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Laser beam Leonard Pieton
Captured by a special camera, a laser beam invisible to the naked eye shoots across the dark expanse of the David Taylor Model Basin at the Naval Surface Warfare Center in Bethesda, Md.

Researchers Transmit Energy Using a Laser

The first long-range, free-space power-beaming device struts its stuff.

Attendees at a three-day tech demonstration at the Naval Surface Warfare Center in Bethesda, Md. got to witness the first long-range, free-space power beaming system. It consists of the two 13-ft-high towers—one a 2-kW laser transmitter, the other a receiver of specially designed photovoltaics. The important part was the laser beaming 400 watts of power across 1,066 ft, but it was invisible to the naked eye.

On one receiving end, the laser beam was being converted to DC power, which an inverter was turning into AC power to run lights, several laptops, and a coffeemaker that Navy organizers were using to make coffee for the attendees.

“Power beaming, the concept, has been around for decades and there’ve been laboratory demonstrations, but this is really a first and a new technology that’s getting fielded,” explained Tom Nugent, chief technology officer of PowerLight Technologies, the hardware provider for the Navy’s Power Transmitted Over Laser (PTROL) project.

The demonstration was two years in the making for PowerLight and Paul Jaffe, an electronics engineer with the U.S. Naval Research Laboratory. During a briefing that preceded the demo, Jaffe had described that day’s demonstration as historic.

Laser transmitter

A 2-kilowatt laser transmitter atop a 13-ft-high tower, part of the long-range, free-space power beaming system. (Credit: Leonard Pieton)

Early power beaming demonstrations took place in 1975: the first in Waltham, Mass. in the Raytheon labs, and the second at the Goldstone Station of NASA’s Deep Space Network in California.

At NRL, Jaffe has been conducting space-based solar energy research for more than a decade, focusing in part on transmitting solar energy from space to Earth. One of the biggest challenges he and others working on the problem have faced is the enormous sizes required for the transmitter and receiver.

“Radio waves have a fairly long wavelength, and to steer them effectively you need a really big antenna,” he explained. “But as the wavelength gets shorter, as it does for infrared light—which is what we're using here today—the transmitter and receiver can be much, much smaller.”

The photovoltaics of the receiver are similar to those of a typical solar panel, Jaffe said, though they are designed to be sensitive to the single color of light of the laser, rather than the broad spectrum of sunlight. They convert that particular wavelength with much greater efficiency than would a regular solar photovoltaic.

Standing beside a monitor showing a live feed from an expensive, highly specialized camera that captured the invisible laser beam as a purple light shooting across the dark expanse of the basin, Jaffe called the power beaming system a remarkable new capability. He said it could unlock all kinds of amazing possibilities for the Department of Defense and the private sector.

Imagine using it to send power to locations that are remote, hard to reach or lack infrastructure, he suggested.

Another potential application of the technology would be powering electric unmanned aerial vehicles (UAVs), whose flight time is currently severely limited by their on-board battery life. The third phase of the PTROL project will involve using power beaming to send power to a flying UAV.

“If you have an electric drone that can fly more than an hour, you're doing pretty well,” Jaffe said. “If we had a way to keep those drones and UAVs flying indefinitely, that would have really far-reaching implications. With power beaming, we have a path toward being able to do that.”

Also present for the demonstration was Eric Follstad with Transformation and Concept Development at U.S. Central Command. He compared the proposed UAV power beaming capability to air-to-air refueling for manned aircraft.

“I think this is just a logical extension of [that] concept,” Follstad said. “Now we can do ground-to-air recharging of some of these electric platforms that we've been flying.”

According to Jaffe, power beaming could transmit power from solar energy-collecting satellites in space to the ground, wherever it’s needed. “If we could capture the boundless sunlight in space, where it’s brighter than anywhere on Earth, we could send it to places that are difficult and expensive to get energy to today,” he said.

The most notable aspect of the demonstration, however, according to Jaffe and Nugent, was the device’s technology safety subsystems. No one in the test facility that day was wearing laser safety goggles or any other safety gear, including the personnel operating the device. To put that in perspective, a typical laser of just 1/2 watt requires protective eyewear.

Nearly all power beaming demonstrations in the past have involved at least the risk of exposure to hazardous power densities, whether optical, radio, or microwave frequencies. The safety of this new system was validated by the Lead Naval Technical Laboratory for Laser Safety (LNTL-LS).

“In this one, the safety subsystems make it effectively impossible for anyone to be exposed to hazardous energy levels,” Jaffe said.

Among the challenges the designers had to grapple with is the effects of snow, rain, and other weather phenomena interfering with the laser beam. But the designers have also given a lot of thought to the prospect of humans or animals crossing through the beam and inadvertently getting a “face full of laser,” as Nugent put it.

To prevent such accidents, the safety subsystem detects objects before they ever reach the laser beam and turn the laser off.

To demonstrate the safety of the laser, a technician would hold a cardboard circle on a stick in front of the receiver’s photovoltaics. Each time he did so, the laser beam would cut off, a fact attendees could confirm by watching the infrared live feed on a nearby monitor.

In the future, researchers at PowerLight plan on increasing the laser’s transmitting wattage, along with the distance the device can send it, and improving the system’s overall efficiency. Nugent said he wants the process of operating it to be as simple as flipping a light switch or plugging in an extension cord.

“You do not need to go through a couple days of training to plug in an extension cord,” Nugent said. “This is a wireless extension cord. So you should not need to go through a whole bunch of training in order to operate it.”

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