piezoelectric energy harvesting of rainfall is presented. Different features have been taken into account in order to define the limits in this energy harvesting.
Harvesting rainfall energy by means of piezoelectric transducer F. Viola, P. Romano, R. Miceli, G. Acciari DEIM – Dipartimento di Energia, Ingegneria dell’Informazione e Modelli Matematici Università degli Studi di Palermo - Viale delle Scienze, Palermo (Italy)
electromechanical model, as well as accurate uncertainty analysis of measured data is required. Another aspect regarding the production of electricity from rain is related to the protection of the soil. In times of sowing, the soil is protected by sheeting to protect the seeds. Sheets must be used to damp the energy of rainfall and prevent the disintegration and the collapse of the rocks which can be turned in hazardous landslides. In the first case the protective sheeting have seasonal character, not compatible with the installation of photovoltaic systems; in the second case the need to protect soil in steep points cannot provide ideal conditions for the deployment of solar panels. The question is if a piezoelectric sheet can provide a solution to these problems.
Abstract— In this paper a detailed study on the piezoelectric energy harvesting of rainfall is presented. Different features have been taken into account in order to define the limits in this energy harvesting. Only commercial transducers have been considered: a lead zirconate titanate and polyvinylidene difluoride transducer. Keywords—energy harvesting, piezoelectric.
I. INTRODUCTION Today, renewable energy sources are widely promoted worldwide [1-10]. Among renewable energy sources, fuel cell use has been deeply investigated for a wide variety of research areas, from handheld devices to household appliances [11-15]. Due to the combined heat-power generation option, fuel cells are the most promising source of energy for residential use, often coupled with other renewable sources as photovoltaic arrays. In recent years, together with a rapidly growing interest in renewable energy sources and their reliable working [1619], much attention has been given to the possibility of generating energy without the use of conventional thermal power or nuclear plants, in order to meet also the growing demand for energy in developing countries. A discussion that it is undertaken relates to convert, by means of piezoelectric plates, the kinetic energy possessed by the drops of rainwater into electrical energy [20-21]. For this application, since a limited amount of environmental energy is drawn, the power conversion efficiency of the whole harvesting system is the key target, even overcoming the conventional performance indicators [22-23]. In the literature, several solutions to improve the power conversion efficiency are proposed [24-28].The works reported in the literature agree that the single drop of water hitting the piezoelectric plates generates voltages less than the tens of volts. The objective of this study is to demonstrate whether similar values can certainly be used for feeding single electronic devices, by means of storage systems, or connected in series and parallel may be desirable for the supply of power equipment. Accurate system modeling is key to a successful conclusion of the overall design process. In the literature, the importance of accurate system modeling as well as the chance of a unique co-simulation environment to match several heterogeneous models is widely addressed [29-30]. For this purpose accurate
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II. PIEZOELECTRIC MATERIALS AND MODEL In the experiments two types of piezoelectric materials have been taken into consideration: piezoelectric ceramic Lead Zirconate Titanate (PZT) and polyvinylidene difluoride (PVDF). This choice was made to try to provide a comprehensive overview of the possibility to extract energy from precipitation. Indeed, the presence of lead in PZT transducer places him among the materials shall not be used to avoid contaminating the environment. Piezoelectricity is a property present in many materials, which, subjected to mechanical stress, develop electrical charges on their surface (direct piezoelectric effect) and, vice versa, subjected to an electric field, are deformed mechanically (effect inverse piezoelectric). The ability of piezoelectric materials to convert electrical energy into mechanical and vice versa depends on their crystalline structure. The necessary condition occurs because the piezoelectric effect is the absence of a center of symmetry in the crystal, which is responsible for charge separation between positive and negative ions and the formation of the Weiss domains, i.e. groups of dipoles with parallel orientation. Applying an electric field to a piezoelectric material, the Weiss domains are aligned in proportion to the field. Consequently, the size of the material change, by increasing or decreasing if the direction of the Weiss domains is the same as or opposite to the electric field. To describe in simplistic terms the direct effect it can be said that by applying an external force to a piezoelectric material the modification of the positions of the ions in the crystal lattice induces a separation of charge that
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produces an electric dipole with a single axis of symmetry. It is beyond the scope of this study provide an exhaustive description of the phenomenon and of the changes that have been developed to optimize the electrical performance and mechanical specifications, interesting discussion can be found in [31]. The piezoelectric transducer can be considered as a charge generator or a voltage generator. When the piezoelectric film is subjected to a pressure, inside charges are generated which give rise to an electric field. The electrodes that are located close to the surface, are affected by this field and accumulate on their faces a quantity of charge proportional to pressure. At this point the role of the transducer may be interpreted differently depending on the type of load that is connected at its ends: if the load has an input impedance very low, the charges that accumulate on the electrodes are poured entirely on it, similarly to a charge generator; if the piezoelectric material is connected to a high-impedance load the charges remain confined on the faces of the sensor thus keeping the electric field unchanged, as in a voltage generator. A suitable equivalent electrical model can be that of a voltage source in series with a capacitor or the equivalent Norton’s one. Also a resistance that connects the two ends of the active component can be used to refine the model, so introducing the electrical loss. In order to define the behavior of the electromechanical transducer also the mechanical part has to be modeled, Fig.1. Lm Rm
signals, since it fails to generate pulses at high pressure when the sheet is very large. If the device is placed in the cavity saturated with air, such as headphones or hearing aids, then the yield is much higher and the low frequency response of the piezoelectric is improved. The presented model depends also on the state of locking of the piezoelectric film: if it is bound by both ends, or only one. In order to improve the response of the model such choice has to be made: PZT can be easily locked by one or two ends, due to their rigid structure, only short PVDF can be locked by only one end, due to their flexible structure. Different studies in the literature show encouraging results with regard to the generation of electricity from water droplets. The piezoelectric transducers can reach tens of volts, but this result does not yet allow to attribute to them the character of power generators. The water drops continuity in the same place is very variable: there may be intervals of seconds (small rainfall) or fractions of seconds (downpour). The previous expressed voltage is a peak-peak voltage, not a continuous voltage, so an equivalent average voltage has to be defined. For a power system the equivalent average current can be obtained by using a bridge rectifier and a smoothing capacity; for the theoretical model initially this approach has been not considered. To evaluate the power output of a piezoelectric transducer is necessary to define a range of possible stresses. The single drop of water can have a diameter that varies between 0.2 to 6 mm. Considering a cruise speed on impact of approximately 2 m/s for the small drop and 9 m/s for the largest, it is possible to estimate the energy input: Emin=3.1J, Emax=0.063J. Also considering the interval of two seconds to have a successive drop, the power is: Pmin=1.5W , Pmax=0. 031W. The energy input is low, so no comparison can be made with a traditional photovoltaic system. The removable power, however, is affected by several factors. The drop, while centering fully the piezoelectric film, is not able to transfer maximum energy as it is subject to the phenomenon of splashing: the collision is not complete since the impact surface are separated some small drops. It must therefore associate an efficiency of a collision. In the same way we should introduce a performance of the electrical-mechanical system. The drop stresses the piezoelectric according to the 31 mode and not all the energy is converted into charges on the plates of the transducer. Finally an electrical performance coefficient is to be introduced to take into account the losses of the rectifying bridge. The output power is given by:
+ stress
Ce Cm
mechanical
V
n electrical
Fig. 1. Equivalent electro-mechanical scheme.
In the mechanical part the inductor Lm represents the equivalent mass and the inertia of the piezoelectric generator, Rm represents the mechanical losses, Cm represents the mechanical stiffness, stress generator is caused by mechanical vibration, n is the transformation ratio of the transformer equivalent, element that relates the physical quantities with those electrical [32]. Ce represents the capacitance of the piezoelectric element and V is the voltage across the piezoelectric transducer. Electric and mechanical parameters depend on the shape of the piezoelectric transducer. By applying pressure or torsion on the material, is created inside an electric field due to the polarization of the material. The application of force brings the internal lattice structure of the piezoelectric element to deform, causing the separation of the centers of molecular gravity, and therefore to the generation of small dipoles, which global effect is taken into account by modeling the transformer. A sheet of piezoelectric material has some limitations in the mechanical-electrical transduction for low-frequency
Pout = Școllision·Șpiezo·Șrect Pmax. The output power is certainly reduced, then the objective is to maximize it. The transducers on which the experiments were conducted are Mide Volture V22B and V22BL and the MEAS LDT1-028K. Volture the sensors were mounted in a suitable support, caged to extremes, due to their fragility. The sensor Meas was mounted as a cantilever in a suitable support.
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primary circuit, with secondary voltage which represents the voltage on the load of the piezoelectric generator. By employing the electromechanical model a first investigation on the shape of the transducer is made: considering a constant rain drop source of stress (5mm), the outgoing power is maximized if w=3.3mm and Le= 25-30mm. This result has been tested on PVDF transducers that have been shaped according to these directives. w
Lb
b Le Lto t
Fig. 2. Lead zirconate titanate (PZT) transducers: V22BL in the top and V22B in the bottom of the picture.
Fig. 4 Geometrical features of the PVDF transducer.
III. EXPERIMENTAL STUDY The piezoelectric transducers were exposed to rain and were measured the voltage values generated by the impact of the droplets. Roughly some typical generated waveforms are definable: transducers bound to the both ends have generated waveforms more regular, in which a first pulse largest is followed by a second smaller and of opposite sign, Fig. 5. Sometimes the second pulse has been followed by a third and sometimes was not present. In some cases we have obtained single negative peaks, this physical behavior has required a bit more attention since the physical stress did not change, but it happened to the tension. It was found that the negative peaks of tension occurred in correspondence with a state of piezoelectric plate already burdened with a water film, as a result of impact the compression status ranged, generating a negative peak of voltage. The voltage levels were maintained with maximum peaks of 6 V during the different tests. The piezoelectric sensor Meas has been used in cantilever configuration. An output voltage is represented in Fig. 6. The output voltage has an oscillating behavior, due to the presence of an underdamped system. The energy of the drop of water is absorbed and then released to the electrical system in a longer interval than that used by the Volture transducers. To characterize the behavior of the transducers were carried out some measures by placing a resistive load connected to the electrodes. Ten measurements for each load were considered useful. The output parameter of the experiments is given by the power exchanged during the impact. This power value is not straightforward to be assessed. It is chosen to bring the voltage pulses, which assume waveforms, similar to those of Fig. 5 and 6, equivalent to continuous values, evaluating the areas subtended by the pulse shapes.
Fig. 3 Meas LDT1-028k polyvinylidene difluoride (PVDF) transducer.
Fig. 2 and 3 show the piezoelectric transducers. The PVDF Meas is customable, the PZT Volture has a rigid structure and no modification can be introduced. The Meas piezoelectric has been deeply studied and here only a part of the study is reported [33]. The value of inductance present in the electromechanical model is: Lm
k1 k2 m,
where m is the mass of the raindrop, k1 and k2 are geometrical coefficient, given by:
k1
b 2 Lb Le 2 I b
k2
2 L3b 3b 2 Lb Le
,
with I(b) is the moment of inertia relative to a homogeneous mass; other quantities are reported in Fig.4. A more detailed study on the model will be published soon, but some important reflections can be reported. In the first place it is possible to observe that the reduction of the thickness b determines a decrease of the damping factor (it affects also the resistance). The magnitude of this reduction, however, is subjected to a particular constraint, a lower limit determined by the fact that it is necessary to fulfill the requirement of sub-damping model, otherwise, the similar system is no longer able to represent the physical one. Secondly, it can be stated that the turns ratio n increases with decreasing the thickness of the single layer PVDF and in general is influenced by the characteristics of the piezoelectric material. It is an important factor, since it relates the physical tension, of which it is made similar to that of electricity in the
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Power (W)
Figure 5 Three signals acquired: the first presents the typical behavior, a large positive pulse followed by a second smaller; the second signal acquired differs from the first one because of two negative pulses; the third signal acquired presents only positive pulse. The waveform of the signals is often conditioned by the presence of the water film on the transducers.
5,00E-06 4,50E-06 4,00E-06 3,50E-06 3,00E-06 2,50E-06 2,00E-06 1,50E-06 1,00E-06 5,00E-07 0,00E+00
single PVDF two PVDF
Fig. 6 Waveform of the voltage given by a PVDF transducer in cantilever configuration. Oscillations are due to the particular structure. Load (kɏ)
In Fig. 7 the power extracted from the single drop of water, in the case of PZT transducers, for to the loads of 10, 22, 47, 100, 220, 470 kȍ is represented. The two transducers have values of comparable powers, this is due to the fact that both have equal amplitude of piezoelectric material, while it varies the length of the material which constitutes the shell. In Fig. 8 the power extracted from the single drop of water, in the case of PVDF transducers, for to the loads of 10, 15, 22, 27, 33, 39, 47, 56, 68, 82, 100, 150, 180, 470 kȍ is represented. Two cases are studied a single PVDF transducer and two set in parallel. Contrary to what could be expected maximum values of power are attributable to the case of single piezoelectric and not to the two parallel.
Fig. 8 Power extracted from single drop of water using a single and a double PVDF transducers.
An explanation can be given to this behavior: the two drops of water do not affect the transducers simultaneously, so generating oscillating pulses in delay or with opposing phases, resulting in a reduction of voltage at the terminals. The shift to higher load resistors in the position of the maximum, in the case of the two PVDF in parallel, is attributable to an adaptation of the system to double capacity for constant pulsation of oscillation. In order to improve the power extractable from a system of two transducers in parallel, a bridge rectifier could be used, even if part of the signal generated by the drop can be absorbed by diodes of the bridge (0.6V).
1,30E-06 Volture V22B
Power (W)
1,10E-06
IV. DISCUSSION
Volture V22BL
If the energy possessed by raindrops is taken into account it is clear that it is not possible to compare the production of electricity between piezoelectric and photovoltaic systems. To make the comparison unfair is also the cost factor, piezoelectric systems for energy harvesting are experimental and their cost exceeds the equivalent photovoltaic for same harvesting area, this is especially felt in the case of PZT transducers. An interesting comparison concerns the materials used: PVDF and PZT can be used for the production of energy, PVDF has a lower cost then PZT, PVDF is not toxic, while PZT is toxic, and PVDF makes available to the electrodes higher power.
9,00E-07 7,00E-07 5,00E-07 3,00E-07 10
22
47
100
220
470
Load (kɏ) Fig. 7 Power extracted from single drop of water using the PZT transducers.
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By comparing the energy and the minimum and maximum power possessed by the drops of water, contained in the second paragraph, with those made available to the terminals of the transducers, it is clear that there is a low conversion efficiency, due to the phenomenon of splashing and also to mechanical losses of the system. A more detailed study will investigate the possibility of improving the performance of the system by acting on the mechanical characteristics of PVDF transducers. Furthermore, particular attention will be paid to partial discharge analysis on this transducer [34-35] A future development will study in detail the use of PVDF transducers for the supply of electronic devices that require power for short intervals and then switch to sleep mode. The feeding of these devices can easily be obtained by interposing an electronic circuit that implements a current pump between the PVDF transducer and the electronic device.
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