[Server-sky] Fwd: [ExI] Power sats again

Michael Turner michael.eugene.turner at gmail.com
Thu Jul 18 04:01:10 UTC 2013

Accidentally sent only to TMS, intended for the list.

---------- Forwarded message ----------
From: Michael Turner <michael.eugene.turner at gmail.com>
Date: Thu, Jul 18, 2013 at 11:09 AM
Subject: Re: [Server-sky] [ExI] Power sats again
To: "tme at asteroidinitiatives.com" <tme at asteroidinitiatives.com>

On Thu, Jul 18, 2013 at 10:57 AM, tme at asteroidinitiatives.com
<tme at asteroidinitiatives.com> wrote:
> On July 17, 2013 at 7:14 PM Michael Turner <michael.eugene.turner at gmail.com>
> wrote:
>> "The problem with hydro in most places is that it is remote, or small,
>> or destructive of habitat, and usually all three."
>> First, something that keeps getting lost in this discussion is that I
>> propose beaming power up from Earth through a microwave relay back
>> down to Earth. Surely, if it's very efficient sky-to-Earth, it's no
>> less efficient Earth-to-sky?
> I don't think so, for two reasons. One, the problem is not really symmetric.
> Antennas are more expensive per square meter up there than down here (at
> least now, and for quite some time to come).

Yes, but it's not just the expense of the receiving microwave antenna
in orbit, it's also the cost-savings of not building the alternative:
a huge solar PV array in orbit.

> ... So, you should expect smaller
> antennas up there radiating to big farms down here. The reverse is not as
> efficient, unless the uplink array is phase coherent (so it acts like one
> large antenna), which is expensive AND subject to atmospheric phase changes
> (which you don't care about in downlink).

OK, that went over my head, so I'll take your word for it.

> .... The real problem is that you want to
> go _two way_.  That multiplies your efficiencies (squares them if they are
> equal up link and down link), plus another factor for whatever link losses
> you have up there. (For example, if you were 50 % efficient going up, and
> 50% going down, the total efficiency is 25%; 25% is definitely less than
> 50%, and that's before you account for losses up there.)

I was under the impression that orbit-to-Earth would be about 85% efficient.

It's really about some formula combining cost-per-delivered watt with
reduced carbon emissions, however. If you can build lots of low-impact
hydro with cheap labor in remote areas of less-developed nations,
incidentally producing significant income (in PPP terms) for those
cheap-labor countries, maybe it works out, even with 60% losses in the
uplink, while featuring some economic development value where it
counts the most.


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