From: wreck@fmsrl7.srl.ford.com (R. Cage)
Subject: Re: SPS feasibility (WAS: SPS fouling astronomy)
Summary: Coffman omits highly significant facts and gets lotsa numbers wrong.
Organization: Ford Motor Company Scientific Research Labs, Dearborn, MI
Date: Thu, 13 Aug 1992 17:47:05 GMT

In <1992Aug13.075037.2707@ke4zv.uucp> gary@ke4zv.UUCP (Gary Coffman) writes:
>Let's look at this a bit closer with some ROM estimates. If we have a 1 TW
>laser beam from the Moon impinging Earth, we add an additional TW to the
>Earth's heat balance. Normal heat input from the Sun for the Earth is
>about 140 TW so the additional 1 TW is less than a 1% change. But due to
>efficiency factors discussed below, actual net heat load would be 3 TW.

You're assuming things, like:

1.)	25% conversion efficiency.  Sorry, but that's too low.
	Photovoltaic cells tuned for monochromatic radiation can
	already go considerably higher, and in the IR, there is
	the possibility of using micro-rectennas.

2.)	Laser is the most efficient transmission method.  Microwave
	is probably more efficient.

Also, you have the earth's energy budget wrong.  The disc area
is about pi * (6.4e6 m)^2, or 1.28e14 square meters.  The solar
constant is about 1360 W/m^2, so we get about 1.75e17 watts, or
175,000 TW of incident sunlight.  Not 140 TW.  You're off by 3
orders of magnitude!

Adding another 4 TW of heat is literally a drop in the bucket.
Spread it out and you'll have a very hard time detecting it.

>By contrast, for Earth based solar collection, there is no net increase 
>in heat flux. The energy is striking the planet anyway whether we use it 
>or not.

3.)	You're assuming that the albedo of the earth's surface and
	a solar collector are the same.
	
In a word, WRONG!  Solar collectors are a lot blacker than most things.

If you counter that the collectors can be interspersed with light
colored areas to reflect light and keep the balance the same, you
can do that without solar collectors too.  It's not an argument
for ground-based.

>Now what's happening on the Moon? Gas lasers are very inefficient devices. 

Free-electron lasers aren't.  Neither are amplitrons.

>I don't have CO2 numbers at hand, but He-Ne efficiencies are around 0.1%. 

I believe CO2 lasers have reached 20%; someone please correct me.

>Now solar energy striking the top of the Earth's atmosphere, or 
>the surface of the Moon, is 1 kW/m^2.

Close enough.

>So our 1 TW delivered to the busbar on 
>Earth requires a solar collector area of 1E14 square meters on the Moon.

This assumes a system efficiency of 1%.  That is about 10x too low.  Solar
cells are over 20%, conversion equipment to microwaves is well over 50%.

>requires a square 10,000 km on a side, or about 6,000 miles on a side. Sorry 
>gentlemen, the Moon isn't that big.

More like 3,000 km on a side.  Just about the area of Luna.

>Now if we discard the laser, the Moonbase 
>with it's 6000 mile on a side collector, and simply use the same collector 
>field we were going to use for the laser on Earth, we still gather in 1E10 
>watts, or 10 GW, and we haven't spent an improbable fortune on the Moon.

You can't use the same collector.  It's out of the sun a lot more than
half the time, has to be built to deal with WATER and WIND, and a whole
lot of other things that limit its life and output.  Vacuum and low G
has the advantage that it's very friendly to most equipment.

>A Lunar collector array would be in darkness 2 weeks out of 4, so storage
>for two weeks would be required on Earth.

Wrong.  You'd put collectors on both sides of the moon.  When power
is only hitting the far side, it's the far side collectors which feed
the transmitters.  Transmission lines are easy in 1/6 G and vacuum.

>Therefore, at least half of
>Earth's capture arrays would be in daylight at any given moment. That's
>at least 7 times better than the Loony Laser.

Wrong.  Receivers not in direct view of Luna could be fed by
reflectors (easy for lasers, a little harder for microwaves).
If a receiver can take a feed from more than one reflector at
a time, then interruptions from the reflector going behind
Earth as seen from Luna will be avoidable.

>But solar *thermal* is workable *today* in certain cases.

Yes, mostly peaking power or fossil displacement.  Not replacement,
not base-load.  The whole point of Lunetta or SPS is base-load power.

IMHO, if there is the capability to build that much collector
area on the moon, a single catapult can put it into orbit and
leave it in sunlight 24 hours per day, all month.  Having many
feed angles and immunity from lunar eclipses and beam
occultation by Earth is another big advantage.  This is why I
think SPS has a better future than Lunetta.

>Gary
-- 
Russ Cage   wreck@fmsrl7.srl.ford.com  russ%rsi.uucp@destroyer.rs.itd.umich.edu
  *  When Ford pays me for my opinions, THEN they can call them theirs. *

                           _Bad_ cop.  No donut.