Advances in Energy Harvesting Methods

Advances in Energy Harvesting Methods
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Advances in Energy Harvesting Methods

Chapter 2Broadband Vibration Energy HarvestingTechniquesLihua Tang, Yaowen Yang, and Chee Kiong SohAbstract The continuous reduction in power consumption of wireless sensingelectronics has led to immense research interests in vibration energy harvestingtechniques for self-powered devices.

Unfortunately, in the vast majority ofpractical scenarios, ambient vibrations are frequency-varying or totally random withenergy distributed over a wide frequency range. Hence, increasing the bandwidth ofvibration energy harvesters has become one of the most critical issues before theseharvesters can be widely deployed in practice. This chapter reviews the advancesmade in the past few years on this issue.

The use of batteries not only leads to their costly replacement especiallyfor sensors at inaccessible locations, but also causes pollution to the environment. Besides, batteries also place limitation on the miniaturization of micro- or nano-electromechanical systems. With the advances in integrated circuits, the sizeand power consumption of current electronics have dramatically decreased. Hence, inthe past few years, ambient energy harvesting as power supplies for small-scaleL. Elvin and A. Erturk eds. Tang et al. Different energy sources existing in the environment around a system, suchas sunlight, wind, and mechanical vibration, can be the options for energy har-vesting.

Among them, pervasive vibration sources are suitable for small-scalepower generation of low-power electronics and thus have attracted more researchattention. Current solutions for vibration-to-electricity transduction are mostly ac-complished via electrostatic [1, 2], electromagnetic [2, 3], or piezoelectric methods[4, 5]. No matter which principle wasexploited, most of the previous research work focused on designing a linearvibration resonator, in which the maximum system performance is achieved at itsresonant frequency. If the excitation frequency slightly shifts, the performance ofthe harvester can dramatically decrease.

Since the majority of practical vibrationsources are present in frequency-varying or random patterns, how to broaden thebandwidth of vibration energy harvesters becomes one of the most challengingissues before their practical deployment. This chapter presents a review of recent advances in broadband vibrationenergy harvesting. However, when the excitation frequencyis unknown or varies in different operational conditions, the harvester with pre-tuned resonant frequency is unable to achieve optimal power output.

Hence, inpractice a conventional linear harvester is expected to incorporate a resonancetuning mechanism to increase its functionality. The active moderequires continuous power input for resonance tuning. While in the passive mode,intermittent power is input for tuning and no power is required when frequencymatching is completed, that is until the excitation frequency varies again. Resonance tuning methods can be categorized into mechanical, magnetic, andpiezoelectric methods. Furthermore, the tuning process can be implemented manu-ally or in a self-tuning way.

Usually, it is more practical to change the stiffnessrather than the mass of the system. Leland and Wright [11], Eichhorn et al. In their experimental test on the prototype with a 7. However, the energy required for the tuning procedure was notaddressed. Furthermore, the resonant frequency could only be tuned unidirectionallysince only the compressive preload was considered.

Eichhorn et al. Figure 2. The arms connected the tip of the beam and two wings. A revolutionof the screw generated compression on the spring, which applied the force on thearms. This force was then applied to the free end of the cantilever through the wings. Below the fracture limit, a resonance shift from to Hz was achieved byapplying preload from 7 N to The quality factor 4. Analytically, Hu et al.

The resonance can be adjusted either higher or lower witha tensile or compressive load, respectively. In their model, it was reported that atensile load of 50 N increased the resonance from Instead of considering the bending mode, some researchers have investigated atunable resonator working in extensional mode, termed XMR [14, 15]. The XMRpresented by Morris et al. For the developed prototype, it was found that a resonance shift between 80 and Hz can be easily achieved with a change of pretension displacement of around1. Morris et al. However, thecapability of self-tuning or sequential tuning during operation of the XMR was notinvestigated.

A similar investigation was pursued by Loverich et al. The resonant frequency could be experimentally varied between56 and 62 Hz by adjusting the boundary location by approximately 0. It was found that thestiffness was nearly linear and the system had a high quality factor Q for lowvibration amplitudes, while the resonance frequency shifted and Q reduced forhigh vibration amplitudes.

This feature of nonlinear stiffness also provided an auto-protection mechanism, which is important when mechanical robustness is requiredfor high vibration levels. Rather than applying the axial or in-plane preload, adjusting the gravity centerof the tip mass is another idea to adjust the resonance of a cantilever.

Wu et al. The gravity center of the whole proof masscan be adjusted by driving the movable screw. Intheir prototype, the adjustable resonant frequency range could cover — Hz bytuning the gravity center of the tip mass up to 21 mm, as illustrated in Fig. To address this problem, some researchers [18, 19] developednovel passive self-tuning harvesters. This cantilever couple was movable laterally and had twooperational phases. The horizontal inertia forces exerted to two equal proof masseschange with the excitation frequency and become maximum when the excitationfrequency matches the resonant frequency.

The difference between the horizontalinertia forces is the key to switch the harvester between the two phases. Thisharvester is self-tunable and no power is required in the tuning procedure. Eachcantilever has two resonant peaks as the excitation frequency changes. Different from previous research on harvesting energy from translational baseexcitation, Gu and Livermore [18] focused on rotational motion.

A passive self-tuning piezoelectric energy harvester was designed in which centrifugal force wasexploited to adjust the stiffness and thus its resonant frequency. The harvesterconsisted of a radially oriented cantilever beam mounted on a rotational body, asshown in Fig. Since the centrifugal force was related to both driving frequencyand resonant frequency of the harvester, the harvester could be designed such thatthe resonant frequency was exactly equal to the driving frequency at Inaddition, within a range of 6. Thus, the harvester always worked at or near its resonance.

Intheir experiment, the self-tuning harvester could achieve a much wider bandwidthof 8. However, this device isonly applicable for rotational motion. Challa et al. All magnets were vertically aligned so that attractive andrepulsive magnetic forces could be generated on each side of the beam. Power output wasundermined as the damping increased during the tuning procedure, as shown inFig.

Given the maximum tuning distance of 3 cm, the required energy wasestimated to be 85 mJ and it would take s for each tuning procedure. Thus suchdevice can only work where the excitation frequency changes infrequently.

Reissman et al. The resonance of the piezoelectric energyharvester could be tuned bidirectionally by adjusting a magnetic slider. The effectivestiffness of the piezoelectric beam was dependent on the structural component Km ,the electromechanical component Ke that varied with external resistive loading Rl ,and the magnetic stiffness Kmagnetic that varied with the relative distance D betweenthe two magnets, i.

Km C Ke. Hence, the total bandwidth of the harvester was A schematic of thetuning mechanism is shown in Fig.

Kundrecensioner

Editors: Elvin, Niell, Erturk, Alper (Eds.) Covers multiphysics problems such as fluidic energy harvesting. Advances in Energy Harvesting Methods presents a state-of-the-art understanding of diverse aspects of energy harvesting with a focus on: broadband energy conversion, new. Advances in Energy Harvesting Methods presents a state-of-the-art understanding of diverse aspects of energy harvesting with a focus on: broadband energy.

The microcontroller woke up periodically,detected the output voltage of the generator and gave instructions to drive a linearactuator to adjust the distance D between the two magnets. In their experimentaltest, the resonant frequency was tuned from At a constant acceleration of 0.

J Micromech Microeng IEEE Micro —42 Exp Fluids — Bryant M, Garcia E Modeling and testing of a novel aeroelastic flutter energy harvester. J Vib Acoust J Fluids Struct — Pobering S, Ebermeyer S, Schwesinger N Generation of electrical energy using short piezoelectric cantilevers in flowing media. J Acoust Soc Am Abstract The continuous reduction in power consumption of wireless sensing electronics has led to immense research interests in vibration energy harvesting techniques for self-powered devices.

Currently, most vibration-based energy har- vesters are designed as linear resonators that only work efficiently with limited bandwidth near their resonant frequencies. Unfortunately, in the vast majority of practical scenarios, ambient vibrations are frequency-varying or totally random with energy distributed over a wide frequency range.

Hence, increasing the bandwidth of vibration energy harvesters has become one of the most critical issues before these harvesters can be widely deployed in practice. This chapter reviews the advances made in the past few years on this issue. The broadband vibration energy harvesting techniques, covering resonant frequency tuning, multimodal energy harvesting, and nonlinear energy harvesting configurations are summarized in detail with regard to their merits and applicability in different circumstances. Portable devices and wireless sensors are conventionally powered by chemical batteries.

The use of batteries not only leads to their costly replacement especially for sensors at inaccessible locations, but also causes pollution to the environment. Besides, batteries also place limitation on the miniaturization of micro- or nano- electromechanical systems. With the advances in integrated circuits, the size and power consumption of current electronics have dramatically decreased.

Hence, in the past few years, ambient energy harvesting as power supplies for small-scale. Tang et al. Different energy sources existing in the environment around a system, such as sunlight, wind, and mechanical vibration, can be the options for energy har- vesting. Among them, pervasive vibration sources are suitable for small-scale power generation of low-power electronics and thus have attracted more research attention.

Current solutions for vibration-to-electricity transduction are mostly ac- complished via electrostatic [1, 2], electromagnetic [2, 3], or piezoelectric methods [4, 5]. Various models, including analytical models [2, 6], finite element models [3, 7] and equivalent circuit models [8, 9], have been established to investigate the energy harvesting capability of each method. No matter which principle was exploited, most of the previous research work focused on designing a linear vibration resonator, in which the maximum system performance is achieved at its resonant frequency.

If the excitation frequency slightly shifts, the performance of the harvester can dramatically decrease. Since the majority of practical vibration sources are present in frequency-varying or random patterns, how to broaden the bandwidth of vibration energy harvesters becomes one of the most challenging issues before their practical deployment. This chapter presents a review of recent advances in broadband vibration energy harvesting. The state-of-the-art techniques in this field, covering resonant frequency tuning, multimodal energy harvesting, and nonlinear energy harvesting configurations, are summarized in detail with regard to their merits and applicability in different circumstances.

When the excitation frequency is known a priori, the geometry and dimensions of a conventional linear harvester can be carefully selected to match its resonant frequency with the excitation frequency. However, when the excitation frequency is unknown or varies in different operational conditions, the harvester with pre- tuned resonant frequency is unable to achieve optimal power output. Hence, in practice a conventional linear harvester is expected to incorporate a resonance tuning mechanism to increase its functionality.

The active mode requires continuous power input for resonance tuning. While in the passive mode, intermittent power is input for tuning and no power is required when frequency matching is completed, that is until the excitation frequency varies again. Resonance tuning methods can be categorized into mechanical, magnetic, and piezoelectric methods. Furthermore, the tuning process can be implemented manu- ally or in a self-tuning way.

Manual tuning is very difficult to implement during operation. From elementary of vibration theory, the resonance of a system can be tuned by changing the stiffness or mass. Usually, it is more practical to change the stiffness rather than the mass of the system. Leland and Wright [11], Eichhorn et al. In their experimental test on the prototype with a 7. The power output was relatively flat over this range and even decreased at low frequencies, which could be explained by the increased damping ratio due to the applied preload, as shown in Fig. However, the energy required for the tuning procedure was not addressed.

Furthermore, the resonant frequency could only be tuned unidirectionally since only the compressive preload was considered. Eichhorn et al. Figure 2. The arms connected the tip of the beam and two wings. A revolution of the screw generated compression on the spring, which applied the force on the arms. This force was then applied to the free end of the cantilever through the wings.

Below the fracture limit, a resonance shift from to Hz was achieved by applying preload from 7 N to Analytically, Hu et al. The resonance can be adjusted either higher or lower with a tensile or compressive load, respectively. In their model, it was reported that a tensile load of 50 N increased the resonance from Instead of considering the bending mode, some researchers have investigated a tunable resonator working in extensional mode, termed XMR [14, 15]. The XMR presented by Morris et al. Hence, the resonant frequency can be tuned by adjusting the link length that symmetrically pretensions both piezoelectric sheets.

For the fabricated XMR prototype with a circular configuration, the frequency response functions were obtained by tuning the preloading screw at three random adjustment positions, as shown in Fig. For the developed prototype, it was found that a resonance shift between 80 and Hz can be easily achieved with a change of pretension displacement of around 1. Morris et al. However, the capability of self-tuning or sequential tuning during operation of the XMR was not investigated. A similar investigation was pursued by Loverich et al.

The resonant frequency could be experimentally varied between 56 and 62 Hz by adjusting the boundary location by approximately 0. Furthermore, they also made use of nonlinearity of the pre-deflected plate. Similar force—deflection characteristics were obtained as Eq. It was found that the stiffness was nearly linear and the system had a high quality factor Q for low vibration amplitudes, while the resonance frequency shifted and Q reduced for high vibration amplitudes. This feature of nonlinear stiffness also provided an auto- protection mechanism, which is important when mechanical robustness is required for high vibration levels.

Rather than applying the axial or in-plane preload, adjusting the gravity center of the tip mass is another idea to adjust the resonance of a cantilever. Wu et al. The gravity center of the whole proof mass can be adjusted by driving the movable screw. In their prototype, the adjustable resonant frequency range could cover — Hz by tuning the gravity center of the tip mass up to 21 mm, as illustrated in Fig. In Sect. To address this problem, some researchers [18, 19] developed novel passive self-tuning harvesters.

Jo et al. This cantilever couple was movable laterally and had two operational phases. The horizontal inertia forces exerted to two equal proof masses change with the excitation frequency and become maximum when the excitation frequency matches the resonant frequency. The difference between the horizontal inertia forces is the key to switch the harvester between the two phases.

This harvester is self-tunable and no power is required in the tuning procedure. Each cantilever has two resonant peaks as the excitation frequency changes. Although the resonant frequency only switches between two phases and thus can not cover a continuous frequency range, such device is still significantly more efficient than a conventional cantilever harvester without a self-tuning mechanism. Different from previous research on harvesting energy from translational base excitation, Gu and Livermore [18] focused on rotational motion.

A passive self- tuning piezoelectric energy harvester was designed in which centrifugal force was exploited to adjust the stiffness and thus its resonant frequency. The harvester consisted of a radially oriented cantilever beam mounted on a rotational body, as shown in Fig. Since the centrifugal force was related to both driving frequency and resonant frequency of the harvester, the harvester could be designed such that the resonant frequency was exactly equal to the driving frequency at In addition, within a range of 6.

Thus, the harvester always worked at or near its resonance. In their experiment, the self-tuning harvester could achieve a much wider bandwidth of 8. However, this device is only applicable for rotational motion. Applying magnetic force to alter the effective stiffness of a harvester is another option for resonance tuning. Challa et al. All magnets were vertically aligned so that attractive and repulsive magnetic forces could be generated on each side of the beam. Power output was undermined as the damping increased during the tuning procedure, as shown in Fig.

Given the maximum tuning distance of 3 cm, the required energy was estimated to be 85 mJ and it would take s for each tuning procedure.

A reality check on renewables - David MacKay

Thus such device can only work where the excitation frequency changes infrequently. Reissman et al. The resonance of the piezoelectric energy harvester could be tuned bidirectionally by adjusting a magnetic slider. This is a much simplified design as compared to the design of Challa et al. The effective stiffness of the piezoelectric beam was dependent on the structural component Km , the electromechanical component Ke that varied with external resistive loading Rl , and the magnetic stiffness Kmagnetic that varied with the relative distance D between the two magnets, i.

Keff D. Km C Ke. By tuning the vertical relative distance Dy of the two magnets, the resonance could be tuned bidirectionally. For a specific Dx , the maximum frequency achieved was Hence, the total bandwidth of the harvester was Zhu et al. A schematic of the tuning mechanism is shown in Fig. The microcontroller woke up periodically, detected the output voltage of the generator and gave instructions to drive a linear actuator to adjust the distance D between the two magnets. In their experimental test, the resonant frequency was tuned from At a constant acceleration of 0.

However, the damping was increased and the output power was less than expected if the tuning force became larger than the inertial force caused by vibration. Additionally, the energy consumed for the tuning procedure in their design was 2. They claimed that the linear actuator and microcontroller would be ultimately powered by the generator itself to form a closed-loop tuning system. However, experimentally, the tuning system was still powered by a separate power supply for preliminary evaluation. Another drawback was that the control system detected the resonance by comparing the output voltage with a predefined threshold.

Thus, such a system could suffer from inefficient detection of the frequency change direction and from mistaken triggering if there was certain change in the excitation amplitude. Following the work of Zhu et al. The phase difference between the harvester and the base was measured in the closed- loop control, which was used to indicate the direction to tune the magnets. A tuning range of However, under the excitation of 0.

However, the energy of 3. Although the above magnetic tuning harvesters implemented automatic control systems, they were not self-powered. The magnets and control systems also increase the complexity of system design and integration. A piezoelectric transducer used as an actuator can alter the stiffness of a system. In fact, the stiffness of the piezoelectric material itself can be varied with various shunt electrical load.

Hence, piezoelectric transducers provide another option for resonance tuning. The energy generation method could be electrostatic, electromagnetic, or piezoelectric conversion. The tunable bandwidth of the generator was 3 Hz from In the two demo tests, the device was excited under a chirp and random vibration from 80 to Hz, an average harvested power of 1.

These results corresponded to an increase of A microcontroller was utilized to sample the external frequency and adjusted the capacitive load to match the external vibration frequency in real time, in other words, the device was tuned actively. Peters et al. Such deformation caused an additional hinge moment and thus a stiffer structure. A discrete control circuit, which exploited the phase characteristic of the resonator, was implemented to actively control the resonance tuning.

However, the power consumption of around mW was supplied externally, which significantly outweighed the harvested power 1. Thus, the development of a low- power CMOS integration control circuit was recommended for practical closed-loop automatic tuning. Roundy and Zhang [10] investigated the feasibility of active tuning mechanism. The electrode was etched to create a scavenging and a tuning part. Lallart et al. The schematic of the system is shown in Fig. An additional piezoelectric sensor and an accelerometer recorded the beam deflection and base acceleration.

Energy harvesting for assistive and mobile applications

The self- detection of frequency change was based on the average product of these two signals, which gave the phase information and instructed the closed-loop control to apply the actuation voltage VS. The most critical part of the required power for tuning in this device was the power for actuation Pact. Other terms can be found in Lallart et al. However, frequency detec- tion and information processing modules of the system worked in real time from a continuous external power supply. The proposed system was estimated to achieve a positive net power output and to increase the bandwidth by a factor of 2 from 4.

This result is different from the conclusion by Roundy and Zhang [10], in which they could wrongly derive the net power by using the maximum power rather than the average power for actuation [28]. However, once the control voltage was disconnected, the frequency drifted away from the initial adjusted value due to leakage of the piezoceramic, as shown in Fig. The charge had to be refreshed periodically to maintain the desired resonant frequency.

Table 2. From Table 2. However, most of the tunable designs using mechanical method required manual adjustment of the system parameters, such as the preload, pre-deflection, or gravity center of the tip mass. Tuning screws were widely used in these adjusting procedures, which makes it difficult to implement automatically during operation. The mechanical work required for tuning was not addressed in the literature reviewed. Only a few self-tuning mechanical methods [18, 19] enabled the harvesters to be self-adaptive to the vibration environment by exploiting the frequency-dependent inertia force.

These devices were capable of automatic tuning during operation without external power input. However, they were applicable for specific conditions.

Description:

For example, the device by Gu and Livermore [18] only worked for rotational vibration, and the design by Jo et al. Using magnets for resonance tuning can achieve moderate tunability. Automatic control and tuning can be implemented to adjust the distance between the magnets by using linear actuators. Thus, automatic tuning can be achieved during operation. Thus they are only suitable for the scenarios where small and infrequent frequency changes occur. The use of magnets and control systems also increase the complexity of system design and integration.

As shown in Table 2. However, since the piezoelectric transducer itself functions as both the controller and tuning component, it is convenient to implement automatic tuning by applying voltage to the transducer or switching the shunt electrical load. In some reported designs [10, 26], the power required for active tuning significantly outweighed the harvested power.

However, Wu et al. The reason for this difference is that the concept in Wu et al. The latter usually consumes much more power [27]. Besides, when voltage is applied to the actuator, the leakage of piezoceramic increases the power consumption. Active tuning is usually implemented by piezoelectric tuning methods.

Generally, it requires more power input than pas- sive tuning, and the tuning power may outweigh the harvested power. However, a net power increase is still possible in active tuning mode if resonance tuning is only required in a very limited range [25, 27]. Passive tuning requires less power input to periodically detect and change the frequency, which is suitable when the excitation frequency varies infrequently. However, if the harvested power can sustain the continuous power required for tuning, an active tuning harvester can work for the excitation with constantly changing frequency or under random excitation, such as the case studied by Wu et al.

In practice, energy harvesters are multiple degree-of-freedom DOF systems or distributed parameter systems. Thus one of the vibrational modes of the harvester can be excited when the driving frequency approaches the natural frequency associated with the particular mode. If multiple vibration modes of the harvester are utilized, useful power can be harvested i.

However, in practice, multimodal energy harvesters are usually implemented by exploiting multiple bending modes of a continuous beam or by an array of cantilevers. One implementation of this idea was the multifrequency electromagnetic harvester developed by Yang et al. Other than this work, most of the reported studies in the literature exploited a multimodal harvester with a cantilever beam configuration, in which the first two bending modes were used in other words, a 2DOF vibration energy harvester.

Tadesse et al. The electromagnetic scheme generated high output power for the first mode, while the piezoelectric scheme was efficient for the second mode. However, the first resonance and the second resonance of such device were far away from each other 20 Hz and Hz. Such discrete effective bandwidth may only be helpful when the vibration source has a rather wide frequency spectrum. The increased size may be another drawback since the permanent magnet is usually difficult to scale down.

Besides, a drastic difference of matching loads for electromagnetic and piezoelectric harvesting presents a difficulty in interface circuit design to combine the power outputs from the two schemes. Ou et al. Although two useful modes were obtained, similar to Tadesse et al. Arafa et al. Besides, it was observed see Fig.

However, the magnifier with a spring beam length of 70 mm and a magnifier mass of Erturk et al. With proper parameter selection, the second natural frequency was approximately double the first, as shown in Fig. However, how to avoid mode-shape-dependent voltage cancelation was a critical issue. Changing the leads from the first piezoelectric segment in a reverse manner could avoid the cancelation of the second mode but this caused the cancelation for the first mode instead. Thus a more sophisticated interface circuit is required to adaptively change the electrode leads or to deliver the energy separately to avoid voltage cancelation.

Berdy et al. The concept of this design is shown in Fig. The fabricated prototype successfully achieved two closely spaced resonant modes at 33 Hz and In a wide bandwidth of Another advantage of this device was that the sensing electronics and circuit board could be used as the distributed proof mass thus achieving a compact system.

Compared to the conventional SDOF harvester, this device was able to generate two close effective peaks in voltage response with properly selected parameters, as shown in Fig. Thus, multimodal energy harvesting was achieved with only a slight increase of the system volume. Besides, significant voltage output could be obtained from the secondary beam Fig. Thus, this device efficiently utilized the material of the cantilever beam. Moreover, as compared to previously reported 2DOF harvester designs, it was more compact and could have two resonant frequencies much closer to each other.

Different from the discrete bandwidth corresponding to the multiple modes of a single beam, multiple cantilevers or cantilever arrays integrated in one energy harvesting device can easily achieve continuous wide bandwidth if the geometric parameters of the harvester are appropriately selected. Similar to the configurations in Sect. Yang and Yang [41] suggested using connected or coupled bimorph cantilever beams for energy harvesting, whose resonant frequencies could be tuned to be very close to each other.

Similar to Wu et al. The amplitude and location of the resonances were found to be sensitive to the end spring and end masses.

Passar bra ihop

Kim et al. Other than these previous two designs of two coupled cantilevers, most of the research attempts were made to develop multimodal devices with more cantilevers to tailor and cover desired bandwidth for specific applications [43—47]. Different from Yang and Yang [41] and Kim et al. Each cantilever was regarded as one substructure of the harvester and thus the first mode of each cantilever was one of the vibration modes of the harvester.

Shahruz [43] designed an energy harvester that consists of piezoelectric can- tilevers of various lengths and tip masses attached to a common base Fig. It was capable of resonating at various frequencies by properly selecting the length and tip mass of each beam and thus provided voltage response over a wide frequency range Fig. They found that the bandwidth of their PB array configuration could be tailored by choosing an appropriate connection pattern mixed series and parallel connections.

Advances in energy harvesting methods

Connecting multiple bimorphs in series could broaden the bandwidth. This shift was due to the change in the electrical boundary condition when increasing or decreasing bimorphs in parallel. Ferrari et al. When excited by mechanical vibrations, the device charged the storage capacitor and regularly delivered the energy to the wireless sensor and measurement transmission module.

Under resonant excitation, i. Conversely, with the complete converter array, the converted energy was high enough to trigger the transmission for all the tested frequency, including f4. Besides, the shorter switching time two measure-and-transmit operations was obtained using the converter array rather than a single cantilever. It was claimed that the wider bandwidth and improved performance were worth the modest increase in size of the proposed array device. Broadband energy harvesters with cantilever array were also implemented compatibly with current standard MEMS fabrication techniques [46, 47].

Liu et al. In their experimental test, a phase difference in voltage output from each cantilever was observed, which impaired the voltage output of this three cantilever device Fig. Thus, the DC voltage across the capacitor after rectification was only 2. To address this problem, separate rectifier for each cantilever was required, which increased the total DC voltage to 3. With the wider bandwidth — Hz and the improved output, such a device was claimed to be promising in applications of ultra-low-power wireless sensor networks.

However, the more complicated rectification circuit may cause significant energy loss in these MEMS-scale devices especially for low-level or off-resonance excitations. Sari et al. The developed device generated power via the relative motion between a magnet and coils fabricated on 35 serially connected cantilevers with different lengths. It was reported that 0. The cantilever size had a very similar scale but the power output from the device by Sari et al.

Furthermore, the voltage level of 10 mV from the harvester by Sari et al. Multimodal energy harvesting can be implemented by exploiting multiple bending modes of a continuous beam or by exploiting a cantilever array integrated in one device where the first mode of each cantilever is one of the vibration modes of the device. Compared with the resonance tuning techniques, multimodal energy harvesting does not require tuning and hence is much easier to implement. The multiple bending modes of a continuous beam are usually far away from one another and thus the effective bandwidth is discrete.

Some novel structures like L-shaped beams [38], cut-out beams [40], and cantilevered meandering beams [39] can be considered to achieve close and effective resonant peaks. However, in general, only the first two modes can contribute to effective multimodal energy harvesting. Multimodal energy harvesting increases the bandwidth but is however accompanied by an increased volume or weight of the device. For example, in the cantilever array configuration, only one cantilever or a subset of the array is active and effective for energy generating while the other cantilevers are at an off-resonance status.

Hence, with the known dominant spectrum of the ambient vibration, the harvester should be carefully designed with a proper number and dimensions of the cantilevers such that the device can cover the targeted bandwidth with the least sacrifice of power density. Multimodal energy harvesting requires more complex interface circuit than that for a single-mode harvester. A critical electrical issue is to avoid mode shape dependent voltage cancelation in a continuous beam or the cancelation due to the phase difference between cantilevers in array configurations.

More sophisticated interface circuits are required to adaptively change the electrode leads or to deliver the energy separately i. An interface circuit is also required to address the drastic difference in matching load for different energy transduction mechanisms in the hybrid energy harvesting scheme based on a continuous beam [35]. Actually these magnets introduce not only a change in the linear stiffness but also a change in the nonlinear stiffness.

The nonlinear behavior becomes apparent when the harvester experiences oscillation with significant amplitude. Such nonlinearity also benefits wideband energy harvesting. As reported in the available literature, nonlinearities in energy harvesters are con- sidered from two perspectives, i. Compared to the nonlinear piezoelectric coupling, which results from the manufacturing process of piezoelectric materials, the nonlinear stiff- ness of a harvester is relatively easier to achieve and control.

This section reviews recent advances in designing broadband energy harvesters with nonlinear stiffness. The dynamics of a general oscillator can be described as. Details on this kind of electrical viscous damping can be found in El-Hami et al. For a piezoelectric generator, the damping caused by piezoelectricity cannot be modeled as a viscous damper [60] and Eq.

The circuit equations for the piezoelectric and the electromag- netic harvesters are quite different due to differences in their internal impedances. They are not given here but they can be readily found in the literature related to electromagnetic and piezoelectric transductions, such as El-Hami et al. Duffing-type nonlinear oscillator For a Duffing-type oscillator, the potential energy function U x can be consid- ered in a quadratic form as [61, 62],.

The potential function U x for different Duffing oscillators is shown in Fig. Other than the Duffing-type oscillator, some researchers also attempted to exploit piecewise-linear stiffness to increase the bandwidth of vibration energy harvesters. Using mechanical stoppers is one common way to introduce the piecewise-linear stiffness [56, 63—65]. A typical setup of a vibration energy harvester with a mechanical stopper and its nonlinear stiffness are illustrated in Fig. This section reviews both Duffing-type nonlinear harvesters and harvesters with mechanical stoppers. Their benefits on improving the performance of vibration energy harvester are discussed in the following parts.

Ramlan et al. Their numerical and analytical studies showed that ideally, the maximum amount of power harvested by a system with a hardening stiffness was the same as the maximum power harvested by a linear system, regardless of the degree of nonlinearity. However, this might occur at a different frequency depending on the degree of nonlinearity, as shown in Fig. Such a device has a larger bandwidth over which the significant power can be harvested due to the shift in the resonant frequency. Mann and Sims [50] presented a design for electromagnetic energy harvesting from nonlinear oscillations due to magnetic levitation.

The derived governing equation has the same form as Eq. At low excitation level, the frequency response Fig. However, at high excitation level, the response curve was bent to the right Fig. Thus, relatively large amplitudes persisted over a much wider frequency range. Both experiment and theoretical analysis captured the jump phenomena near the primary resonance and the multiple periodic attractors, as shown in Fig.

However, such a hardening device only broadened the frequency response in one direction the peak response shifts to the right. Stanton et al. The device consisted of a piezoelectric beam with a magnetic end mass interacting with the fields of oppositely poled stationary magnets, as shown in Fig. By tuning the nonlinear magnetic interactions around the end mass i.

In the experimental validation, a linearly decreasing frequency sweep was performed for the softening case. Different from Ramlan et al. This might be due to the change of damping due to the magnets used in the experiment [20, 22], while a constant damping was used in the analysis by Ramlan et al. Previous monostable designs have a larger bandwidth due to the shift in the resonant frequency. Theoretical predictions include stable solutions solid line and unstable solutions dashed line [50], copyright: Elsevier.

A linearly decreasing or increasing frequency sweep can capture the high-energy attractor and hence improve the bandwidth for the softening and hardening cases. Unfortunately, such conditions cannot be guaranteed in practice. Certain means of mechanical or electrical disturbance or perturbation is required once the nonlinear devices enter low-energy orbits; otherwise little power can be harvested. Previous reported studies did not address the required momentary perturbation if the harvester is in the low-energy branch and the requisite actuation energy. Furthermore, under a White Gaussian excitation, Daqaq [67] demonstrated that the hardening-type nonlinearity failed to provide any enhancement of output power over typical linear harvesters.

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FAQ Policy. About this book Advances in Energy Harvesting Methods presents a state-of-the-art understanding of diverse aspects of energy harvesting with a focus on: broadband energy conversion, new concepts in electronic circuits, and novel materials.