VASIMR

[Giorgio]

Where are we going to get a 200MW (megawatt) space nuclear reactor? Won't that be freakishly huge?

The reactor is small. It is the radiators that are the problem.

True, even if recent advancments in metamaterias and new understandings in the working of "highly mismatched alloys" might soon greatly reduce the need for radiators thanks to new generations of Peltier-Seebeck cells.

http://www.sciencedaily.com/releases/20 ... 113755.htm

[MSimon]

Where are we going to get a 200MW (megawatt) space nuclear reactor? Won't that be freakishly huge?

The reactor is small. It is the radiators that are the problem.

True, even if recent advancments in metamaterias and new understandings in the working of "highly mismatched alloys" might soon greatly reduce the need for radiators thanks to new generations of Peltier-Seebeck cells.

http://www.sciencedaily.com/releases/20 ... 113755.htm

Carnot says it is unlikely to help. It is only when the work is done outside the system (at the power plant) that you get any gain.

[Skipjack]

I think that nuclear thermal is better anyway. It is a simpler concept and you can get a much better T/W ratio.
Polywell would of course be the solution to all our problems
ML thrusters would not hurt either.

[Giorgio]

The reactor is small. It is the radiators that are the problem.

True, even if recent advancments in metamaterias and new understandings in the working of "highly mismatched alloys" might soon greatly reduce the need for radiators thanks to new generations of Peltier-Seebeck cells.

http://www.sciencedaily.com/releases/20 ... 113755.htm

Carnot says it is unlikely to help. It is only when the work is done outside the system (at the power plant) that you get any gain.

You have heat left from the first cycle (power plant) that you just use as a source to power a second/third /fourth power plant each one reducing the amount of heat that must be radiated as waste.

Hybrid power generating solutions like HMA, AMTEC, Thermophotovoltaics will hopefully reduce to a fraction the amount of heat to be rejected at the end of the power cycle.
There is still a lot of work to be done before we will reach that point, but year aftert year we are steadly progressing in that direction.


http://www.osti.gov/bridge/servlets/pur ... 810718.pdf
(Warning, 340 pages PDF!)

http://icapp.ans.org/icapp02/program/abstracts/1138.pdf

http://www.electrochem.org/dl/ma/201/pdfs/0834.pdf

http://www.sciencedirect.com/science?_o ... e6d54a04ae


AMTEC (alkali metal thermoelectric converter):
http://ieeexplore.ieee.org/Xplore/login ... ision=-203
http://en.wikipedia.org/wiki/Alkali-met ... _converter


Thermophotovoltaics:
http://www.technologyreview.com/read_ar ... notech&a=f

[MSimon]

You have heat left from the first cycle (power plant) that you just use as a source to power a second/third /fourth power plant each one reducing the amount of heat that must be radiated as waste.

You don't understand Carnot.

Maximum efficiency is : 1 - Tc/Th

The mass goes up the more you try to extract that last few %.

The best bet is hotter reactors - for which we do not have enough experience to do it in space.

One of the most useful things about a pBj Polywell is that the waste heat can be rejected at high temperatures. And secondly it is not a thermal machine. So on the generation side it is not limited by Carnot. There are of course other problems.

[Giorgio]

You have heat left from the first cycle (power plant) that you just use as a source to power a second/third /fourth power plant each one reducing the amount of heat that must be radiated as waste.

You don't understand Carnot.

Maximum efficiency is : 1 - Tc/Th

I think I do understand pretty well Carnot as my background is mechanical Engineering with a major in Energy.

You did not fully read those links or did not understand the many points expressed there.

http://ieeexplore.ieee.org/Xplore/login ... ision=-203

"the AMTEC cell, being an electrochemical converter of heat to electricity, has no moving parts and is not limited to Carnot-cycle efficiency."


Or maybe this will clarify it better:
http://ieeexplore.ieee.org/Xplore/login ... ision=-203

"Sacred among the mechanical engineers is the “second law of thermodynamics,” which defines the maximum possible efficiency of an engine that converts thermal energy into mechanical power. The second law value is the difference between the engine's heat-source temperature and its heat-sink temperature, divided by the absolute value of the engine's heat-source temperature. For example, an engine setting on 0° C ice and running on steam from 100°C boiling water is not allowed to have more than 26.8% efficiency. Power-generating violators of the second law efficiency-limit range from horses to fuel cells. They do not burn fuel to generate mechanical or electrical power. The latest second law violator is the alkali-metal thermal-to-electric converter."


What we are interested in space application is not the "Carnot Efficiency" of a single power extraction step, but to reduce to the minimum the quantity of heat to be radiated in space, and thus the surface of radiators.
As engineers we can do this in several way, and we are not obliged to use a "Carnot Cycle" machine in every aspect of the design of a spaceship.

For the one interested in better understanding the AMTEC principles, the first 50 pages of this Thesis are a good starting point:
http://etd.lib.ttu.edu/theses/available ... 017807.pdf



And yes, a pB11 Polywell will be a better solution, but, as far as we know, its feasibility is still hypothetical (with a solid theoretical base, but hypothetical nevertheless), while these alternative hybrid systems are progressing.

[MSimon]

It takes energy to raise the temperature of an object. It doesn't matter if you are pumping the heat or just injecting it (you do get some help when pumping it).

That energy has to come from the plant making the heat.

Given the losses in the energy production system it is probably cheaper (mass and energy wise) to just reject the heat at a higher temperature with the attendant losses in plant efficiency.

[taniwha]

One of the most useful things about a pBj Polywell is that the waste heat can be rejected at high temperatures. And secondly it is not a thermal machine. So on the generation side it is not limited by Carnot. There are of course other problems.

Yeah, and the biggest one is shielding for the side-reaction gamma rays and neutrons (for pB).

Assuming gamma power at 0.1% of reactor power, an unshielded 120GW reactor will give a fatal dose to anything within 1km in seconds. And that's just the gamma rays. It takes about 100,000km to get it down to safe levels (safe=adult can stand there pretty much forever).

Why 120 GW? From my calculations/simulations, you're not getting into orbit with much less (because of the shielding), and that's making a lot of assumptions about a Polywell's operation.

[Giorgio]

It takes energy to raise the temperature of an object. It doesn't matter if you are pumping the heat or just injecting it (you do get some help when pumping it).

That energy has to come from the plant making the heat.

That's obvious, the trick is in balancing the system with enough new technologies to improve what you will have left as "available" power.
Like I said, there is still lot of work to be done, but the progress is steady.

Given the losses in the energy production system it is probably cheaper (mass and energy wise) to just reject the heat at a higher temperature with the attendant losses in plant efficiency.

It will not make sense to have a 10MWt Space Nuclear generator system with 20 M2 of radiators if what you have left will only be 50 KW net available power. Same as it will not make sense if you will need 2 KM2 of radiators to get 5MW power.

The size of the radiators is the biggest vulnereability of a spaceship.
The bigger they are the more difficult is to protect them against micrometeorites. The harder is to bring enough spare parts to statistically cover mechanical breakage and fluid losses.
A disabled radiators means that you are on your own in space with no power or a reduced power.

We need a different way of thinking when dealing with space machines.

[taniwha]

Giorgio: Please read this (and the rest of the site). You might like the liquid droplet radiators

What it boils down to is that unless you have no other option (admittedly, our current situation), thermal energy production in space just doesn't make sense. You lose too much energy to Carnot. That's why MSimon is talking about flying Polywells and waste heat (of course, if you can tap some of the waste heat, more power to you).

[Giorgio]

Giorgio: Please read this (and the rest of the site). You might like the liquid droplet radiators

Yes, I know the site, and now that I saw it again I also remembered about the liquid droplet radiator. Every time I see it I get a shiver in my back spine. The logic is great, but the idea to delegate the thermal waste management of a Nuclear Reactor to an open system in space is so scaring that I will probably remove it again from my brain in the next few hours

What it boils down to is that unless you have no other option (admittedly, our current situation), thermal energy production in space just doesn't make sense. You lose too much energy to Carnot. That's why MSimon is talking about flying Polywells and waste heat (of course, if you can tap some of the waste heat, more power to you).

We are starting to have different options to produce energy in space from thermal sources, and they are not all limited by a Carnot cycle. They are still immature, but many of them have great potential. This is what my first post was all about.

[TDPerk]

Msimon, there are energy conversion systems to which the Carnot cycle simply does not apply. Thermoelectric is one of them, to the best of my knowledge.

If one surface of the radiator is very cold, and the other warm, and the radiator is thermoelectric, then there will be net power retained as high usability electricity at what may well be very little marginal cost in mass or $$$.

If the power source is a polywell, that electricity may nevertheless be superfluous.

[MSimon]

These devices can achieve very high fractions of Camot efficiency at
relatively low operating temperatures.

http://etd.lib.ttu.edu/theses/available ... 017807.pdf

[MSimon]

Giorgio,

I'm going to be a little snide here (as is my won't) and claim that you not only didn't get Carnot in school (or statistical thermodynamics for that matter) you didn't even read carefully the beginning of the article you suggested.

I mean seriously guys. Except for a 12 week course in Nuke Power School I have not "officially" studied thermodynamics. None the less every few months I drag out my thermo books for exciting bed time reading. Yeah. I'm strange.

So let me put it to you straight. When it comes to thermal energy you can't beat Carnot. For further reading on the subject may I suggest a look at Maxwell's Demon. Yeah. James Clerk Maxwell.

I think it all comes down to the perniciousness of education. Being uneducated I'm sure I am more than sufficiently ignorant of any topic I might care to study. It keeps me on my toes. Educated people get complacent. It is especially bad when they think they know something that ain't so.

[Giorgio]

These devices can achieve very high fractions of Camot efficiency at
relatively low operating temperatures.

http://etd.lib.ttu.edu/theses/available ... 017807.pdf

Carnot efficency is just used as a reference point.
"Carnot cycle" does not apply to this systems, but "Carnot Efficency" can be used as a reference tool to express their potentiality (in certain way similar to what the mach number is for jetplanes).