CO2 turbines efficient thermal to rotary power

Interesting and technically informative. Around minute 29 begins Sorensen's discussion of CO2 as a working fluid and how small and efficient they are relative to steam. "Sweet spot" at 500 to 650 degrees C. Dense fluid very size efficient, does not need superheating to reach high efficiency. Efficiency already at 45%, well above the best steam at 35 to 40%. No cooling water required. Many other points Sorensen makes. This happens to be his reason for shifting from anti-nuclear to pro, in the interests of saving the biosphere. Take notes, lots of engineering, Sorensen formerly with NASA.

Quote from Longview

Interesting and technically informative. Around minute 29 begins Sorensen's discussion of CO2 as a working fluid and how small and efficient they are relative to steam. "Sweet spot" at 500 to 650 degrees C. Dense fluid very size efficient, does not need superheating to reach high efficiency. Efficiency already at 45%, well above the best steam at 35 to 40%. No cooling water required. Many other points Sorensen makes. This happens to be his reason for shifting from anti-nuclear to pro, in the interests of saving the biosphere. Take notes, lots of engineering, Sorensen formerly with NASA.

And one might ask "how can this turbine be so compact?" Basically it is a function of two things to which Sorensen alludes . A) CO2 is nearly 15 times denser than helium, and 2.5 times the density of water vapor at standard temperatures and pressures. B) the critical point is very unusual for CO2, at any reasonable pressure it is a super-critical fluid above 30.98 degrees C. Only at extreme pressures (thousands of atmospheres) can it form a solid at any temperature above the 30.98 C critical point. Further, above that point it is a fluid, that is its behavior is that of both a gas and liquid, essentially the PV = nRT of a gas, the compressibility of a gas, and the density of CO2 liquid, in this case a bit over 1.5 X that of liquid water.

Please see:

https://www.google.com/search?q=CO2+triple+point&hl=en-US&tbm=isch&imgil=dpfgqbhe5xTIdM%253A%253BAuyzj06x2pIkQM%253Bhttp%25253A%25252F%25252Fdecarboni.se%25252Fpublications%25252Fgood-plant-design-and-operation-onshore-carbon-capture-installations-and-onshore-4&source=iu&pf=m&fir=dpfgqbhe5xTIdM%253A%252CAuyzj06x2pIkQM%252C_&biw=1280&bih=609&usg=__JqF-8S3Uo7ho6IH2IYb4QXprz2o%3D&ved=0CCgQyjc&ei=tqeYVZ3pC8_qoATh3wY#imgrc=dpfgqbhe5xTIdM%3A&usg=__JqF-8S3Uo7ho6IH2IYb4QXprz2o%3D

Longview continues: Essentially CO2 can be a superdense fluid driving a very compact and special type of turbine (sort of a hybrid of both a steam turbine and a hydro turbine). Hot CO2 can oxidize some metals at red heat, but ceramics, no. Practically speaking the strong forces in such a compact turbine would dictate very strong and redox stable components, nothing really exotic though., so one does not want to push beyond the 650 C sweet spot. Sorensen mentions Hastelloy, proprietary nickel alloys of extreme strength and corrosion resistance, often used in the nuclear industry, made by Haynes International.

The most stable of those alloys is still made by Haynes, that is Hastelloy C-22, which is Ni-Cr-Mo-W. See at: http://www.csialloys.com/aboutc22.php

Here for further reading is a review of the CO2 turbines, here called "supercritical carbon dioxide" turbines:

http://breakingenergy.com/2014…arting-to-hit-the-market/

Longview

Quote from Andrea Calaon

I think that now the downscaling is limited by the bearings of the turbine, which, if too small would heat too much. But I may be wrong.
For the adoption on means of transport much depends on the heat source. Remember that, if the heat source is hydrocarbons you will be producing CO2 that you have to store in some way.
If you instead have a nuclear source things change. You can heat up CO2 by radiation (you need some volume here).
If you have a nuclear source, these systems seem ideal for generating mechanical power and electricity.
With LENR they will be put on ships, then trains and large airplanes, and may be later on smaller means of transport as well.
For airplanes you need a volume where to pass the heat from the nuclear source (solid state) to the fluid CO2 through radiation (no pressure loss). I don't know how large it will be. Then you will need to pass the energy to a propeller. Slower then jet, but VERY long lasting flights.

Once the competition will start because the technology will be common knowledge, these systems seem to have all features that will lead to their spreading anywhere you need large amounts of power.

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Downscaling may be limited in today's combustion turbines-- not so much because the technology is not there, but because of environmental issues (sound and pollutants) and the unit costs presently in place are not competitive due to the extremely developed nature of reciprocating engines. One of the virtues of the CO2 (or argon, see below) turbine is exactly the low temperature / high power nexus. Assuming Sorensen's assertion that CO2 turbines work very efficiently from 500 to 650 degrees C. That is way below the limiting temperature of a combustion gas turbine. A high performance jet engine can have a driven turbine (the one right in the hot flame) "inlet" temperatures well over 2000 C, necessitating ceramics and internal cooling of both the rotor and stator components. Stationary gas turbines, such as those used to boost the pressure in long distance natural gas lines, have an "inlet" temperatures no higher than 1400 C, (last I knew) which is near the maximum without the complications of exotic materials, ceramics and internal cooling passages in the turbine blades-- a real technical challenge to be avoided in mass production, I would think. But inlet temperature is a high motivator, since it increases efficiency on a steep exponential. High inlet temperature increases power to weight and "mileage", so to speak.

Sorensen seems to imply there are other "sweets spots" on the temperature curve for CO2. Certainly that could make sense since CO2 remains a very heavy fluid far on up the scale as both pressure and temperature increase. The present "sweet spot" mentioned may have been simply material limitations. Those would be removed in a ceramic system, small ceramic turbines are already in mass production (the driven turbine in turbo-superchargers). I may have mentioned that the reason CO2 and metal may be a problem at say 1400 C, is the redox reactivity of CO2 at those temperatures and pressures-- CO2 can be reduced by hot iron, hot carbon etc to give first carbon monoxide and of course destruction of the metal reductant.

Bearings are not generally the limiting factor today, since those can much more easily be cooled, via the shaft (rotor) and via the housing (stator) and via the lubricant itself. All kinds of high temperature ceramic bearings are already available in small ceramic versions "off the shelf". The loadings in steam and gas turbines generally require sleeve type bearings, and certainly the loadings in a CO2 turbine could well be even higher. Sleeve bearings can be made of exotic materials and/or can be readily cooled. Steady development of inexpensive synthetic lubricants coming first from jet engine applications are now widely available (the high polyester synthetic motor oils for cars are a direct descendent). Silicone and fluoropolymer lubricants of very high temperature tolerance are the next stage. Of course lubricant can be independently pumped through the bearings before any rotation occurs in such applications.... and during normal hot shutdown or emergency.

But seals to protect bearings from hot CO2 might be a durability problem. It appears that all the elastomers listed in the Air Liquide Gas Encyclopedia show issues with CO2. But complete chemical inertness brings up the possibility of argon (Ar) as a working gas-- which at atomic weight near 40 is almost a match to CO2 at 44. That is they would be close for specific impulse. Ar has a critical temperature of minus 122 C. Argon is relatively cheap and would not become corrosive at high temperature-- so for higher end applications might be a preferred medium. Argon is abundant at just under one percent of the Earth's atmosphere (0.93% by wt.). Air Liquide shows all common elastomers are "satisfactory" with Ar. And by the way, most all lubricants are "satisfactory" with either CO2 or Ar.

The direct heat to working fluid application of such turbines in LENR is clear to me anyway. The size and lower "under hood" temperature are nice bonuses. For outer space, one has to consider that virtually all excess heat has to be dissipated radiatively, there typically being no atmosphere to convect or conduct away waste heat. Radiative dissipation is a 4th power exponential with respect to temperature, so there we have to think of higher temperature turbines as an advantage, rather than a drawback. There again argon might be a good candidate. I know mercury (Hg) has long been suggested in that context -- no one wants to service that, even in outer space., even it is much smaller than a CO2 turbine for the same power.... (Hg atomic wt. ~200)-- let's let the robotic systems build and use those, if needed....

Andrea Calaon wrote:

Quote from Andrea Calaon

You can heat up CO2 by radiation (you need some volume here).
If you have a nuclear source, these systems seem ideal for generating mechanical power and electricity.
With LENR they will be put on ships, then trains and large airplanes, and may be later on smaller means of transport as well.
For airplanes you need a volume where to pass the heat from the nuclear source (solid state) to the fluid CO2 through radiation (no pressure loss). I don't know how large it will be. Then you will need to pass the energy to a propeller. Slower then jet, but VERY long lasting flights.

Yes, exactly with respect to conventional / thorium nuclear. With respect to LENR, if it is aneutronic, and say under something like the multitude of Lipinski UGC WIPO experiments with proton to Li 7, or like other aneutronics such as deuteron to Li 6, there is only the product of energetic alphas whose kinetic energy on impact heats the quite thin containment. The containment can be double walled with any heat transfer medium between the walls. Supercritcal Argon or CO2 would be very workable there. If there is a need for highest efficiency, then perhaps some sort of counter current flow (low temperature wall or tube heats incoming low temperature fluid, and high temperature heats the hotter stuff from the lower temperature section) basic heat transfer stuff.

Under the analogy of a high-bypass turbofan (all modern passenger jets today), there is near sonic flight without concern for propeller tip speeds going supersonic. These are in fact a ducted-fan / stator set, where the fans are driven by a smaller coaxial gas turbine engine. "All" that is necessary is to substitute an argon LENR turbine to coaxially drive the ducted fan. The hot, high pressure working fluid must get from the LENR device to the turbine--- a best location might also be coaxial and ahead or behind the working inert gas turbine chain. If LENR were very compact, it might be internal to the working inert gas turbine itself, which in turn is coaxial and inside the ducted fan as mentioned.

[Below is my earlier combined answer to Alain and Andrea Calaon]:

Downscaling may be limited in today's combustion turbines-- not so much because the technology is not there, but because of environmental issues (sound and pollutants) and the unit costs presently in place are not competitive due to the extremely developed nature of reciprocating engines. One of the virtues of the CO2 (or argon, see below) turbine is exactly the low temperature / high power nexus. Assuming Sorensen's assertion that CO2 turbines work very efficiently from 500 to 650 degrees C. That is way below the limiting temperature of a combustion gas turbine. A high performance jet engine can have a driven turbine (the one right in the hot flame) "inlet" temperatures well over 2000 C, necessitating ceramics and internal cooling of both the rotor and stator components. Stationary gas turbines, such as those used to boost the pressure in long distance natural gas lines, have an "inlet" temperatures no higher than 1400 C, (last I knew) which is near the maximum without the complications of exotic materials, ceramics and internal cooling passages in the turbine blades-- a real technical challenge to be avoided in mass production, I would think. But inlet temperature is a high motivator, since it increases efficiency on a steep exponential. High inlet temperature increases power to weight and "mileage", so to speak.

Sorensen seems to imply there are other "sweets spots" on the temperature curve for CO2. Certainly that could make sense since CO2 remains a very heavy fluid far on up the scale as both pressure and temperature increase. The present "sweet spot" mentioned may have been simply material limitations. Those would be removed in a ceramic system, small ceramic turbines are already in mass production (the driven turbine in turbo-superchargers). I may have mentioned that the reason CO2 and metal may be a problem at say 1400 C, is the redox reactivity of CO2 at those temperatures and pressures-- CO2 can be reduced by hot iron, hot carbon etc to give first carbon monoxide and of course destruction of the metal reductant.

Bearings are not generally the limiting factor today, since those can much more easily be cooled, via the shaft (rotor) and via the housing (stator) and via the lubricant itself. All kinds of high temperature ceramic bearings are already available in small ceramic versions "off the shelf". The loadings in steam and gas turbines generally require sleeve type bearings, and certainly the loadings in a CO2 turbine could well be even higher. Sleeve bearings can be made of exotic materials and/or can be readily cooled. Steady development of inexpensive synthetic lubricants coming first from jet engine applications are now widely available (the high polyester synthetic motor oils for cars are a direct descendent). Silicone and fluoropolymer lubricants of very high temperature tolerance are the next stage. Of course lubricant can be independently pumped through the bearings before any rotation occurs in such applications.... and during normal hot shutdown or emergency.

But seals to protect bearings from hot CO2 might be a durability problem. It appears that all the elastomers listed in the Air Liquide Gas Encyclopedia show issues with CO2. But complete chemical inertness brings up the possibility of argon (Ar) as a working gas-- which at atomic weight near 40 is almost a match to CO2 at 44. That is they would be close for specific impulse. Ar has a critical temperature of minus 122 C. Argon is relatively cheap and would not become corrosive at high temperature-- so for higher end applications might be a preferred medium. Argon is abundant at just under one percent of the Earth's atmosphere (0.93% by wt.). Air Liquide shows all common elastomers are "satisfactory" with Ar. And by the way, most all lubricants are "satisfactory" with either CO2 or Ar.

The direct heat to working fluid application of such turbines in LENR is clear to me anyway. The size and lower "under hood" temperature are nice bonuses. For outer space, one has to consider that virtually all excess heat has to be dissipated radiatively, there typically being no atmosphere to convect or conduct away waste heat. Radiative dissipation is a 4th power exponential with respect to temperature, so there we have to think of higher temperature turbines as an advantage, rather than a drawback. There again argon might be a good candidate. I know mercury (Hg) has long been suggested in that context -- no one wants to service that, even in outer space., even it is much smaller than a CO2 turbine for the same power.... (Hg atomic wt. ~200)-- let's let the robotic systems build and use those, if needed....

Quote from Longview

Andrea Calaon wrote:

Yes, exactly with respect to conventional / thorium nuclear. With respect to LENR, if it is aneutronic, and say under something like the multitude of Lipinski UGC WIPO experiments with proton to Li 7, or like other aneutronics such as deuteron to Li 6, there is only the product of energetic alphas whose kinetic energy on impact heats the quite thin containment. The containment can be double walled with any heat transfer medium between the walls. Supercritcal Argon or CO2 would be very workable there. If there is a need for highest efficiency, then perhaps some sort of counter current flow (low temperature wall or tube heats incoming low temperature fluid, and high temperature heats the hotter stuff from the lower temperature section) basic heat transfer stuff.

Under the analogy of a high-bypass turbofan (all modern passenger jets today), there is near sonic flight without concern for propeller tip speeds going supersonic. These are in fact a ducted-fan / stator set, where the fans are driven by a smaller coaxial gas turbine engine. "All" that is necessary is to substitute an argon LENR turbine to coaxially drive the ducted fan. The hot, high pressure working fluid must get from the LENR device to the turbine--- a best location might also be coaxial and ahead or behind the working inert gas turbine chain. If LENR were very compact, it might be internal to the working inert gas turbine itself, which in turn is coaxial and inside the ducted fan as mentioned.

[Below is my earlier combined answer to Alain and Andrea Calaon]:

Downscaling may be limited in today's combustion turbines-- not so much because the technology is not there, but because of environmental issues (sound and pollutants) and the unit costs presently in place are not competitive due to the extremely developed nature of reciprocating engines. One of the virtues of the CO2 (or argon, see below) turbine is exactly the low temperature / high power nexus. Assuming Sorensen's assertion that CO2 turbines work very efficiently from 500 to 650 degrees C. That is way below the limiting temperature of a combustion gas turbine. A high performance jet engine can have a driven turbine (the one right in the hot flame) "inlet" temperatures well over 2000 C, necessitating ceramics and internal cooling of both the rotor and stator components. Stationary gas turbines, such as those used to boost the pressure in long distance natural gas lines, have an "inlet" temperatures no higher than 1400 C, (last I knew) which is near the maximum without the complications of exotic materials, ceramics and internal cooling passages in the turbine blades-- a real technical challenge to be avoided in mass production, I would think. But inlet temperature is a high motivator, since it increases efficiency on a steep exponential. High inlet temperature increases power to weight and "mileage", so to speak.

Sorensen seems to imply there are other "sweets spots" on the temperature curve for CO2. Certainly that could make sense since CO2 remains a very heavy fluid far on up the scale as both pressure and temperature increase. The present "sweet spot" mentioned may have been simply material limitations. Those would be removed in a ceramic system, small ceramic turbines are already in mass production (the driven turbine in turbo-superchargers). I may have mentioned that the reason CO2 and metal may be a problem at say 1400 C, is the redox reactivity of CO2 at those temperatures and pressures-- CO2 can be reduced by hot iron, hot carbon etc to give first carbon monoxide and of course destruction of the metal reductant.

Bearings are not generally the limiting factor today, since those can much more easily be cooled, via the shaft (rotor) and via the housing (stator) and via the lubricant itself. All kinds of high temperature ceramic bearings are already available in small ceramic versions "off the shelf". The loadings in steam and gas turbines generally require sleeve type bearings, and certainly the loadings in a CO2 turbine could well be even higher. Sleeve bearings can be made of exotic materials and/or can be readily cooled. Steady development of inexpensive synthetic lubricants coming first from jet engine applications are now widely available (the high polyester synthetic motor oils for cars are a direct descendent). Silicone and fluoropolymer lubricants of very high temperature tolerance are the next stage. Of course lubricant can be independently pumped through the bearings before any rotation occurs in such applications.... and during normal hot shutdown or emergency.

But seals to protect bearings from hot CO2 might be a durability problem. It appears that all the elastomers listed in the Air Liquide Gas Encyclopedia show issues with CO2. But complete chemical inertness brings up the possibility of argon (Ar) as a working gas-- which at atomic weight near 40 is almost a match to CO2 at 44. That is they would be close for specific impulse. Ar has a critical temperature of minus 122 C. Argon is relatively cheap and would not become corrosive at high temperature-- so for higher end applications might be a preferred medium. Argon is abundant at just under one percent of the Earth's atmosphere (0.93% by wt.). Air Liquide shows all common elastomers are "satisfactory" with Ar. And by the way, most all lubricants are "satisfactory" with either CO2 or Ar.

The direct heat to working fluid application of such turbines in LENR is clear to me anyway. The size and lower "under hood" temperature are nice bonuses. For outer space, one has to consider that virtually all excess heat has to be dissipated radiatively, there typically being no atmosphere to convect or conduct away waste heat. Radiative dissipation is a 4th power exponential with respect to temperature, so there we have to think of higher temperature turbines as an advantage, rather than a drawback. There again argon might be a good candidate. I know mercury (Hg) has long been suggested in that context -- no one wants to service that, even in outer space., even it is much smaller than a CO2 turbine for the same power.... (Hg atomic wt. ~200)-- let's let the robotic systems build and use those, if needed....

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Glad to skip the advertisements (for Biotech no less) being fronted by the two previous one and two time "beginners". It would be interesting to know how many dozens of Forums they may have managed to insert themselves on behalf of their [bogus?] company. I have to imagine they are not even aware of the subject matter here, if they are humans at all.

Quote from acalaon

Dear Longview,
Argon does not absorb IR and must be heated by convection. This means to me too high pressure losses.
Prof. Gianfranco Angelino back in the '60 studied many gases and CO2 was recognized as optimal. So Argon would not be the best choice from the efficiency point of view either.
Bearing are not a limiting factors for certain dimensions, but going below the MW power would mean reactors that are so minuscules that the bearings would absorb too much energy and limit very much the efficiency. I am not an expert, but I read this:
http://nextbigfuture.com/2013/…ritical-co2-turbines.html

Very interesting points Acalaon. Thanks. I see the IR absorption could be an issue if the transmission of heat were not direct. In radiators, and in steam boilers, the transmission is often direct through a metallic wall--- that is from one medium to another via the conductive barrier. IR is an interesting issue, perhaps even in such fluid to fluid transfers. But I suspect that when you are transmitting heat via a metal or ceramic surface from the reactor to the working fluid and to a mechanism such as a turbine, IR is a modest contributor to overall heat flux. Please note, the CO2 and Argon cases may well be supercritical fluids... both with very high conductiviity and convective heat transfer coefficients, regardless of the IR absorption spectra for either of those fluids... which I don't yet see good online examples for, by the way. Of course a supercritical fluid is likely to have IR extinctions hundreds of times greater than a dilute STP gas... simply from molar/atomic density or concentration (Beer's Law).

Further there may be other interactions of IR with dense fluids that just don't show in the spectrum of a dilute gas-- and those being colligative may apply to Ar as well as CO2. We need to see such fluid spectra to know. But even so, I suspect IR alone is not enough of an issue to kill or advance any particular supercritical fluid, at least in the model situation I give here:

Using the Lipinski UGC device as a exanple that may well be first to market, at least for long range drones or interplanetary flights, we see all of the MeV alpha flux is likely to be directed toward a metallic or ceramic target.... on the other side of that target will be some working fluid, that is for thermo-mechanical energy extraction the heat transfer agent will surely be some gas or liquid or supercritical fluid. The chemical inertness of Ar allows a much higher working temperatures both in the context of the "boiler" and in interaction with turbine blades. Such materials as 300 or 400 series Stainless, Inconel, monel or bronzes and including pure copper are all inert to very high temperature Ar, but may react readily with CO2 at such temperatures as 1000 to 1500 deg C. That inertness I already suggested was important for bearings and seals, which may well be some of the limitations of CO2 to well below 800 deg C well within Sorensen's "sweet spot".

The IR absorption itself is in many ways related to the possible chemical reactivity. For argon there are no molecules at all, and not even much interatomic association in the gas phase-- except possibly in supercritical fluids. There is also no asymmetry in the atomic orbitals or loose electrons or "holes" to participate at shorter wavelengths. The molecular IR (bond stretching, twisting, bouncing etc.) level of the IR EM spectrum is certainly one path for heat transmission for those molecules susceptible. If you look at CO2, it also has little IR absorption, except at a few very specific energies. That does not prevent it from acting as an excellent heat transfer fluid. But, I still find your suggestion interesting, at least in theory, that is IR absorption might well make heat transfer somewhat more efficient in a molecular fluid, if only that the heat has one more ways to get into the fluid medium. But I am sure the lack of IR spectral activity is not a "killer" for any heat transfer medium, particularly at fluid densities approaching 1 g/cc. I would never want to underestimate mere "convection" and "conduction", both are exceedingly important in heat transfer processes seen industrially, regardless of the medium.

Quote from GlowFish

Just a question and a thought. I would assume in this case a working fluid or gas would be piped through the core of a reactor and the gas itself used to drive the turbine? Is there a huge disadvantage of using a heat exchanger like conventional reactors where the coolant is kept separate from the working fluid due to radiation concerns etc.? In that case, can you use CO2 as the coolant with whatever advantages it has in terms of IR absorption etc, but then transfer that heat to an Argon working fluid using convection? Heat would not magically vanish unless you have poor insulation and lose it to the environment.

Thanks for that interesting point, GlowFish.

To my knowledge, most of the gas phase Ni-Li-H style reactors require a particular gas composition and probably relatively low pressure in the actual reactor (applies to Lipinski- UGC as well). So with that constraint you are right, there is a need for a staged system. Lipinski's is inherently staged, or so my reading of their WIPO application leads me to conclude. In theirs the directionality of the resultant alphas assures that their impact can be on a articular "cooled" metallic surface, thereby generating the transfer locus for any secondary hot working fluid.

There is nothing preventing steam or other working fluid from being run through tubes inside of typical LENR reactors we've seen so far. That would be fine particularly if there was no dangerous radioactivity.... but even then the secondary heat transfer agent, steam, CO2, Ar, ammonia or whatever is essentially shielded by the tubes from lower energy radioactivity. Neutrons and high energy betas and beta +. Nevertheless, under some schemes we might imagine nuclear reactions that could conceivably induce radioactivity in at least those in-dwelling tubes, if not in their fluid contents. Of course much similar to that has been addressed many times in the fission engineering innovation history.

As a general rule the lower the atomic weight of the working fluid, the shorter the lifetime of any induced radioactivity. But that is a rather poor generalization. Generally in LENR claims we have yet to see any substantial transmutations via neutrons or positrons giving rise to much of concern. Perhaps because those particles are at comparatively low energies. The LIpinski alphas are low MeV, but they are massive, so much of the MV squared is in the M, and they effectively devolve to inert helium 4 gas.

If the whole thing is going to be radioactively "hot" it is likely to be a lot less than conventional fission, and certainly much less that "hot" fusion.

Quote from acalaon

What I am suggesting is not that radiation should contribute to the heating of the cycle fluid. I am suggesting to exchange heat ONLY through radiation. With no pressure losses at all. Dense SC CO2 in well engineered chambers can absorb all the energy radiated from LENR heated walls. If the walls are well above 800 [C], let us say 1,100 [C] I "feel" the density of transferred power per unit volume would be enough for engineering a system with only radiation exchange. Instantaneous thermal energy flux control, no losses. Possibly the LENR devices could be very rapidly controlled through a change of SC CO2 flux.
The heat exchange system would be something like a reheating furnace with many walls and SC CO2. That is why I was saying "you need some volume here".

Without fully comprehending what you are saying, I project that in a Ni-Li-H system as we have seen examples so far, one should be able to get around the "space" issue by having small tubes running through the reactor itself. That places the working fluid, whatever it is, in close proximity to the thermal energy presumably being generated. It also allows (as you suggest) "agile" thermoregulation by simply controlling the flow rate of the fluid cycle through those small tubes. In this, it looks a lot like what in the US I am fairly certain is called a steam superheater.

With Lipinski there is no "space" issue, the energy is delivered to a two dimensional surface, metal or ceramic, it could even be focused to some extent by using a curved lithium proton target as the alpha source, shrinking the alpha particle target area toward a desired or tolerable energy / flux intensity.

Any CO2 usage is limited in temperature to the survival of the alloys, CO2 is an oxidant for many metals at sufficient temperature and pressure. Argon has no such issues. Also please look at the Air Liquide guide for compatibility of hot CO2 v. hot Ar with seals Viton, fluoroelastomer, slicone, EPDM etc., likely an essential component regardless of the type of bearing used.

It does seem that we are talking past each other a bit on this subject, Acalaon. But I respect and appreciate your persistence.