Author Topic: Power options for a Mars settlement  (Read 146818 times)

Offline Semmel

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Re: Power options for a Mars settlement
« Reply #580 on: 08/20/2018 07:55 am »
How do big flywheels handle planetary rotation? Also you could double the energy density by filling them with batteries.

Offline speedevil

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Re: Power options for a Mars settlement
« Reply #581 on: 08/20/2018 09:29 am »
7GPa T1100G carbon fibres in unidirectional layup enables almost 2500000m²/s² specific strength = ~300Wh/kg thin-cylindrical flywheels, but more realistically perhaps 150-200Wh/kg, which is pretty competitive with state of the art lithium ion batteries (that can't be cycled too deeply anyway).

If you want large reserve capacity, and keep them cool, even current lithium-ion may be good enough.
Reserve capacity of more than a day at 'hotel' loads pretty much inherently means you're cycling the batteries less.
Keeping them cool - 10C - also helps moderately.
Charging the whole thing only to 80% capacity - keeping a reserve which could be charged if an upcoming storm is spotted - helps a lot in terms of life.

In these sorts of conditions - cells that last a decade are not a huge stretch.

To provide 500kW for 8 hours is 'only' 20 tons of lithium-ion, with 100% depth of discharge, or 35 or so with enough reserve to last a decade.


Offline speedevil

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Re: Power options for a Mars settlement
« Reply #582 on: 08/20/2018 09:32 am »
How do big flywheels handle planetary rotation? Also you could double the energy density by filling them with batteries.
That doesn't work.
The energy is (if you look at it the right way) stored as the flywheel material being stretched, with a really weird rotary gearbox.
Adding mass to the flywheel reduces energy storage by slowing down the maximum RPM in exact proportion to the proportion of non structural mass added.
In other words, it's no better than a flywheel without the extra mass.
Planetary rotation is almost irrelevant - the rate is low enough that the forces on the bearing are quite small.
The forces are zero if you line up the gyro axis with the planets rotation.

Offline RotoSequence

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Re: Power options for a Mars settlement
« Reply #583 on: 08/20/2018 10:53 am »
Steve Hoeser did an interesting writeup on the power budget needed to produce liquid oxygen and methane for the return trip home, looking at a variety of processes and techniques. Whatever the methodology, the energy requirements to produce fuel and oxidizer for BFR are considerable.

Part 1
Part 2
Part 3

The final estimated power requirement is 16 gigawatts to extract, refine, produce, and condense the fuel and oxidizer needed for one BFR return.

Offline speedevil

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Re: Power options for a Mars settlement
« Reply #584 on: 08/20/2018 10:59 am »
The final estimated power requirement is 16 gigawatts to extract, refine, produce, and condense the fuel and oxidizer needed for one BFR return.
Gigawatt-hours. (1.8 megawatts continuous)

Offline RotoSequence

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Re: Power options for a Mars settlement
« Reply #585 on: 08/20/2018 11:28 am »
The final estimated power requirement is 16 gigawatts to extract, refine, produce, and condense the fuel and oxidizer needed for one BFR return.
Gigawatt-hours. (1.8 megawatts continuous)

I figured the "hours" part was implied.  ;) Whether you do it in 26 months or 12, it's still a nice round number to build on. What I'm wondering about at the moment is how one would store the refined product. Liquid methane and oxygen make for somewhat demanding pressure vessels, and the vessels, or components to fabricate them, need to fit inside of BFR(s). In-situ welding would be a tough sell to whoever was responsible for putting those things together.

Offline ThereIWas3

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Re: Power options for a Mars settlement
« Reply #586 on: 08/20/2018 12:30 pm »
Just as one would be jumped on here for confusing "propellant" with "fuel", or "thrust" with "ISP", as a EE I feel compelled to correct misuse of "Watts" when you mean "Watt hours".   They are entirely different things.   Using the traditional water analogy, Watts is flow (how big are my solar panels?  How thick do the wires have to be?) while watt-hours is volume (how big do my batteries have to be?)
"If you want to build a ship, don’t drum up people to collect wood and don’t assign them tasks and work, but rather teach them to long for the endless immensity of the sea" - Antoine de Saint-Exupéry

Offline meekGee

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Re: Power options for a Mars settlement
« Reply #587 on: 08/20/2018 01:34 pm »


The final estimated power requirement is 16 gigawatts to extract, refine, produce, and condense the fuel and oxidizer needed for one BFR return.
Gigawatt-hours. (1.8 megawatts continuous)

I figured the "hours" part was implied.  ;) Whether you do it in 26 months or 12, it's still a nice round number to build on. What I'm wondering about at the moment is how one would store the refined product. Liquid methane and oxygen make for somewhat demanding pressure vessels, and the vessels, or components to fabricate them, need to fit inside of BFR(s). In-situ welding would be a tough sell to whoever was responsible for putting those things together.

"hours" implied?   Why not "seconds"?

That's like saying the pressure was a kg.

I get 19000 hours in 26 months so more like 1 MWatt continuous.

Either way it is not as large sounding as "16 GWatts"...



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Offline spacenut

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Re: Power options for a Mars settlement
« Reply #588 on: 08/20/2018 01:56 pm »
RobLynn is right about the flywheel concept.

In the 1970's power plants were thinking of building 300' (100m) diameter flywheels.  They would put them in the ground in case of breaking or destruction.  The wheels would be made from steel cable wire tightly wound.  A vacuum would be pulled on it to reduce air friction.  The idea was to run existing nuclear, hydro, and natural gas power plants at full capacity all the time whenever possible.  Excess power would turn the flywheels.  Then during peek air conditioning times, the flywheels would kick in to carry the extra load.  The flywheels would be placed near these large power plants.  It was estimated to be able to produce 1/3 more power for the grid without new power plants.  The idea was to get rid of the coal plants. 

Now on Mars this sounds like a very good idea coupled with solar, by using simple flywheel power at night.  Martian iron could be made into flywheels without bringing the heavy wire from earth.  Carbon from the atmosphere could be mixed with the iron to make steel wire.  Initially it might be cheaper to bring some wire until forging equipment can be brought from earth.  This would be cheaper than lithium batteries brought from earth.  It might not be as efficient, but it would work. 

In the 1970's flywheels were also considered in cars with an engine/generator combo.  At night you would plug it into your outlet at home, flywheel would spin up to max overnight.  Drive with flywheel produced power during the day.  The idea never really caught on because flywheels were as heavy as a big V8 engine, and the prevailing idea was to make cars lighter and smaller with smaller engines, less raw material. 

Offline envy887

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Re: Power options for a Mars settlement
« Reply #589 on: 08/20/2018 02:11 pm »
For the flywheel concept, UHMWPE fiber is stronger than steel cable and probably easier to make.

Offline RotoSequence

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Re: Power options for a Mars settlement
« Reply #590 on: 08/20/2018 06:31 pm »
"hours" implied?   Why not "seconds"?

That's like saying the pressure was a kg.

I get 19000 hours in 26 months so more like 1 MWatt continuous.

Either way it is not as large sounding as "16 GWatts"...



If you want it in a year, you need ~2 megawatts continuous. Time is still an independent variable if you want to send any ships back, without passengers, between synods. Whatever the launch interval, there's a need to figure out how to generate 16 Gigawatt Hours for fuel production between each launch from Mars.

Offline meekGee

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Re: Power options for a Mars settlement
« Reply #591 on: 08/20/2018 06:58 pm »
"hours" implied?   Why not "seconds"?

That's like saying the pressure was a kg.

I get 19000 hours in 26 months so more like 1 MWatt continuous.

Either way it is not as large sounding as "16 GWatts"...



If you want it in a year, you need ~2 megawatts continuous. Time is still an independent variable if you want to send any ships back, without passengers, between synods. Whatever the launch interval, there's a need to figure out how to generate 16 Gigawatt Hours for fuel production between each launch from Mars.
I was going with 26 month since that's the frequency of launch...  The time that the ship is on the surface should not matter as long as there's enough storage tankage.



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Offline RotoSequence

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Re: Power options for a Mars settlement
« Reply #592 on: 08/20/2018 07:11 pm »
I was going with 26 month since that's the frequency of launch...  The time that the ship is on the surface should not matter as long as there's enough storage tankage.

I suspect storage tank assembly and production on Mars will be somewhat tricky. Metal cryogenic storage vessels need to be rather stout and leak proof. How do you bring tankage to Mars to store the produced fuels long enough for reflight? If there's any significant leak losses over 26 months, the power budget overhead stands to go up considerably.

Offline envy887

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Re: Power options for a Mars settlement
« Reply #593 on: 08/20/2018 07:13 pm »
I was going with 26 month since that's the frequency of launch...  The time that the ship is on the surface should not matter as long as there's enough storage tankage.

I suspect storage tank assembly and production on Mars will be somewhat tricky. Metal cryogenic storage vessels need to be rather stout and leak proof. How do you bring tankage to Mars to store the produced fuels long enough for reflight? If there's any significant leak losses over 26 months, the power budget overhead stands to go up considerably.

Fortunately, if you arrive in a BFS you brought your own storage tanks with you.

Offline RotoSequence

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Re: Power options for a Mars settlement
« Reply #594 on: 08/20/2018 07:18 pm »
Fortunately, if you arrive in a BFS you brought your own storage tanks with you.

Is that actually doable? It'll be desirable to keep the skin temperature of BFR above the freezing point of CO2, while the vehicle itself needs to be insulated enough to keep the re-liquification demands of the tanks to a minimum.

Offline Patchouli

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Re: Power options for a Mars settlement
« Reply #595 on: 08/20/2018 07:30 pm »
Another thing to keep in mind a BFS should require less propellant to get home than it did to for the outward trip since it would be carrying less payload and you can better make use of aerobraking at Earth.
Fuel consumption on the return leg also can be reduced with a lunar flyby in some scenarios.
« Last Edit: 08/20/2018 07:32 pm by Patchouli »

Offline RotoSequence

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Re: Power options for a Mars settlement
« Reply #596 on: 08/20/2018 07:36 pm »
Another thing to keep in mind a BFS should require less propellant to get home than it did to for the outward trip since it would be carrying less payload and you can better make use of aerobraking at Earth.
Fuel consumption on the return leg also can be reduced with a lunar flyby in some scenarios.

For Mars to Earth downmass below 50 tons, I suppose, but the launch architecture does budget itself with that amount of return mass in mind. They probably want to bring back a lot of samples.  ;D



Offline envy887

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Re: Power options for a Mars settlement
« Reply #597 on: 08/20/2018 08:39 pm »
Fortunately, if you arrive in a BFS you brought your own storage tanks with you.

Is that actually doable? It'll be desirable to keep the skin temperature of BFR above the freezing point of CO2, while the vehicle itself needs to be insulated enough to keep the re-liquification demands of the tanks to a minimum.

It will need insulation anyway, to some extent, for TPS and to avoid ice formation on the launchpad, and to minimize heat transfer into the header tanks during coast and to minimize boiloff while being refueled in LEO. I'd think the additional mass penalty of insulating the main BFS tanks is much lower than that of bringing separate better-insulated tanks all the way form Earth just for that purpose.

Offline matthewkantar

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Re: Power options for a Mars settlement
« Reply #598 on: 08/20/2018 08:43 pm »
To put it in familiar terms, the power requirement to refuel a BFS for Mars departure is roughly what would be required to run an average hand heeled hair dryer continuously for two years?

Offline speedevil

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Re: Power options for a Mars settlement
« Reply #599 on: 08/20/2018 08:54 pm »
To put it in familiar terms, the power requirement to refuel a BFS for Mars departure is roughly what would be required to run an average hand heeled hair dryer continuously for two years?
No.
Two thousand years.