Energy harvesting (EH) which scavenges electric power from ambient, otherwise wasted, energy sources has been explored to develop self-powered portable electronic devices. Vibration energy, a widely available ambient energy source, can be converted into electric power using a piezoelectric energy harvester that generates electric potential in response to applied mechanical strains. As a compact and durable design paradigm, a piezoelectric energy harvesting skin (PEH skin) which can be directly attached onto the surface of a vibrating engineered system has been proposed to scavenge electric power from vibration energy. The goal of this chapter is to describe the core technologies for the realization of the PEH skin from a system integration perspective as four parts: (a) modeling, (b) design, (c) manufacturing, and (d) demonstration. The readers will be able to learn the entire procedure of developing the PEH skin and applying it to self-powered wireless sensor network (WSN) through this chapter.
TopIntroduction
The advances in wireless communications and low-power technology promote the use of wireless sensors. However, the limited life expectancy and high replacement cost of batteries make it difficult to use wireless sensors, although they have lots of benefits more than wired sensors (Anton & Sodano, 2007). Energy harvesting (EH) technology, which scavenges electric power from ambient, otherwise wasted, energy sources, has been explored to develop self-powered portable or wireless electronic devices and possibly eliminate battery replacement cost for wireless sensors (H. S. Kim, Kim, & Kim, 2011).
Vibration energy, one of widely available ambient energy sources, can be converted into electric power using piezoelectric, electromagnetic, electrostatic, and/or magnetostrictive transduction mechanisms. Various vibration sources with their maximum acceleration magnitude and frequency range were well listed by Roundy et al. (Roundy, Wright, & Rabaey, 2003). Among vibration-based energy harvesting mechanisms, piezoelectric energy harvesting has been most preferred due to its high energy density and no need for external equipment. A piezoelectric energy harvester can produce alternating current (AC) in response to applied mechanical strains.
Even though piezoelectric energy harvesters have been primarily designed as cantilever beams, they have some drawbacks from a practical point of view: (a) an additional space required for a proof mass and clamping device, (b) a great deal of vibration energy loss when clamping conditions become loosened after long time use, and (c) a fatigue failure expected due to excessive strains at a clamping part. As an alternative design paradigm of the cantilever beam, a piezoelectric energy harvesting skin (PEH skin) has been proposed which can be directly attached onto the surface of a vibrating engineered system and thus requires no need for clamping fixtures and proof mass (Lee & Youn, 2011b), as shown in Figure 1. This work has been featured on the website (PhysOrg.com, 2011) as an innovative design for a piezoelectric energy harvester.
Figure 1. Piezoelectric energy harvesting skin (PEH skin) for powering to operate portable and/or wireless electronic devices
It is of great importance to systematically design and fabricate the PEH skin to generate the sufficient amount of electric power for operating portable electronic devices in real time. For the purpose of designing the PEH skin and selecting best sites for installation, it is essential to preliminarily quantify harvestable electric power under a given vibration condition. So it is required to develop an electromechanical model of the PEH skin with the high predictive capability based on rigorous theories and mechanics. In addition, because the PEH skin is fabricated by laminating thin piezoelectric layers onto a vibrating engineered system, the scientific and theoretical rationale for designing the PEH skin is needed to determine the optimal placement of the piezoelectric layers. Furthermore, the resonance frequency of the PEH skin and impedance matching should be considered to maximize harvestable electric power. On the other hand, the manufacturing process, such as a laser cutting, bonding, and curing of piezoelectric layers, should be standardized to fabricate reliable PEH skins while reducing production uncertainty for better performance. To successfully realize the PEH skin, therefore, it is necessary to thoroughly understand its core technologies, such as modeling, design, and manufacturing, and make a connection between them for establishing self-powered electronic devices in reality.