OFMM Development: Power Systems

[Bj]

Working on the power generators for this mission, I have ran into a few technicalities that need to be kinked out.

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First off, I found nice reactor design(s)

http://fti.neep.wisc.edu/neep533/SPRING2004/lecture23.pdf

called SP-100

Now I came to the power design, which brings me to a few questions:

I don't think you can directly plug in any electronic device directly into a nuclear reactor so we will need to bring along some batteries.

How much power do you think we need? -specifically how many kWh a day will we will consume? -This will determine the size and the number of nuclear power generators we bring along.

What would be the maximum ampere in the day do we will need? -thinking of if we run a high power item, (such as air conditioner, cooking unit, or some science experiment or especially the machine that creates our rocket fuel) will we run over the watts provided by the nuclear reactor. If we do run over for the brief period we have to have deep cycle long lasting batteries to counter the demand, then an excess from the generator to re-charge the used batteries.

http://www.eia.doe.gov/ask/electricity_faqs.asp

In 2008, the average annual electricity consumption for a U.S. residential utility customer was 11,040 kWh, an average of 920 kilowatt-hours (kWh) per month. Tennessee had the highest annual consumption at 15,624 kWh and Maine the lowest at 6,252 kWh.

wiki says ISS makes 524.8 kW but I assume we will need much more because of the number of people we are bringing (BTW, how many is that) and the kind of equipment we need to bring along. ie. excess freezers/refrigerators, computers, ECLSS ext...


If something goes wrong, we will need enough power to have a supply for communications + heating for I say 90 days. We should bring along some additional NiH2 batteries that has more than 90 days (129,600 minutes) of power to the only most vital of systems. (or is that too unrealistic?) I would personally prefer (especially if I was going) enough power for an entire trip between Mars -Earth, but that is really unlikely that will happen.

Some quick calculations:

ISS says it has user-required 124 V DC

Given:
[math]W = V * A[/math][math]Wh = V * Ah[/math]
then:
[math]100,000 = 124 * A[/math]
Amps (from 100kW generator) = ~806.45

charge/discharge time:
[math]\alpha = \left(\frac{Ah}{A} \right)[/math]
[math]Ah = A*\alpha[/math]
by Reserve capacity definition (discharge at 25 ampere)
[math]RC = \left(\frac{Ah}{25} \right)[/math]
More practically: Am =
[math]RC = \left(\frac{Ah}{A_m} \right)[/math]
------------

So for just a demo using my numbers:

[math]129,600= \left(\frac{Ah}{50} \right) [/math] = 6,480,000 Ah, = 52,258.064 Wh.

[ame="http://en.wikipedia.org/wiki/Nickel_hydrogen_battery"]NiH2 batteries[/ame] have 75 Wh/kg and 60 Wh/dm^3 or .06 Wh/m^3 so

0.87096 m^3 of batteries & 696.77kg to have 90 day backup? Did I do that all right? That doesn't seem that large or that heavy at all.

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Anyway boils down to estimates for:

For standard batteries

• Highest ampere rate (and how long)
• Low ampere rate- to reserve power for charging batteries.
• Average kWh a day

For backup batteries

• How long of a reserve should we leave
• Lowest ampere rating -amperes with only critical systems on.

 

[Urwumpe]

You can plug things directly into a nuclear reactor - you need batteries if you have quickly changing loads on the reactor, because nuclear reactors need time for changing power output.

 

[Bj]

You can plug things directly into a nuclear reactor - you need batteries if you have quickly changing loads on the reactor, because nuclear reactors need time for changing power output.

Oh I get it, similar to a small Honda power generator, it can be plugged in, but if some heavy load comes on, it has to crank hard for a sec. Same with reactor I guess, only it takes much longer?

 

[Urwumpe]

Oh I get it, similar to a small Honda power generator, it can be plugged in, but if some heavy load comes on, it has to crank hard for a sec. Same with reactor I guess, only it takes much longer?

Well, nuclear reactors are pretty much like huge steam engines, like in a steam train: You need to be shoveling coal like hell, if you need the extra power in 10 minutes.

Reactors are pretty similar, if you need more power in 3 hours, you need to slowly ramp the power output up from now on. And if you have less power demand, you need to slowly reduce power or completely shut the reactor down.

That is why nuclear reactors usually do base load, that means, they always run at 100% power, and use more agile power plants for the fast reactions.

 

[Izack]

That is why nuclear reactors usually do base load, that means, they always run at 100% power, and use more agile power plants for the fast reactions.

So in other words...not great for a small-scale precision operation like a Mars base?

 

[fireballs619]

I'm not quite sure about our power needs, but have we considered solar power?

 

[Bloodworth]

So in other words...not great for a small-scale precision operation like a Mars base?

Not necessarily. Several satellites have used nuclear reactors for their power systems. Both of the Voyager missions did, and I believe that the space shuttle launched at least 1 nuclear reactor equipped satellite.

 

[Izack]

Not necessarily. Several satellites have used nuclear reactors for their power systems. Both of the Voyager missions did, and I believe that the space shuttle launched at least 1 nuclear reactor equipped satellite.

You're thinking of [ame="http://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator"]RTG[/ame]s. Very different things. Nuclear fission power has never been used in space to my knowledge.

 

[Bloodworth]

I see your point. That wouldn't give the power requirements needed.

 

[Bj]

I'm not quite sure about our power needs, but have we considered solar power?

Solar power is really hard and inefficient in massive quantities at the distance of Mars. It would take a huge area covered in panels just to get the same from a reactor. Also, what would you do at night or when theres a sand storm?

to the base load question, I would guess it would be like a power gennerator, you can turn the gen on high even though there is no load on the gen. I guess the same for this. If not though the difference can be made with some starting batteries.

So far i calculated using in situ resources, we will need a lot of power to convert and make our fuel. A 1000 kWe gennerator can be solely dedicated to making fuel alone. Working out the details now though, should be done soon

 

[kwan3217]

Even though they were RTG, not reactors, the Voyagers still had this problem. As far as I know, Voyager had no batteries, so its RTGs were sized to run everything simultaneously. When it doesn't run everything, its RTGs are still providing full power (can't be throttled), which is dumped overboard by running the extra power through a resistor heater attached to a radiator panel.

Our reactors could do the same thing, be sized for peak power and dump the excess power when not needed. Maybe into a battery, maybe into an ISRU system, maybe just into the atmosphere. You will still need to plan ahead for heavy loads, but with this dump you can cut back as much as you want whenever you want.

 

[Izack]

Or, if energy is managed closely enough, into heat-lamps in the biodomes? Say, when some things shut down for the night, route the extra watts into those heaters since there is no longer solar energy for the plants.

Of course, this is only one mode of energy-dumping. You don't want to fry your plants when you flick your lights off. :lol:

 

[Bj]

Ok Orbit wiki:

O20000 Produce propellants in-situ for four lift-offs per season

I said in another post:

LOX/CH4 engines have ISP of 322-381 seconds. Assume 351 ISP seconds

LH2 has 33.3 kWh/kg

Assume 50% efficiency, 66.6kWh/kg to create LOX/CH4 fuel.

lander at dry 5,000kg would take roughly 13,500 kg per launch?

The ISRU would take roughly 899,100 kWh

Which for a single 1,000 kWe generator, it would take 37.5 days to create a full load of fuel.

However to fill the demand of so many launches. A quick guesstimate says we need 11 landings/launches + the number of power genns we need with 3-4 landings first before fuel production can even begin.

[math]\theta = 899,100 / power[/math] result in hours for production.

[math]\theta = (899,100 / hours)/1000[/math] result in number of power generator required for production.

Obviously we cannot wait 1.5 years to get enough fuel so the question becomes, pack the fuel with, or pack along more power generators. If we pack 18?... means about 50 hours per launch. Also note the ISRU's performance doesn't depend on size, but on quantity. More ISRUs will be needed for high fuel production.

Pros of packing ISRU:

• It will create the ability to have numerous launch and landings in the future with little cost of weight
• Improves independence from Earth's resources (though still requires hydrogen, which I suppose could be found in areas around Mars)
• The return portion of the OSHV is not dependent of the fuel produced by ISRU, it uses pre loaded fuel so no matter the timing, the OSHV can leave Mars, but it is a one way ticket...

Con

• First mission will be hard on the weight budget, also all this equipment needs to get to the surface, so fuel is still needed for multiple landings.

 

[T.Neo]

Nuclear fission power has never been used in space to my knowledge.

See SNAP-10A and [ame="http://en.wikipedia.org/wiki/RORSAT"]RORSAT[/ame].