2014 IEEE Students’ Conference on Electrical, Electronics and Computer Science
Piezoelectric Energy Harvester Design and Power Conditioning Dhananjay Kumar, Pradyumn Chaturvedi and Nupur Jejurikar Samrat Ashok Technological Institute, Vidisha (MP), India-464001
[email protected],
[email protected],
[email protected] Abstract— Energy harvesting is fascinating area of research now when the whole world is looking for green energy as an alternative source. This paper describes the design of energy harvester prototype and the power conditioning circuit. The optimization of extracted power out of the piezoelectric tile has been presented. The generation of electric energy when some load is applied on the sensors either in the form of direct strain or ambient vibration depends upon various factors such as number of piezoelectric transducers, electromechanical coupling coefficient of the piezoelectric sensors, amount of load applied, and also on the scheme of arrangement. Energy harvester floor tile has been designed with very inferior quality piezoelectric diaphragms which are used in buzzers. An efficient way has been presented to capture the generated energy via dedicated IC and boost it by a converter to get regulated output for charging the batteries of smart phones. The complete charge cycle has been studied for the developed system. The simulation and experimental studies have been successfully carried out. The model design and testing was purely for studying the energy generation and capturing phenomenon in an efficient manner. It can be implemented to generate large power by suitably considering the several factors mentioned above and implementing it on the large scale. Index Terms—Energy harvesting, piezoelectric sensors, power conditioning, Storage device, Boost converter.
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I. INTRODUCTION
HE SUN is the most important source of energy for the life on the earth either in direct or derivative form. Dependency on nonrenewable sources decreasing these sources day by day and in near future it may get exhausted completely. Hence it is required to explore for alternative sources and shift our dependency on renewable sources. This will conserve nonrenewable sources and produce clean energy. These renewable sources include solar cells (Solar energy), wind mills (Wind energy), geothermal power plants (Geothermal energy), tidal turbine (Tidal energy) etc. Solar power provides a considerable amount of energy per area and volume, but unfortunately is limited to applications that are actually sunlit. We utilize a large part of our muscular energy for moving from one place to other and also the infrastructure like roads, railways, runway bears a large amount of mechanical strain energy. This energy i.e. muscular or mechanical strain on various infrastructures gets wasted. But it is possible to convert this mechanical energy in to electrical pulse form with the help of piezoelectric transducers. These electrical pulses, which are alternating in nature, can be directly utilized or may
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be captured by a storage device for further utilization. Efforts have been put in this work to harvest energy from mechanical stress using the principle of piezoelectric energy conversion. For a harvesting system of constant thickness, the generated power increases with increase in applied force. The output power of harvester depends on increase in the thickness [1]. Various models of piezoelectric generators are given in [2-4], [6-8]. The output power obtained from piezoelectric generators depends on various factors like which piezoelectric sensor has been used, it’s packing density, type of strain applied to it, electronic circuitry to process the pulse generated, storage device, and load connected to it. When a simple rectifier is used the output power generated greatly depends upon the load connected [5]. The important criteria for maximizing the output power are to match the optimal load of the harvester to that of converter circuit [9]. Several techniques are available for converting mechanical vibration energy to electrical energy. The most prevalent methods among them are electrostatic, electromagnetic and piezoelectric conversion [11]. A majority of current research has been done on piezoelectric conversion due to low complexity of its analysis and fabrication. Most of research however has targeted a specific device scale [12]-[15]. The latest advancement in the micro-electromechanical system systems (MEMS) and wireless technology, the portable electronics and wireless sensors are in great demand. These portable devices must have their own power supply. If this supply is a conventional battery, then using this type of power supply will be problematic as their life span is finite. In portable electronics, replacing the battery may destroy the electronics any time. For sensors which are planted in the remote locations or in the host body, if battery has been discharged the sensor must be retrieved and the battery should be replaced. Because of remote location of the mobile host body, it is quite difficult to retrieve the sensor and replacing the battery. If a sensor is embedded inside a civil infrastructure then it is not possible to replace the battery. If the adequate energy in the surrounding medium could be obtained, then it can prove as the substitute of the battery. One method is to use the piezoelectric material to obtain the energy lost due to vibration of the host structure. This captured energy can be processed and could be used to prolong the life of the power supply or to provide the endless energy to a device. The host structure may be a mobile floor, roadway, pedestals, rail, runway etc where a continuous strain is experienced and this strain or vibration energy which was wasted earlier may be transformed in to usable electrical energy to power up the low power electronic and electrical devices.
2 Piezoelectric energy harvesters are device which convert the mechanical strain in to electrical form. Centimeter scale piezoelectric elements are generating milliwatts range electric power using ambient vibrations for a frequency below 1 KHz. They are the perfect solution for extended life micro power generator as they generate enough power to drive low power electronic devices such as smart wireless sensor which dissipates less than few milliwatts [10],[15]-[19]. A vibrating piezoelectric element electrically behaves as a capacitive ac source [20], [5] which is rectified at later stage at a desired dc voltage level to be useful for powering electronic devices. This paper presents the basic arrangement of six numbers of double sided piezoelectric diaphragms along with a shaker modal with energy harvesting circuitry, generating variable rectified output between 1- 5 volts, a boost converter to get regulated output of 5 Volts for load utilization. This boosted DC output is then used to charge the smart phone. Energy harvester firstly designed with Diode Bridge and electrolyte capacitor as the storage and then the diode bridge was replaced by energy harvesting IC and electrolyte capacitor was replaced by ultra capacitor. It is found that piezoelectric energy harvester faces low drop with dedicated IC than Diode Bridge and also the ultra capacitor response to store energy is quite fast. The generated power can be scaled by designing a robust piezoelectric load bearing mechanical structure comprising very strong piezoelectric discs arranged in multilayer stack. II. FUNDAMENTAL OF PIEZOELECTRIC ENERGY HARVESTING Piezoelectric materials belong to a wider class of materials called ferroelectrics. Ferroelectric material has a property that their molecular structure is oriented in such a way that material exhibit local charge separation, known as an electric dipole. These electric dipoles are randomly oriented throughout material composition, but when the material is heated above a certain point known as Curie temperature, and a very strong electrical field is applied, the electric dipoles reorient themselves relative to the electric field; this process is called polling. After the material is cooled, the dipoles maintain their orientation and the material is said to be poled. After the completion of the polling process the material will exhibit the piezoelectric effect. The mechanical and electrical behavior of a piezoelectric material can be formulated by two linear constitutive equations. These equations contain two mechanical and two electrical variables. The direct effect and the converse effect can be modeled by the following matrix equations (IEEE Standard on Piezoelectricity, ANSI Standard 176-1987): {S} + [ (1) Direct piezoelectric effect: Converse piezoelectric effect: {T} = [ ] {S} – [e] {E} (2) Here, {D} is the electric displacement vector, {T} is the stress vector, [e] is the dielectric permittivity matrix, [ ] is the matrix of elastic coefficients at constant electric field strength, is the dielectric matrix at {S} is the strain vector, [ constant material strain, and {E} is the electric field vector. After the material has been poled, an electric field can be applied in order to induce an expansion or contraction of the
material. However, the electric field can be applied along any surface of the material, each resulting in a potentially different stress and strain generation. Therefore, the piezoelectric properties must contain a sign convention to facilitate this ability to apply electric potential in three directions. Piezoelectric material can be generalized for two cases. The first is stack configuration that operates in the -33 mode and the second is the bender, which operates in -31 mode.
Fig. 1 Direct piezoelectric effect- Electromechanical Conversion [1]
The earliest experimental result on crystals of tourmaline, quartz, topaz, cane sugar and Rochelle salt by Pierre and Jacques Curie in 1880 showed a great scope. Quartz and Rochelle salt exhibited most piezoelectricity. From 1880 to First World War the mathematics of direct and converse piezoelectricity has been developed. During Second World War the ferroelectric ceramic (Barium Titanate) was invented. Subsequently PZT (Lead Zincronium Titanate) was reported by Shirane at the Tokyo Institute of Technology. Various version of PZT subsequently became the prevalent piezoelectric ceramic material due to their major advantage over barium titanate (BaTiO ) ceramics, better reproducibility and higher speed of propagation. A majority of piezoelectric generators that has been fabricated and tested use some variation of PZT. Typically PZT is used for piezoelectric energy harvester because of its large piezoelectric coefficient and dielectric constant, allowing it to produce more power for a given input acceleration. In 1969, strong piezoelectricity was observed in PVDF (Polyvinylidene Fluoride) [1-10], [15]. The most easily available piezoelectric sensor is PZT and we have used two of its form one is in the form of rounded diaphragm and other is a PZT sheet. For a piezoelectric material to induce maximum charge it must be strained between its self resonant frequency (SRF) ranges.
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Fig. 2 Piezoelectric Diaphragm
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Fig. 3 Equivalent Circuit
A piezoelectric transducer under self resonant frequency range can be approximated to an electrical equivalent circuit having a sinusoidal current source i in parallel with a high value of resistance R and capacitance C as shown in Fig. 2 [8, 22].
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3 Each current source is assumed of few milliamps at 10 Hz with a high value of resistance and capacitance in parallel with it. This is how the equivalent circuit has been drawn. III. HARVESTER MODEL AND SIMULATION
the filter capacitor ( ) and with purely resistive load is considered, secondly the filter capacitor is applied and output is measured across load resistor. The simulation circuit diagram and corresponding output waveforms are shown in figure 5 and figure 6, respectively.
A. Harvester Mechanical to Electrical Modeling The harvesting principle of electrical energy from mechanical energy is shown in Fig. 4. The piezoelectric transducer remains in direct contact with the source of vibration. When the vibration occurs, the piezoelectric transducer induces the electric charge. The rate of change of these induced charges with respect to time gives the alternating current pulses. A static converter is used before feeding the storage unit or the electrical load. Mechanical Load
Fig. 5 Simulation model for equivalent piezoelectric energy harvester circuit with bridge rectifier and output capacitor Static Converter
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Fig. 4 Schematic diagram of a vibrating piezoelectric Harvester model
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The transducer used in this model are piezoelectric diaphragms or bender plate that consists of a piezoelectric ceramic plate (PZT), with electrodes on both sides, attached to a metal with conductive adhesive shown in Fig. 2. The resonant frequency of these diaphragms is given by Helmholtz’s equation [23]
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B. Simulation model and Results for Harvester Circuit An approximated model of piezoelectric harvester has been drawn in MATLAB by considering the electrical equivalent model of piezoelectric transducer assuming the suitable constants. The output of a piezoelectric transducer is an AC signal. It must be converted to DC for load or storage cell utilization. A full bridge rectifier is used to convert the AC voltage produced by piezoelectric diaphragm to DC voltage [5]. It is observed that during each load impact on the piezoelectric tile at least six piezoelectric transducers are simultaneously actuated. Therefore, a parallel grid of six units of the transducer has been used at the input of rectifying units and primary storage unit ( ). Firstly a rectifier circuit without
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Where is the resonant frequency (Hz), is the velocity of energy wave, is the radius of ceramic diaphragm (cm), d is the diameter of the support, is the thickness of support and is the material constant. It is considered that piezoelectric transducers are operated under self resonant frequency so that maximum charge can be induced.
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Fig. 6 AC output waveform of Piezo equivalent model (Top), Rectified output without (Middle) & Output with a rectifying capacitor of 100 (Bottom)
The harvester circuit output power can be represented as the sum of the output power generated by each individual PZT (Piezoelectric Diaphragm-Lead-Zirconium-Titanate). As the PZT are connected in parallel, Kirchhoff’s law can be applied to find the equivalent circuit. Here the source (I) can be taken as the sum of the individual current source of PZT and is given by equation (4). 4 The total resistance (R) of the PZT is taken as the parallel combination of individual units given by equation (5). (5) Also the total capacitance of the piezoelectric grid can be represented by (6) The power output of a full bridge rectifier with a single transducer is given in equation (7) [8].
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2
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4 is the total power output of the bridge rectifier unit Here, is the plate capacitance with one piezoelectric diaphragm, is the voltage at rectifier of the piezoelectric transducer, is the excitation frequency of the transducer, is output, is the open circuit voltage at the output of PZT unit and the the diode voltage drop. The grid equivalent of the six transducers has the frequency of excitation to be half of actual excited frequency and is given by equation (8). 2 Thus, the total output power ( transducer grid can be given as 2
frequency. LTC3588 is the energy harvesting IC programmed for low power generation that integrates the bridge rectifier and the efficient energy storage hardware algorithm. The output of IC is low ripple containing DC with 51.33 % ripple factor.
8 ) of the harvester with the /2
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represents the total voltage at rectifier output, is Here the total open circuit voltage at the output of PZT grid connected in parallel. IV. PROTOTYPE OF ENERGY HARVESTER SYSTEM To verify the principle of power generation and power conditioning, the experimental setup consisted of 6 number of PZT diaphragms connected in parallel and it has been pasted on wooden board. The arrangement of PZT diaphragms and the shaker arrangement are shown in figure 8.
Fig. 8 AC output voltage observed on DSO (Scale: x-axis 100ms/div, y-axis: 5 Volts/div)
The RMS value of the AC output voltage of the piezoelectric harvesting tile is 1.58 Volts. The output of the bridge rectifier is 1.8 Volts which is the average voltage depending upon the strain applied. Firstly the variable DC output of the bridge rectifier is stored in a NOKIA BL-4C, a Li-ion battery of 3.7 Volts, 860 mAh which also provides the input to the step up converter with minimum voltage of 0.9 volts to drive the boost converter. The charging time of the NOKIA BL-4C battery was very high. It took 6 to 7 hours if we were applying 5 strokes per second and also the current rating was low for intermittent load. The output of step-up converter was 5.6 Volts, 200mA i.e. the regulated DC which was utilized for charging the smart phone. Now we have modified our harvester circuit with IC LTC3588 and ultra capacitor as the storage device. The circuit diagram with hardware arrangement is shown below. 100
Fig. 7 Piezoelectric Energy harvester tile design with shaker modal
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The hardware model of energy harvesting tile consists of its wooden base pasted with piezoelectric transducers connected in parallel. The base is a square wooden board covered with rubber sheet to provide a uniform and elastic plane. Piezoelectric diaphragms arranged in 2×3 matrix with shaker arrangement as shown in figure 8. The ac output voltage when a variable strain is applied on the tile is shown in figure 9. The voltage obtained without bridge rectifier is of alternating nature of frequency below 10 Hz. The magnitude of ac output obtained depends on the various factors such as packing density of piezoelectric transducer, frequency of excitation, and type of strain applied on the surface [23]. The AC voltage obtained is further processed via energy harvester circuit that consists of the rectifier IC LTC3588. Earlier the bridge rectifier has been used with electrolyte capacitor as the storage but it caused the drop of generated power across the diode and electrolyte capacitor. The electrolyte capacitor has been replaced by the ultra capacitor but it was not charging since the frequency of the harvested power was very low. Then we have used an IC which not only rectifies with low drop but also multiplies the
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Fig. 9 Circuit diagram of Energy Harvester
Fig. 10 Energy harvester circuit hardware implementation picture
The output of the harvester IC is stored in the ultra capacitor of 4.0 F/5.5 V which is an efficient storage device with high
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D outpu
5 horizontal axis show the time taken and the vertical axis show the voltage. 4.0
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current rating. The step up converter is designed to take the variable input DC voltage obtained after rectification and gives a regulated DC supply for load utilization. The rectified DC output obtained after harvester IC is shown in figure 12.
Fig. 11 Rectified DC output of the Harvester circuit observed on DSO (Scale: x-axis: 10ms/div, y-axis: 1 Volts/div)
The time required for charging the fully discharged NOKIA BL-4C battery was initially too large since the vibrations applied were of random nature. But when the tile was strained at resonant frequency, the charge time has been reduced quite significantly. The resonant frequency can be calculated by using the Helmholtz equation given in equation 3. The ultracapacitor is charged by rectified and filtered DC output of the harvester circuit. The charge time for the ultra capacitor is very less in comparison to the Li-Ion battery. It took 2.5 hours to get full charge which is less than half the time required by the Li-Ion battery. The discharge time is also large under no load condition. This ultracapacitor stores the charge and when the charge exceeds over 0.9 volts the boost converter turns ON and regulated output is obtained across the load. Table I shows various quantities and their measured value.
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Fig. 12 Charge cycle of the ultra capacitor
The charge cycle graph study shows that during the first hour of applied vibration the ultracapacitor charges steadily. During first and second hour the ultracapacitor charges very quickly and beyond second hour the ultracapacitor charges slowly and saturates slowly after 2.5 hours. This ultracapacitor is suitable for the storage of the energy generated by our harvester modal. The boost converter circuit hardware is shown in figure 14 and boosted output is viewed on digital oscilloscope shown in figure 15.
Table I: Experimental Results
Quantity Peak to Peak AC voltage generated, Frequency Rectified Output, % Ripple Input to Boost converter Output of Boost Converter Charging Time of Ultra Capacitor
Measured Value
Fig. 13 Boost converter
4.0 Volts peak to peak 5.0 Hz 1.2 volt, (1.8 volt maximum) 51.33% (3 % more than FB rectifier) 0.9 Volt to 5.5 volt 5.0 volt, 10mA with 3.0 volt LED load 2.5 Hrs, with 5 strokes per second
When charging a battery, the most important electrical factor of the power supply is that is it being able to provide a fairly significant amount of current. The charge time of a rechargeable battery is directly dependent on the amount of current supplied to it. The current rating supplied to the Li-Ion battery was quite low so the charging time was more. The current rating supplied from harvester circuit to the ultra capacitor is large so the charging time is quite less. The charge cycle of the ultra capacitor has been shown in figure 12. The
Fig. 14 Boosted output
The output of the harvester tile can be scaled up by considering several factors in our design. As stated earlier the output of the piezoelectric energy harvester tile depends on the number of the piezoelectric diaphragm per unit area, the electromechanical coupling coefficient of the piezoelectric material, plan of arrangement. The output voltage obtained can be scaled up by taking high coupling coefficient piezoelectric material, increasing the number of piezoelectric diaphragm per unit area and using series parallel configurations of piezoelectric diaphragms. The rectified output can further scaled up by suitable step up converter. V. CONCLUSION AND FUTURE SCOPE A piezoelectric energy harvester has been simulated, designed and implemented experimentally. A constantly increasing impulse strain is applied every time to the entire unit. It is observed that the output increases initially and after sometimes it saturates at some particular value. It has been a great experience to harvest the electrical energy from mechanical strain. The equivalent circuit model is developed in MATLAB and the expected result is obtained. The
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6 performance and theory has been experimentally verified. The developed energy harvester can be applied to supply low powered electronics like wireless sensors, bugging devices, weather monitoring devices, aircraft power supply and many more low powered MEMS (Micro electromechanical Systems) devices. There is a wide scope of improvement of this type of harvesting technique because of increased demand of portable micro powered electronics. The all round development of self powered electronics depends upon the highly efficient energy harvesting systems. Some improvements have been done in this model to reduce the voltage drop at rectifier stage using dedicated IC. Further improvements may be done to minimize the loss and to accumulate the optimum power.
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