Net Energy and Waste Heat Recovery

[Henning]

Brian H wrote:

…
Not even a turbine and generator.
…

Turbine? Generator? Nosuch nonesuch present. That’s kind of the whole point. No steam or spinnin’ magnets.

You’re still on the mode, that direct conversion is enough. That the efficency of induction and photovoltaics is good enough to get out more than is put in. It won’t. At least in the beginning we need a steam cycle (helium in primary cycle, water or anything else in the secondary). In ten, twenty years MAYBE we’re good enough. But that’s a big maybe, which puts more “maybe” on, than with FF being a big maybe in itself.

And then there’s still excess heat, even with photovoltaics working close to optimum.

Read some discussions with Rematog. Just what he writes, ignore the others (you, me, anybody). Here is one discussion with him: https://focusfusion.org/index.php/forums/viewthread/283/

@redsnapper:
A steam cycle is required (with helium as primary coolant), and the number of a constant 300Hz operation (because of the 60Hz grid) you read in the forums is also bogus. A beam only produces energy for a few microseconds. A capacitor or coil to smoothen out the peaks is required anyway. More likely the frequency will even vary within that 50Hz/60Hz output cycle, more shots during the maximum/minimum of the grid cycle, less when it’s close to zero.

[Henning]

redsnapper wrote: Henning,
Is there any info available on the size and composition of the X-ray converter? I saw that there was a patent issued, but my experience with patents is they’re unlikely to give you that kind of detail. (They’re supposed to be easily understood by someone “skilled in the art,” but that implies that they don’t tell you anything they absolutely don’t have to, presuming that you already know and/or are skilled enough to guess what they don’t say. That probably doesn’t cover me, in this context. :-)) I did actually take the link to the patent and I scanned the opening page – a couple of weeks ago – but really didn’t try to digest any of the content at the time.
Also, mjv1121 answered that the energy distribution between the two ion beams might only be determined after tests have gone to the next level – but I can hardy believe that there aren’t already some pretty concrete expectations – and there should be some fairly fundamental physics dictating the gross effects. (I took a plasma-physics course in grad school 33 years ago, and I hate to admit how much I’ve forgotten – though I’d feel worse if it was something I’d actually found a use for in the meantime!) In fact, I’d assume that if a Rogowski coil can be applied to the He nuclei, it could also be applied to the electron beam. If the electrons only emit 80% of their energy in X-rays, surely that remaining 20% is well-ordered kinetic energy worth going after with another Rogowski coil? The more energy you can get back directly as electricity, the better. Heck, because they’re moving electrons, they’re already electrcity (much more so than the He ions). Is there an even simpler and obvious way to funnel them into a conductor? Maybe that’s already part of the design – so self-evident that nobody’s mentioned it in one of these posts?

My interpretation of the x-ray photovoltaics:
The x-ray converter is a layer of thousands of conducting foils separated by non-conducting foils. Inner foils are made of beryllium, outer foils are made of metal of larger Z (proton count). Also the outer layers are thicker than their inner counterparts. An x-ray photon should pass several layers depending on its energy, and realease its energy as it passes its final layer. Together with the innermost layer it builds up a electrical potential. It’s more or less a capacitor (as I understand), with energy put in by photons. Anyone else to comment?

The best picture we have is the rendering by Torulf. That’s the logo on the Focus-Fusion Facebook-Group, or that rendering in LPP’s web-site: http://lawrencevilleplasmaphysics.com/

If you make the electrodes of about 10-20cm long, the whole shebang might have a diameter of 50cm, plus induction coils.

With the energy of the electron ray: I remember reading somewhere that the electron ray carries less than 1% of the energy. But I might be wrong here.

Currently the electron ray is lost on the anode. Maybe it’s possible to collect the electron energy with a third electrode within the anode’s hole — but I suspect it can’t be insulated enough from the anode, so a electrode that is close to mass would drain all of the anode.

[Brian H]

Henning wrote:

…
Not even a turbine and generator.
…

Turbine? Generator? Nosuch nonesuch present. That’s kind of the whole point. No steam or spinnin’ magnets.

You’re still on the mode, that direct conversion is enough. That the efficency [efficiency] of induction and photovoltaics is good enough to get out more than is put in. It won’t.
…

Well, that’s the basic LPP claim, and I’m sticking with it till I see good reason not to. The cost of steam-cycle generation is many times higher than direct conversion, so if direct conversion has a Q>1, it is impossible to justify spending a cent on Carnot cycle stuff.

IMO.

[redsnapper]

I don’t understand where you’d put a He cooling cycle, anyway? If the best picture of the reactor core (including X-ray converter and Rogowski coil) is what’s on the LPP page, then my initial concerns about cooling the core stand. It’s not that big! There’s no way to remove that kind of thermal energy from the inside (because that’s where the plasma is). But if you wait till the heat gets to the outside of the onion, you don’t need an X-ray transperent cooling fluid, because the X-ray’s are already gone; therefore you may as well use water impingement (something cheaper than He, I presume). In any event, either you can cool it adequately or you can’t, and the decision of whether to actually use that waste heat is an economic one. (Except if Henning’s fears are realized, the economics require it, because otherwise you can’t break even.) I’d certainly prefer to remain optimistic, however. 🙂

[Henning]

That x-ray transparent fluid (helium gas, I don’t know at which pressure it gets fluid, but I suspect the pressure has to be extremely high at 1000K) is required for cooling the electrodes (canals within the electrodes), and also the onion (also with canals).

Maybe also lithium would be possible: https://focusfusion.org/index.php/forums/viewthread/457/#4213

The patent has a drawing of the onion with canals in it for cooling (figure 13). Look for a reference in the text to figure 13.

[zapkitty]

Henning wrote: That x-ray transparent fluid (helium gas, I don’t know at which pressure it gets fluid

It doesn’t, not even at the multi-megapascal pressures that were researched for certain other fission and fusion reactors. “Fluid” in this usage is a generic term the same as “working fluid”… the He will still be gaseous

I think those extreme He pressures required for other, more widely known applications were why Lerner-hakase specifically pointed out that in an FF the He pressure would only be a few bar.

[rashidas]

Waste Heat Recovery: Has the issue of waste heat been addressed adequately in the design of a working aneutronic reactor? So much energy being produced in a very small space will make waste heat a significant issue so I believe this question should be addressed now. There are many potential uses for this heat, including food processing, space heating, horticulture and aquaculture as examples.

[Brian H]

rashidas wrote: Waste Heat Recovery: Has the issue of waste heat been addressed adequately in the design of a working aneutronic reactor? So much energy being produced in a very small space will make waste heat a significant issue so I believe this question should be addressed now. There are many potential uses for this heat, including food processing, space heating, horticulture and aquaculture as examples.

Lots of discussion, but the basic problem is just dumping it fast enough. Low-grade uses might be economic, but any significant equipment expenditure to “exploit” the heat immediately runs up against competition with just using ultra-cheap electricity from the FF to do the same thing.

Perhaps warm air pumped/fed to local buildings (local space heating) etc. might pay, but anything much more elaborate is iffy. Horticulture and aquaculture are possibilities. But remember, if they can use electricity at ¼-½¢/kwh more economically direct from an FF generator, that’s what they will and should do.

[Aeronaut]

Based on googling “how are molding machines cooled?”, you’ll find an entire industry dedicated to throwing away excess heat as quickly as possible. I submit that we have a much larger cooling system budget than we’ve been thinking, and that plant owners will even expect to see that type of auxiliary equipment near their DPF generators.

[Brian H]

Check the total efficiency, fuel therms consumed to therms wasted. I think you’ll find that FF is throwing away far less heat than any competitive energy gen method per MW generated, from shovel or well to output.

The problem is the SIZE of the FF rig: it’s too small! So the temp rises fast. I put it to you that the real problem is high electrode temperatures, not total heat disposal. Once extracted from the core, the only heat-handling equipment that will pay its way is about the level of ducts and fans. Maybe hypothetical hi-efficiency thermocouples, if they’re cheap and durable enough.

[zapkitty]

Brian H wrote: Check the total efficiency, fuel therms consumed to therms wasted. I think you’ll find that FF is throwing away far less heat than any competitive energy gen method per MW generated, from shovel or well to output.

Topical example: Each of the units at the afflicted Fukushima plant had to reject 3 times as many megawatts worth of heat as they produced of electricity. 33%.

Coal and oil fired plants on average range from 30% to 49%.

Gas turbine plants average about 50% with very expensive heat recovery steam generators (HRSG) in their exhaust systems boosting that to 60% in one plant in Wales at the current highest level.

If an FF unit can produce only twice as much heat as it does electricity, 50%, without needing to resort to steam, turbines, or HRSG then it has all major players beat hands down and walking away on cost vs. thermal efficiency as well as cost per MW… and global adoption would actually lower the current total heat output of power stations worldwide.

And of course the heat produced by all power stations combined is irrelevant to the warming caused by their CO2 emissions.

Brian H wrote: The problem is the SIZE of the FF rig: it’s too small! So the temp rises fast. I put it to you that the real problem is high electrode temperatures, not total heat disposal. Once extracted from the core, the only heat-handling equipment that will pay its way is about the level of ducts and fans. Maybe hypothetical hi-efficiency thermocouples, if they’re cheap and durable enough.

Yes, once boron fusion is demonstrated the first issue in designing a practical power generator will not be the ion coil(s) or even the x-ray conversion “onion”… it will be to cool the core. How much He is needed at what pressure in what kind of internal electrode design?

I look forward to it 🙂

[redsnapper]

(My apologies if this is a sort-of duplicate – something weird happened when I tried to do the spell check!)

Well, yes, the total efficiency theoretically is very competitive, but let’s face it, until there’s a working core, and a working X-ray converter, and a working Rogowski coil, all playing together with a humongous capacitor bank and switches, that efficiency remains theoretical. Given that the efficiency numbers pan out, I agree that the problem is the (small) size of the FF rig. I’m not quite sure what you mean by the high electrode temperature being a problem (don’t confuse heat with temperature) – indeed, the theoretical Carnot efficiency of a heat engine will only improve as the high-side temperature goes up, so in that sense, the higher the electrode temperature, the better. (Part of the reason the theoretical efficiency of the FF device is so high is precisely because its energy originates at 1Billion degC – somewhat higher than the hottest fossil-fuel fired power plant. :-)) Further, the higher the permissible electrode temperature, the more feasible it becomes to remove that excess heat using real materials between the electrode and the outside environment. Remember, the waste heat isn’t generated because the electrodes are hot, the electrodes are hot because they’re adjacent to a billion-degree plasma, and because there’s a phenomenal amount of energy passing through them.

As for high efficiency thermocouples (or any other device depending on the thermoelectric effect), I wouldn’t hold my breath. Current state of the art is less than 10% efficiency. My guess is that once you’ve got a working reactor, you’ll get a far better bang-for-the-buck by incremental increases in X-ray conversion, or the Rogowski coil efficiency, or incremental improvements in the electrodes and FF device itself such that more energy comes out in the ion beam or Xrays. One day there will probably be a breakthrough design for the electrodes themselves (for example: highly-conductive, Xray-transparent, ceramic electrodes that can handle 2000 degC; better yet, virtual electrodes that can handle 500MdegC). Scavenging waste heat (excluding the obviously direct applications of waste heat, e.g. heating a hotel or a swimming pool) probably adds cost faster than saleable energy output. As you point out, FF already promises to be a very efficient process by fossil-fuel standards. (BTW – we have trouble building conventional photovoltaic devices with efficiencies higher than 25% – and we’ve been working at this, perhaps only semi-seriously, for the last 40 years. It does seem to me we might be ridiculously optmistic to think we can convert 80% of Xrays into electricity as early as three years from now. Is the argument that the entropy of Xrays is so much lower than that of visible light, that there’s reason for optimism?)

I think it’s going to come down to permissible “waste heat density” – so to speak. If a breadbox-sized 5MW-net-output DPF core simply can’t be maintained (or can’t simply be maintained) cool enough for the electrodes to survive, just divide that 5MW across however many cores you can keep cool. Or if the core is already far-and-away the most expensive fraction of the cost of the powerplant (it might not be, once you’ve factored in the cost of the “minimal” cooling system ), just scale down the power (cycles/second of operation). Yes that raises the cost/MW, but I assume that the afore-mentioned incremental improvements in efficiency will eventually allow you to run the power back up. You were already so far ahead of the conventional power cost that you can afford to miss the original target by a substantial margin. So what if FF1.0 can only generate enough power for 200 houses instead of 2000? Somewhere in the world, that’s just the right size. Or a yacht instead of a cruise ship? There’ll be plenty of market for fusion power, in whatever increment is available at any given time. It’ll only improve with FF2.0.

[Henning]

redsnapper wrote: electrodes themselves (for example: highly-conductive, Xray-transparent, ceramic electrodes that can handle 2000 degC; better yet, virtual electrodes that can handle 500MdegC)

Read: quite-conductive (36 nΩ·m, compared to 16.78 nΩ·m of copper at 20 °C), x-ray-transparent, beryllium electrodes that can handle 2400 °C

The electrodes never get into contact with the 500 M°C (actually 2 G°C) pinch.

[Henning]

redsnapper wrote: BTW – we have trouble building conventional photovoltaic devices with efficiencies higher than 25% – and we’ve been working at this, perhaps only semi-seriously, for the last 40 years. It does seem to me we might be ridiculously optmistic to think we can convert 80% of Xrays into electricity as early as three years from now. Is the argument that the entropy of Xrays is so much lower than that of visible light, that there’s reason for optimism

The problem with visible light is, it comes with low energy that needs to push up the electron one valence layer. See band gap for more details.

The x-ray photovoltaic works differently: it uses a potential gap (if I understood it correctly) of the several layers of the onion.

Might not explain it completely. Maybe someone else has better words for that…

[redsnapper]

Henning wrote:

electrodes themselves (for example: highly-conductive, Xray-transparent, ceramic electrodes that can handle 2000 degC; better yet, virtual electrodes that can handle 500MdegC)

Read: quite-conductive (36 nΩ·m, compared to 16.78 nΩ·m of copper at 20 °C), x-ray-transparent, beryllium electrodes that can handle 2400 °C

The electrodes never get into contact with the 500 M°C (actually 2 G°C) pinch.

I think you missed my point. I was hypothesizing an improved electrode. Beryllium melts at 1278degC, and what I’ve been reading about FF is that electrode temperature would be limited to 700degC – perhaps somewhat arbitrarily. Seems like I remember the words “electrode erosion” or something like that – I’m sure that the higher the electrode temperature, the quicker it sublimates into the plasma – however hot the plasma is that surrounds the electrodes. (Resistivity tends to go up with temperature, as well, though I don’t know the specifics of Be. That could be a design factor, too.) Still, a limit is a limit, which if raised, tends to open new opportunities. So I was hypothesizing that some future ceramic (ceramics typically handle much higher temperatures than metals) might be developed that had the resistivity and Xray transparency of Be, but because of the higher temperature allowed, would relax the cooling challenge. This is called science fiction, or literary license – take your pick. 🙂 You do remember Scotty’s amazing exposition of transparent aluminum in the “save the whales” Star Trek movie?

And you’re right. Any physical surface representing the cooling boundaries (walls) never sees even the bulk fluid temperatures (using the word fluid here very fluidly – meaning plasma), because there must be a boundary layer gradient if there is indeed any heat transfer to the wall; further, I understand that the highest temperatures are within the plasmoids, which are microscopic and deep within the inner electrode. That’s why I was guessing that the plasma actually contacting the walls would be much cooler than the plasmoid temperatures. 500M vs. 1G or 2G seemed like an illustrative swag. But the bulk plasma is at such a low density that the heat transfer coefficient to the walls is such that the walls might indeed be as cool as 700degC, or 2400degC, or whatever – a few orders of magnitude lower than the “bulk” plasma temperature. Of course, my basic understanding of this system is feeble. Perhaps the “bulk” plasma temperature is only 700degC. After all, suppose a single shot (capacitor discharge) lasts only a microsecond – that’s plenty of time to take a few micrograms of localized plasmoid from 700degC up to 2GdegC – and if time-to-next-shot is a few milliseconds, there’s plenty of time for the residual plasma (not counting the hot ions that shot out the end of the core into the Rogowski coil) to begin to equilibrate and smooth out the temperature to a much-lower average. I assume also that only the teensiest fraction of the plasma actually fuses, so virtually all the plasma that’s heated during the pre-ignition phase remains at the required temperature until the plasmoid dissipates. I think if I saw quantitative simulation data of plasma temperature profiles in the DPF core I might quit making such terrible assumptions. Animated color CGI of the simulation is great, but it doesn’t show what I need to know to avoid gross error.

[Author]

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