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Energy harvesting finds spot in sensor nets

Posted: 05 Dec 2007 ?? ?Print Version ?Bookmark and Share

Keywords:energy harvesting? sensor? wireless nets?

At least two energy harvesting companies are reporting sharp increases in sales to customers who are finding all types of ordinary uses for power harvesting devices in advance of widespread deployment of wireless sensor networks.

Energy harvesting for wireless sensor networks enable remote nodes to run unattended for years. They work by storing enough charge from an environmental transducer to periodically power-up, take a reading and transmit the reading. The market remains small, but is predicted to grow at a rate of 65 percentexceeding 200 million units by 2010, according to Darnell Group Inc.

"Sales of our energy harvesting modules has increased by 78 percent," said Bob Chao, CEO of Advanced Linear Devices Inc. "The demand for energy harvesting modules is coming from a wide range of industries."

One of the most active areas for energy harvesting is replacing light switches with wireless versions that don't need batteries. For instance, the new 57-story Torre Espacio skyscrapper in Madrid uses 4,200 wireless buttons to control its 4,500 lights, thereby enabling all interior walls to be moveable without rewiring any light switches.

Architects used wireless buttons from EnOcean GmbH that transduce the motion of pushing them into an RF signal that turns on a light. All building lights were wired into a network that supplies power and control signals to turn the them on and off. Wireless receivers accepting the coded signals from wireless buttons then route the control signal to the correct light.

'Ocean of energy'
"Our main competitor today is the wire," said Jim O'Callaghan, VP of sales and marketing at EnOcean. "There is an ocean of energy to be harvested, but the challenge is to create devices that work as well as battery-powered devices, minus the battery."

Many companies have tried to create wireless networks to control simple electrical devices such as turning lights on and off, but they required batteries in the button. Battery-powered buttons also present a quandry to users whenever a light's button didn't work: Users couldn't tell whether it was a burned out bulb or a dead battery.

Enocean was spun off from Siemens to solve the "dead-battery problem" with buttons that harvest the energy of pushing them. Pushing a wireless button sends a "start" signal; releasing it sends an "end" signal, enabling their routers to set a dimmer by measuring the time between pushing and releasing.

"The force of merely pushing a button can produce enough RF energy to travel about 100ft through walls," said O'Callaghan. "We design our switches to last at least 20 years in the field without maintenance."

Enocean also has a line of other energy harvesters that transduce motion, sound, vibrations, temperature, ambient light and other energy sources. Enocean designs its buttons to work for five days at a time between recharges. The company is also branching out into heating, ventilation and air-conditioning systems that use wireless controllers to manage fans and ducts.

So far, Enocean has about 60 OEMs, each of which ships switches, routers and other modules to Enocean for interoperability certification. The company is also trying to create an open standard from its proprietary system.

"We are currently forming an alliance," said O'Callaghan. "We are setting a standard which will be ready next year."

Nanoamp draw
For the emerging wireless sensor network market, EnOcean is currently developing its own SoC that it hopes will rival other energy harvesting companies like Advanced Linear Devices. ALD's energy harvesting modules use proprietary, zero-threshold MOSFETs hat turn on at 0.01V instead of the typical 0.7V, resulting in circuitry that consumes only nanoamps of power to work.

"We are moving to a system-on-chip to lower energy consumption to 60nA," said O'Callaghan. "We also want to lower our prices. We are aiming at four dollars per module."

- R. Colin Johnson
EE Times

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