converter controlled by a very low consumption circuit effectively maximizes the power flow into a ... reduced voltage at the DC side and the voltage stresses.
International Review of Electrical Engineering (I.R.E.E.), Vol. 7, N. 2 ISSN 1827- 6660 March-April 2012
Obtaining Renewable Energy from Piezoelectric Ceramics Using Sheppard-Taylor Converter Ahmet Karaarslan
Abstract – This paper presents a harvesting vibrational energy with piezoelectric ceramics using a single phase Sheppard-Taylor converter. In this study, the renewable energy is obtained by the piezoelectric ceramics and the converter regulates the power flow to the desired load. The control of the Sheppard-Taylor converter satisfies optimal working points using vibration-powered piezoelectric generators as a source. The circuit describes the generator’s power dependence and helps the definition of the load behavior for power optimization. The performance of the energy harvesting circuit is investigated by Matlab/Simulink program and laboratory conditions. Experimental results show that the converter controlled by a very low consumption circuit effectively maximizes the power flow into a 12 V. The rechargeable Lithium-ion battery charger connected to the converter output. The converter’s efficiency is above 80% for input voltages between 0.5 and 12.8 Vrms, and for output powers between 1.2 mW and 25 W. The presented circuit and control strategy can be used as well for power optimization of piezoelectric energy harvesting devices. Copyright © 2012 Praise Worthy Prize S.r.l. - All rights reserved.
Keywords: Piezoelectric Ceramic, Renewable Energy, Energy Conversion, Sheppard-Taylor Converter, Battery Charger
Nomenclature dp gp d33 g33 K 3t A tx r 0
Cpiezo ipiezo Vpiezo vp Cs d CCM CVM C0 fs fline
The ratio of short circuit charge density The ratio of the open circuit electric field Induced polarization in direction 3 Induced electric field in direction 3 Factor for electric field in direction 3 Plate area Thickness The relative permittivity of the material Air permittivity, 0 = 8.85… × 10 12 F/m Electrode capacitance The magnitude of the polarization current Open-circuit voltage Rectified voltage The value of super capacitor The duty cycle of switches S1 and S2 Continuous conduction mode Continuous voltage mode Clamped electrical capacitance Switching frequency Line frequency
I.
Introduction
In recent years, there has been an increasing demand for low-power and portable energy sources due to the development and mass consumption of portable electronic devices. Manuscript received and revised March 2012, accepted April 2012
Furthermore, the portable energy sources have to be associated with environmental issues and imposed regulations. These demands support research in the areas of portable energy generation methods. In this scope, piezoelectric ceramics become a strong candidate for energy generation and storage in future applications. Motion energy or vibrations are an attractive source for powering miniature energy harvesting generators [1]. Vibration energy can be converted into electrical energy through piezoelectric [2], electromagnetic [3], and electrostatic [4] devices. This paper focuses on usage of piezoelectric materials has been in particular investigated for converting ambient mechanical vibrations into electrical energy [4]-[8]. Currently, a significant amount of research has been done on piezoelectric generators, on the electronic circuits processing and storing electrical energy due to their small size and noninvasive harvesting method. A vibrating piezoelectric device differs from a typical electrical power source in that its internal impedance is capacitive rather than inductive in nature, and that it may be driven by mechanical vibrations of varying amplitude and frequency. While there have been previous approaches to harvesting energy with a piezoelectric device [6]-[8], there has not been an attempt to develop a circuit that maximizes power output. Energy harvesting by piezoelectric devices has great potential applications in self-powered sensor networks, portable electronic devices and other areas. Copyright © 2012 Praise Worthy Prize S.r.l. - All rights reserved
3949
Ahmet Karaarslan
It uses piezoelectric effect to convert mechanical vibration or the strain variation with time into electric energy and store it in energy storage devices such as super capacitors and rechargeable batteries. Various piezoelectric generators and interface electronic circuits between the piezoelectric generator and energy storage component have been researched to improve the energy-harvesting capability. The output power of the piezoelectric generators can be more than 100 mW, and each of them has its advantage under certain applications. The method of managing the rectified voltages from piezoelectric component has been investigated experimentally. The output power of such devices can be used for micromachining techniques and are limited from mill watts up to only a few watts. The output voltage of an energy harvester, generally, is not directly compatible with what is needed to power the load electronics; moreover, the power transfer from an energy harvester, generally, can be maximized by optimizing the load impedance connected to the harvester. Thus, a power processing circuit needs to be connected between harvester and load. For this purpose, single phase Sheppard-Taylor converter [10]-[13] that is controlled by a PIC16F877 microcontroller is used to maximize the power harvested from the piezoelectric ceramic. This topology allows a better current tracking at the AC side, with a relatively reduced voltage at the DC side and the voltage stresses on the switches are significantly reduced. Results showed that usage of the converter increased the power to the electrochemical lithium-ion rechargeable battery, by 300% as compared to when the battery was directly charged with a piezoelectric ceramic. The relatively low power levels of a single piezoelectric ceramic, however, prohibited a circuit implementation that could power the PI control circuitry while providing enough power for an additional electronic load. Operation states of a Sheppard-Taylor converter used as power optimization interface are analyzed, and conditions are defined. Then, simulation and practical implementation of a 25 W power optimization circuit with a PI control is demonstrated. Robustness of the control, prototype of the converter circuit and possible use for optimizing electromagnetic energy harvester are discussed. The experimental results are given to verify the simulation study before the conclusion.
A piezoelectric generated voltage of the same polarity as the poling field occurs, due to a compressive force applied parallel to, or a tensile force applied perpendicular to, the polar axis. Reversing the direction of the applied forces reverses the polarity of the generated voltage. The positive electrode on the finished ceramic shape is usually identified by a polarity mark. This is the electrode to which the positive voltage is applied during the poling operation. The lead zirconatetitanates are the most extensively used material for piezoelectric ceramics [8]. The electrical and mechanical unit conversions of piezoelectric ceramic are given in Table I: dQ dt
i
e
L
di dt
L
d 2Q
f
dt 2
(1)
dv dt
M
M
d 2s dt 2
(2)
TABLE I ELECTRICAL AND MECHANICAL UNITS OF PIEZOELECTRIC Electrical Units Mechanical Units e Voltage (Volt) F Force (Newton) i Current (Ampere) v Velocity (meter/second) Q Charge (Coulomb) s Displacement (Meter) C Capacitance (Farad) Cv Compliance (Meter/Newton) L Inductance (Henry) M Mass (kg) Z Impedance ( ) ZM Mechanical Impedance
The equations of capacitance piezoelectric are given as follows: K
C
0 rA
rA
tx
Q
CV
tx ve
Q
and
A tx AV tx
charge
of
(3)
(4)
The equations of electrical displacement and electric field are given as follows: D
E
II.
ds dt
v
V tx
Q A
V tx D
(5)
E
(6)
Piezoelectric Ceramic Description
The piezoelectric effect is exhibited in a certain group of crystalline solid materials whose unit cells do not possess a center of symmetry. These materials, when mechanically stressed, will produce an electrical charge. Currently the most widelyused piezoelectric transducer materials are polycrystalline ceramics based on lead-zirconate-titanate and barium-titanate compositions [14], [15].
Copyright © 2012 Praise Worthy Prize S.r.l. - All rights reserved
Coupling is a key constant used to evaluate the "quality" of an electro-mechanical material. This constant represents the efficiency of energy conversion from electrical to mechanical or mechanical to electrical. This constant is obtained in Eq. (7) [8]: k2
Mechanical _ Energy _ to _ Electrical _ Charge (7) Mechanical _ Energy _ Input
International Review of Electrical Engineering, Vol. 7, N. 2
3950
Ahmet Karaarslan
Piezoelectric constants (dp and gp) are given in Eqns. (8)-(9) respectively: Coulomb / metre 2
Ch arg e _ Density Stress
dp
gp
Newton / metre2 Volt / metre2
Electric _ Field Stress
Newton / metre
2
(8)
(9)
The equations of frequency, capacitance, generated voltage and strain that are obtained by lead-zirconatetitanate piezoelectric ceramic are given as follows [9]: Nt tx
f piezo
C piezo
V piezo
4
K 3T 0 tx
4 d p2
t x piezo
(10)
d p2
(11)
tx
d 33V
Values 0.04 0.002 300x10 -12 26.1x10-3 1300 2030 1.015 7.23 12.8 7.68x10-9 2 500
The electrical characteristics of vibrating piezoelectric elements can be modeled as a sinusoidal current source in parallel with its electrode capacitance Cpiezo. The magnitude of the polarization current ipiezo depends on the mechanical excitation level of the piezoelectric element (as characterized by the piezoelectric element’s unloaded or open-circuit voltage Vpiezo), but is assumed to be independent of the external loading conditions (Fig. 1). An ac-dc rectifier is connected to the output of the piezoelectric ceramics and the single phase SheppardTaylor converter of the piezoelectric harvesting device is shown in Fig. 2.
(13)
TABLE II THE PARAMETER VALUES OF PIEZOELECTRIC CERAMIC Parameters
III. Circuits Description and Operation
(12)
g33 F
The 5400 Navy piezoelectric material is used in simulation and experimental studies. The parameter values of piezoelectric ceramic are shown in Table II. It shows the structure and dimensions of the piezoelectric generator that is developed in this study.
dp tx d 33 g 33 K 3t Nt fpiezo Cpiezo Vpiezo txpiezo Cs F
The series branch R, L, and C represents the converted mechanical properties—effective mass, compliance and mechanical loss. This basic circuit is applicable at frequencies only near the first fundamental resonance, well-removed from any other resonant states.
[m] [m] [m/V] [Vm/N] [Const.] [Hz m] MHz nF V m F N
The electromechanical characteristics of a piezoelectric ceramic can be represented in the simplest form by the equivalent circuit in Fig. 1.
Fig. 1. Electrical circuit of piezoelectric ceramic
Copyright © 2012 Praise Worthy Prize S.r.l. - All rights reserved
Fig. 2. Electrical connections of piezoelectric harvesting device with Sheppard-Taylor converter
The most significant advantage of the super capacitors is their ability to be charged and discharged continuously without degrading. The super capacitors will supply power to the system when there are surges or energy bursts since super capacitors can be charged and discharged quickly. The output power of the piezoelectric element is the product of the output current and the rectifier super capacitor voltage. The magnitude of the polarization piezoelectric current generated by the piezoelectric element, and hence the optimal rectifier voltage, may not be constant as it depends upon the level and frequency of the mechanical vibrations. This creates the need for flexibility in the circuit, i.e., the ability to change the output voltage of the rectifier as the mechanical excitation changes to achieve and maintain the maximum power flow. To accomplish this, a single phase Sheppard-Taylor converter is placed between the rectifier and the Lithium-ion battery charger load as shown in Fig. 2 and Fig. 3. The current of the Lithium-ion battery charger must be put into about 0.1A until 4.1 volts is reached for each battery. Then, the voltage tightly is regulated at 4.1 V until the charge rate drops to 10 percent (0.01A) by PIC16F877 microcontroller. This charger uses a regulated supply for both parts of the charging. Only the charging current is monitored since the supply voltage is maintained at 4.1 V [16], [17]. The charging current is controlled using variable pulse width of the voltage feed.
International Review of Electrical Engineering, Vol. 7, N. 2
3951
Ahmet Karaarslan
The single phase Sheppard-Taylor converter operates in two types; discontinuous input current and continuous input current.
Following these explanations, applying the state-space averaging modeling technique in continuous current and voltage mode yields the following averaged model of the converter [14]: L1
diL1 dt
vp
L2
diL 2 dt
dvc
C
dvc dt
Fig. 3. Lithium-ion battery charger circuit
There are two operating states during a switching period when converter input current is in continuous mode. The different switching operating states (on/off) of the converter in CCM are shown in Figures 4(a) and 4(b), respectively [18], [19].
C0
1 2d iL1 diL 2
(16)
1 d iL 2
iL 2
_
vc
Y
(b) _
A
Copyright © 2012 Praise Worthy Prize S.r.l. - All rights reserved
(15)
iL1
X
State 2(dTs 0 always stand; these assumptions are justified by a suitable choice of inductor L2 and intermediate capacitor C. The models corresponding to time intervals of 0
