# A New Prospect in Road Traffic Energy Harvesting Using Lead-Free Piezoceramics

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## Abstract

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## 1. Introduction

_{2}emission to the atmosphere. For context, the 2018 annual report [1] about the Spanish electrical system shows that 19.8% was wind production, 13.8% was hydraulic, and 4.8% was solar (thermal, 1.8%, and photovoltaic, 3%).

## 2. The New Piezoelectric Characterization System

^{™}graphical language. The software commands the acquisition of the measured piezoelectric signal of the PDUT at the first routing stage to obtain the active piezoelectric simulation model, according to the periodical operation of the test bench. The software obtains the transient and the steady state of the energy harvesting measured voltage. The power and load regulation graphs are obtained by applying different loads. The open load voltage and the output equivalent impedance of the energy harvesting capacitor filtered rectifier circuit are computed. An example of the acquisition of four electrical signals from a PZT PDUT using the oscilloscope is presented in Figure 3a. In Figure 3b, our developed software user interface acquires channel number 1 of the piezoelectric response shown in Figure 3a.

- (1)
- The PDUTs are electrically characterized. Their impedance is measured with an impedance meter. The piezoelectric elements are placed in the test bench.
- (2)
- The test bench is set in action. The piezoelectric voltage is acquired and its active Fourier model is calculated. The active Fourier model is obtained by calculating each Fourier component of the inner piezoelectric generators, taking into account the input impedance of the measurement equipment and the impedance of the PDUTs.
- (3)
- The active Fourier model is sent to the LabVIEW
^{®}PSpice-based software module. An iterative process is started. The harvesting circuit formed by a capacitor-filtered rectifier stage is simulated for n different load resistance values. The high accuracy of the active Fourier models achieves a low simulation error. - (4)
- The VI computes the voltage–current and power graphs. A first estimation of the open circuit voltage (V
_{oc}) and the equivalent output resistance (R_{o}) of the harvester in the maximum power zone is obtained. - (5)
- The next step is to verify the accuracy of the first estimation obtained for the key parameters V
_{oc}and R_{o}. Analyzing the simulation results, a pair of appropriate values for the load resistance (R_{load1}and R_{load2}) are chosen. These resistor values are connected in the HEH module. - (6)
- The test bench is set in action. The voltage, current, and power are registered for both load resistance values.
- (7)
- The practical values of output resistance (R
_{o}), open circuit voltage (V_{oc}), and maximum power point (Po_{max}) are obtained and empirically verified.

#### 2.1. Piezoelectric Ceramic Material Characterization under Harvesting Conditions

_{pz}and the piezoelectric impedance Z

_{pz}are the elements of the active electrical model of the piezoelectric ceramic materials. The impedance of the measurement equipment is a key factor to calculate the active Fourier electrical model that predicts its behavior in whatever energy harvesting application. In this case, the measurement oscilloscope probe (Z

_{_meas}in Figure 6) has an equivalent input impedance of 10 MΩ in parallel with a capacitance of 4 pF when it is connected to the input impedance of the oscilloscope (which is of 1 MΩ in parallel with a capacitance of 11 pF).

_{pz}, when the spectrum of frequencies of the measured voltage V

_{o}is computed by the VI. Equation (3) calculates the measurement impedance Z

_{_meas}with C

_{p}and R

_{p}being the capacitive and resistive values of the probe connected to the oscilloscope, respectively.

#### 2.1.1. Impedance of the PDUTs

#### 2.1.2. Piezoelectrically Active Electrical Model

_{o}in Figure 6) was recorded in the VI to compute their Fourier spectrum. The modulus of the PZT Fourier analysis is shown in the Figure 9. The voltage V

_{o}measured with the oscilloscope and the modulus of the active generator from the spectral Fourier analysis, │V

_{pz}│, calculated by the VI are presented in Figure 10 for the PZT and the lead-free piezoceramics.

_{o}voltage is on the tens of volts range; meanwhile, the amplitude of the components in the inner active piezoelectric generator (V

_{pz}, see Figure 6) is on the order of magnitude of a thousand volts. The effect of the load impedance and the high impedance of the PDUTs explains this behavior in practical energy harvesting applications.

## 3. Energy Harvesting Results

_{_load}in Figure 11) is varied in successive simulations from 100 Ω (practical zone of short circuit) to 1000 GΩ (practical zone of open load) to obtain the voltage and current load graph. The practical graphic results are presented in Figure 12 for the PZT and lead-free PIC700 ceramic.

_{o}(see Figure 11) versus load current (Io) in resistor R

_{_load}is presented for tests at 58 km/h of simulated speed using PZT and PIC700 lead-free ceramics. The parameters R

_{o}(output resistance, calculated as the slope of the linear zone where maximum power is achieved) and V*

_{oc}(open circuit voltage: Intersection of the ordinate axis with the extended line of the linear maximum power zone) are the key factors to estimate the maximum power point of the harvesting power.

_{_load}applied equals the output equivalent (R

_{o}) resistance of the piezoelectric harvesting circuit. The parameter R

_{o}is previously unknown and is of significant relevance to design energy harvesting systems that achieve the maximum energetic efficiency. Our methodology calculates R

_{o}and estimates V*

_{oc}with high precision.

_{_load}is approximately three times lower in the lead-free ceramic.

_{oc}) of approximately √3 times greater. This conclusion opens the way to the ecological materials in alternative energy generation.

_{_load}values were selected to be in the linear zone of maximum harvesting power. The practical values of the accumulated voltage V

_{o}in the energy harvesting circuit are presented in Figure 13 for the PZT material. Table 4 calculates the practical parameter R

_{o}and the relative error (Er) between empirically validated data and previous results from simulations.

_{o}and the relative error achieved between previous results from simulations and test validated data.

## 4. Conclusions

_{oc}) of approximately √3 times greater. This conclusion opens the way to the ecological materials in alternative clean energy generation.

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 3.**(

**a**) Piezoelectric electrical signals from four lead-containing lead zirconate titanate (PZT) piezoelectric devices under test (PDUTs). (

**b**) Software interface acquiring one channel of electrical PDUT response to obtain the active electrical model.

**Figure 6.**Electrical equivalent circuit needed to obtain the active piezoelectric model in energy harvesting road traffic applications.

**Figure 7.**(

**a**) Open view of the commercial piezoelectric housing; (

**b**) exploded view of the commercial piezoelectric showing the lever mechanical amplifier and the piezoelectric material outside the holder; (

**c**) bottom view of the commercial piezoelectric placement in the test bench; (

**d**) top view of the PDUTs in the inner path of the road traffic simulator.

**Figure 10.**(

**a**) PZT material measured voltage; (

**b**) Fourier spectrum (modulus) of the active piezoelectrical generator for the PZT ceramic material; (

**c**) lead-free ceramics measured voltage; (

**d**) Fourier spectrum (modulus) of the active piezoelectrical generator for the PIC700 ceramic material.

**Figure 12.**Comparative results: PZT vs. lead-free piezoceramics: (

**a**) Regulation graph; (

**b**) power generated graph.

**Figure 13.**Transient response and steady state of the accumulated voltage in the capacitor (C

_{_load}= 1 μF) of the harvesting circuit when the PZT piezoelectric material is utilized in the Test Bench: (

**a**) Output voltage recorded by virtual instrument (VI) when using a set of resistors of equivalent R

_{_load1}= 300 MΩ; (

**b**) output voltage when R

_{_load2}= 50 MΩ.

**Figure 14.**Transient response and steady state of the accumulated voltage in the capacitor (C

_{_load}= 1 μF) of the harvesting circuit when PIC700 lead-free piezoelectric material is utilized in the Test Bench: (

**a**) Output voltage recorded by VI when using a set of resistors of equivalent R

_{_load1}= 300 MΩ; (

**b**) output voltage when R

_{_load2}= 50 MΩ.

Published [Reference] | Contribution |
---|---|

2010 [5] | Finite elements theoretical and simulation study of the application of cymbal-type housing for piezoelectric materials. 1.2 mW generated at 20 Hz |

2012 [6] | Several piezoelectric packages are studied using the finite elements technique for asphalt inlay highlighting cymbal and bridge for its efficiency in energy conversion |

2015 [7] | Three encapsulation options for bridge-type housing are studied to minimize the fracture of the piezoelectric material by fatigue. It is concluded that the arch bridge is optimal for burying on asphalt. An applied pressure of 0.7 MPa generated 286 V |

2016 [8] | A prototype consisting of 4, 8, or 16 piezoelectric disks sandwiched between two copper plates was assembled in-between asphalt mixtures. A uniaxial compression test was performed to measure the output power directly on a resistor |

2016 [9] | Based on the Ph. D. thesis of the first author, piezoelectric degradation measurements in an USA real road installation are presented. Over 14% of the asphalt stress produced by the vehicles is transmitted to the road-embedded prototypes producing 3.106 mW of harvested power |

2016 [10] | Two prototypes formed by stacked prismatic or cylindrical piezoelectric elements are tested in the laboratory. Assuming daily moderately busy USA Interstate highway traffic of 30,000 vehicles/day, the first prototype will produce 9.66 Wh per year and the second one 240.95 Wh |

2016 [11] | A cymbal structure is modified in seven piezoelectric parallelized sections. In a laboratory test over a 400 kΩ resistor, 2.1 mW of power is produced |

2016 [12] | An association of piezoelectric cantilevers produces 184 µW over an empirically optimized resistor of 70 kΩ. A Universal Test Machine (UTM) performs the laboratory tests |

2016 [13] | Wheel tracking tests are performed assuming a continuous rate of traffic. Several recommendations are obtained to adjust the geometry and composition of the piezoelectric material in order to maximize the extracted power in response to variable speed and distance between vehicles |

2017 [14] | Up to 60 PVDF layers are associated in parallel to generate 200 mW of peak power. Viability of using flexible material is shown |

2017 [15] | A new structure formed by a layer of piezoelectric material embedded between two layers of conductive asphalt generates 1.2 mW in UTM tests |

2018 [16] | A stacked array type of piezoelectric energy harvester is field-tested, generating a voltage between 250 and 400 V when a test vehicle is passes. The obtained piezoelectric energy lights LED signs |

2018 [17] | A new prototype of 11 stacked piezoelectric elements is presented and compared to the prototype results presented in [8]. The energy output estimated per prototypes I and II was 360 and 171 Wh annually |

**Table 2.**Piezoelectric (g

_{33}and d

_{33}) and elastic (s

_{33}

^{D}; or Y

_{33}= 1/s

_{33}) coefficients, dielectric permittivity and losses (K

_{33}

^{T}and tan δ), and electromechanical coupling factors (

**k**) of the lead-containing, hard lead titanate zirconate (Navy I-type PZT; APC International, Ltd., Mackeyville, PA, USA) and lead-free, tetragonal bismuth sodium barium titanate (BNBT) (PIC700; PI Ceramic GmbH, Lederhose, Germany) commercial ceramic materials (longitudinally poled cylinders of 6 mm diameter and 15 mm length). The catalog values are shown for PZT, and PIC700 was characterized using the resonance method (f

_{33}_{s}= 148.3 kHz, f

_{p}= 160.1 kHz).

Material | g_{33} (10^{−3} Vm/N) | d_{33} (10^{−12} C/N) | s_{33}^{D} (10^{−12} m^{2}/N) | K_{33}^{T} | tan δ (%) | k_{33} |
---|---|---|---|---|---|---|

PZT | 26 | >260 | 12.5 | 1280 | 0.6 | >0.68 |

BNBT | 16 | 98 | 7.5 | 710 | 0.4 | 0.40 |

Parameter | PZT | PIC700 |
---|---|---|

R_{o} (GΩ) | 2.36 | 5.57 |

V*_{oc} (V) | 5640 | 4800 |

Po_{max} (mW) | 3.4 | 1.03 |

Measurements | Simulations | Er % |
---|---|---|

${R}_{o}=\frac{\left|{V}_{o1}-{V}_{o2}\right|}{\left|{I}_{o1}-{I}_{o2}\right|}=\frac{645-115}{\left|2.12-2.35\right|\xb7{10}^{-6}}=2.30\mathrm{G}\Omega $ | 2.36 GΩ | −2.54 |

Measurements | Simulations | Er % |
---|---|---|

${R}_{o}=\frac{\left|{V}_{o1}-{V}_{o2}\right|}{\left|{I}_{o1}-{I}_{o2}\right|}=\frac{250.7-43.6}{\left|0.835-0.873\right|\xb7{10}^{-6}}=5.45G\Omega $ | 5.57 GΩ | −2.15 |

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**MDPI and ACS Style**

Vázquez-Rodríguez, M.; Jiménez, F.J.; Pardo, L.; Ochoa, P.; González, A.M.; de Frutos, J.
A New Prospect in Road Traffic Energy Harvesting Using Lead-Free Piezoceramics. *Materials* **2019**, *12*, 3725.
https://doi.org/10.3390/ma12223725

**AMA Style**

Vázquez-Rodríguez M, Jiménez FJ, Pardo L, Ochoa P, González AM, de Frutos J.
A New Prospect in Road Traffic Energy Harvesting Using Lead-Free Piezoceramics. *Materials*. 2019; 12(22):3725.
https://doi.org/10.3390/ma12223725

**Chicago/Turabian Style**

Vázquez-Rodríguez, Manuel, Francisco J. Jiménez, Lorena Pardo, Pilar Ochoa, Amador M. González, and José de Frutos.
2019. "A New Prospect in Road Traffic Energy Harvesting Using Lead-Free Piezoceramics" *Materials* 12, no. 22: 3725.
https://doi.org/10.3390/ma12223725