Hiroshi Maiwa

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/64162

#### **Abstract**

The piezoelectric material selection and the circuit design in vibrational energy harvesting are discussed. The performances of the energy-harvesting unimorph devices that captured frequencies of 60 Hz by using piezoelectric PZT-based and BT-based ceramics were evaluated. Output voltages and power from the devices depend on the amplitude and the frequency of the oscillations, and depend on the load resistance. Generally, PZTbased ceramics are superior for piezoelectric energy-harvesting applications. The figures of merit of the materials are discussed in order to provide the guidelines of the piezoelec‐ tric material selections. Piezoelectric voltage coefficient, *g*31, is considered to be good parameter to predict the maximum voltages. On the other hand, *d*31*g*31/tanδ, *k*312 *Q*m and *d*31*g*31 are close to the behavior of the maximum power. Combination of the piezoelectric unimorph and power management circuit produced the constant voltage output, which would be used as the power sources.

**Keywords:** energy harvesting, piezoelectricity, lead-free, power management circuit

## **1. Introduction**

Energy harvesting (EH) is the process of capturing small amounts of energy from external natural energy sources, accumulating and storing them for later use. In many cases, EH devices convert ambient energy into electrical energy. By combining suitable electronics, EH devices can be used for creating a self-sufficient energy supply system. The merits of the system include the replacement or supplement of the batteries and the minimization of the associated mainte‐ nance expenditure, and the replacement of the power supply cables. Major application target of EH is for independent sensor networks [1]. The aspect of the sensor network is illustrated in **Figure 1**. The sensor network is consisted of the wireless sensors placed on various places, such as human body, vehicles, and buildings, in order to monitor the physical or environmental

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conditions, such as temperature, humidity, sound,pressure, etc.Thedata are gatheredfrom the sensor nodes to data center through the gateway sensor node. The compositions of the sensor node are illustrated in the upper right side of **Figure 1**. Sensor node is consisted with a radio transceiver with an antenna, a microcontroller, an electronic circuit for interfacing with the sensors, and an energy source. EH is attracting an attention for the embedded energy source of sensor nodes and is considered to be one of the key technologies of Internet of things (IoT). In the last decade, the field of EH has increasingly become important as illustrated in the rising numbers of publications in **Figure 2**.

**Figure 1.** The architecture of the sensor network. The configuration of sensor network consisting of sensors placed on various places. The compositions of the sensor node are illustrated in the upper right side.

**Figure 2.** Year-to-year comparison of the numbers of papers on energy harvesting, 1958–2015.

The available energy from the environment includes solar power, thermal energy, wind energy, salinity gradients, and kinetic energy. Solar energy has the capability of providing large power density outdoors; however, it is not easy to capture the adequate solar energy in indoor environment. Mechanical vibration is the most attractive alternative [1, 2]. Vibrationelectrical energy harvesting using piezoelectric effects has been explored for possible use in sensor network modules [1–11]. In order to harvest energy from the environment, it is necessary to capture vibrations with frequencies less than 200 Hz, because such frequencies are dominant in normal life and in vehicles, as shown in **Table 1** [12, 13].


**Table 1.** Acceleration and frequency of the vibration sources in the environment [13].

conditions, such as temperature, humidity, sound,pressure, etc.Thedata are gatheredfrom the sensor nodes to data center through the gateway sensor node. The compositions of the sensor node are illustrated in the upper right side of **Figure 1**. Sensor node is consisted with a radio transceiver with an antenna, a microcontroller, an electronic circuit for interfacing with the sensors, and an energy source. EH is attracting an attention for the embedded energy source of sensor nodes and is considered to be one of the key technologies of Internet of things (IoT). In the last decade, the field of EH has increasingly become important as illustrated in the rising

**Figure 1.** The architecture of the sensor network. The configuration of sensor network consisting of sensors placed on

various places. The compositions of the sensor node are illustrated in the upper right side.

**Figure 2.** Year-to-year comparison of the numbers of papers on energy harvesting, 1958–2015.

numbers of publications in **Figure 2**.

130 Piezoelectric Materials

**Figure 3.** Typical components of the vibrational energy-harvesting systems. Mechanical energy is converted to electri‐ cal energy by energy harvester and is adjusted the output by the power management circuit.

Considering the application to the power source, appropriate power management circuit design to adjust to the requirement of the application devices. The block diagram of the vibrational energy-harvesting systems is illustrated in **Figure 3**. Energy harvester is vibrated by the excitation force of the vibration source, then the mechanical energy is converted to electrical energy by piezoelectrics in energy harvester. The generated electrical energy is consumed in the application circuits; finally, the optimum control of an electrical output in accordance with a load condition of the applications is required. Therefore, the power management circuit plays an important role in the system. The block diagram of the power management circuit is shown in **Figure 4**. Since the voltage and current of the electricity generated by the piezoelectric energy harvester are alternating, diode rectifier has required to produce direct current (DC) power supply. And the electricity from the piezoelectric power generator is large amplitude and frequency fluctuations; therefore, regulation circuit is required. DC-DC convertor is controlled by regulation circuit to adjust the requirement of the applications. The output voltage from the piezoelectric generator, the output voltage after rectification, and the controlled voltage output are shown in **Figure 5(a–c)**, respectively. Alternating voltage having a waveform of almost positive negative symmetry is generated by piezoelectric generator. By full wave rectifying circuit, the generated voltage was converted to one of constant polarity (positive or negative). Smoothing capacitor or filter is required to produce the steady direct voltage. The regulated circuit including capacitor and DC-DC convertor produced constant voltage output. In order to enhance the performance of the energy-harvesting circuit, a nonlinear processing technique "synchronized switch harvesting on inductor" (SSHI) [5], or active full-wave rectifier by using CMOS (complementary metal oxide semiconductor) technology [14], and power conditioning circuit with maximum power point tracking (MMPT) have been proposed [15].

**Figure 4.** Block diagram of the power management circuit in the energy-harvesting systems. The power management circuit is composed of the AC-DC converter, DC-DC convertor, and the controller to adjust the requirement of the ap‐ plications.

In order to obtain energy harvesters with high performances, material selections are important problems. From the view point, figures of merit of the materials have been discussed thus far. Priya has provided dimensionless figures of merit (DFOM) for a 3–1 mode transducer under on-resonance and off-resonance conditions, as follows.

DFOM = (*k*312 *Q*m/*s*11E)on-resonance

DFOM = (*d*31*g*31/tanδ)off-resonance

Here, *k*312 , *Q*m, *s*11E, *d*31, *g*31, and tanδ are the electromechanical coupling factor, mechanical quality factor, piezoelectric strain coefficient, piezoelectric voltage coefficient, and loss tangent, respectively [9].

Takeda et al. have suggested that power output from vibration-based generators should be expressed as linear functions of the term composed of electromechanical coupling coefficients *k*sys2 and the mechanical quality factor *Q*m\* of the generator, which enables output estimation using material constants *k*31<sup>2</sup> and *Q*m [10]. However, the aspects of the ambient vibration and required electrical characteristics as converters are diverse, and obtaining a full understanding of the appropriate material properties for various applications is still challenging.

generated by the piezoelectric energy harvester are alternating, diode rectifier has required to produce direct current (DC) power supply. And the electricity from the piezoelectric power generator is large amplitude and frequency fluctuations; therefore, regulation circuit is required. DC-DC convertor is controlled by regulation circuit to adjust the requirement of the applications. The output voltage from the piezoelectric generator, the output voltage after rectification, and the controlled voltage output are shown in **Figure 5(a–c)**, respectively. Alternating voltage having a waveform of almost positive negative symmetry is generated by piezoelectric generator. By full wave rectifying circuit, the generated voltage was converted to one of constant polarity (positive or negative). Smoothing capacitor or filter is required to produce the steady direct voltage. The regulated circuit including capacitor and DC-DC convertor produced constant voltage output. In order to enhance the performance of the energy-harvesting circuit, a nonlinear processing technique "synchronized switch harvesting on inductor" (SSHI) [5], or active full-wave rectifier by using CMOS (complementary metal oxide semiconductor) technology [14], and power conditioning circuit with maximum power

**Figure 4.** Block diagram of the power management circuit in the energy-harvesting systems. The power management circuit is composed of the AC-DC converter, DC-DC convertor, and the controller to adjust the requirement of the ap‐

In order to obtain energy harvesters with high performances, material selections are important problems. From the view point, figures of merit of the materials have been discussed thus far. Priya has provided dimensionless figures of merit (DFOM) for a 3–1 mode transducer under

quality factor, piezoelectric strain coefficient, piezoelectric voltage coefficient, and loss tangent,

Takeda et al. have suggested that power output from vibration-based generators should be expressed as linear functions of the term composed of electromechanical coupling coefficients *k*sys2 and the mechanical quality factor *Q*m\* of the generator, which enables output estimation

, *Q*m, *s*11E, *d*31, *g*31, and tanδ are the electromechanical coupling factor, mechanical

point tracking (MMPT) have been proposed [15].

on-resonance and off-resonance conditions, as follows.

*Q*m/*s*11E)on-resonance

DFOM = (*d*31*g*31/tanδ)off-resonance

plications.

132 Piezoelectric Materials

DFOM = (*k*312

respectively [9].

Here, *k*312

**Figure 5.** The output voltages (a) from the piezoelectric generator (b) after rectification with a full-wave rectifier using four diodes and (c) controlled by using power management circuit.

In our previous study [16, 17] the unimorphs—cantilevers with an active piezoelectric layer and an inactive elastic layer—were fabricated, and, prelaminar results on the performance were reported. Although EH devices fabricated by using thin film piezoelectrics are advanta‐ geous in the miniaturization and mass-productivity, EH devices fabricated by using bulk ceramics are superior in power output. Moreover, in the case of the unimorph EH devices by using bulk ceramics, material selections are easy by just replacing the ceramics. While, in the case of EH devices by using thin films, film deposition process is depending on the materials, comparative study of materials is relatively difficult. From the perspective of minimizing the environmental load by avoiding the use of lead-containing materials, consideration of leadfree piezoelectric materials is valuable for energy-harvesting devices [13]. Therefore, in addition to PZT-based ceramics, barium titanate (BT)-based ceramics was evaluated for piezoelectric materials in this chapter. The performance of piezoelectric energy-harvesting devices that captured frequencies of 60 Hz for PZT-based and BT-based ceramics was evalu‐ ated and the figures of merit of the materials have been discussed in order to provide the guidelines of the piezoelectric material selections. The results using power management circuit are included for evaluating the performance as the power source.
