Supercritical CO2 Tech Team | Hacker News Reader

Supercritical CO2 Tech Team

energy.gov

76 pointscinquemb50 comments

"Clean coal" - are we really going to carry on flogging this dead horse ?

Indeed. I cringe each time I see the phrase.

There is "Clean coal" - a political lie that coal from US swing states is more "clean" than coal from China.

However this article describes CO2 capturing and storage technology. Something that could definitely help us in the coming decade.

Am I looking at the right article?

The one I read was about using CO2 in generators instead of steam.

edit: ah it was text on an image at the top that mentioned "clean coal" I didn't find it by text search so was confused.

Article has nothing to do with CO2 Capture and Storage.

You've got to make an actual argument rather than dismissing a technology out of hand. I'm put in mind of immortal quotes like "While most companies dumped gasoline in rivers (this was before the automobile was popular), Standard used it to fuel its machines" [0].

I'm not holding my breath, but there is nothing wrong with an energy department facilitating new energy technologies.

[0] https://en.wikipedia.org/wiki/Standard_Oil

Argument: Coal is made of carbon, and any combustion of it contributes to carbon dioxide, the primary driver of anthropogenic climate change.

Putting the word "clean" behind it refers only to reduction of sulfur/nitrogen oxides, and does nothing to solve the carbon dioxide. It's a solution to the acid rain problem from several decades ago, but has minimal bearing on the current problem of climate change. At this point, it's a marketing term designed to confuse and confuddle the issue, and to imply that it's even possible for coal to be "clean".

There is everything wrong with an energy department facilitating the same old energy technologies that have no substantial differences in the ways that matter.

But given that we appear to be burning the carbon anyway, it does make sense to burn it at the highest attainable efficiencies. Ignoring clever tech just means more coal burnt for nothing, in defiance of the fact that 85% of our energy comes from fossil fuel.

If the tech doesn't reduce carbon emissions then it isn't exactly a dead horse. They just aren't investigating it in an effort to reduce carbon emissions.

This is turbine technology. Turbines are energy source agnostic (gas, nuclear, solar concentration, etc) and will continue to be used for many years. They are basically the only viable way to reliably turn a heat gradient into electricity. Any advancement in this fundamental technology is welcome.

The mention of clean coal in an image at the top of the article is irrelevant except for political messaging reasons, which I agree is a dumb reason for it to be there but here we are.

The article has absolutely nothing to do with clean coal.

No idea about the technical details... but wow the size difference between a 300MW steam and sCO2 turbine - mind-blowing.

Is storing a lot of dense, pressurised CO2 not a major liability? If it leaks, it floods whole areas with suffocating gas.

> storing a lot of dense, pressurised CO2

In my understanding, there's only enough CO2 to use as a working medium in the closed loop. No storage as such.

Still, if it leaks out it will displace a lot of air, just because of how dense it will have to be.

It will, but it's easy for people to detect and needs to be around 10% to kill someone, about a hundred times more than hydrogen sulfide or chlorine or carbon monoxide.

If the baseline is superheated steam, I'm not very worried.

One that, and two, emergency oxygen masks work really well against CO2. It's only toxic at concentrations far above what you notice, not at all toxic at concentrations you clearly notice it (and take care to fix the seat/seal of your oxygen mask), and you shed it very quickly once you get a low-CO2 breathing atmosphere.

It's literally the one thing you don't necessarily need a sensor for if you get regular training to remember how you perceive dangerously elevated CO2.

How is elevated CO2 perceived? Does it have a smell?

When you hold your breath, it's not decreased oxygen that triggers the burning sensation in your lungs and the desperate need for air; it's elevated CO2. So it's pretty easy to perceive if you're at all aware of the signs.

However, reliably detecting 1000~2000 ppm isn't hard, but will require some training.

And that's the level where you want to leave the place in a state that's not dangerous, followed by getting out of there and waiting for someone with an oxygen mask to go leak searching.

But yeah, CO2 is the thing that makes you want to open a window long before acute toxicity.

Bad air/fatigue. It's the headache/mind-numbing effect you remember from walking into a poorly ventilated classroom or such.

It's not hard to detect, especially if sudden or strong, but to reliably act you need to regularly train to judge actionable concentrations, so you remember the feelings/perception for the specific actionable levels.

We have chemical plants all around the world where a leak of one kind or another can (and has) cause a terrible catastrophe. We've gotten to the point where large scale incidents are at least rare even dealing with highly reactive chemicals.

The sudden decompression will cool down the CO2 rapidly. To the point where it will become dry ice. This will slow down the release speed significantly. It is still not nice and probably a risc, but not als large as you estimate.

Actually you can make dry ice like this: get a CO2 cinder upside down and open valve into a cloth bag.

Having anything pressurised is a liability, be it CO2 or steam.

I am puzzled how the turbine can be 20 times smaller. That would mean 20 times the load or even more. What is it made of?

Steam turbines are HUGE compared to gas turbines. Think of the turbines on an airliner.

The main reason they haven’t built many Coal burning or nuclear power plants since 1980 is that steam turbines are uneconomical compared to gas turbines.

It has to do with the density at it's operating state, which is a lot closer to water than steam. It's the same reason a 1m radius water turbine can produce about 1MW of energy, but an air turbine of equivalent power might have a radius of 50m.

Ahh, that makes sense, thanks!

I wonder if it might be possible to develop miniaturized low power turbines, say anything under a megawatt range? it's already icebox size, can we make a high efficiency 100kW (133HP) turbine the size of a briefcase? (of course we'll also need some fairly decent sized radiators too).

I actually think this technology can be perfect in combination with new nuclear power.

FlibeEnergy hopes to use it, and I know the other vendors are not planning to use it, but if it came about they would love it.

Yes, I think reducing the cost of the non-nuclear part of a NPP is essential if nuclear is to have any sort of future. The problem is sCO2 needs a higher temperature reactor than LWRs, which introduces its own host of problems.

Does it? I thought sCO2 could operate at small temperature gradients. In fact, I thought sCO2 can work for industrial heat pumps.

The applications for nuclear under investigation involve temperatures of 550 C to 800 C.

https://www.energy.gov/sco2-power-cycles/sco2-power-cycles-n...

This chart shows temperatures for nuclear applications as low as 350 C, but that's still hotter than today's LWRs.

https://www.energy.gov/sites/prod/files/2015/01/f19/NEACsCO2...

This second pdf shows the problem: replacing steam means not just building sCO2 turbines, but doing all the incremental optimizations on them that steam turbines already have.

Most next generation reactors already are high temperature.

All the major companies at the forefront of reactor commercialization have high temperature reactors.

The optimal today for a reactor is having one reactor with output X and then add 3X worth of turbines and a way to store hot solar salt. This way you can produce huge amount of energy at the times when prices are high.

Of course it also means the nuclear part of the project would only be a small part of the overall cost.

> Most next generation reactors already are high temperature.

Yes, and as I said this introduces a host of new problems. Not only do these reactors need higher temperature materials, they need materials that can with standard corrosion and radiation at those temperatures.

And they also remove far more problems. In fact they are far simpler then PWRs.

It is far, far, far easier to design a reactor to withstand high temperature and corrosion then operating a reactor under gigantic amount of pressure.

> Closed Brayton cycles for power generation have a commercial history dating to the late 1930’s when the first of a series of power generation plants using the closed Brayton cycle made use of air as a working fluid. These closed air Brayton units operated from the late 1930’s through the 1970’s using a variety of energy sources including coal, gas, and waste heat. These recuperated Brayton cycle installations achieved good reliability and relatively high efficiencies as compared to other power generation technologies of the era.

So why did we stop using them?

My guess: expensive. Gas turbine power plants are not cheap, but fuel wasn't that expensive back then.

I'd expect the replacement to either use hydrogen with hydrogen-resistant steel, or, more likely, helium. I think carbon-carbon composite can handle extreme temperatures in a helium atmosphere, beyond what you can reasonably reach with a fuel flame. However, the temperatures should be feasible for fusion power plants, which could use helium or another noble gas (in case helium diffuses too fast) to cool the chamber-facing tiles.

The light gas would be for power density by way of having a high speed-of-sound.

Also, we mostly switched to Rankine cycle for external combustion power plants. Steam turbines are quite good at what they're doing.

Gas turbine power plants (specifically, combustion turbines) are actually very cheap. The reason is they avoid heat exchangers. Simple open cycle combustion turbines don't need heat exchangers at all, neither before the turbine nor after it (to condense/cool a working fluid). A simple cycle gas turbine power plant might be as little as $400/kW, vs $10,000/kW for a nuclear power plant.

> to other power generation technologies of the era.

Emphasis added. These days we can design significantly better steam turbines due to advancements in computational fluid dynamics and air as a working fluid is just not as nice (thermodynamically speaking) as steam. The Brayton cycle is still used in jet engines and CCGT power plants though.

I guess here we want a fluid with high heat capacity, but which can perform well under lower pressure than an equivalent amount of steam. Good candidates are substances with low mollecular weights and which don't react with the pipes and turbine materials. Alternatives here can be helium or argon etc, but CO2 is far cheaper.

Supercritical CO2 or helium also perform better when the heat source is hotter, which is suitable for the Brayton cycle. It's also why they are frequently mentioned when talking about high temperature next-gen nuclear, or concentrated solar.

The operating pressures here are actually much higher than steam.

Supercritical fluids are extremely interesting because they have some helpful characteristics of liquids, like density, and some helpful characteristics of gasses, like heat expansion. For this reason, supercritical fluids in general are the subject of a lot of heat engine research. I believe supercritical CO2 comes to the table because it becomes supercritical at a low enough temperature and pressure that it won't take major breakthroughs in materials science in order to build a heat engine with it.

Aren't trees a very efficient and cheap way to capture CO2 from air and store it in a safe way?

Only unless they burn down...as Microsoft had to realize in the North East just recently.

It’s just temporary storage, as the trees die and release all the carbon back as they decompose (worse, they release a bunch of methane in the process).

Also they burn as the climate changes: the Dixie fire destroyed a bunch of carbon offset forest (putting all that carbon back into the atmosphere). Which can be replanted and new certificates issued for the same land. Brilliant!

I have nothing against planting trees and they can have some temporary (but slow) value, but they don’t really make a dent on fossil fuel consumption which is centuries of sequestered carbon being released every year.

Well, trees have lifespans that are counted in decades, so they should offer a good buffer for humanity until we sort our energy source issues.

And, if you have programs that are like rolling tree planting systems, then you can build a pretty long lasting buffer.

Trees are nice, and have a bunch of benefits in terms of moisture, cooling, aesthetics, and as something you can burn in a camp fire. They are great and we should have more of them.

But as a form of carbon capture they are a cruel joke and people whe supply credits for that are fraudsters.

Even if your plan were to plant trees, cut them down when mature and dump them at the bottom of the ocean (away from the continental shelves) and replace them, they could not keep up with fossil fuel burning which releases about 400 years of global carbon production per year.

It’s like putting a 64-byte L1 cache in front of your CPU.

If I understand correctly, fossil fuels are ancient swampy bogs. Trying to imagine a situation where hundreds of vertical feet of carbon rich soils can accumulate, I think of a river delta slowly extending to sea, or an ancient lake eutrophying.

Perhaps a geologist will come along who can explain better.

It's probably useful for this as well:

https://www.inceptivemind.com/energy-dome-co2-battery-promis...

Here's the HN discussion about that https://news.ycombinator.com/item?id=28025121

this looks like a sweet way to build ship engines

CO2 growth is mostly something that happens in China and India. Unless this technology can be exported and used there, it is pointless.

"Pointless" is a massive and silly exaggeration. Regardless of growth, over a third of CO2 is emitted by developed countries. Reducing CO2 is valuable whether or not it happens in China and India.

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