Why don't they use refrigerant in a stirling engine?

[fleabag]

I've been trying to wrap my head around the stirling engine, I've seen models made on youtube perfectly functioning but I also read (on wikipedia) that stirling engines have a lot of optimization to be done. It seems like a very underdeveloped device as I cannot find a preassembled stirling engine, but kits instead which are more for amusement purposes than anything else. "Since the Stirling engine is a closed cycle, it contains a fixed mass of gas called the "working fluid", most commonly air, hydrogen or helium."

Also the stirling engine sounds more like a refrigerator than anything else and I think there is proof of that with this tidbit found in the wiki article "What appears to be the first mention of a Stirling cycle machine using freely moving components is a British patent disclosure in 1876 [36]. This machine was envisaged as a refrigerator (i.e., the so-called reversed Stirling cycle) and the piston was therefore driven externally. The very first consumer product to utilize a free-piston Stirling device was a portable refrigerator manufactured by Twinbird Corporation of Japan and offered in the US by Coleman in 2004."

But I've yet to hear of anything using a stirling engine whatsoever besides this and definately never heard of anyone using a refrigerant instead of hydrogen or helium. What would be the reason they'd use hydrogen or helium anyways? According to my dad, hydrogen is the worst substance you could possibly use as a refrigerant, that even air would be much better.


Or how about they use ammonia?

 

[Modelworks]

Ammonia is already used in certain types of refrigeration , mainly RV, which also uses hydrogen.
One of the best refrigerants , though not used as much because of the dangers, is propane.
Propane is actually better than r134a which is what most refrigerators now contain.

 

[BrownTown]

Well helium and hydrogen have EASILY the highest specific heats. I'm not sure why that would really be a good thing though, you would have to ask someone with a better thermodynamics background. But saying that hydrogen is the worst refrigerant out there is misleading, depening on what you are talking about it is actually by far the best in terms of removing heat. I know they use it to cool the generators at big power plants and such.

 

[CycloWizard]

The ideal working fluid for a heat engine would have zero heat capacity. This would require minimal energy input/output to achieve the desired temperature change. If you use a fluid with a very high heat capacity, then you have to pump in lots of energy to achieve the same temperature change, so things become more complicated.

Refrigerants are generally not used in Stirling engines because it's a one-phase system and refrigerants are primarily used in phase change processes. A refrigerant will be selected for a phase change process (like conventional engines) if its boiling point temperature is at the process' working pressure.

edited for clarity

 

[BrownTown]

Originally posted by: CycloWizard
The ideal working fluid for a heat engine would have zero heat capacity. This would require minimal energy input/output to achieve the desired temperature change. If you use a fluid with a very high heat capacity, then you have to pump in lots of energy to achieve the same temperature change, so things become more complicated.

yeah, thats what I though which is why you wouldn't think hydrogen or helium would be used, but on the other hand since they are the lightest gases, the specific heat per mole of gas isn't nearly so high. And the ideal gas law would say that the pressure would increase based on the number of moles, not the number of grams of working fluid, so maybe when you factor that it its not bad.

 

[CycloWizard]

Originally posted by: BrownTown
yeah, thats what I though which is why you wouldn't think hydrogen or helium would be used, but on the other hand since they are the lightest gases, the specific heat per mole of gas isn't nearly so high. And the ideal gas law would say that the pressure would increase based on the number of moles, not the number of grams of working fluid, so maybe when you factor that it its not bad.

This is true. However, very small gas molecules tend to deviate from ideal gas behavior at relatively low pressuers due to their compact structures. Thus, if you consider a simple compressibility factor equation of state (z=PV/RT, where z is the compressibility factor and z=1 recovers the ideal gas equation), the compressibility factor for these small molecules is dependent on the molecular size of the gas. Sort of.

 

[Zorba]

Originally posted by: CycloWizard
The ideal working fluid for a heat engine would have zero heat capacity. This would require minimal energy input/output to achieve the desired temperature change. If you use a fluid with a very high heat capacity, then you have to pump in lots of energy to achieve the same temperature change, so things become more complicated.

If you are running a working fluid with a low specific heat, yeah it'll take less energy to heat it up to temperature but it will also have a lot less energy available to extract in a turbine/piston/etc. Thereby causing you to have to run much higher mass flow rates to get the same energy extraction in the turbine/piston.

Also a zero heat capacity fluid would not be able to hold any energy, making it useless for an engine.

That said, I could definitely see the benefits of a low specific heat fluid for something like the stirling engine, depending on its heat source.

 

[CycloWizard]

Originally posted by: Zorba
If you are running a working fluid with a low specific heat, yeah it'll take less energy to heat it up to temperature but it will also have a lot less energy available to extract in a turbine/piston/etc. Thereby causing you to have to run much higher mass flow rates to get the same energy extraction in the turbine/piston.

Also a zero heat capacity fluid would not be able to hold any energy, making it useless for an engine.

That said, I could definitely see the benefits of a low specific heat fluid for something like the stirling engine, depending on its heat source.

Obviously there is no fluid with zero heat capacity. In general, it's much better to have a low heat capacity fluid, so I just used "zero" to indicate an asymptotically low heat capacity. A zero capacity fluid has the theoretical benefit of allowing 100% efficiency since the temperature reservoirs will be infinitely different in temperature. I also considered the idea that the flowrate in such a process would have to be much higher, but the costs for moving a working fluid are almost always negligible compared to the gains seen by achieving a higher process efficiency.

 

[Zorba]

Originally posted by: CycloWizard

Originally posted by: Zorba
If you are running a working fluid with a low specific heat, yeah it'll take less energy to heat it up to temperature but it will also have a lot less energy available to extract in a turbine/piston/etc. Thereby causing you to have to run much higher mass flow rates to get the same energy extraction in the turbine/piston.

Also a zero heat capacity fluid would not be able to hold any energy, making it useless for an engine.

That said, I could definitely see the benefits of a low specific heat fluid for something like the stirling engine, depending on its heat source.

Obviously there is no fluid with zero heat capacity. In general, it's much better to have a low heat capacity fluid, so I just used "zero" to indicate an asymptotically low heat capacity. A zero capacity fluid has the theoretical benefit of allowing 100% efficiency since the temperature reservoirs will be infinitely different in temperature. I also considered the idea that the flowrate in such a process would have to be much higher, but the costs for moving a working fluid are almost always negligible compared to the gains seen by achieving a higher process efficiency.

I am not sure how you are getting that it is more efficient to have a lower specific heat. Either way you have to put the same amount of energy into a system to get the same amount of work out. For a heat exchanger it is most efficient to match the heat capacity (m(dot)*Cp) of the fluid on each side of the exchanger.

Maybe I am missing something but I have never seen specific heat in any fundamental efficiently equation, like carnot, etc. before or in relation to heat transfer efficiency except for matching m(dot)*Cp. If you want to argue that you can get a lower specific heat fluid hotter easier, I might see the point you are trying to make a little more, but all other things the same (including operating temperatures) I think a REAL system would be more efficient with a high heat capacity working fluid, assuming there is a high quality heat source (ie boiler, combustor, etc not a car tailpipe).

 

[Zorba]

also, if lower specific heat made the best working fluid why do the vast majority of power plants run water as a working fluid instead of Air?

 

[CycloWizard]

Originally posted by: Zorba
I am not sure how you are getting that it is more efficient to have a lower specific heat. Either way you have to put the same amount of energy into a system to get the same amount of work out. For a heat exchanger it is most efficient to match the heat capacity (m(dot)*Cp) of the fluid on each side of the exchanger.

Maybe I am missing something but I have never seen specific heat in any fundamental efficiently equation, like carnot, etc. before or in relation to heat transfer efficiency except for matching m(dot)*Cp. If you want to argue that you can get a lower specific heat fluid hotter easier, I might see the point you are trying to make a little more, but all other things the same (including operating temperatures) I think a REAL system would be more efficient with a high heat capacity working fluid, assuming there is a high quality heat source (ie boiler, combustor, etc not a car tailpipe).

Heat capacity doesn't appear in any efficiency equations. However, you can easily see how it plays a role if you consider how power cycles are designed. Plants are specified as requiring a given power rating (e.g. 1 GW). Thus, to maximize the temperature difference between the hot and cold reservoirs at constant power, thereby maximizing efficiency, one needs to minimize heat capacity.

 

[Zorba]

Originally posted by: CycloWizard

Originally posted by: Zorba
I am not sure how you are getting that it is more efficient to have a lower specific heat. Either way you have to put the same amount of energy into a system to get the same amount of work out. For a heat exchanger it is most efficient to match the heat capacity (m(dot)*Cp) of the fluid on each side of the exchanger.

Maybe I am missing something but I have never seen specific heat in any fundamental efficiently equation, like carnot, etc. before or in relation to heat transfer efficiency except for matching m(dot)*Cp. If you want to argue that you can get a lower specific heat fluid hotter easier, I might see the point you are trying to make a little more, but all other things the same (including operating temperatures) I think a REAL system would be more efficient with a high heat capacity working fluid, assuming there is a high quality heat source (ie boiler, combustor, etc not a car tailpipe).

Heat capacity doesn't appear in any efficiency equations. However, you can easily see how it plays a role if you consider how power cycles are designed. Plants are specified as requiring a given power rating (e.g. 1 GW). Thus, to maximize the temperature difference between the hot and cold reservoirs at constant power, thereby maximizing efficiency, one needs to minimize heat capacity.

You can get to the same temperature regardless of heat capacity :roll:. The limiting factor on max temperature is your heat source and the materials of your system. To get the same temperature you would just decrease the mass flow rate of the higher heat capacity fluid. Again the most efficient mode of heat transfer in a HX is when the mass flow rate times the specific heat is the same on both sides of the exchanger.

Again if your argument was true at all then why would major power plants use water as a working fluid instead of air?

I am sure you will come back with "It is more complicated to heat a higher specific heat fluid." Which is complete bull. If easy thing in the real world it is easier to heat a high specific heat fluid because the mass flow rates required would be so much lower resulting in much smaller boilers/ ombustors/HXes.

Not to mention parasitic losses go way up with increased mass flow rates, especially if you get the higher flow with high velocities (instead of larger/more piping).

 

[CycloWizard]

Originally posted by: Zorba
You can get to the same temperature regardless of heat capacity :roll:. The limiting factor on max temperature is your heat source and the materials of your system. To get the same temperature you would just decrease the mass flow rate of the higher heat capacity fluid. Again the most efficient mode of heat transfer in a HX is when the mass flow rate times the specific heat is the same on both sides of the exchanger.

You can get the same temperature difference with any heat capacity at steady state, but not in transient conditions. A higher heat capacity fluid will always require a longer startup time during which efficiency is lower. Therefore, overall process efficiency will always be lower. :roll: Oh, and your "most efficient mode of heat transfer" is simply conservation of energy, as well as another reason why my suggestion of a low heat capacity working fluid could be a good one since a low mass flowrate of a high capacity exchange fluid could be used.

Again if your argument was true at all then why would major power plants use water as a working fluid instead of air?

Because power plants use phase change cycles to produce electricity. Stirling cycles, which we're talking about here, are not phase change cycles. I think you're getting a little carried away with my mistakenly saying "heat engines" rather than "Stirling engines" in one post.

I am sure you will come back with "It is more complicated to heat a higher specific heat fluid." Which is complete bull. If easy thing in the real world it is easier to heat a high specific heat fluid because the mass flow rates required would be so much lower resulting in much smaller boilers/ ombustors/HXes.

ou have to use strawman arguments? Are we in P&N now?

 

[Zorba]

Originally posted by: CycloWizard

Originally posted by: Zorba
You can get to the same temperature regardless of heat capacity :roll:. The limiting factor on max temperature is your heat source and the materials of your system. To get the same temperature you would just decrease the mass flow rate of the higher heat capacity fluid. Again the most efficient mode of heat transfer in a HX is when the mass flow rate times the specific heat is the same on both sides of the exchanger.

You can get the same temperature difference with any heat capacity at steady state, but not in transient conditions. A higher heat capacity fluid will always require a longer startup time during which efficiency is lower. Therefore, overall process efficiency will always be lower. :roll: Oh, and your "most efficient mode of heat transfer" is simply conservation of energy, as well as another reason why my suggestion of a low heat capacity working fluid could be a good one since a low mass flowrate of a high capacity exchange fluid could be used.

Why not just use a lower mass flow rate on both sides? Also if m(dot)*Cp is the same it will take just as long for either to heat up to temperature. If you are talking about the same amount of mass, then yes the lower specific heat fluid would heat up quicker.

You can conserve energy without m(dot)*Cp being equal on both sides of an a heat exchanger, btw. You would just end up with one side responding much more quickly than the other.

Again if your argument was true at all then why would major power plants use water as a working fluid instead of air?

Because power plants use phase change cycles to produce electricity. Stirling cycles, which we're talking about here, are not phase change cycles. I think you're getting a little carried away with my mistakenly saying "heat engines" rather than "Stirling engines" in one post.

This all got started because you said "heat engine" implying all heat engines. I said early on that for a Stirling engine I could see the benefits of a lower specific heat working fluid.

I am sure you will come back with "It is more complicated to heat a higher specific heat fluid." Which is complete bull. If easy thing in the real world it is easier to heat a high specific heat fluid because the mass flow rates required would be so much lower resulting in much smaller boilers/ ombustors/HXes.

ou have to use strawman arguments? Are we in P&N now?

Strawman, no, you said that heating a higher specific heat fluid was more complicated in an early post. That wasn't a straw man argument at all.

then you have to pump in lots of energy to achieve the same temperature change, so things become more complicated.

 

[myocardia]

Originally posted by: Zorba
Again if your argument was true at all then why would major power plants use water as a working fluid instead of air?

That's easy. All large-scale electrical generators on the planet use steam to turn the turbines, including the nuclear generators. Water (turned to steam) is many times more efficient than trying to use heated air, especially since the boilers and the turbines/generators aren't in the exact same location. They're close to each other, yes, but heated air would lose it's velocity (because it cools too quickly) much faster than steam does, making it less efficient.

 

[CycloWizard]

Originally posted by: myocardia
That's easy. All large-scale electrical generators on the planet use steam to turn the turbines, including the nuclear generators. Water (turned to steam) is many times more efficient than trying to use heated air, especially since the boilers and the turbines/generators aren't in the exact same location. They're close to each other, yes, but heated air would lose it's velocity (because it cools too quickly) much faster than steam does, making it less efficient.

At some point in this thread, all context was lost in an effort by some guy to make himself look smarter than everyone else. This seems to be a recent trend here at AT.

 

[Zorba]

Originally posted by: myocardia

Originally posted by: Zorba
Again if your argument was true at all then why would major power plants use water as a working fluid instead of air?

That's easy. All large-scale electrical generators on the planet use steam to turn the turbines, including the nuclear generators. Water (turned to steam) is many times more efficient than trying to use heated air, especially since the boilers and the turbines/generators aren't in the exact same location. They're close to each other, yes, but heated air would lose it's velocity (because it cools too quickly) much faster than steam does, making it less efficient.

I know why they use water instead of air for power plants. His argument was that lower the specific heat the more efficient, and air is arounf 1/4 the specific heat, so if there were gains to be had I think they would be exploted. It is also much more engery intestive to compress air than pressurize water, which is another big driver for using water on a Rankine cycle instead of using a gas with on Brayton cycle. So yeah, it was probably a stupid question for me to ask, but I was trying to battle a stupid argument.

Again, how fast something cools or heats is a function of mass*specific heat, so they air could retain its heat just as well as steam if the mass was high enough. Assuming all other heat transfer parameters remain constant.

 

[Zorba]

Originally posted by: CycloWizard

Originally posted by: myocardia
That's easy. All large-scale electrical generators on the planet use steam to turn the turbines, including the nuclear generators. Water (turned to steam) is many times more efficient than trying to use heated air, especially since the boilers and the turbines/generators aren't in the exact same location. They're close to each other, yes, but heated air would lose it's velocity (because it cools too quickly) much faster than steam does, making it less efficient.

At some point in this thread, all context was lost in an effort by some guy to make himself look smarter than everyone else. This seems to be a recent trend here at AT.

I don't get why you are taking this so personally. Why don't you prove that you are right? If I am the dumb one here it should be easy to prove I am wrong.

I also thought the whole point of this forum was to debate on "Highly Technical" topics. But I guess some people can't take being questioned.

 

[CycloWizard]

Originally posted by: Zorba
I don't get why you are taking this so personally. Why don't you prove that you are right? If I am the dumb one here it should be easy to prove I am wrong.

I also thought the whole point of this forum was to debate on "Highly Technical" topics. But I guess some people can't take being questioned.

No one ever said you were dumb, nor am I here to prove anyone wrong. The point of the HT forum is to help people understand HT topics better, not to shout each other down. I already said I mistakenly said "heat engines" rather than "Stirling engines," which should have been the end of it. Unfortunately, you can't seem to let it go.

 

[Zorba]

Originally posted by: CycloWizard

Originally posted by: Zorba
I don't get why you are taking this so personally. Why don't you prove that you are right? If I am the dumb one here it should be easy to prove I am wrong.

I also thought the whole point of this forum was to debate on "Highly Technical" topics. But I guess some people can't take being questioned.

No one ever said you were dumb, nor am I here to prove anyone wrong. The point of the HT forum is to help people understand HT topics better, not to shout each other down. I already said I mistakenly said "heat engines" rather than "Stirling engines," which should have been the end of it. Unfortunately, you can't seem to let it go.

Okay, I agree with you there could be some certain advantages for using low specific heat with most applications of a stirling engine.

BTW: I didn't mean to put you down or make it personal at all, I am sorry that I did a little. :beer:

 

[CycloWizard]

Originally posted by: Zorba
Okay, I agree with you there could be some certain advantages for using low specific heat with most applications of a stirling engine.

BTW: I didn't mean to put you down or make it personal at all, I am sorry that I did a little. :beer:

No worries. Apparently I'm a little distracted and can't keep my thoughts straight while typing anymore, so it's to be expected.

 

[BladeVenom]

Originally posted by: CycloWizard

Originally posted by: BrownTown
yeah, thats what I though which is why you wouldn't think hydrogen or helium would be used, but on the other hand since they are the lightest gases, the specific heat per mole of gas isn't nearly so high. And the ideal gas law would say that the pressure would increase based on the number of moles, not the number of grams of working fluid, so maybe when you factor that it its not bad.

This is true. However, very small gas molecules tend to deviate from ideal gas behavior at relatively low pressuers due to their compact structures. Thus, if you consider a simple compressibility factor equation of state (z=PV/RT, where z is the compressibility factor and z=1 recovers the ideal gas equation), the compressibility factor for these small molecules is dependent on the molecular size of the gas. Sort of.

Smaller molecules are also more likely to leak.