[Engineering Feature]
Energy Scavenging Offers Endless Power Possibilities
With proper management, developers can power small systems for a lifetime using energy drawn from the application environment itself.
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.
Excellent article and very fruitful design concepts, which could greatly benefit if complemented with the cornerstone idea of alternative energy revolution: Capacitive Energy storage replacing electrochemical batteries of any types (as it was introduced and then detailed in a Green Electricity (GEL) Initiative, topping Google search for many years (follow the link: www.alexanderbell.us/Initiative/GEL.htm , or just Google on “GEL Initiative” if link is not displayed properly).
Kudos to Rich for his contribution.
Alexander Bell, NY, USA
Alexander Bell -April 13, 2009
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