[ExI] extropy-chat Digest, Vol 107, Issue 19

[Keith Henson]

On Thu, Aug 16, 2012 at 5:00 AM,  John Clark <johnkclark at gmail.com> wrote:

> On Tue, Aug 14, 2012 at 2:19 PM, Keith Henson <hkeithhenson at gmail.com>wrote:
>
>> It does as well per area as PV does in the brightest part of a clear day,
>> but a rectenna produces that level of power virtually all the time.
>>
> Considering the vast effort involved I'd really expect it to do better than
> that. One of the main problems with solar is the vast amount of land
> required due to the dilute nature of sunlight, and power satellites do not
> solve that problem.

Power sat rectennas are about 100 km^2 and produce 5 GW.  or 50 W/m^2,
1.2 kWh per day per square meter.  There are ways to cut the area by a
factor of 4 by putting several together at the cost of awfully large
concentrations of power.  At 200 kWh/person per day ~ 170 square
meters.  For 60 million people, 10,000 km^2.  That's about 4 % of the
UK land area, where 46% used for pastures and 25% used for
agriculture.  Rectennas, because they let 95% of the light through,
should not interfere with either use.

It would take 5 times as much area to so the same with PV, you cannot
reuse the land under PV for growing food, and you have a huge storage
problem.

>> > The worst rain storm on record was analyzed back in the 70s for power
>> sats.  It does take some energy out of the beam, but it's a relatively
>> small fraction.  I don't remember the exact numbers but it is nothing to
>> worry about.
>>
>
> It depends on the microwave frequency, below about 4 GHZ rain doesn't
> hinder communication satellites much but we're talking about power
> satellites; if the power of the signal from a communications satellite
> suddenly drops in half it's hardly noticed,  but if the output from a big
> power plant suddenly drops in half it would cause havoc.

Not really.  You have to understand how cheap these things are.  They
produce power at less cost than any other way.  If they can't be made
that cheap, then there is no point in building them at all.  So we
over build them to supply the maximum power demand.  What do you do
with the difference between maximum and current demand?  You feed it
to hydrogen plants and feed the hydrogen into making synthetic fuel.
So if the output of one drops by half (which takes a really rare
rainstorm) and they cut back a couple of GW being fed into making
hydrogen.

> And for
> frequencies higher than 4 GHZ or so rain starts to cause problems even for
> communication.
>
>>>> and if you have a grid, then we can "cross the beams" to keep the grid
>>>> fed from power sats out of the shadow.
>>>>
>>>
>>> > > This problem like all problems is solvable, but it's going to take
>>> even more money
>>>
>> > This trick has no cost.
>>
> Directional antennas that are adjustable cost more than the non-adjustable
> type,

A circle on the earth at 45 deg latitude is about 26,650 km long.  The
eclipse at 70 minutes would be 1300 km wide, the distance to GEO
~40,000 km.  The angle from GEO is about 2 deg.  So crossing the beams
to keep some of the rectennas supplied with power from outside the
eclipsed zone in GEO would mean taking power from satellites 2 deg off
from straight to GEO.  At the receiving end, that's not enough to be
concerned about, considering the rectenna is 45% off axis from the
power sat due to latitude.

At the transmitting end, the antenna is mechanically pointed toward
the receiving station on the ground.

I don't understand where directional antennas are needed.

> and 2 power satellites cost more than one.

That's obviously true, but how does it apply?  All the power
satellites are used all the time possible.  The question is: Can we
cope with a reduction to half the generation in a time zone for a few
weeks in the spring and fall of the year around local midnight?

With a little forethought as to where the power sats and rectennas
pairs are located, it seems it would cause no problem even if SBSP was
used to supply 100% of electric power.

>> Do you have a number on how long the thorium will last?  I be it isn't
>> very long if you try to use it as the primary energy source.
>>
> In the Earth's crust Thorium is about 4 times as abundant as Uranium which
> makes it about as common as lead. And today all commercial Uranium reactors
> are non-breeders, that means they only use U235 which is about one part in
> 143 of natural uranium, the remainder being U238 which can be used to breed
> Plutonium for power but for several very good reasons it usually isn't, and
> so nearly all of natural Uranium is just dead weight. By contrast natural
> Thorium comes in only one isotope and reactors burn up 100% of it,  not
> just .7% of the Uranium as in existing reactors; so the human race can
> expect to get 4*143= 572 times as much energy from Thorium as they get from
> Uranium.  And Uranium reactors have many problems but acute uranium
> shortage is not near the top of the list.

That wasn't the question.  If the human race were pulling the majority
of power from thorium reactors, how long would it last?  I think the
answer will concern you

I have seen this derived and remember the order of magnitude, though I
don't remember where.

Keith