Volume 85, Issue 9 - Energy Harvesting

This paper analyzes the dynamic characteristics of broadband tristable energy harvesters to reveal their response mechanism via a bifurcation diagram, the corresponding frequency spectral analysis and the phase portrait topology. The bifurcation diagram of response voltages shows that tristable energy harvesters orderly undergoes singly periodic intrawell oscillation, singly periodic interwell oscillation, triply periodic interwell oscillation, singly periodic interwell oscillation, double-periodic interwell oscillation, chaotic oscillation, singly periodic interwell oscillation, multi-period oscillation, and finally enters into chaotic oscillation range, as the increase of the excitation amplitude. The frequency spectral analysis demonstrates that sub-harmonics and super-harmonics numerically and experimentally exist in the response voltages of tristable energy harvesters. In addition, it is found that both the first harmonic and the third harmonic are main frequency components in the response voltages.

Harvesting energy using the pyroelectric effect has seen growth as a potential energy source for low power applications, such as self-powered and autonomous wireless sensor networks. The scavenged energy is generally at low power levels, from mW to less than µW. While the voltages generated by pyroelectrics can be appreciable, the electric currents can be low in the order of nano-amps. In the case of pyroelectric harvesting the frequency of operation can also be low, typically much lower than 1 Hz, due to the slow temperature oscillations and transients in systems of large thermal mass. The combination of low power levels and low frequency of operation means that methods of storing electrical energy generated by pyroelectrics and the influence of inherent second order losses is of importance to create efficient harvesting devices. This paper examines the second order characteristic effects of practical capacitors and diodes for storage. The stored energy decay characteristics for commercially available components are examined and the data is used to characterise the second order effects. Selected components are then used in a pyroelectric harvesting system to determine potential improvements by appropriate selection of components with low loss.

A hybrid energy harvester combining piezoelectric and electromagnetic transduction mechanisms is designed to scavenge vibration energy. The system comprises of a cantilever beam, a piezoelectric harvester and a magnetic mass hung through a spring at the free end. The beam with piezoelectric harvests electrical energy due to the strain induced in the piezoelectric patch. The hung mass oscillates in and out a solenoid to harvest energy due to electromagnetic induction. The system can generate power from any vertically oscillating vibrating host structure. This paper studies the power harvested from the hybrid harvester under harmonic excitation using experimental and analytical evaluations. Comparisons are made with the standalone piezoelectric and electromagnetic harvester under the same excitation environment. The study shows that the present hybrid harvester can harvest energy at a broad range of frequencies. Furthermore few parametric studies are carried out for understanding the device performance. Finally, the efficiency of the proposed hybrid energy harvester is compared with the existing hybrid energy harvester.

This paper presents a novel approach to design and optimize geometric nonlinear springs for wideband vibration energy harvesting. To this end, we designed a spring with several folds to increase its geometric nonlinearities. A numerical analysis is performed using the Finite Element Method to estimate its quadratic and cubic spring stiffness. A nonlinear effective spring constant is then calculated for different values of the main folding angle. We demonstrate that this angle can increase nonlinearities within the structure resulting in higher bandwidths, and that it is possible to control the behavior of the system to have softening-type or hardening-type response depending on the choice of the folding angle. Based on the Lindstedt-Poincaré perturbation technique, a first order approximation is determined to predict the frequency-response of the system. In order to validate the perturbation analysis, numerical solutions based on long-time integration method and mixed VHDL-AMS/Spice simulations are presented. Finally, this method is applied to a previously published device and shows a good agreement with experiments.

Vibration-based energy harvesting has the main objective to convert available environmental mechanical energy into electrical energy. Piezoelectric materials are usually employed to promote the mechanical-electrical conversion. This work deals with a numerical investigation that analyzes the influence of nonlinear effects in piezoelectric vibration-based energy harvesting. Duffing-type oscillator that can be either monostable or bistable represents mechanical nonlinearities. A quadratic constitutive electro-mechanical coupling model represents piezoelectric nonlinearities. The system performance is evaluated for different system characteristics being monitored by the input and the generated power. Numerical simulations are carried out exploring dynamical behavior of energy harvesting system evaluating different kinds of responses, including periodic and chaotic regimes.

A novel magnetoelectric (ME) vibration energy harvester employing magnetostrictive and piezoelectric laminate composite transducers is presented for potentially powering wireless sensor systems. The harvester consists of two four-layered Terfenol-D/PZT laminate composite and a magnetic circuit composed by two parallel magnetic springs and two rectangular magnets. The repulsive forces are realized by a magnetic spring for more robustness. In order to realize a high power density, a multiple transducer design with a lateral configuration is proposed. The magnetic flux density and the induced displacement in the magnetostrictive layers are investigated by finite element analysis to determine the optimal relative position of the twin transducers at the static equilibrium. Furthermore, the output characteristics of the harvester are experimentally studied and compared to the case when only a single transducer is used. The experimental results show that the twin lateral converter can provide a higher power outcome especially if operated at resonance. In addition, doubling the amplitude of vibration from 0.5 mm to 1 mm leads to a voltage output which is four times higher at resonance.