China Building Solar Station in Space Could Generate Practically Endless Power

Is that what you mean when you say real estate? That it's not a lot of area on the ground? Or that the space based part is small relative to the sky?

edit: the article makes no mention of this part. The UK proposal mentions that the ground antenna is the size of what was the largest offshore wind farm until 2018:

https://spaceenergyinitiative.org.uk/space-based-solar-power...

The cheaper that solar and wind and batteries get I think the worse this looks.

Maybe for near the poles? Seems like it'll always be comparitvely expensive and niche.

The orbital elements have a trade-off over altitude: too low, and they must pick between not working at night, or being repeater stations for things not in shadow (with exponential losses per hop on repeater, and thermal issues from those losses). Too high, and then not only must each transmitter be bigger to focus on the ground station, but also each station has a view over an increasing fraction of the globe — which is bad because geopolitics, I'll get to specifics later.

Even just for the equator, the lowest you can go without needing to pick between repeater stations and power cuts is a filling an orbit at (sqrt(2) * Earth radius) altitude[0]. But that's for a completely filled orbit, as any given satellite will pass into shadow just as it comes over the horizon — and the reason that only works on the equator, is that the same argument actually means nobody more than 45° from the equator can ever see a satellite, while people just a bit off from the equator will still get dark periods.

So, what's wrong with high altitude? The problem is that the minimum needed to get nighttime coverage means that other parts of the world get many satellites at the same time. Why's that a problem? Because geopolitics means you have to engineer this not with the assumption that everyone's nice and friendly, but that they're potentially actively hostile. Even if these are all, by agreement, built with large ground stations where the maximum power density is low, does the USA trust that China hasn't attached a laser and some Hubble-scale optical elements and given themselves an orbital weapon? Does China trust that the USA didn't? Even in geostationary orbits (so you don't have to worry about someone else's going over your own cities), China putting lasers on a power station like this, means that China has space- and air-supremacy over most of the hemisphere.

But it's worse than that: the ground stations themselves are the other half of the problem. They have to be contiguous, and the smaller the ground station, the bigger the transmitter has to be to focus that well. Halve the radius of the ground station, double the radius of the transmitter. Exact numbers depend on the frequency and altitude, but for sake of examples, the 1978 NASA SPS study required a 1 km diameter geostationary transmitting antenna and a 10 km diameter receiving rectenna for a microwave beam at 2.45 GHz — these are bound by physics, and if you make the receiver 100 meters then the transmitter has to be 100 km diameter, ~7800 km^2.

Also, you've got a size trade-off based on the maximum power density you're willing to have: are these small receiver stations, say the size of a city block and getting a gigawatt or so to power the nearest million or so people? Or are they continental? If the receivers are small, then the power needs to be highly concentrated at transmission time: the same geopolitics that means you have to worry about lasers mounted on the transmitters, now also applies to just redirected microwave energy. If the receivers are big, then you need a large contiguous patch of land where nobody's going to object you tiling with rectennas made of metal that you could otherwise have used for a major power grid upgrade.

If you can get past all the geopolitics and weaponisation options, it's much easier to build a contiguous 1m^2 cross section aluminium hoop around the world. Either in orbit or on the ground. Such a ring would have 1 Ω electrical resistance the long way around, and make it easy to power your winter nighttime from your antipode's summer sun; if it's in orbit, then it doubles as an orbital ring[1] and would massively reduce the cost to orbit.

Right now, China — and only China — is making enough aluminium to do that. Geopolitics is what's holding us back from friendly cooperation to build such a power grid on the ground, not manufacturing limits.

That said, if we do start building a city on Mars, I absolutely wouldn't be surprised if that first city gets beamed power. Global dust storms, less geopolitical issues, etc.

[0] draw a circle for the Earth, parallel lines for the sunlight just touching it, the point of the Earth at solar midnight has direct line of sight at 90° to the edge of the shadow, forming a square; the side length is 2 * Earth radius, the corner of intersection forms a right triangle so sqrt(2) * Earth radius.

[1] https://en.wikipedia.org/wiki/Orbital_ring