Energy From Thorium Discussion Forum

It is currently Nov 22, 2018 5:56 am

All times are UTC - 6 hours [ DST ]




Post new topic Reply to topic  [ 78 posts ]  Go to page Previous  1, 2, 3, 4, 5, 6  Next
Author Message
PostPosted: Jun 19, 2009 12:20 am 
Offline

Joined: Jan 24, 2007 2:24 pm
Posts: 436
Location: Montreal, Quebec CANADA
fnord wrote:
I can't get over how much smaller the turbomachinery is for SCO2. What sort of capital savings will this enable, ceteris paribus?

How much further will you be able to push overall thermal efficiency?


A supercritical CO2 turbine cycle can achieve a very high thermal efficiency at medium turbine inlet temperatures of ~600C at a pressure of about 20Mpa (2900 psi) using a dual expiation set up with a HP section followed by an LP section (single spool).

The Muto and Kato paper: Gas Turbines for Nuclear Power Systems, reports that a thermal efficiency of 50% are possible with this design.

Capital savings (if any) are difficult to predict as high pressure and high RPM have their own cost penalties.


Top
 Profile  
 
PostPosted: Jun 19, 2009 1:41 am 
Offline

Joined: Dec 31, 2008 12:09 am
Posts: 236
Location: Berkeley, CA
David wrote:
There is a quote from Dostal's work that confused me a bit.

Quote:
The supercritical CO2 cycle at 550 C achieves 46% thermal efficiency which is the same as the helium Brayton cycle at 800 C (if all losses are taken into account)


Does anyone know what he meant by taking losses into account? The typical efficiency we see quoted for helium Brayton with molten salt cooled reactors is 48% if the salt peak temperature is 705 C and the peak helium at turbine inlet is 670 C. Was Dostal perhaps referring to Brayton cycles with less intercooling or are there "losses" that are being ignored when people quote Brayton efficiencies?


The quote on the helium Brayton cycle efficiency is inaccurate, because it applies to the efficiency of a single-expansion He Brayton cycle for modular helium reactors. LFTR and PB-AHTR reactors would use multiple reheat helium Brayton cycles. The cross-over in efficiency between SCO2 and multiple reheat helium Brayton cycles occurs around 650°C (multiple reheat Brayton being better above this). Thus LFTR and PB-AHTR are really in the sweet spot where either SCO2 or multiple reheat Brayton cycles would work with almost equal efficiency. This is great, since it means that there are two different options available, and better chance that at least one can be developed to successful commercial deployment.


Top
 Profile  
 
PostPosted: Jun 19, 2009 5:47 am 
Offline
User avatar

Joined: Nov 30, 2006 9:18 pm
Posts: 1938
Location: Montreal
What happens to the turbine when the temperature drops and the SCO2 starts condensing into liquid droplets ? ....rapid erosion & failure ?


Top
 Profile  
 
PostPosted: Jun 19, 2009 9:23 am 
Offline

Joined: Mar 07, 2007 11:02 am
Posts: 909
Location: Ottawa
Per Peterson wrote:
David wrote:
There is a quote from Dostal's work that confused me a bit.

Quote:
The supercritical CO2 cycle at 550 C achieves 46% thermal efficiency which is the same as the helium Brayton cycle at 800 C (if all losses are taken into account)


Does anyone know what he meant by taking losses into account? The typical efficiency we see quoted for helium Brayton with molten salt cooled reactors is 48% if the salt peak temperature is 705 C and the peak helium at turbine inlet is 670 C. Was Dostal perhaps referring to Brayton cycles with less intercooling or are there "losses" that are being ignored when people quote Brayton efficiencies?


The quote on the helium Brayton cycle efficiency is inaccurate, because it applies to the efficiency of a single-expansion He Brayton cycle for modular helium reactors. LFTR and PB-AHTR reactors would use multiple reheat helium Brayton cycles. The cross-over in efficiency between SCO2 and multiple reheat helium Brayton cycles occurs around 650°C (multiple reheat Brayton being better above this). Thus LFTR and PB-AHTR are really in the sweet spot where either SCO2 or multiple reheat Brayton cycles would work with almost equal efficiency. This is great, since it means that there are two different options available, and better chance that at least one can be developed to successful commercial deployment.



Thanks Per, I realize you are one of the main, (first?), proponent of multiple reheat Brayton cycles. Nice to see you so fair and open minded about the competition. It is indeed nice to have a choice of great designs.

As I've mentioned before, there are potential advantage to running LFTRs at a little lower peak temperature, mainly the possibility of using simple unclad stainless steels for the entire primary loop and heat exchangers. In those cases (peak 550 to 600), SCO2 is likely the clear choice.

David L.


Top
 Profile  
 
PostPosted: Jun 19, 2009 9:30 am 
Offline

Joined: Mar 07, 2007 11:02 am
Posts: 909
Location: Ottawa
jaro wrote:
What happens to the turbine when the temperature drops and the SCO2 starts condensing into liquid droplets ? ....rapid erosion & failure ?


The term "supercritical CO2" is actually a bit of a misnomer. In this turbine design the CO2 never actually passes into the supercritical stage. At the compression stage at low temperature it gets very close to supercritical so the gas gets very dense but it never actually becomes liquid or even supercritical. It is mainly the big savings on compression power that give the great overall efficiencies. (If memory serves anyhow, its been many months since I read Dostal's work).

Maybe you were talking about one time only events were the CO2 might condense by mistake?

David L.


Top
 Profile  
 
PostPosted: Jun 19, 2009 10:48 am 
Offline
User avatar

Joined: Nov 30, 2006 9:18 pm
Posts: 1938
Location: Montreal
David wrote:
Maybe you were talking about one time only events were the CO2 might condense by mistake?

Yes.


Top
 Profile  
 
PostPosted: Jun 19, 2009 11:16 am 
Offline

Joined: Dec 31, 2008 12:09 am
Posts: 236
Location: Berkeley, CA
jaro wrote:
What happens to the turbine when the temperature drops and the SCO2 starts condensing into liquid droplets ? ....rapid erosion & failure ?



One of the potential disadvantages of the SCO2 cycle is that its operation is sensitive to the heat rejection temperature. I'm pretty sure that condensation is not a problem, but one can depart pretty rapidly from optimal efficiency if one's heat sink temperature moves from the optimal.

Also, SCO2 operates with a smaller gas temperature change for heat rejection, so it is not as suitable for thermal desalination and dry cooling applications as multiple reheat helium Brayton cycles.

I think that both cycles can and should be developed through to commercialization, since they will have different advantages and disadvantages. In the very long term, when natural gas supplies have been fully depleted, we will no longer have an economical source of helium and SCO2 will likely be the most practical power conversion fluid. But in the intermediate term it makes sense to develop both.


Top
 Profile  
 
PostPosted: Jun 20, 2009 3:58 pm 
Offline

Joined: Jul 28, 2008 5:01 am
Posts: 461
Location: Teesside, UK
Kirk Sorensen wrote:
........There's nothing about SCO2 that precludes multiple reheat.

True, but Dostal doesn't think it's worth the trouble (page 176-179 of the thesis, discussing the case of a lead cooled reactor, which he states earlier (p156) is essentially identical to any liquid metal or molten salt cooled system for this analysis.) For a 550°C top temperature, the thermal efficiency is 45% for his best non-reheated cycle, and about 46.5% for a single reheat. Further reheats make almost no difference. The extra electricity scarcely pays for the heat exchanger, and won't pay for the extra turbine and piping. In contrast, at the same temperature your java sim for He gives 29.5% for a non reheated regenerated cycle, 34.2% for 1 reheat, 36.6 for 2.

I would tend to prefer to develop S-CO2 first, because it works so well with likely first generation LFTRs operating at modest temperatures. Higher temps come later. I'm further biased because direct sea water cooling is permitted here (UK for sure, and much of the rest of Europe too, I think), so there is reliably cold cooling to keep S-CO2 systems going. In the southern US, lack of cooling could shut it down just when it is most needed, so air cooled He cycle could be better, even if it is a bit less efficient in early systems.

Luke


Top
 Profile  
 
PostPosted: Jun 20, 2009 4:22 pm 
Offline
User avatar

Joined: Nov 30, 2006 3:30 pm
Posts: 3871
Location: Alabama
Those efficiency differences have more to do with the pressure ratio than anything else.


Top
 Profile  
 
PostPosted: Jun 20, 2009 6:32 pm 
Offline

Joined: Jul 28, 2008 5:01 am
Posts: 461
Location: Teesside, UK
Kirk Sorensen wrote:
Those efficiency differences have more to do with the pressure ratio than anything else.

I'm probably using the sim incorrectly then. I set it to 'realistic triple reheat', adjusted the turbine efficiency up to 90% to match Dostal's assumption, and temps to 822/309 K, then changed the pressure ratio to find the maximum efficiency. Then changed the turbomachinery configuration to two machines, then one, re-optimising the pressure ratio in each case. The point I was trying to make is that adding reheat seems to help He cycles much more than S-CO2 ones. The first reheat gives nearly 16% more output from the same reactor for He, but only 3% extra for S-CO2

Luke


Top
 Profile  
 
PostPosted: Jun 21, 2009 6:51 am 
Offline

Joined: Dec 05, 2008 8:50 am
Posts: 335
A question I posted in the blog, too : is it CO2 as coolant compatible with the temps range typical of a LFTR, > 700 °C? I remember one of the reason for the choice of helium (instead CO2 used in older Magnox/AGR) as coolant in HTR was the compatibility of helium vs CO2 with the operating temps of HTGR (in that case, 850 °C and even higer)


Top
 Profile  
 
PostPosted: Jun 21, 2009 7:19 am 
Offline
User avatar

Joined: Nov 30, 2006 3:30 pm
Posts: 3871
Location: Alabama
Alex P wrote:
A question I posted in the blog, too : is it CO2 as coolant compatible with the temps range typical of a LFTR, > 700 °C? I remember one of the reason for the choice of helium (instead CO2 used in older Magnox/AGR) as coolant in HTR was the compatibility of helium vs CO2 with the operating temps of HTGR (in that case, 850 °C and even higer)


I think it will be a different situation since the gaseous coolant in a gas-cooled reactor has to go through the core and be compatible with the neutronic flux and materials found there, whereas in LFTR the gas coolant only circulates through the turbomachinery and the gas heaters. In the gas heaters there will be coolant salt on one side and gas on the other and no neutron flux or gamma radiation.


Top
 Profile  
 
PostPosted: Jun 21, 2009 7:35 am 
Offline

Joined: Dec 05, 2008 8:50 am
Posts: 335
Kirk Sorensen wrote:
Alex P wrote:
A question I posted in the blog, too : is it CO2 as coolant compatible with the temps range typical of a LFTR, > 700 °C? I remember one of the reason for the choice of helium (instead CO2 used in older Magnox/AGR) as coolant in HTR was the compatibility of helium vs CO2 with the operating temps of HTGR (in that case, 850 °C and even higer)


I think it will be a different situation since the gaseous coolant in a gas-cooled reactor has to go through the core and be compatible with the neutronic flux and materials found there, whereas in LFTR the gas coolant only circulates through the turbomachinery and the gas heaters. In the gas heaters there will be coolant salt on one side and gas on the other and no neutron flux or gamma radiation.


Yes, I knew it, but besides this I think an issue with CO2 is it tends to decompose/dissociate with higher temps (unlike helium, instead), I meant, is it a problem for the temps range of LFTRs, too?


Top
 Profile  
 
PostPosted: Jun 21, 2009 8:22 am 
Offline
User avatar

Joined: Nov 30, 2006 3:30 pm
Posts: 3871
Location: Alabama
This applet seems to indicate that CO2 dissociation is not an issue below 1500K:

Carbon Dioxide Dissociation Applet


Top
 Profile  
 
PostPosted: Jun 21, 2009 9:51 pm 
Offline

Joined: Dec 31, 2008 12:09 am
Posts: 236
Location: Berkeley, CA
jagdish wrote:
I have always wondered why only CO2, N2 and He are considered as gas coolants. What's wrong with argon and neon?


Helium is considered attractive because it has a very high thermal conductivity. Neon is very expensive. Argon's properties are no better than nitrogen, which is cheaper and has a very large turbomachinery experience base (from air open Brayton power cycle systems). For closed cycles that are proposed to make use of air-based turbine experience, some helium is commonly mixed with nitrogen to increase the thermal conductivity (important in affecting the heat transfer and pressure drop in heat exchangers). CO2 is special because its critical pressure and temperature are relatively low and a supercritical cycle is possible.

These are the basic reasons that nitrogen (air), helium and CO2 end up being the primary candidates for closed gas power cycles.


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 78 posts ]  Go to page Previous  1, 2, 3, 4, 5, 6  Next

All times are UTC - 6 hours [ DST ]


Who is online

Users browsing this forum: No registered users and 1 guest


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
cron
Powered by phpBB® Forum Software © phpBB Group