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7/22/10 9:55 PMEnergy From Thorium Discussion Forum :: View topic - Supercritical CO2 Closed-Cycle Gas Turbines

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Author: Kirk Sorensen [ Jul 18, 2008 10:52 am ]

Post subject: Supercritical CO2 Closed-Cycle Gas Turbines

I didn't see a thread on this subject, so I thought I'd start one.

The supercritical CO2 cycle is intriguing. The basic advantage of the approach is that itis easiest to compress a gas when it's dense, and CO2 near the critical point has somereally nice advantages. I've been reading more and more on this cycle recently andwondering if it is advantageous over an ideal-gas Brayton cycle using helium.

Much of the recent work in this field was done by Dr. Vaclav Dostal, who was a studentat MIT but is now a professor at the Czech Technical University. Here is a link to hisdissertation:

http://dspace.mit.edu/handle/1721.1/17746

Author: Cyril R [ Jul 18, 2008 1:06 pm ]

Post subject: Re: Supercritical CO2 Closed-Cycle Gas Turbines

I've linked to that study a few days ago in the thread about effects of lowering peaksalt temp.

Benefits are obvious:

- in the right supercritical regime, higher efficiency with lower temps- more compact than helium cycles - can be more simplified than helium cycles- CO2 is much less prone to creeping and leaking than helium, it's easier to containwhich allows lower cost simplified equipment (bearings, valves, connectors etc)- CO2 is abundant and easy to obtain (not that I think helium shortage will occur - theamounts required are relatively low).

Author: David [ Jul 22, 2008 4:50 pm ]


Kirk,

http://www.energyfromthorium.com/forum/

http://www.energyfromthorium.com/forum/viewtopic.php?f=17&t=834

http://people.fs.cvut.cz/?criteria=12107

http://dspace.mit.edu/handle/1721.1/17746



Good idea for a separate thread as this topic has popped up numerous times. I am notsure if there is a free way to download the link you posted to Dostal's Ph.d thesis but Ifound another link to Dostal's work through the Wiki entry for Supercritical CO2 (firstlink).

It really does sound like a promising route. It needs much higher pressure and CO2 canbe a little corrosive but the savings in size, cost and complexity seem huge over theBrayton cycle. It is sometimes pointed out that the helium Brayton is further along indevelopment due to the South African work on Pebble Beds but I wonder in general ifthere might be even less R&D money that would need to be spent to develop aproduction turbine (due to simplicity of design and lower temp for blade material).

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 sameas 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 efficiencywe see quoted for helium Brayton with molten salt cooled reactors is 48% if the saltpeak temperature is 705 C and the peak helium at turbine inlet is 670 C. Was Dostalperhaps referring to Brayton cycles with less intercooling or are there "losses" that arebeing ignored when people quote Brayton efficiencies?

I'll try to read more of Dostal's work but does anyone know off hand if the 550 C inlettemperature is a real lower limit on inlet temperature? I realize the phase transitioncomes into play but I wonder if the cycle works a little lower, say for a 500 C inlettemp. Even if this meant a drop to 40% (from 46% at 550 C) it might still be veryattractive. I have been looking into the prospects of designs with alternate carrier saltssuch as NaF-BeF2 with much lower melting points than Flibe (2LiF-BeF2). The bigadvantage being the ability to use common stainless steels throughout the primarycircuit, including in core (instead of Hastelloy N which is rather expensive and doesn'thold up well to a high neutron flux).

P.S. I just skimmed through the Wiki Dostal link and it does seem like 500 C inlet oreven lower is possible with supercritical CO2.

Here is a link to the thread I started discussing the advantages of slightly lowering theoperating temperature of a molten salt reactor.


Author: Kirk Sorensen [ Jul 22, 2008 11:52 pm ]





I got in touch with Dostal today and he says they're looking at building a 500 kWsupercritical CO2 rig in the Czech Republic.



I read or skimmed through much of Dostal's work. He includes very useful summarysections to each chapter which is of much help. One thing that can be confusing is thefact that it is almost always thermal efficiency is being discussed, whereas overall cycleefficiency is the value of importance. A good example on that is his discussion of theSouth African pebble bed work, their thermal efficiency is about 51% due to a highinlet temp of 900 C. However, when you add in all the losses the overall cycle dropsdown to 41 to 42% (see page 255 of the Wiki link). It is quite hard to find reference tooverall cycle efficiency in Dostal's work.

He also addresses the multiple stage Brayton cycle that Per Peterson, Charles Forsbergand others are promoting for molten salt "cooled" designs. Dostal's opinion is that evenwith 3 stages of reheat and intercooling (and 3 extra compressors and turbines) thatsupercritical CO2 still is a better efficiency for the 500 to 700 C inlet range. He alsocontends that the incremental improvement in efficiency for each extra stage onlywould warrant adding 1 extra stage if the costs are properly included. He claims this iswhy the South African work only has a single extra stage. See Chapter 12 starting onpage 254 for all the details, it is quite a compelling argument.

Again though, since Dostal rarely gives "overall cycle" efficiency it can be a bit hard tocompare since he typically quotes the thermal efficiency when comparing heliumBrayton to the supercritical CO2. About the only example I found was page 103-104where a simplified version gave a thermal efficiency of 41.5% at 550 C but a "net"efficiency of ~37%. A 4.5% drop is pretty big. For comparison, in the multistage heliumBrayton work the thermal efficiency at 600 C was 45.8% with a net efficiency of 44%(less than half the "drop" and why so much less a drop than the South African work?).Thus one has to be careful in going by thermal efficiency comparisons between heliumand CO2.

Dostal's best case for 550 (without spending too much money on reheaters etc) was45.3% thermal efficiency. If it has the same drop of 4.5% as the other more simplifiedCO2 case, then we are looking at a net efficiency of about 40.8%. I wish he wouldhave mentioned the overall cycle efficiency much more often which perhaps indicatesthe real losses are a bit of an unknown.





Remember efficiency isn't everything in a Brayton cycle as well....I would contend thatnet work is nearly as important a metric.

Author: USPWR_SRO [ Jul 24, 2008 11:59 am ]


I look at Reactor thermal power vs net MWe to the grid.

Example:

San Onofre Unit 2Reactor = 3438 MW thermalMain Generator output = 1180 MWePlant loads = ~50MWeMW electrical to grid = 1130 MWe

Overall efficiency = 1130 / 3438 * 100 = 32.9%

Even a jump to 40% efficiency would be an enormous benefit.

Author: jaro [ Jul 24, 2008 5:40 pm ]


USPWR_RO wrote:Even a jump to 40% efficiency would be an enormous benefit.

....and so would productive use of what is currently "waste heat."



I also found a report from MIT regarding how to integrate the supercritical CO2 cyclewith a reactor in an indirect cycle (with reactor cooled by gas, lead or molten salt).There are lots of choices of how to put together a Power Conversion Unit (PCU). Theywere looking to a horizontal arrangement with everything except the generator within asingle pressure vessel maintained at the lowest pressure of the cycle 8 Mpa (peak is 20to 25). A single vessel for just 250 MWe is almost as big as a PWR vessel but only halfthe pressure or so. Much smaller and simpler than the massive "steam island" currentlyneeded but still some big pieces of hardware needed. The CO2 turbines andcompressors are tiny but it is the heat exchangers (precoolers, recuperators etc) thattake up all the space.



By the way, does anyone know how CO2 would interact with our molten salts? Not thatwe`d probably ever want to do away with the intermediate loop but good to know whatthe behavior might be. I think I recall reading something about how it would interactbut can`t recall what was said.

Author: Per Peterson [ Jan 02, 2009 8:19 pm ]

Post subject: Re: Closed-Cycle Gas Turbine Development

As Kirk notes, the largest advantage of closed gas cycles for liquid fluoride and otherliquid salt systems is that it becomes much easier to prevent freezing and control overcooling transients.

The big issue for selecting the best gas for a closed gas cycle is the compressor specificspeed. The threshold for getting an axial compressor to operate near its optimalperformance and stability limits, for a 3000 to 3600 rpm speed occurs when the massflow rate of gas corresponds to a thermal power of around 900 MWth, where heliumbecomes the most attractive gas (rather than nitrogen at lower power levels). This iswhy PBMR (400-500 MWth) has a gear box and GT-MHR (600 MWth) uses frequencyinversion.

Even for lower power levels where nitrogen is the best fluid, one still adds some heliumto increase the thermal conductivity of the gas and reduce the size of the heatexchangers. This is why Areva has picked an indirect cycle with nitrogen and helium asthe working fluid.

Supercritical CO2 cycles look good in the temperature range that sodium fast reactorsproduce, but closed gas Brayton cycles (likely helium) look best at the highertemperatures delivered by the LFTR/AHTR/LIFE. Helium scarcity may become a bigissue once we've used up most of our natural gas, but we'll have economical helium fora longer period of time if we can develop nuclear energy more rapidly to displace theuse of natural gas for electricity, process heat, and hydrogen production.

For tritium control, the big issue is the fact that tritium diffuses rapidly through metalsat high temperature, and thus it goes rapidly through the IHX to the intermediate salt,and through steam generator tubes into water. But as soon as one goes to closed gascycles for power conversion, the entire gas cycle pressure boundary operates at lowtemperature (it is in contact with the low temperature gas coming from thecompressors so that inexpensive materials can be used to contain the pressure), andthe only interface with water is at the precooler and intercoolers where temperaturesand tritium diffusion are very low. One must still recover tritium, but it's much easierbecause one can tolerate higher partial pressures in the primary and intermediatecoolants without having to worry that it will be driven into water, where it is expensiveto get out (e.g., CANDU's).



On the other hand, NGNP is now looking seriously at steam generation, mainly becauseit turns out that co-generation of electricity and process steam is what thepetrochemical industry wants most in the near term. This is ok, since PBMR willdemonstrate helium Brayton cycle technology earlier anyhow. In our 900 MWth PB-AHTR design work we have adopted the PBMR/Mitsubishi turbomachinery design withvery small modifications (essentially ganging 3 power conversion trains together to get3 expansion stages with reheat and 6 compression stages with intercooling, givingapproximately 46% power conversion efficiency with a core inlet temperature of 600°Cand outlet temperature of 704°C.

A more detailed design of this 410 MWe power conversion system design was developedby our NE 170 senior design class project,

Attachment:

PB-AHTR_Design_Isometric.jpg [ 92.53 KiB | Viewed 208 times ]

They also developed a preliminary physical arrangement and building structural designthat includes seismic base isolation, aircraft crash mitigation, HVAC zoning design,beryllium control, and radiation shielding analysis for the pebble transfer and storagesystems. Most of this can of course be adapted directly for the LFTR as well.

-Per

Author: atomicrod [ Jan 03, 2009 3:55 am ]


Per Peterson wrote:As Kirk notes, the largest advantage of closed gas cycles for liquid fluoride and other liquid saltsystems is that it becomes much easier to prevent freezing and control over cooling transients.

The big issue for selecting the best gas for a closed gas cycle is the compressor specific speed. The

http://www.energyfromthorium.com/forum/download/file.php?id=270&mode=view



threshold for getting an axial compressor to operate near its optimal performance and stability limits,for a 3000 to 3600 rpm speed occurs when the mass flow rate of gas corresponds to a thermal powerof around 900 MWth, where helium becomes the most attractive gas (rather than nitrogen at lowerpower levels). This is why PBMR (400-500 MWth) has a gear box and GT-MHR (600 MWth) usesfrequency inversion.

Even for lower power levels where nitrogen is the best fluid, one still adds some helium to increasethe thermal conductivity of the gas and reduce the size of the heat exchangers. This is why Areva haspicked an indirect cycle with nitrogen and helium as the working fluid.

Per - thank you for the detailed post. Like Kirk, I would like to be in your design class; it sounds likeyou and your fellow students have done some interesting and useful work.

I have approached closed cycle nuclear gas turbine design from a different angle that has led me tosignificantly different choices. Not claiming my design choices are better, just that they are differentbecause the entering arguments were different. (I am looking for well developed equipment, low initialcapital costs, reliability, power output sized to compete with diesel engines and combustion gasturbines, and modest initial temperatures and pressures. I also plan to minimize the number ofdifferent heat exchangers; simple cycle combustion gas turbines have achieved a great deal ofcommercial success without using recuperation, intercooling, and reheat.)

Unlike aeroderivative combustion gas turbines, nuclear gas turbines are not limited to direct couplingbetween the compressor and turbine and have no design advantage for low cross-sectional area orlow weight. (People designing aircraft engines that hang under the wings, however, take those twocriteria rather seriously.) Tossing out those evaluation criteria along with the much lower maximumtemperature of the system - compared to modern aeroderivative gas turbines - led me to determinethat centrifugal compressors with electric motors would provide a great deal of operational flexibility.

Since I made that selection, I determined that the turbine and generator arrangement best suited formy goals is an integrated turboexpander. (For more information on turboexpanders, here is anexample from GE.http://www.geoilandgas.com/businesses/ge_oilandgas/en/prod_serv/prod/turboexpanders/en/index.htm

With the Adams Engine current conceptual design, the number of compressors can be variedindependently of the number of turboexpanders. The compressor speeds can be varied to aid in powercontrol, which is obtained by controlling the mass flow rate through the system. The only heatexchangers in the system are the reactor itself and a heat sink. (There is consideration for splittingthe heat sink into two separate units, one that provides high temperature discharge appropriate for asteam bottoming cycle and one that is more appropriate for either direct heat discharge or lowtemperature heat recovery for something like desalination.)

Again, no criticism is implied or intended for any other design; I just wanted to share a bit about whyI have made different choices than others in selecting our working fluid, our compressors and ourturbines.

http://www.geoilandgas.com/businesses/ge_oilandgas/en/prod_serv/prod/turboexpanders/en/index.htm



Author: Per Peterson [ Jan 04, 2009 12:31 am ]


Rod,

You are correct, for lower gas flow rates and power levels centrifugal compressors havethe best performance. Axial compressors become optimal at higher gas flow rates andpower levels, so the best selection will depend upon the desired power output of theplant. One issue for turbo-generators is over-speed control if the generator load is lostand there is no place to dump the power. This was one of the issues that caused PBMRto drop their earlier vertical turbomachinery configuration (two turbo-compressors andone turbo-expander) and go to a Mitsubishi-designed horizontal in-line configurationwith the turbine, compressors and generators integrated on a single shaft.

-Per

Author: Kirk Sorensen [ Jun 18, 2009 9:33 pm ]


I just posted on the blog about a paper I attended today at ANS09 on SCO2:

Supercritical CO2 is dense like water

Author: DV82XL [ Jun 18, 2009 10:41 pm ]


Well I'm sold, CO2 is obviously the working fluid of choice, Smaller turbine diametersmean potentially much higher max RPM as well which is also an efficiency multiplier.

Author: Alex Goodwin [ Jun 18, 2009 11:16 pm ]


I can't get over how much smaller the turbomachinery is for SCO2. What sort of capitalsavings will this enable, ceteris paribus?

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

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Author: DV82XL [ Jun 19, 2009 12:20 am ]


fnord wrote:I can't get over how much smaller the turbomachinery is for SCO2. What sort of capitalsavings 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 mediumturbine inlet temperatures of ~600C at a pressure of about 20Mpa (2900 psi) using adual 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 athermal efficiency of 50% are possible with this design.

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

Author: Per Peterson [ Jun 19, 2009 1:41 am ]


David wrote:There is a quote from Dostal's work that confused me a bit.







The quote on the helium Brayton cycle efficiency is inaccurate, because it applies to theefficiency of a single-expansion He Brayton cycle for modular helium reactors. LFTR andPB-AHTR reactors would use multiple reheat helium Brayton cycles. The cross-over inefficiency 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 reallyin the sweet spot where either SCO2 or multiple reheat Brayton cycles would work withalmost equal efficiency. This is great, since it means that there are two differentoptions available, and better chance that at least one can be developed to successfulcommercial deployment.

Author: jaro [ Jun 19, 2009 5:47 am ]


What happens to the turbine when the temperature drops and the SCO2 startscondensing into liquid droplets ? ....rapid erosion & failure ?

Author: David [ Jun 19, 2009 9:23 am ]


Per Peterson wrote:David wrote:There is a quote from Dostal's work that confused me a bit.



The quote on the helium Brayton cycle efficiency is inaccurate, because it applies to theefficiency of a single-expansion He Brayton cycle for modular helium reactors. LFTR andPB-AHTR reactors would use multiple reheat helium Brayton cycles. The cross-over inefficiency 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 reallyin the sweet spot where either SCO2 or multiple reheat Brayton cycles would work withalmost equal efficiency. This is great, since it means that there are two differentoptions 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 reheatBrayton cycles. Nice to see you so fair and open minded about the competition. It isindeed nice to have a choice of great designs.

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

David L.

Author: David [ Jun 19, 2009 9:30 am ]


jaro wrote:What happens to the turbine when the temperature drops and the SCO2 startscondensing into liquid droplets ? ....rapid erosion & failure ?

The term "supercritical CO2" is actually a bit of a misnomer. In this turbine design theCO2 never actually passes into the supercritical stage. At the compression stage at lowtemperature it gets very close to supercritical so the gas gets very dense but it neveractually becomes liquid or even supercritical. It is mainly the big savings oncompression 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 bymistake?

David L.

Author: jaro [ Jun 19, 2009 10:48 am ]


David wrote:Maybe you were talking about one time only events were the CO2 might condense bymistake?

Yes.



Author: Per Peterson [ Jun 19, 2009 11:16 am ]


jaro wrote:What happens to the turbine when the temperature drops and the SCO2 startscondensing into liquid droplets ? ....rapid erosion & failure ?

One of the potential disadvantages of the SCO2 cycle is that its operation is sensitiveto 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 temperaturemoves from the optimal.

Also, SCO2 operates with a smaller gas temperature change for heat rejection, so it isnot as suitable for thermal desalination and dry cooling applications as multiple reheathelium 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 aneconomical source of helium and SCO2 will likely be the most practical powerconversion fluid. But in the intermediate term it makes sense to develop both.



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

There's nothing about SCO2 that precludes multiple reheat.

Author: Luke [ Jun 20, 2009 3:58 pm ]


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) isessentially 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-reheatedcycle, 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 extraturbine and piping. In contrast, at the same temperature your java sim for He gives29.5% 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 firstgeneration LFTRs operating at modest temperatures. Higher temps come later. I'mfurther biased because direct sea water cooling is permitted here (UK for sure, andmuch of the rest of Europe too, I think), so there is reliably cold cooling to keep S-CO2systems going. In the southern US, lack of cooling could shut it down just when it ismost needed, so air cooled He cycle could be better, even if it is a bit less efficient inearly systems.

Luke



Those efficiency differences have more to do with the pressure ratio than anything else.

Author: Luke [ Jun 20, 2009 6:32 pm ]


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', adjustedthe turbine efficiency up to 90% to match Dostal's assumption, and temps to 822/309K, then changed the pressure ratio to find the maximum efficiency. Then changed theturbomachinery configuration to two machines, then one, re-optimising the pressureratio in each case. The point I was trying to make is that adding reheat seems to helpHe cycles much more than S-CO2 ones. The first reheat gives nearly 16% more outputfrom the same reactor for He, but only 3% extra for S-CO2

Luke

Author: jagdish [ Jun 21, 2009 4:16 am ]


I have always wondered why only CO2, N2 and He are considered as gas coolants.What's wrong with argon and neon?

Author: Alex P [ Jun 21, 2009 6:51 am ]




A question I posted in the blog, too : is it CO2 as coolant compatible with the tempsrange typical of a LFTR, > 700 °C? I remember one of the reason for the choice ofhelium (instead CO2 used in older Magnox/AGR) as coolant in HTR was the compatibilityof helium vs CO2 with the operating temps of HTGR (in that case, 850 °C and evenhiger)

Author: Kirk Sorensen [ Jun 21, 2009 7:19 am ]


Alex P wrote:A question I posted in the blog, too : is it CO2 as coolant compatible with the tempsrange typical of a LFTR, > 700 °C? I remember one of the reason for the choice ofhelium (instead CO2 used in older Magnox/AGR) as coolant in HTR was the compatibilityof helium vs CO2 with the operating temps of HTGR (in that case, 850 °C and evenhiger)

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

Author: Alex P [ Jun 21, 2009 7:35 am ]


Kirk Sorensen wrote:Alex P wrote:A question I posted in the blog, too : is it CO2 as coolant compatible with the tempsrange typical of a LFTR, > 700 °C? I remember one of the reason for the choice ofhelium (instead CO2 used in older Magnox/AGR) as coolant in HTR was the compatibilityof helium vs CO2 with the operating temps of HTGR (in that case, 850 °C and evenhiger)

I think it will be a different situation since the gaseous coolant in a gas-cooled reactorhas to go through the core and be compatible with the neutronic flux and materialsfound there, whereas in LFTR the gas coolant only circulates through theturbomachinery and the gas heaters. In the gas heaters there will be coolant salt onone 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 todecompose/dissociate with higher temps (unlike helium, instead), I meant, is it aproblem for the temps range of LFTRs, too?








Author: Kirk Sorensen [ Jun 21, 2009 8:22 am ]


This applet seems to indicate that CO2 dissociation is not an issue below 1500K:

Carbon Dioxide Dissociation Applet

Author: Per Peterson [ Jun 21, 2009 9:51 pm ]


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 isvery expensive. Argon's properties are no better than nitrogen, which is cheaper andhas a very large turbomachinery experience base (from air open Brayton power cyclesystems). For closed cycles that are proposed to make use of air-based turbineexperience, some helium is commonly mixed with nitrogen to increase the thermalconductivity (important in affecting the heat transfer and pressure drop in heatexchangers). CO2 is special because its critical pressure and temperature are relativelylow and a supercritical cycle is possible.

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

Author: Leviathan [ Jul 25, 2009 10:47 am ]


I understand the design constraints of synchronous operation at 3600 rpm. I alsounderstand that our low power LFTR would like to run at higher RPM to get thecompressor specific speed into the right range. Per mentioned the alternatives; use agear box to convert to 3600 rpm or an electrical inverter to down convert thefrequency.

This may be another alternative and I'd like your comments on it. Since so much of our



http://www.engr.colostate.edu/~allan/thermo/page11/co2/CO2reaction.html



shaft work is internal between the turbine and compressor, has someone considered afree turbine design? As I understand their application in aircraft engines, free turbinedesigns can follow rapid power changes better than fixed turbine designs, but freeturbines have a lower peak efficiency.

In a free turbine, the first turbine stages drive the compressor. The turbine andcompressor can run asynchronously, meaning not at 3600 RPM. This is often called the"gas generator section" since all it does is deliver high pressure gas to the powerextraction section of the engine. The power extraction turbine is on a separate shaftand runs at 3600 rpm. Even if the power extraction turbine runs at higher RPM anduses a gear box, a separate output turbine allows for much lower rotating inertia and amuch simpler gear box. You do NOT want resonances between the turbine andgenerator...or between the turbine and the grid for that matter. These requirementsconstrain the lower torsional stiffness of the gearbox and turbine shafting.

Does someone here have experience with small turbine generators in our power rangeof 100MW? We call that a low power LFTR, but to put it in perspective it is 130thousand output horsepower. A 3600 RPM 130,000 HP gear box must be something tosee and quite something to pay for. I am not complaining about geared designs; Iappreciate their art. Then again I have not seen a 100MW inverter either.

Do free turbines simplify the turbine design?

Author: DV82XL [ Jul 25, 2009 12:00 pm ]


Leviathan wrote:Does someone here have experience with small turbine generators in our power rangeof 100MW? We call that a low power LFTR, but to put it in perspective it is 130thousand output horsepower. A 3600 RPM 130,000 HP gear box must be something tosee and quite something to pay for. I am not complaining about geared designs; Iappreciate their art. Then again I have not seen a 100MW inverter either.

Do free turbines simplify the turbine design?

Modern gas turbine gensets are available in sizes between 50MW and 250MW. All ofthem use reduction gearboxes to drive the final output shaft. These gearboxes are awell understood technology having been used for fifty years on turboprop engines inaviation. Almost all of the larger types are multi-spool designs with the power shaft onits own turbine, the rest coupled to the compressor. So yes they are free turbine types.





Leviathan wrote:I understand the design constraints of synchronous operation at 3600 rpm. I alsounderstand that our low power LFTR would like to run at higher RPM to get thecompressor specific speed into the right range. Per mentioned the alternatives; use agear box to convert to 3600 rpm or an electrical inverter to down convert thefrequency.

If the electrical signal is going to get "washed" through a conversion to DC (forunderwater or long-distance transmission) then reconstructed as AC, then it won'tmatter much what frequency we started out with--we reconstruct the waveform theway we want it.

Author: DV82XL [ Jul 25, 2009 2:18 pm ]


Kirk Sorensen wrote:If the electrical signal is going to get "washed" through a conversion to DC (forunderwater or long-distance transmission) then reconstructed as AC, then it won'tmatter much what frequency we started out with--we reconstruct the waveform theway we want it.

If we are talking about plants in the 100MW range, not much will be gained by usingDC transmission because this class of generating station would be built near its market.

Again, most of the issues around these aspects of using the Brayton cycle have alreadybeen solved for us by the natural gas turboshaft systems already running.

Author: Cyril R [ Jan 30, 2010 11:36 am ]


So the condensing temperature needs to be fairly constant for the SCO2 cycle. This is asimple matter of condenser design, no? Oversizing the condenser and pumps shouldtake care of this? (unless one uses pure dry cooling).

Author: jaro [ Jan 30, 2010 1:50 pm ]


Cyril R wrote:So the condensing temperature needs to be fairly constant for the SCO2 cycle. This is asimple matter of condenser design, no?



SC is intended for Brayton cycles: when you have a condenser, its no longer Brayton,but Rankine.

Author: Cyril R [ Jan 30, 2010 6:20 pm ]


jaro wrote:Cyril R wrote:So the condensing temperature needs to be fairly constant for the SCO2 cycle. This is asimple matter of condenser design, no?

SC is intended for Brayton cycles: when you have a condenser, its no longer Brayton,but Rankine.

Well OK, heat sink temp. Hey, I'm still stuck in the steam paradigm

Author: Luke [ Jan 30, 2010 7:39 pm ]


The strength of the S-CO2 cycle is also its weakness. By compressing close to thecritical point, the compression requires less work than an ideal gas would under thesame conditions, and the changing heat capacity of CO2 near the critical point meansthat more of the heat is rejected at low temperatures than would be for an ideal gas,making the effective T(cold) of the cycle nearer to the actual lowest temperature, i.emoving it nearer to a true Carnot cycle operating at the same temperatures. But thislocks the design to having the compressor inlet conditions of ~70 bar, 32°C, andhaving a cooling water supply/heat exchanger that can take a lot of heat out at not farabove 30°C. This in turn requires very high pressures at the hot end of the cycle, andreliable access to cold water. It's great for Northern Europe, Canada, Russia, maybe theNew England states. For the South-western US, helium (or He-N2) Brayton cycles areless likely to be knocked out by cooling water shortages in hot summers, ie just whenthe power is most needed.

Author: iain [ Jan 31, 2010 12:59 am ]


Thank you, for the first concise description I've seen yet for why a supercrticial CO2cycle is interesting.

Best post I've seen in a while.



Author: Cyril R [ Jan 31, 2010 5:19 am ]


Luke wrote:The strength of the S-CO2 cycle is also its weakness. By compressing close to thecritical point, the compression requires less work than an ideal gas would under thesame conditions, and the changing heat capacity of CO2 near the critical point meansthat more of the heat is rejected at low temperatures than would be for an ideal gas,making the effective T(cold) of the cycle nearer to the actual lowest temperature, i.emoving it nearer to a true Carnot cycle operating at the same temperatures. But thislocks the design to having the compressor inlet conditions of ~70 bar, 32°C, andhaving a cooling water supply/heat exchanger that can take a lot of heat out at not farabove 30°C. This in turn requires very high pressures at the hot end of the cycle, andreliable access to cold water. It's great for Northern Europe, Canada, Russia, maybe theNew England states. For the South-western US, helium (or He-N2) Brayton cycles areless likely to be knocked out by cooling water shortages in hot summers, ie just whenthe power is most needed.

Keep in mind that this potential efficiency penalty becomes smaller with increasing peakgas temps. I suppose with cooling water under 25 degree C things should be fine. LikeI asked above, it could make sense to have a higher cost, higher performance coolingsystem.

Cooling water shortage is typically the result of poor design and planning.

Author: Lars [ Jan 31, 2010 4:59 pm ]


Can you give some examples of this?My impression is rather different (perhaps skewed by the general water shortage insouthern california).Access to water is of high value to many people. The fight over water shut down farmsall through the central valley this past summer.The proposed nuclear power plant in Fresno (California) proposed to collocate with awaste water treatment facility in order to get access to less precious water. It is myimpression that the utilities commission here really wants the power plants to switch toair cooling.

Author: Cyril R [ Jan 31, 2010 5:53 pm ]


Lars wrote:Can you give some examples of this?



My impression is rather different (perhaps skewed by the general water shortage insouthern california).Access to water is of high value to many people. The fight over water shut down farmsall through the central valley this past summer.The proposed nuclear power plant in Fresno (California) proposed to collocate with awaste water treatment facility in order to get access to less precious water. It is myimpression that the utilities commission here really wants the power plants to switch toair cooling.

The main thing is that water consumption of power plants is actually quite low.Withdrawal of water can be large for once through cooling, but actual consumption(extra evaporation caused by slightly heating the water) is lower than a standardevaporative wet cooling system. Which is itself very low. So, the more expensive waterdesalination techniques can be used and add very little to the total cost of electricity.Desalination requires heat or electricity, guess what, a powerplant produces just that.That doesn't mean a smart company shouldn't try to minimize cost by using lowergrade water. Geothermal plants can do this too, I think it's the Geysers that disposes ofwaste water while getting lower cost well injection. Turn a problem into a solution.

Agricultural water consumption is no joke. It's so much bigger that the more expensivedesalination would add a lot to the product. If we care about potable water resources,this is the big area for improvement, not powerplants. We can switch all power stationsto dry cooling, if we don't try harder in the agricultural department it won't help muchfor alleviating water shortages. We need to look more to closed systems agriculture(greenhouses and hydroponics), drip irrigation etc. It would be silly to spend (lose) alarge amount of money in mandating dry cooling, when the same amount of money cansave so much more in other water consumption. Agriculture mainly, but householdwater consumption is also large compared to the water consumption required for wetcooling the amount of electricity for one household. That said I think we need to spendsome more in advancing dry cooling, there are interesting developments in lowtemperature foam based cooling systems for example. There are some extraadvantages related to siting with a dry cooling system. Then again I wonder if thelevelised cost wouldn't be lower if we simply pipe in desalinated seawater for arecirculating cooling system. It's probably a good idea to increase such sustainablewater infrastructure in places like Southern Calif anyway.

There is a very unsustainable but common practice in the US to drain aquifers at largefor huge irrigation needs. Water's one of the main reasons why corn ethanol is a stupidplan.

IMHO there's plenty more examples of stupid policy on cooling water use. Look atFrance, building so many nuclear powerplants next to rivers of insufficient flow ratewhen they should be building more on the coast in stead. They should be building onlyonce through seawater cooling with diffusor pipe systems (or recirculating wet cooling



with towers in case of sensitive local ecosystem concerns).

Author: Lars [ Feb 01, 2010 1:31 am ]


Just read over the wikipedia article (http://en.wikipedia.org/wiki/Cooling_tower) - lotsof alternatives in the cooling.Though it looks like cooling towers have been more dangerous to the public (excludingChernobyl) that the nuclear power plants.

This sounds like a decent solution for dryer climates - though not likely to be much helpin south-eastern US in the hot & humid days.

Fortunately, with the higher temperature of LFTR raising the cold temperature hurts theefficiency less than with LWR.




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





Author: Cyril R [ Feb 01, 2010 5:16 am ]


That's a good wiki article. My favorite system is the Heller indirect dry cooling, whichcools the working fluid (or gas) with evaporative wet cooling but then has another drycooling loop to throw the heat out into the air. It is more efficient than air cooledcondensers (direct fan cooling). It also solves the blowdown and freezing issues betterthan standard recirculating wet cooling towers. There are hybrids also which use somewater for emergency hot days, but very little or none during normal temperatures. Thisgets a high performance system while saving water at very low cost. The only downsideis that cooling towers are considered ugly.

Author: iain [ Feb 02, 2010 1:02 am ]


Lars,

I think you had the right idea a long time ago. The biggest cost driver in nuke plants inthe US is licensing. Simplifying the licensing process will be a huge win.

A mechanically driven dry cooling system without the tall passively-vented coolingtower is absolutely the way to go. It has no "nuke" shape stigma, and the leastpossible reliance on the surrounding environment. I would spend a bit of money toensure that the thing is reasonably quiet, without bringing this up as an issue forprotesters to pounce on and make outrageous demands about.

-Iain

Author: FRE [ Feb 02, 2010 3:14 am ]


Legionnaires' disease was first diagnosed about 1976 when some people at aLegionnaires' convention became ill and some died. Almost certainly the disease hadbeen around for many decades prior to that, but had not previously been diagnosed.Since then, it has been diagnosed numerous times in several different countries.

The most common source of Legionnaires' disease seems to be droplets from coolingtowers. In an attempt to prevent it, regulations in many areas require periodicinspections to ensure that the bacteria that causes Legionnaires' disease is not presentin cooling towers. However, it seems unlikely that the problem can be totallyeliminated.





It may be that the risk of Legionnaires' disease could be used to support the argumentfor direct once-through water cooling from the ocean or other body of water, but thatcould backfire if the regulating authority would permit only dry air cooling.



FRE wrote:Legionnaires' disease was first diagnosed about 1976 when some people at aLegionnaires' convention became ill and some died. Almost certainly the disease hadbeen around for many decades prior to that, but had not previously been diagnosed.Since then, it has been diagnosed numerous times in several different countries.



Where did you read this? The recirculating wet cooling technique uses a lot of anti-biofouling and anti-bacteria chemicals, sometimes also corrosion inhibitors - in fact somuch that disposal of the increasingly concentrated brine becomes a disposal problem.Can't imagine that Legionaires' disease stands much of a chance in that environment.



Offhand, I would expect that the exclusionary zone plus some trees should do a prettygood job of keeping the power plant quiet at the boundary of the property.



There's a bunch of new techs that can be applied to a forced air cooling system. Inparticular carbon or metal foam based high surface area radiators, and tubercle vortexhigh efficiency, silent low RPM blades for the fans.

Lars has good ideas using trees on the periphery. Some people had similar ideas for a

biomass plant cooling tower. Who says cooling towers have to be ugly :



Author: Lindsay [ Feb 02, 2010 1:46 pm ]


Beautiful, every home should have one.

Legionella is not a trivial problem, it requires careful management of a well designedsystem. One of the core issues is that Legionella occurs naturally in most environmentsin the air and soil, so there is this constant source of infection being ingested by thecooling tower. Most towers will carry some Legionella, the trick is to manage thebiocide regime to maintain a low bacteria count in the circulating water. Warmconcentrated fresh water fertilised by dust and contaminants from the air is a perfect



breeding ground for Legionella, but by keeping the system physically clean and dosedregularly with biocide, bio-film dispersants and the like, it can be maintained with a lowbug count. This is important to maintain a safe environment for plant staff and thecommunity around the power station.

Author: FRE [ Feb 02, 2010 2:11 pm ]


Cyril R wrote:FRE wrote:Legionnaires' disease was first diagnosed about 1976 when some people at aLegionnaires' convention became ill and some died. Almost certainly the disease hadbeen around for many decades prior to that, but had not previously been diagnosed.Since then, it has been diagnosed numerous times in several different countries.




It's information that I've acquired over a number of years.

When the first diagnosed cases of Legionnaire's disease occurred, the health authoritieshad no idea what the source was. After considerable sleuthing, it was discovered that anear-by cooling tower was breeding the (newly discovered) bacteria. Subsequentinvestigation determined that it was a very common bacterium and that it wascommonly found in cooling towers. Probably cooling towers become infected frombacteria carried by dust. The result was that regulations were enacted all over to ensurethat cooling towers would not breed the organism that caused the outbreak, but theregulations have not been totally effective. Not everyone exposed will become ill, butwhen people do become ill, it is often fatal unless promptly treated with antibiotics.Elderly people are much more likely to become ill than younger people. It may be thatminor exposure seldom results in illness because the body would have more time tofight it off before the bacteria have time to multiply too greatly.

A few years ago, when I was doing some research on more efficient air conditioningsystems for homes, I learned that small cooling towers were available for home air



conditioning systems, the purpose being to increase efficiency by reducing the high-side pressure. I was concerned because I doubt that they'd be as well managed asindustrial cooling towers, so there could very well be a health risk associated with them.

Here in the South-West, many homes use evaporative coolers. I've wondered about therisks of those, but because there is no aerosol, probably bacteria in the coolers neverbecome air-borne, so probably the risk is low.

In any case, cooling towers for power plants are a potential hazard that must berecognized and carefully monitored.

Author: USPWR_SRO [ Feb 02, 2010 2:12 pm ]


Lindsay wrote:Beautiful, every home should have one.

Legionella is not a trivial problem, it requires careful management of a well designedsystem. One of the core issues is that Legionella occurs naturally in most environmentsin the air and soil, so there is this constant source of infection being ingested by thecooling tower. Most towers will carry some Legionella, the trick is to manage thebiocide regime to maintain a low bacteria count in the circulating water. Warmconcentrated fresh water fertilised by dust and contaminants from the air is a perfectbreeding ground for Legionella, but by keeping the system physically clean and dosedregularly with biocide, bio-film dispersants and the like, it can be maintained with a lowbug count. This is important to maintain a safe environment for plant staff and thecommunity around the power station.

Source please?

Author: FRE [ Feb 02, 2010 3:21 pm ]


USPWR_SRO wrote:Lindsay wrote:Beautiful, every home should have one.




Source please?

Probably if you were older, you would not be asking for the source.

When the original outbreak of Legionnaires' disease occurred at a Legionnaires'convention, I was at an unrelated convention in San Francisco. For days, the outbreakwas followed by newspapers all over the country. I remember it very well, as will mostpeople who were old enough at the time (about 1976) to be aware of such things;people born after about 1966 would be less likely to be aware of it. That means thatpeople younger than about 44 probably would not remember the original outbreaksince they would have been younger than 10 years old at the time. I also rememberwhen it was discovered that a near-by cooling tower was infected and was emitting anaerosol that drifted over the immediate area. It was immediately realized that the samehazard could exist in other cooling towers, so heath personnel all over the countrystarted taking samples from cooling towers and discovered that many were infected.Regulations soon followed in an only partially successful attempt to ensure that allcooling towers would be managed in such a way that the risk would be eliminated.

A google search will provide plenty of information from multiple sources.



Older?

I am 47 years old. I remember well the legionaires convention outbreak. I was in highschool.

I wanted a source linking that to power plant cooling towers. Thats the first time I hadever heard of a link between them. Links to HVAC cooling plants yes.....but not topower plant cooling towers. News to me.

Author: Lars [ Feb 02, 2010 5:05 pm ]


Here is a start (http://en.wikipedia.org/wiki/Cooling_tower) .

"French researchers found that Legionella spread through the air up to 6 kilometresfrom a large contaminated cooling tower at a petrochemical plant in Pas-de-Calais,France. That outbreak killed 21 of the 86 people that had a laboratory-confirmedinfection."

This was a cooling tower from a petrochemical plant but it seems reasonable to assumethe same issue must be addressed in all wet cooling towers. It may be an easily solvedissue but it caught me by surprise. I wonder if dry cooling with occasional assist fromwet cooling has such a problem.




Author: FRE [ Feb 02, 2010 5:47 pm ]


USPWR_SRO wrote:Older?



I hadn't heard about the problem with power plant cooling towers either but, since theyuse the same principals of physics as HVAC cooling towers, I would expect theproblems to be the same. In any case, I wouldn't want to be very close to any type ofcooling tower.



USPWR_SRO wrote:Source please?

Source, personal experience as the manager of a 125 MW CCGT Cogeneration plant.Managing Legionella is an issue for anyone running a fresh water cooling tower setup.The worst offenders are not usually the big utility towers, it tends to be the smaller wettowers associated with air conditioning equipment. There was an outbreak oflegionnaires disease in Christchurch, NZ some years ago which was eventually tracedback to the hospital (NOT GOOD)

I don't know if Legionella is an issue in seawater based cooling towers, but it certainlyis for fresh water based ones, due the steady warm conditions and lot of bug food,minerals etc from the air. I don't have any documents to point you to, but there willprobably be many describing best practise and biological monitoring for cooling towersif you did a google search.

Where I used to work they would take dip slides once a week which grow the bugsvisually, and then perhaps once a month take water sample and have it analysed at alab. If the bug count comes back as being higher than a certain threshold, then someform of shock dosing of chlorine or other biocide is done and the bugs further analysedto determine is Legionella is present and in what number. Normally one just looks attotal bug count and manages to that.

Edit: FRE I would not be concerned about being close to a well run tower at a powerstation, but a small wet tower on an office or apartment block next door, that isdefinitely more likely to be a problem, normally because people don't manage small



towers, they just use them.



Lindsay,Is this a problem that should impact the selection of the cooling system or is it onethat just requires proper operations?That is, should we shy away from wet cooling due to this problem or is it perfectlymanageable?










That's a good wiki article. My favorite system is the Heller indirect dry cooling, whichcools the working fluid (or gas) with evaporative wet cooling but then has another drycooling loop to throw the heat out into the air. It is more efficient than air cooledcondensers (direct fan cooling). It also solves the blowdown and freezing issues betterthan standard recirculating wet cooling towers. There are hybrids also which use somewater for emergency hot days, but very little or none during normal temperatures. Thisgets a high performance system while saving water at very low cost. The only downsideis that cooling towers are considered ugly.

Author: iain [ Feb 02, 2010 1:02 am ]


Lars,

I think you had the right idea a long time ago. The biggest cost driver in nuke plants inthe US is licensing. Simplifying the licensing process will be a huge win.

A mechanically driven dry cooling system without the tall passively-vented coolingtower is absolutely the way to go. It has no "nuke" shape stigma, and the leastpossible reliance on the surrounding environment. I would spend a bit of money toensure that the thing is reasonably quiet, without bringing this up as an issue forprotesters to pounce on and make outrageous demands about.

-Iain

Author: FRE [ Feb 02, 2010 3:14 am ]


Legionnaires' disease was first diagnosed about 1976 when some people at aLegionnaires' convention became ill and some died. Almost certainly the disease hadbeen around for many decades prior to that, but had not previously been diagnosed.Since then, it has been diagnosed numerous times in several different countries.









FRE wrote:Legionnaires' disease was first diagnosed about 1976 when some people at aLegionnaires' convention became ill and some died. Almost certainly the disease hadbeen around for many decades prior to that, but had not previously been diagnosed.Since then, it has been diagnosed numerous times in several different countries.






Offhand, I would expect that the exclusionary zone plus some trees should do a prettygood job of keeping the power plant quiet at the boundary of the property.



There's a bunch of new techs that can be applied to a forced air cooling system. Inparticular carbon or metal foam based high surface area radiators, and tubercle vortexhigh efficiency, silent low RPM blades for the fans.

Lars has good ideas using trees on the periphery. Some people had similar ideas for a

biomass plant cooling tower. Who says cooling towers have to be ugly :





Beautiful, every home should have one.

Legionella is not a trivial problem, it requires careful management of a well designedsystem. One of the core issues is that Legionella occurs naturally in most environmentsin the air and soil, so there is this constant source of infection being ingested by thecooling tower. Most towers will carry some Legionella, the trick is to manage thebiocide regime to maintain a low bacteria count in the circulating water. Warmconcentrated fresh water fertilised by dust and contaminants from the air is a perfect



breeding ground for Legionella, but by keeping the system physically clean and dosedregularly with biocide, bio-film dispersants and the like, it can be maintained with a lowbug count. This is important to maintain a safe environment for plant staff and thecommunity around the power station.

Author: FRE [ Feb 02, 2010 2:11 pm ]


Cyril R wrote:FRE wrote:Legionnaires' disease was first diagnosed about 1976 when some people at aLegionnaires' convention became ill and some died. Almost certainly the disease hadbeen around for many decades prior to that, but had not previously been diagnosed.Since then, it has been diagnosed numerous times in several different countries.




It's information that I've acquired over a number of years.

When the first diagnosed cases of Legionnaire's disease occurred, the health authoritieshad no idea what the source was. After considerable sleuthing, it was discovered that anear-by cooling tower was breeding the (newly discovered) bacteria. Subsequentinvestigation determined that it was a very common bacterium and that it wascommonly found in cooling towers. Probably cooling towers become infected frombacteria carried by dust. The result was that regulations were enacted all over to ensurethat cooling towers would not breed the organism that caused the outbreak, but theregulations have not been totally effective. Not everyone exposed will become ill, butwhen people do become ill, it is often fatal unless promptly treated with antibiotics.Elderly people are much more likely to become ill than younger people. It may be thatminor exposure seldom results in illness because the body would have more time tofight it off before the bacteria have time to multiply too greatly.

A few years ago, when I was doing some research on more efficient air conditioningsystems for homes, I learned that small cooling towers were available for home air



conditioning systems, the purpose being to increase efficiency by reducing the high-side pressure. I was concerned because I doubt that they'd be as well managed asindustrial cooling towers, so there could very well be a health risk associated with them.

Here in the South-West, many homes use evaporative coolers. I've wondered about therisks of those, but because there is no aerosol, probably bacteria in the coolers neverbecome air-borne, so probably the risk is low.

In any case, cooling towers for power plants are a potential hazard that must berecognized and carefully monitored.



Lindsay wrote:Beautiful, every home should have one.


Source please?

Author: FRE [ Feb 02, 2010 3:21 pm ]


USPWR_SRO wrote:Lindsay wrote:Beautiful, every home should have one.




Source please?

Probably if you were older, you would not be asking for the source.

When the original outbreak of Legionnaires' disease occurred at a Legionnaires'convention, I was at an unrelated convention in San Francisco. For days, the outbreakwas followed by newspapers all over the country. I remember it very well, as will mostpeople who were old enough at the time (about 1976) to be aware of such things;people born after about 1966 would be less likely to be aware of it. That means thatpeople younger than about 44 probably would not remember the original outbreaksince they would have been younger than 10 years old at the time. I also rememberwhen it was discovered that a near-by cooling tower was infected and was emitting anaerosol that drifted over the immediate area. It was immediately realized that the samehazard could exist in other cooling towers, so heath personnel all over the countrystarted taking samples from cooling towers and discovered that many were infected.Regulations soon followed in an only partially successful attempt to ensure that allcooling towers would be managed in such a way that the risk would be eliminated.

A google search will provide plenty of information from multiple sources.



Older?





Here is a start (http://en.wikipedia.org/wiki/Cooling_tower) .

"French researchers found that Legionella spread through the air up to 6 kilometresfrom a large contaminated cooling tower at a petrochemical plant in Pas-de-Calais,France. That outbreak killed 21 of the 86 people that had a laboratory-confirmedinfection."

This was a cooling tower from a petrochemical plant but it seems reasonable to assumethe same issue must be addressed in all wet cooling towers. It may be an easily solvedissue but it caught me by surprise. I wonder if dry cooling with occasional assist fromwet cooling has such a problem.




Author: FRE [ Feb 02, 2010 5:47 pm ]


USPWR_SRO wrote:Older?



I hadn't heard about the problem with power plant cooling towers either but, since theyuse the same principals of physics as HVAC cooling towers, I would expect theproblems to be the same. In any case, I wouldn't want to be very close to any type ofcooling tower.



USPWR_SRO wrote:Source please?

Source, personal experience as the manager of a 125 MW CCGT Cogeneration plant.Managing Legionella is an issue for anyone running a fresh water cooling tower setup.The worst offenders are not usually the big utility towers, it tends to be the smaller wettowers associated with air conditioning equipment. There was an outbreak oflegionnaires disease in Christchurch, NZ some years ago which was eventually tracedback to the hospital (NOT GOOD)



Edit: FRE I would not be concerned about being close to a well run tower at a powerstation, but a small wet tower on an office or apartment block next door, that isdefinitely more likely to be a problem, normally because people don't manage small



towers, they just use them.



Lindsay,Is this a problem that should impact the selection of the cooling system or is it onethat just requires proper operations?That is, should we shy away from wet cooling due to this problem or is it perfectlymanageable?








Author: Kirk Sorensen [ Feb 02, 2010 10:15 pm ]


I learn so much from you guys!



Lars wrote:Lindsay,Is this a problem that should impact the selection of the cooling system or is it onethat just requires proper operations?That is, should we shy away from wet cooling due to this problem or is it perfectlymanageable?

Wet cooling towers are an extremely effective and economic means of rejecting heat tothe environment and even the most efficient thermal power cycles have significant heatrejection requirements. Full wet or hybrid wet and dry towers are ideal for this purpose.

Regarding Legionella and other bacteria that are prone to grow in these towers, thoseissues are easily managed by initial good design combined with regular monitoring andappropriate biocide treatments. It is very easy to get this right, but it does requiresome regular attention.

The only black mark (if you can call it that) against hybrid wet/dry towers is that theyconsume water, on all other counts they are very benign and cost effective. I personallydo not like dry cooling systems and would try to avoid using them if possible.



Lindsay wrote:USPWR_SRO wrote:Source please?

Source, personal experience as the manager of a 125 MW CCGT Cogeneration plant.





Managing Legionella is an issue for anyone running a fresh water cooling tower setup.The worst offenders are not usually the big utility towers, it tends to be the smaller wettowers associated with air conditioning equipment. There was an outbreak oflegionnaires disease in Christchurch, NZ some years ago which was eventually tracedback to the hospital (NOT GOOD)



Edit: FRE I would not be concerned about being close to a well run tower at a powerstation, but a small wet tower on an office or apartment block next door, that isdefinitely more likely to be a problem, normally because people don't manage smalltowers, they just use them.

Very interesting information thanks.

Plant I work at has no cooling towers. We use once through ocean cooling. We do addbiocide to the incoming seawater but not for any concern about stuff in this thread, butactually to enhance heat transfer of condenser tubes by preventing slime coatingbuildup on them.



Thanks for that. Yes, direct once through cooling is nice if you can get it, get those nicelow condenser pressures, especially on a LWR where the condensers have a lot of workto do.

I have just found this reference for anyone who would like to read more, it is put outby the NZ version of OSHA. There some further links within that web page. Enjoy.http://www.osh.govt.nz/publications/series/hb-20-legionnaires.html


http://www.osh.govt.nz/publications/series/hb-20-legionnaires.html




Thanks, that was my general impression.

Author: FRE [ Feb 03, 2010 1:54 am ]


Even if it is perfectly manageable, it might be best to leave doubt in the minds of thoseobjecting to once-through cooling. That might make it possible to avoid cooling towersand use once-through cooling instead. They might be induced to fear the health risks ofthe cooling tower more than the possible environmental damage caused by once-through cooling.

That might be seen as stooping to their level.

Author: David [ Apr 15, 2010 2:50 pm ]


Just been reviewing this thread again, so much great information. I am lost on many ofthe fine engineering details but am still trying to sort out a preference. From myviewpoint I still think it is a pretty even race between all candidates of He multi-reheat,Supercritical CO2, Nitrogen and good old steam Rankine (likely ultra supercritical butmaybe even just superheated steam).

I thought I`d add a quick summary and ask a couple more questions. Please chime in ifyou disagree with things (and why).

First is just good old steam. The big advantage is it is a well known system and off theshelve (with modifications needed). I`d also add that while the compact nature of gasturbines tend to make them look so much cheaper than steam, when you find reportscomparing He or S-CO2 to steam the gas cycles don`t seem to come out all that muchcheaper AND the authors are not including huge R&D costs of either He or S-CO2. Hereis a good short paper that does some cost comparisons between S-CO2, He and Steam.They would not be comparing to the best multi-reheat helium cycles but stillinteresting.

Attachment:

300kwePCU.pdf [1.72 MiB] Downloaded 31 times

The disadvantages to steam are that tritium control becomes a bigger issue and thatthe high melting point of typical intermediate coolant salts means feedwater returnheating is a problem. ORNL had solutions in place for both issues and in the mid 70s

http://www.energyfromthorium.com/forum/download/file.php?id=775



tests with NaF-NaBF3 looked quite good for trapping tritium. There are other solutionsto tritium as well, such as changing carrier salts to not produce it or adding a thirdcoolant loop (nitrate salts or He gas, see ORNL TM 5325 and ORNL TM 3428).Feedwater heating was to be by steam injection which would add cost and complexityto the standard Supercritical Steam Rankine (Based on the Bull Run Coal Plant).

In general I think most of us tech nerds turn our noses up at steam but I am certainlyfar from convinced this is still not the way to go, especially for the first number ofplants so we are not asking investors to also fund turbine development (which is whatmany people are saying killed the South African Pebble Bed project). I`d also add thatevery major group promoting molten salt systems (French TMSR/MSFR, RussianMOSART, Japanese FUJI) still assume a steam cycle.

Multi-Reheat He.

Has clear advantages if you can keep your inlet He temp high enough. If you keep thesalt to its traditional 700 C limit you need pretty big heat exchangers to try to get yourfinal He temp as close to this as possible. I couldn`t find it right now but I think Per`ssystem only has about a 35 C drop from peak primary salt temp to peak He temp(compared to a steam temp of 565 for MSBR and tiny IHXs). Helium (and any gas) alsomakes the tritium issue easy to deal with and provides a "soft" linkage betweenturbines and reactor (good for stable reactor control). Helium also seems fairly easy tointegrate dry cooling by raising the waste heat rejection temperature. Can anyone givean idea of efficiency loss in this case since I often see 30 C as the quoted rejectiontemp?

Supercritical S-CO2

Seems to have clear advantages if we can`t get peak gas temperature up near 700 Cor higher. Excellent efficiencies between 500 and 650 and extremely compactequipment. However, the need to get rid of heat near the critical point seems to ruleout dry cooling, can someone confirm this (it has been mentioned on and off). I wouldguess that even more R&D would be needed than He but that is only a guess.

Nitrogen

I need to learn much more here but by Rod Adam`s and Per`s exchange it would seemthat Nitrogen has some clear advantages if your total power output is lower. Cansomeone provide some numbers here? What would be the ideal output for Rod`ssystems? If they can compete on a capital cost per MWe output I don`t see a bigproblem with having more turbines per plant. The R&D input certainly seems to belower and to me this is a huge advantage. Very hard to convince people that we needhundreds of millions to develop our reactors and at the same time hundreds of millionsto match gas turbines to them.



Anyhow, let me know what you think. I personally think it still a very close race...

David LeBlanc

P.S. I didn`t mention, but if you have a peak salt temp around 700 C there is actuallyvery little difference in the potential thermal or net efficiency of He, S-CO2 and Steam(not sure about N2).

Author: NNadir [ Apr 15, 2010 6:30 pm ]


At the right time, under the right conditions, particularly in the presence of any carbonin any form, carbon dioxide is actually a fairly decent oxidant.

It's certainly by no means a show stopper, but something worth keeping in the back ofone's mind. The topic of carbon dioxide as an oxidant is on my mind quite a bit.

Supercritical carbon dioxide, usually with complexants or other additives, is now thesubject of international attention as a so called "green" solvent, including solventextraction in actinide/fp chemistry, but not limited to that kind of chemistry.

Author: jagdish [ Apr 16, 2010 1:36 pm ]


CO2 cooled reactors are in use in the UK but there are corrosion problems limit thetemperature. Nitrogen would be similar. The economical substitute for He is only Argon.It is also close to air or CO2 density.

Author: Lindsay [ Apr 16, 2010 6:22 pm ]


jagdish wrote:Nitrogen would be similar.

Are you sure about that? Most nickel based superalloys that I am familiar with performquite well in the presence of nitrogen. Like most things, care would need to be takenthat all materials are compatible with the other materials they come into contact withduring the material selection process. As an aside I wonder how well a stable formxenon would perform, but the trick might be what happens to it when it becomeirradiated, and I have no idea how that side of things looks.

Author: Lars [ Apr 16, 2010 7:05 pm ]




The turbine gas should not be irradiated.

Author: Cyril R [ Apr 19, 2010 11:12 am ]


jagdish wrote:CO2 cooled reactors are in use in the UK but there are corrosion problems limit thetemperature. Nitrogen would be similar. The economical substitute for He is only Argon.It is also close to air or CO2 density.

The UK's Magnox machines have CO2 in high flux environment and IIRC in directcontact with graphite (carbon). We have neither for the gas turbine cycle in the LFTR.

Alloys with a lot of carbon could give trouble rather similar to steam in steam Rankinecycles.

Titanium in the Hastelloy I think will react with nitrogen to form a fairly inert titaniumnitride. But you're talking very high temperatures and even then it may not bedetrimental. Anyone want to try this stuff out in a lab?

Author: friend2all [ May 26, 2010 6:57 pm ]


Thorium Molten Salt Reactors using S-CO2 turbo machinery are not just non-GHGproducing, they actually take CO2 out of the atmosphere and put as yet undeterminedvolume of CO2 at an undetermined pressure to work in the outer power generationloop. In this respect the combination of Thorium Molten Salt Reactors and S-CO2 turbo-machinery is not CO2 neutral but is actually CO2 removing or CO2 negative.

Can anyone estimate/guess the volume of the outer loop of a commercial 1000 MW(e)LFTR using supercritical CO2 turbo-machinery and the amount of CO2 working fluid inpounds that would be required to fill that outer loop while the LFTR and S-CO2 turbineoperate?

Author: Cyril R [ May 27, 2010 3:00 am ]


friend2all wrote:Thorium Molten Salt Reactors using S-CO2 turbo machinery are not just non-GHGproducing, they actually take CO2 out of the atmosphere and put as yet undeterminedvolume of CO2 at an undetermined pressure to work in the outer power generationloop. In this respect the combination of Thorium Molten Salt Reactors and S-CO2 turbo-machinery is not CO2 neutral but is actually CO2 removing or CO2 negative.



Can anyone estimate/guess the volume of the outer loop of a commercial 1000 MW(e)LFTR using supercritical CO2 turbo-machinery and the amount of CO2 working fluid inpounds that would be required to fill that outer loop while the LFTR and S-CO2 turbineoperate?

It's peanuts. A helium Brayton requires around 10 metric tons per GWe of helium. Evenif a CO2 cycle would require 10x that in CO2 inventory, it's still nothing. At most a one-time couple thousand dollars, compared to a multi hundred million dollar per yearrevenue for power sales...

Author: friend2all [ May 31, 2010 1:02 am ]


Cyril R wrote:friend2all wrote:It's peanuts. A helium Brayton requires around 10 metric tons per GWe of helium. Evenif a CO2 cycle would require 10x that in CO2 inventory, it's still nothing. At most a one-time couple thousand dollars, compared to a multi hundred million dollar per yearrevenue for power sales...

Thanks Cyrill!The combination of LFTR with S-CO2 turbine-generator technology can directly replacecoal fired power plants on a one for one basis. From a Green House Gas generationstandpoint the most important impact of the LFTR/S-CO2 turbine is that is obviates theneed for burning many thousands of tons of coal. The additional CO2 that would beremoved from the environment by using CO2 to provide the working fluid for ClosedBrayton Cycle turbines is just another small additional GHG reduction.




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