Solar concentration mirrors in the outer solar system

Spider Jerusalem wrote:

Point well taken. I suppose they could live on spirulina grown in
tanks at 3 kw/person, but that wouldn't be much of a life, not having
any real food.



http://www.spirulina.com/

It's the pic on the left that gets me - I think these people intend to
terraform the _Moon_!

Marshall Savage of _The Millennial Project_ did sing the praises of spirulina,
as well as proposing a development plan which wasn't supposed to produce much
visible stuff for a few years yet. It'd be nice to think that Spirulina.com
is actually part of an on-track Plan.

-xx- Damien X-)

Ian Stirling wrote:
In sci.space.policy WLM wrote:

Point well taken. I suppose they could live on spirulina grown in
tanks at 3 kw/person, but that wouldn't be much of a life, not having
any real food.

As far as the cooling is concerned, that would probably be a power
*source* instead of requiring power. The temperature of the space

For this to be true, you've got to have much larger radiators.
Say the heat source is at 273K.
The heat loss per square meter of radiator is of the order of
500W/m^2 or so.
Let's say you want to run the heat engine between 273K and 137K (50%
efficiency with an ideal engine), your radiators need to be 16 times as
large, assuming the same heat output.

On the flip-side, your energy gathering area can be 50% smaller.

Could you, or someone, expand on what was going on here? "Radiators need to
be 16 times as large" -- as large as what? The 137K radiators after heat
recovery would have to be 16 times larger than the 273K radiators with more
energy loss? Energy = k * Temperature^4? Wait, that doesn't seem to make
sense...

-xx- Damien X-)

In sci.space.policy Damien R. Sullivan wrote:
Ian Stirling wrote:
In sci.space.policy WLM wrote:

Point well taken. I suppose they could live on spirulina grown in
tanks at 3 kw/person, but that wouldn't be much of a life, not having
any real food.

As far as the cooling is concerned, that would probably be a power
*source* instead of requiring power. The temperature of the space

For this to be true, you've got to have much larger radiators.
Say the heat source is at 273K.
The heat loss per square meter of radiator is of the order of
500W/m^2 or so.
Let's say you want to run the heat engine between 273K and 137K (50%
efficiency with an ideal engine), your radiators need to be 16 times as
large, assuming the same heat output.

On the flip-side, your energy gathering area can be 50% smaller.

Could you, or someone, expand on what was going on here? "Radiators need to
be 16 times as large" -- as large as what? The 137K radiators after heat
recovery would have to be 16 times larger than the 273K radiators with more
energy loss? Energy = k * Temperature^4? Wait, that doesn't seem to make
sense...

It's not energy which is proportional to T^4, but energy emission from a
radiating surface in general.

Google "stephan-boltzmann"

Basically, as the temperature of the radiator goes down, the amount it can
radiate per unit area (given a 0K environment) goes as the fourth power of
the temperature.
So, to remove a constant amount of heat, you need the radiator to be
larger.

(Damien R. Sullivan) wrote in message ...

3Kw/person is probably a bit low.

My conservative estimate for a comfortable colony would be a full megawatt per
person. The Earth gets about 10^16 watts, and has not quite 10^10 people.
Divide. You could argue some of that energy is supporting "waste" ecosystems,
but they're part of what makes the world nice, and we're said to be
intercepting 40% of primary productivity as it is, while at the same time
supplementing that with fossil and nuclear power.

I suppose if we just consider food... photosynthesis is about 1% efficient,
right? So that 40% is actually .4*10^14 watts, at best. Going the other way,
human bodies are roughly 100 W heat lamps, so 10^10 humans need 10^12 W of
food energy. Which seems about right, consider losses as you climb the
food chain and our eating a lot of meat.

So yea, 3-10 kW is probably a minimum just for food, but there's also the
elemental recycling (though at least some of that would be handled by the food
production), and industrial uses, and some amount of transportation and
"parkland", depending on whether your habitat is more like a nuclear submarine
or Soviet apartment block on one end, or an O'Neill park cylinder or Culture
GSV on the other.

*I* want a megawatt.

-xx- Damien X-)

At first, I thought you must be Amrican, and a MW for a human is
overkill. On Earth, that amount of sunlight is taken on average by
about 4,000m2, but I'd expect the population density to be higher.

That said, a basic O'Niell colony with 100km2 of surface area needs
about 10GW of light. Such a colony could support 10,000 to 1 million
people, giving a lighting of about 10 KW to 1MW / person. Add in farm
areas, and you could maybe double that. But 10,000 would be real
luxury, and lots of space isn't the most important contributor to
luxury.

However, when I do some of the calcualtions for high speed travel, the
energy requirements boggle the mind. A 10,000 ton space ship
travelling at .1C needs 45E20 Joules, or what we currently burn in 140
years. But then we're talking advanced fusion, or massive solar arrays
around mercury.

(Alex Terrell) wrote:
(Damien R. Sullivan) wrote in message
...

My conservative estimate for a comfortable colony would be a full megawatt
per person. The Earth gets about 10^16 watts, and has not quite 10^10
people. Divide. You could argue some of that energy is supporting "waste"
ecosystems, but they're part of what makes the world nice, and we're said
to be intercepting 40% of primary productivity as it is, while at the same
time supplementing that with fossil and nuclear power.

At first, I thought you must be Amrican, and a MW for a human is
overkill. On Earth, that amount of sunlight is taken on average by
about 4,000m2, but I'd expect the population density to be higher.

I am American, though one who rides a bicycle and has no car, for whatever
that may matter. But I didn't think my calculation was related to that: we're
already pushing the comfortable limits of our ecology, so I divided the energy
of the ecology by number of humans.

As for density, the Netherlands, which I presume are one of the denser
countries of the world, are at 2000 m^2/person (using CIA figures of 33,000
km^2 land and 16 million people.) And again, that's with importing a lot of
oil. With urban-style habitats the population density of the habitable volume
proper might be a lot higher, but remember that cities draw on lots of land
and water few people are living on; here that's displaced to the solar panels
or mirrors. Urban densities aren't sustainable by themselves.

You may be able to get under a megawatt a person, certainly, but will people
*want* to, past the early days. "We could build another habitat, or we could
build a park for this one."

And as you note, moving the habitat around will take more energy.

-xx- Damien X-)

On 12 Sep 2004 15:17:08 GMT, Ian Stirling
wrote:

In sci.space.policy Damien R. Sullivan wrote:

Could you, or someone, expand on what was going on here? "Radiators need to
be 16 times as large" -- as large as what? The 137K radiators after heat
recovery would have to be 16 times larger than the 273K radiators with more
energy loss? Energy = k * Temperature^4? Wait, that doesn't seem to make
sense...

It's not energy which is proportional to T^4, but energy emission from a
radiating surface in general.

Google "stephan-boltzmann"

Basically, as the temperature of the radiator goes down, the amount it can
radiate per unit area (given a 0K environment) goes as the fourth power of
the temperature.
So, to remove a constant amount of heat, you need the radiator to be
larger.

Black body radiation is sigma T**4, where T is in deg absolute (is
that Kelvin or Rankin these days?). This is one of the few formulae I
still remember from engineering school after thirty-plus years.

You can radiate more if you can change sigma, which is a constant for
each type of material. However, the changes aren't huge compared to
the effect of the temperature differences.

Mary

--
Mary Shafer Retired aerospace research engineer

Mary Shafer wrote:

Black body radiation is sigma T**4, where T is in deg absolute (is
that Kelvin or Rankin these days?). This is one of the few formulae I
still remember from engineering school after thirty-plus years.

The SI unit is the kelvin.

You can radiate more if you can change sigma, which is a constant for
each type of material. However, the changes aren't huge compared to
the effect of the temperature differences.

sigma is a universal constant. You're thinking that the full equation
is

P = e sigma A T^4,

where e is the emissivity, and A is the surface area, T is the
thermodynamic temperature, and sigma is a fundamental constant.

--
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