Giggi
Well-Known Member
Firstly, a short description of supercritical carbon dioxide (frequently abbreviated as “sCO2”) for people like me who aren’t chemical engineers:
Carbon dioxide goes supercritical when it is subjected to pressures and temperatures exceeding those of its critical point: ~7.4 MPa (~1070 psi) and ~31 C (~88 F) respectively. Under these conditions, the physical properties of its liquid and gas phases become so similar that the phases merge together into a ‘supercritical fluid’ that exhibits properties of both liquids and gases simultaneously. It becomes extremely dense, having a density over half that of liquid water at the critical point and becoming denser as pressure increases, while at the same time it expands to fill its container like it does in its gaseous state. In addition, sCO2 exhibits some properties that are unlike either liquid or gaseous CO2 - it is a potent solvent that loves to eat things made of organic polymers (such as many types of seals and lubricants), and when near the critical point, small changes in its temperature have unexpectedly large effects on its pressure, and vice versa.
(Note: Nearly every substance on earth can enter a supercritical state, but very few of them have a combination of properties that make them viable for use as a working fluid. The viable ones include CO2, SF6, several different hydrocarbons and most of the noble gases; but CO2 especially is cheap, non-corrosive/flammable/highly toxic and also has some of the most favorable supercritical properties among them.)
One of the latest and greatest, most cutting-edge concepts in the field of power generation right now is the supercritical carbon dioxide Brayton cycle engine - essentially a recirculating external-combustion gas turbine that uses sCO2 as its working fluid instead of air. It’s more often compared to steam turbines than gas turbines though, partly because it physically bears more resemblance to a steam turbine, and also because its primary anticipated use is as a compact alternative to the steam turbines in power plants and naval vessels and such. I’ll be comparing it to both.
[Image: full-scale mockup of 10 megawatt CO2 turbine rotor]
Article link: https://www.technologyreview.com/s/601218/desk-size-turbine-could-power-a-town/
[Image: functional radial turbine wheel from Barber-Nichols]
Article link: http://www.theenergycollective.com/dan-yurman/84762/supercritical-co2-turbine-being-developed-smrs
Though the science of supercritical power cycles is still evolving and the only existing engines of this type are large and immobile test rigs, I believe (based on the fairly large amount of googling I’ve done, and the word of individuals in the field) that this technology could very well be leveraged to create an auto/aero engine with attributes very different, and much superior, to the more conventional engines that it resembles. To name a few:
sCO2’s liquid-like density, combined with the fact that it never changes phase, allows it to absorb and transfer energy much more effectively than steam, permitting the heat exchangers, power turbine, and practically every component that contacts the working fluid in a sCO2 engine to be made extremely small relative to their counterparts in a similar-powered steam turbine engine - the turbine itself being up to ten times smaller, with the rest of the engine being about 50% smaller.
Because supercritical fluids can undergo great changes in temperature and pressure without vaporizing or liquefying, a supercritical Brayton cycle engine can cool its working fluid down to a near-liquid state before compressing it. This makes its compressor work more like the pump in a rankine cycle engine, allowing it to draw a much smaller percentage of the turbine’s power than the air compressor in a normal jet engine. This and other smaller factors enable even small sCO2 engines to achieve thermal efficiencies of around 40-50%, higher than almost any other mobile powerplant in existence.
There are quite a few factors that positively influence the (theoretical) reliability of an sCO2 engine. Among these factors are:
I've got a bunch of technical papers downloaded as pdfs, but since I’ve already spent a month or two failing to write this post in a way I like, I don’t have the get-up-and-go to find all the links right now. But if there’s anything y’all want specifically I’ll do my best to find it.
I haven't yet discussed this subject at length with people who actually know what they're talking about, so I'd love to hear what you guys think about it :emb:
Carbon dioxide goes supercritical when it is subjected to pressures and temperatures exceeding those of its critical point: ~7.4 MPa (~1070 psi) and ~31 C (~88 F) respectively. Under these conditions, the physical properties of its liquid and gas phases become so similar that the phases merge together into a ‘supercritical fluid’ that exhibits properties of both liquids and gases simultaneously. It becomes extremely dense, having a density over half that of liquid water at the critical point and becoming denser as pressure increases, while at the same time it expands to fill its container like it does in its gaseous state. In addition, sCO2 exhibits some properties that are unlike either liquid or gaseous CO2 - it is a potent solvent that loves to eat things made of organic polymers (such as many types of seals and lubricants), and when near the critical point, small changes in its temperature have unexpectedly large effects on its pressure, and vice versa.
(Note: Nearly every substance on earth can enter a supercritical state, but very few of them have a combination of properties that make them viable for use as a working fluid. The viable ones include CO2, SF6, several different hydrocarbons and most of the noble gases; but CO2 especially is cheap, non-corrosive/flammable/highly toxic and also has some of the most favorable supercritical properties among them.)
One of the latest and greatest, most cutting-edge concepts in the field of power generation right now is the supercritical carbon dioxide Brayton cycle engine - essentially a recirculating external-combustion gas turbine that uses sCO2 as its working fluid instead of air. It’s more often compared to steam turbines than gas turbines though, partly because it physically bears more resemblance to a steam turbine, and also because its primary anticipated use is as a compact alternative to the steam turbines in power plants and naval vessels and such. I’ll be comparing it to both.
[Image: full-scale mockup of 10 megawatt CO2 turbine rotor]
Article link: https://www.technologyreview.com/s/601218/desk-size-turbine-could-power-a-town/
[Image: functional radial turbine wheel from Barber-Nichols]
Article link: http://www.theenergycollective.com/dan-yurman/84762/supercritical-co2-turbine-being-developed-smrs
Though the science of supercritical power cycles is still evolving and the only existing engines of this type are large and immobile test rigs, I believe (based on the fairly large amount of googling I’ve done, and the word of individuals in the field) that this technology could very well be leveraged to create an auto/aero engine with attributes very different, and much superior, to the more conventional engines that it resembles. To name a few:
- Higher power-to-weight ratio (versus steam turbines)
sCO2’s liquid-like density, combined with the fact that it never changes phase, allows it to absorb and transfer energy much more effectively than steam, permitting the heat exchangers, power turbine, and practically every component that contacts the working fluid in a sCO2 engine to be made extremely small relative to their counterparts in a similar-powered steam turbine engine - the turbine itself being up to ten times smaller, with the rest of the engine being about 50% smaller.
- Better fuel efficiency (versus gas turbines)
Because supercritical fluids can undergo great changes in temperature and pressure without vaporizing or liquefying, a supercritical Brayton cycle engine can cool its working fluid down to a near-liquid state before compressing it. This makes its compressor work more like the pump in a rankine cycle engine, allowing it to draw a much smaller percentage of the turbine’s power than the air compressor in a normal jet engine. This and other smaller factors enable even small sCO2 engines to achieve thermal efficiencies of around 40-50%, higher than almost any other mobile powerplant in existence.
- High reliability (versus all other engine types)
There are quite a few factors that positively influence the (theoretical) reliability of an sCO2 engine. Among these factors are:
- Carbon dioxide, unlike the water that’s present in all internal combustion and steam engines, is harmless to metal (so long as it’s completely dry). Also, since any CO2 engine would be closed-cycle, and possibly also hermetically sealed using a magnetic transmission, the working fluid would never become contaminated with combustion products or any other substances like in a gas turbine.
- Fewer moving parts than piston engines, and less complicated moving parts than other turbines due to the fact that sCO2 turbines are smaller and have fewer stages.
- Since conventional lubricants can’t be used, contactless fluid bearings would be a natural design choice. This eliminates mechanical contact between moving parts, along with the needs of cooling/lubricating/maintaining the bearings.
- sCO2 engines operate at lower temperatures than gas turbines, meaning the metal is less prone to deforming or creeping if the engine is run at high load for long durations.
I've got a bunch of technical papers downloaded as pdfs, but since I’ve already spent a month or two failing to write this post in a way I like, I don’t have the get-up-and-go to find all the links right now. But if there’s anything y’all want specifically I’ll do my best to find it.
I haven't yet discussed this subject at length with people who actually know what they're talking about, so I'd love to hear what you guys think about it :emb:
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