Stirling Engines with Supercritical Fluids?

[blade]

Have Stirling Engines that utilize a supercritical fluid as the working gas been built? Supercritical fluids are dense compared to gases and less dense than liquids. Supercritical fluids share many characteristics of gases and typically have a very high thermal conductivity and heat capacity. Supercritical CO2 seems like a candidate for use in Stirling Engines, and I am curious whether anyone has constructed an engine using supercritical CO2. In certain temperature and pressure regimes, supercritical CO2 has almost twice the thermal conductivity of hydrogen.

[Ian S C]

Blade, I'v never heard of supercritical fluids until now. What sort of viscosity do they have (all gasses and fluids have viscosity), one of the main advantages of either hydrogen, or hellium, is their very low viscosity. As the gas gets pressurised, the viscosity increases, and to make full advantage, high pressure is required in a high powered motor. Ian S C

[vamoose]

Blade
Interesting topic. I'm not familiar with supercritical fluids either but hope this is of some help.

I'm guessing if that once your above and beyond the critical pressure and critical temperature then the fluid is at supercritical state. Also if the critical pressure increases/decreases the the critical temperature decreases/increases (and vice versa for temp to pressure).

If you were able to tune/find the right operational conditions for temp and pressure you maybe able to use supercritical fluid in this manner-
As the fluid heats and moves from the regenerator into the hot side it would vigorously/critically expand from a state of supercritical fluid into a gas state (also the cycle volume increases), and when cooling, moving through the regenerator and to the cold side it would revert back to supercritical fluid and then into fluid state, vigorously contracting (also the cycle volume decreases).

[hr]
wiki...
'In addition, there is no surface tension in a supercritical fluid, as there is no liquid/gas phase boundary. By changing the pressure and temperature of the fluid, the properties can be "tuned" to be more liquid- or more gas-like.'
http://en.wikipedia.org/wiki/Supercritical_fluid
[hr]

Also if you wanted to adjust the supercritical fluid properties you could combine other forms of supercritical fluids to get the right property mixture of temp and pressure...

[hr]
wiki...
All supercritical fluids are completely miscible with each other so for a mixture a single phase can be guaranteed if the critical point of the mixture is exceeded. The critical point of a binary mixture can be estimated as the arithmetic mean of the critical temperatures and pressures of the two components,

wiki...
Supercritical fluid in power generation

The efficiency of a heat engine is ultimately dependent on the temperature difference between heat source and sink (Carnot cycle). To improve efficiency of power stations the operating temperature must be raised. Using water as the working fluid, this takes it into supercritical conditions. Efficiencies can be raised from about 39% for subcritical operation to about 45% using current technology.[16] Supercritical water reactors (SCWRs) are promising advanced nuclear systems that offer similar thermal efficiency gains. Carbon dioxide can also be used in supercritical cycle nuclear plants, with similar efficiency gains.[17] Many coal-fired supercritical steam generators are operational all over the world, and have enhanced the efficiency of traditional steam-power plants.
http://en.wikipedia.org/wiki/Supercritical_fluid
[hr]

video
http://www.youtube.com/watch?v=yBRdBrnIlTQ

vamoose

[blade]

According to Wikipedia, supercritical fluids have between 5 and 10 times the viscosity of gases.

[NerdyEE]

Hi Blade,

I helped a buddy with a supercritical gas project a few months back and was introduced to it then. They filled a container via a supercritical process that blew my mind. A seal for gas/liquid CO2 might not seal with supercritical CO2. I wish I could say more on this…

I did find a SUPERCOOL video on supercritical fluid Heilum for you/everyone… (bad pun intended!)

http://www.youtube.com/watch?NR=1&featu ... Z6UJbwxBZI

Cheers,
-J

[rustybarrel]

Hi
I dont know about using super critical fluids. What I know is at certain pressure and temperature water also acts as supercritical fluid. You can find the pressure temperature from PV (pressure volume) diagram for a given gas.
Also have anyone tried to use CFC or HCFC or any other refrigerent as a working fluid? I tried it once with R22 but my seals were not good and the gas would escape out quickly.
thanks
rustybarrel

[blade]

NerdyEE,
I believe superfluid helium is different from a supercritical fluid(or supercritical helium). I believe superfluid helium only exists at very low temperatures, which makes it useless for Stirling engines. However, supercritical fluids exist at high temperatures, which allow them to be used as working fluids.

[blade]

rustybarrel,
yes. even water exists as a supercritical fluid at very high pressures and temperatures. The main reason I was interested in supercritical CO2 is that the supercritical fluid occurs at lower pressures, which makes it more practical to actually use. Furthermore, work on Brayton engines is being done with supercritical CO2.

[Ferraccio]

If I've well understood.
1) Molecular weigh of CO2 is very high (good), but the dimension of the molecule is very large (very bad). So exchange clearances have to be very large (fkuid do not flow well) This contrast at all the condition of "good surface of exchange" (many VERY small ducts, with enormous surfaces with very small quantity of gas), in External combustion engines as Stirling, the main problem is to transfer heat outside-inside and again outside the engine. Good performances so are excluded.
(In fact is demonstrated that hydrogen is the best, as you can see in cooling of large electrical generators of electrocal generation units).

2) The supercritical fluids (as CO2) are interesting for theyr "change of phase" liquid-gas in an interesting range of temperatures, in CO2 this range is lower than water steam. As in the steam Rankine engine this change of phase is the bearer happening of the steam generation system. This is NOT the case of the Stirling engine. For definition the Stirling is an engine, external combustion, gas runned.

[DIYStirlingGuy]

Here's a good video demonstration of a supercritical fluid transition. It looks a lot like the superfluid transition in the liquid helium video.
[youtube]http://www.youtube.com/watch?v=yBRdBrnIlTQ[/youtube]