[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.
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.
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
Your Comments:
Enter the text from the image below
Please refresh the page if you have trouble reading this text.
Search Electronic Design
Email Newsletter
Sponsored By:
The Find Power Products monthly newsletter brings you the most important new developments within the world of power design. The newsletter includes exerpts from industry leader Sam Davis's exclusive blog, as well as overviews of the latest new products.
Enter Email to Subscribe
Web Seminar
Sponsored By:
Title: Exploring How Good GUIs Drive Adoption in the Digital Power Management Space
Reprints
