Vibrational Energy Harvesting

Energy harvesting is an attractive way to power electronic systems such as wireless sensors without using batteries or other energy storages with limited lifetime. Among the energy harvesters proposed from different sources (e.g. light, thermal gradient, strain, vibrations, electromagnetic field, air flow and pressure variations), vibrations pervasively available in different environments (indoor and outdoor) represent an attractive option for the development of adequate sources for low power supplying or for extending the autonomy of remote sensors and portable electronics. Power harvested from mechanical vibrations represents a very promising energy source with estimated power in the μW–mW range [ROU 04a]. Vibration-powered generators are typically inertial spring and mass systems (Figure 6.1) which employ three main transduction mechanisms to extract energy from vibrations: piezoelectric, electromagnetic and electrostatic. Piezoelectric generators employ active materials that generate a charge and, therefore, a voltage when mechanically stressed. Electromagnetic generators harvest energy from vibrations by employing the electromagnetic induction arising from the relative motion between a magnetic flux gradient and a conductor. Electrostatic generators use the vibration-induced relative movement between electrically isolated charged capacitor plates against the electrostatic force to generate energy. Other solutions employing either electrets or magnetostrictive materials for the mechanical energy conversion were also proposed [WAN 08, KAR 08a]. Microelectromechanical systems (MEMS) technology was largely investigated to achieve vibration energy harvesters that can be potentially integrated with low power applications such as wireless sensor networks (WSN) nodes [AMM 05,ERI 05, JEO 05, BEE 06, ROU 03]. Piezoelectric transducers have often been proposed to implement easily exploitable energy harvester solutions mainly because of their low-cost manufacturing process and the potential integration with complementary metal–oxide semiconductor (CMOS) technology. Nevertheless, electromagnetic transducers were also explored due to their complementary advantages compared to piezoelectric transducers, and solutions combining both transduction harvesters were proposed to improve the energy density and the conversion efficiency [BEE 07a]. In order to be used in practical applications, the energy harvesters have to deliver a minimum output power and voltage, which are required by power converters to operate with acceptable efficiency. Unfortunately, this is not always the case, and several energy harvesters proposed in the literature have low output power and voltages, were large and bulky, and their efficiency was shown to peak only in a very narrow frequency range, thus making such devices unsuitable to scavenge energy from actual ambient vibrations. For this reason, research activities have been oriented to improve the power efficiency and the output power of the vibration energy harvester, to decrease the size of the transducers, to decrease the operating frequency, to match the low frequency ambient vibrations and to widen their bandwidth to maximize the energy collection (ambient vibrations rarely never occur at exact frequencies) [MUR 09]. This chapter will present a short overview of the MEMS energy harvesters employing both piezoelectric and electromagnetic effects both proposed in the literature and developed in the framework of the Nanofunction project. A short overview of state-of-the-art vibration energy transducers employing the piezoelectric effect is presented in section 6.2, where MEMS prototypes realized in the framework of the Nanofunction project will also be presented. Near-field characterization techniques as well as electromechanical modeling and simulation required for the design of the energy harvesting transducers will be illustrated. Electromagnetic generators presented in the literature including large-scale discrete devices and integrated versions are reviewed in section 6.3, where the results achieved on vibration energy harvester exploiting both electromagnetic and piezoelectric effects will be derived. Modeling and simulation results will be presented to demonstrate the feasibility of the proposed device concepts.