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The Defense Advanced Research Projects Agency (DARPA), affiliated with the US Department of Defense, announced last September that it had selected 3 teams. To design aerial power relay systems required in this…type of projects.

Light energy in the atmosphere

One can imagine how laser beams interact with solar cells, and how the energy transfer process is carried out. A laser directs its rays into a network of solar cells, whose job it is to convert light into electricity.

DARPA’s project is not aimed at providing electricity to homes, but rather to provide power to places where it is difficult, expensive or dangerous to build it. Because its infrastructure requires the export of fuel or batteries.

In an interview reported by the website of the American magazine “Electrical Engineers”, Paul Jaffe, an electronic engineer who oversees the “Continuous Wireless Optical Energy Transmission” initiative at DARPA, said: “Energy supports everything we do, even in defense operations. A wireless power network can reduce logistics and deliver power in a more flexible, efficient and scalable manner.

DARPA selected three teams to design and build the energy transfer systems: RTX in Arlington, Virginia; “WoDraper” in Cambridge, Massachusetts; and Beamco in Orlando, Florida. The agency set a goal for the three groups: launch laser beams into orbital systems capable of sending these beams to their targets.

The project prefers to use optical or infrared rays, whose relatively short wavelengths require small relays that can be easily installed on aerial platforms.

Aerial platforms

Jaffe says it’s too early to tell how the three teams’ approaches differ. But there are several strategies that teams can adopt in designing atmospheric arrays to send light to its targets, such as reflection, diffraction, refraction, or a combination of the three.

The first phase of the initiative requires teams to develop theoretical designs for air platforms capable of absorbing a portion of the energy released. This strategy will lead to the design of smaller and less expensive aircraft in the future, as the new platforms will help drastically reduce the vehicle’s engine and fuel consumption. Instead of requiring trips to refuel or recharge, this type of vehicle can harvest energy from the emitted energy.

The second step involves integrating relay technologies into cabins carried on conventional aircraft. In the third and final phase, the agency’s goal is to place the laser at a ground facility to send a 10-kilowatt beam toward a horizontal ground receiver 200 kilometers away, using 3 air relays.

“This will demonstrate that we have a way to deliver energy over very long distances to places that are currently difficult to reach,” says Jaffe.

The principle of energy transfer has not achieved much till date. For example, the first successful attempt to send laser energy into space in 2023 led by the US Naval Research Laboratory on the International Space Station only extended a distance of 1.45 meters.

“The current focus will be on expanding energy transfer to an order of magnitude twice that demonstrated to date,” Jaffe points out.

Implementation challenges

The principle of energy transfer may be simple, but the challenges facing its application on the ground are many, but recent technological advances may be close to achieving it.

Jaffe explains that the largest losses encountered in power transmission tests “often occur at the carrier level.” However, the development of laser technology in the last decade has not only contributed to the development of more efficient transmissions, but also to the improvement of broadcast quality. “Only the quality of the broadcast determines its level of concentration. The more focused the broadcast, the better the energy it can deliver,” emphasizes the engineer.

Additionally, advances in lidar technology (a laser device for tracking and measuring distances) used in automated vehicles have contributed to the development of photodiodes that are more effective at converting light into electricity. “Laser energy conversion can be as high as 50 percent, and at low temperatures as high as 75 percent,” explains Jaffe.

In previous techniques, each relay system in power transmission networks received light, converted it to electricity, and then used that electricity to charge a laser and direct it to the next point, but these conversion steps were ineffective. In contrast, the new scheme aims to direct light from one relay to another using optical techniques, avoiding switching losses.

Finally, we still don’t know how effective energy transfer efforts will be, especially since energy transfer testing on the International Space Station showed an efficiency of no more than 11 percent between the nodes of the network.