[AR] Re: space based solar
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Wed, 20 Mar 2019 13:07:52 -0400 (EDT)
On Wed, 20 Mar 2019, Keith Henson wrote:
...You can redirect the light before it reaches the array, you can
redirect the power beam after it leaves the array, or you can play
orbit games to reduce the range of motion required...
Most of the current designs, including mine, redirect the light to
deal with the once a day rotation. The exception is Ian Cash's
distributed design which rotates the microwave power beam.
I was rather taken with his previous design, which used the third option:
put the powersat in a Molniya orbit instead of GSO, and now the power beam
is more or less perpendicular to the plane of the Sun's motion around the
sky(*), and you can put a no-moving-parts sandwich powersat at a 45deg
angle to both the beam and the sunlight. (In GSO the beam is roughly
in-plane, so there is no simple equivalent.) But the Molniya orbit isn't
geostationary, alas, which adds its own headaches.
(* Visualizing such problems and their solutions often becomes *much*
easier if you choose your coordinate frame carefully to simplify the
problem. The right frame is often a rotating frame; there is nothing
wrong with that, despite such frames often being considered vaguely
disreputable... )
Even if you don't have rotation, the lines to take 10 GW of power out
to the transmitter are a hard problem...
Powersats unfortunately have a bad scaling law for power lines: power
output scales with the square of size, while if you want to keep the
line-loss percentage the same, line mass scales with the fourth power of
powersat size. (If you double the powersat size while keeping the design
otherwise the same, each line has to carry four times as much current, so
it needs four times the cross section to keep a constant loss per unit
length. But the line is also twice as long, so it needs *another* factor
of two in cross section to hold end-to-end loss percentage constant. And
twice the length adds yet another factor of two in mass.) This is one
place where sandwich concepts win big.
This might be the best solution available.
https://new.siemens.com/global/en/products/energy/high-voltage/power-transmission-lines/gas-insulated-lines.html
High-voltage transmission unfortunately is a poor fit to solar arrays,
whose operating voltage tends to be limited by the need to avoid arcing
through the surrounding plasma. About 200V is considered the practical
limit for LEO; I'm not sure what it would be for GSO.
(This has gotten rather off-topic for AR; perhaps we should move this
discussion offline.)
Henry
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