> What’s the bullish story for why solar panels in space with microwave beaming is better than solar panels on earth?

Space-based solar power (SBSP) has three advantages over terrestrial solar: flexibility, footprint and theoretical efficiency.

The classic case for flexibility is disaster zones; it's easier to set up receiving meshes than build new power plants. The problem is humanitarian crises don't pay. The real sell is military and mining. Beaming power to remote operations simplifies logistics.

Footprint is minor, but worth mentioning: the receivers for space-based solar are smaller and could be laid over e.g. farmland.

Last, but not least, the atmosphere absorbs a lot of solar energy. In this model, the space-based rig is a lens, converting the well-absorbed frequencies that generate power into microwaves to which dry air is more transparent. If you can get a focussed beam, a SBSP rig could also be placed far enough out that it's less subject to Earth's day-night cycles. These factors boost its theoretical efficiency.

I haven't run the numbers for several years. But they only become compelling if you have ISRU and in situ manufacturing of panels. I don't think SBSP is a beachhead market for asteroid mining. But it's feasible if we have it.

> The problem is humanitarian crises don't pay.

That’s only a problem if profit/exploitation is more important than humanity.

IMHO, space based technology needs to be public owned (not corporate) and not allowed to profit, with its goals and designs fit to benefit humanity before profit.

One other important note is that is possible to put solar arrays in orbit that are always facing the sun, so they can deliver power 24 hours a day.

You'd need them to be in geosynchronous orbits unless you plan on powering the planet instead of the United States. And orbits that always face the sun are polar orbits, so you'd need a polar geosynchronous orbit. This would likely mean that the distance from the ground stations is way too large for meaningful power transfer. It would also require the whole installation to be somewhat movable. to correct its orbit as it slips over time because the installation is so large that different parts of it will be in slightly different orbits!

Quick quibbles:

1. Geostationary orbits are in 24/7 sunlight except for a short period each day near the equinoxes

2. Geostationary polar orbits do not exist. Geostationary orbits are, by definition, 0° inclined. A polar geosynchronous orbit, like you mentioned, would not offer much besides being over the same point on the Earth at the same instant every day.

> A polar geosynchronous orbit, like you mentioned, would not offer much besides being over the same point on the Earth at the same instant every day.

It would offer a lot: the ability to send down an absolutely massive amount of power without having the corridor shift around so that it can be reliably routed around by air traffic. Having the equivalent of a microwave scythe moving across a large area would be kind of counterproductive.

Which is why a geostationary orbit would be much more feasible. Constant solar exposure with a minimal ground-track footprint that can be reliably pointed at the same ground station 24/7.

In fact, if we're already beaming power (assuming this works reliably enough) it might make sense to have a set of birds "surfing the terminator" in a polar orbits that can then transmit to geostationary birds which in turn transmit to ground stations.

It makes for good science fiction. But the footprint of the spot on the receiving end would be such that most of the energy would be lost. Even at an opening angle of only 10 degrees (which would be pretty good) from LEO you'd be looking at a 50 km spot size.

A satellite could be illuminated over 99% of the time and be in Earth's shadow a maximum of only 72 minutes per night at the spring and fall equinoxes at local midnight.[44] Orbiting satellites can be exposed to a consistently high degree of solar radiation, generally for 24 hours per day, whereas earth surface solar panels currently collect power for an average of 29% of the day.[45]

That is the big potential with solar satellites, 24 hour a day power.

https://en.m.wikipedia.org/wiki/Space-based_solar_power#:~:t....

Two reasons. 1. A microwave receiver on earth is much smaller than many solar farms, so we don't use a lot of our finite land for energy generation 2. This could actually bring solar to some geographies which aren't well suited to solar due to weather

To achieve the energy transition, you need SO MANY solar panels and they will take up a lot of land. That land is most expensive near the very population centers which need the most power.

According to Jesse Jenkins, to power America, "the solar farms are an area the size of Connecticut, Rhode Island and Massachusetts."[1] (this isn't the most precise source I could find, but it's reputable)

[1] https://www.nytimes.com/2022/09/20/podcasts/transcript-ezra-...

No, it's bs. Because of the double conversion you're never going to outperform solar here on earth and the cost of deployment undoes any advantage you might have had anyway. But: cool project and nice of them to get it bankrolled.

> To achieve the energy transition, you need SO MANY solar panels and they will take up a lot of land.

This isn't true.

> According to Jesse Jenkins, to power America, "the solar farms are an area the size of Connecticut, Rhode Island and Massachusetts.

The estimates are that it's anywhere from 10000 to 30000 square miles which is compared to the 3.8 million square miles of the continental USA an absolute pittance. Doing this by tightbeaming the output of an equivalent solar installation in space is ridiculous.

Agriculture is 44% of US land, PV on an area <1% is enough to power America. And this 1% can (and should) be deployed in deserts

Not to mention we can make dual use of existing land in the form of roof top solar.

The roof of every single new big box store should be covered in solar panels and we should start covering large parking lots in them too. Shaded parking that powers the area would be sweet.

Come to South Africa where the government is unable to provide reliable electricity to the country. Almost all new developments come with rooftop solar, and quite a few large parking lots in my hometown got covered with solar arrays for shade.

> Agriculture is 44% of US land, PV on an area <1% is enough to power America

There is even a potential convergence: https://en.wikipedia.org/wiki/Agrivoltaics

Sure, on a per square foot basis, but the requirements and limitations are decidedly different. I wouldn't dismiss it out of hand, at least not on a 10-30 year time frame.

Not to mention you get a 30% boost in solar insolation by snagging the light before it goes through the atmosphere, plus you don't have the resistive losses due to heat building up in a land-based solar cell.

And how do you propose to move that power losslessly across your downlink? Those losses will likely be far larger than the 30% gain.

Getting rid of waste heat in space is harder than it is on earth!

> you need SO MANY solar panels and they will take up a lot of land

If we used the area that is currently producing ethanol and "biodiesel" for solar panels we would be 80% done with the energy transition.

If we used that and the land for cattle feed, maybe we could power space with our microwave transmitters.

Unfortunately #1 isn't correct. The minimum size of a ground station is fixed by the size of the transmitter, the wavelength, and the distance.

One standard design had a 10km contiguous(!) diameter receiver for a 1km diameter transmitter in GEO. You can lower the transmitter to about 6371 km[0] before having to worry about night, but that's only going to give you a factor of about 6 improvement. Different wavelengths can do better antenna gain, but start to need to care about atmospheric absorption. You could in principle also relay the power around the same sort of way we do for data, though that way you get a percentage loss from each relay (might still be worth it, might not, I've not tried to cost it).

You're also limited by the maximum safe power density; from what I've heard this is surprisingly close to what you'd get from the yearly average output of PV, which on the one hand means that although it can be an alternative to storage when the question is "what about night and winter?" it is fairly unhelpful when the question is "can we use less land"?

> That land is most expensive near the very population centers which need the most power.

Yes, but wire-based transmission is better than most give it credit for. And even if it was that bad, if you perfectly solved wireless transmission by any mechanism you like, it's a shorter route from the ground on the opposite side of the planet, to low orbit, around the planet, and back down again (~20e6m), than from geostationary orbit (~36e6m).

> According to Jesse Jenkins, to power America, "the solar farms are an area the size of Connecticut, Rhode Island and Massachusetts."[1] (this isn't the most precise source I could find, but it's reputable)

Sounds about right: https://www.wolframalpha.com/input?i=%28Connecticut%2C+Rhode... (20% efficient cells, 10% capacity factor).

Those are 2.9e11, 6.3e10, 5.5e11 W, compared to recent US use of 4.6e11 W.

But they're also fairly small states, with 1%, 0.3%, and 2% of the US total population: https://www.wolframalpha.com/input?i=%28Connecticut%2C+Rhode...

[0] If I've done the maths right:

1. The radius of the inscribed circle of an equilateral triangle is r = (sqrt(3)/6)*a

2. The altitude of the triangle from any side is h = (sqrt(3)/2)*a = 3*r

3. So the closest you can get the satellites while being sure at you never have a period where the only visible satellites are themselves not in shadow is:

triangle_altitude - diameter_earth = 3*r - 2*r = r

> [0] If I've done the maths right:

I now think my assumption is wrong, that the closest you can get without worrying about night is minimal bounding square, not a minimal bounding triangle, so the orbital altitude should be:

(sqrt(2) * radius) - radius = (sqrt(2) - 1) * radius ~= 2639 km

so a factor of about 14 better than GEO.

I haven’t done the math or really thought about it all that much but one reason could be that space has a lot of, well, space. You can build a huge array without wasting land that could be used for something else, condense the power down to a beam and send it to earth.

The power it can provide is much more reliable than power provided by solar panels on earth, so it's more economically valuable than power by solar panels on earth. It would provide power at night and if the right microwave transmit frequencies are used, even if it's raining on the receiver. The loss through a thunderstorm isn't that bad.

It's competitive for locations where the alternative is flying in diesel for generators. Some Northern locations are like that, although most have land or sea access on a seasonal basis. But the military flies a lot of diesel / kerosene.

Haven't you played SimCity 2000?

There isn't. It's just another excuse to justify spending lots of money on things that make no sense.

Space Solar is really coming from a DARPA initiative to build microwave beams from space as a weapon system to intercept missiles and other targets. The idea of selling solar power from space is another contrived commercial dual-use application to justify the technology development. A similar history was followed for Starlink which is not commercially viable without its basis in furthering SDI technology.

https://en.wikipedia.org/wiki/Starlink#Military_capabilities

Starlink is separate and viable without Starshield. They are independent programs: https://www.space.com/spacex-starshield-space-force-contract

Starlink performance trends have shown it is not viable alone.

https://en.wikipedia.org/wiki/Starlink#Subscribers

nothing about that screams "not viable" to me.

Those charts show each of the newest satellites buys ~400 paying subscribers (per additional satellite). Build, launch, operating for one satellite--while low for SpaceX--is too expensive for that to make sense.

If they try to do more than 400 users per satellite, the service throughout plummets.

I get 500 customers/satellite (2,300,000/4,500) since 2021, and 1000 new customers per satellite for Q3 2023. Using the lower number of 500 at $120/month, that is $7,200 per customer, or $3.6 Million per satellite over a 5 year lifetime. 4600 operational satellites (out of 5700) over 134 launches equals 34/ launch or $124 million revenue. or $250 million/launch if we want to use thee 1000 users/satellite number.

Space X sells commercial falcon 9 launches for $ 67 million (to GTO opposed LEO) and this already includes a profit margin.

It still isn't obvious to me.

This math is completely wrong with even a trivial napkin check.

They announced 2 million users with fewer than 4k sats in orbit (presumably even fewer serving).

Let’s assume 2 mil by 4k for simple math, which yields 500 users per sat.

However, the vast majority of the sats are not where the bulk of their customer base is (US and Europe) at any given moment. That means that the 5% of sats over user dense areas at any given point in time are capable of carrying 100% of the current load.

Stated differently, they have sats capable of serving the US user density sitting over every other country in the latitude bands of those orbits just waiting for licensing. I.e. User count could 10x on the current constellation if the growth is outside of the existing markets.

A trend of a network being so popular the user growth outpaces capacity?

> A similar history was followed for Starlink which is not commercially viable without its basis in furthering SDI technology.

This is complete nonsense.

"calling bullshit" is a reasonable act and we all should be able to wield that power.

That said I often wish there was an effective way to communicate whether one calls bullshit because of gut feeling or not convinced by the readily available evidence and/or priors, or whether one calls bullshit because they know more details and are just deflecting actual well known bullshit but for some reason preferred to omit the details (e.g. because they value terseness).

Terseness is beautiful, but not when it actually hinders communication

A person can use Google for 5 minutes to realize that a company does not launch 5000 satellites (surpassing all other launcher companies on Earth combined by mass launched in the process) as part of some kinda US research subsidy.

Of course, if a person is already poisoned by twitter discourse to the point where they substitute their own reality just to keep hating Musk, then all hope is lost...

Yeah I agree, as a layperson with the ability to google 5 minutes and have my own priors I can reach the same conclusion.

What I'm saying OTOH is that I have no way to know whether you have some more insightful things to say about this since you just called bullshit without any further details. I hoped you had more details

We agree actually. SpaceX is not doing this for some research project, or for a commercial internet service. It's a military project-- see the Wikipedia link.

I'm not agreeing or disagreeing, I'm just making a meta-point about trying to better qualify one's assertions so that a reader can better judge the information content of the statement.

Apparently you didn't read the link.

in the near term (the next three orders of magnitude of human energy usage growth) it probably isn't practical

rectenna arrays might have been a reasonable idea when solar panels were expensive. they're still reasonable if you're willing to use a microwave beam that's concentrated enough to be a devastating weapon of war. sunlight is 1000 watts per square meter, and that's already enough to heat up a tin roof or the interior of a car to temperatures that will burn or kill. if you boost that by a factor of 10 or 100, you could get a higher-density "power plant" on the ground that's cheaper than the equivalent solar farm

but you still have to build the solar farm, you just build it in space. right now that's much more expensive, and, as jacquesm points out, rejecting waste heat in space is more difficult. there are a few possible ways that building it in space could be better:

1. if robots are mining asteroids for minerals, they can build solar panels in space that don't have to survive earth weather, or for that matter launch or reentry. plausibly they could be under a micron thick, thus using thousands of times less material than earth solar panels. conceivably you could even do this with terrestrial factories if you have cheap enough and gentle enough launch, which you don't, but maybe with bfr you will

2. if barbarians are mobbing your castle with pitchforks and torches, banning borax and methyl ethyl ketone, coercing you into swearing loyalty oaths, or threatening to put your family in ovens because of your ethnicity, you might prefer to have your solar panels somewhere they can't reach

3. to make the transition from a kardashev type 1 civilization to a kardashev type 2 civilization, obviously covering the entire earth with solar panels is insufficient, so you have to put them somewhere else

4. you can put the solar panels much closer to the sun than the earth is, so they produce much more power per unit area

obviously none of these are relevant right now, but sooner or later they will be

Your arguments are good, but waste heat rejection is not a problem for space solar power. The PV arrays double as their own radiators. I mean, this is how satellite PV arrays already do it. You do need radiators for the microwave transmitters, but these are spread out as well.

Now, if you wanted to build a nuclear power plant in space and beam its power, then heat dissipation would be a big problem.

thanks!

i think this depends in part on how high a temperature you can tolerate; at 20° the stefan–boltzmann emissivity is only 420 watts per square meter for an ideal black body, a third of the above-the-atmosphere solar irradiance. the blackbody equilibrium stefan-boltzmann temperature for 1300 watts per square meter is 116°, which is a bit toasty for efficient solar panels; even at half that (if you're emitting infrared from the back of the pv array as well as the front) you're at 54°. and if you want to move your power plant closer to the sun to harvest that sweet, sweet radiation, the temperature goes up further

so plausibly some heatpipes painted black sticking far out the backside would be helpful

Or, just go with PV that tolerates higher temperature better. GaAs PV cells are used in some satellites for this reason, I think. Here on Earth, CdTe is used in place of Si in some hotter environments for this reason as well. CdTe has the advantage of being extremely thin (the absorption length of photons in the material is something like 0.1 microns) which could be an advantage in space, where the PV arrays could be deployed as extremely thin sheets, much thinner than modules on the ground that have to stand up to wind, rain, and gravity.

i was thinking carborundum, diamond, or materials that aren't even semiconductors at room temperature, like zirconia. i don't think cadmium telluride will survive sunlight at the orbital radius of mercury, but plainly some solids do

Microwave receiving antenna arrays aren't solid like solar panels on earth - they would essentially be a big grid of wire loops suspended above ground over maybe a square kilometre or so. While you probably wouldn't want to live underneath this, plants would grow and animals would not be harmed wandering in and out underneath it.

So the benefit would be that substantially larger surface areas of renewables could be deployed with fewer maintenance concerns (i.e. thin antennas don't really care about dust, and present little cross section to being struck by debris).

Only half of our 500M square kilometre planet faces The Sun at any one time, and only half the surface (barren land and, more likely, oceans) could be used for solar panels. That gives us 125km2 of usable area.

The atmosphere attenuates our 1300W/m2 of solar energy down to 300W/m2 so for a 125M km2 planet you only get 40PW of available power, or 5PW after 80% loss in solar panels and distribution.

In terms of global energy demand we’re currently using 25PWh per year which is the equivalent of 3TW. Assuming oil runs out, politics annuls nuclear options, and we cover most of the planet in solar panels then peak terrestrial solar is three orders of magnitude away.

Covering over everything with panels would presumably have devastating effects on the environment just as much as fossil fuels but if we limit to 1% coverage then we’re only one order of magnitude away from terrestrial solar being unable to power the world. It’s neither sunny nor midday all the time either so solar is starting to look dangerously close to not cutting it, and yet harvesting solar energy could be our only option for power for the next ten thousand years.

I pulled all these numbers from Wikipedia and crunched them with GNU units — this is HN after all — but it doesn’t sound unreasonable to conclude that anything planet bound is going to be bounded by planet size. Moreover, if the goal is for power generation to have zero impact on our biome then we should probably not generate power within the biome itself.

On that scale simply using power on earth will have a devastating impact on the environment and microwaving more power in makes it even worse.

Yes and this goes into some awkward Thanos did nothing wrong territory. I’m sure there’s serious research out there on what the fundamental population limit of Earth should be if we are to bound it based on minimal ecological impact.

"Malthus" is taught in high school. The idea comes up all over intellectual history and has yet to pan out. One of these days.

What's taught in high school is generally simplified to the point of falsehood.

What Malthus wrote about was basically the lived experiences of those around him for basically all of what he knew of to be recorded history: improved farming meant more people, not happier people.

Now, I'm not qualified to even judge if Jared Diamond's even fair let alone correct, but he wrote in Collapse (2005) that the Rwandan Genocide was brought about in part due to excessive population pressures, that it "illustrates a case where Malthus's worst-case scenario does seem to have been right."

On the other hand, it also seems plausible to me that widespread access to contraceptives may break the assumptions behind Malthus.

May? All developed countries have subreplacement fertility [1] (and falling) the world population is predicted to peak before the end of the century.

https://en.m.wikipedia.org/wiki/Sub-replacement_fertility

For now, yes; there is also a distinct possibility that the sub-populations who have more kids do so because of some genetic tendency (singular or plural) which would, if it actually exists and isn't mere hypothetical, rapidly reverse this.

There probably is such research, but "minimum ecological impact" is highly subjective and technology dependent.

We had some impact as hunter-gatherers at 10M population, probably made some species extinct from hunting. If I assumed c. 1940 tech was the limit and wanted to minimise human ecological impact, this population would be the maximum sustainable given CO2 emissions.

Conversely, if we put submarine cities in deep water, powered by PV on the surface that are ready to withdraw when storms come[0] and feed ourselves on the most efficient foods we already know how to grow in sealed environments, we could probably support trillions of people… but at that level we might possibly need to find some asteroids rich in, IDK, phosphorus or something.

I have seen larger maximum population numbers, but they're for scenarios where you fairly strongly disregard the ecology — if you're willing to do weird stuff like putting up an L1 sun shade to block wavelengths of sunlight that serve only to heat us up because they get fully absorbed by the atmosphere, but somewhere in the 10-100 trillion range our direct heat emissions become dominant.

[0] ignore the expense, this is a thought experiment

> I’m sure there’s serious research out there on what the fundamental population limit of Earth should be if we are to bound it based on minimal ecological impact.

There is but there is a 7/8ths probability that you're not going to like the answer.

https://en.wikipedia.org/wiki/Sustainable_population

>Moreover, if the goal is for power generation to have zero impact on our biome

I don't know if it is. The impacts could be positive, or at least counteract other negative impacts.

For instance, shading some ocean might counteract the heating of it.

Oceans are where co2 is turned into oxygen. We really dont want to be deliberately hampering that cycle. Put the panels on land, preferably atop sand or bare rock.

Primary production is mostly concentrated in the costal regions. The following map's colour bands are on a log scale: https://upload.wikimedia.org/wikipedia/commons/4/44/Seawifs_...

There's other reasons not to put PV in the middle of the oceans, like the storms, and that if you're willing to make suitable transmission lines of that kind of distance, you can also perfectly well connect the Nevada and Namib deserts and put panels in each so the time zones are powering each other after sunset/before sunrise).

But would kill light dependent life and increase some bacterial and fungi populations that might not be healthy for animals.

Does beaming power to earth change the energy in the system? At least with solar all energy was already being absorbed by the earth's atmosphere, just being converted to another form and ultimately back to heat. By beaming energy that would not otherwise have passed through the atmosphere we are effectively warming the planet. Though I've no idea if the number is material or not.

> Only half of our 500M square kilometre planet faces The Sun at any one time, and only half the surface (barren land and, more likely, oceans) could be used for solar panels. That gives us 125km2 of usable area.

You're off by a factor of 1,000.

By a factor of a million in my typo of “125M km2”, right? The available solar power calculation is still correct, I believe.

Indeed. As for the available solar power insolence is about 1000 W / square meter at solar noon with panels aimed at the sun, depending on tracking you either have zero, one or two axis that are going to be 'off' and then there is panel efficiency to account for, so in practice a 100 square meter area of panels will provide out of the 50 KW available only about 17 KW peak, and usually less than that.

> The atmosphere attenuates our 1300W/m2 of solar energy down to 300W/m2 so for a 125M km2 planet you only get 40PW of available power, or 5PW after 80% loss in solar panels and distribution.

I think you've got a mistake and a bad assumption here, too. I think the atmosphere attenuates by not to 300W/m^2, but you might also mean that the earth being spherical rather than flat geometrically reduces the solar irradiation to an annual average of 340W/m^2? But then you'd be double-counting the effect of night, and ignoring weather. The currently installed PV systems produce about 10% of their nameplate capacity, so I tend to just use that in my approximations of all the different effects together, so a 20% efficient cell[0] would produce 1kW/m^2 * 10% (capacity factor) * 20% (efficiency) gets 20 W/m^2 in practice.

1% of land with those percentages gets us 29.79 TW[1] which is indeed only about 1 order of magnitude away from current use (and we definitely want to use more energy than we do as most people don't have the luxury of energy abundance found in developed nations), but that already accounts for it being "neither sunny nor midday all the time either".

But also, 1% of Earth's land area is already built up[2], so I think this may be too tight a constraint (after all, PV can be used as a surface covering for buildings and even vehicles).

With sufficient political will (yes yes, I know that's wishful thinking), it's not unreasonable to build a thick enough set of conductors to make even a global power grid have negligible resistance — you'd need a 1m^2 cross section for an equatorial ring of aluminium[3] to have 1Ω resistance, and while this would be a big project today, it's not so large as to be absurd for a decade-long project.

[0] The record for efficiency is 47.6%, though this will only matter if we ever ran out of land: https://en.wikipedia.org/wiki/Solar-cell_efficiency

[1] https://www.wolframalpha.com/input?i=land+area+on+earth+*+1%...

[2] https://ourworldindata.org/land-use

[3] Don't put 20 TW through a single ring. If I did the maths right, the magnetic field at the surface will be in the order of 1 tesla depending on how you choose to distribute current and voltage.

Also, I have no idea how much energy such a thing would absorb from a CME, but I think the big CMEs can be in the range of 5e25 J, and if that's efficiently absorbed it would vaporise even a ring that big.

Apparently, the main reason there is interest in this is because China is doing it too.

It makes the power available 24/7, regardless of night and season. It also enables dispatch of the power to regions with higher demand, in ways that would be difficult with terrestrial transmission lines (for example, between northern and southern hemispheres.)

The downside is cost and scale. Launching enough mass to power the world would also approach pollution limits for injection of water into the stratosphere and mesosphere.

I have imagined it might be useable at small scale in specialized applications first, if the power is beamed by laser. This requires improvement in laser efficiency and cost, but it could be very nice to power long distance aircraft (at altitude above clouds) this way. The challenges are considerable, of course.

Large scale use of power in space would not need transmission of that power, so could be an easier step. Latency insensitive servers there? The heat needs to be dissipated.