The universe is full of energy, and efforts to harvest that ambient energy are as old as the windmill and sailing ships. The convergence of three exponentially improving technologies, however, is creating striking new opportunities for ambient energy harvesting that can power applications unthinkable only a few years ago. The key to unlocking these opportunities is effectively managing minuscule amounts of power.
Talk about extracting energy from the environment, and most people will imagine large solar panels, geothermal power plants, and giant propellers on towers scattered across the landscape. Such large-scale power-generation opportunities are rare, however, and of no special interest to electronic system developers.
What’s becoming increasingly common—and interesting—are opportunities to scavenge ambient energy on small, almost nano, scales to power electronic devices. While such energy harvesting may or may not involve a battery, it does free a device from both line power and the need for periodic battery replacement, opening a vast range of new applications for portable electronic systems.
Imagine instrumenting a highway bridge with a network of strain gauges to monitor its structural integrity. Wiring such a network, which would require thousands of nodes, would be prohibitively costly and time-consuming.
Using a wireless network protocol such as ZigBee could be cost-effective, but running the nodes on battery power would create a maintenance nightmare in trying to keep all of the nodes powered. If node power could come from the energy in the vibrations generated by traffic, though, it would eliminate the need for battery maintenance and bring the application within practical reach.
As far-fetched as such an installation may seem, emerging technologies make this and many more once impractical applications not only feasible, but delivered. The German company EnOcean, for instance, has placed thousands of wireless light switches in buildings across Europe. The switch sends coded radio messages to turn light fixtures on and off, getting its power solely from the mechanical energy the user provides by pressing the switch. By eliminating the need to wire the switches to the lighting power, adopters have realized substantial savings in both wiring cost and installation time.
The growing opportunity for developing such “zero power” applications stems from exponential trends in three separate technologies. First, each new generation of microcontrollers can accomplish more and more for less and less power. Second, wireless networking is evolving radios and protocols that carry increasing amounts of information at decreasing power levels. Finally, the ability to capture and utilize minute amounts of power by various means has expanded dramatically. This harvesting ability has now surpassed the falling power demands for many small systems, opening the door to myriad possibilities.
THREE FREE ENERGY TYPES
An energy harvesting system has two key elements: electricity-producing energy converters, and power-management blocks that condition and sometimes store the electrical power for application use.
Energy converters can utilize radiant, mechanical, or thermal energy as their source to produce electrical currents and voltages. Converters for each energy type are now available, with more in active development. According to industry analysts IDTechEx, more than 200 organizations in 22 countries are actively involved in energy-harvesting development.
The silicon-based photovoltaic (PV) cell is by far the most well-known and widely available energy converter, harvesting radiant energy in the form of ambient visible light. The PV generator is moving beyond this traditional, crystalline “solar cell,” though. In fact, development is under way at companies such as Konarka and Sony to create organic and dye-sensitized PV cells that can harvest ultraviolet or infrared light.
In addition, companies like AIST Tsukuba Japan are creating flexible, transparent PV films to convert light to power. Intel Research Seattle Labs recently demonstrated still another type of radiant energy converter, the WARP (wireless ambient radio power). It was able to gather 60 mW from the RF transmissions of a television tower 4 km away.
Mechanical energy converters use electromagnetic (EM) or piezoelectric (PZE) effects to turn movement into electricity. The EnOcean ECO 100 converter, for example, uses the linear movement of a switch throw to move a magnet through a field coil and generate a current burst. Similarly, the AdaptivEnergy AdaptivTouch switch uses ruggedized laminated piezo (RLP) technology to generate a burst of power from a finger press.
Cyclic movement such as vibration is even more popular as a source of mechanical energy. Many energy converters work with such motions. The Ferro Solutions VEH-360, for instance, uses EM generation techniques to harvest the energy of 60-Hz vibrations in motors and similar machinery.
On the PZE front, several companies use proof masses and moment arms in resonant configurations to convert wide- or narrow-band vibration to PZE-generated electricity. AdaptivEnergy’s Joule Thief, the Volture system from Mide Technology, and the Perpetuum PMG series all harvest the vibrations in their installed environment to generate electricity for wireless sensor and other applications (Fig. 1).
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