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PostPosted: Sep 20, 2012 6:58 am 
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KitemanSA wrote:
All good, but you will have to be near water while with a Brayton you don't. Oh, and if near sea water, no desal.

Just a thought.


One bird in the hand... better than 10 in the sky.

The steam cycle is a bird in the hand. In terms of locations, most new generation will have to be concentrated in existing sites, which already have cooling water. Dry cooling is also possible with Rankine cycles, the higher the steam temperature the less it hurts the efficiency. Certainly the efficiency of a dry cooled superheated steam cycle is much better than your 25% Braytons.

It is not easy to design a salt-air heater for a Brayton cycle, even if it's an open cycle, that only solves some of your compressor issues. The HX issues are still there, and are in fact magnified for the open Brayton cycle. In particular, there are two serious problems. First, the heat transfer figures of merit between molten salt and air are so far apart that it is very difficult to engineer (thermal shock, startup/shutdown issues, etc.). Second, freezing is a very serious design issue with an open air Brayton, as you're putting in cold air - perhaps freezing in winter - on one side and have high freezing temp salt on the other. And lots of surface area and little flow passages that can get frozen easily. Simply micro-managing the freezing issues (local subcooling, powerup/powerdown, etc.) throughout the heat exchangers alone, will probably be an insurmountable problem on its own.


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PostPosted: Sep 20, 2012 9:36 am 
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Cyril R wrote:
KitemanSA wrote:
All good, but you will have to be near water while with a Brayton you don't. Oh, and if near sea water, no desal.

Just a thought.
.....
.....
It is not easy to design a salt-air heater for a Brayton cycle, even if it's an open cycle, that only solves some of your compressor issues. The HX issues are still there, and are in fact magnified for the open Brayton cycle. In particular, there are two serious problems. First, the heat transfer figures of merit between molten salt and air are so far apart that it is very difficult to engineer (thermal shock, startup/shutdown issues, etc.). Second, freezing is a very serious design issue with an open air Brayton, as you're putting in cold air - perhaps freezing in winter - on one side and have high freezing temp salt on the other. And lots of surface area and little flow passages that can get frozen easily. Simply micro-managing the freezing issues (local subcooling, powerup/powerdown, etc.) throughout the heat exchangers alone, will probably be an insurmountable problem on its own.
The new graphite foam fluid-gas HXs should help fix that problem.

And doesn't the compression stage heat up the gas before reaching the F-G HX?

Will a 25MW steam cycle TG fit in a Conex Box? LM2500s do. A modified one might.

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PostPosted: Sep 20, 2012 11:31 am 
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KitemanSA wrote:
The new graphite foam fluid-gas HXs should help fix that problem.


They are cool and it's interesting you bring them up (no one seems to talk about this important innovation in HXs). But will they solve the problems? It certainly helps to have a high surface area solid conductor so that the salt passage can be relatively thick. But it's still cold air on the other side of the tubing in between the graphite foam. Compared to a nitrate-steam HXs, which would be fed regenerative feedwater at temperatures above the nitrate salt melting point... and a fluoride salt - nitrate salt HX is relatively easy to design (simple liquid-liquid HX and roughly similar heat transfer figures of merit).

KitemanSA wrote:
And doesn't the compression stage heat up the gas before reaching the F-G HX?


Yes, but if start with ambient temperature, the final compression gas temperature is quite low. If you start with hot gas...

I have never heard of an operating high temperature compressor. Something to keep in mind - do these things exist at all?


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PostPosted: Sep 20, 2012 11:56 am 
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KitemanSA wrote:
And doesn't the compression stage heat up the gas before reaching the F-G HX?


Cyril R wrote:
Yes, but if start with ambient temperature, the final compression gas temperature is quite low. If you start with hot gas...

I have never heard of an operating high temperature compressor. Something to keep in mind - do these things exist at all?


Whaaa? The engine on a commercial jet sucks in air at -40 C and compresses it more than half way to the melting point of the turbine. The air coming out of that compressor is HOT. You want that... adding combustion heat to cold air makes a lot of entropy. A major issue in jet engine design is keeping down the temperatures during takeoff, when the inlet temperature can be 40C or more. If I understand correctly, the solution is to drop the compression ratio substantially.

If you use a heat exchanger instead of combustion, all the temperatures (and thus compression ratio and efficiency) have to scale down, but I would think the air compressor output temp would still be much higher than 0 C, even in (say) Finland in winter. Designing it so that the compressor outlet temp was higher than the salt melting point could be done, but might not be the most thermodynamically efficient design point.

-Iain


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PostPosted: Sep 20, 2012 3:31 pm 
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The discharge temperature from the compressor is a function of pressure ratio (PR), compressor efficiency and inlet temperature. Without referring to any references I would expect that temperature to be in the range of 230 - 330C for various reasons. For high PR machines like the LM6000 the compressor discharge temperature can be as high as 540C, for molten salt driven systems the optimal PR for best efficiency is much lower than 30:1, therefore it is closer to that 230 - 330C range.

A quick word on HX for air and gases, these are pretty big things even when dealing with compressed air, but for land based power generation I don't see that as a major problem, but it would be more convenient if they were smaller.


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PostPosted: Sep 20, 2012 4:36 pm 
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iain wrote:
KitemanSA wrote:
And doesn't the compression stage heat up the gas before reaching the F-G HX?


Cyril R wrote:
Yes, but if start with ambient temperature, the final compression gas temperature is quite low. If you start with hot gas...

I have never heard of an operating high temperature compressor. Something to keep in mind - do these things exist at all?


Whaaa? The engine on a commercial jet sucks in air at -40 C and compresses it more than half way to the melting point of the turbine. The air coming out of that compressor is HOT. You want that..


No you don't. You want to increase pressure with as little temperature increase as possible. That is, you want an efficient compressor, ideally just isentropic. The compressor's power is diverted from the main shaft, so adding more heat there is inefficient (you'd be taking already converted shaft power back into the heat engine). With an open Brayton cycle the air is also your heat sink. You don't want to heat it up at the front end of things. It is similar to having colder inlet gas temperature: output is increased.

But good point about the high compressor outlet temp of a jet engine. Looking it up finds references to up to 580 degrees Celsius. This is much higher than I thought. And I checked the Berkeley helium Brayton development, which has lower compressor operating temperatures, because of the intercooling-multiple stages, only 100-130 Celsius. Good. The issue then would be with the high temp salt to air HX. This would be a pretty serious R&D effort, seems even worse than the development needed to get reliable combustion chambers for combustion Braytons.


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PostPosted: Sep 21, 2012 6:08 pm 
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The ideal Pressure Ratio (PR) for best efficiency is linked to firing temperature to turbine inlet temperature (TIT). The higher the TIT, the higher the PR tends to be and the higher the compressor discharge temperature as a consequence. (Note this statement does not hold for systems using intercooled compressors).

For salt driven gas turbine-like machines the optimal PR will be low because the TIT is low relative to combustion turbines, so compressor discharge temperature tends to be lower. This also helps us join the following dots: gas fired CCGT's can be 60% efficient on a LHV basis, but we can't do that with any salt driven power conversion system until were are well above 900C. The reason is we don't have the higher temperatures achievable by combustion turbines (1350C and higher).

We know that we have MSR friendly materials for up to 704C, we know that getting to 900C at the heat engine is going to take much longer to get there and while 54% conversion efficiency is highly desirable, maybe that's best considered for the Evolution II MSR. If we can have a subcritical or supercritical steam system coupled to a reactor operating at 704C outlet temperature and a net efficiency of 47% or better, that is very appealing in terms of how that might fit with near term MSR development, especially if the core design is low cost. Please note however as already noted earlier on this thread, smaller machines operate at lower steam conditions and have lower internal or isentropic efficiency. To achieve 45%+ efficiency you need to be dealing with bigger machines, 300 MWe and larger, by my estimate.


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PostPosted: Sep 22, 2012 3:50 am 
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Lindsay, what do you think about the supercritical water turbine proposed by Burns and Roe? It is rather large (1600 MWe) but is very simple with no reheat, no superheat, just a simple once through operation. It gets 44.8% efficiency net, with just 500 degrees Celsius "steam" temperature.


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PostPosted: Sep 22, 2012 6:37 am 
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Cyril R wrote:
Lindsay, what do you think about the supercritical water turbine proposed by Burns and Roe? It is rather large (1600 MWe) but is very simple with no reheat, no superheat, just a simple once through operation. It gets 44.8% efficiency net, with just 500 degrees Celsius "steam" temperature.

There is a reheater powered by main steam at 500C. There is a moisture separator where 120.57 kg/s of condensate is taken off (looks like 10.6% of turbine exhaust flow), which is followed by a two stage reheater that brings the steam temperature up to 363C from 188C. As for simplicity it is at a similar level of complexity to thermal reheat steam turbine, but working at lower temperatures.

It's a bit unusual, but it seems to work ok based on the efficiency achieved, I probably would have tried for higher pressure and temperature directly downstream of the reheater, but I've never looked at a low temperature SC cycle before, it is quite different to thermal SC where you have much higher temperatures available and a lot less moisture to deal with. 1600MWe is at the top end of the size range, but that seems to be the way things are going and have been going for along time, as soon as someone has the capability of building bigger units, they do, because they seem to work out cheaper in $/kW. Being an 1800 rpm machine it allows them to go for a large exhaust area without having to have too many LP turbines.

For an MSR driven STG, the temperatures would be higher and the turbine parameters (pressures and temperatures) would be exactly the same as those seen in a modern coal plant, very common and very well known, no moisture separators and the reheater would be salt driven, not steam driven. Whereas this SC steam turbine is quite special by comparison, but still well within the bounds of current designs and materials.


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PostPosted: Sep 22, 2012 7:28 am 
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But the reheater is placed between the turbine stages, isn't this called a moisture seperator-reheater (yes, the acronym is MSR :lol: )?

Do you mean that a higher temperature supercritical cycle wouldn't need such a MSR? That would change things.

I don't know if the reheater can be salt-driven. It might risk freezing. ORNL seemed very concerned about it, I think they went for steam heated reheat and had extra feedwater heating via booster pumps. But it seems ok for a nitrate salt loop.

The reason why the Burns and Roe steam cycle is unusual is because it is for a supercritical water reactor, and they wanted to keep the reactor simple, so a once through "boiler" to keep things simple in the reactor, but it was ok to put complexity downstream (the MSR).

Also, I noticed that superheat doesn't do much for a supercritical cycle efficiency. May not be worth the added complexity?


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PostPosted: Sep 22, 2012 4:37 pm 
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Cyril R wrote:
But the reheater is placed between the turbine stages, isn't this called a moisture seperator-reheater (yes, the acronym is MSR :lol: )?

Yes, but there is a reheating step in there, it's just arranged quite differently to a coal plant.

Cyril R wrote:
Do you mean that a higher temperature supercritical cycle wouldn't need such a MSR? That would change things.

Correct, if the steam temperatures are high enough as it goes into the turbine that it only become saturated in the last few stages of the LPT.

Cyril R wrote:
I don't know if the reheater can be salt-driven. It might risk freezing. ORNL seemed very concerned about it, I think they went for steam heated reheat and had extra feedwater heating via booster pumps. But it seems ok for a nitrate salt loop.

On closer inspection, I think that you'll find that the reheater is ok as it is dealing with superheated steam at a higher temperature than boiling water, also that reheater steam behaves more like a gas. The hard part is the steam generator especially in the zone where boiling takes place, because in a nucleate boiling regime the metal temp will be within a couple of degrees of the boiling temp of the water, which will normally freeze the salt on the outside of the cold metal tube. That's the most difficult part of the salt driven steam generator.

Cyril R wrote:
The reason why the Burns and Roe steam cycle is unusual is because it is for a supercritical water reactor, and they wanted to keep the reactor simple, so a once through "boiler" to keep things simple in the reactor, but it was ok to put complexity downstream (the MSR).

That's right, it is a unique problem, requiring a unique solution and it looks pretty good on paper, although I did spot a couple of apparent minor anomalies.


Cyril R wrote:
Also, I noticed that superheat doesn't do much for a supercritical cycle efficiency. May not be worth the added complexity?

I don't believe that's correct, temperature is usually everything, if we took the high isentropic efficiency that this machine probably has and cranked up the steam temperatures, I'm confident that you would see some significant improvement in cycle efficiency.


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PostPosted: Sep 23, 2012 7:05 am 
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Apart from what Sandia is doing, the development of supercritical CO2 turbines seems to be gaining traction, also for non-nuclear applications. NREL is cooperating with Dresser-Rand on a 10MW version:

http://www1.eere.energy.gov/solar/sunsh ... rbine.html

Toshiba is cooperating with the Shaw Group and NET Power:

http://ir.shawgrp.com/phoenix.zhtml?c=6 ... highlight=

http://www.netpowerllc.com/

I wonder when the established turbomachinery players (Alstom, GE, Siemens, etc.) get into this game. What bottlenecks can be expected when scaling up supercritical CO2 brayton cycles ? The heat exchangers ?


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PostPosted: Sep 25, 2012 12:26 am 
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That NREL/Dresser-Rand point is interesting, considering NREL/Dresser-Rand have worked with RamGen and their supersonic CO2 compressor (which is basically the front half of RamGen's rotary ramjet engine).


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PostPosted: Nov 21, 2012 9:38 am 
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ORNL 4541 talks about subcritical versus supercritical as well.

http://energyfromthorium.com/pdf/ORNL-4541.pdf

Table 5.2.

Net efficiency of supercritical: 44.5%
Net efficiency of subcritical: 41.1%

That's a big hit! Apparently, a big issue is the high power required to heat up the feedwater in the subcritical cycle, to prevent salts freezing.

Considering that, I guess that added nitrate loop looks great for a subcritical unit. Standard feedwater temperature could be used.


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PostPosted: Nov 21, 2012 10:22 am 
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The HX to the buffer salt would naturally be at a lower temp and using a lower melting temp salt - wouldn't that help?


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