Energy Harvesting using Piezo-electric SensorsPayal A. Lathi

Electrical DepartmentIndian Institute of Technology Bombay

Mumbai, Indiapayalllathi@gmail.com

Anamika SharmaElectrical Department

Indian Institute of Technology BombayMumbai, India

anamikasharma2512@gmail.com

Abstract—With the increase in demand of power supply anddecreasing non-renewable sources of energy, the need for newsource of energy has been arising. This paper presents a model forenergy harvesting using piezo-electric sensor. The kinetic energyreleased during walking is used to generate useful electricalenergy. The idea is to incorporate piezoelectric senors in thesole of the shoes and generate energy by walking which storedin a capacitor.

I. INTRODUCTIONIn India, the primary sources of energy are[1] from non-

renewable energy sources, in which, thermal energy con-tributes major part. But, these sources are depleting and causeenvironmental pollution. The solution to this problem is theuse of renewable energy sources like solar, hydro, geothermal,etc. But, their use is also restricted by several factors, e.g.Energy cannot be harvested using solar energy in the absenceof solar light; Geothermal energy emits poisonous gases andgeneration of hydro energy requires huge amount of water i.e.without water we cannot generate energy. There is a need ofalternate source of energy whose production is not restricted.

Everyday, our body releases energy during activities likewalking, typing, arm movements, etc. The energy releasedwhile walking is absorbed by the ground and gets lost inthe environment. This wasted kinetic energy can be used togenerate useful electrical energy with the help of piezoelectricsensors. When human being walks or runs, force is exertedon the ground by virtue of their weight. This force is appliedon the piezo-electric sensors(Fig.1.) which in turn produceselectrical voltage which can be stored and utilised.

Fig. 1. Piezo-electric sensor

II. SYSTEM DESCRIPTIONA. Piezo Sensor

Piezo Sensors convert mechanical stress applied onto it toElectrical energy. They are made up of crystals which are polarin nature without electrical field being applied. When no forceis applied on the piezo, the charges are exactly balanced andthe net dipole moment is zero producing zero electrical voltageacross its terminals. When mechanical stress is applied, thecharges go out of balance. A net non-zero dipole momentbuilds up producing electrical voltage. Piezo electric sensorcan be modeled as an alternating current source(I) connected inparallel with a Capacitance(C) and Resistance(R) as depictedin Fig.2.

Fig. 2. Electrical equivalent of a Piezo-electric sensor

B. Characterisation of Piezo Sensor

Experiments were conducted to study the characterisitics ofa piezo-electric sensor.

1) AC output: The output of a Piezo-electric sensor isalternating in nature. This is observed on the Cathode RayOscilloscope(CRO)(Fig.3.) when pressure is applied on thepiezo sensor. This alternating output cannot be used forstorage purpose. Hence, this output must be rectified beforetransferring into a energy storage device. Therefore to make ita stable DC, we make use of a bridge rectifier circuit whichconverts AC to DC.

2) Configuration: The output from a single piezo sensoris low, hence a combination of five piezo sensors is used.They are investigated for parallel and series configurations of2,3 and 5 piezo’s. The Fig.4. depicts the voltage output of 2piezo sensors connected in parallel and series. The mechanicalstress is provided using standard weights of 5gm, 10gm and

Fig. 3. AC output of a Piezo-electric sensor

20gm dropped from a certain height. It is clear from the figurethat series connection results in better voltage output as itadds up the output voltage. But the impedance of the parallelconfiguration would be less as compared to when connectedin series. Hence the parallel configuration produces higheroutput current. Therefore, we connect the five piezo sensorsin parallel.

Fig. 4. Voltage vs Weights plot of series and parallel configurations of twopiezo-electric sensors

3) Hysteresis: The internal capacitance of the piezo ex-hibits hysteresis which is clear from the Fig.5. In this experi-ment, we placed weight on the piezo one after the another from0gm to 50gm and then unloaded from 50gm to 0 gm. From thereadings, it has been observed that while unloading from 50gm to 0 gm, the output of sensor didnt follow the same pathas that of loading. In Fig.5. black colored line shows voltagereadings while loading and unloading. However, the currentoutput of the sensor is in nA (which was not measurable onthe Digital multimeter(DMM) and hence it is shown as 0.1uAconstant.

III. METHODOLOGY

A. Block Diagram

The overall circuitry(Fig.6.) consists of five piezoelectricsensors are connected in parallel. The output is given to a

Fig. 5. Hysteresis

bridge rectifier circuit of four diodes for rectification. The DCoutput of the bridge rectifier is connected to energy storagedevice.

B. Positioning of the sensors

The piezo sensor used in the project is made up of PZT(LeadZirconate Titanate). The piezo sensors are small circular inshape which fits comfortably in the sole of the shoes. Also,the conditioning circuitry(Fig.7.) is small that it can be easilyintegrated with the shoes. During walking, the mechanicalstress applied on the piezo is converted into electrical voltagewhich is given to a energy storage device.

Fig. 6. Block Diagram of Piezo-Electric energy harvesting process

Fig. 7. Circuit Diagram

C. Storage Device

The harvested energy can be stored in a battery or acapacitor. Battery requires constant voltage for charging withminimum intermittent vibrations. It has higher energy storageper unit weight and slower discharge response. Also, battery

Fig. 8. Positioning of the sensors in the shoe-sole

takes long time to charge but it does not require continuousforce change on piezo.

Capacitors, on the other hand don’t require minimum volt-age for charging. They have low energy densities. Capacitorsrequire constant application of force and are charged immedi-ately. Hence, we use capacitor as energy storage device.

For 500 walking steps, different capacitors are charged todifferent voltage levels and they display varied charge anddischarge time.

IV. RESULT AND CONCLUSION

A. Experimental Results

Table I displays the output voltage generated across variouscapacitors for 500 walking steps. Output voltage of 7.6Vwas produced for human weight of 47kg nad 8.4V outputvoltage(Fig.9.) for human weight of 55kg across a 10uFcapacitor for 500 walking steps. The charge stored on thecapacitor was utilised to light a small LED. The LED waslit for a short period of time as the discharge time of ceramiccapacitors is high.

TABLE IOUTPUT VOLTAGE STORED IN VARIOUS CAPACITORS FOR 500 WALKING

STEPS

Capacitance Voltage10uF 7.6V4.7uF 5.36V100uF 1.40V

B. Piezoelectric energy

In low frequency conditions, a piezo electric element canbe approximated as a parallel plate capacitor. The electricalenergy of the piezo electric sensors can be given by

U = 1/2× C × V 2

For the capacitance value of 10uF, 500 walking stepsproduced an output voltage of 7.6 V. The piezo electric energycan be approximated as

U = 1/2× 10u× (7.6V )2 = 288.8 uJTherefore, energy is extracted by walking, converted to

electrical energy and is stored in the capacitor. The output

Fig. 9. Voltage output after 500 walking steps

current generated is less, which may increase the chargingtime of a battery. This project is low cost approach to exhibitthe application of piezo electric sensors to harvest electricalenergy from human activity. The possible sources of errors inour project could be measurement error while taking readings,human errors while dropping weights from certain height.

V. FUTURE SCOPE

The idea can be extended to generate electrical energy forhigher load applications. It can also be used in real timecharging of portable devices. Thus, a clean and reliable formof energy can be harvested from the piezoelectric sensors.Also, the energy generated is proportional to the weight fo theperson and speed of the motion. Punning would produce moreoutput than walking. Hence, the sensors can be employed atthe crowded public spots as shopping mall, airports, railwaystations, roadways so as to generate energy from renewablesource and use it efficiently.

VI. ACKNOWLEDGMENT

We would like to thank Prof. Siddharth Tallur for hisconstant support and guidance for the project. Also, we thankWEL lab and its staff for providing the required circuitcomponents.

REFERENCES

[1] Link: http:///www.marinebuzz.com/2008/07/19/why-indo-us-nuclear-deal-when-oceans-can-give-clean-energy

[2] Steven R Anton and Henry A Sodano, A review of power harvesting us-ing piezoelectric materials (20032006), Topical review, Smart materialsand structures,IOP Publishing

[3] Yamuna. M. B1, K. Shanmukha Sundar, Design of Piezoelectric EnergyHarvesting and Storage Devices, International Journal of AdvancedResearch in Electrical, Electronics and Instrumentation Engineering,Vol.3, Issue 8, August 2014

[4] J. John Livingston, M. Hemalatha, Charging an Electronic Gadget usingPiezoelectricity, Indian Journal of Science and Technology, Vol 7(7),945948, July 2014