Laser Lunar Solar Power Satellite

[charlesrwest]

Hello!

I'm a laser newbie, but I'm looking into the feasibility of building a laser solar power satellite to help power stuff on the Moon. I'm trying to figure out if you could build a 200-2000 kg package that would be able to deliver meaningful amounts of power to a target on the Moon, such as an industrial site. The Moon takes a ton of fuel to land on due to the lack of atmosphere and is a pretty horrible operating environment, so if you could keep most of your power systems from needing to land, that would be a pretty big win.

The furthest range I've seen discussed in power beaming literature so far is ~10 km. While you could potentially have a lunar orbit which went that low, it would have problems with being in shadow a lot, requiring power storage and having to have really fast tracking/high power lasers to dump the power to the ground before it was out of sight. This would be less of a problem if you had a bunch of satellites, but most of the point of using lasers in this scenario is to be able to achieve a useful amount of power without sinking in a ridiculous amount of resources first.

An ideal location would be the Earth/Moon L1 point, but that is about ~61,350 km from the Lunar center. That's far enough away that it's probably impractical.

Essentially, the longer the range you can get while maintaining relatively power conversion efficiencies the better a single satellite or small constellation is likely to work out. I've hear that diode lasers have crappy divergence but high efficiencies solid state lasers have crappy efficiency but low divergence.

Do you guys have any suggestions on what sort of laser system would be best for this sort of application and what sort of range you could reasonably expect?

Thanks!

 

[gazer101]

All I know is that your standard laser pointer beam, when pointed at the moon, would diverge to the size of Texas by the time it got there

 

[Sowee7]

All I know is that your standard laser pointer beam, when pointed at the moon, would diverge to the size of Texas by the time it got there

depends on the divergence, some common dpss laser modules have a divergance of up to 0.2mrad!

 

[bostjan]

Hello!

I'm a laser newbie, but I'm looking into the feasibility of building a laser solar power satellite to help power stuff on the Moon. I'm trying to figure out if you could build a 200-2000 kg package that would be able to deliver meaningful amounts of power to a target on the Moon, such as an industrial site. The Moon takes a ton of fuel to land on due to the lack of atmosphere and is a pretty horrible operating environment, so if you could keep most of your power systems from needing to land, that would be a pretty big win.

The furthest range I've seen discussed in power beaming literature so far is ~10 km. While you could potentially have a lunar orbit which went that low, it would have problems with being in shadow a lot, requiring power storage and having to have really fast tracking/high power lasers to dump the power to the ground before it was out of sight. This would be less of a problem if you had a bunch of satellites, but most of the point of using lasers in this scenario is to be able to achieve a useful amount of power without sinking in a ridiculous amount of resources first.

An ideal location would be the Earth/Moon L1 point, but that is about ~61,350 km from the Lunar center. That's far enough away that it's probably impractical.

Essentially, the longer the range you can get while maintaining relatively power conversion efficiencies the better a single satellite or small constellation is likely to work out. I've hear that diode lasers have crappy divergence but high efficiencies solid state lasers have crappy efficiency but low divergence.

Do you guys have any suggestions on what sort of laser system would be best for this sort of application and what sort of range you could reasonably expect?

Thanks!

I'm not sure what sort of efficiency you are expecting. Best case, your laser is 20% efficient at turning electricity into light, and your solar panels are 15% efficient at turning light into electricity, so your transfer would be 3% efficient, at the most.

Then, is there going to be an unobstructed path between your satellite and the surface of the Moon, or is the Earth going to be in the way some large portion of the time?

Whatever solar panels you have shipped to the Moon- couldn't you just point those at the Sun instead, and get more power just as often without an expensive satellite orbiting Earth and being maintained?

Not sure it's a worthwhile venture.

 

[RedCowboy]

Yea, solar panels are the obvious answer, or nuclear batteries for your moon based power needs, but as far as what wavelength laser to traverse air and space for sending power from Earth to moon or from orbit to either you can easily research that, then you have to consider conversion efficiency, operating lifespan and maintenance, possibly waste heat recovery, but rather than a laser how about a maser or microwave system ?

https://www.forbes.com/sites/arielcohen/2021/03/29/space-lasers-the-truth/?sh=3bceb43a6d46

 

[charlesrwest]

I'm not sure what sort of efficiency you are expecting. Best case, your laser is 20% efficient at turning electricity into light, and your solar panels are 15% efficient at turning light into electricity, so your transfer would be 3% efficient, at the most.

Then, is there going to be an unobstructed path between your satellite and the surface of the Moon, or is the Earth going to be in the way some large portion of the time?

Whatever solar panels you have shipped to the Moon- couldn't you just point those at the Sun instead, and get more power just as often without an expensive satellite orbiting Earth and being maintained?

Not sure it's a worthwhile venture.

@bostjan:

Thank you for replying! To address the raised issues:
1. I've heard diode lasers can reach efficiencies of 40-50% while solid state is more like 10-20%. If the divergence can be made small enough, diodes are obviously preferable.
2. If you know you are going to be receiving a particular frequency, tuning the solar cells for it can actually bring the efficiency up to 40-50% for the receiver. This was demonstrated by Lasermotive (now Power light) some years ago in a NASA competition.
3. You will be obscured by the earth sometimes, but how much can be mitigated by choosing a good orbit.
4. The big problem with the "just use solar cell on the lunar surface" is the two week long lunar night. It's really hard to bring/make enough storage to make sure you don't freeze to death.

Also, I just finished reading a NASA proposal from 2019 that delivered 1 watt/kg. Existing solar panel tech in free space can deliver 100+ watts/kg, so your efficiency could be pretty bad and still be competitive with that.

 

[charlesrwest]

Yea, solar panels are the obvious answer, or nuclear batteries for your moon based power needs, but as far as what wavelength laser to traverse air and space for sending power from Earth to moon or from orbit to either you can easily research that, then you have to consider conversion efficiency, operating lifespan and maintenance, possibly waste heat recovery, but rather than a laser how about a maser or microwave system ?

@RedCowboy :
I agree with the solar except for the night issue. There are a very small number of places (high up and on the poles) that get sunlight most of the time. Nuclear could work, but the dev time and political concerns tend to be killer. Might make commercialization difficult.

I agree that microwave power beaming is a better solution, if you solar power satellite is large enough. The big problem with using microwaves is that your focusing is diffraction limited, which means you need an extremely large and synchronized antenna to be able to get any sort of reasonable focus.

The reason I'm looking into using lasers is because the initial power needs aren't going to be that big relative to on Earth. The proposal I mentioned earlier was for 35 kilowatts. This stands in stark contrast to gigawatt scale needs on Earth. The big potential of laser is of potentially mass producing small satellites (200 kg - 1000 kg) instead of needing to make a big one. It makes the amount of money you need to put in before you get some sort of benefit much smaller (though if you are interested in terrestrial uses of microwave solar power satellites, there is some REALLY interesting work coming out of Caltech right now and most of the papers are online).

For me, the big question is what sort of divergence angle is achievable with diode lasers and solid state lasers. The whole thing gets a lot more practical if you can keep you distance from the Moon high (less delta-v required to get there (meaning more satellite mass for the same cost), coverage of more locations and less shadow time).

Thanks!

 

[bostjan]

@bostjan:

Thank you for replying! To address the raised issues:
1. I've heard diode lasers can reach efficiencies of 40-50% while solid state is more like 10-20%. If the divergence can be made small enough, diodes are obviously preferable.
2. If you know you are going to be receiving a particular frequency, tuning the solar cells for it can actually bring the efficiency up to 40-50% for the receiver. This was demonstrated by Lasermotive (now Power light) some years ago in a NASA competition.
3. You will be obscured by the earth sometimes, but how much can be mitigated by choosing a good orbit.
4. The big problem with the "just use solar cell on the lunar surface" is the two week long lunar night. It's really hard to bring/make enough storage to make sure you don't freeze to death.

Also, I just finished reading a NASA proposal from 2019 that delivered 1 watt/kg. Existing solar panel tech in free space can deliver 100+ watts/kg, so your efficiency could be pretty bad and still be competitive with that.

1. Well, you are either going to get the poor divergence of the diode laser or the poor efficiency of the solid state laser. I don't see how you can get the best of both worlds without some sort of major yet-unknown discovery.
2. The 40-50% GaAs PV cells are quite expensive and can easily be destroyed by overexposure or just a little banging them around. I think that might be the part that would require the most work. It's feasible to maybe achieve 50% efficiency at high power in a more robust device, but the other challenges might make it a waste of resources.
3. Not sure how you'd mitigate that effectively. Not to mention, the smallest amount of vibration would make transfer of energy impossible. You're talking about focusing a spot the size of what, maybe a football field over the distance from the Earth to the Moon, something like 6.75 x 10^-14 sr (your divergence would need to be less than 15 microdegrees, which is sort of laughably impossible). Again, it might someday be possible to do, but the size of the satellite with that much stabilization and the cost of pulling off such a feat with today's technology would be kind of silly, even if you could manage the divergence.
4. I agree that standard solar panels would be too error-prone to rely on them for life support on a Moon settlement. But that doesn't take away from the fact that they'd still be far less precarious than relying on a laser that could very easily be obscured by debris, fail due to misalignment, fail due to hardware issues, etc.

Long story short. If you can make this happen with any sort of practical application, with current technology, you'd be eligible for at least one Nobel Prize in Physics for the advancement in laser technology necessary to make it work.

 

[charlesrwest]

Hey @bostjan !

1. Fair, though it looks like there might be some higher efficiency solid state lasers nowadays. I've seen claims of 40%.
2. I didn't know that (thank you!) Apparently the commonly used space solar cells are around 30% efficient, so that's probably the number to use. Upon reflection, it might be difficult to get people to use nonstandard cells anyway.
3. Earth/Moon distance is ~400,000 km. The highest lunar orbit I am looking at is 5,000 km, so the divergence and pointing are a bit easier.
4. You can mitigate most of those risks by having multiple satellites, as long as nothing physically covers up the ground receivers.

Thanks!