Energy From Thorium Discussion Forum

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PostPosted: Dec 31, 2010 12:36 am 
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Infinia sells 4.5m diameter solar dishes with a 3.2Kw free-piston Stirling generator.
The generator takes 2200F (1200C) heat and cool air (5 to 55C),
and makes 50 or 60Hz power at 220 or 240VAC.

The combined heat-engine and generator has two moving parts, and operates for 25 years without service.
(They've actually operated one that long; The same company developed them for radioisotope generators, and other applications. The bearings are flexures, metal disks. No lubricants, no exotic materials. The working fluid is Helium, and the generator is hermetically sealed. It looks like a very optimized Harwell thermoacoustic generator.)

I figured the efficiency as 23% (Mojave sunlight at 896W/m2), low, I guess, but nuclear heat is cheap: needing 4GWth only doubles the thorium use to 1.6 tonnes/GW-year).

So, this is 312,000 generators/GWe. At a quarter of a cubic meter each, they would fit in a 50m cube, a "generator hall." I think you could drive them from a secondary salt loop, and get outside air to them with fans. And that's it. You're done. There's no water use.

They won't talk about the $/W, but the generator is about 5% of the dish by weight, and the dish prototype project cost about $33/W, so the generator is about $1.65/W prototyped, so it should be well below $0.87/W mass-produced. (Lousy estimate, but... real as far as it goes. It helps that there's no exotic stuff in the generator.)

So, it's a bargain, already comparable to real steam plants at $1.80/W, with lots of room to go down.

And, it should be remarkably more reliable. If 4% fail every year, with a 6 month service cycle the power conversion equipment is going to be >98% up all the time.

I think the generator could bolt to a standard mount. If it takes a half man day to replace one, that's about 17 guys full time per GWe. Automated replacement looks really possible: Give it a plug like a power meter, with a single bolt and a couple of valves, and handling equipment mounted on a robot like the ones that rack boxes in automated warehouses.


Last edited by rgvandewalker on Dec 31, 2010 7:52 pm, edited 2 times in total.

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PostPosted: Dec 31, 2010 1:30 am 
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rgvandewalker wrote:
They won't talk about the $/W, but the generator is about 5% of the dish by weight, and the dish prototype project cost about $33/W, so the generator is about $1.65/W prototyped, so it should be well below $0.87/W mass-produced.


I suspect that the $/kg for the generator is substantially higher than the $/kg for the reflector dish. The generator could be half the cost of these units, even if it is only a small fraction of the total mass. The efficiency is also terrible - with a 1200°C source the efficiency should be at least 50% and 60+% is possible. Thorium may be cheap, but it's a lot harder to pay off the costs of building and running a high temperature reactor when you are throwing 3/4 of the heat away.


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PostPosted: Dec 31, 2010 1:31 am 
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A thorium molten salt reactor with an outlet temperature of 1200C; I would love to see one in the worst way…but we won’t see that for 200 years if ever!!!

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PostPosted: Dec 31, 2010 2:03 am 
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Yeah, it does double the cost of the kernel, but as so many have pointed out, that's a small part of the system cost. This scheme eliminates water use so any site is suitable, and mass-produces all the expensive heat engines and low temperature heat exchangers.

It scales for any size reactor, too. Did you notice?

The temperature: Can't we use TZM? 1300C, Corrosion resistant. http://www.hcrosscompany.com/refractory/molybdenum.htm

The generator is manufactured with "standard automotive techniques" and it's about the size and complexity of an automotive air conditioning compressor. So, maybe $1,000 full-up with absurd retail profits. That would put it at $0.32/W.

Another way of coming at it is that mass produced stuff from ordinary materials costs about $10/lb ($20/Kg), so to get to $1/W, the engine only has to get to 0.013 Hp/Lb (0.02 Kw/Kg). Marine diesels for container ships (the heaviest engine, ever) get 0.02 Hp/Lb (0.03 Kw/Kg), so this should be very possible.
http://en.wikipedia.org/wiki/Power-to-weight_ratio

These ratios are never seen in big turbo machinery, because it can't get economies of scale, but this scheme absolutely can.

I'm going to stick by my estimates. This is a real-life antithesis to big, expensive, impossible-to-develop heat engines, right?


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PostPosted: Jun 25, 2013 7:40 pm 
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Stirling engines work very well for solar thermal, when you have a laser-like focus of IR radiation on the hot side and limitless atmospheric air on the cold side.
The problem with them, especially for high power applications when the engine is being used as a heat tranferer between 2 fluid loops is that the hot and cold ends are very close together, so heat leaking around the engine is unavoidable, even in an alpha engine where there are separate hot and cold cylinders, most of the gas has to move between them, so the pipe connecting them can't be very long.

There is also the Ericsson engine which has a few extra valves and pipes, but has a hot expansion chamber and cylinder separate from a tank where cool gas is stored after being pressurized at the other end of the cylinder. It powered a ship once and seems to me, at least, to be much better for large-scale applications than the stirling. The Ericsson cycle is also the limit for the Brayton cycle, which Ericsson actually invented.
http://en.wikipedia.org/wiki/Ericsson_cycle

A modern version might be big and expensive, but it was developed in the 1850s, so it can't be hard to engineer with modern meterials.


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PostPosted: Jun 28, 2013 3:29 pm 
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J. Perry, excellent reasoning, but the whole point of this proposal is to deprioritize heat-engine efficiency in order to reduce other costs. If we prioritize thermal efficiency, the logical proposal (as described elsewhere) is a large multistage intercooled helium turbine. The result is billion-dollar development costs for the first plant, and such bad numbers result that no entrepreneur could ever get funding for the second plant.

The proposal is to deploy cheap, available heat engines. The whole point of the cute little stirlings is that you can buy them now. If we need a custom stirling, developing it is a couple of million dollars at most. The Ericson engine has now existing vendors, or at least they're not on the web.


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PostPosted: Jun 29, 2013 5:29 am 
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rgvandewalker wrote:
J. Perry, excellent reasoning, but the whole point of this proposal is to deprioritize heat-engine efficiency in order to reduce other costs. If we prioritize thermal efficiency, the logical proposal (as described elsewhere) is a large multistage intercooled helium turbine. The result is billion-dollar development costs for the first plant, and such bad numbers result that no entrepreneur could ever get funding for the second plant.

The proposal is to deploy cheap, available heat engines. The whole point of the cute little stirlings is that you can buy them now. If we need a custom stirling, developing it is a couple of million dollars at most. The Ericson engine has now existing vendors, or at least they're not on the web.


As was pointed out before, thermal efficiency has a big influence on total power cost. This is because thermal efficiency affects almost every part of the plant cost, so even if the power cycle is more expensive, it reduces the size (cost) of most other components per watt-electric, even down to building cost, foundations etc as you get more power out of the same size thermal equipment (all things being equal).

Free piston Stirling cycles are awesome, but they don't scale up well to LFTR sizes (need >10 MWe units). Hundreds or even thousands of little Stirling engines will cost a lot more than one or two steam turbines, it doesn't take a scientist to see this. Supercritical or superheated Rankine cycles scale much better, having up to around 47% net efficiency in state of the art modern machines, the technology is available commercially, and the required temperature is lower allowing bigger temperature drops across heat exchangers (smaller heat exchangers).


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PostPosted: Aug 11, 2013 7:01 pm 
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Cyril, very logical, but I already made realistic estimates of the cost, see above. It's comparable or less. Steam is an old technology. Westinghouse turbines are massive, precisely balanced precision-machined objects. The Stirling generators are much cheaper per pound, stamped from sheet-metal, and this, combined with cheap nuclear heat, overcomes their efficiency disadvantage.

I did not cost the generator hall. I think it could be comparable to parkign structures, which are about $30/square foot.


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PostPosted: Aug 12, 2013 3:40 am 
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Stirling engines are absolutely precision machinery. More so than steam turbines. For one thing, the working gas for Stirlings is usually a light gas such as helium or hydrogen. That is difficult to contain and requires very precise machining in things like bearings, cylinder bores/piston mating, etc.

It is basically a question of the economics of mass manufacturing versus the economics of better efficiency and larger scale.

By the way, what's the efficiency of these Stirling engines? At 650 C not at 1200 mind you (1200 would require >1200 salt which is not feasible as Axil noted). If it's 23% @ 1200 C Thot then that's bad news for 650 C. I'm wildly guessing you'd be around half that at 650C, 12%. That alone makes it a loser.


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PostPosted: Aug 12, 2013 10:07 am 
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I would imagine the larger size of modern plants wins out.

Just like I know of only one commercial PWR that has more than one turbine shaft (Sizewell B) and that was the result of there being no British manufacturers for ~1000MWe steam turbine sets at the time. Using smaller sets allowed the use of designs derived from ~500MWe CEGB models.


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PostPosted: Aug 12, 2013 9:40 pm 
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E Ireland wrote:
I would imagine the larger size of modern plants wins out.

Just like I know of only one commercial PWR that has more than one turbine shaft (Sizewell B) and that was the result of there being no British manufacturers for ~1000MWe steam turbine sets at the time. Using smaller sets allowed the use of designs derived from ~500MWe CEGB models.

The field is evolving all the time, once upon a time you couldn't get 1000 MW fossil plant on one shaft, now you can, and Siemens is of the view that 1200 MWe as a fossil fuelled turbine can be done a tandem compound (one shaft).

Large steam turbines operating at high temperature can be pretty efficient and the smaller ones aren't too far behind. Stirling Engines may have applications in small scale, but on a total $/kW installed basis for the overall MSR NPP, I struggle to see how Stirling engines could be cheaper or produce cheaper power and especially not if the efficiency is 23% or less.

If the required power is kW Stirling engines may win through. What $/kWe and real-world efficiency values are we talking about? Those are the key parameters that will drive the PCU selection for civilian power generation.


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PostPosted: Mar 28, 2014 9:49 pm 
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Guys, there's no doubt that you're right about the safe, sane approach. I just wanted to point out an unsafe insane approach that seemed to have half the cost, given really cheap nuclear heat.


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PostPosted: Mar 29, 2014 7:32 am 
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But, why wouldn't you just buy an off the shelf GE gas turbine and run an open air cycle heated by the secondary salt of a LF-MSR cooled reactor. Look at some of Dr Forsberg's (MIT) presentations at conferences. An off the shelf gas turbine seems very simple, current technology, does not NEED water, for 40-45% efficiency, but if you put a bottoming steam cycle on you would get 60% efficiency and only 40% waste heat (i.e. water usage), vice 67% for conventional LWR reactors.

Ignore the fact that Forsberg is collaborating with a solid fueled MSR, vice a liquid fueled one. That is irrelevant to the BOP, except that you need that secondary loop for the LF and not for the SF.


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PostPosted: Mar 29, 2014 10:01 am 
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The main problem with that approach would be that tritium would be released to the environment at levels that would be unacceptable from a licensing perspective.


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PostPosted: Mar 29, 2014 4:53 pm 
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Kirk Sorensen wrote:
The main problem with that approach would be that tritium would be released to the environment at levels that would be unacceptable from a licensing perspective.


Interestingly, Dr. Peterson's group on the PB-FHR mk1 seems to be convinced that some barrier coatings, combined with sorption on graphite in the fluoride salt, will be sufficient.


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