With the advancement in the technologies around the world over the past few years, the microelectromechanical systems (MEMS) have gained much attention in harvesting the energy for wireless, self-powered and MEMS devices. In the present era, many devices are available for energy harnessing such as electromagnetic, electrostatic and piezoelectric generator and these devices are designed based on its ability to capture the different form of environment energy such as solar energy, wind energy, thermal energy and convert it into the useful energy form. Out of these devices, the use of a piezoelectric generator for energy harvesting is very attractive for MEMS applications. There are various sources of harvestable energy including waste heat, solar energy, wind energy, energy in floating water and mechanical vibrations which are used by the researchers for energy harvesting purposes. This paper reviews the state-of-the-art in harvesting mechanical vibrations as an energy source by various generators (such as electromagnetic, electrostatic and piezoelectric generators). Also, the design and characteristics of piezoelectric generators, using vibrations of cantilevered bimorphs, for MEMS have also been reviewed here. Electromagnetic, electrostatic and piezoelectric generators presented in the literature are reviewed by taking into an account the power output, frequency, acceleration, dimension and application of each generator and the coupling factor of each transduction mechanism has also been discussed for all the devices.
Over the past few years, the use of energy harvesting devices for harvesting the useless surrounding energy has grown tremendously for empowering small portable as well as importable units. Many researchers have focused on the different available environmental energy sources including wind energy, solar energy, thermal gradients and mechanical vibrations [
Electromagnetic generators employ electromagnetic induction arising from the relative motion between a gradient and a conductor whereas electrostatic generators employ the relative motion between electrically isolated charged capacitors plates to generate energy and piezoelectric generators employs piezoelectric materials that generate a charge when subjected to external load. Piezoelectric generators have the advantage that they can be more easily implemented with MEMS technology. Generators using piezoelectric materials can be called either as actuators, when the design of the device is optimized for generating stress or strain using the converse piezoelectric effect, or as sensors when the design of the device is optimized for the generation of an electric signal by employing the direct piezoelectric effect, in response to the mechanical input [
This review article is about energy harvesting concepts, different types of harvesting generators which convert mechanical movement (energy) present in the application environment into usable electrical energy. It also discusses the design and characterization of piezoelectric vibration in electricity converters.
Energy harvesting refers to the generation of energy from sources such as ambient temperature, vibration or air flow, solar energy and wind energy. As shown
Converting the available energy from the environment allows a self-sufficient energy supply for MEMS devices. Energy harvesting requires a transduction mechanism to generate electrical energy from motion and the generator will require a mechanical system that couples environmental displacements to the transduction mechanism. The design of the mechanical system should maximize the coupling between the energy source and the transduction mechanism and will depend entirely upon the characteristics of the environmental motion. The transduction mechanism itself can generate electricity by exploiting the mechanical strain or relative displacement occurring within the system. The strain effect utilizes the deformation within the mechanical system and typically employs piezoelectric materials whereas in case of relative displacement, either the velocity or position
can be coupled to a transduction mechanism. Velocity is mainly associated with electromagnetic transduction while relative position is associated with electrostatic transduction [
Energy harvesting devices are those which are used for converting the available surrounding energy, which may go waste as shown in
An electrostatic generator is a generator that produces static electricity, or electricity at high and low current. These generators can be operated through manual power to transform mechanical work into the electrical energy. Electrostatic generators develop electrostatic charges of opposite signs rendered to two conductors, using only the electric forces, and the work done by using the moving plates, drums, or belts to carry electric charge to a high potential electrode. The charge is generated by one of two methods: either by using the triboelectric effect (friction) or electrostatic induction. Electrostatic converters have the advantage that they can be more easily implemented with MEMS technology. Meninger et al. [
When the piezoelectric ceramics, activated mechanically with pressure or vibration it has the capacity to generate electric voltages sufficient to produce spark across an electrode gap. The advantage of the piezoelectric energy harvester is that it has high energy density and can be integrated with electronic devices by MEMS technology. The voltage of the output energy harvester can be more than 10 volts, so up-conversion is not necessary. The disadvantage is that the resonant frequency of the piezoelectric energy harvester is higher than that of the
ambient vibration sources normally available when the energy harvester is miniaturized. The latter has a typical value below 200 Hz, and the former is well above 1 kHz. Also the PZ material is brittle, so long term mechanic wear out may limit the lifetime of the energy harvester. Two common applications of piezoelectric generators are in the push button cigarette lighters and gas barbecue grills. In these applications, pressing a button causes a spring-loaded hammer to apply a mechanical force to a rod-shaped single-layer piezoelectric ceramic. As a result of the piezoelectric effect, the ceramic element produces a voltage that passes across a small spark gap causing the fuel source to ignite. Electrical energy in a rod-shaped single-layer piezo generator is released very quickly, is very high voltage, and very low current. Piezoelectric ignition systems are small and simple, long lasting and require little maintenance. Multilayer piezo generators consist of a stack of very thin (sub-millime- ter-thick) piezoelectric ceramics alternated with electrodes. The electrical energy produced by a multilayer piezo generator is of a much lower voltage than is generated by a single-layer piezo generator. On the other hand, the current produced by a multilayer generator is significantly higher than the current generated by a single-layer piezoelectric generator. Because they do not create an electromagnetic interference as multilayer piezo generators are excellent solid-state batteries for electronic circuits. Due to advancements in micro-electronic systems many consumer devices have decreased in size. Smaller electronic systems require less power to operate. As a result, the solid state multilayer piezoelectric generators have become a feasible power source for some applications. Current applications for multilayer piezo generators are energy sources for munitions and wireless sensors, such as sensors that monitor tire pressure in automobiles [
Electromagnetic generators works on the principle of electromagnetic induction which was first discovered by Faraday in 1831, is the generation of electric current in a conductor located within a magnetic field. The conductor typically takes the form of a coil and generates electricity because of the change in the magnetic field or due to the relative movement of the magnet and coil. The amount of electricity generated within a coil depends upon the strength of the magnetic field, number of turns in a coil and the velocity of the relative movement between the magnet and the coil. One of the most effective methods for energy harvesting is to produce electromagnetic induction by means of permanent magnets, a coil and a resonating cantilever beam [
An electrostatic generator, shown in
electrostatic energy conversion is the variable capacitor. The variable capacitance structure, which will be fabricated with MEMS technology, is driven by mechanical vibrations and oscillates between a maximum capacitance (Cmax) and a minimum capacitance (Cmin). If the charge on the capacitor is constrained, the voltage will increase as the capacitance decreases. If the voltage across the capacitor is constrained, charge will move from the capacitor to a storage device or to the load as the capacitance decreases. In either case, mechanical kinetic energy is converted to electrical energy [
Piezoelectric generators have been used for many years to convert mechanical energy into electrical energy. The following sections describe the range of piezoelectric generators described in the literature to date. For the purposes of this review, piezoelectric generators have been classified by its size, power output, voltage and its applications on both macro scale and micro scale It begins with a brief description of piezoelectric theory in order to appreciate the different types of generator and the relevant piezoelectric material properties.
Piezoelectric generator works on the principle of piezoelectricity or piezoelectric effect developed by Pierre Curie brothers in 1880 which describes a relationship between stress and voltage. Piezoelectric materials can becomes electrically polarized or undergoes a change in polarization when subjected to a stress, as shown in
Size constraints play an important role in the selection of a piezoelectric generator configuration. Hence, it is necessary to specify the size first while selecting a configuration. A bending element is considered as the basis for a generator because of two reasons: a) low resonance frequency and b) can attained higher strains. A bending element can be mounted in several ways to form a generator; a cantilever structure having piezoelectric material attached to the top and bottom surfaces along with a center shim layer is an attractive geometry for harvesting energy from mechanical vibrations. This configuration of a generator has been chosen for two reasons: a) cantilever configuration provides highest possible strain and the power output is directly proportional to the strain and b) cantilever configuration results in low resonance frequency which is essential for low frequency target vibrations. Roundy [
Author name | P (µW) | F (Hz) | A (ms−2) | M (g) | Volume (mm3) | Details |
---|---|---|---|---|---|---|
Tashiro [ | 36 | 6 | 1 | 780 | - | Aluminum |
Arakawa [ | 6 | 10 | 3.9 | - | 800 | Glass |
Mitcheson [ | 3.7 | 30 | 50 | 0.1 | 750 | Silicon |
PZT-5A shim to each side of a steel center beam with a cubic mass made from an alloy of tin and bismuth was attached to the end of the generator and allowed to resonate at 120 Hz, it was shown that the prototype produced a maximum power output of nearly 80 µW with 2.5 m/s2 input acceleration. Lee [
Junhui [
White [
Coupling modes employed for piezoelectric generators, as shown in
mode and b) “33” coupling mode. “31” coupling mode implies that the direction of the applied force or strain is perpendicular to the polarization axis whereas “33” coupling mode indicates that the direction of the applied force or strain is parallel to the polarization axis. Conventionally, the “31” mode has been the most commonly used coupling mode. Baker [
Authors name | Piezoelectric configuration | Advantages/disadvantages |
---|---|---|
Mateu and Moll [ | Rectangular cantilever and triangular cantilever | Triangular configuration capable of higher strains and higher power generation |
Roundy [ | Trapezoidal cantilever | Trapezoidal configuration allows strain to be evenly distributed increasing efficiency |
Baker [ | Rectangular and trapezoidal cantilever | Trapezoidal beam produced 30% more energy than rectangular |
Commonly used vibration sources, as given in
Vibration source | Acceleration (m/s2) | Frequency (Hz) |
---|---|---|
Drilling machine | 0.93 | 178 |
Lathe machine | 1.36 | 68 |
Bearing test bed | 10.57 | 200 |
Refrigerator | 0.14 | 110 |
Washing machine | 0.82 | 62 |
Cloth dryer | 4.21 | 59 |
Microwave oven | 0.49 | 40 |
A/C compressor | 2.14 | 59 |
Car engine | 0.56 | 30 |
Truck engine | 1.98 | 37 |
Author name | Generator dimension | Materials used | Generator configuration | Coupling mode | Input | Output | Application |
---|---|---|---|---|---|---|---|
Lu [ | (5 × 1 × 0.4) mm | PZT-PIC 255 PZN-8% PT | Piezoelectric laminated generator | “31” coupling mode | Amplitude on seismic mass (µm) = 30 | PZT-PIC 255 P = 0.64 µW PZN-8% PT P = 0.31 µW | MEMS |
Roundy [ | (17 × 3.6 × 0.38) mm | PZT-5A PZT-5H Steel center shim Brass center shim | Cantilevered Bimorphs | “31” coupling mode | Input Acceleration = 2.5 ms−2 Input Frequency = 120 Hz | 375 µW from 1 cm 3 generator | Wireless transceiver |
Erturk [ | (50.8 × 31.8 × 0.52) mm | - | Cantilever based energy harvester | - | Input frequency = 45.6 Hz | 23.9 mW/g2 @ 35 KΩ | - |
Lee [ | (3000 × 1500 × 11) µm | PZT, Si, SiO2, Ti/Pt | Cantilever type generator | “31” and “33” mode MEMS generator made by a silicon process | For “31” mode Resonant frequency = 255.9 Hz Operating Load = 150 KΩ For “33” mode Resonant frequency = 214 Hz Operating Load = 510 KΩ | For “31” mode P = 2.099 µW Voc = 2.415 V Vsh = 1.587 V For “33” mode P = 1.288 µW Voc = 4.127 V Vsh = 2.292 V | MEMS |
Kim [ | (53 × 31 × 6.6) mm | - | Cantilever beam piezoelectric generator | - | Input Acceleration = 0.2 ms-2 Input Frequency = 34.57 Hz | 60 µW @ 27.6 KΩ | - |
Anam Khalid,Amit Kumar Redhewal,Manoj Kumar,Anupam Srivastav, (2015) Piezoelectric Vibration Harvesters Based on Vibrations of Cantilevered Bimorphs: A Review. Materials Sciences and Applications,06,818-827. doi: 10.4236/msa.2015.69084