NBF: Supercritical Carbon Dioxide Brayton-Cycle Turbines

[93143]

Uh, anyone know how insulating supercritical CO2 is at these conditions ??

Should be about 40-55 mW/m·K for pressures around 8 MPa and temperatures from 550-800 K (Scalabrin et al., 2006).

Viscosity is around 25-35 μPa·s in that range (Fenghour & Wakeham, 1998). I don't have a reference EOS handy, but if the unmodified Peng-Robinson EOS (Peng & Robinson, 1976) is any guide, the specific heat should be in the vicinity of 1150-1200 J/kg·K, giving us a Prandtl number of roughly 0.72-0.77. Nothing too out of the ordinary there...

The low-pressure numbers in the back of my heat transfer textbook are quite similar to the above figures...

Hydrogen at the same conditions seems to have thermal conductivities between 290-380 mW/m·K or so and viscosities between 13-17 μPa·s, with Cp ~14.5-14.7 kJ/kg·K, for Prandtl numbers between 0.69-0.67, roughly (Incropera & DeWitt, 4th ed.). The effect of pressure on transport properties this far above hydrogen's critical temperature is small.

[Giorgio]

http://www.physorg.com/news/2011-03-sup ... bines.html

The density of energy extraction would lend itself to cooling polywells...

Uh, anyone know how insulating supercritical CO2 is at these conditions ??

Scalabrin paper on Thermal Conductivity Equation for Carbon Dioxide:
http://www.nist.gov/data/PDFfiles/jpcrd723.pdf

[Giorgio]

Lot of interesting tibids from the Testing section of the coming "2011 Supercritical CO2 Power Cycle Symposium"

http://www.sco2powercyclesymposium.org/ ... er/testing

[KitemanSA]

67% efficienciy at 300° C?? Not in this world!

In space with a very large surface area radiator at ~180k, maybe!

I suspect this is a situation of inadequate editing by NBF. The 300° C and the 67% efficiency (450MW, 300MWe) were by different organizations and probably had different assumptions.

More nit-picking when time permits.

Oh, and this doesn't fit the definition of "Brayton Cycle" as was defined to me in my college thermo course. A Brayton cycle is a GAS cycle, not a supercritical fluid cycle.

Supercritical H2O turbines are typically considered Rankine cycles.

[cc]

67% efficienciy at 300° C?? Not in this world!

Looks like an error, with the 50% applied in the wrong direction. The diagram is less than ideal for comparison purposes: it doesn't show the electrical output for the steam turbine, only the steam turbine is shown with casing, and the numbers that are included are clearly suspect.

Oh, and this doesn't fit the definition of "Brayton Cycle" as was defined to me in my college thermo course. A Brayton cycle is a GAS cycle, not a supercritical fluid cycle.

As I understand it, it does, as it has a compression stage. Wether it is gas or a supercritical fluid, it is still compressible; the Rankine cycle uses a condenser and pumps the resulting liquid.

Supercritical H2O turbines are typically considered Rankine cycles.

If they are, perhaps the water is not supercritical throughout the entire cycle?

[crj11]

67% efficienciy at 300° C?? Not in this world!

It is not the best wording, but when they say a 50% increase in efficiency (in the case of a steam turbine), they mean that the efficiency goes from 30% to 45%, i.e. 1.5 * 30% = 45%.

This does mean that you get 50% more electricity from the same amount of heat.

[KitemanSA]

67% efficienciy at 300° C?? Not in this world!

It is not the best wording, but when they say a 50% increase in efficiency (in the case of a steam turbine), they mean that the efficiency goes from 30% to 45%, i.e. 1.5 * 30% = 45%.

This does mean that you get 50% more electricity from the same amount of heat.

If you look at the size comparison picture they have a label under the standard Helium Brayton showing 330MW, 167MWe. This is the expected ~50% efficiency expected from a high temperature BC. Under the SC BC the label reads 450MW, 300MWe which certainly implies a 67% efficiency. But who knows.

[chrismb]

It is not the best wording, but when they say a 50% increase in efficiency (in the case of a steam turbine), they mean that the efficiency goes from 30% to 45%, i.e. 1.5 * 30% = 45%.

This does mean that you get 50% more electricity from the same amount of heat.

..or that you waste 22% less heat!

Ahhh!!... Statistics!!..

[93143]

Of course, if you want constant output power, you waste 48% less heat going from 30% to 45%, or 56% less heat going from 40% to 60%...

[GIThruster]

Here's an update:

http://www.bizjournals.com/albuquerque/ ... oosts.html

IIRC, this Gen4 company is the same that was involved with testing at BLP?

In any event, this system is so efficient and so small that it appears it would be useable in some mobile applications. Obviously ships, but even craft like the MV-22. The combined output of the MV-22's turbines is significantly less than 20MW. This system ought to be useable with next generation superconducting electric ducted fans and with a miracle power source such as a working LENR, such craft would have almost unlimited range. The stuff you saw in Avatar. . .

The need to duct fuel to the tilting nascells would be removed in an electric version, offering several significant imrovements.

[paperburn1]

They are small enough to be transported as whole unity over land on an oversized flatbed.

I also think that this could be awesome for nuclear submarines and nuclear aircraft carriers. Especially on submarines, space is always and issue and the steam turbines are loud.

you could bring back Trains as well.

[Skipjack]

you could bring back Trains as well.

If people want them. But if they can use this to get the prices down far enough...

[zDarby]

I learned of this quite some time ago....I think from the Sornensen's Thorium facebook page. Among my first thoughts was: could it be used in lighter than air craft? Zeppelins? Strange, I know, and will probably mark me as backwards-thinking. Still, LTA travel and rigid zeppelins particularly have always fascinated me since I was a child. For certain applications LTA is still a viable transport methodology....Though I'll not argue that here and now.

Another thought was whether using a gas with a lower critical temperature (like nitrogen @-146.9C & 33.5 atm [wikipedia]) would do as well for lower temperature differences; eg, a warm day and 10' underground. Or, perhaps, it would work well as the working fluid for a brayton cycle refrigerator....Which, really, is to ask if it's supercriticallity or the CO2 that's important for the improved efficiency....And, of course, the more I think of it, the more I believe it's both. I still wonder how nitrogen would fair.

Yet another question I had was whether S-CO2 would work well in a thermoacoustic engine/refrigerator.

[GIThruster]

Which, really, is to ask if it's supercriticallity or the CO2 that's important for the improved efficiency.

There are two very important aspects to this so far as I understand it. First is that supercritical fluids don't go through a phase change in this sort of operation, but they do go through enormous changes in density. Because there are no phase changes the conditions can be taylored to the specifics of the fluid and vice versa. So for instance all surface tension is removed and there's far less loss in the system due to friction. This is the kind of control that allows one to bring the efficiency up.

The second issue is that because supercritical CO2 going through its change in density (not phase like with a steam turbine) generates more force over less distance than water/steam for the same energy exchange, the fluid needs to move shorter distances so the entire jacket can be smaller. This is what makes it such a compact turbine.

Personally I think the second of these is the more important if indeed they can get 20MW from 4 cubic meters. (The engines on the MV-22 are only about 4.6MW@.) That kind of power density is enough to replace the Rolls-Royce turbines on an MV-22 and get much more power, with less weight and have twice the range on the craft. I already sent a friend at DIA a note to see they follow this up and find out if such mobile applications are possible. I haven't seen anything from the DOE people about mobile applications, so there may be complications. When they say "4 cubic meters" they may not be counting important parts of the system.

In any case, this is a perfect fit for space-based power systems of every kind and will generate savings here in earth as we replace older systems.