Traditionally, batteries are used as the electrical energy power sources to power wireless sensors and embedded electronics. WSN technology has gained increasing importance in industrial automation, structural health monitoring, healthcare, agriculture, and civil and military applications. Thousands of spatially distributed wireless sensors can be developed which can be embedded virtually anywhere in civil structures, bridges, or in the human body. The task of each node is to collect and transmit data to the outside world via a radio link. A wireless sensor node consists of low power microcontroller unit, radio frequency transceiver and microelectromechanical- (MEMS-) based sensor. The advances in low power electronics, and wireless sensor networks (WSNs) in particular, have driven numerous researches in the field of energy harvesting in the past decade. This paper presents a concise review of piezoelectric microgenerators and nanogenerators as a renewable energy resource to power wireless sensors. Among the ambient energy sources such as solar energy, heat, and wind, mechanical vibrations are an attractive ambient source mainly because they are widely available and are ideal for the use of piezoelectric materials, which have the ability to convert mechanical strain energy into electrical energy. Energy harvesting raises the possibility of self-powered systems which are ubiquitous and truly autonomous, and without human intervention for energy replenishment. The process of harnessing and converting ambient energy sources into usable electrical energy is called energy harvesting. Moreover, the embedded nature of some of the sensors and hazardous sensing environment make battery replacement very difficult and costly. One of the major limitations on performance and lifetime of WSNs is the limited capacity of these finite power sources, which must be manually replaced when they are depleted. The spatial distributed nature of WSNs often requires that batteries power the individual sensor nodes. The recent advances in ultralow power device integration, communication electronics, and microelectromechanical systems (MEMS) technology have fuelled the emerging technology of wireless sensor networks (WSNs).
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