Hybrid energy harvesters could power handheld electronics
The handheld-electronics market has grown remarkably, with realization of diversified products including mobile phones, personal digital assistants, cameras, and healthcare devices. This market drives significant progress in the development of low-power electronics and wireless-communication technology. Harvesting energy from ambient vibrations to recharge the batteries in such devices is a potential route to realizing handheld electronics with infinite lifespans, i.e., self-powered electronics. The human body itself is a potential source of energy for harvesting. Other sources of vibrations include equipment and engines used in household appliances.
State-of-the-art micro-electromechanical systems (MEMS) energy harvesters1,2 offer intriguing possibilities to realize such self-renewing power sources. Energy extracted from vibrations is stored in chip-compatible, rechargeable batteries such as thin-film lithium-ion types, which further power the loading application (e.g., a wireless-sensor node) by means of a regulator circuit. Vibration-based MEMS energy harvesters use one of three energy-transduction mechanisms: electrostatic, electromagnetic, or piezo-electric. Electrostatic energy harvesters collect energy from capacitance changes during the vibration cycle,3,4 while electromagnetic energy harvesters collect energy from the current generated in coils by variations in magnetic flux induced by the movement of a permanent magnet.5–7 On the other hand, piezo-electric materials are perfect candidates for harvesting power from ambient vibration sources, because they can efficiently convert mechanical strain to an electrical charge without any additional power.8–10
Most reported approaches work by employing one of these three mechanisms. In this study, we explore a hybrid approach that combines two mechanisms. To harvest energy from vibrations, an inertial mass is needed to collect kinetic energy. Magnets that provide this inertial mass are placed at the end of a piezo-electric cantilever (see Figure 1). We used co-fired multilayer piezo-electric cantilever elements from Piezo Systems Inc. These cantilevers are built up from a number of lead zirconate titanate (PZT) layers, each 30μm thick, and screen-printed electrodes a few micrometers thick between each pair of layers in a piezo-electric ceramic pattern. The cantilever's length, width, and total thickness are 22, 9.6, and 0.65mm, respectively. When the device is excited by external vibrations, their kinetic energy can be harvested through both electromagnetic and piezo-electric transduction mechanisms.
The resonant quality factor of electromagnetic energy harvesters is essentially proportional to the resonant frequency. In contrast, that of piezo-electric energy harvesters is proportional to the square of the frequency. Thus, the optimum output power of piezo-electric mechanisms diminishes rapidly with increasing frequency. At high frequencies, their output power degenerates more rapidly than in the electromagnetic case. Thus, piezo-electric energy harvesters generally outperform their electromagnetic equivalents at low frequency, and vice versa at high frequency. Piezo-electric energy harvesters usually produce high voltages and lower current. In comparison, electromagnetic energy harvesters tend to produce relatively low AC voltage, and the voltage output decreases when the size is scaled down. Previously reported data indicates that piezo-electric energy harvesters present a high power density and are more suitable for microsystem applications, while electromagnetic energy harvesters are good at relatively large applications.
We explored the effect of the relative positions of coils and magnets on the PZT cantilever end and that of the poling direction of the magnets on the output voltage of the energy harvester. When the poling direction is normal to the plane of the coils, the coils yield the highest output voltage. The maximum output voltage and power from the PZT cantilever and the coils are 0.84V and 176μW, while they are 0.78mV and 0.19μW, respectively, under vibrations resulting from a 2.5g acceleration at 310Hz. The power densities of piezo-electric and electromagnetic mechanisms for a type III device are 790 and 0.85μW/cm3, respectively.
In summary, the major advantages of our device are low cost and the capacity to harvest energy by both piezo-electric and electromagnetic mechanisms. The position and weight of the magnets can be used to optimize the output power and modify the resonant frequency of the PZT cantilever. In the future, this hybrid energy harvester could be an alternative power source for wireless-sensor nodes.
Chengkuo Lee received his PhD degree in precision engineering from the University of Tokyo (Japan) in January 1996. He is currently an assistant professor.
