [8]

Figure 8. Buck, Boost and Buck-boost DC-DC Converters.

G. DC-DC Converters

There are different topologies of the DC-DC converters such as: buck (step-down), boost (step-up) and buck-boost (step-up/step-down) as shown in Figure 8. The boost converter is the most suitable for low excitation levels and for high efficiency, and it works at input voltage below the output voltage. Assuming no power losses and continues mode of operation as the duty ratio (D) is between zero and one (0 ≤ D ≤ 1), the boost converter will step up the voltage. The boost converter is used to increase low level dc voltage output from the bridge rectifier.

H. Energy Storage

Super-capacitors: capacitors are designed with varying power levels and can be composed of any of a wide assortment of materials, including glass, ceramics, metal film, and aerogel. Super-capacitor is a capacitor with remarkably high power and energy density, giving them much higher efficiency. Super-capacitors can also be described as mechanical batteries, due to their similarities to chemical batteries, and are very small in size. Super-capacitors have a high power- and energy-density, particularly compared with traditional capacitors. Super-capacitors are compact in more than just their size; they can store far more energy than traditional capacitors and can release their stored energy either slowly or quickly, depending upon the needs of the application.

Rechargeable batteries: a rechargeable battery or storage battery is a group of one or more electrochemical cells. Rechargeable batteries come in many different shapes and sizes, ranging anything from a button cell to megawatt systems connected to stabilize an electrical distribution network. Several different combinations of chemicals are commonly used, including: lithium ion "Li-ion" (3.6V), lithium ion Polymer "Li-Po" (3.7V), nickel Cadmium "NiCd" (1.2V), and nickel Metal hybrid "Ni-MH" (1.2V). In comparing between the rechargeable batteries and supercapacitors, the rechargeable batteries have more advantage as they have higher energy stored per unit weight and it has a slow discharge response.

Table 2 highlights the best design selections among the alternatives for PEH with italic and bolded characters.

TABLE 2. Design parameters for PEH Alternative Solutions

Design

parameters

Option3 Option2 Option1

MFC Quick pack PZT Material

Triple Morph

Bi-Morph Uni-morph Structure

d33 d31 Loading Modes

- Parallel Series Electrical connection

- Simple beam

Cantilever Beam

Fixation

Voltage doubler

Full wave rectifier

(Synchronous rectifier)

Half wave rectifier

AC-DC Converter

Buck-Boost

Boost Buck DC-DC Converter

- Rechargeable Battery

Super-capacitor Storage

III. STANDARDS FOR PEH

The International Society of Automation (ISA) is an organization that is setting standards for automation by helping over 30,000 worldwide members and other professionals solve difficult technical problems. This organization is founded in 1945. The ISA 100.18 power sources group works to match the requirements of the wireless sensor networks of third generation for active RFID, where every tag is a reader [27]. In this case, the sensor is referred as node and the network is called WSN. The standards ISA100.11a are defined for industrial wireless sensors. The ISA100.18 Working Group is preparing standards and information documents on power sources for WSNs. Key objectives are to define specifications for the interchangeability of various power sources, including batteries, energy harvesters, and other possible types, such as 4-20mA loops, and to define performance specifications so users can compare different harvesters and choose the optimum power source for each application. The working group is cooperating with different organizations, including VDI and NAMUR on battery standards for WSNs and other organizations using 802.15.4, such as WirelessHART and Zigbee as well as other low power wireless protocols [28].

The first draft of standard on vibration energy harvesting (VEH) is discussed in 2009; where the annual energy harvesting workshop was held in Virginia Tech (http://cpe.vt.edu/ehw). The committee was formed consisting members from academia, industry, and federal labs. This committee was assigned the task of compiling current practices used to characterize the vibration energy harvesting devices and come up with a metric which can allow the comparison of all prototype harvesters [8]. The committee stated the parameters to describe the vibration source as follows: � The acceleration values for vibration source should be

reported as peak-to-peak g level. The preferred unit for acceleration is in m/s2 described in terms of “g” where 1 g = 9.8 m/s2. Acceleration can be further categorized as

low (less than 10mg), mid (10–100mg), and high (above 100mg).

� The median frequency for vibration source should be reported in unit of Hertz. Frequency can be categorized as low (less than 10Hz), mid (10–120Hz), and high (above 120Hz).

The theoretical model to describe the vibration energy harvesting can use Williams-Yates method or Erturk–Inman method [8]. The testing of the VEH should include following three measurements [7]: � RMS power as a function of vibration frequency at fixed

acceleration (1g) and matching load, � Power as a function of acceleration at fixed frequency

(60 Hz) and matching load, � Power as a function of load at fixed acceleration (1g) and

frequency (60Hz). Characterization of the conditioning circuit [8]: � If the described system includes power conditioning, then

the rate at which conditioned power can be delivered to a defined load should be specified, namely, DC voltage level, load impedance, and current available to that load.

� It should be mentioned that how much of the harvested power is consumed by the power-conditioning circuit.

� Describe the output on-time rating in X msec at Y mA.

IV. CONCLUSION

Piezoelectric energy harvesting systems provides an elegant and infinitely renewable source of power to replace conventional batteries in wireless sensor nodes. The recent development of low power wireless sensors had led to harvest energy from ambient environment for self powered devices. Wireless remote monitoring of mechanical structures, self powered wireless sensors, and biomedical sensors are candidate strongly for piezoelectric energy harvesting applications. Piezoelectric energy harvesting has the following advantages over other alternative techniques: large power density and ease of applications and capability to fabricate the piezoelectric material to harvest the vibrating energy into different scales: macro, micro, and nano scale. The piezoelectric generator has a limited power and maximization is necessary to achieve the highest functionality per unit volume of the sensor node. The maximum power extraction (mechanical/electrical) depends on:

� The matching of resonance frequency between the harvester and the vibration source

� The harvester’s mechanical and electrical properties � The matching of impedance between the piezoelectric

generator and the sensor load � The maximum power point tracking MMPT in the DC-

DC converter design Therefore, the adaption to the system changes should be at

all the levels of the supply and consumption interactions. A good energy characterization of loads is essential to help effectively in power adaptation problem. More research is needed to handle this problem in run time. The power generated by piezoelectric materials is far too small to power most electronics. Therefore, methods of increasing the amount of energy generated by the power harvesting device or developing new and innovative methods of accumulating the

energy are the key technologies that will allow energy harvesting to become a power source for portable electronics and wireless sensors.

Additionally, the continuous research to minimize the circuit losses in power electronics is necessary to increase the power flow from the piezoelectric generator to the load. Power storage such as the use of rechargeable batteries or super-capacitor or both has to be effectively developed to improve the overall system efficiency before energy harvesting technology will see widespread use.

The future research is directed towards the integration of the piezoelectric transducer, its circuitry, power management, energy storage, and the RF sensor into flexible thin film substrates to develop smart active labels. Piezoelectric MEMS-based transducer is recommended to develop micro sensors for different applications. Recently, nano-based materials are available to harvest energy from light sources. The integration of this technology with piezoelectric technology can increase the generator power output from different sources like solar, RF, and vibration.

ACKNOWLEDGMENT

This work is supported by NPST program by King Saud University, Project Number 10-NAN1036-02.

REFERENCES [1] C. B. Williams and R. B. Yates, "Analysis of a Micro-Electric Generator

for Microsystems", Sens. Actuator A52:8-11, 1996. [2] P. D. Mitcheson, T. C. Green, E. M. Yeatman, and A. S. Holmes,

“Power Processing Circuits for Electromagnetic, Electrostatic and Piezoelectric Inertial Energy Scavengers,” Microsystem Technologies, Vol. 13, pp. 1629–1635, 2007.

[3] S. P. Beeby, M. J. Tudor and N. M. White, "Energy Harvesting Vibration Sources for Microsystems Applications", Measurement Science and Technology, Vol.17, PP.175-195, 2006.

[4] K. A. Cook-Chennault, N. Thambi, and 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, 043001 (33pp), 2008.

[5] Energy Harvesting Systems: A Block Diagram (July 16, 2010). Holistic Energy Harvesting, Available online: http://www.holistic.ecs.soton.ac.uk/res/eh-system.php.

[6] A. Erturk and D. J. Inman, "Piezoelectric Energy Harvesting", John Wiley & Sons, 2011.

[7] T. J. Kazmierski and S. Beeby (Editors), "Energy Harvesting Systems: Principles, Modeling and Applications", Springer Science, 2011.

[8] S. Priya and D. J. Inman, "Energy Harvesting Technologies", Springer Science, 2010.

[9] S. P. Beeby and N. White (Editors), "Energy Harvesting for Autonomous Systems", Artech House, 2010.

[10] M. R. Mhetre, N. S. Nagdeo, and H. K. Abhyankar, "Micro Energy Harvesting for Biomedical Applications: A Review", 3rd IEEE Int. Conf. on Electronics Computer Technology (ICECT), pp.1-5, 8-10 April, 2011.

[11] A. Harb, "Energy Harvesting: State-of-the-art", Renewable Energy 36 (Elsevier), pp.2641-2654, 2011.

[12] S. Saadon and O. Sidek, "A Review of Vibration-based MEMS Piezoelectric Energy Harvesters", Energy Conversion and Management 52, pp.500–504, 2011.

[13] J. Paulo and P.D. Gaspar, "Review and Future Trend of Energy Harvesting Methods for Portable Medical Devices", Proceedings of the World Congress on Engineering WCE2010 Vol II, , London, U.K., June 30 - July 2, 2010.

[14] A. Khaligh, P. Zeng, and C. Zheng, "Kinetic Energy Harvesting Using Piezoelectric and Electromagnetic Technologies—State of the Art", IEEE Trans. On Industrial Electronics, , Vol. 57, No. 3, pp. 850-860, MARCH 2010.

[15] D. JIA and J. LIU, "Human Power-Based Energy Harvesting Strategies for Mobile Electronic Devices", Energy Power Eng. China, 3(1): pp.27–46, 2009.

[16] R. Bogue, "Energy Harvesting and Wireless Sensors: A Review of Recent Developments", Sensor Review (29/3), pp. 194-199, 2009.

[17] S. Chalasani and J. M. Conrad, "A Survey of Energy Harvesting Sources for Embedded Systems", IEEE Southeastcon, pp.442-447, April 3-6, 2008.

[18] T. H. Guilherme, M. A. Galhardi , and V. L. Junior, " A Review of Power Harvesting Circuits and Applications", 7th Barazilian Conference on Dynamic, Control, and Applications, pp. 1-9, May 7-9, 2008.

[19] P. D. Mitcheson, E. M. Yeatman, G. K. Rao, A. S. Holmes, and T. C. Green, "Energy Harvesting From Human and Machine Motion for Wireless Electronic Devices", Proceedings of the IEEE Vol. 96, No. 9, pp.1457-1486, September 2008.

[20] G. Park, T. Rosing, M. D. Todd, C. R. Farrar, and W. Hodgkiss, "Energy Harvesting for Structural Health Monitoring Sensor Networks", Journal of Infrastructure Systems, Vol. 14, No. 1, pp. 64-79, March 2008.

[21] M. Pereyma, "Overview of the Modern State of the Vibration Energy Harvesting Devices", MEMSTECH’2007, Lviv-Polyana, UKRAINE, pp.107-112, May 23-26, 2007.

[22] M. T. Penella and M. Gasulla, "A Review of Commercial Energy Harvesters for Autonomous Sensors", Instrumentation and Measurement Technology Conference - IMTC 2007 Warsaw, Poland, May 1-3, 2007.

[23] L. Mateu and F. Moll, "Review of Energy Harvesting Techniques and Applications for Microelectronics", VLSI Circuits and Systems II, Proceedings of the SPIE, Volume 5837, pp. 359-373, 2005.

[24] H. A. Sodano, D. J. Inman, and G. Park, "A Review of Power Harvesting from Vibration using Piezoelectric Materials", The Shock and Vibration Digest, Vol. 36, No. 3, pp. 197-205, May 2004.

[25] S. O. Reza Moheimani,"A Survey of Recent Innovations in Vibration Damping and Control Using Shunted Piezoelectric Transducers ", IEEE Trans. On Control Systems Technology, V. 11, No. 4, JULY 2003.

[26] H. A. Sodano H. A. and D. J. Inman, "Comparison of piezoelectric Energy harvesting devices for recharging Batteries", LA-UR-04-5720, Journal of Intelligent Material Systems and Structures, 16(10), 799-807, 2005.

[27] P. Harrop, "Standards Will Speed Energy harvesting Adoption ", Energy Harvesting Journl, 2010. Available online at http://www.energyharvestingjournal.com/articles/standards-will-speed-energy-harvesting-adoption-00002342.asp?sessionid=1.

[28] R. Freeland, "Energy harvesting: A Practical Reality for Wireless Sensing", 2011. Available online at http://www.isa.org/InTechTemplate.cfm?template=/ContentManagement/ContentDisplay.cfm&ContentID=87300.