Micropower Energy Harvesting
More than a decade of research in the field of thermal, motion, vibration and electromagnetic radiation energy harvesting has yielded increasing power output and smaller embodiments. Power management circuits for rectification and DC–DC conversion are becoming able to efficiently convert the power from these energy harvesters. This paper summarizes recent energy harvesting results.
The low power consumption of silicon-based electronics has enabled a broad variety of battery-powered handheld, wearable and even implantable devices. All these devices need a compact, low-cost and lightweight energy source, which enables the desired portability and energy Autonomy. Today batteries represent the dominant energy source for many devices and alike. In spite of the fact that energy density of batteries has increased by a factor of 3 over the past 15 years, in many cases their presence has a large impact, or even dominate, size and operational cost. For this reason alternative solutions to batteries are the subjects of worldwide extended investigations. One possibility is to replace them with energy storage systems featuring larger energy density, e.g., miniaturized fuel cells. A second possibility consists in providing the energy necessary to the device in a wireless mode; this solution, already used for RFID tag, can be extended to more power hungry devices, but it requires dedicated transmission infrastructures. A third possibility is harvesting energy from the ambient by using for example, vibration/ energy, thermal energy, light or RF radiation. For each type of sources, different ambient situation are considered. They correspond to various level of available power, and hence of generated electrical power. Wireless sensor networks are made of large numbers of small, low-cost sensor nodes working in collaboration to collect data and transmit them to a base station via a wireless network. They are finding growing application in body area networks and health monitoring of machine, industrial and civil structures. These networks are intended in many cases to operate for a period of years. Because of the large numbers of devices and of their small size, changing the battery is unpractical or simply not feasible. Increasing the size of the battery to ensure energy autonomy during the lifetime of the system would increase system size and cost beyond what is tolerable. The combination of an energy harvester with a small-sized rechargeable battery (or with another energy storage system like a thin-film rechargeable battery or a super capacitor) is the best approach to enable energy autonomy of the network over the entire lifetime. Super capacitors bridge the gap between batteries and conventional capacitors. This technology is especially suited for applications where a large amount of power is often needed for fractions of a second to several minutes. In the following sections we will focus on emerging methods for power generation through energy harvesting and on power management.
Energy harvesting approaches
Harvesting energy from motion and vibration
Schematic view of a vibration harvester electrostatic energy harvesters
For converting motion or vibration, the established transduction mechanisms are electrostatic, piezoelectric or electromagnetic. In electrostatic transducers, the distance or overlap of two electrodes of a polarized capacitor changes due to the movement or to the vibration of one movable electrode. This motion causes a voltage change across the capacitor and results in a current flow in an external circuit. In piezoelectric transducers, vibrations or movement cause the deformation of a piezoelectric capacitor thereby generating a voltage. In electromagnetic transducers, the relative motion of a magnetic mass with respect to a...
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