Using common radio waves for power, sensor networks may soon
be able to eliminate the need for ac sources and batteries.
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technology, introduced late last year, enables sensors and microcontrollers to
draw small amounts of electrical current from nearby radio frequency (RF)
transmitters, and then use the power to operate autonomously for years.
Developers of the technology say it could offer significant advantages for
hotels, hospitals, schools and office buildings, as well as for users of
wearable medical and sports devices.
looking at a range of applications, including building automation, HVAC control
and small defense systems," says Harry Ostaffe, vice president of marketing and
business development for Powercast Corp.
maker of the new RF energy harvesting technology. "A lot of potential users
have great interest in being able to deploy sensor systems that don't need to
Powercast's RF technology joins
new energy harvesting power sources
- including solar, thermal and
vibration -- that are drawing interest from makers of electronic building
control systems and manufacturers of mobile electronic products. Trane
for example, recently employed Powercast's Powerharvester
in a hospital HVAC system, where they used it to send power to controllers that
"watch" the temperature, humidity, oxygen and CO2
individual rooms. By employing the Powerharvester, Trane engineers say they
were able to cut power consumption, reduce labor costs and eliminate the need
to send technicians into hospital rooms.
to be able to get the temperature or humidity information from a room where
there might be three or four patients, and be less invasive and not interrupt
medical procedures," says Scott Repsher, a controls project engineer for Trane.
"This way, we can do it without ripping down walls, running conduit or drilling
that by employing Powercaster RF transmitters in the hospital hallways near the
rooms. The 3W transmitters broadcast RF signals at 915 MHz (a frequency
commonly used in cordless phones and baby monitors) from the hallways to
antennas in the rooms. Powerharvester wireless receivers in the rooms then convert
the RF signals to dc current, storing the current in capacitors for use by a
wireless sensor system. The sensor system, which includes a PIC24F XLP
from Microchip Technology
, uses the current from the Powerharvester to collect data from the sensors.
It then sends the data out over a 2.4-GHz wireless signal to the hospital's
building management system.
system enables users such as Trane to work without batteries or ac power. For
those who are willing to use batteries, Powercast also offers a separate Powerharvester
that enables users to store onboard energy.
engineers, who are still evaluating the technology, say they like its potential
to eliminate batteries. "Batteries lead to service life issues," Repsher says.
"If you look at it from a service point of view, do you really want to send techs
out in the field and have them change 300 batteries?" Bright Future
Powercast engineers say that one of the keys to the
technology's success is the availability of low-power microcontrollers and
transceivers to run the sensor applications. That's why the company teamed with
Microchip late last year to roll out a specialized development
. The kit includes a wireless sensor module with low-power microcontroller
and transceiver, an RF transmitter, evaluation and development boards, and a
components are critical for industrial and consumer applications because the
amount of electrical current available from RF transmitters is miniscule,
especially as they move farther away from the application. Engineers say it's
not unusual for such energy-harvesting systems to deliver microwatts of power. For
that reason, the two companies' development kit includes a microcontroller that
consumes as little as 8 Ķ in its least power-hungry run mode, and as little as
20 nA in sleep mode.
potential applications for the technology are multiplying fast. Powercast says
it's talking to defense contractors, server farms, energy providers, and even
structural engineers for applications on bridges and buildings. They also
believe the technology could be used in medical monitoring bracelets and
support applications where they use rechargeable batteries or where they don't
want to use batteries at all," Ostaffe says. "In some applications, we can
potentially eliminate the batteries by using RF."
experts say that the best strategy for such applications is still to back them
up with a battery. "My recommendation to anyone who's doing (energy) scavenging
is to throw in a battery," says Kris Pister, chief technology officer of Dust Networks
and a professor of
electrical engineering and computer science at the University of
California-Berkeley. "That way, if you're scavenging goes away, you've still
got the battery to give you years of lifetime. And if you've still got your
scavenging, then you're not draining the battery."
that some lithium thionyl chloride coin-sized cells
offer extremely low leakage, making them strong candidates for energy-scavenging
applications. One manufacturer of the technology, Tadiran Batteries
, reports that its
batteries have self-discharge levels of less than 1 percent per year, and
claims that its batteries routinely last more than 25 years.
or without batteries, however, the future of energy scavenging in wireless
sensor networks is believed to be bright. "Low-power wireless sensors are going
to be everywhere," Pister says. "There's hardly an industry that wouldn't
benefit from more sensor data. And having energy harvesting to power them makes
a lot of sense."