[AR] Re: space based solar
- From: Keith Henson <hkeithhenson@xxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Tue, 19 Mar 2019 08:20:00 -0700
Jordan Trewitt <Jordan.Trewitt@xxxxxxxxxxxxxxxx> wrote:
I just want to see what the radiators will be like on either laser or
microwave transmission satellite. Even if it operates at 50% efficiency (may
have mentioned 65%) as the paper claims, that's still a megawatt of radiators
(this assumes 100% duty cycle though).
Large space radiators have been a concern of mine since Eric Drexler
and I wrote a paper on using dust as the heat transfer medium clear
back in 1979.
In more recent years, I designed a light radiator system using plastic
tubes and condensing low-pressure steam. (The path was through cooling
lasers.) Such radiators operate at around 20 deg C. They are big but
mass about a kg per kW of radiation. They taper to keep the steam
velocity constant as about 80% condenses. There is a video out there
called beamed energy bootstrapping that illustrated the radiator
design.
Henry Spencer wrote:
On a microwave powersat, they needn't be too impressive, because the
conversion of DC to microwaves can be pretty efficient.
Also, the transmitter disk has a lot of area so it does not take a
very high temperature to radiate the waste heat.
And separate radiators might disappear altogether with a "sandwich"
structure, in which
the solar array and the transmitter array are one and the same.
This gets tricky. Some of the designs which show large concentrations
of sunlight are not realistic (they would get too hot). There is a
variation on Ian Cash's design where the photovoltaic surface and the
transmitter are the same. (Sunlight is reflected down on a surface
tilted 45 deg with respect to the earth.) I tend to prefer
transmitters somewhat larger than the minimum because you can shape
the transmit beam to a "top hat" configuration. This allows the whole
rectenna to operate at the per square meter power limit.
One problem with distributed designs is self-powering out to GEO. If
you are using efficient arcjets, they need concentrated power in the
GW range for reasonably fast transit.
There are a bunch of engineering problems with power satellites, among
them how to avoid the space junk. We can ship up the parts in compact
form and build them above the junk, but that puts us in high
radiation, not good for robots or humans. One proposal is to build a
laser with enough power (tens to hundreds of MW) to ablate the junk.
The bigger stuff can be deorbited this way, the smaller pieces can be
evaporated.
Keith
PS There is another problem, making the power satellites resistant to
big CMEs. Those show up in the carbon 14 record every several
centuries. It would not do to have much of the energy for the world
turned off when the sun burps.
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