February 3, 2011 -- Going wireless in applications such as sensor nodes or PC peripherals requires not only wireless communication but also an alternative energy source to an ac wall plug. It makes little sense to eliminate wired communication if your design still requires a power cord. Harvesting the ambient energy, which can be solar or artificial lighting or mechanical energy, such as equipment vibration, provides an energy source for ultra-low-power devices. Devices such as wireless sensor nodes conform to their ultra-low-power budget by spending most of their time asleep, waking up briefly to take a sensor reading and transmit it in a burst to their node network.

However, the burst of power necessary for communication, although brief, requires more energy than ambient sources can provide, so some form of energy reservoir must provide this short, intense burst, making energy storage the other side of the coin for energy harvesting. Capacitors, including supercapacitors, and batteries are the main forms of electrical energy storage for energy harvesting. Both device types have constraints that energy-harvesting circuits for wireless sensor networks must accommodate to make wireless nodes practical. Those constraints are high leakage current for supercapacitors and charge/ discharge-cycle lifetimes for batteries. New lithium-ion technologies promise to expand the scope of both supercapacitors and batteries.

By Margery Conner, EDN Magazine Technical Editor.