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 Figure 1, there are various sources and different types of losses during energy conversion.

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 [9] .

Energy harvesting devices are those which are used for converting the available surrounding energy, which may go waste as shown in Figure 2, into the usable electrical energy. Electromagnetic, electrostatic and piezoelectric generators are some of the widely used energy harvesting devices. Electromagnetic generator is an inductive energy scavenger that transforms mechanical energy into electrical energy, using the interconnected principles of magnetism and electricity. The process by which an electromagnetic generator produces current or electricity is known as electromagnetic induction, which basically means that an electric current is induced within a conductor through the use of a magnet. Most of the electromagnetic generators work on electromagnetic induction, and some of these use renewable sources such as water power and wind power to create the initial mechanical energy. Mechanical energy can basically be thought of as kinetic energy or movement energy [10] . However, nearly all the reported electromagnetic (EM) micro power generators are silicon based devices. Therefore these harvesters result in poor scavenging performance. In order to improve the performance, the most common printed circuit board material, FR4 (Flame Retardant 4) is used as an alternative element for energy harvesting applications. Hatipoglu [11] presented the use of FR4 as the most commonly used PCB material for body worn sensors and intelligent tire sensors instead of silicon MEMS devices. Kim [12] presented two types of EM power generators exploiting direct conversion of airflow into mechanical vibration.

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. [13] have designed an electrostatic converter fabricate with MEMS technology. Their simulations result shows that 8.6 µW can be converted from a device of 1.5 cm × 1.5 cm in size. Roundy [14] focused on electrostatic vibration to electricity converter made by using MEMS technology; their simulations result shows that an output power density of 116 µW/cm³ is possible from input vibrations of 2.25 m/s² at 120 Hz. Electrostatic machines are typically used in science classrooms to safely demonstrate the electrical forces along with high voltage phenomenon. The elevated potential differences achieved have been also used for a variety of practical applications, such as operating X-ray tubes, medical applications, sterilization of food, and nuclear physics experiments [15] .

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 [16] .

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 Figure 5, because the slight change in the dimension of a piezoelectric material results in the variation in bond length between cations and anions caused by stress. Conversely, a piezoelectric material will have a change in dimension when it is exposed in an electric field. This inverse mechanism is called electrostriction. Those devices utilizing the piezoelectric effect to convert mechanical strains into electricity are called piezoelectric generators [25] .

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 [26] developed a piezoelectric cantilever beam generator as shown in Figure 6, where the cantilever used was of constant width which simplifies the modeling and fabrication of the beam but results in an unequal distribution of strain along the length of the generator. A prototype generator was fabricated by attaching a

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 [27] presented the development of two piezoelectric MEMS generators “31” mode and “33” mode for scavenging mechanical energy of ambient vibrations and converting it into the useful electrical energy. The two piezoelectric MEMS generators were of cantilever type made by a silicon process and can transform mechanical energy into electrical energy through its piezoelectric PZT layers. A PZT deposition machine was developed which uses an aerosol deposition method to fabricate the high quality PZT thin film efficiently. The result shows that “31” mode generator possesses a maximum open circuit voltage of 2.675 V_p-p and a maximum output power of 2.765 µW with 1.792 V_p-p output voltage excited at a resonant frequency of 255.9 Hz under a 2.5 g acceleration level and the “33” mode device possessed a maximum open circuit output voltage of 4.127 V_p-p and a maximum output power of 1.288 µW with 2.292 V_p-p output voltage at its resonant frequency of 214 Hz at a 2 g acceleration.

Junhui [28] proposed a piezoelectric generator to increase its vibration energy harvesting capability based on a cantilever beam which not only uses the strain change of piezoelectric components bonded on a cantilever beam but also employs the weight at the tip of the cantilever beam to hit piezoelectric components located on the two sides of weights. It shows that the piezoelectric components operating in the hit mode can substantially enhance the energy harvesting of the piezoelectric generator based on a cantilever beam. Two methods were suggested for increasing vibration energy harvesting capability of a piezoelectric generator. In one of them the DC voltages from rectifiers are connected in series and hence the total DC voltage is applied to a capacitor. As a result it was found that 22.3% of the harvested energy is wasted in case of series connection but in another connection, the DC voltage from each group is applied to different capacitors and in this case the total output electric energy could be 43 nJ for one vibration excitation applied by spring with initial vibration amplitude of 18 mm and frequency of 18.5 Hz.

White [29] presented an approach for modeling and designing of vibration generator, in which a thick film micro generator was described and the corresponding test was performed. In this paper, the micro generator used is a resonant mechanical structure based on a cantilever beam design, seismic mass and piezoelectric ceramic PZT-5H. It was shown that the proposed generator produces the maximum power output of about 2 µW which is too small for actual applications F Lu presented a simple design modeling and analysis of the “31” transverse mode type piezoelectric micro generator for MEMS applications. The output power and energy conversion efficiency of laminated type micro-generators using PZT-PIC 255 and single crystal PZN-8% PT was tested for comparison. It was found that optimal external resistance gives the maximum output power. Furthermore, increasing the frequency of the vibration can improve the output power. While beyond a certain value, further improvement cannot be achieved by simply increasing the vibration frequency. At the higher frequency, single crystal PZN-8% can achieve much higher output power in comparison to piezoelectric material PZT-PIC 255. The performance of PZN-8% PT was more sensitive to operational frequency and that of PZT-PIC 255 was more sensitive to external resistance [30] .

Coupling modes employed for piezoelectric generators, as shown in Figure 7 are of two types: a) “31” coupling

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 [31] shows that “31” mode has a lower coupling coefficient as compared to “33” mode (Table 2). By comparing a piezoelectric stack operating in “33” mode to a cantilever beam operating in “31” mode, it was observed that, although the stack was more robust and had a higher coupling coefficient, the cantilever produced more power when subjected to the same force. It was concluded that in a small force environment “31” mode proved most efficient, but in a high force environment “33” mode would be more durable and produce useful energy., Yang et al. [32] have shown that analytically, for a piezoelectric plate operating in the “33” mode, the power output is directly proportional to the coupling coefficient as well as dielectric constant. It means that the devices with higher coupling coefficient will produce more power Roundy [26] concluded that the resonant frequency of a system operating in “31” mode is much lower, making the system more likely to be driven at resonance in a natural environment, thus providing more power.

Commonly used vibration sources, as given in Table 3 include the casing of a microwave oven, base of a milling machine, drilling machine, bearing test bed, A/C compressor, washing machine, cloth dryer etc. Other vibration sources measured include a notebook computer casing, cooling ducts, large exterior windows, mobile phones, the floor in high traffic areas of office buildings, and several household appliances.

Table 4 summarizes the dimension, material used, configuration, coupling mode, etc. of different piezoelectric vibration harvesters, reported in literature.