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Sandia progressing to demo stage with supercritical CO2 Brayton-cycle turbines; up to 50% increase in efficiency of thermal-to-electric conversion

Simple diagram of the heated un-recuperated supercritical CO2 Brayton loop. Source: Wright et al. 2010. Click to enlarge.

Sandia National Laboratories (SNL) researchers are progressing to the demonstration phase of a supercritical CO2 (S-CO2) Brayton-cycle turbine system for power generation, with the promise that thermal-to-electric conversion efficiency will be increased as much as 50% for nuclear power stations equipped with steam turbines, or 40% for simple gas turbines. The S-CO2system is also very compact, meaning that capital costs would be relatively low.

Research focuses on supercritical carbon dioxide (S-CO2) Brayton-cycle turbines, which typically would be used for bulk thermal and nuclear generation of electricity, including next-generation power reactors. The goal is eventually to replace steam-driven Rankine cycle turbines, which have lower efficiency, are corrosive at high temperature and occupy 30 times as much space because of the need for very large turbines and condensers to dispose of excess steam. The Brayton cycle could yield 20 MW of electricity from a package with a volume as small as four cubic meters.

The Brayton cycle, named after George Brayton, originally functioned by heating air in a confined space and then releasing it in a particular direction. The same principle is used to power jet engines today.

This machine is basically a jet engine running on a hot liquid. There is a tremendous amount of industrial and scientific interest in supercritical CO2 systems for power generation using all potential heat sources including solar, geothermal, fossil fuel, biofuel and nuclear.

—principal investigator Steve Wright of Sandia’s Advanced Nuclear Concepts group

The supercritical properties of carbon dioxide at temperatures above 500 °C and pressures above 7.6 MPa enable the system to operate with very high thermal efficiency, exceeding even those of a large coal-generated power plant and nearly twice as efficient as that of a gasoline engine (about 25%). As a result, there has been research interest in producing a commercially viable S-CO2 Brayton-cycle turbine for power generation, especially nuclear; however, much of the work has been largely analytical. A number of DOE labs, including Idaho National and Argonne National, have contributed to the body of work.

Turbo-alternator-shaft design for the SNL S-CO2 test loop. This configuration uses gas-foil bearings and includes a small turbine (red). Source: Wright et al. 2010. Click to enlarge.

Sandia’s effort is hardware-focused and requires the development of turbo-alternator-compressor technologies capable of operating with supercritical CO2 at very high power densities, high speeds, high pressures, and high fluid densities.

Sandia currently has two supercritical CO2 test loops. The key component of these loops is the turbo-alternator-compressor unit (TAC) and the technologies used in its design. In its final configuration, the TAC uses gas foil bearings, a high speed permanent magnet motor/alternator and labyrinth gas seals to reduce the rotor cavity pressure. Because of the extremely high power densities and fluid density, Sandia has filed a Technical Advance for the TAC design.

A power production loop is located at the Arvada, Colo., site of contractor Barber Nichols Inc., where it has been running and producing approximately 240 kW of electricity during the developmental phase that began in March 2010. It is now being upgraded and is expected to be shipped to Sandia this summer.

A second loop, located at Sandia in Albuquerque, is used to research the unusual issues of compression, bearings, seals, and friction that exist near the critical point, where the carbon dioxide has the density of liquid but otherwise has many of the properties of a gas.

Immediate plans call for Sandia to continue to develop and operate the small test loops to identify key features and technologies. Test results will illustrate the capability of the concept, particularly its compactness, efficiency and scalability to larger systems. Future plans call for commercialization of the technology and development of an industrial demonstration plant at 10 MW of electricity.

A competing system, also at Sandia and using Brayton cycles with helium as the working fluid, is designed to operate at about 925 °C and is expected to produce electrical power at 43% to 46% efficiency. By contrast, the supercritical CO2 Brayton cycle provides the same efficiency as helium Brayton systems but at a considerably lower temperature (250-300 ° C). The S-CO2 equipment is also more compact than that of the helium cycle, which in turn is more compact than the conventional steam cycle.

Sandia’s S-CO2 Brayton cycle program is supported by DOE with funding from the Labs’ Laboratory Directed Research & Development (LDRD) program.


  • S. A. Wright et al. (2010) Operation and Analysis of a Supercritical CO2 Brayton Cycle. (SAND2010-0171)

  • Anton Moisseytsev and James J. Sienicki (2009) Investigation of alternative layouts for the supercritical carbon dioxide Brayton cycle for a sodium-cooled fast reactor. Nuclear Engineering and Design. Volume 239, Issue 7, Pages 1362-1371 doi: 10.1016/j.nucengdes.2009.03.017

  • C.H. Oh et al. (2006) Development Of A Supercritical Carbon Dioxide Brayton Cycle: Improving VHTR Efficiency And Testing Material Compatibility. Final Report (INL/EXT-06-01271)

  • C.H. Oh et al. (2004) Development of a Supercritical Carbon Dioxide Brayton Cycle: Improving PBR Efficiency and Testing Material Compatibility (INEEL/EXT-04-02437)



This kind of massive improvement should reduce costs of nuclear even further.
The Chinese build them for around $1565kw, so with this you might get near to $1000kw, and if you used annular fuel even lower.


Yes Davemart. This is very interesting potential efficiency improvement for thermal (coal, NG, SG, Oil, Biomass etc) and nuclear power stations.

This is a retrofit that should be positively supported, if not imposed, specially on fossil fuel power stations to reduce GHG and fuel consumption and e-power production cost.

Of course, there are other competing technologies with similar results that should also be considered.


I have been waiting for this. It's big.

Unfortunately, it appears to me that this probably can't be retrofit to existing nuclear powerplants. Anything which depends on e.g. boron in the coolant to regulate the reactivity isn't going to work with such a radical change in the coolant. Being able to hike the efficiency and output from existing plants by such a large amount would be great, though.

The real issue, though, is what you get when you use this with combustion fuels. Conventional gas turbines operate at turbine inlet temperatures up to 1360°C. If the same could be used with SCO2 as the working fluid, the thermal efficiency could go well over 60%.


I like the idea of hybrid solar combined cycle plant using CSP to add extra steam to the cycle. But if the steam cycle part of the system could be replaced with SCO2, you could possibly even run the gas turbine on the same shaft

I also wonder if in a similar way to CAES compressed CO2 could be stored


For natural gas I prefer the idea of pumping it to the home or office, then putting it through a fuel cell. Panasonic and others reckon they can bring the price down to reasonable levels by 2013.
That way of course the waste heat gets used to heat water or for space heating.
We might be able to get a 30% or so better use out of our natural gas that way.


One application that pops into mind would be a Genset for a class 8 truck that operates as a serial PHEV. Scaling in a linear fashion, that means that said Genset would occupy a volume of 2/10 of a cubic meter for a 500kw Genset. In reality, scaling wouldn't be linear but you could still end up with something in the neighborhood of 0.5m^3 while weighing significantly less than a turbo diesel of equivalent output. Factor in weight reductions from the elimination of exhaust treatment systems and the transmission and there's plenty of slack to add batteries and e-motors.

Commercial freight vessels would also benefit from such an engine as would trains.


Also in the category of "hmmmm, interesting" would be a serial electric airplane! Or perhaps direct drive!


A perfect match for the LFTR. Liquid Fluoride Thorium Reactors are a proven technology, research was terminated in 1975 because it wasn't suitable for making bombs. LFTRs burn at higher temperatures and the Brayton cyclo with better heat exchangers make this even more efficient. LFTRs can burn our stockpile of radio-active waste from existing nuclear power plants. (Because LFTRs are so efficient it would take over 100 years to do this). LFTRs produce little long term radio active waste, or products suitable for making bombs. The radio-active waste produced has a short half life and requires only 300 years of storage as compared to the uranium waste which has to be stored for 10,000 years. There is also much less radio-active waste, 0.3% for equivalent power from uranium. Thorium is plentiful, there is enough in coal ash and mine tailings to power the world for 100 years, and a million years supply can be dug out of the earth. See:
and also:


Liquid-metal and molten-salt reactors are specifically mentioned in Vaclav Dostal's PhD thesis on SCO2 turbines.

Dave, I believe the H2O byproduct from CH4 combustion in a system designed along the lines of an SCO2 turbine would be an issue at the cold end; you'd wind up with some very different specific heats on opposite sides of the regenerator, causing pinch points and losing efficiency. This suggests that methane would be best used in something else anyway.


the whole thing is hard to understand, but the thermodynamic is based on exchange of heat through variation of heat capacity rather than exchange of heat through temperature variation. They use the outstanding properties of super critical state. So as a result they don't need a very high temperature source, amazing the efficiency is very high and the compactness is better because you work with liquid and not gaz. Wow this is truly a revolution


What the supercritical cycle does is exploit the very high specific heat (and low ratio of specific heats, Cp/Cv) in the region of the point of criticality to reduce the back-work of compression. This has other complications specifically related to regeneration/recuperation (the specific heat of the fluid leaving the compressor is much greater than the same mass-flow of fluid at higher temperature coming from the turbine) but there are ways to deal with this, e.g. the recompression cycle.

Johnny B Good

“Test results will illustrate the capability of the concept, particularly its compactness, efficiency and scalability to larger systems. Future plans call for commercialization of the technology and development of an industrial demonstration plant at 10 MW of electricity.”
Why the focus on larger systems and not smaller compact 30kw units.
Are we forgetting about distributive power. What is wrong with putting power production directly in the hand (and control) of the individual end user. Does anyone remember how data processing was delivered in the 60s and early 70s. Back then we had “big IBM” and it was not until the advent of desk top PC’s (and Mac’s) that we had the freedom to make our lives/society better. The same case & history can be considered for how we communicate. Going from large centralization to decentralization is usually more – better – faster.

Henry Gibson

High efficiency is not needed in a nuclear power plant because even with the costs of fabrication of fuels there is only a small fraction of the power cost as delivered that is due to the cost of fuel. When the first power reactors were proposed the power industry executives knew that if coal were delivered to existing power plants at no cost, the cost of electricity at the consumer would be reduced by only 25 percent.

The efforts to build larger and larger nuclear power plants to increase the efficiency of the turbine and generator has only resulted in much higher costs.

The highest costs result from lies told about the dangers of nuclear radiation by people who don't know that every living thing has always had radioactive potassium in every cell, and that cell mechanisms exist to repair damage from this built in radioactivity as well as the radioactivity always coming from the universe and the earth. Much more damage is being done to the cells by the large concentration of oxygen in the air that gets into the cells and the iron that is necessary in very small amounts for metabolism. The cells also repair this damage most of the time. If the damage to a cell is not repaired, it dies or is killed most of the time by its neighbors. Cancer almost never happens and tumours are almost always caused by something else besides nuclear radiation, and it can never be shown that a particular tumour was caused by radiation or if it was caused by natural radiation.

Most of the atoms of potassium in live and dead things that were alive are not radioactive but modern technology allows them to be replaced by mostly radioactive potassium atoms because the body is constantly changing them. It is likely that such an action would kill the test subject which would show that people can have an average life span even with a few but not many radioactive atoms in them.

If the subject did not die soon with mostly radioactive potassium atoms in it, It would show that the cells do good repairs. People and animals live in some naturally highly radioactive areas without obvious damage. One mold is known to grow near intense radiation in power plants, and it likely feeds on it like plants do on sunlight.

All rocks and soil have some level of radioactive natural uranium and natural potassium in them, so if a few extra atoms are added as they are with any good fertilizer it is not a significant risk. This also means that any radioactive material could be mixed into such fertilizers or spread separately on the ground if the additional radiation provided was a small fraction of a percent. If the highly energetic cosmic rays don't kill you and they don't, then a few rays from added materials in the ground will not either.

Natural or synthetic Radioisotopes can be immediately be made non dangerous by very highly diluting them in water and spreading them out on the land or the sea where they join the far more common all natural radioisotopes; Live things demonstrate this all the time by coexisting with the about 50 potassium atom explosions per second per kilogram of body weight and the additional rays from space and rocks and soil and plants and dogs and cats and people.

CANDU reactors are among the smallest and fastest standard reactors to build and they should be ordered until even smaller standard ones become available. They can operate on a thorium cycle to get rid of long lasting transuranic isotopes including surplus materials from weapons and other reactors.

Fuel reprocessing has been shown to be necessary because the US government has failed to use the used fuel funds according to the signed contracts. The US WIPP can be used to reliably store the small volume of fission products produced every year. Once reprocessing factories are in full use, the costs of operating them can be reduced by experience and cheaper processes. They are necessary for thorium cycles.

Every nuclear power site should order one or more of the smaller reactors proposed for delivery, and they can be used for emergency cooling power all the time for the big reactors. Excess power from them is fed into the grid during normal operation for more revenue.



Henry... you're off the rails again.

Why the focus on larger systems and not smaller compact 30kw units. Are we forgetting about distributive power.
sCO2 systems require fairly cool heat sinks and run at very high pressure, so they aren't very well suited to portable, residential and small commercial use. Organic Rankine cycles can have higher efficiency at lower temperatures, too.

Henri Gibson, when will you stop to pollute this nice site with your disinformation and outdated technology promotion like zebra battery and Candu reactors ?


I just did a blog post about LFTRS and the Brayton cycle. Nothing in depth as I'm just learning about them myself but they do seem quite promising.


For natural gas I prefer the idea of pumping it to the home or office, then putting it through a fuel cell. Panasonic and others reckon they can bring the price down to reasonable levels by 2013.
That way of course the waste heat gets used to heat water or for space heating.
We might be able to get a 30% or so better use out of our natural gas that way.

Davemart still trying to shoehorn fuel cells into every discussion. Instead of pumping natural gas inefficiently to every home then using it in a fuel cell why not just use it in a combined cycle generating plant at higher efficiency and transmit the power through existing lines?


Because you use the waste heat for the home.


He's claiming 30% energy recovery from NG using a fuel cell with waste heat recovery yet combined cycle generating plants can easily beat that.


"50% for nuclear power stations equipped with steam turbines"

SOFCs are 50% efficient and you can recover more than 30% of the heat created for the home.

Account Deleted

i am wondering why CO2 is used. if compared to water, CO2 specific heat capacity is less and the constant R for the ideal gas law is also less.

CO2 absorbs less heat and it also expands less if compared to water


I apologize for mentioning combustion and giving DaveMart an excuse to bring fuel cells into the discussion. Fuel cells are not heat engines and cannot be run on nuclear or solar heat. They are irrelevant.

i am wondering why CO2 is used.
sCO2 has worthwhile properties around the critical point; the specific heat skyrockets, so the temperature increases much less during compression. This eliminates the need for e.g. intercooling and reduces the compressor back work. At higher temperatures it behaves like an ideal gas. It can be used in a cycle with greater efficiency and lower cost than e.g. helium.
CO2 absorbs less heat and it also expands less if compared to water
That's an advantage. Steam turbines have to be enormous to handle the large volumes of fluid, which makes them expensive. They are also much less efficient.

Account Deleted

for CO2 to be closer to liquid state, it needs to be cooled or pressurized. this may not be preferable.

water on the other hand can be manipulated easily to exist in either liquid or vapor form. To maximize heat absorption, water should exist in liquid state. Once the time comes for it to do work, it should be in gaseous state. Of course, as it changes state from liquid to gas, latent heat of evaporation, this will further absorb the excess heat.

"That's an advantage. Steam turbines have to be enormous to handle the large volumes of fluid, which makes them expensive. They are also much less efficient. " well i beg to differ, water with very high specific heat capacity and high gas constant R, it is the other way around actually. Furthermore, density of CO2 is always lower than water and many of the thermodynamics equation uses m (mass) thus it does make a big difference.


Liquid CO2 requires lower temperatures which may be difficult to achieve.

You're ignoring the disadvantages of water. The large expansion requires large (expensive, difficult to transport) machinery to handle it. The large change in entropy in condensation means large heat rejection (loss).

Making claims about the density of CO2 ignores the details of practical cycles. I suggest you review Vaclav Dostal's PhD thesis (available on-line) to see just how this might mislead you from the traits which are more relevant, namely cost and efficiency.

Account Deleted

i disagree with you, steam expansion in heat engine is only needed to move a turbine or piston. Piston and turbine sections are normally confined in a small volume which is easy to manage. When work is not needed, water should exist in liquid form.

Try doing so with carbon dioxide. You need a lot of pumping work to move the carbon dioxide around.

if i can remember when i was studying engineering in Boston, brayton cycle normally has lower efficiency. it's not a surprise why nuclear and coal power plant uses water as the working medium.

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