Advanced Solar Energy in Space: Part II (Turbines!)

[PB666]

3 hours ago, YNM said:

Good luck with that, I'm definitely not cooling my spacecraft down with bunch of passive coolers tied by ropes. A car radiator would still be better.

The ISS has radiators with coolant you know. And that's not having any super hot parts.

 

I'm sorry, I know it's meant to be "tough SF", but my very bad sense tells there's a constant disregard to proper thermodynamics. IRL thermodynamics are very important - even to fields not really concerning with actually utilizing heating/cooling/advanced fluid mechanics - so only seeing the good sides and disregarding their bad sides is just soft "magic".

For instance, it kept being said that the turbines can run at very high efficiency. But the fact that it needs cooling means some of that energy is simply lost yet again. Very high efficiency engines don't have coolers - for instance, Stirling engine - but they're impractical indeed.

Just get ribbons of PVs or something.

The crux of the problem forks into two areas, the kinetics/thermodynamics and the mass aspects. And I really do not understand why we are still discussing this as if it were a new topic, we have hashed this out to the Nth degree in other threads. Matter Beam seems to be dead-set on ignoring this.

You have a heat generator, it can be chemical, it can be nuclear, it could be a black hole it doesn't matter . .  switching from nuclear or chemical or black hole does not change this issue. Once you have heat you can use entropy to create energy.

So here is the basic solution on land. You build a medium size power generator on land and flow air through it, you may have even evaporative cooling towers to increase the effectiveness. Or you at larrger scale build next to a lake and borrow water from the lake which then radiates the heat over the surface of the lake, again most is passively transfer to the atmosphere that flows out and radiates it to a huge area of space. I have to repeat the obvious how many times? There is no air in space, there is no lakes in space, the radiative mass has to be entirely provided by the ship. You cannot use evaporative cooling towers in space. All heat needs to be radiated by the mass of the ship.

So the thermodynamics states that in order to generate power there has to be a energy differential. I start with alot of energy here, it drops in a transfer, and part of the energy is extracted in a different form, electricity, and the rest is radiated. Without a radiator the efficiency is zero, the ship glows until its read hot and there is no electricity generated except in the plasma generated during its unplanned disassembly. So for example, thermocouples create electricity at the rate of k/'K (where K is degree Kelvin and k is the efficiency constant for the thermocouple)  if the power plant is 1500 degrees and the radiators surface is 100 degrees, it can generate more heat than if the power plant is 1500 degrees and the radiator is 1300 degrees.  At some heat thermocouple wont work at all, you cant have the power plant at 10000 degrees and the radiator at 1300 degrees and expect it to generate more power it just burns up the thermocouples. Right now the very best reactor in space would even with modern technology be about 20% efficient, 25 if we pushed thermocouple technology to the limit. Steam generation is essentially an unproven technology in space for large scale use. But why isn't steam used in space?

The kinetics really addresses this problem if you don't have a place to sink heat into rapidly, then why have an translocative-base energy system if you don't have someplace to drop the energy particular if you don't want the mass of the steam system if you don't need it. For example if your reactor is boiling water at 150'C and your radiator is 130'C, your radiator is not going to transfer the heat from steam. But to get the surface temperature of the radiator down to 75'C you might need double the amount of a very heavy radiator. So really good generators need a good heat sink and this enables high energy heat carriers like steam to do much work. But in space steam makes almost no sense. I say almost for one reason. If you had a static station such as one of these circumsolar space colonies or a dyson swarm, steam would come in very handy at moving power at its source over long distances were it can be cooled and condense and return. Remember the above, no air or lakes in space, we have to radiate over large surfaces, if the source, say fusion, is by necessity concentrated in one spot, then we have to move the heat to all the distal mass of the ship. Steam moves very quickly, even under low pressure, and it carries heat very effectively. In this system the failure of system is secondarily contained, you don't lose the water because you simply have a metal transfer plate for the last half inch of travel to the outside of the ship. In that peculiar instance you don't care that the ship is heavy because you are not moving it and its heavy for 100 other reasons related to its function.  But for most ships going from one SME isoquant to another, steam makes no sense given the added weight it adds. There are other ways of doing this, you could have a staged system were heat near the reactor is cooled to say 250'C with sodium or some other liquid metal and this transfers to steam across the first thermocouple which is carried to the outside of the ship and drops to say 50'C before being pumped back to the core.

The mass issue. The problem with mass is two fold. One is function and other is structure. Lets talk structure, lets assume we have a powerful reactor (choose 1) and we have a radiator of a given size, as the radiator grows in size the amount of structure per unit of radiator falls at the beginning, but then starts to rise and at some point the structural mass would be greater than the radiator. For example the mass of a solar panel on a small space craft is much smaller per KW than the mass of the structure and panels on the ISS. Included are the trusses that hold the radiators, struts and binding trusses. Since the distance of the radiator is a distributive surface the transporter element (Steam, sodium, whatever) would be carried from large diameter to smaller diameter, as Size increase so do the number of sections of large diameter of pipe. For a decent size nuclear reactor this can equal a million gallons of water per minute in native cooling water. So we are talking about a sizable amount of pipe that increases faster then the energy that it is carrying.  The heat transfer system does not have to be a liquid you could have the reactor like a brake caliper on two balanced wheels. The reactor could transfer heat to the rolling wheels and as the wheels spin around the heat goes through the thermocouple and radiate, So for example you could have a sun facing side pick up heat from the reactor and then the heat is transferred to the space side over thermocouples which cool it before it reaches the reactor again. Wither it is a sphere, rectangle or disk the surface area grow with the square of radius, which allows more power, but also  increases the structural issues to bind it together and to the ship. One benefit of the wheel based system is that if the disk were neutron absorbing metals, you could dump your bad waste on the wheel and allow the waste to generate heat and use the good weight for thrust. Again, this might afford period of just running off of ambient electricity. Since the waste is on the sun side, as long as the neutrons are grabbed it should be of no further effect to the ship. In any case we are talking about huge masses.

If we break the mass problem down into radiator weight and use this logic.
-Solar panels have intrinsic radiators (except in a Mercurial environment) and the more efficient they get the less radiators they need (which is academic except inside the orbit of Venus)
-Nuclear power and chemical power of any kind has the same basic issue, they need radiators that currently are not efficient converters of heat. Given that radiative based systems are generally less efficient at capturing power than solar energy you are almost never going to use nuclear or chemical power to generate electricity when solar power is available 24/7 (I say almost because of the latest Mars rover).  As solar becomes more efficient, 40  . . . 50% it becomes harder and harder for nuclear power of any kind to compete. As I previously stated we don't need better nuclear power plants, we need better heat converters, converters that can turn 70% instead of 20% of the heat to electricity. This means, if you could do it, the drop in radiator surface area from 100 units to 37.5 %, for a space ship that alone would increase acceleration by ~11 fold. What this means is that at the low end of efficiency in a electric power system, for each percent of efficiency added you gain 2% in dV and this payback increases efficiency gets past 50%. This is good, but only if someone has a novel idea how to increase efficiency.

-Finally if we consider the break even point for a chemical energy or nuclear based electric engine, we look at the amount of fuel that we need to generate the dV, as weight continues to rise due to combined inefficiency and nomimal radiation the amount of dV extracted, the fuel drops and the amount of mass-ejection fuel we need to carry, eventually with a radiative cooled source power system we have a situation where there are no target in our dV range that could be reached, IOW we could neither carry enough fuel to get us anywhere from LEO because we could not leave LEO. When you try to move a nuclear based system bigger is not better, its worse. If you had a tiny reactor, say 10kw which generated 3 kw of power and had one HiPEP on it and you used the ships ION drives and other surfaces to radiate heat, you would have potentially something, but big, the ship is all radiator, forget. Im going to get back to these small HiPEP systems, I have reread the original papers and a found a few key observations that make this maybe a unique drive system.

Conclusion
My point to Matter Beam is this, what he is advocating is trying like using a pair of pruning sheers to chop down a Sequoia. The systems he advocates are either unsuited for use in space or are not really efficient enough to benefit a scenario where we say . . . we have solar.. . lets abandon it and go nuclear. OF course the flip side of that is if you have no sun, then you also do not abandon nuclear and go solar. In either direction the markets for Solar are likely to remain so indefinitely (unless you see some crazy efficiency appear), the markets for nuclear in space now are likely to remain so (unless you have a type of solar panel that runs of latent engine heat).

 

 

 

 

 

[MatterBeam]

7 hours ago, PB666 said:

But the blog, which he referred to, is not, he asked for critiques. I didn't tell him that his artwork or text was $#!^. If you ask someone who has refereed 1000s of papers for a critique are you then going to say to that person, but don't use any of the skills you developed as a reviewer or journal? If you capsulize what a person on the editorial staff of  journal does . . . .its about content, logic, syntax, spelling, symbolism, peer-view (and their authorit), authority, imagery, referencing, style. We don't pick and choose what we want to review, we have to review all. I have even criticized K2, here when the symbolism was inadequate. I expect others to criticize me.  The core editors of a publication has to look at a work and decide somewhat subjectively, whether it will benefit the readers of the journal, that's really it in a nutshell. Publishing something puts it as a part of the published sphere forever. And when it is not, like the silencing of heretical gospils or the burning of the library at alexandria, society generally has regrets. Blogs used to be like a diary, more or less like mendels studies of peas,  but now they are closer to publications spirit of the process becomes zen of blogging.

There is a particular philosophy that I carry with regard to publication which I wish was shared by all scientist. Your best friend is your most hostile critic who is close to you, the closer they are to you and more critical they are, the more, ultimately, they will benefit your work as long, in most cases, that the work is not rejected. I have seen many a time when people clamp their mouths when they saw something they knew was in error. It generally does not benefit the authors, the folks who, in my younger days I clamped my mouth, are no longer in the field and left the field unexpectedly quickly. It was fortunate that my graduate work was in a lab with a very critical and vocal advisor that encouraged critique.

I think you're getting lost in your own argument. What I asked was... nothing. I just copied over the blog post for anyone who would want to "read and comment on". Whatever 'critiques' mantra you've got going on, and whatever logical hoops you want us to jump through, such your personal history and whatever the library of alexandria has got to do with anything, I'm not interested. 

6 hours ago, YNM said:

Good luck with that, I'm definitely not cooling my spacecraft down with bunch of passive coolers tied by ropes. A car radiator would still be better.

The ISS has radiators with coolant you know. And that's not having any super hot parts.

 

I'm sorry, I know it's meant to be "tough SF", but my very bad sense tells there's a constant disregard to proper thermodynamics. IRL thermodynamics are very important - even to fields not really concerning with actually utilizing heating/cooling/advanced fluid mechanics - so only seeing the good sides and disregarding their bad sides is just soft "magic".

For instance, it kept being said that the turbines can run at very high efficiency. But the fact that it needs cooling means some of that energy is simply lost yet again. Very high efficiency engines don't have coolers - for instance, Stirling engine - but they're impractical indeed.

Just get ribbons of PVs or something.

A car radiator is used in a car, on Earth, in an atmosphere. It is different from a space radiator used in a spaceship, in space. Does that make more sense?

I believe the rest of your comments are simply a misunderstanding of what thermodynamics are, coupled with a lack of anything more than a superficial reading of what was written. For example, you state that very high efficiency engines don't have coolers... which is completely wrong. 

[PB666]

1 minute ago, MatterBeam said:

I think you're getting lost in your own argument. What I asked was... nothing. I just copied over the blog post for anyone who would want to "read and comment on". Whatever 'critiques' mantra you've got going on

That is simply avoiding the issue, not dealing with it. You believe what you believe, you asked for critiques, but deep in your heart you really don't want them. I have noticed this behavior in past discussions. But the problem is throwing off the problem as someone else's Mantra won't fix them. Either your currency is in the world of science and engineering or your currency is in the world of myth and fake facts.

There is an observation of people here that somehow that if they dream badly enough to want something to happen, that it can somehow happen. This is the problem with Isaac Arthur's work, that somehow all the dreams of science fiction or fantasy could in some distortion of reality become fact. They frequently hinge on one notion . . . if a warp drive becomes a thing someday then.  . . .if we get fusion finally working then. . . . . .But frequently the thing they think will solve all the problems is only the beginning of much more severe problems. (And I want to point out that I agree with alot of what Isaac says in his video, but it relies heavily on what people want him to talk about (something they saw in sci-fi) and not really what engineering in science would predict.

The proof of any pudding is in the eating. That which will likely work in the future is that which is currently working now that can be improved upon, why would you try an 4-stroke (oxygen breathing) engine in space if they are hard to engineer for the task and we have both sources of electricity and thrust that are magnitudes better suited for space. A simpler version of the logic is why put a screen door on a submarine.