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Comment by h2odragon | Hacker News Reader

dont miss the bit on "remotely beamed microwave power."

"silly idea" it may be but now i want a mega-maser as described for my own sporting purposes.

I'm not trained in STEM at all but these discussions are always a little confusing for me because if our goal is to bootstrap mining/manufacturing in space, why does everyone jump to expensive, complicated and inefficient solutions like "energy lasers" when we've had power lines for like 150 years and can make them on-site much more easily?

it might be a weight constrain?

This question is answered in the article and I guarantee you, most STEM specialities will never discuss this particular problem.

Anyways, the TL;DR is all those cables weigh a lot, so launching them in to space isn’t cheap.

Well, cables are going to be cheaper at some distance and power beaming will be cheaper at a greater distance, and the question is, where's the crossover.

Look at the efficiency numbers on contact free charging pads. "Beaming" fails for power transmission immediately; it needs other factors to justify the efficiency hit; and once you're out of near field its much much worse.

Microwave transmission of power hits 70 or 80% efficiency http://large.stanford.edu/courses/2011/ph240/shu2/

What you're seeing with phones isn't microwave, but inductance, very different technology with very different benefits/drawbacks.

Solar panels are quite useful, and they are essentially just beamed power receivers. And if you focus the beam instead of just using a glowing ball of hydrogen, go with a more efficient wavelength and remove all the atmospheric losses, it actually becomes quite efficient.

> Solar panels are quite useful, and they are essentially just beamed power receivers

Yes, but they are because their power source is unbelievably large and free. If you look at their efficiency from the perspective of what the sun puts out, they're laughable.

Problem is, with the moon beams we don't have infinite and free source power to waste on inefficiency.

> And if you focus the beam instead of just using a glowing ball of hydrogen, go with a more efficient wavelength and remove all the atmospheric losses, it actually becomes quite efficient.

Focusing the beam is not that easy, you need to hit a moving spot of minimal size with an extremely powerful laser. Avoiding all atmospheric losses is probably not going to work, either. Lastly, you need to get those transmitters built on earth, which, as the OP points out, quite land intensive.

It is a theoretical option, but I would not call it efficient.

> Yes, but they are because their power source is unbelievably large and free

Power sources on earth are also plentiful and almost free, compared to building stuff on the moon (at least until we have a lot more infrastructure up there)

The article mentioned why very quickly while discussing building multiple solar farms so that at least some aren't in shadow at the same time: sending enough cable up on rockets to connect all of them would be more expensive than the "expensive, complicated and inefficient solutions".

Its likely because bringing a smelter and wire drawing machine don’t feel “space age”. Also most people think of small bases where its not cost effective to bring a bunch of manufacturing capacity as quite a few processes don’t scale down well. Mining is another issue as you would need to find good sources of each major metal within transportable distance of your base.

With the current capital cost of ~$50/W to beam power, its reasonable to think about for small endeavors, but for a base designed for 100k people making power lines onsite is better, as you will need that manufacturing capacity for other things anyway.

Its probably more realistic to put a few satellites in orbit of the moon and beam power from them. This keeps the distances down so the transmitter and receivers can be more reasonably sized.

If you also switch to laser power beaming, they can illuminate the existing solar farm so a separate receiver is not needed.

I'm totally in favor of any plan that has multiple megawatt SPACE LASERS as part of it, yes.

Wouldn't you just use earthlight and normal solar panels? The internet says moonlight is 1/345th the power of sunlight but once you factor in Earth's larger radius and larger albedo you get 1/9th the power of sunlight. IE you just use 10 times as many solar panels to get you through the night.

> The internet says moonlight is 1/345th the power of sunlight but once you factor in Earth's larger radius and larger albedo you get 1/9th the power of sunlight.

That sounds weird. As per Wikipedia, Sun's apparent magnitude is around -26.7, full Moon's is -12.7, so the Sun is 400000 times brighter than the Moon under the best circumstances. Earth being larger can't correct this by three orders of magnitude.

Even if you are off by orders of magnitude, there may still be a useful thought in here.

Find a large symmetrical crater pointing directly at Earth. Silver its surface with very cheap to transport BoPET (Mylar). Once installed, there's no need to worry about the sheets flying off or dust blowing over since there is no wind. Place a solar / thermal collector at the crater's focal point. Reap power from earth-light.

If you choose a large crater, an MVP system can go live with just a small collector and small portion of the crater silvered. It can subsequently scale as your growing operations demand more power.

It can do double-duty in broadcast communications too.

You won't ever be able to concentrate eartlight enough to start a fire... I doubt that you can get any real power generation from such a system.

We don't need the heat for fire. Maybe for comfort though. The moon gets mighty cold at night. Its temperature range is about 80 to 370 Kelvin.

You could use the same kit to radiate away some of that intense daytime heat too. Spread it across all of Earth.

Now as to solar PV, in principle it might be doable I guess. But I am not able to make the necessary calculations. How large would the crater need to be for 10MW say?

Even as a heat engine, you might get useful power out. If you could achieve say 150 - 200 celsius difference between your concentrator and night time temperature of the surrounding rocks. Which rocks serve as both the engine's heat-sink and a relatively comfy foundation for your habitat.

Finally, how cool it would be for someone on Earth to look at that crater with a telescope. Especially when an astronaut turns up at its focal point.

> moonlight is 1/345th the power of sunlight

I'm pretty sure you are off by several orders of magnitude here as all sources I find list the sun as ~400,000 times brighter than the moon.

https://what-if.xkcd.com/145/

Do they expect to outdo the sun?

In one very specific frequency band, at one comparatively small place, yes. by a large amount. Ocean vs firehose.

Or, better: In the same way every "daytime headlight" you see is outdoing the sun.

Which 'frequency band'? https://www.researchgate.net/figure/Spectral-power-density-o...

microwave

> Achieving a beam that narrow (1/2000000 rad) from the Earth would require a phased array spaced out over a much larger area, at least 200 km at 5 Ghz (6cm).

I doubt they'd want a very large band but in microwave work I dunno what counts as "large".

and i'm a little afraid to go look deeper into this because i'll wind up tearing up junkyard microwave ovens and building something i shouldn't.

The headlight vs. sun comparison (which I think was an edit?) is irrelevant because we're comparing the amount of energy deliverable over distance. The energy delivered by the sun (at a tenable band) vs. the energy delivered by the remote transmitter on earth, after the 'link budget' has been applied (taking into account distance, atmospheric attenuation, and so on) are the two things that need to be compared.

I beg you pardon, im not arguing for the idea of a earth to moon microwave power link as viable. or sensible. I think it's beautifully insane, i have severe doubts about the actual practicality of such a thing, etc.

But thats the thing: now i want to do a mad max maser on a truck with maybe dozens of magnetrons if i could tow a generator...

It fits with my "Orbital Slingshot" project so well, too.

What is your Orbital Slingshot project?

I'm glad you asked :)

It's a totally silly idea to built a big ass slingshot that throws things as high as possible. I figure calling it an "Orbital Slingshot" makes it at least as viable and investment worthy as some of the other efforts [1] and [2] forex.

If I can get my daughter to do some artwork I might fire up a web page for it finally.

[1] https://www.space.com/23015-slingatron-reusable-launch-syste...

[2] https://www.nextbigfuture.com/2018/02/spinlaunch-is-using-la...

Ok, check my math.

basics:

  solar constant: 1.36 kW / m^2
  earth-to-moon range: 400 km

parameters from the article:

  frequency: 5 GHz -> wavelength: 6cm
  earth antenna array linear dimension: 200 km
  transmit power density: 100 W/m^2

Let's assume that the earth antenna array elements are 50m wide, and spaced out such that they cover 1% of the total 200km * 50m area, for a total antenna aperture of 1e5 square meters (10% of the SKA). Combining the stated transmit power density of 100 W/m^2 with the antenna area, we get a total transmit power of 10 MW.

Throw it at Friis:

  power density at moon = transmit power * earth antenna area / (range^2 * wavelength^2)
    = 10e6 watts * 1e5 m^2 / ( 400e3^2 m^2 * 0.06^2 m^2)
    = 1.73 kW / m^2

Atmospheric attenuation at 5 GHz is pretty minimal. If we conservatively assume 20% loss, I think we still end up with a higher power density at a single frequency than from the sun across the entire spectrum.

I don't think anything like this will ever be built, but I don't see why it is impossible. Where's the mistake?

> earth-to-moon range: 400 km

Three zeros seem to be missing.

yikes, thank you

The one that the receiver is the most sensitive to. The fact that sunlight comprises many wavelengths is the reason why single-junction cells have comparatively low efficiency.

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