Instead of using wires to deliver power, the Defense Advanced Research Projects Agency (DARPA), part of the U.S. Department of Defense, wants to wirelessly beam power over hundreds of kilometers. In September, the agency announced that it has chosen three groups to design the aerial relays required for such an endeavor.
Anyone familiar with both laser beams and solar cells might imagine how power beaming might work: A laser can shine its beam at a distant solar array, which can convert that light to electricity.
DARPA’s aim is not to bring electricity to homes but to deliver energy to places where it would prove difficult, expensive, or dangerous to reach with grid infrastructure or fuel or battery shipments.
“Energy underpins every activity we do, including defense,” says Paul Jaffe, an electronics engineer who leads the Persistent Optical Wireless Energy Relay (POWER) power-beaming initiative at DARPA. A wireless energy web “could simplify logistics and fuel supply in an extremely flexible and efficient manner that’s quickly reconfigurable,” Jaffe says.
In this artist’s conception of an energy web platform, power is beamed across multiple aerial platforms to its final destination.DARPA
Now DARPA has chosen three teams to design and develop power-beaming relays: RTX in Arlington, Va; Draper in Cambridge, Mass; and BEAM Co. in Orlando, Fla. The goal for each team is to fire lasers at aerial relays that can then route the beams to their destinations.
The project will employ optical or infrared light, as their relatively short wavelengths only require comparably small relays. These can more easily fit on aerial platforms.
Jaffe says it was too early to specify how the three teams might differ in their approaches. But he notes there are multiple strategies the groups might pursue for the aerial relays to route light to their destinations—reflection, diffraction, refraction, or some combination of those.
In phase one, which the project is now entering, the three teams will develop benchtop demonstrations of the relays. The first phase is expected to last 20 months, with the potential for three extra months to address the riskiest parts of each design.
Phase one will also have the groups develop conceptual designs for aerial platforms that can siphon a fraction of the beamed energy to power themselves. This strategy could lead to smaller, less-expensive future aircraft, since it could dramatically reduce the volume they would need for their engines and fuel. Such aircraft could also harvest energy from the beamed power instead of requiring trips for refueling or recharging, which could result in essentially unlimited range and endurance.
Phase two will integrate the relay technologies into pods carried on conventional aircraft, culminating in low-power airborne tests. In the third and final phase, the goal is for a laser in a ground facility to beam 10 kilowatts of power to a ground receiver horizontally 200 kilometers away using three aerial relays.
“This would demonstrate that we will have a means to deliver energy over very long distances to places that would otherwise be difficult to deliver it to,” Jaffe says.
“Laser power conversion can reach more than 50 percent, and even as high as roughly 75 percent at very low temperatures.” —Paul Jaffe, POWER program leader
Up until now, power beaming has achieved relatively modest demonstrations. For example, the first successful instance of laser power beaming in space, which the U.S. Naval Research Laboratory achieved on board the International Space Station in 2023, only worked over a distance of 1.45 meters.
“Our focus right now is extending power beaming to a distance that is two orders of magnitude greater than demonstrated to date,” Jaffe says.
Power beaming may be a simple concept, but many challenges have long kept it from becoming practical. However, recent technological advances may have brought it closer to reality.
In general, the biggest losses that power beaming experiences “tend to be on the transmitter side,” Jaffe says. However, advances in fiber laser technology in the past decade have led not only to more efficient transmitters but also higher beam quality. “One thing that determines how well a beam stays focused is the beam quality,” he says. The more focused a beam can be, the better it can deliver energy.
Moreover, advances in lidar for autonomous vehicles have led to more efficient photodiodes for converting light to electricity. “Laser power conversion can reach more than 50 percent, and even as high as roughly 75 percent at very low temperatures,” Jaffe says.
In addition, previous strategies for power-beaming networks had each relay receive light, convert it to electricity, and use that electricity to power a laser fired at the next node. These conversion steps proved inefficient. In contrast, the new project aims to use optical strategies to route light from relay to relay, avoiding the losses from conversion.
Furthermore, the aerial nature of the relays enables transmission at high altitudes. These allow longer ranges and greater efficiency than would be achieved beaming power in the thicker and more turbulent lower atmosphere.
Continuously tracking aerial relay positions in order to fire laser beams at them is also not a trivial matter. Still, research into directed energy weapons means scientists can now track uncooperative targets “with pretty high precision,” Jaffe says. In contrast, power beaming involves cooperative receivers, “so the problem is greatly simplified.”
It remains uncertain whether these power-beaming efforts will prove very efficient—the International Space Station power-beaming test displayed an end-to-end efficiency of roughly only 11 percent. Still, the previous head of POWER, U.S. Air Force Col. Paul Calhoun, noted that he used to fly tankers of fuel to destinations, and in the process “burned hundreds of thousands of pounds of fuel to deliver thousands of pounds of fuel,” Jaffe says. “So clearly our existing paradigm is extraordinarily inefficient.”
This research may have military applications and also prove useful for solar-power satellites. These proposed orbital solar farms could harvest sunlight in space and beam it down to Earth in the form of microwaves, avoiding challenges that earthbound solar farms face such as nighttime and weather. Although solar-power satellites are not the goal of POWER, “if successful, our results could well be extendable and applicable to solar-power satellites,” Jaffe says.
