From: hal@finney.org
Date: Thu Aug 26 1999 - 11:24:09 MDT
Robert's essay was very interesting and enlightening. I haven't
seen this kind of detailed analysis before. But 5 years to build a
Gates-style mansion? I had no idea it would take so long.
A couple of quibbles:
> Then you get your open-source nanoseed to assemble solar collectors
> over most of the property. At an insolation of ~1000 W/m^2 and
> 0.2 conversion efficiencies (pretty conservative), that gives you
> 400,000 watts of power during the day. Assuming a mass manipulation
> cost of ~15,900 kJ/mol of sapphire (perhaps the highest cost), that lets
> you nanoassemble ~10 kg of nanomaterial per hour.
I wonder if this energy cost isn't off by a couple of orders of magnitude.
There is not much discussion in Nanosystems of the actual energy costs
to build the products. It seems that every synthetic step which Drexler
analyzes produces energy rather than consumes it, which doesn't seem
right, but I think he is assuming high-energy feedstocks. If you are
starting with oxidized carbon, aluminum and silicon (like you'd find
in the air and in the soil) then you first have to expend energy to get
these things ready to use.
But if I do try to use the figures for energy dissipation on p441,
I find that we should be able to construct 1.1 kg of product in one
hour dissipating 1.1 kW. This is about 3.6 x 10^6 Joules of energy
dissipated per kg. However on p397 Drexler says that it should be
possible to dissipate only about 1.5 x 10^6 Joules per kg. Not a big
deal, but a factor of >2 inconsistency is puzzling, when Drexler is
giving us 2 significant figures.
In terms of sapphire, Al2O3, molecular weight about 100, a mole would be
about 100 grams, so 1 mole would be about .1 kg. We should be able to
construct a mole for .1 the kg cost, or 1.5 x 10^5 J if we use Drexler's
lower estimate. Robert has 1.5 x 10^7 which is two orders of magnitude
higher.
I want that factor of 100 back! Then I can build my mansion in 20 days
rather than 5 years.
> It turns out that you probably don't want to assemble more than
> 10 kg/hour because if everyone on the planet is doing it you
> start to interfere with the heat carrying capacity of the planet [3].
How would you interfere with the heat carrying capacity if you're getting
all your energy from sunlight? Isn't that virtually all dissipated as
heat now anyway, minus a tiny fraction captured chemically, in both
biological and nonbiological processes? It would seem that all we
could hope to do is to capture it more efficiently and bind more of
it chemically, so that if anything there would be less heat radiated.
I don't see how we could make the earth hotter than it is already, just
working with the already-present solar energy.
Or is that the issue? By "interfere with the heat carrying capacity"
is it meant that we might make the earth too *cold*? (Sucking CO2 out
of the atmosphere to get carbon for construction purposes might further
cool things off.) That ought to be easy to fix, by sending up a few
solar collecting satellites.
The total solar energy impinging on earth is about 2 x 10^17 Watts, which
with a population of 10^10 gives everyone over 10^4 kW to play with.
That would be enough to assemble 10^4 kg/hour, four orders of magnitude
more, and lets us build our mansion in 4 hours instead of 5 years.
However this would require an infrastructure to capture solar energy
over much of the globe and pump it where needed.
Hal
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