Powering drones with a laser beam, infinite air time?

[charles.ouweland]

According to this article: https://www.scmp.com/news/china/science/article/3205885/chinese-scientists-develop-laser-powered-drone-stay-aloft-forever they have developed a drone that can be powered by a laser beam and can stay in the air forever.

According to the article, no data were released about the efficiency of this application. It aims at military applications, so I guess the energy efficiency is not the first of their concerns.

It reminds me of the solutions for wireless charging of mobile phones at a distance. They have to closely track the position of the drone to be able to direct the beam exactly at the drone. Hmm... interesting

[jonpaul]

laser strong enough to power a flying drone will have 100s watts, useful as a weapon.  Power supply and dériver will occupe à truck.

Rain or for or clouds will block it

Efficiency 0.00001%

j

[Stray Electron]

   Unless your aircraft is just a very tiny unmanned model aircraft it would require kilowatts, or ten kilowatts, or hundreds kilowatts of laser power at the receiving end to power it. Not only would the laser transmitters require 10x or more power, but there is also no photocells in existence that could convert that much laser power into electricity.  And Lasers of MUCH less power are already a hazard to aircraft and larger lasers would also also be a hazard to satellites and space craft.  And you'd also have to be concerned about lasers reflecting from clouds and aircraft and affecting people back on the ground.

   This whole idea is a non-starter from the git-go.

   In the 1960s they were flouting the same idea but using microwaves instead.  AFIK that idea never went anywhere either.

   Aircraft that have an infinite flight time are already in existence and are already being flown. They use solar cells during the daylight hours to power their engines, and to charge batteries to that they can continue to fly even during darkness.  And let's not forget those old fashioned things called balloons!

[Stray Electron]

  Speaking from first hand experience,  lasers of even tens of watts of output power have a very bad tendency to burn up their mirrors, lens and windows. Even the tiniest speck of dust or any imperfection becomes a hot spot in the lens that in turn gathers MORE energy and the cycle grows until the lens is destroyed. That process operated literally at the speed of light so it only takes nanoseconds to occur.  The mirrors, windows and lens used for very high power lasers are effectively one shot items and certainly couldn't be used to continuously supply large amounts of energy to an aircraft for hours on end.

  There is a HUGE difference between being able to transmit very short, megawatt level laser pulses and being able to continuous transmit even tens of watts of power.

[AlbertL]

   In the 1960s they were flouting the same idea but using microwaves instead.  AFIK that idea never went anywhere either.

See: "History and Status of Beamed Power Technology and Applications at 2.45 Gigahertz"
https://ntrs.nasa.gov/api/citations/19900000834/downloads/19900000834.pdf

[mzzj]

  Speaking from first hand experience,  lasers of even tens of watts of output power have a very bad tendency to burn up their mirrors, lens and windows. Even the tiniest speck of dust or any imperfection becomes a hot spot in the lens that in turn gathers MORE energy and the cycle grows until the lens is destroyed. That process operated literally at the speed of light so it only takes nanoseconds to occur.  The mirrors, windows and lens used for very high power lasers are effectively one shot items and certainly couldn't be used to continuously supply large amounts of energy to an aircraft for hours on end.

  There is a HUGE difference between being able to transmit very short, megawatt level laser pulses and being able to continuous transmit even tens of watts of power.

If you keep the laser beam size rather large or use multiple parallel beams the power density should be low enough to keep your optics from frying.

laser strong enough to power a flying drone will have 100s watts, useful as a weapon.  Power supply and dériver will occupe à truck.

Rain or for or clouds will block it

Efficiency 0.00001%

j

Laser diodes have developed quite a bit lately, nowadays you can build battery-powered "hand-held" 100W laser for few hundred bucks. Efficiency is also reasonably good for the laser, around 30-40%
1kW 455nm laser system (with power source) would be possible to build in man-portable format.

[nali]

Using lasers to loft stuff into the sky has been around longer than you may think... from New Scientist 2009
OK, granted, not quite a the same as a drone but sill using laser as the power source:

https://www.newscientist.com/article/dn18122-space-elevator-wins-900000-nasa-prize/

A laser-powered robotic climber has won $900,000 in a competition designed to spur technology for a future elevator to space.

Building a space elevator would require anchoring a cable on the ground near Earth’s equator and deploying the other end thousands of kilometres into space. The centrifugal force due to Earth’s spin would keep the cable taut so that a robot could climb it and release payloads into orbit.

Though building a space elevator might require an initial investment of billions of dollars, proponents say once constructed, it would make for cheaper trips into space than is possible using rockets. But huge technological hurdles must first be overcome, including how to supply power to the robotic climber.

To that end, NASA offered $2 million in prize money in a competition called the Power Beaming Challenge, in which robotic climbers, powered wirelessly from the ground, attempt to ascend a cable as fast as possible.

Now, a robotic climber has made a prize-winning ascent worth $900,000, making it the first to win money in the competition, which has occurred annually since 2005.

[Siwastaja]

Efficiency 0.00001%

Quite pessimistic. Given that consumer grade laser diodes are ~30% efficient and solar cells ~20% efficient, and specialized products (like solar cells working at narrow wavelength) are much better than this.

This only leaves the problem of automatic aiming. But this is only work of endless engineering. You can totally mount the solar cell to an arm with two degrees of freedom and servo the position and direction of the cell so that it's always perpendicular to the beam.  Beam can be aimed using computer vision. Aiming can be also fed back from the drone by wireless communication back to the base station.

This only leaves the power loss caused by dispersion and any dust/rain etc. Something around 5-10% would still be well possible, if well engineered, using endless resources and possibly military technology.

[mikerj]

Efficiency 0.00001%

Quite pessimistic. Given that consumer grade laser diodes are ~30% efficient and solar cells ~20% efficient, and specialized products (like solar cells working at narrow wavelength) are much better than this.

And the losses though absorption and scattering caused by gas molecules and aerosols, atmospheric turbulence, thermal blooming etc?

[Siwastaja]

Efficiency 0.00001%

Quite pessimistic. Given that consumer grade laser diodes are ~30% efficient and solar cells ~20% efficient, and specialized products (like solar cells working at narrow wavelength) are much better than this.

And the losses though absorption and scattering caused by gas molecules and aerosols, atmospheric turbulence, thermal blooming etc?

We all know that during good weather, loss of light in typical drone distances is some single-digit percents, maybe just barely double-digit. Otherwise we could not see the drone properly, it would appear darker or fuzzy. If you can see it, you totally can power it.

I don't know enough to say what happens when you significantly exceed the power density of sunlight (1000W/m^2), i.e., thermal blooming. Does it become non-negligible at 10kW/m^2? 100kW/m^2? 1MW/m^2?

During colossally bad weather, the problem with absorption and scattering in water droplets is obvious. On the other hand, it is not set in stone that a drone that can only fly during good weather is useless. Quite the opposite, most small commercial aircraft is only allowed to fly during so-called visual flying conditions, and yet we do have those airplanes. Whenever visual flying is possible, powering a drone is, too. Line-of-sight is necessary, but I think that is obvious.

Then it's matter of if we have really good weather (say, >90% transfer medium (air) efficiency), or marginal weather (maybe 10% efficiency).

Saying that efficiency is 0.000001% unconditionally, is just bollocks. Maybe that would be the worst case during a rainstorm.

[mikerj]

Efficiency 0.00001%

Quite pessimistic. Given that consumer grade laser diodes are ~30% efficient and solar cells ~20% efficient, and specialized products (like solar cells working at narrow wavelength) are much better than this.

And the losses though absorption and scattering caused by gas molecules and aerosols, atmospheric turbulence, thermal blooming etc?

We all know that during good weather, loss of light in typical drone distances is some single-digit percents, maybe just barely double-digit. Otherwise we could not see the drone properly, it would appear darker or fuzzy. If you can see it, you totally can power it.

I don't know enough to say what happens when you significantly exceed the power density of sunlight (1000W/m^2), i.e., thermal blooming. Does it become non-negligible at 10kW/m^2? 100kW/m^2? 1MW/m^2?

During colossally bad weather, the problem with absorption and scattering in water droplets is obvious. On the other hand, it is not set in stone that a drone that can only fly during good weather is useless.

The effects of thermal blooming vary greatly with wind speed, aerosols and distance; the cleaner the air, the faster the wind and the closer the target the less the effect.  Above a few km it's less of a problem, but for a ground based power source it could be pretty significant.

As an example of the kind of efficiencies you could get have a look at this paper: https://jwcn-eurasipjournals.springeropen.com/articles/10.1186/s13638-020-01874-3