Green Energy Using Piezoelectric Transducers
Shishir Harishanker, Dr. T K Ramesh
Electronics & Communication Engineering
Amrita Vishwa Vidyapeetham
Abstractthe world is on the brink of destruction. Every year there is a massive number of species stepping towards extinction. As educated human beings it is our role to understand the gravity of the situation and make changes accordingly to lead a better life and help other living beings co-exist. Our dependence on fossil fuels has made the earth undergo drastic changes in the past decade and we still seem to have been unaffected by it.
In this project initiative, we came up with a strong idea that could minimize the usage of fossil fuels to generate electricity using piezoelectric transducers. We have developed a low cost and efficient circuit to harness energy that can be used to drive any DC load using a rechargeable battery. Before making the prototype we modelled a piezoelectric diaphragm on ANSYS Academic to observe the total deformation and equivalent stress it undergoes when a weight of 70kg is applied on it.
Keywords Piezoelectric, AC-DC conversion, piezoelectric diaphragm, Energy harvesting circuit,
We humans are dependent on fossil fuels for almost all our activities. Whether it is for transportation, or generation of electricity, usage of fossil fuels is just increasing day by day. The harmful effect of fossil fuels is slowly being felt and noticed. There are countries which have already switched over to driving electric cars to reduce the amount of pollution, to reduce global warming and all other factors associated with it. There are many other methods by which we can reduce our dependency on it. One of the major methods to harness energy is by using piezoelectric devices . These devices are cost efficient, easy to manufacture, and the output is fairly decent dependent on the application. There are researches and experiments going on to increase the efficiency of these devices to yield greater output.
The innovation aspect that comes along with piezoelectric devices and their usage is tremendous. In many countries, these sensors are installed on roads and speed breakers to generate electricity whenever a vehicle passes over it , and this electricity is used to power the street lamps. Another interesting application of these sensors is in Oxygen pumps for divers when they go deep into the sea.
We have exploited the properties of the diaphragm, which converts mechanical energy (any kind of stress or vibration) to electric energy, thereby feeding this electric energy to power any DC load.
II. SYSTEM DESCRIPTION
A. Piezoelectric Sensor
Piezoelectric sensors convert mechanical stress applied onto it to electrical energy. They are made up of crystals which are polar in nature without electrical field being applied. When no force is applied on the sensor, the charges are exactly balanced and the net dipole moment is zero producing zero electrical voltage across its terminals. When mechanical stress is applied, the charges go out of balance. A non-zero dipole moment builds up producing electrical voltage. Piezoelectric sensors can be modelled as an alternating current source (I) connected in parallel with a Capacitance (C) and Resistance (R) as depicted .
B. Characterization of Piezoelectric Sensor
1) AC output: The output of a piezoelectric sensor is alternating in nature. This is observed on the Cathode Ray Oscilloscope (CRO) when pressure is applied on the sensor. This alternating output cannot be used for storage purpose. Hence, this output must be rectified before transferring it into an energy storage device. Therefore to make it a stable DC, we make use of a bridge rectifier circuit which converts AC to DC.
2) Configuration: The output from a single piezoelectric sensor is very low and hence a combination of five sensors is used. They are investigated for parallel and series configurations of 2, 3 and 5. The mechanical stress is provided using standard weights of 5gm, 10gm and 20gm dropped from a certain height. The series connection results in better voltage output as it adds up the output voltage. But the impedance of the parallel configuration would be less as compared to when connected in series. The parallel configuration produces higher output current. Therefore, we connect the five piezoelectric sensors in parallel.
3) Hysteresis: The internal capacitance of the piezoelectric sensor exhibits hysteresis. In this experiment, we placed weight on the piezo one after another from 0gm to 50gm and then unloaded from 50gm to 0gm. From the readings, it has been observed that while unloading from 50gm to 0gm, the output of the sensor did not follow the same path as that of loading. The black coloured line shows voltage readings while loading and unloading. However, the current output of the sensor is in nA (which was not measurable on the Digital multi meter) and hence it is shown as 0.1uA constant.
A. Physical attributes of the diaphragm
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 model of a piezoelectric sensor is shown. We have observed and calculated the deformation it undergoes when subjected to a force in the range of 100 to 500 N. The modelling was done on ANSYS Academic. The material properties were considered as for the commercial use of these sensors. ANSYS finite element methods were used for demonstrating and analysis of piezoelectric materials. The single piezoelectric sheet model was presented. The basic characteristic of the piezoelectric material was analyzed and the affecting factors of characteristics were derived.
The constitutive relation of piezoelectric behaviour are described in Equation (1) & Equation (2),
D ??e?????E, (2)
? – Stress Vector, D – Electric displacements, ? – strain vector, e – stress piezoelectric matrix, ? – dielectric matrix with the coefficients of electric permittivity on its diagonal. Components of electric field intensity E is linked with the electric potential ? by relation shown in Equation (3)
E = -??, (3)
The total deformation and equivalent stress undergone by the diaphragm is observed.
(a) Equivalent Stress
(b) Total deformation
B. Energy harvesting circuit
The overall circuitry consists of 10 piezoelectric sensors connected in parallel. The output is given to a bridge rectifier circuit of four diodes for rectification. The DC output of the bridge rectifier is connected to an energy storage device (Li-battery/Capacitor). But since the charging of a battery has to be constant, we have used a voltage regulator LM317EML in between and an auto-cutoff circuit using the IC 741 in comparator configuration to isolate the battery from the circuit when fully charged.
To design the Voltage regulator, we have used the standard formula
Vout = 1.25V (1 + R1/R2)
The capacitor value for stabilizing the output from the regulator is 0.22uF as mentioned in the data sheet of LM317EML.
This design was constructed keeping cost efficiency in mind. We can use other harvesting circuits as well, but at the expense of added cost. This project aims to reduce the use of fossil fuels and hence this design can be implemented further in spray pumps used by farmers to spray pesticides and fertilizers. The use of a Li-Battery as energy storage device is disadvantage for this application because the charging time will be very high.
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