I've been thinking about steam turbines. Obviously we use

Since you're going to have a lot of heat and compression in a steam engine, using something that's flammable is a bad idea because shit is going to explode

>>8021238
Doesn't Carnot efficiency stop steam engines from being anymore than 40% efficient or something like that?

>>8021244

You mean like with piston engines? I think it would be fine. But yea, the point of using chemicals with very low boiling points was to get a turbine that can generate electricity without producing excess temperatures.

>>8021262

I've got no idea.

>>8021262
No. More like 80%.

Most combined cycle powerplants get about 60%-70%.

>>8021262
how do you not even know what the fucking carnot efficiency is you mongoloid retard

>>8021306
80%? No that's way to high for a steam engine.

>>8021321

You'd be impressed at steam engines. Back in their day people did amazing things with them.

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We have engines that use low boiling point fluids. One interesting one is organic rankine cycle. Where you essentially run on refrigerant. Benefit of this is that you can operate at lower pressures.

Another interesting one is supercritical CO2. Supercritical CO2 is nice because it transfers more heat, so stuff can be more compact.

Now if you really want crazy you can make heat engines that use mercury or potassium as a working fluid.

More weird working fluids here
http://douglas-self.com/MUSEUM/POWER/oddfluid/oddfluid.htm

>>8021306
>1300K water
Nigger you what

>>8021340

Mercury is just too toxic for us to work with though. Yea we can tough it out but any accident will make expensive laborers retarded through heavy metal toxicity.

>>8021415
But we built one. It was a dumb idea, but someone built one.

>>8021238
>> heat dissipation in space.
So you wanna generate power from heat in space to solve heat dissipation? Won't fucking work.

You have to reject heat to somewhere for heat engines to work. In space you have to reject heat via radiation or throwing mass overboard. Heat engines will only increase the thermal resistance.

>>8021238
I thought shit froze in space?

Though it never made sense to me since the vacuum would have no particles to transfer thermal energy to.

>>8021436

It was very dumb, but it had to be done. We had to know. I know very little, mercury is second best to liquid nitrogen in my book in terms of low temperature turbine generators.

>>8021451

It does freeze, but because it's a vacuum it's hard to dissipate heat that's generated frequently. That's why I made this thread about liquid nitrogen turbine generators.


I've only worked in fast food lol I'm not with NASA I'm just curious.

I need realistic next gen solutions to make my starfighter fantasies in my mind be realistic enough for me to daydream about.

>>8021454
Heat engines will not help with heat dissipation. Read more on this here: http://www.projectrho.com/public_html/rocket/thermodynamics.php

>> mercury best for low temperature
Oh lord no. A big part of the reason for using mercury is that you can run at higher temperatures
More info here:
http://www.douglas-self.com/MUSEUM/POWER/mercury/mercury.htm

>>8021511

The point of a liquid nitrogen turbine is that liquid nitrogen boils 300 degrees below waters freezing point..

So it would cause no heat dissipation issues in space, right?

And yea, mercury is to liquid nitrogen like lead is to water in nuclear reactors for some applications

Water has a very high heat capacity, which means steam wont condense to liquid water very easily. A turbine would essentially be ruined by this condensation, so heat capacity is one of the, if not THE, most important factors in development of these devices.

It doesnt hurt that water is nontoxic, nondamaging to the environment, condenses/evaporates at near-ambient conditions, and is near-free.

>>8021238
>Obviously we use water because it's so common

we use water because it has a stupid high specific heat and is an incredibly stable and predictable working fluid.

that being said, your though process regarding nitrogen isn't totally off. alot of research is being done for other working fluids, and the most promising is supercritical CO2.

>>8021306
>Most combined cycle powerplants get about 60%-70%

you wish. more like 40-55 at best. only cutting edge power plants with really elaborate topping cycles can hit 65%+

>>8021697
Well 80% efficiency is normal for CHP (combined heat and power) cycle

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>>8021238
>>8021340
>>8021697
SCO2 is definitely coming if just for the power density improvements.

>GE has a prototype 10 Megawatt supercritical CO2 turbine that is ten times smaller than the equivalent steam turbine

>GE sees its new supercritical carbon dioxide turbine as a strong rival to batteries for storing power from the grid. GE Global Research is testing a desk-size turbine that could power a small town of about 10,000 homes. The unit is driven by “supercritical carbon dioxide,” which is in a state that at very high pressure and up to 700 °C exists as neither a liquid nor a gas. After the carbon dioxide passes through the turbine, it's cooled and then repressurized before returning for another pass.

>It’s about one-tenth the size of a steam turbine of comparable output, and has the potential to be 50 percent efficient at turning heat into electricity. Steam-based systems are typically in the mid-40 percent range; the improvement is achieved because of the better heat-transfer properties and reduced need for compression in a system that uses supercritical carbon dioxide compared to one that uses steam. The GE prototype is 10 megawatts, but the company hopes to scale it to 33 megawatts.

http://nextbigfuture.com/2016/04/ge-has-prototype-10-megawatt.html

>>8022887
You can make an engine more efficient by setting a kettle on top of it to boil water.

>>8022887
heat and power is way different than just power generation. the efficiencies go up because the waste heat that you can't generate electricity from is just used to heat the town around the plant. thats where the 80% efficiency comes from.

its not an 80% conversion from thermal energy to electric energy.

>>8022990

I can't believe this, it's just too impressive

>>8021238
>Would that be useful for space applications?

No. Lower boiling point doesn't really help all that much. Water is used not just because it's so common; water is actually one of the better working fluids that exists:

- Water has a very high heat capacity, and a high energy of vaporization, both of which are useful for a working fluid
- Even the phase change isn't all bad. In one case, it does reduce turbine life cycle dramatically, but it is much easier to pump a liquid than it is to pump a gas, so the 'compression' part of the cycle benefits from this.

There are, however, other working fluids which are researched occasionally, with their own benefits and drawbacks. Two that I've read about or worked with are ammonia and supercritical carbon dioxide.

>>8021244
>Since you're going to have a lot of heat and compression in a steam engine, using something that's flammable is a bad idea because shit is going to explode

This matters somewhat for safety reasons, but it's not a huge factor. You can use dangerous stuff, because turbines are always going to be closed systems. Besides, it's not like the pressurized steam that is currently used in steam turbines isn't dangerous. If any power plant turbine were to suddenly rupture, you can bet your ass that anyone nearby is going to die, fire or no fire.

>>8021262
>Doesn't Carnot efficiency stop steam engines from being anymore than 40% efficient or something like that?

Yeah, pretty much. It depends mostly on how hot you can get the hot side of the cycle. Some other working fluids could get hotter, but I think the temperature is usually limited by the lubricants used in the turbines themselves.

>>8021305
>You mean like with piston engines? I think it would be fine. But yea, the point of using chemicals with very low boiling points was to get a turbine that can generate electricity without producing excess temperatures.

This doesn't make any fucking sense.

(Continued in next reply...)

>>8021306
>No. More like 80%.

The 40% number is for electrical conversion efficiency (the actual useful comparable number), the 80% number is for co-generation plants, where the heat itself is one of the products. It's not a very good way of comparing efficiencies.

>>8021327
>You'd be impressed at steam engines. Back in their day people did amazing things with them.

Jesus, you're dumb.

>>8021340
>Another interesting one is supercritical CO2. Supercritical CO2 is nice because it transfers more heat, so stuff can be more compact.

You're close, but not quite spot on. The biggest benefit of supercritical co2 is that there is no phase change, meaning there's no risk of cavitation ruining your turbines. Additionally, at high low temperatures, it behaves more like a liquid than a gas, so you can still take advantage of the efficiency difference between pumps (for liquids) and compressors (for gases).

>>8021451
>Though it never made sense to me since the vacuum would have no particles to transfer thermal energy to.

You're on the right track! Things do cool off in space up until they reach thermal equilibrium with their surroundings (around 3 kelvin), but the problem is that they only cool via radiation. Look up 'black body radiation'.

>>8021454
>I need realistic next gen solutions to make my starfighter fantasies in my mind be realistic enough for me to daydream about.

Your starfighter doesn't actually need a lot of power, except for its main drive. Your main drive needs to eject mass no matter what (with some exceptions), in order for your starship to actually accelerate. Therefore, the best way to deal with waste heat in space is to literally eject the hot material -- no need for cooling, no need for thermodynamic cycles at all. A small turbine can be attached to the rocket exhaust to generate what little power the starship needs that's not thrust.

(continued...)

>>8021604
>The point of a liquid nitrogen turbine is that liquid nitrogen boils 300 degrees below waters freezing point..

Not actually helpful. The problem is that any thermodynamic cycle is just that -- a cycle. You would need some way to cool the nitrogen back down to liquid form after you heated it up to send it through the turbine. At this step, you still need to dissipate just as much total heat as you do for any other working fluid.

>>8021697
This guy's not dumb.

>>8023106
>I can't believe this, it's just too impressive

Because it works in theory. Supercritical carbon dioxide cycles are still in very preliminary development. I don't think anyone has built one yet that has surpassed (or even approached) any water-based systems.

>>8021238
>Obviously we use water because it's so common
And becaouse it has an outrageously high specific heat capacity. Even if it weren't common it'd still be a fantastic working fluid.
>but what about using alcohol
Good for waste heat recovery, I suppose.
https://en.wikipedia.org/wiki/Organic_Rankine_cycle
>or even liquid nitrogen?
Nitrogen is supercritical at normal and elevated temperatures. You can't liquify it without chilling it to cryogenic levels. That said, you could use an all-gaseous cycle (i.e. Brayton or Stirling) with gaseous pressurized nitrogen.
>I'm trying to think of a solution to the heat dissipation problems in space.
WHAT problems? I don't see how this really helps with any of the thermal problems specific to space.
>A liquid nitrogen turbine is interesting to think about.
I suppose. If you have some need for regulated, open-circuit cryocooling anyways (i.e. cooling the sensor of an infrared telescope), putting a turbine inline with the vent could be a way of extracting mechanical energy from the cooling system.
>I've got no idea if you could condense enough of it for recycling to make it worthwhile at all.
Yeah, probably not. Unless you head way out past the asteroid belt you're actually gonna need to put energy INTO the system to get the N2 cool enough to liquify.

>>8021238
liquid nitrogen is quite cold. It makes materials brittle. Cryogenic pumps and turbines definitely exist in LOX/LH rocket engines.

But also remember that spacecraft have heaters to keep their electronics within a temperature tolerance.

>>8023147
>Your main drive needs to eject mass no matter what (with some exceptions), in order for your starship to actually accelerate.
I never thought of this, but assuming a spaceship uses a propulsion method that follows Newton's third law, it should always leave a traceable trail of exhaust behind it when travelling through the vacuum of space.

>>8021454
>I need realistic next gen solutions to make my starfighter fantasies in my mind be realistic enough for me to daydream about.
I've got something for you. It's not exactly next-gen, but it's sort of an extreme measure to cooling that's also highly effective and applicable to the whole "space combat" paradigm in more ways than one.

On top of conventional radiators, your starfighter should have an open-circuit cooling system for specific circumstances where either heat production is extraordinarily high (i.e. intense combat or maneuvering where weapons and propulsion systems may be producing extreme levels of heat) or possibly when thermal signature must be minimized at all costs (i.e. during a stealthy flyby of a known threat). Compared to radiators, open-loop cooling systems offer far greater (nearly unlimited) peak heat rejection rates and are capable of cooling the exterior temperature of the spacecraft well below the local equilibrium temperature. However, their greatest drawback compared to radiators is that they consume a finite supply of coolant while in use, exhausting it overboard, and can therefore only function for a limited period of time.
>>8023147
>Things do cool off in space up until they reach thermal equilibrium with their surroundings (around 3 kelvin)
3 kelvin is the background radiation level. To reach an equillibrium temperature that low you pretty much need to be in interstellar space, distant from any star. Within the solar system the equillibrium temperature is significantly higher, especially within the inner solar system.
>Your main drive needs to eject mass no matter what (with some exceptions), in order for your starship to actually accelerate. Therefore, the best way to deal with waste heat in space is to literally eject the hot material -- no need for cooling, no need for thermodynamic cycles at all.
The only real issue with this is that is that propulsion requirements and cooling requirements don't always coincide.

>>8023225

OP here, my point is maintaining a turbine cycle in SPACE might be easier with fluids with a lower boiling temp. I'm not gonna lie, I'm stupid and uneducated in these fields. But if a turbine can operate a thousand degrees cooler with the same efficiency, it might solve problems and make compact spaceship designs more feasible.

>>8023106
SCO2 is hell on seals and materials in general. there are still a bunch of hurdles to overcome.

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just use divining rods

science can't explain it but it works every time

>>8023482
nah man, the way these things work, you want to have as big of a temperature differential as possible.

>>8023482

I think the part that's confusing you is a mix-up between the temperature of the working fluid and the amount of waste heat.

They're not as related as you think. If an engine is 50% efficient, and it produces 100 watts of electrical power, then it must also produce 100 watts of waste heat. This is true whether the working fluid is at 1000 kelvins or if it's at 10 kelvins.

The only coupling between the two quantities (waste heat and temperature of working fluid) comes into play in the calculation of efficiency, and in fact lower temperature working fluids lead to *lower* efficiency!

This means that, everything else the same, if you use a working fluid at a lower temperature, your engine will actually produce MORE heat.

>>8023482
>OP here, my point is maintaining a turbine cycle in SPACE might be easier with fluids with a lower boiling temp.
It isn't. Remember, cooling things off is difficult to do in space - especially if they're already fairly cold. It's much easier to work at elevated temperatures, where waste heat can be radiated away much more easily.
>But if a turbine can operate a thousand degrees cooler with the same efficiency, it might solve problems and make compact spaceship designs more feasible.
Quite the opposite, actually. High-temperature materials allow you to get more power and efficiency out of smaller machinery.

>>8023465
the answer to your open loop question is compressibility.

Materials science is nowhere near the level needed for that to be viable, unless you like refueling every 20 minutes.

>>8023912
>>8023583
You need to explain to OP how radiative heat transfer behaves instead of simply telling him it won't work that way.

A more simple explanation:
Computer graphics cards.

The "famed heat producing" AMD 290X was designed (with its reference cooler) to operate at 95C

Heat dissipates more quickly the higher the source's temperature is compared to the ambient (in open air, not space) temperature.
A graphics card programmed to thermally throttle at 95C will be able to dissipate more heat to the atmoshpere, rather to dissipate that heat faster, than that same graphics card if it were programmed to thermally throttle at 70C

Now, this example obviously does not apple so well to the vacuum of space.
But detailing out just a bit should tell people who don't know just how things work.

>>8023965
Sorry, double posting.

What I mean to explain is that, for instance, an object heated to 70C might take as much time to reach, say, 40C, as an object at 95C takes to reach 50C

Assuming air, or particle, flow is the same.