Net Energy and Waste Heat Recovery

[mjv1121]

Hello redsnapper,
I don’t think you’ve quite understood where the “heat” is.

The capacitor bank puts electrical energy in – 100% in, lets call it 100x in
The plasmoid thru various atomic and plasma processes converts generates extra energy from the fuel and the amount of energy increases to 180x (perhaps over 200x).
The plasmoid releases this energy in 2 main ways:
1) The ion beam away from the electrodes, this is going to be quite focused. Because the beam is charged particles in motion, it is effectively already electricity. Using a special induction coil transformer thingy the ion beam will be converted into “usable” electricity. Hopefully this can be done at 80% efficiency. So the ion beam removes an energy of 90x from the plasmoid and gives us 72x in electricity and 18x in waste heat.
2) The other side of the plasmoid produces an electron beam. These electrons lose most of their energy by emitting x-rays. The x-rays are then converted directly to electricity by the “photo-electric onion” thingy. So the electron beam removes an energy of 90x from the plasmoid via x-rays and gives us 72x in electricity and 18x in waste heat.

So we have 144x in electricity and 36x in waste heat. The 144x electric will be split 100x back into the capacitor bank for the next pulse and 44x electric. The stated target was to generate 5MW electric, so the 44x equates to 5MW. So the 36x equates to about 4MW – of course thermal and electric are not quite the same thing, but the ion beam and x-rays are not “heat” either. This is the main advantage – fusion is almost a smokescreen to the real beauty of the device, and that is, generating electricity without using a thermal system.

So there’s going to be 2MW of megawatts of heat in the ion beam transformer and another 2MW in the x-ray onion. Although most of the energy from the plasmoid is removed by the ion beam and the electron x-rays there’s going to be losses and possibly side reactions, enough to heat the central electrode to 1000K. Some heat is needed to maintain the fuel as a gas, but the big problem is keeping the electrode cool. Unfortunately I haven’t been able to glean any detail on thermal heat numbers other than its going to be made of beryllium, approximately 25mm in diameter and 150mm long and will be at about 700C.

My thoughts with the thermo-electrics was to try to raise the overall efficiency of the machine with an additional direct-to-electricity method. With a total “waste” of about 5MW, it seemed like a good idea to try to re-coup say 500kW. It would ease the pressure on the ion beam transformer and x-ray onion design teams or reduce the pulse rate and prolong the wear-and-tear.

[AaronB]

MJV and redsnapper both have some well-thought-out points. We already know that cooling will be our major limiting factor once things get going. Overcoming that challenge will be Phase 2 in this project. There will be a lot of “waste” heat that can be used for other applications. For example, a big hotel could have two generators (for redundancy during maintenance), and the waste heat can provide space and water heating, or be used for air conditioning (I know that sounds crazy, but it’s true.) The same could be said for a cruise ship or university campus. Anyway, when we hit Phase 2, we’ll have to hire a bunch of scientists and engineers to figure this out and make it work. The ideas you come up with now will certainly help them get off to a good start.

[redsnapper]

mjv,

Thanks for the helpful post! It helps me tremendously – and even after my post Friday I realized there were huge holes in my grasp of where the heat actually appears. For this post, then, for the sake of argument I’ll accept that there isn’t a significant thermal load on the outer electrode, and I’ll temporarily set aside the questions of cooling the inner electrode, which won’t be a trivial problem, but perhaps is still not the biggest concern. With your correction, I think this is where it leaves me:
4MW thermal instead of 5MW (same ballpark)
of that, 2MW appears in the onion skin X-ray converter (somewhat bigger than a breadbox, I presume, since a breadbox sits inside it)
and 2MW appears as heat in the walls/wires/whatever of the nucleon decelerator (also bigger than a breadbox, I presume, but this structure remains totally vague in my mind)

Basically, we’ve spread the heat over two ballparks instead of just one. So instead of being 20x over the thermal budget following my original misguided assumption that the sizes of things were immediately related to the DPF core itself, we’re what – 5x over? 2x over? The Be core might actually have handled higher temperatures than these peripherals, too, so maybe the heat is being generated in somewhat larger volumes, but can the X-ray converter handle 1000K? Can the nucleon decelerator? The other crucial questions, of course, are exactly how large are these peripherals? Because we’ve still got a tremendous heat load in a relatively confined space. Until someone can give me some hints as to these size scales, there’s not a whole lot else to do at this stage for the local cooling challenge, other that recognize it’s still not a slam dunk.

Actually, there are a couple of other “sanity check” calculations I’d appreciate somebody following for me and commenting, because I’d like to be sure that I’m not missing something else. Following mjv’s description of the overall energy balance, then for a 5MW machine operating at 300Hz, each capacitor pulse must deliver 38kJ of energy. If the pB reaction adds 80% of that (30kJ), we then have (38+30)/2, or 34kJ exiting the core in two equal ion beams. (BTW, off the top of my head I don’t remember my basic plasma physics well enough, but would we expect equal energy between the two beams, or equal momentum? Or something in between? I guess the ion and electron temperatures are different, but surely they don’t scale in inverse proportion to the ion mass, do they? The He nuclei outweigh the electrons by 3600:1 – 1 He nucleus at 4*1800 for 2 electrons – surely the electron temperature isn’t 3600 that of the He? Regardless, the total’s all that matters for the present calculation.) If we extract 80% from the two beams (X-ray capture for the electrons, electric-field decelerator for the nucleons – again it doesn’t matter for this calculation whether it’s equally divided), that’s 55kJ back in electricity, of which 38kJ goes back to the caps for the next pulse, net 17kJ useful electricity. The 20% waste heat, if equally divided between the X-ray capture and the beam decelerator, is 6.8kJ each peripheral. If you do this 300 times every second, we have net output of 17kJ*300=5MJ/s (i.e., 5MW electric), and 6.8kJ*300=2MJ/s waste heat in each peripheral, with 11.4MW recirculating in and out of the caps.

I understand that the prototype device (i.e. the capacitor bank) will be delivering something on the order of 2MA of current, and the voltage is 45kV. If we assumed square pulses and a full capacitor discharge each cycle, that would suggest pulse power of 90GW (shades of Doc Emmett Brown in Back to the Future!) – and the pulse length would be 420ns. (I suppose the return pulse is spread out somewhat, but it’s got 3ms available at 300Hz.) If the caps don’t discharge fully each cycle, you’ve still got to get 38kJ, so the pulse gets wider accordingly. There are also bound to be some conduction losses in getting 2MA through the wiring, but I’m not sure where to start – I’ll assume they’re small compared to the overall energy balance. But for example, suppose you lost only 100V of the 45kV in conduction – that gives you 50kW RMS power loss in the wiring (between the ignition and return pulses). That’s a minor cooling problem in its own right.

So if somebody (mjv? AaronB?) would comment on the foregoing, I’d appreciate it. In the mean time, I’m going to work on a separate post regarding the macro-cooling problem at the “environment” end of the reactor. Because although the details of the DPF core itself may be somewhat vague at this point, you’ve still got to dissipate that 4MW to the environment, and that’s my other major concern. (Ironically, I actually spent an hour writing a second post on this subject Friday night, then went to preview the post, made a booboo of opening another tab in IE, went back to the original tab, thinking I was still on the new tab, and lost everything! The “back” button took me to an *empty* reply window, and I couldn’t find any way to get back to my original text! Argh! If I’m smart, I’ll do all my creative writing in another text editor first, and then paste it in here. Webmaster, is there any hope of avoiding this problem otherwise?!)

[zapkitty]

Do note that the exhaust temp of 200 C for the cooling system was for a proposed portable 2 MWe FF unit housed in a shipping container. That’s an entirely different setup than the classical 5 MWe power unit you are using for your examples.

200 C was appropriate for the lower output unit and enabled a generator that could be placed almost anywhere. A fixed installation such as the 5 MWe station “the size of a two-car garage” would have a higher upper limit on its exhaust… temps in excess of 700 C 🙂

And in the “garage” style station the cooling system need not be crammed into the same shipping container as an FF. So the intermediate water or oil loop can also be omitted. Direct air cooling for the heat sink where the helium dumps the core heat..

[Henning]

redsnapper wrote: Ironically, I actually spent an hour writing a second post on this subject Friday night, then went to preview the post, made a booboo of opening another tab in IE, went back to the original tab, thinking I was still on the new tab, and lost everything! The “back” button took me to an *empty* reply window, and I couldn’t find any way to get back to my original text! Argh! If I’m smart, I’ll do all my creative writing in another text editor first, and then paste it in here. Webmaster, is there any hope of avoiding this problem otherwise?!

I often edit my articles in a text-editor, before pasting it to the browser’s input field. But this forward-and-back thing I just did myself a few minutes ago with a different article, but luckily Firefox remembered it. So one way of getting around this problem is using Firefox and/or an external text-editor.

[Henning]

redsnapper wrote: nucleon decelerator (also bigger than a breadbox, I presume, but this structure remains totally vague in my mind)

This vague nucleon decelerator is a rogwski coil.

See the design of the device on the left side at http://lawrencevilleplasmaphysics.com/ , with DPF, onion and coil. That’s about the size you would expect. Yes, shoe-box-sized.

Those pictures I’ve attached are stolen from wikipedia. They show the general idea of these coils. Hope that helps a bit. They just show a single turn of the coil.

Attached files

[mjv1121]

zapkitty has, perhaps unwittingly, made an very good point – The easiest way to improving cooling would be to decrease output.

Take a 1MW device – that is 1000 FoFu’s per GW. Eric was hoping that these things could be mass produced for $300,000. So let’s guesstimate an average install cost of $500,000 per MW – that’s $500,000,000 per GW.
Coal and Natural Gas are struggling to hit $2,000,000,000 per GW installed. Add to that the cost advantage to grid systems – placing generator’s near the load – FoFu’s could put a serious dent in the pylon industry – would be especially in emerging markets.

Of course there’s still merit in having 5MW generators or 6, 7 or even 10MW if it can be done – I’m just saying that a 2 or 3MW device is far from being a failure. A 2MW generator (including a reduction on transmission costs) would represent a saving of 8 to 10 times less than all other options currently available – a 5MW generator, 20+ times cheaper than any other option currently available.

redsnapper

The proportion of energy released from each beam is dependent of several variables – fuel gas pressure, magnetic field strength, quantum mechanical effects – shit like that. Also the size of the electrodes plays a big role. The basic idea at this stage is to get the plasmoid as dense as possible to create conditions for pB11 fusion. Until they get past the experimental stage its entirely clear exactly what size the electrodes will be and consequently the “combustion” chamber and how much and in what proportions the energy will be released and recovered. Seeing as no one has put forward any objection to my numbers they’re probably as good as any right now.

[Brian H]

mjv;
I expect you meant to say “entirely unclear exactly what size …”.
Just to be clear that it’s not clear.
😉

[mjv1121]

what I meant to say/type was “not entirely clear”. I hope that’s clear. If its unclear or if you’re not clear about what I meant to say, then don’t hesitate to ask for clarification.

[Rezwan]

At least that was not entirely unclear.

Thanks!

Henning wrote:

Ironically, I actually spent an hour writing a second post on this subject Friday night, then went to preview the post, made a booboo of opening another tab in IE, went back to the original tab, thinking I was still on the new tab, and lost everything! The “back” button took me to an *empty* reply window, and I couldn’t find any way to get back to my original text! Argh! If I’m smart, I’ll do all my creative writing in another text editor first, and then paste it in here. Webmaster, is there any hope of avoiding this problem otherwise?!

I often edit my articles in a text-editor, before pasting it to the browser’s input field. But this forward-and-back thing I just did myself a few minutes ago with a different article, but luckily Firefox remembered it. So one way of getting around this problem is using Firefox and/or an external text-editor.

Yes, I use firefox, too. My stuff is always there when I go back. I wouldn’t know what to begin to do to fix it for explorer. So sorry for the inconvenience!

[redsnapper]

wait a sec – was that unclear or nuclear?

[redsnapper]

Let’s take a possible scenario – granted, this is again making some ballpark assumptions:
1) The 4MW of waste heat originates at 1000K (probably not this high – because probably the X-ray converter and the ion decelerator are made of material more delicate than solid Be or any other solid material – but for the sake of argument, let’s suppose they can handle 1000K)
2) Ambient is 300K
3) We “split the difference” between some sort of local conduction/convection/impingement whatever cooling at the heat source itself, so that 4MW emerges in the form of something like steam or another coolant capable of carrying that heat somewhere we can dispose of it safely. That means (for instance), we’ve got 4MW of power at 650K (375degC).
4) Based on the above, that means we need an “environmental” heat exchanger capable of unloading 4MW with a delta-T of 350degC. (Again, in thermal resistance terms, that’s on the order of 0.0009degC/W.)

Lowest tech, cheapest solution:

Free convection from large fins. A bit of circular/iterative computation is in order, but textbook values for free convection film coefficients at this scale (TBD) turn out to be anywhere between 3 (laminar) and 10 (turbulent) W/m^2/degC. Again, at this scale and temperature difference, we’re well into the turbulent regime. Indeed, textbook correlations yield a film coefficient (vertical, heated flat plate) of 9.2 W/m^2/degC, so at the 350degC delta-T, this implies a heat exchanger surface area of 1200m^2! Now, coming from the semiconductor business, I hardly qualify as a power-plant cooling expert, but at first, I thought cramming 1200m^2 of fin area into a two-car-garage model seemed a bit of a stretch. But maybe not: for instance, if you had 20 fins, 3m tall and 10m long, spaced at 1m, you’d have 1200m^2 of total fin area (counting both sides of each fin) – in a space 3x10x20 m^3. Sounds like a two-car garage to me. So I guess we’re not outside the realm of feasibility. Obviously there are other logistics involved, like the volume of fluid you have to pump from the compact heat source (DPF reactor core, Rogoski coils, X-ray converter), and so forth – but it’s probably doable. Remember, too, a heat exchanger using air as the external working fluid it going to suck a lot of air, and that air is going to come out hot. The same calculations above show that the minimum volume flow rate of air is going to be 15m^3/s (STP), and that’s if it comes out at the maximum fin temperature (i.e. 375degC, in this case). If you don’t want to dump air that hot into the environment, you’ll have to move a lot more air. (Actually, for free convection, the amount of air that moves isn’t an independent variable. That 15m^3/s figure isn’t based on free convection principles, it’s simply based on the heat capacity of air. So the actual value is likely to be a much higher figure, but it won’t be a back-of-the-envelope calculation with stuff I carry around in my head.) You can also lower the fin temperature, but at the expense of needing more fin area. But I think my original impression that the physical size of the final-stage heat exchanger was ridiculously large was unnecessarily pessimistic for low tech cooling methods. (I don’t think it will “fit on a truck.”)

I do remember studying the design of large cooling towers (passive, free convection cooling) in one of my grad-school classes. It can certainly be done, and should be considered in more detail. If you do have a source of water and can otherwise justify it environmentally, you can also vastly reduce the size of the cooling system. I mentioned in my first post that it only takes 2kg/s of water to absorb 5MW, if you take it through the liquid-vapor phase change. That’s 40,000 gal/day. For many reasons, that wouldn’t be my first choice for the final-stage cooling, but it’s also something to consider. (In the context of my first post, that was a possibility for cooling the core – and that water would be in a closed loop, not a continuous external supply.)

Also trying to put the waste-heat figures into perspective – sanity check on the above rough figures: a 5MW power plant is large enough to supply the needs of approximately 2000 homes. Each of those homes ultimately dumps all that energy as waste heat into the environment. So we’re taking 80% of that figure (i.e. 4MW ), which would be the equivalent of 1600 homes, and injecting it into the environment “locally” to the reactor. That’s a pretty dense compression of power into a small space. It should be no surprise that this takes some fairly large fins. (A relatively small “neighborhood” power plant – mainly oil-fired – near my home is a 110MW output plant. The whole facility covers hundreds of acres and has some very large cooling towers. It most certainly does not make optimal use of its real estate, but there’s a reason they gave it so much room!)

Actually, AaronB made a comment about using the waste heat from a 5MW reactor for something as mundane as heating at a hotel. That’s actually a great idea, and in my mind, the sort of thinking that should be pursued. Any operation for which a few MW is the expected low-entropy power requirement, if you can think of other high-entropy needs that could simultaneously be met, a 5MW-sized fusion reactor would be amazing. Come to think of it, any open-ocean applications – like cruise ships (AaronB) – would be excellent candidates, because you’ve got all that water to use for cooling. Cruise ships dump their waste heat into the environment anyway, and the fusion source of energy is so much cleaner in every other respect!

Don’t get me wrong – I still think there are issues at the core end, and knowing sizes and materials of the Rogowski coils and X-ray converter would help better establish the parameters there.

[Henning]

redsnapper wrote: I thought cramming 1200m^2 of fin area into a two-car-garage model seemed a bit of a stretch.

That two-car-garage number didn’t include any heat exchangers, I think. Not even a turbine and generator. And I don’t know whether that’s included in the 300000 USD / 5 MWe calculation. Those numbers were only for direct conversion (I believe). But Rematog made it clear, that a conventional steam-cycle is still required.

If you do calculations, probably also include Helium as main heat conductor. It makes it invisible to x-rays (because of low Z – only two protons), as water shades the hard x-rays (8 protons in oxygen). We want the energy mainly in the photovoltaics, not just as heat. It gets heat anyway.

There are some discussions about helium as coolant in the forum.

[redsnapper]

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?

[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.

@redsnapppppped;
You deserve to be nuked for that! >:(

[Author]

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