Since the first waterwheel and windmill, people have been “harvesting” energy from one source and applying it to another. Today, the sources of the harvest have exploded to reap extremely tiny amounts of electrical power from ambient sources like the sun, wind, tides, and of course, hydropower that currently produces more than 15% of the world’s electricity.
In general, the goals of energy harvesting are to create large amounts of power to reduce reliance on fossil fuels, or to enable small devices such as wearables and the sensors in IoT networks—someday perhaps eliminating batteries in these devices. Among the diverse array of technologies employed in energy harvesting, capacitors and current-sense amplifiers often play an outsized role in the long-term life of the system.
Capacitors, for example, are core components of photovoltaic and wind-power generation, where they’re used in inverters that convert dc to ac electricity, and to store power for moments of months depending on the application. In wind-power systems, the trend is toward direct-drive systems that eliminate gearboxes by connecting the rotor directly to the generator. This approach not only eliminates a large, heavy, expensive, and frequently troublesome component, but reduces maintenance as well.
Turning the Tide in Capacitors
Although electrolytic capacitors were first used in wind-power applications, film and polymer-type capacitors are replacing them, as they combine small size with high efficiency and longer operating lives, and in some cases the ability to “self-heal” to repair dielectric faults (Fig. 1). Ceramic types don’t need self-healing since they’re inherently robust, but for metallized film, tantalum capacitors provide that capability.