Geostationary orbit is further away than the most distant part of the Earth. If you have a way to transmit power efficiently over that distance, it’s still going to be cheaper to put the panels on the ground in Nevada, the Sahara, Saudi Arabia, the Gobi, and Australia, and transmit the power from there. (Microwave towers, LEO satellite relays, superconducting wire, whatever).

It’s likely to be really hard to make it efficient to transmit the power, because either it will be microwaves or it will be lasers; lasers can be blocked by clouds, microwaves need an huge antenna at each end (the numbers I’ve seen say that the one on the ground has to be big enough that people will ask why you’re not just using building a PV solar array on the ground instead of the antenna). If the antenna isn’t big enough, transmission losses from beamed power is proportional to distance squared, whereas resistance loss in wires is only proportional to distance (P = I²R, R = distance * some_design_specific_number Ω/m).

And that’s without the geopolitical implications of anyone worrying if the enormous directed microwave/laser source could be weaponised. Just saying “trust us” isn’t going to be good enough.

Why not PV on the ground: because wire antenna is a lot cheaper.

Why people don't have to trust us: because the technology isn't physically capable of maintaining even that low energy density at the target, without the reference signal from the ground.

Why put the PV in space instead of on the ground: because there's 5.4X as much energy available and it's 24/7.

There's real engineering behind this design, detailed in the book and the SPS-Alpha final report to NASA: https://www.nasa.gov/pdf/716070main_Mankins_2011_PhI_SPS_Alp...

I'm not aware of any real engineering studies for things like transmitting power via LEO satellite relays from the Sahara. Personally I don't find it plausible, but if you can provide such a study I'm willing to change my mind.

> Why people don't have to trust us: because the technology isn't physically capable of maintaining even that low energy density at the target, without the reference signal from the ground.

From what I see that’s a design choice not a fundamental aspect of reality, but even if it isn’t: what stops a hostile actor creating their own guide signal, either to direct the power towards an vulnerable location, or to directed away from the receiver, causing wide-scale power outages in the process?

> I'm not aware of any real engineering studies for things like transmitting power via LEO satellite relays from the Sahara. Personally I don't find it plausible, but if you can provide such a study I'm willing to change my mind.

This is because beamed power in general is a bad idea compared to simple wires, however the Sahara is a lot closer than Geostationary orbit and therefore any technology that works for Geostationary orbit will also work for the Sahara, and at guaranteed lower cost regardless of the technology.

Sure, it's the physical design of the device. But that's a consequence of making something cheap from lots of small identical parts that self-assemble. You could make something that didn't require the reference signal, but it'd cost significantly more. And you could always allow inspections if people are really concerned.

Changing the beam target would require a repointing of the satellite in addition to the new guide signal. If you somehow do that militarily, it's still a pretty weak weapon; it'd slightly increase people's body temperature and they'd have to evacuate unless the guide transmitter could be found quickly. It probably wouldn't be hard since it's sending a signal that can be detected 36,000 miles away.

I'd like to see a real engineering cost estimate of transmitting power from one side of the planet to the other, by wires or any other method. I really doubt it can be done for four cents/kWh. And it would certainly introduce a possibility for hostiles to cause wide-scale power outages.

> Changing the beam target would require a repointing of the satellite in addition to the new guide signal.

I can believe either-or, not both. If a guide signal is important, antenna orientation isn’t; if orientation is important, guide signals are not.

> I'd like to see a real engineering cost estimate of transmitting power from one side of the planet to the other, by wires or any other method. I really doubt it can be done for four cents/kWh. And it would certainly introduce a possibility for hostiles to cause wide-scale power outages.

Do I have to? Geostationary is 36,000 km, pole-to-pole is 20,000 km over the surface. All I am claiming here is that any tech you use would also be able to solve my alternative more easily.

The guide signal is what you need for a tight, coherent beam. Direction is up to the satellite, which can choose which of multiple receivers to target.

The advantage of transmitting power from space instead of the other side of the planet is that there's a straight line with no planet in the way. To go around the planet you need relay stations or wires. Plus there's the 5.4X more energy collected by each square meter of panel. So yes, some supporting evidence would be nice. Maybe even take a quick look at the NASA report I provided, too.

> The advantage of transmitting power from space instead of the other side of the planet is that there's a straight line with no planet in the way. To go around the planet you need relay stations or wires.

Which is why I said 20,000 km not 12,000 km. Even if the circuit approximates the Earth as a square and goes up 6,000 km, turns 90 degrees at one relay, goes 12,000 km, makes another 90 turn at a second relay, and then 6,000 km down, that’s still a shorter path (though I’d expect the important distance in that instance to be 12,000 km not 24,000 km because it’s related to the inverse square law between any two antennas not over the whole distance).

> Maybe even take a quick look at the NASA report I provided, too.

I did. Now that I no longer at work I have time for a more detailed (though still brief) look at the contents rather than just skim read.

• That research was an attempt to turn a TRL-1/2 into a TRL-3.

• It is suggesting that — with further work — it could deliver electricity rate of 9 cents per kilowatt hour, whereas the Lazard 2017 price estimate for utility-scale ground mounted PV electricity was about half that (4.3-5.3) and the record (unless it’s been beaten since) is 2.4 cents/kWh.

• What I have read about the Retro Directive Phased Array is that it is it an assistant for good actors, not that it is a safety mechanism. By analogy, it seems that saying it makes this system “safe” is like claiming the passive aerodynamic stability of a 747 will prevent it crashing into a building — not the claim made by the inventor.

RDPA looks to me like a nice improvement to a traditional phased array antenna. Phased arrays can be re-aimed dynamically without physical rotation — which is, I think, why it’s being used in this design.

• The link itself suggests using its own wireless power transmission tech as a substitute for wires (page 50)

• Their own estimate for an initial full-scale (1 GW) is “far term (20-30 years)” (page 65) whereas their “mid- to far-term” market opportunities are the $0.5-$3.0 per kWh range (page 51)

$0.5/kWh is worse than combining batteries (~$0.18/kWh) with 5.4 times as much ground-based PV.

• The construction costs for the 2 GW plant “With aggressive tech advances“ are estimated at $16 per Watt (page 83), compared to current ground mounted costs of $0.103-$0.278 per Watt according to PV EnergyTrend on October 31 last year.

• Check out that risk-impact matrix for the 2 GW “mature” design on page 95

• The ground receiver is estimated to cost $10/m^2 on page 79; Unfortunately page 70 says “Determination of the actual power received will require additional, more detailed analysis“, which makes me wonder what the claimed costs per kilowatt hour even refer to. (Transmission? But there are estimates of beamed power efficiency…)

I’ve not seen estimates for the size of the ground station in that document, I’m going to have to handwave this one and say that the ground station is a 10 km diameter circle (because that’s the value that I’ve seen in other analyses of space-based solar power), which gives 2 GW / (π (5000m^2)) ≈ 25 W/m^2, which is both a bit higher than I’d be comfortable with and simultaneously so low that at $10/m^2 = 2.5 W/$ = $0.4/W, even the ground station alone is significantly more expensive than PV ($0.13/W).

So bear in mind the study was done before SpaceX did much. The $0.09/kWh and $16/watt on page 83 assume $500/kg launch cost. I wouldn't suggest SPS except in a scenario where SpaceX achieves the goals that Musk laid out Saturday night: $50/kg launch and total launch capacity thousands of times higher than the world has now, within the next several years. That kind of capability drastically advances the possible schedule, and drops the cost quite a bit too.

Meanwhile, the comparable cost from solar is the total required for dispatchable power 24/7, not just the cost of PV alone.

Also bear in mind that it involves technology which hasn’t been sufficiently developed yet, technology which would make ground-based solar power cheaper (robotic assembly, beamed power), and that even with the relevant technology the ground station (whose costs are necessarily not impacted by lower launch costs) is more expensive on its own then an equivalent ground PV system including the cost of a battery for nighttime power.

Aren't there some pretty serious health and ecological problems with such large-scale microwave transmission?

Not really. Even at the intended target, it's diffuse enough not to raise body temperature much. Birds could fly through without harm. Under the antenna, there'd be very little microwaves leaking through. Aim anywhere without a homing signal and you lose the coherent beam, making it much more diffuse.

iirc, Novar Conrols/Novar Electronics out of Barberton Ohio (at the time) was testing the phased arrays using bees and their hives and what ever else got in the area. Oh, yeah, and apparently some of the engineers and the boss.

(I used to work in a lab there next to some of the equipment and one day I asked my good friend about it - he had been there for decades.)

Perhaps, but several orders of magnitude less bad than health and ecological problems from sunlight.

How can you get more power from an orders of magnitude less powerful beam than from an orders of magnitude more powerful beam (sunlight) over an equal surface area? And if you can’t, why do it? This blows my mind.

Disclaimer: I do not know the specifics, but I guess neither do you. The gist of it is probably that more energy gets transmitted per surface unit, but in a way that's less dangerous than sunlight.

Firstly, you have 24/365 production, always at peak power. All things equal, you need a lot less capacity for equivalent power generation.

Then, I am fairly certain that the amount of power that can be collected depends a lot on antenna size, just like standard wireless transmission: the antenna needs to be "tuned". And a few hundred meters of steel cable would likely absorb a lot more energy than an 80 L water bag.

And finally (but related), microwaves are likely a lot less dangerous than UV: it isn't ionizing, because individual photons cary a lot less energy. That also means that they penetrate more "cooking" (increasing temperature) more deeply the objects in its path. Instead of getting your skin burnt from the energy, you feel slightly warmed up (and would likely be able to dissipate the small extra heat in the atmosphere). A sizeable portion of this energy would likely end up in the ground, if no antenna is present to collect it.

This technology is interesting, as it offers a lot of new possibilities: it's easy to relocate, for instance. We might also be able to split the beam to power multiple places simultaneously, reducing grid losses (and load balancing at the satellite).