Going Supercritical!

The weird, revolutionary world of supercritical CO2.

Incautious Optimism Sep 26 ∙ Preview

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Our world is powered by the gas turbine and the steam turbine.

I often feel that the above sentence should be more commonly known, but it isn’t, and I shouldn’t be cross: People have busy lives, and who really has the time to ponder where all the electricity comes from anyway? From the moment that we’re woken by the alarm (or a heavy snorer, or “mama, I’m thirsty”, or even “bababa-OOO-Badebedebede” from the next room) we’re pretty much full-on until we slump in front of the TV at 9PM wondering what normal people do when they’re not busy all the time. There’s not much time in there for reflections on power generation.

So I’m here to help you out, and you can have a read of this instead! It’s probably better for you than watching The Summer I turned Pretty or streaming an American serial killer docu-series on Netflix, so why not give it a go?

I’ve probably lost 10% of the readership right there by reminding people that serial killer documentaries exist, but oh well.

In any case, the power that brings you this message, fires up your alarm clock (the non-human one anyway) and fills your eyeballs with televised crime voyeurism has to be generated from somewhere, and for the most part it’s a gas turbine or a steam turbine that does it: An austere triumph of metallurgy, plasma coating and machining that just sits there spinning insanely fast at temperatures that would make you into mush in seconds. It’s ubiquitous; in coal power plants, gas-fired plants, oil, nuclear, geothermal… at some point you’re either shoving superheated steam through a turbine, or superheated air through a turbine. 

Except for last Saturday night in Ireland, I guess, where half of our power was generated by the wind, using a different type of big spinny thing entirely. That’s an exception though, and to really drive the point home I made sure to ironically light the wood & coal stove that night for the first time this year. Clean power indeed.

And there’s hydro dams too, I guess, but I can’t focus on every exception when I’ve broad sweeping brush-strokes to make…

Anyway, in a world long since dominated by a selection of gas & steam turbine-based technologies, in which thousands of engineers make tiny iterative improvements for tiny iterative gains, wouldn’t it be nice to try something completely different for a change? That’s what this article is all about, for instead of plodding along chasing one or two per-cent improvements on a mature technology, it might be possible to blow the doors right off if you re-imagine the most fundamental part of turbine-based generation…

…Why not use something other than air or steam?

In a time where carbon emissions reduction is the secular religion, and the prime commandment of the era is ‘efficiency’, wouldn’t it be nice if we could ratchet the techno-irony all the way to the top, and save the world by using the very gas we’re not supposed to emit in the first place. Why not run a power-generating turbine on supercritical CO2?

Smaller, faster, better, cleaner…

Get ready, ladies & gentlemen. We’re going deep on something properly revolutionary today. 

1: The Orthodoxy of Air & Steam.

Near the vast dust-stained halls of coal pulverisers, where wan sodium lights stain a Dante descent, lies a strange room. In it, modern lights shining across polished floors, lies an odd orange creature of massive blocky cylinders, connected by organic-looking stainless steel pipes terminating in polished manifolds. In the orange beast, sessile and slug-like on the floor, vast energies are channelled in a steady, powerful roar..

The vast majority of power stations on Earth are heat engines. This means essentially that they heat a coolant, either directly or indirectly, that expands and performs work which is transformed into electricity, typically through a turbine. The two big generating cycles that are used for this are gas turbines (the ‘Brayton cycle’) and steam turbines (the ‘Rankine cycle’), each with their own advantages and disadvantages. 

We’ll start with gas turbines, which is what you see on jet aircraft and on rapid-response ‘peaker’ plants used to shore up an electrical grid with seconds to spare. In a gas turbine, the turbine and compressor stages are connected through a central shaft, and the compressor is used to compress air before entry into a combustion chamber where fuel (typically kerosene or methane, though there are others) is mixed and ignited in the air and then proceeds to expand through the turbine stages, doing useful work, powering the compressor and generating power. 

There’s a lot to like about gas turbines, and they’re ideally suited to aircraft where their light weight is useful, and the high velocity (sometimes high pressure) flow leaving the engine generates thrust. They can also burn extremely hot, since the coolant is also the oxygenating medium for the fuel, and air tapped from the compressor stage can be used to film-cool the turbines, letting you run hotter. For power generation gas turbines lack something however: The weight benefits of burning fuel inside the coolant (air), and having a coolant that can be pulled straight from the atmosphere are less beneficial for a ground-based installation, and atmospheric air has its own problems that limit the gas turbine’s efficiency.

-For one, gas is easily compressible. This is useful in the turbine stage (lots of expansion) but wasteful during compression, where the pressurization of the incoming gas causes waste heating that makes gas turbines less efficient than you’d expect. All that hot air jetting out of the back is energy wasted; more on that later.

By contrast you can use the Rankine cycle, which is steam-powered: The Rankine cycle uses highly pressurised water as the primary coolant, which boils when exposed to heat through a boiler, whatever your heat source. This means that you can pressurize a liquid (which is incompressible and therefore efficient), and expand a gas in the turbine (which is compressible and good at expansion). Surely then, this is the best of both worlds? Well, almost.

Unfortunately, steam turns back to liquid again when you expand it enough, and too many water droplets, delivered at crazy-high speed, will knock the hell out of turbine blades, so you can never cool it all the way down. 

You’re also very much limited on temperature with a water & steam solution, and since engine efficiencies benefit from high turbine entry temperatures (TET), this places you in a bind. You can see the trend by looking at this chart; if you bring all the cycles together in one place you see the clear trend that increasing TET usually brings increasing efficiency… and the higher efficiency, the less fuel you need to burn, the less your electricity costs and the happier all the polar bears are with you. Win-win, right?

Up until recently, the best performer available has been something called a combined-cycle gas turbine (CCGT), typically methane-burning, which can be thought of as a Rankine cycle powerplant bolted onto the back of a Brayton-cycle powerplant: A gas turbine solution that uses the waste heat from the gas turbine to boil pressurized water and run a steam turbine off the back. These are just about the most fuel efficient plants in existence, but they pay for it by being big, complex beasties that can’t be ramped up & down quickly. Baseload beasts in a variable world, CCGTs are amazing but not perfect.

The gas & steam duo have dominated power generation for over a century, but I wouldn’t be writing this if that’s all we had to say: Let’s up-end the whole thing and introduce a new contender that can change everything.

Supercritical CO2.

2: Going Supercritical!

Last year the SouthWest Research Instituted (SwRI) in San Antonio did something very special, generating electricity from a closed-cycle supercritical CO2 powerplant: A super compact and super-efficient form of electricity generation that knocks other turbine-based systems arrogantly into a cocked hat. They did this by unlocking the potential behind something that has been common knowledge in fossil fuel generating circles for some time:

There is another state of matter beyond solid, liquid and gas.

Let’s boil this one down. If you heat a liquid, like water, it will eventually boil into steam. This is intuitive enough. If you pressurize it (in, say, a pressure cooker) it will remain liquid for much longer and transition to steam at much higher temperatures, to a point at least. But if you crank up the temperature and pressure high enough something special happens: Beyond 218 atmospheres of pressure and 374 Celsius, the ‘critical point’, water enters its supercritical fluid phase, neither a liquid nor a gas but with characteristics of both. In this phase, characteristics such as heat transfer and compressibility change smoothly without cliff-edge phase changes, and can be exploited at both ends of this transition: Easier to compress when cool and easier to expand when hot. Many coal plants already use supercritical steam generation, so this is a well-understood technology..

OK, so that’s water. What about carbon dioxide, CO2? Isn’t that the enemy?

Well CO2 can also be made into a supercritical fluid, but it has special properties that make this far more transformative: The critical point of CO2 is at a mere 31 degrees Celsius and 74 standard atmospheres of pressure, which is already much friendlier than supercritical water, and because the lower limit is near ambient temperature, this can potentially allow for a full temperature recovery and a fluid that stays supercritical throughout. 

OK, OK: Lots of technical stuff, but how is this actually useful? Well the primary factor is that although supercritical CO2 has a lower specific heat capacity than water, that’s more than made up for by being much, much more dense than steam, which means that all your expensive turbomachinery can shrink to a fraction of the size. CO2 is also a lot less corrosive than water at high temperatures, which means that you can really turn up the heat.

And if you combine these two features together, you can make high-efficiency miracles out of all kinds of applications, meaning that you can get the tremendous efficiency (59%) of a combined cycle gas turbine plant, but with a tiny compact single-cycle installation that can react quickly to demand spikes. Handy!

And there’s a special freaky variety of this that burns fuel directly in carbon dioxide, and makes dirty fuel sources clean again in an act of mechanical wizardry. We’ll get to that in a little while, but first…

3: The mote in God’s eye: Concentrated solar power.

Across certain parts of Northern Africa are strange fields, where oceans appear to lie on dry land and the eye picks out highlights on waves made of glass…

Concentrated solar power, such as the monstrous Noor plant in Morocco, is unique in being a form of renewable energy with its own self-contained energy storage, held in tanks of molten solar salts, superheated by the concentrated desert sun. Vast banks of mirrors concentrate heat onto tubes of solar oil or even sky-scraping central towers where molten salt is heated by the combined efforts of vast fields of controllable mirrors, creating a second sun on Earth as they focus onto the tower top. 

Concentrated solar is unique and eye-catching, but a niche player in energy markets despite its useful overnight storage capabilities, and that’s largely due to cost and efficiency: Not only is it more expensive to set up than equivalent photovoltaic solutions (think ‘solar panels’), but it also has lower full-system efficiency at converting solar energy into electricity: Only about 15% efficient, compared with 20%-25% for photovoltaics. A lot of this is down to limitations on the heat at which they can operate at: Remember that the hotter the turbine entry temperature is, the more efficiency you can crank out of the system. 

Supercritical CO2 could bypass this limit completely, an excellent heat conductor with low flow resistance and stable to extremely high temperatures (unlike many molten salts), it could allow truly high temperature concentrated solar power plants spinning compact, high efficiency supercritical CO2 turbines directly. 

More power from the sun!

And this is only one of supercritical CO2’s many applications. Let’s keep going…

4: The Cooling Conundrum...

© 2025 Jordan Taylor
548 Market Street PMB 72296, San Francisco, CA 94104
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