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PostPosted: Feb 15, 2013 1:30 pm 
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I was thinking about working fluids and gases this morning, and found several links to technology developed by GE in the 1920's that used mercury as the working fluid in a power plant. I know you're horrified by that. But:

http://en.wikipedia.org/wiki/Mercury_vapour_turbine

Eventually the improving efficiency of steam overtook the early advantages that mercury held, research in mercury seemed to have ended in 1937. The reason Mercury caught my eye, is that its able to move a great deal of thermal energy at low pressure. In the link below, it says that a 15MW turbine can be operated by mercury vapor at 113 PSI and 507 C. The relatively low pressure but high temperature can then go on to boil water for a secondary steam cycle. http://www.aqpl43.dsl.pipex.com/MUSEUM/ ... ercury.htm

In between the primary mercury turbine and the secondary steam cycle, there could be a MHD generator. (http://en.wikipedia.org/wiki/MHD_generator) that could further boost the thermal efficiency of the plant.

GE also demonstrated the ability for the mercury and steam turbines to drive a single shaft for a generator.

http://www.scribd.com/doc/30081705/Alte ... ure-plants

I know mercury wont be a popular working fluid for a nuclear power plant - and I can see a regulator's head explode at the suggestion - but I hadn't heard of the technology before today, and thought it might provide some interesting food for thought and fodder for discussion.


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PostPosted: Feb 15, 2013 6:38 pm 
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Even without the toxicity, mercury is a fairly rare and expensive element, and not that efficient compared to supercritical steam.

But I think the toxicity is a real showstopper. People are freaked out by 0.007 gram quantities in CFLs, and require recycling.

Mercury is troublesome, and has no real advantages to show for it that alternatives cannot do better.


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PostPosted: Feb 15, 2013 7:40 pm 
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Occxium wrote:
I know mercury wont be a popular working fluid for a nuclear power plant - and I can see a regulator's head explode at the suggestion - but I hadn't heard of the technology before today, and thought it might provide some interesting food for thought and fodder for discussion.
Welcome to the EFT forum.

I don't see mercury as having any legs based on toxicity issues as Cyril mentioned. As you say steam has overtaken mercury binary cycles fairly quickly. Modern steam turbines are available for main steam conditions sub or supercritical and 600C with a single reheat to 620C. Add to that careful design and implementation of regenerative feedwater heating and you're left with a pretty efficient and low cost package using non-toxic fluids and materials. Adding any kind of exotic 'topping cycle' to that doesn't actually add a lot, certainly not enough to warrant the expense and complication.

I would like to see how a sodium binary cycle works out on paper, practically it's a nightmare that one should run away from, but as an academic exercise it might be interesting.

If one is in the market for a topping cycle for high temperature nuclear and adapted combined cycle configuration has some merit. In this arrangement the Brayton (gas turbine) provides the topping cycle.


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PostPosted: Feb 15, 2013 11:45 pm 
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Lindsay wrote:
Occxium wrote:
I know mercury wont be a popular working fluid for a nuclear power plant - and I can see a regulator's head explode at the suggestion - but I hadn't heard of the technology before today, and thought it might provide some interesting food for thought and fodder for discussion.
Welcome to the EFT forum.

I don't see mercury as having any legs based on toxicity issues as Cyril mentioned. As you say steam has overtaken mercury binary cycles fairly quickly. Modern steam turbines are available for main steam conditions sub or supercritical and 600C with a single reheat to 620C. Add to that careful design and implementation of regenerative feedwater heating and you're left with a pretty efficient and low cost package using non-toxic fluids and materials. Adding any kind of exotic 'topping cycle' to that doesn't actually add a lot, certainly not enough to warrant the expense and complication.

I would like to see how a sodium binary cycle works out on paper, practically it's a nightmare that one should run away from, but as an academic exercise it might be interesting.

If one is in the market for a topping cycle for high temperature nuclear and adapted combined cycle configuration has some merit. In this arrangement the Brayton (gas turbine) provides the topping cycle.


Thanks, I have been lurking for a long time.

I am not a fan of supercritical steam, mostly because it must be under such high pressure, over 3200 PSI. Id prefer lower pressure systems because its easier and cheaper to over design. Its difficult to imagine the massive pipes needed to hold back 3200 PSI, let alone add on 50% or more safety factor... Thats gotta be massive massive pipes. I liked the mercury technology, because it was low pressure, high temperature - seemed to fit into the theme of a lftr reactor.

I know I shouldnt but I like sodium for a secondary coolant. Sodium is just so slutty that any leakage or contamination of the system could quickly lead to big trouble, but I think sodium also has the potential to be a low pressure high temperature alternative. -- edit -- I think I was thinking more of sulfur when I wrote this, but sodium can react with water to produce sodium hydroxide, and spontaneously ignite in air.


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PostPosted: Feb 17, 2013 1:34 am 
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Sodium is quite nice till it catches fire. Ask people in Japan, UK, France or US who have burnt their fingers, and confidence, in sodium fires. It may be worthwhile looking up a low melting salt eutectic and liner materials to handle it.


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PostPosted: Feb 17, 2013 9:29 am 
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Lower pressure steam is certainly possible. Big saturated steam cycles and also superheat steam cycles are much lower pressure than supercritical water cycles.

But, interestingly, the supercritical systems have better economics, despite the higher pressure, due to a combination of improved efficiency and improved compactness. Yes, that high pressure first stage turbine needs a thick pressure vessel, but the pressure and temperature drops fast down the turbine stages. The next turbine, medium pressure turbine of a supercritical unit is very similar in pressures to the first turbine unit of a PWR.

The molten salt to supercritical water generator only needs thin tubes, with the molten coolant salt on the shell side. With modern compact printed circuit, diffusion bonded heat exchangers, even 500 atmospheres is no longer a problem, and the plates are very thin due to the small hydraulic size of the flow passages. If you make the thing out of Inconel 718, there's near zero corrosion. You wouldn't even have to inspect the internals anymore, as all potential failure modes are eliminated.


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PostPosted: Feb 19, 2013 1:39 am 
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Cyril R wrote:
Lower pressure steam is certainly possible. Big saturated steam cycles and also superheat steam cycles are much lower pressure than supercritical water cycles.

But, interestingly, the supercritical systems have better economics, despite the higher pressure, due to a combination of improved efficiency and improved compactness. Yes, that high pressure first stage turbine needs a thick pressure vessel, but the pressure and temperature drops fast down the turbine stages. The next turbine, medium pressure turbine of a supercritical unit is very similar in pressures to the first turbine unit of a PWR.

The molten salt to supercritical water generator only needs thin tubes, with the molten coolant salt on the shell side. With modern compact printed circuit, diffusion bonded heat exchangers, even 500 atmospheres is no longer a problem, and the plates are very thin due to the small hydraulic size of the flow passages. If you make the thing out of Inconel 718, there's near zero corrosion. You wouldn't even have to inspect the internals anymore, as all potential failure modes are eliminated.


Yes, Ive found out more about super critical steam. The BBC had a documentary, that took viewers on a tour of Britain's largest coal power plant. As I imagined the plumbing is massive. (the episode is on youtube: https://www.youtube.com/watch?v=w6T89Ec6w3U ) Interestingly they - in passing - admit that patents stifle innovation. Which I think is very true - as soon as Microsoft hired lawyers to protect its patents instead of engineers to design new stuff - they've been stuck in stagnation ever since.

I'll have to read up on the bonded heat exchangers - I havent heard of them before. I think of the heat exchangers as tube-in-tube machines probably from the 1950's or 60's.


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PostPosted: Feb 19, 2013 4:01 am 
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The plumbing in coal plants is large due to the combustion gas having a low energy density. A molten salt to supercritical water generator in a diffusion bonded heat exchanger is very compact.

But massive plumbing is something we can't avoid if we want to generate gigawatts of power.

It's true that patents can stifle innovation. But without patents you also have a negative situation. One of secrecy and lack of supply chain efficiency. Patents can protect the IP so that you can work together with suppliers and let them think along with your process.

Here's one company that builds compact heat exchangers:

http://www.heatric.com/

This should allow about 10x more compact supercritical water generators compared to PWR steam generators, for the same electric rating.

The most recent innovations have been in airfoil-shapes replacing the zig-zag corrugations, resulting in about 20x lower pressure drop.


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PostPosted: Feb 21, 2013 6:29 am 
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Occxium wrote:
Thanks, I have been lurking for a long time.

I am not a fan of supercritical steam, ...

Neither am I, the main driver of thermodynamic performance is temperature and good isentropic efficiency, so a subcritical STG with eight stages of feedwater heating and steam conditions of 165 bara, 600/620C is not far behind the supercritical version running 250 bara and the same steam temperatures.


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PostPosted: Feb 21, 2013 7:03 am 
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Lindsay wrote:
Occxium wrote:
Thanks, I have been lurking for a long time.

I am not a fan of supercritical steam, ...

Neither am I, the main driver of thermodynamic performance is temperature and good isentropic efficiency, so a subcritical STG with eight stages of feedwater heating and steam conditions of 165 bara, 600/620C is not far behind the supercritical version running 250 bara and the same steam temperatures.


The reality is that most modern larger powerplants use supercritical steam. Reheat complicates the cycle for not much gain, to make it worth it you need a high reheat temp which results in other problems. A recent Sandia study on molten salt steam generators concluded reheat wasn't worth it and actually reduced the efficiency. Supercritical allows lower temperatures. Machines with 550 C peak turbine inlet temp have a better and more extensive track record than 620 C superheat machines.

Supercritical cycle with only regeneration (FW heating) is more efficient than subcritical cycle with regeneration and reheat. The former seems much easier to design for, considering the lack of technology base for molten salt - steam generators, you want to keep things as simple as possible. A single once through supercritical water generator looks very attractive, especially with a compact PCHE, pressure isn't such a big deal anymore. And I think it was you who also said the difference between 150 or 250 bar isn't such a big deal.


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PostPosted: Feb 21, 2013 7:11 am 
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Cyril R wrote:
...A recent Sandia study on molten salt steam generators concluded reheat wasn't worth it and actually reduced the efficiency. Supercritical allows lower temperatures. Machines with 550 C peak turbine inlet temp have a better and more extensive track record than 620 C superheat machines.

Cyril do have a link for that Sandia work? I'd be interested to take a look at that.

For a PCHE steam generator how do you get past the salt freeze challenge?

For a once through supercritical without reheat how do you deal with the excess moisture that is created as the steam expands?


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PostPosted: Feb 21, 2013 8:08 am 
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Lindsay wrote:
Cyril R wrote:
...A recent Sandia study on molten salt steam generators concluded reheat wasn't worth it and actually reduced the efficiency. Supercritical allows lower temperatures. Machines with 550 C peak turbine inlet temp have a better and more extensive track record than 620 C superheat machines.

Cyril do have a link for that Sandia work? I'd be interested to take a look at that.

For a PCHE steam generator how do you get past the salt freeze challenge?

For a once through supercritical without reheat how do you deal with the excess moisture that is created as the steam expands?


Here's the link:

http://prod.sandia.gov/techlib/access-c ... 106978.pdf

I'd be very interested in your opinion about this Sandia work.

The freeze challenge I would preferably solve the Sandia way: with a molten NaNO3-KNO3 eutectic loop. This has a melting point of 221 degrees Celsius, so if your feedwater temperature is higher than that you never get freezing of the salt. Ideally the feedwater temperature would be much above 221 degrees Celsius so that a single feedwater train failure doesn't freeze the salt (or cause immediate reactor or system trip). I've heard an optimal feedwater heating temperature is above 250 degrees Celsius for supercritical cycles anyway, so probably a conventional arrangement can be used. The regenerative cycle would use only steam bled off from the turbine for feedwater heating, avoiding salts there.

As added advantages, the nitrate loop mops up tritium and better matches steam (doesn't even react with it in case of leaks).

The moisture problem we discussed recently, I think we concluded based on what you said that you either need a downstream moisture seperator-reheater or an upstream reheat, correct? But at least with downstream moisture-seperator-reheaters we don't have to use salts, as compared to an upstream reheater...


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PostPosted: Feb 21, 2013 5:22 pm 
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Hmm, I had a quick skim of that Sandia report, there is some weird stuff in there, it will take me a little time to rank and analyse the weird observations. Without doing the work it's hard to explain, but there are some inappropriate parameter combinations in there, Figure 7 for example, a single expansion from 250 bara 550C will cause excessive wetness in the back end of the turbine, I can say that with confidence without doing the analysis. Figure 9 the reheat pressure is too low for 600C steam which makes the exhaust dry saturated or slightly superheated and carrying too much energy. In these cases the authors do not seem to have applied an acceptable range of turbine exhaust steam dryness to constrain or guide their parameter selection/analysis, which is weird, that's a rookie error.

Nitrate salts, yes that would work well, especially when used in conjunction with regenerative feedwater heating to say 250C or as close as you can get. For MS applications the nice thing about feedwater heating is not only does it improve cycle efficiency and make your condenser smaller, it gives you additional margin on salt freeze for the steam generator.

Quote:
The moisture problem we discussed recently, I think we concluded based on what you said that you either need a downstream moisture seperator-reheater or an upstream reheat, correct? But at least with downstream moisture-seperator-reheaters we don't have to use salts, as compared to an upstream reheater...

That will work, one certainly has to do something about the build up of water droplets that would otherwise occur or they will simply eat the turbine from the inside out. I tend to start with salt driven reheaters as they are the simplest things out there. Because there is no boiling water, salt freeze is so much easier to deal with. Depending on the steam cycle design the cold reheat steam temperature can easily be above the salt freeze temp even with fluoride salts. That said, you can definitely take live steam and use that to provide reheat after a moisture separator. That comes at the cost of having to build a bigger steam generator and increased feed pump power.


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PostPosted: Mar 01, 2013 8:28 pm 
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The more I read about (non nuclear) pressure vessels the less concerned I become about their safety. (having nuclear materials under pressure still strikes me as a bad idea)

For example I was recently reading about gas bottles for CNG cars. (ISO 11439) they routinely operate at pressures above 200 bar and must not rupture until they exceed 225% of their operational pressure... Amazing! a 100 L (volume measured at 1 atm pressure) can weigh as little as 40Kg. (according to ISO 11439 - type 4 is the light weight version)

Iam now convinced that the efficiencies promised by exotic gas cycles aren't necessary to make LFTR fly. (metaphorically) I think my objections to super critical steam where mostly formed around 2 prejudges, firstly presentism - this is how things are presently done, so therefore must be improved or 'fixed' and secondly just being oblivious to the mechanical world around me, not realizing how much pressure we use in systems and containers all around us.

(I also looked into common hydraulic systems - like those that I operated as a teen on a farm tractor - and found they also run into the thousands of PSI)


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PostPosted: Mar 02, 2013 4:33 am 
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It's not the current energy conversion technology that needs a radical change. Steam turbine technology has advanced greatly. In many ways, pressure is easier to deal with than higher temperature. Higher pressure means thicker materials, but you can use the same materials. Higher temperature means you need new materials - a huge development problem. Helium cycles require higher temperatures to get to the same efficiency as steam, so I don't think there's a good tradeoff here despite the lower pressure. And even with helium, higher pressure still means higher efficiency.

In the light of all the advanced conversion technology we have - advanced Rankine and Brayton cycles - it is very strange that we still use such a primitive energy source to power these machines. We're using advanced conversion technology, but fuelling them with long dead plant, the same stuff that fuelled James Watt's steam engine.

There's our problem. It's not the conversion technology that needs alternatives, it's the fuel it runs on.


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