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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Waste heat can also be a source of power for electronic devices by leveraging the Seebeck effect to produce an electrical current from thermal energy flowing through the generator. Micropelt has developed technology that can generate as much as 15 mW with a temperature differential across the generator as low as 10 Kelvin. Its evaluation products include forms for bolting into engine blocks, immersion in fluids, and clamping around ducts and pipes to extract heat energy and generate a continuous, regulated power flow.
EnOcean also has a thermal energy-harvesting kit, with a generator capable of producing power from only a few Kelvins of differential. Start-up Thermo-Life Energy has a thin-film thermopile technology that can glean as much as 30 µW at 3.3 V from a 5-K differential and 135 µW at 5 V with 10 K. Several applications targeted by the company aim to power devices from human body heat.
MANAGING MICRO POWER
Each of these thermal, mechanical, and radiant energy converter approaches to generating system power comes with significant design challenges. Thus, the need arises for the second key element of an energy-harvesting system: the power-management block. The details of this block can vary widely, depending on the type of energy converter in use.
PV cells generate a relatively low voltage (<0.6 V) with current proportional to incident light intensity (Fig. 2). As a result, the power-management block must be able to operate with its source at the half-volt level. Cells can easily be wired in parallel to increase current. However, wiring in series to increase voltage is problematic.
PZE cells present the opposite problem. Their current output is relatively small, and the voltage varies with the amount of strain placed on the PZE element. The raw output can range from a few volts to more than a thousand, forcing a need for protective circuitry in the power-management block.
Also, they deliver the energy in bursts rather than as a steady flow. Furthermore, vibration PZE converters can produce voltages of either polarity—forcing a need for rectification. EM converters are similar in that they additionally require rectification, but they can be designed to limit their output voltage.
Thermal energy converters have fixed electrical impedance and generate current proportional to the temperature differential across them. As a result, their output voltage is low and varies, but not as wide-ranging as PZE converters. Although they don’t reverse polarity in typical installations, the possibility exists and should be accommodated in the power block.
Power-management blocks share a need to convert whatever electrical signal the converter produces to a steady voltage that the application electronics can use—typically 1.3 to 5 V. Power management must also be able to handle the uncertain nature of harvested energy. PV cells, for example, may become shaded or experience full darkness, cutting off the power flow.
Similarly, vibrational and linear mechanical converters only generate power when movement is occurring, and that movement may slow or cease under various circumstances. In the case of systems like the ActivTouch switch, power generation events are certain to be few and far between. Even thermal converters depend on a temperature differential that may not always be present, like when the engine or process for a thermally powered sensor monitor has been shut down for a while.
FORMING AN ENERGY RESERVOIR
In many cases, energy-harvesting system designs will address this power uncertainty by incorporating an energy storage element of some kind in the power-management circuitry. In fact, energy storage is essential for dealing with PZE converters in applications that don’t have strictly periodic movement.
The ability to store harvested energy opens several possibilities for system design options. The system could use stored energy to support a controlled shutdown when power is lost rather than simply stop operating. With enough stored energy, the system could continue operating normally for sustained periods even in the absence of input energy.
Energy storage also helps address what can be a chicken-or-egg dilemma with some types of energy conversion. In such instances, the energy generator’s raw output voltage is too low to drive the power-management circuitry.
A single solar cell, for example, produces barely enough voltage even at maximum output to reach the threshold level on most transistors. Similarly, thermal and PZE converters in some applications may not have enough input energy to create the required drive voltages.
Having energy storage as part of the circuitry, however, allows the power-management circuit to use its own output voltage as its power source. All that’s needed is enough stored energy in the output stage to get things started. Then, the circuit can use a portion of the converted energy to maintain its operation.
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The most common use for energy storage, though, is as a reservoir for powering low duty-cycle activities that require more instantaneous power than the energy converter is able to supply. In fact, the entire application can be run in cycles. This enables the energy converter and power-management circuit to collect energy for long periods of time before running a cycle of the application.
As long as the continual losses due to leakage, power-management circuit operation, and standby application power draw combined are lower than the average energy-harvesting rate, cyclic applications can be successfully powered by extremely small energy sources. Just reduce the duty cycle until the average power demand drops below the net harvest.
Storage in energy-harvesting systems can use supercapacitors for smaller storage needs but are increasingly turning to batteries for greater capacity. Given that one of the promising advantages of energy harvesting is freedom from batteries—with their associated bulk, environmental hazard, and replacement issues—using a battery for energy storage may seem counterproductive.
However, an emerging generation of ultra-small rechargeable batteries can remain installed for decades if not the entire life of the system, restoring that promise of freedom. Many of these new-generation batteries are solid-state, thin-film lithium cells comparable in size to a large IC package. Now available, these thin-film batteries provide many design options in terms of storage capacity, output voltage, and size.
Cymbet’s EnerChip CBC3150 is a 9- by 9-mm, 3.3-V battery with 50 µA-H of capacity, and its CBC012 provides 12 µA-H at 3.8 V in a 5- by 5-mm package. Solicore provides the larger 26- by 29-mm Flexion battery with 3-V output and up to 14 mA-H of capacity. Similarly, Infinite Power Solutions is offering its Thinergy batteries, which are just entering the product shipment phase. More introductions are likely to be forthcoming from companies that are now licensing thin-film battery technology developed at Oak Ridge Micro Energy.
LOW-VOLTAGE OPTIONS EXPAND
Along with these expanding options for energy storage, there’s considerable industry activity to address other power issues in energy-harvesting applications. The chicken-and-egg problem, for example, is being solved by newly arriving power-conditioning circuits for reduced supply voltage operation.
Freescale Semiconductor recently announced an ultra-low-voltage dc-dc converter that can operate with a supply voltage as low as 320 mV, allowing the converter to draw its power directly from a single solar cell and operate without startup assistance from stored energy. Also, Advanced Linear Devices is leveraging its EPAD (electrically programmable analog device) transistor technology to develop an energy-harvesting power-management module targeting operation with less than 100 mV (Fig. 3). The company expects to have a production version available later this year.
Meanwhile, power demands on the application side continue to drop. The latest-generation Texas Instruments MSP430 microcontroller consumes a mere 160 µA/MHz when operating.
To highlight the resulting energy-harvesting possibilities, TI packaged the MSP430 along with its CC2500 RF transceivers and a Cymbet EnerChip battery in a development kit targeting solarpowered wireless sensor applications (see the opening photo).
The system can operate at even indoor light levels, transmitting more than 400 messages even in total darkness if the battery is fully charged.
INGENUITY STILL REQUIRED
The key elements are thus in place for an explosion of applications powered by harvested energy. But turning the potential of energy harvesting into practical reality still requires considerable design ingenuity.
TI’s MSP430 product marketing engineer Adrian Valenzuela points out that applications developers must be energy-aware in their design approach. He indicates that fully understanding and leveraging the various low-power modes offered by a processor is essential to keeping application power draw at a minimum.
Valenzuela also recommends that developers manage their RF transmissions carefully, noting that RF transmissions are orders of magnitude more power-hungry than the processing. Designers can minimize the energy lost during wireless protocol synchronization and handshaking, for instance, by collecting multiple data packets for transmission in one long bundle rather than sending them individually. Similarly, data compression can help keep total energy usage down, with the energy used in the CPU for processing more than offset by the savings due to reduced transmit time.
Developers also should be aware of, and weed out, small voltage and energy losses that would be negligible in more conventionally powered systems, says Valenzuela. Fractional voltage drops in board traces and package leads can represent significant fractions of the total harvested energy available to the system, so high levels of integration can be important in making design choices. Similarly, impedance matching between the power source and application for efficient power transfer becomes critical.
AdaptivEnergy’s CEO Jim Vogley recommends that developers stop thinking about current draw in their circuits and start thinking in terms of joules consumed. He points out that in most energyharvesting applications, there isn’t enough power available at any given time to drive the application electronics continually. Instead, energy must be collected over time and released in bursts. Thus, he continues, designers should evaluate standby needs, average power needs, and peak current draw. This will ensure that the energy harvester, power management, and energy storage elements will meet application demands.
It’s a new approach to design, but fortunately there are more and more opportunities for developers to learn. Companies such as the Darnell Group and IDTechEx have created conferences specifically for energy-harvesting topics: Darnell’s nanoPower Forum, May 18-20, 2009, in San Jose, Calif., and IDTechEx’s Energy Harvesting and Storage Conference, June 3-4, 2009, in Cambridge, the U.K. On the more academic side, the Center for Energy Harvesting Materials and Systems (CEHMS) holds an annual workshop at Virginia Polytechnic Institute and State University (VPISU) in Blacksburg, Va.
Successfully applied energy harvesting makes very real the prospect of small electronics systems such as wireless sensors that are self-powered, maintenance-free, and virtually unrestricted in their placement. With careful power management and energyefficient design, developers can now effectively address applications that were totally impractical only a few years ago. And this is just the beginning, as reducing power needs and increasing harvesting options perpetually broaden the range of possibilities.