The Supercritical CO2 Turbine: Waterless Wonder

For over a hundred years, steam turbines have generated power using water and steam.

Over the years, that steam got hotter and more pressurized. It

made the turbines more efficient, but also made them big and complicated.

Now here comes a different type of turbine. Radically smaller.

Drastically simpler. These turbines have CO2 running through their veins instead of steam.

In today’s video, supercritical carbon dioxide

turbines. The waterless wonder that may be 10 times smaller than their counterparts.

## Spin Up The Turbine Steam turbines run on the Rankine cycle.

In Rankine-based turbines, water is turned into hot pressurized steam inside a boiler.

That steam cools and expands when it hits a set of turbine

blades. Shenanigans and electricity follow.

In the end, we condense the steam back into the liquid water phase

so that the party can get started all over again.

The Rankine's key trait is that it uses a liquid as the turbine’s primary working

fluid - with that liquid changing phases to gas and back in a loop.

A heat engine’s efficiency depends on the temperature differential of the fluid going

in and out of said engine. Since the outlet temperature tends to be fixed,

we have to raise the inlet temperature.

So to make our turbines more efficient, we - and I mean we as in the human race,

not me and you personally - have worked to make the incoming steam hotter.

As part of this strategy, we now compress and heat the water beyond its critical limits to

make it a supercritical fluid - a weird state of matter with traits of both liquids and gases. Once

supercritical, the water turns directly into steam without boiling, letting us heat it up even more.

Today’s top Ultra-supercritical turbines have efficiency rates of about 43-48%.

Utilities want efficient turbines because it means getting more energy

for burning the same amount of coal. Since their single biggest operating

expense is the cost of the fuel, you want to get the most out of it.

One can argue that the Rankine cycle is quite efficient on paper. But achieving

those efficiencies in reality can be tricky. After passing through the blades,

the steam still has residual energy that can only be recaptured with another set of fan blades.

In the end, we only add so many rows of fan blades until it gets a bit ridiculous. Some

Rankine steam turbines have 30+ stages of very large fan blades.

## Brayton Cycle So the Rankine is one of two major philosophies

for running a turbine. But as Yoda says, there is another.

The second is the Brayton cycle - or Joule cycle as some like to call it.

It is the basis of jet engines and the gas-fired turbine. The key aspect of

the Brayton is that the working fluid remains in the gas phase throughout the whole cycle.

The gas turbine takes in air, compresses it, and then adds the natural gas as fuel. Then we

ignite that sucker using a combustor to bring it to a high-energy state.

The high-energy air hits the turbine blades - losing pressure and cooling down in the process.

After passing the turbine, the hot gas is often expelled out the back

like a bad customer. So the gas-fired turbine is an open cycle Brayton system.

There is a variant of this system called the Combined Cycle Gas Turbine,

where the exhaust's residual heat is used as a heat source for an

attached Rankine steam turbine to garner yet more efficiency gains.

I should also note here that we have a whole rogues' gallery of

Brayton variants - all with their own tradeoffs.

The use of gas has certain benefits. With steam turbines,

the superhot steam can degrade the metals in the turbine blades due to moisture-induced

oxidation and "creep" - where the steel deforms over time due to extreme stresses.

Dry gas is more forgiving on the blades at higher temperatures. And thus indeed, modern gas turbines

can operate with inlet temperatures between 1300 and 1500 degrees Celsius.

Such incredibly high inlet temperatures let gas-based Brayton cycle systems operate at

higher efficiencies. Up to 60%, higher than those of top Rankine-based ultra-supercritical turbines.

## The Closed-cycle Brayton

In addition to the rather common Open-cycle Brayton,

there is the closed-cycle Brayton. Which is a bit rarer.

In a closed cycle Brayton, the gas is compressed and heated up indirectly by

some heat source via a heat exchanger. And after passing through the turbine blades,

it is sent back to the heat exchanger to extract some heat before being put

back into the fold like how Mufasa tells it in the Circle of Life.

A closed-cycle Brayton can use any gas as its working fluid. One of the most closely

studied gases is helium. The pluses of Helium is that it is safely inert,

conducts heat much better than air, and flows cleanly.

Helium has serious downsides though. It has a low molecular weight, which means it

easily leaks through shaft seals, imperfect welds, and even small cracks in the casing.

Since it is also kinda expensive, we need some extra engineering to keep it sealed.

The gas's low density also means it takes quite a bit of energy to compress. Power-hungry gas

compressors inflict a considerable penalty on the helium Brayton turbine's real-life efficiency.

Hence to compensate, the Helium Brayton system needs very high inlet temperatures of something

in the neighborhood of 900 degrees Celsius to achieve efficiency rates comparable to that of top

Rankine turbines. Certainly possible for an open cycle system but for a closed cycle a lot tougher.

## Supercritical Carbon Dioxide

So despite helium's strong advantages, people have shifted their attentions to carbon dioxide.

Carbon dioxide is well studied - particularly since it was used as a coolant for early

era nuclear reactors - so people are familiar with it.

It is quite stable at turbine-level temperatures of 1,500 degrees Celsius

or more. It is also non-toxic, less likely to leak than helium,

and is cheap and abundant. Perhaps a bit too abundant nowadays. But yeah.

When supercritical, carbon dioxide's fluid density is 50% higher than that of steam.

Depending on your assumptions, that implies a turbine to be some 10 times physically

smaller than a Rankine-cycle turbine due to smaller parts and fewer stages.

Another important thing is that carbon dioxide's critical pressure

limit of 7.38 mega-pascals is just about a third that of water’s.

Moreover, carbon dioxide’s density rises as it approaches that critical limit - making

it more incompressible like a liquid. Which is a big deal because it is easier to compress an

incompressible liquid than a low-density gas. In Rankine steam turbine systems,

pressure is applied with simple water pumps.

Since gas compressors like I said use so much energy, CO2 getting more liquid-like

as pressures approach the critical point means that the gas compressors can work less.

So all in all, a supercritical CO2 turbine can be just as efficient with inlet temperatures of

550 degrees Celsius as a helium Brayton turbine with inlet temperatures of 900 degrees Celsius.

## How It Works

The simplest supercritical CO2 Brayton system goes as we talked about earlier:

A compressor first compresses the CO2 to near the critical limit.

Then the CO2 is heated up. First, heated up inside the recuperator

with heat from the gas exiting the turbine later on down the line.

And then heated up secondly from the primary heat source - nuclear

or otherwise - indirectly via a primary heat exchanger.

The carbon dioxide then enters the turbine where it expands

and cools. The turbine turns the generator. Electricity results.

Now we must recuperate heat from the carbon dioxide exiting the turbine. We send it to

the recuperator system to transfer its heat to the gas exiting the compressor.

Then it goes back to prepare for the cycle to restart all over again. We

call this the "simple recuperated supercritical CO2 power cycle".

Note how the gas travels in a single flow throughout the cycle.

This single flow works and is simple enough,

but frankly is not all that efficient because the temperature difference between the gases

exiting the compressor and turbine is not that great. We are not recuperating enough heat.

So engineers have developed a variant called the split-flow.

They split the CO2 gas flow exiting the turbine into two, running the twins through additional

components like a compressor to maximize heat recuperation and thus efficiency.

There are so many variants, all tailoring the

cycle to fit certain applications or circumstances. For instance,

they might blend the CO2 with additional gases to tweak the cycle's overall commercial viability.

And then there is the Allam cycle, which is an interesting expansion on the concept. In it,

we burn natural gas and pure oxygen together to get carbon dioxide. This

carbon dioxide is all captured and run through a supercritical CO2 turbine to

generate power. After that, the gas is sent to be sequestered.

## Supercritical History

The idea of using supercritical carbon dioxide for a closed-cycle Brayton dates back to 1948.

That year, a Swiss industrial engineering firm called Sulzer Brothers filed a patent for a

Brayton turbine utilizing supercritical carbon dioxide. They never did anything with it however.

Then in the 1960s, an American engineer at Douglas Aircraft named Ernest Feher

published a series of papers that led him to propose the supercritical power cycle,

choosing carbon dioxide as the working fluid.

The supercritical power cycle is similar to the Brayton cycle. The carbon dioxide

operates throughout the whole system at above its critical limit. To sidestep the challenge

of pressurizing carbon dioxide gas, we compress it to near the critical point.

Other individuals working on the supercritical cycle or something like it in the late 1960s

include D.P. Gokhshtein and G.P. Verkhivker of the Soviet Union and G. Angelino of Italy.

All of their work showed that supercritical CO2 turbines have

generally higher thermal efficiencies than existing Rankine steam turbines:

Anywhere from 48 to 50%, which is an impressively high number.

Plus, their smaller size and simpler designs make them favorable for

constrained environments like ships or nuclear power plants.

Unfortunately, interest in developing supercritical CO2 closed-cycle Brayton

systems petered out. Open-cycle systems were more mature, with far higher temperatures and

efficiencies. Closed-cycle systems were still working out their technical issues.

And inside a terrestrial thermal power plant,

the technology’s potential size advantages were not all that relevant. Fossil fuels also

generate higher temperatures, which more favors steam turbines or Helium Braytons.

## The Nuclear Opportunity

Fortunately, the technology was found to hold interesting potential for nuclear energy.

Nuclear power plants share similar technical traits with fossil fuel thermal plants. As in

they both heat up water to turn a turbine. But their economics are quite different.

A fossil fuel plant's biggest operating cost is that of the coal or oil fuel. So

utilities add certain complexities to their steam turbines like reheat

cycles to squeeze as much energy as possible out of their coal.

But with nuclear energy, the cost of the uranium fuel is small compared to

the plant's immense upfront cost. About 30% of that is the turbine. So the supercritical

CO2 turbine’s potential simplicity and smaller size can impact those build costs.

There are also potential safety benefits with a closed-cycle turbine - as the

sealed cycle could possibly prevent the accidental release of any fissile material.

In 1970, Feher collaborated on a small 150 kilowatt-electrical supercritical

CO2 loop. Not a full turbine but a loop. Evidently to investigate its use

for nuclear energy generation. So people recognized the potential very early on.

The problem that people hit at the time was that supercritical CO2

turbines achieved their ideal thermal efficiency with inlet temperatures of

about 550 degrees Celsius. But the light water reactors of the 1960s and 1970s

produced temperatures between 200-300 degrees. So short of what was ideal.

## A 2000s Revival

The technology gained new energy in the late 1990s and early 2000s due to a general

revitalization of nuclear energy thanks to high oil prices and US government support.

Technology changes also played a factor. New energy sources were emerging. Most notably,

the Generation IV nuclear reactors.

Stuff like the molten salt reactor or very high temperature reactor. These can achieve

temperature ranges of about 550 degrees Celsius or higher - which is the turbine's sweet spot.

Researchers also identified synergies with the Small Modular Reactor, the ever-so-memed SMR.

SMRs are smaller advanced fission reactors with power capacities under 300 megawatt-electrical,

but can be prefabricated in a factory and integrated into a site.

Supercritical CO2 turbines jive with the SMR's overall goals for simplicity,

smaller size and lower capital costs without compromising too much on thermal efficiency.

## Technical Challenges

Supercritical CO2 technology is not alien. The oil and gas

industry already compresses and pumps carbon dioxide, so there is a precedent.

But there are major technical challenges.

Perhaps the most foreboding one being understanding and handling the changing

physical characteristics of the carbon dioxide as it transitions inside the turbine machinery.

Small changes in gas temperature and pressure can lead to big changes in the CO2's density,

viscosity and compressibility. This has major consequences on the machine's

components like its compressor - which tend to be designed for fixed conditions.

The supercritical carbon dioxide flowing throughout the turbomachinery is dense,

putting immense loads on the machinery's bearings and seals.

Broken seals cause leaks and what are called windage losses - significant

efficiency losses that can be as large as 2% of the whole thing.

Another challenge concerns the turbine’s materials - same as

with advanced steam turbines. Metals have to maintain their integrity in

a high-heat environment over the 30 or so years of its lifetime.

Studies on how stainless steel and nickel alloys will perform in such conditions are limited.

Risks include carburization - where carbon diffuses into the metal; Sensitization - where

carbon reacts with steel elements like chromium; As well as high-temperature corrosion and erosion.

## Projects

There are a lot of supercritical CO2 turbine projects around the

world. We have been developing this stuff since the 1960s, remember.

The first post-2000 projects began in the United States with collaborations between

MIT and national government labs like Sandia. Many were test loops that were

too small to show the true physics and mechanics of scaling to commercial size.

After 2015, Sandia realized that they needed to go bigger - and they sponsored projects

to build a 10 megawatt-electrical turbomachine power system, hoping