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« C-R-Newsletter #37 | Main | Googling for Nanotechnology »

January 28, 2006

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Tom Craver

Chris:

Why powered aircraft? Very large hot-air balloons should be more efficient (large so heat losses through the skin are small compared to their volume). A spherical balloon might collect about 200W/sq-m of cross-sectional area, on average. A 1km balloon, with cross-sectional area of 785375sq-m might average 157MW from an airship massing perhaps 300 metric ton.

Also - if the airships self-produce out of recyclable nanoblocks, simply landing and being disassembled for re-use would effectively deliver energy.

Sigma

Why use airships and balloons, when you could put permanent solar collection arrays on the moon? Then you could beam the energy back as microwave energy.

Inaddition you could also setup bases and collect He3 for use in nuclear fusion reactors(which I assume could be constructed with molecular nanotech).

Chris Phoenix, CRN

I didn't want to get into fusion, because we don't yet know how to do it. MM should certainly enable much faster research--imagine if you could build a new Tokamak every month for $1 million or less. But until fusion is working, we won't know how efficient or convenient it will be.

Hot air balloons are an option if you don't care where they drift, and you don't need them to land very often. Basically, I'm not too worried about efficiency; with super-light materials, it will be possible to build a plane that uses a tiny fraction of incoming energy to stay aloft.

Any kind of beamed power needs a big receiver and will produce losses (heat) at the receiving end. Hot air balloons would need either beamed power or flying shuttle-craft--or, if they have even a limited mechanosynthetic capability onboard, they would be able to simply drop gliders full of synthesized fuel. (Gliders, because large balloons will be drifting rather than station-keeping.)

I don't like the idea of any aircraft designed to build copies of itself in the upper atmosphere. It's too close to a "gray goo" scenario. It would be pretty easy to clean them up (shoot them down) if a software bug made them produce too many copies... but why go there at all? Build them on the ground and launch them as needed.

Chris

Tom Craver

Chris:
If this system is the primary means of energy collection, I think you do care about efficiency - e.g. if the system uses half it's energy to stay aloft, and if we are at the limit of our heat budget, that means humanity gets only half as much energy as it could otherwise have.

Also, if it's a global power system, the fact that they drift around the world isn't much of an issue - in fact, its free fuel distribution.

I think the biggest problem with balloons is that they'll have a large area to payload ratio - they'll be able to gather their fuel-lift capacity rather quickly. (But would powered craft do better? Possibly they just take longer to gather the same amount of energy. And as they get near the end of a production cycle, carrying more fuel as cargo, their production rate would drop.)

Instead of self-copying, what if they grow to a certain size (using their energy to produce nanoblocks, mostly energy storing blocks that can double as structural blocks), then rebuild a large fraction of themselves into powered robotic landers? (There's no reason they have to be gliders - powered craft will be safer and better insure they can make it to landing areas.)

Phillip Huggan

What I see here is a description of tradeoffs between nearby atmospheric volumes and non-local airspace. I don't know which will be more valued post-MM.

I like desert solar cells generating power for a hydrogen storage system. But real estate might be too scarce for it to be viable. So then you've got balloons which use up a minimal amount of surface area, but consume a large local amount of airspace. Overall, Chris's planes use up a greater atmosphere cross section than do the balloons, but that airspace can be really far away from the airport.

Phillip Huggan

I was thinking tethered balloons in my analysis. For drifting balloons the tradeoff is fixed airspace volumes for planes versus balloons that drift into the airplane flight paths, crashing them into my desert solar panels and wrecking all three power systems.

Chris Phoenix, CRN

When I said I wasn't worried about efficiency, I meant that I thought the system would be quite efficient, not that I thought it would be inefficient but didn't care.

Build an airplane with a glide ratio of 100:1, and fly it at 20 kph. It'll tend to fall at a rate of 200 meters per hour--thus requiring only that much energy to fly level.

I haven't done the aeronautical engineering, but my numbers aren't that far off from what's achievable today. And almost all the mass that would be structure today could be power storage in MM-built aircraft.

I do have a question about heat dissipation at high altitudes. Would that warm the earth as much as ground-level heat dissipation? I'd think that at least half the heat would be radiated straight into space.

Heat dissipation might matter if the captured energy was being used for mechanosynthesis, which may not be particularly efficient, at least at first. Or if the solar collection technology was inefficient.

Chris

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