EPSRC Energy Harvesting Network

Relevant Publications

This page contains a range of publications (journal articles, conference papers and book chapters) relevant to the field of energy harvesting.

The EH Network resources have been contributed by members. If you are a member, please contribute.


Michele Pozzi, (2016) Magnetic plucking of piezoelectric bimorphs for a wearable energy harvester. In: Smart Materials and Structures, 25(4), p.45008.

Abstract: A compact and low-profile energy harvester designed to be worn on the outside of the knee-joint is presented. Frequency up-conversion has been widely adopted in recent times to exploit the high frequency response of piezoelectric transducers within environments where only low frequencies are present. Contactless magnetic plucking is here introduced, in a variable reluctance framework, with the aim of improving the mechanical energy transfer into the transducers, which is sub-optimal with contact plucking. FEA and experiments were used to design an optimal arrangement of ferromagnetic teeth to interact with the magnets fixed to the piezoelectric beams. A prototype was made and extensively tested in a knee-joint simulator controlled with gait data available in the literature. Energy and power produced were measured for walking and running steps. A power management unit was developed using off-the-shelf components, permitting the generation of a stable and regulated supply of 26 mW at 3.3 V during walking. Record levels of rectified (unregulated) electrical power of over 50 and 70 mW per walking and running steps, respectively, were measured.

Derek Arthur, Steve Beeby, (2014) A miniature coupled bistable vibration energy harvester. In: Journal of Physics: Conference Series, 557, 012124.

Abstract: This paper reports the design and test of a miniature coupled bistable vibration energy harvester. Operation of a bistable structure largely depends on vibration amplitude rather than frequency, which makes it very promising for wideband vibration energy harvesting applications. A coupled bistable structure consists of a pair of mobile magnets that create two potential wells and thus the bistable phenomenon. It requires lower excitation to trigger bistable operation compared to conventional bistable structures. Based on previous research, this work focused on miniaturisation of the coupled bistable structure for energy harvesting application. The proposed bistable energy harvester is a combination of a Duffing’s nonlinear structure and a linear assisting resonator. Experimental results show that the output spectrum of the miniature coupled bistable vibration energy harvester was the superposition of several spectra. It had a higher maximum output power and a much greater bandwidth compared to simply the Duffing’s structure without the assisting resonator.

Kantida Pancharoen, Steve Beeby, (2014) A hip implant energy harvester. In: Journal of Physics: Conference Series, 557, 012038.

Abstract: This paper presents a kinetic energy harvester designed to be embedded in a hip implant which aims to operate at a low frequency associated with body motion of patients. The prototype is designed based on the constrained volume available in a hip prosthesis and the challenge is to harvest energy from low frequency movements (< 1 Hz) which is an average frequency during free walking of a patient. The concept of magnetic-force-driven energy harvesting is applied to this prototype considering the hip movements during routine activities of patients. The magnetic field within the harvester was simulated using COMSOL. The simulated resonant frequency was around 30 Hz and the voltage induced in a coil was predicted to be 47.8 mV. A prototype of the energy harvester was fabricated and tested. A maximum open circuit voltage of 39.43 mV was obtained and the resonant frequency of 28 Hz was observed. Moreover, the power output of 0.96 �¼W was achieved with an optimum resistive load of 250�©.

Peter Woolliams, Mark Stewart, Paul Weaver, Professor Markys G Cain, FIMMM, CPhys, Joe Briscoe, (2014) Improved performance of p-n junction-based ZnO nanogenerators through CuSCN-passivation of ZnO nanorods. In: Journal of Materials Chemistry A, Vol. 2, p. 10945.

Abstract: Self-powered piezoelectric systems are vital components to harvest ambient waste energy for applications such as autonomous self-powered sensors. ZnO nanorod-based devices are gaining wide attention for energy harvesters as they are easily synthesized at low temperature onto a range of substrates. However, losses related to screening of piezoelectric polarisation charges by free carriers in ZnO nanorods can significantly reduce the output of these devices. This paper reports using copper thiocyanate (CuSCN) to reduce ZnO surface-state-induced parasitic losses and improve power density of ZnO nanorod-based energy harvesters by a factor of 10. The surface modified energy harvester generated 1.07 V as an instantaneous peak open circuit voltage giving a maximum calculated 434 uW/cm^2 peak power density associated with release at an acceleration of 50 g. Impedance analysis determined that the device RC time constant (t_RC), correlated with device peak voltage, demonstrating a relationship between the screening effects in ZnO and t_RC.

Steve Beeby, John Tudor, Neil White, Nick Harris, (2013) A novel miniature airflow energy harvester for wireless sensing applications in buildings. In: IEEE Sensors Journal, 13, (2), 691-700.

Abstract: This paper presents a novel miniature airflow energy harvester for wireless sensing applications. The energy harvester consists of a wing that is attached to a cantilever spring. The wing oscillates in response to a steady airflow. An electromagnetic transducer is used to extract electrical energy from the airflow induced oscillations. Both vertical and horizontal orientations have been studied. Experiments have shown that such generator can operate at airflow speeds as low as 1.5m/s which compares well to turbines. When the airflow speed is over 2m/s, the average output power exceeds 90uW, which is sufficient for powering wireless sensor nodes in Heat, Ventilation and Air Condition (HVAC) systems in buildings.

Paul Mitcheson, Eric Yeatman, John Tudor, Steve Beeby, (2013) Magnetic tuning of a kinetic energy harvester using variable reluctance. In: Sensors and Actuators A: Physical , 189, 266-275.

Abstract: In this paper we present a new technique for tuning the resonant frequency of an energy harvester using a variable reluctance device which changes the strain in a cantilever beam to alter its spring constant. In order for energy harvesting devices to be able to operate reliably in many applications they must be able to generate energy as the input excitation frequency changes. Most harvesters are resonantly tuned mass-spring-damper devices and therefore it is important that their resonant frequency is tuneable during operation. Several mechanical methods have previously been demonstrated for accomplishing this task which operates by altering the stress in a cantilever beam by altering the distance between a fixed tuning magnet and a magnet on the moving cantilever. Here, we demonstrate a new actuation method for manipulating the stress in a beam. The actuation mechanism alters the magnetic reluctance between the cantilever mounted and the fixed magnet. The investigation has highlighted the importance of the design of the magnetic circuit and choice of materials in order to avoid eddy current damping and asymmetrical forces. The method presented here has demonstrated a maximum tunable frequency range of 11.1 Hz and may be more suitable for microfabrication than the previously reported techniques due to the reduced tuning force which makes microfabricated actuators feasible.

Peter Glynne-Jones, Neil White, Nick Harris, Russel Torah, Steve Beeby, (2013) Screen printed piezoelectric films for energy harvesting. In: Advances in Applied Ceramics, 112, (2), 79-84.

Steve Beeby, (2013) A broadband electromagnetic energy harvester with a coupled bistable structure. In: PowerMEMS 2013, London, GB, 03 - 06 Dec 2013.

Abstract: This paper investigates a broadband electromagnetic energy harvester with a coupled bistable structure. Both analytical model and experimental results showed that the coupled bistable structure requires lower excitation force to trigger bistable operation than conventional bistable structures. A compact electromagnetic vibration energy harvester with a coupled bistable structure was implemented and tested. It was excited under white noise vibrations. Experimental results showed that the coupled bistable energy harvester can achieve bistable operation with lower excitation amplitude and generate more output power than both conventional bistable and linear energy harvesters under white noise excitation.

Steve Beeby, (2013) A coupled bistable structure for broadband vibration energy harvesting. In: Transducers 2013 , Barcelona, ES, 16 - 20 Jun 2013..

Dr Martin Judd, (2013) Harvesting Energy From Magnetic Fields to Power Condition Monitoring Sensors. In: IEEE Sensors Journal, Vol. 13, Issue 6, pp. 2263-2270.

Abstract: Condition monitoring is playing an increasingly important role within electrical power networks, where its use can help to reduce maintenance costs, improve supply reliability, and permit increased utilization of equipment capacity. In this context, energy harvesting may have a role to play in that it offers the possibility of realizing autonomous, self-powering sensors that communicate their data wirelessly. In the vicinity of electrical transmission and distribution equipment, alternating magnetic fields at the power frequency offer a potential source of energy that does not require hard-wiring or batteries. There are many potentially useful locations for sensors where the level of magnetic flux density may be sufficient to provide enough power for a low-power wireless sensor node. This paper describes a free-standing inductive harvester for use in positions where there is an ambient magnetic field due to conductors that are remote and/or inaccessible. Using data from surveys of magnetic flux density levels at two substations, optimum core and coil designs for the harvester are obtained through theoretical analyses and experiments. A demonstrator is then constructed in which a wireless sensor becomes self-powering when immersed in a 50-Hz magnetic field. Laboratory results show that this system can deliver a useful average power of 300 uW when placed in a magnetic flux density of 18 uTrms.

Dr Martin Judd, (2013) Optimization of Voltage Doublers for Energy Harvesting Applications. In: IEEE Sensors Journal, Vol. 13, Issue 12, pp. 4904 - 4911.

Abstract: Energy harvesting is increasingly enabling the expansion of wireless sensor networks in challenging applications by replacing batteries in low power sensors. Many forms of energy harvester suffer from low output voltage that can be partially compensated for by the use of a Cockcroft-Walton voltage doubler ahead of a dc-dc converter. Impedance matching of energy harvesters is critical to achieve high output power/unit volume. This paper explores optimum impedance match for an energy harvester with a voltage doubler and dc-dc converter. Formulas are derived and experimentally confirmed, which calculate optimum impedance match between the harvester and a load, and calculate voltage at the input to the dc-dc converter for a given wireless sensor power consumption. Further, the formula for optimum impedance match is validated against independently published results.

Joe Briscoe, Nimra Jalali, Peter Woolliams, Mark Stewart, Paul Weaver, Professor Markys G Cain, FIMMM, CPhys, (2013) Measurement techniques for piezoelectric nanogenerators. In: Energy & Environmental Science, 6, 3035-3045.

Abstract: Electromechanical energy harvesting converts mechanical energy from the environment, such as vibration or human activity, into electrical energy that can be used to power a low power electronic device. Nanostructured piezoelectric energy harvesting devices, often termed nanogenerators, have rapidly increased in measured output over recent years. With these improvements nanogenerators have the potential to compete with more traditional micro- or macroscopic energy harvesting devices based on piezoelectric ceramics such as lead zirconate titanate (PZT), polymers such as polyvinylidene fluoride (PVDF) or electrostatic, electret or electromagnetic kinetic energy harvesters. Power output from a nanogenerator is most commonly measured through open-circuit voltage and/or short-circuit current, where power may be estimated from the product of these values. Here we show that such measures do not provide a complete picture of the output of these devices, and can be misleading when attempting to compare alternative designs. In order to compare the power output from a nanogenerator, techniques must be improved in line with those used for more established technologies. We compare ZnO nanorod/PMMA and ZnO nanorod/PEDOT:PSS devices, and show that despite an open-circuit voltage nearly three times lower the ZnO/PEDOT:PSS device generates 150 times more power on an optimum load. In addition, it is shown that the peak voltage and current output can be increased by straining the device more rapidly and therefore time-averaged power, or time-integrated measures of output such as total energy or total charge should be calculated. Finally, the internal impedance of the devices is characterised to develop an understanding of their behaviour and shows a much higher internal resistance but lower capacitive impedance for the ZnO/PMMA device.

Steve Beeby, John Tudor, Nick Harris, (2012) Vibration energy harvesting using the Halbach array. In: Smart Materials and Structures, 21, (7), 075020-.

Abstract: This paper studies the feasibility of vibration energy harvesting using a Halbach array. A Halbach array is a specific arrangement of permanent magnets that concentrates the magnetic field on one side of the array while cancelling the field to almost zero on the other side. This arrangement can improve electromagnetic coupling in a limited space. The Halbach array offers an advantage over conventional layouts of magnets in terms of its concentrated magnetic field and low-profile structure, which helps improve the output power of electromagnetic energy harvesters while minimizing their size. Another benefit of the Halbach array is that due to the existence of an almost-zero magnetic field zone, electronic components can be placed close to the energy harvester without any chance of interference, which can potentially reduce the overall size of a self-powered device. The first reported example of a low-profile, planar electromagnetic vibration energy harvester utilizing a Halbach array was built and tested. Results were compared to ones for energy harvesters with conventional magnet layouts. By comparison, it is concluded that although energy harvesters with a Halbach array can have higher magnetic field density, a higher output power requires careful design in order to achieve the maximum magnetic flux gradient.

Joe Briscoe, Mark Stewart, Melvin Vopson, Professor Markys G Cain, FIMMM, CPhys, Paul Weaver, (2012) Nanostructured p-n Junctions for Kinetic-to-Electrical Energy Conversion. In: Advanced Energy Materials, Vol. 2, pp. 1261-1268.

Abstract: Piezoelectric ZnO nanorods grown on a flexible substrate are combined with the p-type semiconducting polymer PEDOT:PSS to produce a p-n junction device that successfully demonstrates kinetic-to-electrical energy conversion. Both the voltage and current output of the devices are measured to be in the range of 10 mV and 10 uAcm-2. Combining these figures for the best device gives a maximum possible power density of 0.4 mW cm-3. Systematic testing of the devices is performed showing that the voltage output increases linearly with applied stress, and is reduced significantly by illumination with super-band gap light. This provides strong evidence that the voltage output results from piezoelectric effects in the ZnO. The behaviour of the devices is explained by considering the time-dependent changes in band structure resulting from the straining of a piezoelectric material within a p-n junction. It is shown that the rate of screening of the depolarisation field determines the power output of a piezoelectric energy harvesting device. This model is consistent with the behaviour of a number of previous devices utilising the piezoelectric effect in ZnO.

Joe Briscoe, Emiliano Bilotti, (2012) Measured efficiency of a ZnO nanostructured diode piezoelectric energy harvesting device. In: Applied Physics Letters, 101, 093902.

Abstract: We used controlled bending of a ZnO/poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) diode at known strain rates to measure the mechanical-to-electrical energy conversion efficiency. The mechanical energy input into the nanostructured diode was measured as 330�±�2�nJ�cm�2. The electrical energy output was calculated by integrating the product of the short-circuit current and open-circuit voltage over time. This gives a measured external efficiency of the device at a bending rate of 500�mm/min of 0.0067%. The efficiency increased exponentially with bending rate, though this increase must slow as the mechanical coupling efficiency is approached, which gives a maximum possible efficiency of 23% for ZnO.

Wei-Hsin LIAO, (2012) A self-sensing magnetorheological damper with power generation. In: Smart Materials and Structures, 21, 025014.

Abstract: Magnetorheological (MR) dampers are promising for semi-active vibration control of various dynamic systems. In the current MR damper systems, a separate power supply and dynamic sensor are required. To enable the MR damper to be self-powered and self-sensing in the future, in this paper we propose and investigate a self-sensing MR damper with power generation, which integrates energy harvesting, dynamic sensing and MR damping technologies into one device. This MR damper has self-contained power generation and velocity sensing capabilities, and is applicable to various dynamic systems. It combines the advantages of energy harvesting�reusing wasted energy, MR damping�controllable damping force, and sensing�providing dynamic information for controlling system dynamics. This multifunctional integration would bring great benefits such as energy saving, size and weight reduction, lower cost, high reliability, and less maintenance for the MR damper systems. In this paper, a prototype of the self-sensing MR damper with power generation was designed, fabricated, and tested. Theoretical analyses and experimental studies on power generation were performed. A velocity-sensing method was proposed and experimentally validated. The magnetic-field interference among three functions was prevented by a combined magnetic-field isolation method. Modeling, analysis, and experimental results on damping forces are also presented.

Li Yin CHAU, Wei-Hsin LIAO, (2012) A knee-mounted biomechanical energy harvester. In: Proceedings of International Conference on Adaptive Structures and Technologies, In press.

Abstract: A novel knee-mounted biomechanical energy harvester was proposed and developed. The average harvested power of one harvester could be 3.6 W at normal human walking speed. This device could harvest the biomechanical energy from both the knee flexion and extension action. It could significantly improve the energy harvesting efficiency of the current biomechanical energy harvesters by 60-80%. In addition, this energy harvester could provide extra safety to the wearers from its specially designed mechanical structure. This device was also demonstrated a good performance under different conditions such as walking, running and stepping stairs.

Yang Yaowen, (2012) A Nonlinear Piezoelectric Energy Harvester with Magnetic Oscillator. In: Applied Physics Letters, Vol.101, No.9, 094102.

Abstract: This letter proposes a magnetic coupled piezoelectric energy harvester (PEH), in which the magnetic interaction is introduced by a magnetic oscillator. For comparison purpose, lumped parameter models are established for the conventional linear PEH, the nonlinear PEH with a fixed magnet, and the proposed PEH with a magnetic oscillator. Both experiment and simulation show the benefits from the dynamics of the magnetic oscillator. In the experiment, nearly 100% increase in the operating bandwidth and 41% increase in the magnitude of the power output are achieved at an excitation level of 2m/s^2.

Yang Yaowen, Soh Chee Kiong, (2012) Improving Functionality of Vibration Energy Harvesters Using Magnets. In: Journal of Intelligent Material Systems and Structures, online first.

Abstract: In recent years, several strategies have been proposed to improve the functionality of energy harvesters under broadband vibrations, but they only improve the efficiency of energy harvesting under limited conditions. In this work, a comprehensive experimental study is conducted to investigate the use of magnets for improving the functionality of energy harvesters under various vibration scenarios. First, the nonlinearities introduced by magnets are exploited to improve the performance of vibration energy harvesting. Both monostable and bistable configurations are investigated under sinusoidal and random vibrations with various excitation levels. The optimal nonlinear configuration (in terms of distance between magnets) is determined to be near the monostable-to-bistable transition region. Results show that both monostable and bistable nonlinear configurations can significantly outperform the linear harvester near this transition region. Second, for ultra-low-frequency vibration scenarios such as wave heave motions, a frequency up-conversion mechanism using magnets is proposed. By parametric study, the repulsive configuration of magnets is found preferable in the frequency up-conversion technique, which is efficient and insensitive to various wave conditions when the magnets are placed sufficiently close. These findings could serve as useful design guidelines when nonlinearity or frequency up-conversion techniques are employed to improve the functionality of vibration energy harvesters.

Wu Hao, Yang Yaowen, Soh Chee Kiong, (2012) A Compact 2 Degree-of-Freedom Energy Harvester with Cut-Out Cantilever Beam. In: Japanese Journal of Applied Physics, Vol.51, 040211.

Abstract: In this work, a novel 2 degree-of-freedom (DOF) vibration energy harvester is proposed. The harvester comprises one main cantilever beam and one secondary cantilever beam cut out within the main beam. By varying the proof masses, the first two resonances can be tuned close to each other, while maintaining significant magnitudes, thus providing a useful wide bandwidth for energy harvesting. Unlike previous 2-DOF harvesters, the proposed harvester is compact and utilizes the beam more efficiently by generating energy from both the main and secondary cantilevers. Therefore, the proposed harvester is more adaptive and functional in practical random or frequency-variant vibrational circumstances.

Wei-Hsin LIAO, (2010) A self-powered, self-sensing magnetorheological damper. In: Proceedings of 2010 IEEE International Conference on Mechatronics and Automation, 5589157, 1364-69.

Abstract: Magnetorheological (MR) dampers are promising for semi-active vibration control of various dynamic systems. In the current MR damper system, separate power supply and dynamic sensor are required. This paper is aimed to propose and investigate a self-powered, self-sensing MR damper, which integrates energy harvesting, sensing and MR damping technologies into one device. This multifunctional integration will bring great benefits such as size and weight reduction, energy saving, lower cost, higher reliability, and less maintenance for the use of MR damper systems. It will advance the technology of various dynamic systems such as machine tools, robots, prosthetics, and suspension systems. In this paper, a prototype of self-powered, self-sensing MR damper was designed, fabricated, and tested. Theoretical analysis and experimental study on power generation were conducted. The velocity-sensing method was investigated. The MR damping forces were also studied experimentally. Finally, the interactions among three functions were discussed.

Dr Martin Judd, Leigh Fraser (National Grid), (2010) A Novel Inductive Electromagnetic Energy Harvester for Condition Monitoring Sensors. In: Proc. 3rd Int. Conf. on Condition Monitoring and Diagnosis (CMD2010), August 2010, Tokyo, Japan.

Abstract: As the operation of electrical power networks becomes increasingly sophisticated, the role of condition monitoring is expanding. The burden of implementing additional condition monitoring will be eased if self-powered, fully autonomous sensors can be used to reduce installation and maintenance costs. Changing batteries is inconvenient and standard mains power is often not available where sensors are needed. Existing commercial inductive harvesters to power sensors must be fitted around high voltage transmission lines, which requires either a power outage or live line installation. In this paper, an alternative harvester is presented which can be installed at any location where there is sufficient magnetic field. Magnetic flux densities within a cable tunnel are considered, from which a suitable target is defined for the magnetic flux density range over which the harvester must provide power to the sensor. Optimisation of output power per unit volume limits cost and allows placement of sensors in locations with restricted space. Coil parameters to achieve high output power per unit volume are discussed and experimental results are presented that demonstrate effective energy harvesting. A coil design for a typical cable tunnel is proposed.

Emma Worthington , Ashutosh Tiwari , (2010) Design Study of Piezoelectric Energy Harvesting Devices for Generation of a Higher Electrical Power Using a Coupled Piezoelectric-Circuit Finite Element Method. In: IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control, vol. 57, no.2, 427-437.

Abstract: This paper presents a design study on the geometric parameters of a cantilever-based piezoelectric energy harvesting devices (EHD), which harvests energy from motion (vibration), for the purpose of scavenging more energy from ambient vibration energy sources. The design study is based on the coupled piezoelectric-circuit finite element method (CPC-FEM), previously presented by Dr. Zhu. This model is used in the paper to obtain the following simulation results for variations in geometric parameters such as the beam length, width and thickness, and the mass length, width and height: (1) the current flowing through and the voltage developed across the load resistor, (2) the power dissipated by the resistor and the corresponding vibrational displacement amplitude, and (3) resonant frequency. By studying these results, straightforward design strategies that enable the generation of more power are obtained for each geometric parameter, and a physical understanding of how each parameter affects the output power is given. It is suggested that, in designing with the aim of generating more power, the following strategies be used: (1) for the beam, a shorter length, larger width and lower ratio of piezoelectric layer thickness to total beam thickness are preferred in the case of a fixed mass. (2) for the mass, a shortened mass length and a higher mass height are preferred in the case of variation in the mass length and the mass height with mass width and mass value remain fixed, and a wider width and small mass height are preferred in the case of variation in mass width and height (mass length and value remain fixed, and (3) for the case of a fixed total length, a shorter beam length and longer mass length are preferred. With the design strategies, output powers from the device can reach above 1-2mW/cm3, much higher than the 200micro-W/cm3 currently achieved in the published literature. This is an encouraging prospect for enabling a wider range of applications of the EHDs.

Mike Friswell, (2010) Sensor shape design for piezoelectric cantilever beams to harvest vibration energy. In: Journal of Applied Physics, 108[1], pp. 014901:1-6.

Abstract: Energy harvesting for the purpose of powering low power electronic sensor systems has received explosive attention in the last few years. A common device uses the piezoelectric effect for a cantilever beams at resonance to harvest ambient vibration energy. However most of these devices have a rectangular piezoelectric patch covering all or part of the beam. This paper considers the optimum design of such a device, and in particular investigates the effect that the size and shape of piezoelectric sensor has on the harvested energy. It is shown that significant increases in harvested energy may be obtained by optimising the sensor design

Ivo N Ayala-Garcia, Steve Beeby, M J Tudor, (2010) Tunable kinetic energy harvester with dynamic over range protection. In: Smart Materials and Structures, vol 19, 115005.

Abstract: This paper describes the development and implementation of a self-powered control system that autonomously adapts the resonant frequency of an electromagnetic vibration-based energy harvester to ambient vibration frequency. The tuning mechanism adjusts the harvester's spring stiffness by varying the axial tensile force between two permanent magnets. The system adjusts the resonant frequency of the harvester from 64 to 78 Hz, increasing the operational bandwidth of the harvester from 0.26 to 14 Hz, using a single structure. The same tuning principle is also applied to protect the harvester from over range acceleration which could cause physical damage to its structure. The closed loop control uses the phase difference between the harvester output signal and ambient vibration, measured by an accelerometer attached to the vibration source, to adjust the tuning mechanism.

A Cammarano, Dr Steve Burrow, D Barton, A Carrella, L Clare, (2010) Tuning a resonant energy harvester using a generalized electrical load. In: Journal of smart materials and structures, vol. 19, 055003, March 2010, DOI: 10.1088/0964-1726/19/5/055003.

Abstract: A fundamental drawback of vibration-based energy harvesters is that they
typically feature a resonant mass/spring mechanical system to amplify the
small source vibrations; the limited bandwidth of the mechanical amplifier
restricts the effectiveness of the energy harvester considerably. By
extending the range of input frequencies over which a vibration energy
harvester can generate useful power, e.g.\ through adaptive tuning, it is
not only possible to open up a wider range of applications, such as those
where the source frequency changes over time, but it is also possible to
relax the requirements for precision manufacture or the need for
mechanical adjustment in~situ. In this paper, a vibration-based energy
harvester connected to a generalised electrical load (containing both real
and reactive impedance) is presented. It is demonstrated that the reactive
component of the electrical load can be used to tune the
harvester system to significantly increase the output power away from the
resonant peak of the device. An analytical model of the system is
developed, which includes non-ideal components arising from the physical
implementation, and the results are confirmed by experiment. The -3dB
(half-power) bandwidth of the prototype energy harvester is shown to be
over three times greater when presented with an optimised load
impedance compared to the same harvester presented with an optimised
resistive-only load.

D Barton, Dr Steve Burrow, L Clare, (2010) Energy harvesting from vibrations with a nonlinear oscillator. In: Journal of Vibrations and Acoustics, 132, 021009, 2010, DOI:10.1115/1.4000809.

Abstract: In this paper we present a nonlinear electromagnetic energy
harvesting device that has a broadly resonant response. The
nonlinearity is generated by a particular arrangement of magnets
in conjunction with an iron-cored stator. We show the resonant
response of the system to both pure-tone excitation and
narrow-band random excitation. In addition to the primary resonance,
the super-harmonic resonances of the harvester are also
investigated and we show that the corresponding mechanical upconversion
of the excitation frequency may be useful for energy
harvesting. The harvester is modeled using a Duffing-type equation
and the results compared to the experimental data.

Steve Roberts, Michael John Tudor, Steve Beeby, (2010) Design and experimental characterization of a tunable vibration-based electromagnetic micro-generator. In: Sensors and Actuators A: Physical, 158 (2). pp. 284-293.

Abstract: Vibration-based micro-generators, as an alternative source of energy, have become increasingly significant in the last decade. This paper presents a new tunable electromagnetic vibration-based micro-generator. Frequency tuning is realized by applying an axial tensile force to the micro-generator. The dimensions of the generator, especially the dimensions of the coil and the air gap between magnets, have been optimized to maximize the output voltage and power of the micro-generator. The resonant frequency has been successfully tuned from 67.6 to 98 Hz when various axial tensile forces were applied to the structure. The generator produced a power of 61.6-156.6uW over the tuning range when excited at vibrations of 0.59ms^2. The tuning mechanism has little effect on the total damping. When the tuning force applied on the generator becomes larger than the generator's inertial force, the total damping increases resulting in reduced output power. The resonant frequency increases less than indicated from simulation and approaches that of a straight tensioned cable when the force associated with the tension in the beam becomes much greater than the beam stiffness. The test results agree with the theoretical analysis presented.

Dibin Zhu , Michael John Tudor, Steve Beeby, (2010) Strategies for increasing the operating frequency range of vibration energy harvesters: a review. In: Measurement Science and Technology, Vol. 21 022001.

Abstract: This review presents possible strategies to increase the operational frequency range of vibration-based micro-generators. Most vibration-based micro-generators are spring-mass-damper systems which generate maximum power when the resonant frequency of the generator matches the frequency of the ambient vibration. Any difference between these two frequencies can result in a significant decrease in generated power. This is a fundamental limitation of resonant vibration generators which restricts their capability in real applications. Possible solutions include the periodic tuning of the resonant frequency of the generator so that it matches the frequency of the ambient vibration at all times or widening the bandwidth of the generator. Periodic tuning can be achieved using mechanical or electrical methods. Bandwidth widening can be achieved using a generator array, a mechanical stopper, nonlinear (e.g. magnetic) springs or bi-stable structures. Tuning methods can be classified into intermittent tuning (power is consumed periodically to tune the device) and continuous tuning (the tuning mechanism is continuously powered). This review presents a comprehensive review of the principles and operating strategies for increasing the operating frequency range of vibration-based micro-generators presented in the literature to date. The advantages and disadvantages of each strategy are evaluated and conclusions are drawn regarding the relevant merits of each approach.

Dr Martin Judd, John Fitch (National Grid), (2009) Development of magnetic induction energy harvesting for condition monitoring. In: Proc. 44th Int. Universities Power Engineering Conference (UPEC 2009), Glasgow, UK, September 2009.

Abstract: Condition monitoring can play an important supporting role in the supply of electrical power. Autonomous wireless condition monitoring sensors have the potential to ensure reliability, at lower cost, as the power supply system complexity increases. This paper explores the practical issues surrounding the use of magnetic induction to power sensors, from which design drivers are developed. The relationship between available output power and energy harvester volume is analysed and verified by experiment. The impact of core shape and core material choice is demonstrated by the results, and a design approach is presented to benchmark and optimise volume efficiency. These results are used to show how the harvester could be scaled to suit different combinations of conductor current and output power.

Emma Worthington , (2009) Design and Testing of Piezoelectric Energy Harvesting Devices for Generation of Higher Electric Power for Wireless Sensor Networks. In: Proceeding IEEE Sensors 2009, Chrischurch, New Zealand, 25-28 Oct., 2009, pp.699-702.

Abstract: This paper reports our design and testing results on the electric output performance of a vibration-based piezoelectric energy harvesting device (PEHD). The PEHD is a cantilever with a sandwich structure and seismic mass attached to the tip. The geometric parameters of the device are based on optimization design with a volume of around 1cm3 and at a targeted resonant frequency of 80-100 Hz. A maximum output power of 370�¼W at 15.5 volts into a 325k�© resistive load is generated at the resonant frequency of 87Hz and under an acceleration of 0.23g. Quite remarkably, this power is a very encouraging power figure that gives the prospect of being able to power a wider range of applications than is currently possible in wireless sensor network.

Dr. Kok Swee Leong (Stephen), Neil White, Nick Harris, (2009) Fabrication and characterisation of free-standing thick-film piezoelectric cantilevers for energy harvesting.. In: Measurement Science and Technology, Volume 20, Number 12 .

Abstract: Research into energy harvesting from ambient vibration sources has attracted great interest over the last few years, largely as a result of advances in the areas of wireless technology and low-power electronics. One of the mechanisms for converting mechanical vibration to electrical energy is the use of piezoelectric materials, typically operating as a cantilever in a bending mode, which generate a voltage across the electrodes when they are stressed. Typically, the piezoelectric materials are deposited on a non-electro-active substrate and are physically clamped at one end to a rigid base. The presence of the substrate does not contribute directly to the electrical output, but merely serves as a mechanical supporting platform, which can pose difficulties for integration with other microelectronic devices. The aim of this paper is to describe a novel thick-film free-standing cantilever structure that does not use a supporting platform and has the advantage of minimizing the movement constraints on the piezoelectric material, thereby maximizing the electrical output power. Two configurations of the composite cantilever structure were investigated: unimorph and multimorph. A unimorph consists of a pair of silver/palladium (Ag/Pd) electrodes sandwiching a laminar layer of lead zirconate titanate (PZT). A mulitmorph is an extended version of the unimorph with two pairs of Ag/Pd electrodes and three laminar sections of PZT.

S. Olutunde Oyadiji, (2009) Multiple Resonances Piezoelectric Energy Harvesting Generator. In: ASME 2009 Conference on Smart Materials, Adaptive Structures and Intelligent Systems (SMASIS2009) , September 21-23, 2009 , Oxnard, California, USA.

Abstract: Energy harvesting is the process of converting low level ambient energy into usable electrical energy, so that remote electronic instruments can be powered without the need for batteries or other supplies. Piezoelectric material has the ability to convert mechanical energy into electrical energy, and cantilever type harvesters using this material are being intensely investigated. The typical single cantilever energy harvester design has a limited bandwidth, and is restricted in ability for converting environmental vibration occurring over a wide range of frequencies. A multiple cantilever piezoelectric generator that works over a range of frequencies, yet has only one Piezo element, is being investigated. The design and testing of this novel harvester is described.

Zijing Wong, Ashwin A. Seshia, (2009) A multi-degree-of-freedom electrostatic MEMS power harvester. In: the 9th international workshop on micro and nanotechnology for power generation and energy conversion applications (Power MEMS 2009), Dec 1-4, 2009, Washington DC, USA, pp. 300-303.

Abstract: This paper reports the design, modeling and experimental characterization of a multi-degree-of-freedom electrostatic vibration energy harvester. The potential of harvesting vibrational energy at multiple input frequencies and achieving displacement amplification in the response is investigated. To better understand the device performance at a system level, a numerical model which incorporates mechanical and electrical analysis in a charge constrained conversion circuit is developed. Experimental results on a microfabricated prototype demonstrate a maximum measured output power of 0.076 �¼W at 1.4 kHz for an external load of 5.1 M�©. We also experimentally demonstrate that the output power varies with the square of input acceleration and DC bias voltage.

Dr Steve Burrow, L clare, (2009) Open-loop power conditioning for vibration energy harvesting. In: Electronics Letters, Vol.45, Issue 19, pp999-1000. September 2009.

Abstract: Energy harvesters feature a high impedance output that complicates subsequent power conditioning because operation in the peak power region violates the stability criterion for converters with voltage regulation. By using a flyback converter in open-loop discontinuous mode and a shunt voltage regulator it is demonstrated that a harvester can be operated in an optimal manner whilst ensuring stability. A circuit is described that draws unity power factor from the harvester, provides output voltage regulation and consumes 180μW of quiescent power.

Alex Weddell, Dr Neil Grabham, Nick Harris, Neil White, (2009) Modular Plug-and-Play Power Resources for Energy-Aware Wireless Sensor Nodes. In: Sixth Annual IEEE Communications Society Conference on Sensor, Mesh and Ad Hoc Communications and Networks - SECON 2009, 22-26 June 2009, Rome, Italy.

Abstract: Wireless sensors are normally powered by non-rechargeable batteries, but these must be replaced when depleted. Recent developments in energy harvesting technology allow sensors to be powered by environmental energy where it is present, but the wide range of situations where sensors are deployed means that it is desirable for the energy components of a sensor node (i.e. batteries, supercapacitors, and power generation devices) to be selected and configured at the time of node deployment. Previous energy harvesting-powered systems have been designed for specific energy hardware and been difficult to adapt for different resources. Energy-awareness is useful for state-of-the-art network algorithms, but present systems do not provide a standardized or straightforward way for nodes to monitor and manage their energy hardware.

The developments reported in this paper deliver a reconfigurable energy subsystem for wireless autonomous sensors. The new system permits energy modules to be selected and fitted to the sensor node in-situ, in a plug-and-play manner, without the need for reprogramming or the modification of hardware. The node can monitor and intelligently manage its energy resources and assess its overall energy status by analyzing its level of stored energy and rate of power generation. These activities are facilitated by a proposed common hardware interface (which allows multiple energy modules to be connected) and an electronic datasheet structure for the energy modules. The system has been verified through the development and testing of a prototype wireless sensor node which operates from a mix of energy sources.

Dr. Kok Swee Leong (Stephen), Neil White, Nick Harris, (2008) Free-standing thick-film piezoelectric device. In: Electronics Letters , Volume 44, Issue 4 .

Abstract: A free-standing thick-film cantilever sensor structure is presented. Such
devices find use in applications such as vibration detection or energy
harvesting. The structure was fabricated by screen printing layers of
lead zirconate titanate between silver/palladium electrodes and cofiring
the layers together with a carbon sacrificial layer (deposited
underneath) in an air environment at a temperature of 850 C. The
free-standing structure, of dimensions 18 mm long by 9 mm wide
and thickness of 50 mm, was found to produce electrical powers of
up to 95 nW at an acceleration level of 9.81 m/s2 (1 g), when driving
a 60 kOhm load resistance.

M. T. Penella-López, Manel Gasulla, (2007) A review of commercial energy harvesters for autonomous sensors. In: Proceedings IMTC 2007, Warsaw, Poland, May 1-3, 2007.

Abstract: Current commercial autonomous sensors are mainly powered by primary batteries. Batteries need to be replaced and hence can become the largest and most expensive part of the system. On the other hand, our environment is full of waste and unused energy such as that coming from the sun or mechanical vibrations. As a result, commercial energy harvesters are increasingly available to power autonomous sensors. This work presents and analyses commercial energy harvesters currently available. First, environmental energy sources are classified and described. Then, energy harvesting principles are described and some guidelines are given to calculate the maximum power consumption allowed and the energy storage capacity required for the autonomous sensor. Finally, commercial energy harvesters are evaluated to determine their capability to power a commercial autonomous sensor in some given circumstances.

Steve Beeby, John Tudor, Nick Harris, (0) Increasing output power of electromagnetic vibration energy harvesters using improved Halbach arrays. In: Sensors and Actuators A Physical, 203, 11-19.

Abstract: This paper extends previously published studies into the performance of Halbach arrays for electromagnetic vibration energy harvesting. A Halbach array is a specific arrangement of permanent magnets that concentrates the magnetic field on one side of the array while cancelling the field to almost zero on the other side. This arrangement can improve electromagnetic coupling in a limited space. Previous research showed that although the Halbach array has higher magnetic field density compared to normal magnet layouts, its magnetic flux change rate is not necessarily high. Thus, output powers of energy harvesters with standard Halbach arrays are not always greater than those with normal magnet layouts. Two improvements to the Halbach arrays that lead to increased output power of electromagnetic vibration energy harvesters are presented in this paper. Test results showed that the proposed improved Halbach arrays can increase the output power of energy harvesters by a factor of seven compared to the standard Halbach array.


Neil J Grabham, Steve Beeby, John M Tudor, (2013) Review of the application of energy harvesting in buildings. In: Measurement Science and Technology, doi:10.1088/0957-0233/25/1/012002.

Abstract: This review presents the state of the art of the application of energy harvesting in commercial and residential buildings. Electromagnetic (optical and radio frequency), kinetic, thermal and airflow-based energy sources are identified as potential energy sources within buildings and the available energy is measured in a range of buildings. Suitable energy harvesters are discussed and the available and the potential harvested energy calculated. Calculations based on these measurements, and the technical specifications of state-of-the-art harvesters, show that typical harvested powers are: (1) indoor solar cell (active area of 9 cm2, volume of 2.88 cm3): ~300µW from a light intensity of 1000 lx; (2) thermoelectric harvester (volume of 1.4 cm3): 6 mW from a thermal gradient of 25°C; (3) periodic kinetic energy harvester (volume of 0.15 cm3): 2 µW from a vibration acceleration of 0.25 m s�2 at 45 Hz; (4) electromagnetic wave harvester (13 cm antenna length and conversion efficiency of 0.7): 1 µW with an RF source power of 25 dBm; and (5) airflow harvester (wind turbine blade of 6 cm diameter and generator efficiency of 0.41): 140 mW from an airflow of 8 m s-1. These results highlight the high potential of energy harvesting technology in buildings and the relative attractions of various harvester technologies. The harvested power could either be used to replace batteries or to prolong the life of rechargeable batteries for low-power (~1 mW) electronic devices.

Steve Beeby, Leran Wang (Phil), Alex Weddell, Geoff Merrett, Bernard Stark, Bashir M Al-Hashimi, (2013) A comparison of power output from linear and non-linear kinetic energy harvesters using real vibration data. In: Smart Materials and Structures, 22, (7).

Abstract: The design of vibration energy harvesters (VEHs) is highly dependent upon the characteristics of the environmental vibrations present in the intended application. VEHs can be linear resonant systems tuned to particular frequencies or non-linear systems with either bi-stable operation or a Duffing-type response. This paper provides detailed vibration data from a range of applications, which has been made freely available for download through the Energy Harvesting Networks online data repository. In particular, this research shows that simulation is essential in designing and selecting the most suitable vibration energy harvester for particular applications. This is illustrated through C-based simulations of different types of VEHs, using real vibration data from a diesel ferry engine, a combined heat and power pump, a petrol car engine and a helicopter. The analysis shows that a bistable energy harvester only has a higher output power than a linear or Duffing-type nonlinear energy harvester with the same Q-factor when it is subjected to white noise vibration. The analysis also indicates that piezoelectric transduction mechanisms are more suitable for bistable energy harvesters than electromagnetic transduction. Furthermore, the linear energy harvester has a higher output power compared to the Duffing-type nonlinear energy harvester with the same Q factor in most cases. The Duffing-type nonlinear energy harvester can generate more power than the linear energy harvester only when it is excited at vibrations with multiple peaks and the frequencies of these peaks are within its bandwidth. Through these new observations, this paper illustrates the importance of simulation in the design of energy harvesting systems, with particular emphasis on the need to incorporate real vibration data.

Nick Harris, Steve Beeby, (2012) Performance of Linear Vibration Energy Harvesters under Broadband Vibrations with Multiple Frequency Peaks. In: Eurosensors XXVI, Krakow, Poland, 09 - 12 Sep 2012..

Steve Beeby, John Tudor, Nick Harris, (2011) A credit card sized self powered smart sensor node. In: Sensors and Actuators A: Physical , 169 (2), 317-325.

Abstract: This paper reports a self powered smart sensor node (also called �smart tag�) consisting of a piezoelectric vibration energy harvester, a power conditioning circuit, sensors and an RF transmitter. The smart tag has dimensions similar to a credit card and can be easily integrated into various applications such as the surface of the aircraft. The smart tag is powered by an integrated bimorph piezoelectric generator that extracts energy from ambient vibrations. The generator is fabricated using thick film printing technology. Experimentally, the generator produced a maximum RMS output power of 240 μW when excited at vibration with a frequency of 67 Hz and peak amplitude of 0.4 g (3.9 m s�2). This generated power is sufficient to enable periodic sensing and transmission. Details of the experimental results of the piezoelectric generator and the power conditioning circuit are presented. Test shows that the waiting time of the system between two consecutive transmissions is around 800 s.

Alex Weddell, Geoff Merrett, Bashir Al-Hashimi, (2011) Photovoltaic Sample-and-Hold Circuit Enabling MPPT Indoors for Low-Power Systems. In: IEEE Transactions on Circuits and Systems I: Regular Papers, (in press).

Abstract: Photovoltaic (PV) energy harvesting is commonly used to power autonomous devices, and maximum power point tracking (MPPT) is often used to optimize its efficiency. This paper describes an ultra low-power MPPT circuit with a novel sample-and-hold and cold-start arrangement, enabling MPPT across the range of light intensities found indoors, which has not been reported before. The circuit has been validated in practice and found to cold-start and operate from 100 lux (typical of dim indoor lighting) up to 5000 lux with a 55cm2 amorphous silicon PV module. It is more efficient than non-MPPT circuits, which are the state-of-the-art for indoor PV systems. The proposed circuit maximizes the active time of the PV module by carrying out samples only once per minute. The MPPT control arrangement draws a quiescent current draw of only 8uA, and does not require an additional light sensor as has been required by previously-reported low-power MPPT circuits.

Dr Carol Featherston, Greg Waring (Cardiff University), (2009) Thermoelectric Energy Harvesting for Wireless Sensor Systems in Aircraft. In: Key Engineering Materials, 413-414, 487-494 .

Abstract: The use of structural health monitoring in the aerospace industry has many benefits including improved safety, reduced maintenance and extended aircraft lifecycles. A major focus of current research in this area is the development of wireless sensor 'nodes' which rely on batteries as a power source, severely limiting the product lifespan. This paper presents the results of work carried out to examine the feasibility of replacing or supplementing existing battery power supplies using thermoelectric energy conversion from ambient temperature differences in aircraft. An average power demand of 1mW over a typical sensor duty cycle is identified for current wireless sensor hardware. Temperature differentials between the wing fuel tanks and external air are determined and a theoretical model for thermoelectric energy harvesting potential is developed. Results indicate that average power outputs sufficient for the intended application of 6.6-22mW could be achieved during flight, based on a commercially available thermoelectric module of 30x30x4.1mm. An experimental investigation of the performance of this module when subjected to appropriate temperature conditions, using a Ranque-Hilshe vortex tube to generate easily controlled temperatures to -25C is described. Excellent consistency is demonstrated between theoretical predictions and experimental results, confirming the accuracy of the theoretical model.

M. T. Penella-López, J. Albesa, Manel Gasulla, (2009) Powering wireless sensor nodes: primary batteries versus energy harvesting. In: Proceedings IMTC 2009, Singapore, 5-7 May 2009, pp. 1625-1630.

Abstract: Wireless sensor networks (WSNs) are increasingly used in many fields. Still, power supply of the nodes remains a challenge. Primary batteries are mainly used but energy harvesting offers an alternative, although not free of problems. This paper compares the use of primary batteries against solar cells. Basic principles are first enunciated, then generic design examples are presented and finally actual deployed nodes of a WSN are illustrated.

Alex Weddell, Geoff Merrett, Nick Harris, Bashir Al-Hashimi, (2008) Energy Harvesting and Management for Wireless Autonomous Sensors. In: Measurement + Control, 41 (4). pp. 104-108. ISSN 0020-2940.

Abstract: Wireless autonomous sensors that harvest ambient energy are attractive solutions, due to their convenience and economic benefits. A number of wireless autonomous sensor platforms which consume less than 100uW under duty-cycled operation are available. Energy harvesting technology (including photovoltaics, vibration harvesters, and thermoelectrics) can be used to power autonomous sensors. A developed system is presented that uses a photovoltaic module to efficiently charge a supercapacitor, which in turn provides energy to a microcontroller-based autonomous sensing platform. The embedded software on the node is structured around a framework in which equal precedent is given to each aspect of the sensor node through the inclusion of distinct software stacks for energy management and sensor processing. This promotes structured and modular design, allowing for efficient code reuse and encourages the standardisation of interchangeable protocols.

Modelling and Simulating

Nikolaos Pappas, Marios Kountouris, Jeongho Jeon, Anthony Ephremides, Apostolos Traganitis, (2013) Network-Level Cooperation in Energy Harvesting Wireless Networks. In: 1st IEEE Global Conference on Signal and Information Processing (GlobalSIP) 2013, Symposium on Energy Harvesting and Green Wireless Communications, December 2013, Austin Texas.

Abstract: We consider a two-hop communication network consisted of a source node, a relay and a destination node in which the source and the relay node have external traffic arrivals. The relay forwards a fraction of the source node's traffic to the destination and the cooperation is performed at the network level. In addition, both source and relay nodes have energy harvesting capabilities and an unlimited battery to store the harvested energy. We study the impact of the energy constraints on the stability region. Specifically, we provide inner and outer bounds on the stability region of the two-hop network with energy harvesting source and relay.

(2013) Analytical and Experimental Comparisons of Electromechanical Vibration Response of a Piezoelectric Bimorph Beam for Power Harvesting. In: Mechanical Systems and Signal Processing, March 2013, pp. 66�86.

Abstract: Power harvesters that extract energy from vibrating systems via piezoelectric transduction show strong potential for powering smart wireless sensor devices in applications of health condition monitoring of rotating machinery and structures. This paper presents an analytical method for modelling an electromechanical piezoelectric bimorph beam with tip mass under two input base transverse and longitudinal excitations. The Euler�Bernoulli beam equations were used to model the piezoelectric bimorph beam. The polarity-electric field of the piezoelectric element is excited by the strain field caused by base input excitation, resulting in electrical charge. The governing electromechanical dynamic equations were derived analytically using the weak form of the Hamiltonian principle to obtain the constitutive equations. Three constitutive electromechanical dynamic equations based on independent coefficients of virtual displacement vectors were formulated and then further modelled using the normalised Ritz eigenfunction series. The electromechanical formulations include both the series and parallel connections of the piezoelectric bimorph. The multi-mode frequency response functions (FRFs) under varying electrical load resistance were formulated using Laplace transformation for the multi-input mechanical vibrations to provide the multi-output dynamic displacement, velocity, voltage, current and power. The experimental and theoretical validations reduced for the single mode system were shown to provide reasonable predictions. The model results from polar base excitation for off-axis input motions were validated with experimental results showing the change to the electrical power frequency response amplitude as a function of excitation angle, with relevance for practical implementation.

Steve Roberts, Thomas Mouille, John Tudor, Steve Beeby, (2012) General model with experimental validation of electrical resonant frequency tuning of electromagnetic vibration energy harvesters. In: Smart Materials and Structures, 21, (10), 105039.

Abstract: This paper presents a general model and its experimental validation for electrically tunable electromagnetic energy harvesters. Electrical tuning relies on the adjustment of the electrical load so that the maximum output power of the energy harvester occurs at a frequency which is different from the mechanical resonant frequency of the energy harvester. Theoretical analysis shows that for this approach to be feasible the electromagnetic vibration energy harvester�s coupling factor must be maximized so that its resonant frequency can be tuned with the minimum decrease of output power. Two different-sized electromagnetic energy harvesters were built and tested to validate the model. Experimentally, the micro-scale energy harvester has a coupling factor of 0.0035 and an untuned resonant frequency of 70.05 Hz. When excited at 30 mg, it was tuned by 0.23 Hz by changing its capacitive load from 0 to 4000 nF; its effective tuning range is 0.15 Hz for a capacitive load variation from 0 to 1500 nF. The macro-scale energy harvester has a coupling factor of 552.25 and an untuned resonant frequency of 95.1 Hz and 95.5 Hz when excited at 10 mg and 25 mg, respectively. When excited at 10 mg, it was tuned by 3.8 Hz by changing its capacitive load from 0 to 1400 nF; it has an effective tuning range of 3.5 Hz for a capacitive load variation from 0 to 1200 nF. When excited at 25 mg, its resonant frequency was tuned by 4.2 Hz by changing its capacitive load from 0 to 1400 nF; it has an effective tuning range of about 5 Hz. Experimental results were found to agree with the theoretical analysis to within 10%.

Nikolaos Pappas, Jeongho Jeon, Anthony Ephremides, Apostolos Traganitis, (2012) Optimal Utilization of a Cognitive Shared Channel with a Rechargeable Primary Source Node. In: IEEE/KICS Journal of Communications and Networks (JCN) Special Issue on Energy Harvesting in Wireless Networks, Vol. 14, No. 2, April 2012. .

(2012) Electromechanical Piezoelectric Power Harvester Frequency Response Modelling Using Closed-Form Boundary Value Methods. In: IEEE/ASME Transactions on Mechatronics, October 2012 , In press.

Abstract: The conversion of mechanical vibration to electrical energy has shown great promise for extending battery life of smart sensor wireless devices for various engineering applications. This paper presents novel analytical models of a piezoelectric bimorph, using the closed-form boundary value (CFBV) method, for predicting the electromechanical power harvester frequency response. The derivations of the coupled electromechanical dynamic response of the transverse-longitudinal (CEDRTL) form based on the CFBV method were developed using the reduced strong form method of the Hamiltonian principle. The equations from CEDRTL can be reduced to give the coupled electromechanical dynamic response of the transverse (CEDRT) form. The electromechanical frequency response functions with variable load resistance were also given in detail using Laplace transformation. The two theoretical studies are compared together and validated with an experimental study. For some cases, when the load resistance approached open circuit, the difference between CEDRTL and CEDRT tended to be more pronounced. Conversely, the CEDRTL and CEDRT models tended to overlap when the load resistance approached short circuit. Nyquist plots are used to demonstrate the shifting frequency and amplitude changes due to variable resistance. Overall, the experimental and CEDRTL model results were very close to each other.

(2012) Analytical techniques for broadband multielectromechanical piezoelectric bimorph beams with multifrequency power harvesting . In: IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, November 2012, pp. 2555 - 2568 .

Abstract: This paper presents the multifrequency responses of multielectromechanical piezoelectric bimorph beams using a novel analytical model based on the closed-form boundary value method reduced from the strong form of Hamiltonian's principle. The reduced constitutive multielectromechanical dynamic equations for the multiple bimorph beams connected in series, parallel, and mixed series-parallel connections can be further formulated using Laplace transformation to give new formulas for power harvesting multifrequency response functions. The parametric case studies based on the change in geometrical structures of the multiple bimorphs with and without tip masses are discussed to analyze the trend of multifrequency power harvesting optimization under resistive load. Nyquist responses based on varying geometrical structures and load resistances were used to analyze the multifrequency power amplitudes in the complex domain. Overall, the trend of system response using multiple tiers consisting of multiple bimorphs was found to significantly widen the multifrequency band followed by increasing the power amplitudes.

Wu Hao, Yang Yaowen, Soh Chee Kiong, (2012) A Novel Two-Degrees-of-Freedom Piezoelectric Energy Harvester. In: Journal of Intelligent Material Systems and Structures, online first.

Abstract: Energy harvesting from ambient vibrations using piezoelectric effect is a promising alternative solution for powering small electronics such as wireless sensors. A conventional piezoelectric energy harvester usually consists of a cantilevered beam with a proof mass at its free end. For such a device, the second resonance of the piezoelectric energy harvester is usually ignored because of its high frequency as well as low response level compared to the first resonance. Hence, only the first mode has been frequently exploited for energy harvesting in the reported literature. In this article, a novel compact piezoelectric energy harvester using two vibration modes has been developed. The harvester comprises one main cantilever beam and an inner secondary cantilever beam, each of which is bonded with piezoelectric transducers. By varying the proof masses, the first two resonant frequencies of the harvester can be tuned close enough to achieve useful wide bandwidth. Meanwhile, this compact design efficiently utilizes the cantilever beam by generating significant power output from both the main and secondary beams. An experiment and simulation were carried out to validate the design concept. The results show that the proposed novel piezoelectric energy harvester is more adaptive and functional in practical vibrational circumstances.

Yang Yaowen, (2012) A Multiple-Degree-of-Freedom Piezoelectric Energy Harvesting Model. In: Journal of Intelligent Material Systems and Structures, online first.

Abstract: Conventional vibration energy harvesters have been usually studied as single-degree-of-freedom models. The fact that such harvesters are only efficient near sole resonance limits their applicability in frequency-variant or random vibration scenarios. In this article, a novel multiple-degree-of-freedom piezoelectric energy harvesting model is presented. First, a two-degree-of-freedom model is analyzed, and its two configurations are characterized. In the first configuration, the piezoelectric element is placed between one mass and the base, and in the second configuration, it is placed between the two masses. It is shown that the former is advantageous over the latter since with a slight increase of overall weight to the single-degree-of-freedom model, we can achieve two close and effective peaks in power response or one effective peak with significantly enhanced magnitude. The first configuration is then generalized to an n-degree-of-freedom model, and its analytical solution is derived. This solution provides a convenient tool for parametric study and design of a multiple-degree-of-freedom piezoelectric energy harvesting model. Finally, the equivalent circuit model of the proposed n-degree-of-freedom piezoelectric energy harvesting model is developed via the analogy between the mechanical and electric domains. With the equivalent circuit model, system-level electric simulation can be performed to evaluate the system performance when sophisticated interface circuits are attached.

Nikolaos Pappas, Jeongho Jeon, Anthony Ephremides, Apostolos Traganitis, (2011) Optimal Utilization of a Cognitive Shared Channel with a Rechargeable Primary Source Node. In: IEEE Information Theory Workshop 2011, Paraty, Brazil.

(2011) Analytical modeling of self-powered electromechanical piezoelectric bimorph beams with multidirectional excitation. In: International Journal of Smart and Nano Materials, 09 Aug 2011, pp. 134-175.

Abstract: Unused mechanical energies can be found in numerous ambient vibration sources in industry including rotating equipment, vehicles, aircraft, piping systems, fluid flow, and even external movement of the human body. A portion of the vibrational energy can be recovered using piezoelectric transduction and stored for subsequent smart system utilization for applications including powering wireless sensor devices for health condition monitoring of rotating machines and defence communication technology. The vibration environment in the considered application areas is varied and often changes over time and can have components in three perpendicular directions, simultaneously or singularly. This paper presents the development of analytical methods for modeling of self-powered cantilevered piezoelectric bimorph beams with tip mass under simultaneous longitudinal and transverse input base motions utilizing the weak and strong forms of Hamiltonian's principle and space- and time-dependent eigenfunction series which were further formulated using orthonormalization. The reduced constitutive electromechanical equations of the cantilevered piezoelectric bimorph were subsequently analyzed using Laplace transforms and frequency analysis to give multi-mode frequency response functions (FRFs). The validation between theoretical and experimental results at the single mode of eigenfunction solutions reduced from multi-mode FRFs is also given.

Yang Yaowen, (2011) Analysis of Synchronized Charge Extraction for Piezoelectric Energy Harvesting. In: Smart Materials and Structures, Vol.20, No.8, 085022.

Abstract: In the past few years, various power conditioning circuits have been proposed to improve the efficiency of piezoelectric energy harvesting, among which the synchronized charge extraction (SCE) technique has been enthusiastically pursued. In the literature, the SCE technique is investigated based on the uncoupled or in-phase assumptions. The uncoupled assumption is only valid for weak electromechanical coupling and the in-phase assumption is not applicable for energy harvesting at off-resonance. In this paper, we derive an accurate analytical solution for the piezoelectric energy harvesting systems with the SCE technique. Based on this solution, we investigate the applicability of the SCE technique for different cases, i.e. the piezoelectric energy harvester (PEH) with various degrees of electromechanical coupling and the PEH excited at various frequencies. Circuit simulation is also conducted with an accurate circuit model derived for PEHs and the results validate the analytical outcomes. Both the accurate analytical solution and the circuit simulation show that the SCE technique cannot improve or even reduces the power output at resonance if the coupling of the PEH is not negligible. The SCE technique is found capable of significantly boosting the efficiency of energy harvesting only for the PEH vibrating at off-resonance frequencies or with weak coupling.

A. Abdelkefi, Fehmi NAJAR, A. H. Nayfeh, S Ben Ayed, (2011) An energy harvester using piezoelectric cantilever beams undergoing coupled bending-torsion vibrations. In: Smart Materials and Structures, vol. 20, p. 115007.

Abstract: Recently, piezoelectric cantilevered beams have received considerable attention for vibration-to-electric energy conversion. Generally, researchers have investigated a classical piezoelectric cantilever beam with or without a tip mass. In this paper, we propose the use of a unimorph cantilever beam undergoing bending-torsion vibrations as a new piezoelectric energy harvester. The proposed design consists of a single piezoelectric layer and a couple of asymmetric tip masses; the latter convert part of the base excitation force into a torsion moment. This structure can be tuned to be a broader band energy harvester by adjusting the first two global natural frequencies to be relatively close to each other. We develop a distributed-parameter model of the harvester by using the Euler-beam theory and Hamilton's principle, thereby obtaining the governing equations of motion and associated boundary conditions. Then, we calculate the exact eigenvalues and associated mode shapes and validate them with a finite element (FE) model. We use these mode shapes in a Galerkin procedure to develop a reduced-order model of the harvester, which we use in turn to obtain closed-form expressions for the displacement, twisting angle, voltage output, and harvested electrical power. These expressions are used to conduct a parametric study for the dynamics of the system to determine the appropriate set of geometric properties that maximizes the harvested electrical power. The results show that, as the asymmetry is increased, the harvester's performance improves. We found a 30% increase in the harvested power with this design compared to the case of beams undergoing bending only. We also show that the locations of the two masses can be chosen to bring the lowest two global natural frequencies closer to each other, thereby allowing the harvesting of electrical power from multi-frequency excitations.

Alex Weddell, Geoff Merrett, Tom Kazmierski, Bashir Al-Hashimi, (2011) Accurate Supercapacitor Modeling for Energy-Harvesting Wireless Sensor Nodes. In: IEEE Transactions on Circuits and Systems II: Express Briefs, (in press).

Abstract: Supercapacitors are often used in energy-harvesting wireless sensor nodes (EH-WSNs) to store harvested energy. Until now, research into the use of supercapacitors in EH-WSNs has considered them to be ideal or over-simplified, with non-ideal behavior attributed to substantial leakage currents. In this brief, we show that observations previously attributed to leakage are predominantly due to redistribution of charge inside the supercapacitor. We confirm this hypothesis through the development of a circuit-based model which accurately represents non-ideal behavior. The model correlates well with practical validations representing the operation of an EH-WSN, and allows behavior to be simulated over long periods.

Maria Gorlatova, Aya Wallwater, Gil Zussman, (2011) Networking Low Power Energy Harvesting Devices: Measurements and Algorithms. In: IEEE INFOCOM'11, Shanghai, China, April 2011.

Abstract: Recent advances in energy harvesting materials and ultra-low-power communications will soon enable the realization of networks composed of energy harvesting devices. These devices will operate using very low ambient energy, such as indoor light energy. We focus on characterizing the energy availability in indoor environments and on developing energy allocation algorithms for energy harvesting devices. First, we present results of our long-term indoor radiant energy measurements, which provide important inputs required for algorithm and system design (e.g., determining the required battery sizes). Then, we focus on algorithm development, which requires nontraditional approaches, since energy harvesting shifts the nature of energy-aware protocols from minimizing energy expenditure to optimizing it. Moreover, in many cases, different energy storage types (rechargeable battery and a capacitor) require different algorithms. We develop algorithms for determining time fair energy allocation in systems with predictable energy inputs, as well as in systems where energy inputs are stochastic.

Michele Pozzi, (2011) Plucked piezoelectric bimorphs for knee-joint energy harvesting: modelling and experimental validation. In: Smart Mater. Struct., (20) 055007.

Abstract: The modern drive towards mobility and wireless devices is motivating intensive research in energy harvesting technologies. To reduce the battery burden on people, we propose the adoption of a frequency up-conversion strategy for a new piezoelectric wearable energy harvester. Frequency up-conversion increases efficiency because the piezoelectric devices are permitted to vibrate at resonance even if the input excitation occurs at much lower frequency. Mechanical plucking-based frequency up-conversion is obtained by deflecting the piezoelectric bimorph via a plectrum, then rapidly releasing it so that it can vibrate unhindered; during the following oscillatory cycles, part of the mechanical energy is converted into electrical energy. In order to guide the design of such a harvester, we have modelled with finite element methods the response and power generation of a piezoelectric bimorph while it is plucked. The model permits the analysis of the effects of the speed of deflection as well as the prediction of the energy produced and its dependence on the electrical load. An experimental rig has been set up to observe the response of the bimorph in the harvester. A PZT-5H bimorph was used for the experiments. Measurements of tip velocity, voltage output and energy dissipated across a resistor are reported. Comparisons of the experimental results with the model predictions are very successful and prove the validity of the model.

Yang Yaowen, Soh Chee Kiong, (2010) Towards Broadband Vibration-based Energy Harvesting. In: Journal of Intelligent Material Systems and Structures, Vol.21, No.18, pp.1867-1897.

Abstract: The dramatic reduction in power consumption of current integrated circuits has evoked great research interests in harvesting ambient energy, such as vibrations, as a potential power supply for electronic devices to avoid battery replacement. Currently, most vibration-based energy harvesters are designed as linear resonators to achieve optimal performance by matching their resonance frequencies with the ambient excitation frequencies a priori. However, a slight shift of the excitation frequency will cause a dramatic reduction in performance. Unfortunately, in the vast majority of practical cases, the ambient vibrations are frequency-varying or totally random with energy distributed over a wide frequency spectrum. Hence, developing techniques to increase the bandwidth of vibration-based energy harvesters has become the next important problem in energy harvesting. This paper reviews the advances made in the past few years on this issue. The broadband vibration-based energy harvesting solutions, covering resonance tuning, multimodal energy harvesting, frequency up-conversion and techniques exploiting nonlinear oscillations, are summarized in detail with regard to their merits and applicability in different circumstances.

G. Litak, Mike Friswell, (2010) Magnetopiezoelastic energy harvesting driven by random excitations. In: Applied Physics Letters, 96[5], pp. 214103:1-3.

Abstract: This letter considers a nonlinear piezomagnetoelastic energy harvester driven by stationary Gaussian white noise. The increase in the energy generated by this device has been demonstrated for harmonic excitation with slowly varying frequency in simulation and validated by experiment. This paper considers the simulated response of this validated model to random base excitation and shows that the system exhibits a stochastic resonance. If the variance of the excitation were known then the device may be optimized to maximize the power harvested, even under random excitation

Mike Friswell, (2010) Piezoelectric energy harvesting with parametric uncertainty. In: Smart Materials & Structure, IOP, 19 (2010) 105010 (9pp) .

Abstract: The design and analysis of energy harvesting devices is becoming increasing important in recent years. Most of the literature has focused on the deterministic analysis of these systems and problem of uncertain parameters has received less attention. Energy harvesting devices exhibit parametric uncertainty due to errors in measurement, errors in modeling and variability in the parameters during manufacture. This paper investigates the effect of parametric uncertainty in the mechanical system on the harvested power, and derives approximate explicit formulae for the optimal electrical parameters that maximise the mean harvested power. The maximum of the mean harvested power decreases with increasing uncertainty, and the optimal frequency at which the maximum mean power occurs shifts. The effect of the parameter variance on the optimal electrical time constant and optimal coupling coefficient are reported. Monte-Carlo based simulation results are used to further analyse the system under parametric uncertainty.

Zhi Ang Eu, Winston K. G. Seah, (2010) Design and Performance Analysis of MAC schemes for Wireless Sensor Networks Powered by Ambient Energy Harvesting. In: Elsevier Journal of Ad Hoc Networks, Accepted 23 July 2010.

Abstract: Energy consumption is a perennial issue in the design of wireless sensor networks (WSNs) which typically rely on portable sources like batteries for power. Recent advances in ambient energy harvesting technology have made it a potential and promising alternative source of energy for powering WSNs. By using energy harvesters with supercapacitors, WSNs are able to operate perpetually until hardware failure and in places where batteries are hard or impossible to replace. In this paper, we study the performance of different medium access control (MAC) schemes based on CSMA and polling techniques for WSNs which are solely powered by ambient energy harvesting using energy harvesters. We base the study on (i) network throughput (S), which is the rate of sensor data received by the sink, (ii) fairness index (F), which determines whether the bandwidth is allocated to each sensor node equally and (iii) inter-arrival time which measures the average time difference between two packets from a source node. For CSMA, we compare both the slotted and unslotted variants. For polling, we first consider identity polling. Then we design a probabilistic polling protocol that takes into account the unpredictability of the energy harvesting process to achieve good performance. Finally, we present an optimal polling MAC protocol to determine the theoretical maximum performance. We validate the analytical models using extensive simulations incorporating experimental results from the characterization of different types of energy harvesters. The performance results show that probabilistic polling achieves high throughput and fairness as well as low inter-arrival times.

Zhi Ang Eu, Winston K. G. Seah, (2010) Opportunistic Routing in Wireless Sensor Networks Powered by Ambient Energy Harvesting. In: Elsevier Journal of Computer Networks, Accepted 15 May 2010.

Abstract: Energy consumption is an important issue in the design of wireless sensor networks (WSNs) which typically rely on portable energy sources like batteries for power. Recent advances in ambient energy harvesting technologies have made it possible for sensor nodes to be powered by ambient energy entirely without the use of batteries. However, since the energy harvesting process is stochastic, exact sleep-and-wakeup schedules cannot be determined in WSNs Powered solely using Ambient Energy Harvesters (WSN-HEAP). Therefore, many existing WSN routing protocols cannot be used in WSN-HEAP. In this paper, we design an opportunistic routing protocol (EHOR) for multi-hop WSN-HEAP. Unlike traditional opportunistic routing protocols like ExOR or MORE, EHOR takes into account energy constraints because nodes have to shut down to recharge once their energy is depleted. Furthermore, since the rate of charging is dependent on environmental factors, the exact identities of nodes that are awake cannot be determined in advance. Therefore, choosing an optimal forwarder is another challenge in EHOR. We use a regioning approach to achieve this goal. Using extensive simulations incorporating experimental results from the characterization of different types of energy harvesters, we evaluate EHOR and the results show that EHOR increases goodput and efficiency compared to traditional opportunistic routing protocols and other non-opportunistic routing protocols suited for WSN-HEAP.

Yang Yaowen, (2009) System-Level Modeling of Piezoelectric Energy Harvesters. In: Advanced Materials Research, Vols.79-82, pp.103-106.

Abstract: Accurate modeling and computer aided simulation is advantageous during the design stage of a piezoelectric energy harvesting system. In this paper, system-level finite element modeling (FEM) of a cantilevered piezoelectric energy harvester with a resistor is conducted using ANSYS. Considering that practical energy harvesting circuit includes nonlinear electrical elements, which is beyond the modeling capability of ANSYS, an equivalent circuit modeling (ECM) method is proposed to address the problem. After the parameters of equivalent circuit are identified, system-level simulation is conducted in SPICE software.

Minan Zhu, Dr Martin Judd, P. J. Moore, (2009) Energy Harvesting in Substations for Powering Autonomous Sensors. In: Proc. 3rd Int. Conf. on Sensor Technologies and Applications (SENSORCOMM '09), Athens, Greece, June 2009, pp. 246-249.

Abstract: The benefits of enhanced condition monitoring in the asset management of the electricity transmission infrastructure are increasingly being exploited by the grid operators. Adding more sensors helps to track the plant health more accurately. However, the installation or operating costs of any additional sensors could outweigh the benefits they bring due to the requirement for new cabling or battery maintenance. Energy harvesting devices are therefore being proposed to power a new generation of wireless sensors. The harvesting devices could enable the sensors to be maintenance free over their lifetime and substantially reduce the cost of installing and operating a condition monitoring system.

Emma Worthington, James Njuguna, (2009) Analyses of Power Output of Piezoelectric Energy Harvesting Devices Directly Connected to a Load Resistor Using a Coupled Piezoelectric-Circuit . In: IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control, July 2009, vol.56, no.7, 1309-1318.

Abstract: This paper presents, for the first time, a coupled piezoelectric-circuit finite element model (CPC-FEM) to analyse the power output of a vibration-based piezoelectric energy harvesting device (EHD) when it is connected to a load resistor. Special focus is given to the effect of the load resistor value on the vibrational amplitude of the piezoelectric EHD, and thus on the current, voltage, and power generated by the device, which are normally assumed to be independent of the load resistor value in order to reduce the complexity of modelling and simulation. The presented CPC-FEM uses a cantilever with a sandwich structure and a seismic mass attached to the tip to study the following characteristics of the EHD as a result of changing the load resistor value: (1) the electric outputs: the current through and voltage across the load resistor, (2) the power dissipated by the load resistor, (3) the displacement amplitude of the tip of the cantilever, and (4) the shift in the resonant frequency of the device. It is found that these characteristics of the EHD have a significant dependence on the load resistor value, rather than being independent of it as is assumed in most literature. The CPC-FEM is capable of predicting the generated output power of the EHD with different load resistor values while simultaneously calculating the effect of the load resistor value on the displacement amplitude of the tip of the cantilever. This makes the CPC-FEM invaluable for validating the performance of a designed EHD before it is fabricated and tested, thereby reducing the recurring costs associated with repeat fabrication and trials. In addition, the proposed CPC-FEM can also be used for producing an optimised design for maximum power output.

Guoliang Ye, Zijing Wong, Ashwin A. Seshia, (2009) Optimisation of a Piezoelectric System for Energy Harvesting from Traffic Vibrations. In: Proceedings of the IEEE International Ultrasonics Symposium, (IEEE IUS2009), Sept. 20-23, 2009, Roma, Italy.

Abstract: Piezoelectric systems are viewed as a promising
approach to energy harvesting from environmental vibrations.
The energy harvested from real vibration sources is usually
difficult to estimate analytically. Therefore, it is hard to optimise
the associated energy harvesting system. This work investigates
the optimisation of a piezoelectric cantilever system using a
genetic algorithm based approach with numerical simulations.
The genetic algorithm globally considers the effects of each
parameter to produce an optimal frequency response to scavenge
more energy from the real vibrations while the conventional
sinusoidal based method can only optimise the resistive load for
a given resonant frequency. Experimental acceleration data from
the vibrations of a vehicle-excited manhole cover demonstrates
that the optimised harvester automatically selects the right
frequency and also synchronously optimises the damper and the
resistive load. This method shows great potential for optimizing
the energy harvesting systems with real vibration data.

Yang Yaowen, Li Hongyun, (2009) Vibration Energy Harvesting Using Macro-Fiber Composites. In: Smart Materials and Structures, Vol.18, No.11, 115025.

Abstract: The decreasing energy consumption of today's portable electronics has invoked the possibility of energy harvesting from the ambient environment for self-power supply. One common and simple method for vibration energy harvesting is to utilize the direct piezoelectric effect. Compared to traditional piezoelectric materials such as lead zirconate titanate (PZT), macro-fiber composites (MFC) are characterized by their flexibility on large deformation. However, the energy generated by MFC is still far smaller than that required by electronics at present. In this paper, a vibration energy harvesting system prototype with MFC patches bonded to a cantilever beam is fabricated and tested. A finite element analysis (FEA) model is established to estimate the output voltage of the MFC harvester. The energy accumulation procedure in the capacitor is simulated by using the electronic design automation (EDA) software. The simulation results are validated by the experimental ones. Finally, to optimize the efficiency of energy harvesting, the effects of the electrical properties of MFC as well as the geometric configurations of the cantilever beam and MFC are parametrically studied by combining the FEA and EDA simulations.

Yang Yaowen, (2009) Equivalent Circuit Modeling of Piezoelectric Energy Harvesters. In: Journal of Intelligent Material Systems and Structures, Vol. 20, No.18, 2223-2235.

Abstract: Last decade has seen growing research interest in vibration energy harvesting using piezoelectric materials. When developing piezoelectric energy harvesting systems, it is advantageous to establish certain analytical or numerical model to predict the system performance. In the last few years, researchers from mechanical engineering established distributed models for energy harvester but simplified the energy harvesting circuit in the analytical derivation. While, researchers from electrical engineering concerned the modeling of practical energy harvesting circuit but tended to simplify the structural and mechanical conditions. The challenges for accurate modeling of such electromechanical coupling systems remain when complicated mechanical conditions and practical energy harvesting circuit are considered in system design. In this article, the aforementioned problem is addressed by employing an equivalent circuit model, which bridges structural modeling and electrical simulation. First, the parameters in the equivalent circuit model are identified from theoretical analysis and finite element analysis for simple and complex structures, respectively. Subsequently, the equivalent circuit model considering multiple modes of the system is established and simulated in the SPICE software. Two validation examples are given to verify the accuracy of the proposed method, and one further example illustrates its capability of dealing with complicated structures and non-linear circuits.

Mike Friswell, Daniel J. Inman, (18) Piezoelectric energy harvesting from broadband random vibrations. In: Smart Materials & Structures, 18[11], pp. 115005:1-7.

Abstract: Energy harvesting for the purpose of powering low power electronic sensor systems has received explosive attention in the last few years. Most works using deterministic approaches focusing on using the piezoelectric effect to harvest ambient vibration energy have concentrated on cantilever beams at resonance using harmonic excitation. Here, using a stochastic approach, we focus on using a stack configuration and harvesting broadband vibration energy, a more practically available ambient source. It is assumed that the ambient base excitation is stationary Gaussian white noise, which has a constant power-spectral density across the frequency range considered. The mean power acquired from a piezoelectric vibration-based energy harvester subjected to random base excitation is derived using the theory of random vibrations. Two cases, namely the harvesting circuit with and without an inductor, have been considered. Exact closed-form expressions involving non-dimensional parameters of the electromechanical system have been given and illustrated using numerical examples.

Prof. Maryline CHETTO, (0) Optimal Scheduling for Real-Time Jobs in Energy Harvesting Computing Systems . In: IEEE Trans. on Emerging Topics in Computing, 10.1109/TETC.2013.2296537.

Abstract: In this paper, we study a scheduling problem, in which every job is associated with a release time, deadline, required computation time and in addition required energy. We focus on an important special case where the jobs execute on a uniprocessor system that is supplied by a renewable energy source and uses a rechargeable storage unit with limited capacity. Earliest Deadline First (EDF) is a class one online algorithm in the classical real-time scheduling theory where energy constraints are not considered. We propose a semi-online EDF-based scheduling algorithm theoretically optimal (i.e. processing and energy costs neglected). This algorithm relies on the notions of energy demand and slack energy which are different from the well known notions of processor demand and slack time. We provide an exact feasibility test. There are no restrictions on this new scheduler: each job can be one instance of a periodic, aperiodic or sporadic task with deadline.


Jelena Marasevic, Cliff Stein, Gil Zussman, (2014) Max-min Fair Rate Allocation and Routing in Energy Harvesting Networks: Algorithmic Analysis. In: ACM MobiHoc'14, 2014.

Abstract: This paper considers max-min fair rate allocation and routing in energy harvesting networks where fairness is required among both the nodes and the time slots. Unlike most previous work on fairness, we focus on multihop topologies and consider different routing methods. We assume a predictable energy profile and focus on the design of efficient and optimal algorithms that can serve as benchmarks for distributed and approximate algorithms. We first develop an algorithm
that obtains a max-min fair rate assignment for any given (time-variable or time-invariable) unsplittable routing or a
routing tree. For time-invariable unsplittable routing, we also develop an algorithm that finds routes that maximize
the minimum rate assigned to any node in any slot. For fractional routing, we study the joint routing and rate assignment
problem. We develop an algorithm for the time-invariable case with constant rates. We show that the time-variable case is at least as hard as the 2-commodity feasible
flow problem and design an FPTAS to combat the high running time. Finally, we show that finding a max-min fair unsplittable routing or a routing tree is NP-hard, even for a time horizon of a single slot. Our analysis provides insights into the problem structure and can be applied to other related fairness problems.

Junjie Shi, Steve Beeby, (2014) An investigation of PDMS structures for optimized ferroelectret performance. In: Journal of Physics: Conference Series, 557, 012104.

Abstract: This paper reports the ANSYS simulation and fabrication processes for optimising PDMS ferroelectret performance. The proposed model extends the previously published analytical models and compares this with simulation of individual void geometry. The ferroelectret material is fabricated from PDMS using 3D-printed plastic moulds. The
analytical model and Ansys simulation results predict the variation in performance of the PDMS ferroelectret with the different void geometry and surface charge density. The theoretical maximum piezoelectric coefficient d33 achieved was about 220 pC/N. The experimental maximum d33 obtained was 172 pC/N.

Steve Beeby, (2013) The importance of applications in the design and testing of vibration energy harvesters. In: Nanoenergy 2013, Perugia, Italy, 10 - 13 Jul 2013.

M. T. Penella-López, Manel Gasulla, (2010) Runtime extension of low-power wireless sensor nodes using hybrid-storage units. In: IEEE transactions on instrumentation and measurement, March 2010, vol. 59, n. 4, p. 857-865..

Abstract: The sensor nodes of wireless sensor networks remain inactive most of the time to achieve longer runtimes. Power is mainly provided by batteries, which are either primary or secondary. Because of its internal impedance, a significant voltage drop can appear across the battery terminals at the activation time of the node, thus preventing the extraction of all the energy from the battery. Additionally, internal losses can also be significant. Consequently, the runtime is reduced. The addition of a supercapacitor in parallel with the battery, thus forming a hybrid-storage device, has been proposed under pulsed loads to increase the power capabilities and reduce both the voltage drop and the internal losses at the battery. However, this strategy has not yet thoroughly been analyzed and tested in low-power wireless sensor nodes. This paper presents a comprehensive theoretical analysis that extends previous works found in the literature and provides design guidelines for choosing the appropriate supercapacitor. The analysis is supported by extensive experimental results. Two low-capacity (< 200 mAh) batteries were tested together with their hybrid-storage unit counterparts when using an electronic load as a pulsed current sink. The hybrid-storage units always achieved a higher runtime. One of the batteries was also tested using a sensor node. The runtime extension was 16% and 33% when connecting the hybrid-storage unit directly and through a dc-dc switching regulator to the sensor node, respectively.

O. López-Lapeña, M.T. Penella-López, Manel Gasulla, (2010) A new MPPT method for low-power solar energy harvesting. In: IEEE transactions on industrial electronics, September 2010, vol. 57, no. 9, pp. 3129-3138.

Abstract: This paper describes a new maximum-power-pointtracking (MPPT) method focused on low-power (< 1 W) photovoltaic (PV) panels. The static and dynamic performance is theoretically analyzed, and design criteria are provided. A prototype was implemented with a 500-mW PV panel, a commercial boost converter, and low-power components for the MPPT controller. Laboratory measurements were performed to assess the effectiveness of the proposed method. Tracking efficiency was higher than 99.6%. The overall efficiency was higher than 92% for a PV panel power higher than 100 mW. This is, in part, feasible due to the low power consumption of the MPPT controller, which was kept lower than 350 ���¼W. The time response of the tracking circuit was tested to be around 1 s. Field measurements showed energy gains higher than 10.3% with respect to a direct-coupled solution for an ambient temperature of 26 ���ºC. Higher gains are expected for lower temperatures.

A. Cuadras, Manel Gasulla, Vittorio Ferrari, (2010) Thermal energy harvesting through pyroelectricity. In: Sensors and actuators A. Physical, March 2010, vol. 158, no. 1, pp. 132-139..

Abstract: Pyroelectric cells based on fabricated screen-printed PZT and commercial PVDF films are proposed as thermal energy harvesting sources in order to supply low-power autonomous sensors. The cells are electrically modelled as a current source in parallel with output impedance. Heating and cooling temperature fluctuations generated by air currents were applied to the pyroelectric converters. The generated currents and charges were respectively in the order of 10�7 A and 10�5 C for temperature fluctuations from 300K to 360K in a time period of the order of 100 s, which agrees with the theoretical model. Parallel association of cells increased the generated current. The dependence of the generated current on relevant technological parameters has been also characterized. Finally, current from cyclic temperature fluctuations was rectified and stored in a 1 F load capacitor. Energies up to 0.5 mJ have been achieved, enough to power typical autonomous sensor nodes during a measurement and transmission cycle.

Maria Gorlatova, Peter Kinget, Ioannis Kymissis, Dan Rubenstein, Xiaodong Wang, Gil Zussman, (2010) Energy Harvesting Active Networked Tags (EnHANTs) for Ubiquitous Object Networking. In: IEEE Wireless Communications, Vol. 17, No. 6, pp. 18-25, Dec. 2010.

Abstract: This article presents the design challenges posed by a new class of ultra-low-power devices referred to as Energy-Harvesting Active Networked Tags (EnHANTs). EnHANTs are small, flexible, and self-reliant (in terms of energy) devices that can be attached to objects that are traditionally not networked (e.g., books, furniture, walls, doors, toys, keys, produce, and clothing). EnHANTs will enable the Internet of Things by providing the infrastructure for various novel tracking applications. Examples of these applications include locating misplaced items, continuous monitoring of objects, and determining locations of disaster survivors. Recent advances in ultra-low-power circuit design, ultra-wideband (UWB) wireless communications, and organic energy harvesting techniques will enable the realization of EnHANTs in the near future. The harvesting components and the ultra-low-power physical layer have special characteristics whose implications on the higher layers have yet to be studied (e.g., when using UWB communications, the energy required to receive a bit is significantly higher than the energy required to transmit a bit). In this paper, we describe paradigm shifts associated with technologies that enable EnHANTs and demonstrate their implications on higher-layer protocols. Moreover, we describe some of the components we have designed for EnHANTs. Finally, we briefly discuss our indoor light measurements and their implications on the design of higher-layer protocols.

Minan Zhu, Peter Baker, Dr Martin Judd, John Fitch (National Grid), (2009) Alternative Power Sources for Autonomous Sensors in High Voltage Plant. In: Proc. Electrical Insulation Conference (EIC 2009), Montreal, Canada, pp. 36-39, June 2009.

Abstract: Enhanced condition monitoring of the electric grid infrastructure could be achieved by adding more sensors to track the plant health more accurately. However, the installation or operating costs of any additional sensors could outweigh the benefits they bring due to the requirements for new cabling or battery maintenance. High voltage plant, such as substation, contains several potential energy sources. Harvesting the energy from ambient sources to power sensors could support a move towards autonomous wireless condition monitoring sensors while reduced the overall cost of owning and operating an enhanced condition monitoring system. This paper reviews the potential sources of power that may be used to supply autonomous sensor nodes.

Maria Gorlatova , Peter Kinget, Ioannis Kymissis, Dan Rubenstein, Xiaodong Wang, Gil Zussman, (2009) Challenge: Ultra-Low-Power Energy-Harvesting Active Networked Tags (EnHANTs). In: Proc. ACM MobiCom'09 , September 20-25, 2009, Beijing, China.

Abstract: This paper presents the design challenges posed by a new class of ultra-low-power devices referred to as Energy-Harvesting Active Networked Tags (EnHANTs). EnHANTs are small, flexible, and self-reliant (in terms of energy) devices that can be attached to objects that are traditionally not networked (e.g., books, clothing, and produce), thereby providing the infrastructure for various novel tracking applications. Examples of these applications include locating misplaced items, continuous monitoring of objects (items in a store, boxes in transit), and determining locations of disaster survivors. Recent advances in ultra low-power wireless communications, ultra-wideband (UWB) circuit design, and organic electronic harvesting techniques will enable the realization of EnHANTs in the near future. In order for EnHANTs to rely on harvested energy, they have to spend significantly less energy than Bluetooth, Zigbee, and IEEE 802.15.4a devices. Moreover, the harvesting components and the ultra-low-power physical layer have special characteristics whose implications on the higher layers have yet to be studied (e.g., when using ultra-low-power circuits, the energy required to receive a bit is an order of magnitude higher than the energy required to transmit a bit). These special characteristics pose several new cross-layer research problems. In this paper, we describe the design challenges at the layers above the physical layer, point out relevant research directions, and outline possible starting points for solutions.

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