1200°C Brayton cycle efficiency ?

[jaro]

Regarding my last post here, what is the best efficiency we could realistically hope for in a Brayton cycle running with a Thigh of 1200°C ?

I would like to see just how small an HWMSR core would be sufficient to generate, say, 300MWe, 500MWe, or 700MWe.

Thanks for any help, in advance.

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[Kirk Sorensen]

Fire up the Brayton Cycle Simulator and see for yourself. The link's on the blog on the right-hand-side column.

[jaro]

Great -- I knew I should have tried that thing out sooner -- thanks Kirk !

Having tried the various options, with a turbine inlet of 1373K (ie. 100 degree delta-T for the PHT HX), I get efficiencies ranging roughly from 54.4% for the "realistic" system, to 61% for the AHTR-VT model (what is that ?? ...its considerably better than the GT-MHR, at 55.5%).

Looks like a "best realistic figure" is somewhere around 57%, which would lead to 526MW, 877MW and 1228MW as the thermal powers required for producing 300MWe, 500MWe and 700MWe, respectively.

That looks great.

What do you think of the 100 degree delta-T for the PHT HX ? ....sound realistic ?

Next step: how much can we shrink that HWMSR core & piping !

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[Kirk Sorensen]

Make sure the minimum temperature in the gas heaters is comfortably above the freezing temperature of your salt mixture. Otherwise the salt might freeze in the heat exchanger. This turned out to be a big issue in the design choices made for our UTK class project.

[Jim Baerg]

How high a temperature can a gas turbine run at before you get problems with eg: the turbine blades weakening?

Also would the best high temperature materials be way more expensive & increase the cost more that the added efficiency from higher delta-T is worth?

[STG]

How high a temperature can a gas turbine run at before you get problems with eg: the turbine blades weakening?

Also would the best high temperature materials be way more expensive & increase the cost more that the added efficiency from higher delta-T is worth?

Maybe, but with decreasing efficiency your costs for cooling will also be higher.

[jaro]

Make sure the minimum temperature in the gas heaters is comfortably above the freezing temperature of your salt mixture. Otherwise the salt might freeze in the heat exchanger.

Thanks for pointing out my error Kirk.
Any helpful suggestions for fixing it ?

One think I realised later is that the 1200°C (1473K) temperature was my choice for core fuel channel mid-height (that's what I based my thermal radiation calcs on, for heat loss to the calandria tubes).
At core outlet, it would be more like 1265°C (1538K).
This is the temperature that the PHT HX would see at its inlet.
The secondary side peak temp. might then be something like 1450K.
If I want the entire cycle to stay above the UF4 melting point (1308K), that doesn't give a whole lot of area on the T-s diagram.

So I'm thinking maybe some kind of topping cycle -- maybe a fluoride Rankine -- with a much wider liquid temperature range, could reject its heat to a standard Helium Brayton cycle.
That way, one could "kill two birds with one stone," since an intermediate HX loop is probably needed anyway, to ensure that fission products stay out of the Helium circuit.

Ordinary gas turbine generators can get an overall energy conversion efficiency of about 60%, by also combining different cycles.
In that case, the hot cycle is the fuel-air combustion Brayton, which then rejects heat to a steam Rankine.
Sort of reverse of what I'm thinking.

What would be a good candidate for a Rankine topping cycle ? ....maybe FLiNaK, with the KF doing the vapour phase work ?
Any other ideas ?

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[jaro]

How high a temperature can a gas turbine run at before you get problems with eg: the turbine blades weakening?

I checked on the Wikipedia web site, but they don't give any temperatures, as far as I could tell.

I would also be interested in differences between gas turbines an vapour turbines.
I believe that Helium turbines spin at an extremely high speed, exacerbating the stress problem at high temperature.
Heavy vapour turbines, in contrast, probably spin much slower.

Turbine wheels have been fabricated out of single-piece ceramic, but I suspect they might not last long in a high-stress application.
Ceramic turbines in a Rankine cycle might be OK.

.

[rgvandewalker]

The basic limit is the turbine disk's material.
Most superalloys lose strength at about 1100C.
Cooled blades can go much higher, and this is the
normal way to achieve extreme performance
at reasonable costs. (I think GE does this in their
natural gas turbine power plants.)

There are rumours of exotic cobalt alloys
(Cobalt melts at 1495C) used in fighters.
The major aircraft engine manufacturers can make
single-crystal turbine blades, which also helps.

http://www.msm.cam.ac.uk/phase-trans/20 ... lloys.html

[jaro]

There are rumours of exotic cobalt alloys
(Cobalt melts at 1495C) used in fighters.

....and would you believe Thorium, m.p. 1750°C ?

.

[meiza]

afaik some Thorium alloys was used in some very secret projects at a time, with bad health effects to the steel plant workers.

[rgvandewalker]

Thanks guys!

[dezakin]

I've never heard of using molten salts for rankine topping cycles. It sounds like a whole lot of work to make sure you have an environment that isn't hell on the turbines. I know that there was some work done on a mercury topping cycle in the 30's.

It would probably be better to simply use a rankine (or kalina binary fluid ideally since it can have a lower heat rejection temperature and better efficiency) to be a bottoming end.

[Lindsay]

Regarding gas turbine peak temperatures there are plenty of GT's out there running Turbine Inlet Temperatures (TiT) of 1100 to 1250C. GE's new H Class machines are reputedly firing up to 1427C (2600F).

The blades themselves cannot survive at this temperatures for very long, they rely very heavily on sophisticated internal cooling schemes, thermal barrier coatings and being made from oxidation resistant superalloys (usually Ni based) with excellent hot strength characteristics.

Based on what I know of the modern machines I don't see any impediment to running them at 1200C TiT using existing designs and materials.

[Jay Ely]

Current NGCC design uses a firing temp of 1399C (2550F) Advanced Frame F. The best reference I can offer is http://www.netl.doe.gov/energy-analyses ... Report.pdf which has both pulverized coal and natural gas plants summarized. Note that these are very recent, fairly agressive modern designs of about 550 MWE - I doubt any utility would sign up for anything more radical unless extensively underwritten by others.