Technology update: Alternative power sources for instrumentation
Energy scavenging or harvesting methods are becoming more mainstream for low-power instrumentation. Can you use them? At Sensors Expo 2009, energy harvesting took the limelight, using vibration, light, heat, or even radio frequency. Batteries and power management remain important.
Peter Welander, process industries editor
Each year at the Sensors Expo, some technology seems to take the limelight. In recent years it’s been wireless, but in 2009 it was energy harvesting. You probably know what this is about if you have been considering adding wireless instrumentation to your plant. Devices that have no wires to carry data probably have no wires to carry power either, so they depend on self-contained sources. The number of options is growing.
Batteries are an obvious choice, and in many situations they still represent a simple low-cost solution with a proven track record. Non-recharging lithium thionyl chloride battery chemistries can operate for 5, 10, or even 20 years when sized appropriately for the application. Companies such as Tadiran and Xeno Energy offer wide selections of industrial batteries using this long-life technology.
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Even if batteries will last until after you retire, they do go dead eventually. So some users want other technologies to capture the energy available in typical plant environments. Some energy sources are natural, like wind and sun. Others depend on working equipment, like heat, vibration, and even radio waves. Energy harvesting technologies, such as wind and photovoltaic cells, are available in large versions of course. But, for this discussion, we’re talking about very small amounts of power, typically on the order of a few milliwatts, and in some cases, microwatts.
Vibration driven sensors are becoming more common and are available from companies like AdaptivEnergy , Perpetuum , Cerametrics , Cymbet , and KCF Technologies . This is a growing field that includes multiple sub-technologies. Some generators need to operate at a specific vibration frequency, such as 50 or 60 Hz. Others can operate at any frequency. Since output depends on specific conditions, this approach will likely require experimentation. One appealing element of vibration is that it is a continuous supply, as long as the source device is running.
Tiny wind turbines can also deliver power, but like their larger counterparts, they only work when the wind is blowing. Most of these small units need very little wind to operate, but they will probably need protection from the elements. Cymbet and KCF Technologies both offer units.
Cymbet EnerChip EH Solar Energy Harvesting module is the heart of the new Texas Instruments eZ430-RF2500-SEH Wireless Demo Kit. This kit is the world's first complete autonomous Energy Harvesting wireless sensor development tool, Cymbet says.
Photovoltaic cells are made in all shapes and sizes, including tiny units that are around 1 sq. in. These don’t always need to be in direct sunlight but can operate off of artificial sources as well. Given the variety of options, you can get just about any combination of voltage and current you need. Cymbet and KCF Technologies both offer miniature photovoltaic units.
Heat energy scavenging is still developing, but there were units shown by Cymbet and KCF Technologies . One of the difficulties with this approach is creating a large enough temperature differential within the device to be effective. KCF’s unit has a small fan to help dissipate heat, so the generator has to power that as well as the driven device.
Powercast is sending power to devices via RF transmissions, allowing the sensor to collect energy through an antenna. (The company has managed to capture energy from ambient cell phone transmissions, but this typically isn’t enough to be useful.) One 3-watt transmitter can send power to a number of devices, although the distance is limited. The greater the distance, the more the usable wattage falls off. This can be improved using directional antennas, but only so far.
All of these technologies can generate power, but their usefulness depends on careful energy management. The companies listed have all developed hardware and protocols to manage power as well as collect it. This is especially important for those methods that are intermittent by nature. Well-thought-out systems typically incorporate some storage mechanism using capacitors or batteries to accumulate unused power and release it when there is a spike in demand. Another method uses a traditional battery as a backup for interruptions in the primary source.
One mechanism for energy storage is new thin-film batteries that use lithium ion technology. Infinite Power Solutions showed a range, with sizes varying from smaller than postage stamps to half of a business card. A typical unit 1 in. square and 7 mils thick can put out 4 V with 0.5 to 1.0 mAh.
These methods combined with very energy-stingy devices makes for high efficiency and deployment flexibility. Still, there is little in the way of standardization, so expect that you’ll have to do some experimentation.
Also see: Sensors Expo photo gallery: Olympic torch, power harvesting , wireless demo.
—Peter Welander, process industries editor, PWelander@cfemedia.com ,
Control Engineering Process Instrumentation & Sensors Monthly
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