Dibin Zhu

Strategies for increasing the operating frequency range of vibration energy harvesters: a review

This paper reviews 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, non-linear (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 paper 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.

Vibration energy harvesting using the Halbach array

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.

A comparison of power output from linear and nonlinear kinetic energy harvesters using real vibration data

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 Network’s 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.

A novel miniature wind generator for wireless sensing applications

This paper describes a novel miniature wind generator for wireless sensing applications. The generator consists of an aerofoil that is attached to a cantilever spring. The airflow over the aerofoil causes the cantilever to bend, the degree of bending being a function of the lift force from the aerofoil and the spring constant. As the cantilever deflects, the flow of air is reduced by a bluff body and the lift force therefore decreases causing the cantilever to spring back. The aerofoil is hence exposed to the full airflow again and the cycle is repeated. When the frequency of the movement matches the resonant frequency of the structure, the aerofoil has the maximum displacement. A permanent magnet is fixed on the aerofoil and a coil is attached to the base of the generator. The movement of the aerofoil causes the magnetic flux cutting the coil to change, which generates electrical power. The device has dimensions of 12 cm × 8 cm × 6.5 cm. Experiments have shown that the generator can operate at wind speeds as low as 2.5 m·s−1 with a corresponding electrical output power of 470 µW. This is sufficient for periodic sensing and wireless transmission. When the wind speed is 5 m·s−1, the output power is 1.6 mW.

A tunable kinetic energy harvester with dynamic over range protection

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.

Novel Miniature Airflow Energy Harvester for Wireless Sensing Applications in Buildings

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 are studied. Experiments show that such a generator can operate at airflow speeds as low as 1.5 m·s-1, which compares well to turbines. When the airflow speed is over 2 m·s-1, the average output power exceeds 90 μW, which is sufficient for powering wireless sensor nodes in heat, ventilation, and air conditioning systems in buildings.

Vibration Energy Harvesting: Machinery Vibration, Human Movement and Flow Induced Vibration

With the development of low power electronics and energy harvesting technology, selfpowered systems have become a research hotspot over the last decade. The main advantage of self-powered systems is that they require minimum maintenance which makes them to be deployed in large scale or previously inaccessible locations. Therefore, the target of energy harvesting is to power autonomous ‘fit and forget’ electronic systems over their lifetime. Some possible alternative energy sources include photonic energy (Norman, 2007), thermal energy (Huesgen et al., 2008) and mechanical energy (Beeby et al., 2006). Among these sources, photonic energy has already been widely used in power supplies. Solar cells provide excellent power density. However, energy harvesting using light sources restricts the working environment of electronic systems. Such systems cannot work normally in low light or dirty conditions. Thermal energy can be converted to electrical energy by the Seebeck effect while working environment for thermo-powered systems is also limited. Mechanical energy can be found in instances where thermal or photonic energy is not suitable, which makes extracting energy from mechanical energy an attractive approach for powering electronic systems. The source of mechanical energy can be a vibrating structure, a moving human body or air/water flow induced vibration. The frequency of the mechanical excitation depends on the source: less than 10Hz for human movements and typically over 30Hz for machinery vibrations (Roundy et al., 2003). In this chapter, energy harvesting from various vibration sources will be reviewed. In section 2, energy harvesting from machinery vibration will be introduced. A general model of vibration energy harvester is presented first followed by introduction of three main transduction mechanisms, i.e. electromagnetic, piezoelectric and electrostatic transducers. In addition, vibration energy harvesters with frequency tunability and wide bandwidth will be discussed. In section 3, energy harvesting from human movement will be introduced. In section 4, energy harvesting from flow induced vibration (FIV) will be discussed. Three types of such generators will be introduced, i.e. energy harvesting from vortex-induced vibration (VIV), fluttering energy harvesters and Helmholtz resonator. Conclusions will be given in section 5.

Kinetic Energy Harvesting

This chapter introduces principles of normal kinetic energy harvesting and adaptive kinetic energy harvesting. Kinetic energy harvesters, also known as vibration power generators, are typically, although not exclusively, inertial springmass systems. Electrical power is extracted by employing one or a combination of different transduction mechanisms. Main transduction mechanisms are piezoelectric, electromagnetic and electrostatic. As most vibration power generators are resonant systems, they generate maximum power when the resonant frequency of the generator matches ambient vibration frequency. Any difference between these two frequencies can result in a significant decrease in generated power. Recent development in adaptive kinetic energy harvesting increases the operating frequency range of such generators. Possible solutions include tuning resonant frequency of the generator and widening the bandwidth of the generator. In this chapter, principles and operating strategies for adaptive kinetic energy harvesters will be presented and compared.

Energy harvesting study on single and multilayer ferroelectret foams under compressive force

Cellular polypropylene (PP) ferro electret is a thin and flexible cellular polymer foam that generates electrical power under mechanical force. This work investigates single and multilayer ferro electret PP foams and their potential to supply energy for human-body-worn sensors. Human foot-fall is emulated using an electrodynamic instrument, allowing applied compressive force and momentum to be correlated with energy output. Peak power, output pulse duration, and energy per strike is derived experimentally as a function of force and momentum, and shown to be a strong function of external load resistance, thus providing a clear maximum energy point. The possibility of increasing pulse time and reducing voltage to CMOS compatible levels at some expense of peak power is shown. To further increase the output power, multilayer ferro electret is presented. The synchronized power generation of each layer is studied and illustrated using simulation, and results are supported by experiments. Finally, the energy output of single-layer and multi-layer ferro electrets are compared by charging a capacitor via a rectifier. A ten-layer ferro electret is shown to have charging ability 29.1 times better than that of the single-layer ferro electret. It demonstrates energy output that is capable of powering the start-up and transmission of a typical low-power wireless sensor chipset.

Screen-printed piezoelectric shoe-insole energy harvester using an improved flexible PZT-polymer composites

This paper reports improved screen-printed piezoelectric composites that can be printed on fabrics or flexible substrates. The materials are flexible and are processed at lower temperature (130°C). One main PZT particle size (2μm) was mixed separately with smaller piezoelectric particles (0.1, 0.3 and 0.8μm) with different weight ratios to investigate the piezoelectric property d33. The blended PZT powder was then mixed with 40% polymer binder and printed on Alumina substrates. The applied poling field, temperature and time were 8MV/m, 160°C and 10min, respectively. The optimum material gives a d33 of 36pC/N with particle sizes of 2μm and 0.8μm and mixed percentages of 82% and 18%, respectively. A screen-printed piezoelectric shoe-insoles (PSI) has been developed as a self-powered force mapping sensor. The PSI was simulated, fabricated and tested. ANSYS results show that one element of PSI sole can produce an open- circuit voltage of 3V when a human of average weight of 70kg makes a gait strike. Experimental results show that one element produced 2V which is less than the simulated results because of the reduction of poling field for the practical device.