Using diamonds to recharge civilian drones in flight

Using diamonds to recharge civilian drones in flight
Credit: Ecole Polytechnique Federale de Lausanne

A small lab-grown diamond measuring a few millimeters per side could one day enable civilian drones to be recharged in mid-flight through a laser. Thanks to the diamond, the laser beam can remain strong enough over a long distance to recharge photovoltaic cells on the drones' surface. This system, which poses no threat to human health, is being developed by EPFL spin-off LakeDiamond. It could also be used to transmit both power and data to satellites and has just been included in the ten projects supported for two years by of the Swiss Space Office.

Drones are being used for a growing number of purposes. Their designs are ever more efficient, and techniques for flying them are being further refined all the time. But drones still have the same weak point: their battery. This is particularly true of propeller drones, which are popular for information-gathering purposes in dangerous or hard-to-reach regions. These drones can fly for only around 15 minutes at a time because their engines quickly burn through their batteries. One way of addressing this limitation – without weighing the drones down – would be to recharge them while aloft using a power beaming : an energy-rich that is guided by a tracking system and shines directly on on the drones' exterior.

Several labs around the world, including in the US, have been working on this idea in recent years. LakeDiamond, an EPFL spin-off based at Innovation Park, has now demonstrated the feasibility of using a high-power laser for this purpose. What's more, LakeDiamond's laser emits a wavelength that cannot damage human skin or eyes – the issue of safety is paramount, since the system is meant for use with civilian drones. LakeDiamond's technology is built around diamonds that are grown in the company's lab and subsequently etched at the atomic level.

World record for power

Despite appearances, standard laser beams are not as straight as they seem: as they travel, they expand ever so slightly, leading to a loss in density as they go. But LakeDiamond's system produces a laser beam with a wavelength of 1.5 µm that, in addition to being safe, can travel much farther without losing strength. "Systems developed by other companies and labs, often for military applications, employ lasers that are more powerful and thus more dangerous for humans," says Pascal Gallo, CEO of LakeDiamond. His company took the opposite tack: their technology transforms the rays emitted by a simple low-power diode into a high-quality laser beam. Their beam has a larger diameter, and its rays remain parallel over a longer distance – in this case up to several hundred meters.

In LakeDiamond's laser, the light produced by a diode is directed at a booster composed of reflective material, an and a small metal plate to absorb the heat. The breakthrough lies not with this set-up, which already exists, but with the fact that the emitted beam is only a few dozen watts strong. The secret is using a small square lab-grown diamond as the optical component, as this delivers unparalleled performance. LakeDiamond's system holds the for continuous operation using a wavelength in the middle of the infrared range – it delivers more than 30 watts in its base configuration. "That's equivalent to around 10,000 laser pointers," adds Gallo.

The lab-grown diamonds' key properties include high transparency and thermal conductivity. Achieving those things – and mastering the nano-etching process – took the researchers over ten years of development. LakeDiamond grows its diamonds through a process of chemical vapor deposition, an approach that ensures their purity and reproducibility. The surfaces of the resulting tiny square diamonds are then sculpted at the nano level using expertise developed in Niels Quack's lab at EPFL. Thanks to their inherent properties and etched shapes, the diamonds are able to transfer heat to a small metal plate that dissipates it, while at the same time reflecting light in such a way as to create a laser beam.

"To achieve greater power – say to recharge a larger drone – these lasers could easily be operated in series," says Nicolas Malpiece, who is in charge of power beaming at LakeDiamond. The company's remote recharging system works in the lab but will require further development and refinement before it's ready for field use. What would happen if a drone flies behind an obstacle and is cut off from its laser energy source? Several approaches to this problem are currently being explored. A small back-up battery could take over temporarily, or, for information-gathering missions over rough terrain for example, the could simply return to within range of the in order to top up its battery.

This energy transmission system is also interesting for other areas of application. It can for example be used for charging and transmitting data to satellites. The development of the system is included in a support program of the Swiss Space Office, which began on 1 November and runs for two years.

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More information: Carving diamonds for optical components: actu.epfl.ch/news/carving-diam … -optical-components/
Citation: Using diamonds to recharge civilian drones in flight (2018, November 7) retrieved 17 July 2019 from https://phys.org/news/2018-11-diamonds-recharge-civilian-drones-flight.html

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Nov 07, 2018
Interesting. I really want to know where they are getting solar cells that will output enough amperage while mounted on the average drone to actually make charging the batteries worthwhile while the motors are pulling multiple amps of power. While I can see them extending the flight time a little bit, even a large drone doesn't have all that big a body for the cells to mount on to supply enough current to actually extend the time much. I guess that's why companies have been working on the idea for years? I can only see this idea working by making the drone super light with very little batteries (or maybe super caps) but with a large surface area for the solar cells. I read an article somewhere about how that had already been done. The problem is the drone had to be so light it could barely even carry a small ic camera and even a light breeze would make it uncontrollable. I wish the company luck though. Maybe they will figure out a way.

Nov 07, 2018
@24volts: I imagine that the cells on the drone would be similar to cells used in concentrated solar applications? The drone would probably have some heat dissipation issues to deal with too.

Orientation of the cells could be a problem too. If they are mounted horizontally, along the underside of the body, then as the drone gets away from the laser location, the beam will be hitting the cells at an increasingly less perpendicular angle. It might be necessary to return closer to the laser to recharge. If they were suspended under the drone like a vertical fin, then the drone could charge at a distance, but would have to keep the cells aimed perpendicularly to the laser. This might not be too great aerodynamically if not just hovering or there's much wind. Or, maybe there could just be multiple lasers to deal with these issues.

Nov 07, 2018
@ C_U - very perceptive comment!

Similar to CPV cells in several ways - able to handle high-intensity light, and stacked junctions in some cases.

Unlike sunlight, on a CPV cell, a laser is typically monochromatic, so stacking is not needed to get multiple band-gaps to efficiently handle multiple colors. However it still helps efficiency by raising voltage and reducing current and cutting resistive losses. The efficiency record for stacked cells converting laser light is getting close to 70%, if I remember correctly.

Yes, heat dissipation needs to be carefully managed, but high conversion efficiency helps quite a bit (less heat as well as more power).
