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The United States’ Defense Advanced Research Projects Agency (DARPA) recently achieved a new record in transmitting energy over distance. In tests performed in New Mexico, the Persistent Optical Wireless Energy Relay (POWER) program team recorded over 800 watts of power delivered for about 30 seconds with a laser beam crossing 8.6 kilometers.
The greatest distance records previously recorded were 230 watts of average power for 25 seconds at 1.7 kilometers, and an undisclosed amount of power at 3.7 km.
The feat wraps up phase one in a three-phase project.
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The estimated receiver efficiency, says POWER team leader Paul Jaffe, is around 20 percent. In other studies, some industrial lasers have recorded a wall-plug efficiency higher than 50 percent. Higher receiver efficiencies are achievable especially with the use of photovoltaic cells optimized for particular wavelengths—whose production is usually costly and time-consuming.
This was not the case here, Jaffe says. The team used commercial, ready-to-use solar cells placed within a receiver. As it takes in the laser beam, the receiver reflects the infrared radiation from a conical mirror onto the photovoltaic cells—which, in turn, turns that beam into usable electricity. At this stage, the goal, Jaffe says, was not efficiency, but speed. “There’s a number of design decisions that were made in the interest of building something quickly, not efficiently,” he says.
And by quickly they mean a timeline of three months between planning and execution, says Raymond Hoheisel, founder of Teravec Technologies, the company that developed the receiver. “The breakthrough was to prove this technology can be affordable,” he says.
While DARPA did not disclose what the total transmission power was, results show the ensuing output energy was of about 800 W. “And we managed to succeed in tests with the receiver running over more extended periods of time [longer than the reported 30 seconds],” Hoheisel says.
Optical vs. Radio Waves for Power Transmission
Many long-distance power-beaming projects focus on radio (or microwave) frequencies—which means using large transmitters to realize a gain in distance travelled. A process called beamforming is essential for such power transmission to work.
Paul Mitcheson, a professor in electrical energy conversion in the Imperial College London’s Control and Power Research Group, describes it like this: In broadcast television or radio, the objective is to propagate the signal as widely as possible so that many people can tune in a given channel. “That’s exactly what you don’t want to do when you’re beaming power,” he says. In this case, the goal is to beam the signal straight into a receiver with as little loss as possible. “So we need a different structure: the antenna needs to have what we call high gain so that it transmits in one specific direction, with a high degree of directionality.” This is the beamforming process that allows our phones to send a signal to a base station instead of broadcasting it everywhere, Mitcheson adds.
But still, there are signal losses. In different ways, infrared laser (or optical) beams have an edge over radio frequencies, says Eric Yeatman, vice-principal and head of the College of Science and Engineering at the University of Glasgow. “Compared to radio, laser is much more focusable—you can create a narrow beam [almost] without any spreading [in ideal conditions],” he says.
But as optical frequencies still scatter with fog and clouds, microwaves are generally superior for atmospheric transmission, Jaffe says. On the other hand, lasers do not require the large antennas that radio does. In a previous test he was part of at the Naval Research Laboratory, Jaffe says they required a 5 meter transmitter and a 2 m receiver to send 1.6 kW across 1 km via microwave frequencies.
Radio wave wavelengths are much longer than infrared, so beamforming is more difficult. “What you want is a sort of column of waves. [In any given transmission], the output diameter needs to be much larger than the wavelength, and this is what determines whether you can focus something or not,” Yeatman says. Infrared’s shorter wavelengths mean creating a focused beam is much easier.
Because of its high precision, long-range reach, and lighter and smaller equipment required to work, laser technologies are more suitable for building an airborne power relay network. “If it doesn’t work with optical, it doesn’t work at all [for DARPA’s goal],” Jaffe says.
“Though the idea of transferring power by laser isn’t new, what they did was an impressive achievement,” says Yeatman.
The record, Jaffe says, came as a surprise, as it was not the team’s objective. And it’s not the project’s only surprise since it began in 2023. The use of diffractive optics was another of them. People usually think of a mirror or a lens when it comes to redirecting laser beams, Jaffe says. “But one of the things we found is that diffractive optics may be very well suited for this, particularly because they are good at efficiently handling monochromatic wavelengths of light. This was something we didn’t know at the outset and that revealed itself as we moved forward,” he says.
Additively manufactured optics with an integrated cooling system were also something that was not on the script when the project set out. The fact they managed to do it, Jaffe says, “revealed new and intriguing ways to tackle some of the problems that are very likely to have applications far beyond what we’re doing for POWER.”
