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PostPosted: Jul 25, 2009 10:47 am 
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I understand the design constraints of synchronous operation at 3600 rpm. I also understand that our low power LFTR would like to run at higher RPM to get the compressor specific speed into the right range. Per mentioned the alternatives; use a gear box to convert to 3600 rpm or an electrical inverter to down convert the frequency.

This may be another alternative and I'd like your comments on it. Since so much of our shaft work is internal between the turbine and compressor, has someone considered a free turbine design? As I understand their application in aircraft engines, free turbine designs can follow rapid power changes better than fixed turbine designs, but free turbines have a lower peak efficiency.

In a free turbine, the first turbine stages drive the compressor. The turbine and compressor 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 power extraction section of the engine. The power extraction turbine is on a separate shaft and runs at 3600 rpm. Even if the power extraction turbine runs at higher RPM and uses a gear box, a separate output turbine allows for much lower rotating inertia and a much simpler gear box. You do NOT want resonances between the turbine and generator...or between the turbine and the grid for that matter. These requirements constrain the lower torsional stiffness of the gearbox and turbine shafting.

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

Do free turbines simplify the turbine design?

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PostPosted: Jul 25, 2009 12:00 pm 
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Leviathan wrote:
Does someone here have experience with small turbine generators in our power range of 100MW? We call that a low power LFTR, but to put it in perspective it is 130 thousand output horsepower. A 3600 RPM 130,000 HP gear box must be something to see and quite something to pay for. I am not complaining about geared designs; I appreciate 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 of them use reduction gearboxes to drive the final output shaft. These gearboxes are a well understood technology having been used for fifty years on turboprop engines in aviation. Almost all of the larger types are multi-spool designs with the power shaft on its own turbine, the rest coupled to the compressor. So yes they are free turbine types.


Last edited by DV82XL on Jul 25, 2009 1:19 pm, edited 1 time in total.

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PostPosted: Jul 25, 2009 1:11 pm 
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Leviathan wrote:
I understand the design constraints of synchronous operation at 3600 rpm. I also understand that our low power LFTR would like to run at higher RPM to get the compressor specific speed into the right range. Per mentioned the alternatives; use a gear box to convert to 3600 rpm or an electrical inverter to down convert the frequency.


If the electrical signal is going to get "washed" through a conversion to DC (for underwater or long-distance transmission) then reconstructed as AC, then it won't matter much what frequency we started out with--we reconstruct the waveform the way we want it.


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PostPosted: Jul 25, 2009 2:18 pm 
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Kirk Sorensen wrote:
If the electrical signal is going to get "washed" through a conversion to DC (for underwater or long-distance transmission) then reconstructed as AC, then it won't matter much what frequency we started out with--we reconstruct the waveform the way we want it.


If we are talking about plants in the 100MW range, not much will be gained by using DC 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 already been solved for us by the natural gas turboshaft systems already running.


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PostPosted: Jan 30, 2010 11:36 am 
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So the condensing temperature needs to be fairly constant for the SCO2 cycle. This is a simple matter of condenser design, no? Oversizing the condenser and pumps should take care of this? (unless one uses pure dry cooling).


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PostPosted: Jan 30, 2010 1:50 pm 
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Cyril R wrote:
So the condensing temperature needs to be fairly constant for the SCO2 cycle. This is a simple matter of condenser design, no?

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


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PostPosted: Jan 30, 2010 6:20 pm 
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jaro wrote:
Cyril R wrote:
So the condensing temperature needs to be fairly constant for the SCO2 cycle. This is a simple 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 :lol:


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PostPosted: Jan 30, 2010 7:39 pm 
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The strength of the S-CO2 cycle is also its weakness. By compressing close to the critical point, the compression requires less work than an ideal gas would under the same conditions, and the changing heat capacity of CO2 near the critical point means that 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.e moving it nearer to a true Carnot cycle operating at the same temperatures. But this locks the design to having the compressor inlet conditions of ~70 bar, 32°C, and having a cooling water supply/heat exchanger that can take a lot of heat out at not far above 30°C. This in turn requires very high pressures at the hot end of the cycle, and reliable access to cold water. It's great for Northern Europe, Canada, Russia, maybe the New England states. For the South-western US, helium (or He-N2) Brayton cycles are less likely to be knocked out by cooling water shortages in hot summers, ie just when the power is most needed.


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PostPosted: Jan 31, 2010 12:59 am 
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Thank you, for the first concise description I've seen yet for why a supercrticial CO2 cycle is interesting.

Best post I've seen in a while.


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PostPosted: Jan 31, 2010 5:19 am 
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Luke wrote:
The strength of the S-CO2 cycle is also its weakness. By compressing close to the critical point, the compression requires less work than an ideal gas would under the same conditions, and the changing heat capacity of CO2 near the critical point means that 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.e moving it nearer to a true Carnot cycle operating at the same temperatures. But this locks the design to having the compressor inlet conditions of ~70 bar, 32°C, and having a cooling water supply/heat exchanger that can take a lot of heat out at not far above 30°C. This in turn requires very high pressures at the hot end of the cycle, and reliable access to cold water. It's great for Northern Europe, Canada, Russia, maybe the New England states. For the South-western US, helium (or He-N2) Brayton cycles are less likely to be knocked out by cooling water shortages in hot summers, ie just when the power is most needed.


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

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


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PostPosted: Jan 31, 2010 4:59 pm 
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Can you give some examples of this?
My impression is rather different (perhaps skewed by the general water shortage in southern california).
Access to water is of high value to many people. The fight over water shut down farms all through the central valley this past summer.
The proposed nuclear power plant in Fresno (California) proposed to collocate with a waste water treatment facility in order to get access to less precious water. It is my impression that the utilities commission here really wants the power plants to switch to air cooling.


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PostPosted: Jan 31, 2010 5:53 pm 
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Lars wrote:
Can you give some examples of this?
My impression is rather different (perhaps skewed by the general water shortage in southern california).
Access to water is of high value to many people. The fight over water shut down farms all through the central valley this past summer.
The proposed nuclear power plant in Fresno (California) proposed to collocate with a waste water treatment facility in order to get access to less precious water. It is my impression that the utilities commission here really wants the power plants to switch to air 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 standard evaporative wet cooling system. Which is itself very low. So, the more expensive water desalination 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 lower grade water. Geothermal plants can do this too, I think it's the Geysers that disposes of waste 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 expensive desalination 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 stations to dry cooling, if we don't try harder in the agricultural department it won't help much for 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) a large amount of money in mandating dry cooling, when the same amount of money can save so much more in other water consumption. Agriculture mainly, but household water consumption is also large compared to the water consumption required for wet cooling the amount of electricity for one household. That said I think we need to spend some more in advancing dry cooling, there are interesting developments in low temperature foam based cooling systems for example. There are some extra advantages related to siting with a dry cooling system. Then again I wonder if the levelised cost wouldn't be lower if we simply pipe in desalinated seawater for a recirculating cooling system. It's probably a good idea to increase such sustainable water infrastructure in places like Southern Calif anyway.

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

IMHO there's plenty more examples of stupid policy on cooling water use. Look at France, building so many nuclear powerplants next to rivers of insufficient flow rate when they should be building more on the coast in stead. They should be building only once through seawater cooling with diffusor pipe systems (or recirculating wet cooling with towers in case of sensitive local ecosystem concerns).


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PostPosted: Feb 01, 2010 1:31 am 
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Just read over the wikipedia article (http://en.wikipedia.org/wiki/Cooling_tower) - lots of alternatives in the cooling.
Though it looks like cooling towers have been more dangerous to the public (excluding Chernobyl) that the nuclear power plants.


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

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


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PostPosted: Feb 01, 2010 5:16 am 
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That's a good wiki article. My favorite system is the Heller indirect dry cooling, which cools the working fluid (or gas) with evaporative wet cooling but then has another dry cooling loop to throw the heat out into the air. It is more efficient than air cooled condensers (direct fan cooling). It also solves the blowdown and freezing issues better than standard recirculating wet cooling towers. There are hybrids also which use some water for emergency hot days, but very little or none during normal temperatures. This gets a high performance system while saving water at very low cost. The only downside is that cooling towers are considered ugly.


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PostPosted: Feb 02, 2010 1:02 am 
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Lars,

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

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

-Iain


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