low and high temperature radiators?

[jedidia]

First off, I get the general concept. Mixing heats of different temperature is a problem, so it makes sense to divert low-temperature heat to a different set of radiators than high-temperature heat.

What I don't understand about it is the math... More precisely, I haven't been able to find it, which left me a bit puzzled. How exactly do I mix heats of different temperatures mathematically?

Also, is there any significant difference in the construction of low temp and high temp radiators (I understand that high-temp radiators are hypothetical as noone has yet had a reason to build one, but the concept is well enough understood), or is their only difference the context in which they are being used? I assume that both radiate heat according to the Stefan Bolzmann law (approximately at least), or are there different dynamics at work for low temperature radiators?

 

[Urwumpe]

Well, think of what kind of temperature you want to get out of the radiator. Otherwise, it is pretty much like mixing water in a bathtub.

The dynamics are the same, you just use the lower exit temperature for low temperature systems.

 

[jedidia]

Otherwise, it is pretty much like mixing water in a bathtub.

Just that you have joules instead of litres? I remember doing that math in school. No idea anymore how it went, but having worked with it in school makes me pretty confident that I'll be able to work with it now. :lol:

EDIT:

Ok, I just found my misconception. I was always of the impression that heat is somehow pumped into te radiators to raise their temperature as much as possible for better radiation, and completely forgot the basic law of thermodynamics: That heat travels from the higher temperature to the lower. :facepalm:

So in other words, I have to keep the radiators cooler than the parts that pass their heat to them. I.E. they need a pretty high heat capacity (I always thought a low heatcapacity was preferable to more easily let them reach higher temperatures).

Subsequently, if I dump the heat from a high-power device into a radiator, its temperature will rise above the temperature of the low-power producers, meaning they won't be able to pass heat to it anymore, hence I need cooler radiators for those. Seems like I got it now...

 

[Urwumpe]

Exactly that. I would usually not claim to be an expert there, especially not late at night here, but I recently finished a project that involved a 1D simulation of car cooler systems, and there you also have such features like high and low temperature coolers.

One factor that you should include in your model is the temperature difference that your cooler produces, this is just a function of inlet temperature and cooler properties. The higher the inlet temperature, the higher the temperature drop, but: Never lower exit temperatures - that would violate some laws of thermodynamics at once. A bigger cooler can for example produce a higher temperature drop than a small one, at the same mass flow rate.

 

[jedidia]

This isn't exactly about radiators anymore, but it's related to heat, so I thought I'd just put it i the same thread.

I get efficiencies: No matter what you do, some ammount of the energy invested is always lost as heat, e.g. when you convert various forms of power to electrical.

But once the energy is in the system, I'm not quite sure what happens. It does some work, e.g. in electronic systems or whatever. Now, energy doesn't get destroyed, so unless the work the energy does transforms it into another form of energy, I would expect it to turn into heat completely. I.E. all the power for example my computer draws eventually turns up as heat, not just a part of it according to the efficiency of the device.

This would mean that sooner or later, I have to get every single Watt produced in my ship out of the ship again in one way or another, not just the "waste heat". Is that correct?

 

[jangofett287]

Your forgetting light and sound energy.

 

[Urwumpe]

T
This would mean that sooner or later, I have to get every single Watt produced in my ship out of the ship again in one way or another, not just the "waste heat". Is that correct?

yes, everything that is not used for propulsion is essentially waste heat. A bit of it will maybe be dissipated by warming the passive thermal protection system (insulation), maybe you will get heated by the sun more, than you dissipate that way.

Practically, you need to be able to pump every heat inside your spacecraft to the outside.

 

[jedidia]

mkay, got it all figured out more or less (athough I skipped simulating the cooling system, the heat is passed to the radiator directly. It shouldn't get too complex currently...)

Now I have a basic heat managment system, which brings me to those annoying hydrogen tanks. Seriously, how do you keep a hydrogen tank cool??

I know how a fridge works (by compressing a gas to heat it up so you can pass the heat on to something else), but what do we have that stays gaseous when hydrogen is liquid?

 

[RisingFury]

I know how a fridge works (by compressing a gas to heat it up so you can pass the heat on to something else), but what do we have that stays gaseous when hydrogen is liquid?

No, that's not how it works in modern fridges. The idea is the same - compress something, heat it up, let it cool somewhere else and expand it again later, but fridges use a gas that can be liquefied at low pressures. For example, butane under pressure will liquefy and release the energy of evaporation. If you store your working gas near the pressure it takes to liquefy it, you can only increase the pressure a little bit, thus wasting very little energy, but gaining a huge return because the heat of evaporation is generally high.

Another way of cooling is to let a liquid boil away by decreasing the pressure of gas above it and thus lowering its boiling point. Such a system would require you to capture the evaporating gas and is generally quite inefficient.

 

[jedidia]

If you store your working gas near the pressure it takes to liquefy it, you can only increase the pressure a little bit, thus wasting very little energy, but gaining a huge return because the heat of evaporation is generally high.

Ok, got it. Now the question is still, what gas do they use to cool something like liquid hydrogen (as it has to be gaseous in the cooling loop). Hydrogen that is slightly less compressed?

 

[Marvin42]

How your radiators should be:

A fridge works like this: the fluid needs a pressure drop. With it, comes a temperature drop. To do that you can use an expansion valve or a difference in the pipe's sections (comming from a small diameter to a large one). The fluid (a mixure of gas an liquid) is passed through a heatexchanger an it starts to absorb heat. It can absorb a lot of it only if it uses that heat to something. We are using freon because at normal outside temperatures it evaporates. So the freon, entering in the heatexchanger (it's called evaporator and is inside your fridge) will evaporate consuming all the heat it absorbs to change it's state from liquid to gas. It remains cold because all energy is used for the state change. Now, because we have only gas, we can compress it to raise the pressure again so the system can become a cycle. The problem is that after we compress it we still have only gas. We need also liquid (before we expand it again). So we need a heat source to condense it (the heatexchanger to do that is on the back of your fridge). During condensing we give away the heat from the compression but all that energy reduction is used by freon to change in liquid so it remains hot. Hot is good, because it means we still have high pressure so we can expand it again to cool it. Also, there are more than one way to remove heat. The problem is that most systems used on earth need gravity to work (a compressor needs oil to lubrificate, etc). Most heat-exchangers are also made considering earth's gravity in their advantage. In space there will be a different problem. In a low gravity system, for a high temperature differences an absortion system would work better because it has no moving parts. [ame="http://en.wikipedia.org/wiki/Absorption_refrigerator"]Absorption refrigerator - Wikipedia, the free encyclopedia[/ame]. Although it is not an effective way to cool on earth with it (freon systems are much better), in space is a better alterantive. Modified versions are used on ISS. As source it can use heat ot run. Another way is to use a passive cooling for low temperature differences.

 

[RAF92_Moser]

Now I have a basic heat managment system, which brings me to those annoying hydrogen tanks. Seriously, how do you keep a hydrogen tank cool??

I know how a fridge works (by compressing a gas to heat it up so you can pass the heat on to something else), but what do we have that stays gaseous when hydrogen is liquid?

To liquefy hydrogen or helium, you have to intially cool the gas. You cannot use a throttling process (where you isothermally compress the gas into a liquid state, and then let it cool by decompression) for hydrogen and helium at room temperature.

The attractive interactions for these gases are very weak. At higher temperatures (such as room temperature) the gas molecules are moving too fast to really experience such an interaction. They still collide in which their is a high potential energy. When the gases expand, the collisions occur less frequently, so this high positive potential energy decreases and the kinetic energy increases (thus you actually increase the gas's temperature).

So you have to initially cool the hydrogen or helium using precooled gases. The temperature has to decrease til the point where attractive forces are more significant than repulsion forces. This temperature is called the inversion temperature:

For H: 204 K
For He: 43 K

Hydrogen can be liquified by using liquid air for precooling (first done in 1898). Once hydrogen is liquified, you can use it to precool He gas, and thus liquify He using throttling processes.

Source: Introduction to Thermal Physics by David Schroeder, pg. 142.

PS: I know the original question was on how to keep it cool, not create liquid hydrogen, but perhaps this would help. You can keep liquid hydrogen cool by using liquid air and some really good insulation. Create the liquid air by throttling processes (see Hampson-Linde cycle specifically).

 

[jedidia]

For H: 204 K

Wait... you're telling me that LH2 can be stored at 200 K?? Nah, I probably just got something backwards...

 

[RAF92_Moser]

Wait... you're telling me that LH2 can be stored at 200 K?? Nah, I probably just got something backwards...

Hahaha, we wish!

That is the inversion temperature. That is the temperature at which you can begin to cool H2 via throttling. If you try to cool H2 above 204K via throttling, you actually heat it up!

 

[jedidia]

Ah, like that. Thanks!

 

[jedidia]

After some more searching, I can't find enough data to make much sense of it all. I'm still looking for LH2 storage methods.

For space, I think compressed storage seems to be the only real option, as everything else means lots of dead weight. Still, the only thing I learned with certainty is that the liquid hydrogen is stored at 20.3 K. What I didn't find out is how the pressure is kept when the tank gets emptied (carrying the same volume of some other liquid or sufficient amounts of gas with you seems very unpracticle).

The other thing I still don't quite get is how to take heat out of it when it heats up. Ok, I have to run a gas by it that is obviously lower in temperature than 20.3 K (a challenge in and of itself, I guess Helium would be used, but I have really no Idea). This gas takes heat from the hydrogen, then gets directed back to the cooling system where it is liquified by pressure, releasing its evaporation heat (on second thought, Helium has an awfully low evaporation heat. Maybe not suited so well after all), which goes... where? Just into the surroundings despite them having a higher temperature?

I assume so, since this is strictly speaking an energy release and not a heat transfer, so "travels from higher to lower temperature" probably doesn't apply. I'd like to have confirmation for that before starting to code, though... Did I get that bit right?

 

[Capt_hensley]

Space Station uses circulated ammonia. I figured on using roughly the same system for my liquid storage facilities. I'll be compressing and cooling on the same order as an earthbound cryogenic storage facility, except hardened for space operations. \

Insulation will provide the bulk of the work, and a stir system will keep the fluid in motion to prevent solidification.

Boil off will not be preventable, but at the resupply rate I have in mind, it will not be a problem either.

I ran into another problem though, if the tank capacity is not kept at at least 85%, expansion perpetuates boil off. Two ways to reduce this are a. Use smaller tanks with more insulation, b. change the size of the tank on the fly with a flexible bladder.

The radiators for the cooling system have a limitation as well. Efficiency of evaporation(which is different in space) depends on the entire system working at a constant rate. The radiators must remain out of the sunlight to remove the heat from the emerging coolant, thus dissipating the heat collected. Efficiency is about 15% with this method, but you must have a system with at least 65% to work effectively to keep the temp of the coolant down enough to make a difference. A pre-cooler in multiple stages can work, but at this point the mechanical equipment starts to become more complicated and increases you failure points along with a higher chance of failure.

The cooling game in space is a trade off, the more complicated the system, the higher the cooling rate, but also the higher chance of failure. Good luck.

 

[MaverickSawyer]

After some more searching, I can't find enough data to make much sense of it all. I'm still looking for LH2 storage methods.

For space, I think compressed storage seems to be the only real option, as everything else means lots of dead weight. Still, the only thing I learned with certainty is that the liquid hydrogen is stored at 20.3 K. What I didn't find out is how the pressure is kept when the tank gets emptied (carrying the same volume of some other liquid or sufficient amounts of gas with you seems very unpracticle).

Have you considered mixing in a bit of methane to gel the LH2? It will prevent convection losses.
Hell, switch to methane entirely, and you just need to deal with a fuel that is about the same temperature as LOX.