If Direct Conversion works...

Discuss the technical details of an "open source" community-driven design of a polywell reactor.

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Nik
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If Direct Conversion works...

Post by Nik »

I've done some reading of the FAQ references, scratched my head appropriately...

Okay, assuming that WB_9, 10, 11 or whatever reliably exceeds break even, and a significant quantity of power can be extracted by direct conversion...

What do you do with such very, very high voltage, low current power ?

Yes, yes, feed directly to international 'DC' super-grid, which makes it some-one else's problem and pays the bills...

There are many 'SciFi' uses of course, of course, most of them off-planet. Happens I've a fondness for plasma drives and very big free-electron lasers...

I was thinking of terrestial and/or industrial applications...

If your local power station could feed your business deep-discounted 'direct conversion' power, what could you use it for ??

---

added: This is in 'Design' as, if there's no economic use, no point designing for it...

kunkmiester
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Post by kunkmiester »

This has been covered a bit. There's ways to shift the voltage down reasonably efficiently. Someone else will have to go into detail on that.

On the other hand, if you can pulse your fuel feed so you get a pulsed output, you can then just run the output through the proper transformers to step down the voltage, and to get AC. That was my dad's idea, I recall someone mentioning a criticism of the idea, but I don't remember what it was.
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chrismb
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Post by chrismb »

It's not an issue. 800kV is now routinely used for commercial HVDC power transmission systems. If you can set up a couple of grids, each putting out their 800kV contributions, they can be tapped separately from electrostatic screens, in an analogous way to tapping a transformer.

The issue I think is the actual direct energy conversion. I think it is barking mad [given the distributions of particle energies you'd get] and is the kind of thing only an armchair inventor could come up with and believe could work

D Tibbets
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Post by D Tibbets »

I don't know if there would need to be much voltage stepdown, but anything working off direct current might be a canidate. With cheap power, some processes carried out by other means may be more economical.
Things I can think of- any distillation or cracking process that currently uses part of the feedstock for heating- gasoline production, agricultural ethanol production, algal drying (I believe that is the major problem with extracting agal oil economically)
Electrical blast furneses, aluminum production, other electrolytic processes, desailination, d/c moter driven trains, ...

Dan Tibbets
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Stoney3K
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Post by Stoney3K »

D Tibbets wrote:I don't know if there would need to be much voltage stepdown, but anything working off direct current might be a canidate. With cheap power, some processes carried out by other means may be more economical.
The European international power grid is 400kV/50Hz three-phase.

Given the output voltage of a DC Polywell (say, 1000MW output) could be somewhere in the dozens of MV, possibly with 100A-kA currents, stepping that down to 'grid' power and subsequently inverting it will be an engineering challenge.

It's not impossible, though, but you need to find the right switching systems that can cope with the enormous voltages and currents involved. Most MOSFETs or IGBTs break down at anything over 1MV. Encasing the entire super-HV installation in a vacuum to prevent any arcing or air breakdowns would probably be a good idea as well. :)

HVDC itself is not rocket science, neither are superconducting cables these days. So offloading the conversion process to an adjacent building and transferring that to several sub-stations with HTSC cable, will decrease the load on those substations since they need to handle much less power densities than the direct conversion that's first in line.

In the long run, it might even be more practical to change over to longer runs of DC distribution, and get rid of the 60Hz or 50Hz grids that were only practical because they could be run with 'conventional' power plants (which have enormous generator rooms). Higher frequency power grids will reduce transformer footprints and will not pose health hazards from ELF radiation.

Most household appliances won't even notice the difference, since 9 out of 10 devices these days don't rely on grid frequency anymore and rectify the input power straight out of the wall jack.
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chrismb
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Post by chrismb »

Stoney3K wrote: Most MOSFETs or IGBTs break down at anything over 1MV.
I've no idea where you get that figure, but the 'standard' 800kV HVDC distribution systems that are coming on line use 3 to 5 kV thyristors stacked in series, and that's a lot of thyristors in series!.. but it is done that way.

I did ask someone at Areva (one of the companies that install these systems) how they simultaneously switched all on at the same time without putting a huge pd across the last one to switch on (seeing as I've tried this myself and it ain't easy) but he only told me that it was their 'trade secret'.

They used to use mercury tubes, but these proved to be somewhat unreliable and required regular maintenance.

There are many advantages of HVDC, mainly from being able to alter the transmission voltage within one cycle, and efficiency due to reduced capacitance of the line (yeah, it begins to make a big difference even at 50Hz with a 1000km transmission line). The lower peak voltages and currents also means less cable mass and can be closer to the ground. It's a more compact system and is a better cost option than AC for transmissions over a few hundred miles.

This is all 'old-hat'. Most of it you can search for yourself. The big issue is actually managing to perform this mystical only-ever-talked-about direct conversion.

Stoney3K
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Post by Stoney3K »

chrismb wrote:
Stoney3K wrote: Most MOSFETs or IGBTs break down at anything over 1MV.
I've no idea where you get that figure, but the 'standard' 800kV HVDC distribution systems that are coming on line use 3 to 5 kV thyristors stacked in series, and that's a lot of thyristors in series!.. but it is done that way.
But now you're mentioning series of several devices, stacked gate to source. I can't recall any MOSFET or IGBT that will survive a gate-source voltage of 1MV or greater by itself.
There are many advantages of HVDC, mainly from being able to alter the transmission voltage within one cycle, and efficiency due to reduced capacitance of the line (yeah, it begins to make a big difference even at 50Hz with a 1000km transmission line). The lower peak voltages and currents also means less cable mass and can be closer to the ground. It's a more compact system and is a better cost option than AC for transmissions over a few hundred miles.
The major disadvantage of using DC is safety, though. If a DC circuit runs a short, it will keep going even if the main breaker is thrown (the current will simply arc over that). A lot of situations regarding railway catenaries were actually caused by that. AC doesn't have that problem, since both the voltage and current cross zero every x times per second, unable to sustain the arc.
This is all 'old-hat'. Most of it you can search for yourself. The big issue is actually managing to perform this mystical only-ever-talked-about direct conversion.
I still think that maintaining a power density of 1000MW or greater running through a single circuit coming from the reactor chamber will be quite the challenge as well! :)
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Nik
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Slightly OT...

Post by Nik »

I remember seeing opto-isolated IGBTs, and even some that were designed to be driven by fibre-optics...

By curious coincidence, they were advertised beside an article on measuring HV by, IIRC, the change of polarisation in a doped fibre-optic filament threaded through insulator...

Uh, my original question was for 'local' applications that did not need transmission / conversion of HVDC.

IIRC, smelting, electrolysis, electroplating, homopolar motors, superconductor magnets etc need mega-current rather than mega-voltage.

Other than driving a free-electron laser, what could you do with raw HV-DC ??

== Added

Other than jump-starting more polywells ??

KitemanSA
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Post by KitemanSA »

Stoney3K wrote: Given the output voltage of a DC Polywell (say, 1000MW output) could be somewhere in the dozens of MV, ...
IIRC, the voltage of a direct conversion unit (used mainly for pB) would have a peak voltage UNDER 2MV while 2/3 of the current would be closer to 1.3MV. This is not seriously higher than current technology.

Only if you talk direct conversion of DD or DT do you get that high a voltage, and most folks seem to think that thermal conversion is needed to capture the energy of those reactions, since much of the energy comes out as neutrons, not charged particles.

krenshala
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Post by krenshala »

Stoney3K wrote:I still think that maintaining a power density of 1000MW or greater running through a single circuit coming from the reactor chamber will be quite the challenge as well! :)
That is probably not that difficult. Since RNebel said the resulting particles would exit through the cusps, the first thing that came to my mind was to have each "section" of the enclosing collection grid feed its own substation. I'd say a minimum of 6, and possibly more (assuming a trunc-cube polywell).

choff
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Post by choff »

This is probably dumb, but I was thinking about the vacuum problem again. After the Alpha's have exited the cusps and the energy extracted by collectors, wouldn't the discharged Alpha's simply drop to the floor of the containment vessel under gravity. If its spherical, it would concentrate making extraction easier through a drain.
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Stoney3K
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Post by Stoney3K »

choff wrote:This is probably dumb, but I was thinking about the vacuum problem again. After the Alpha's have exited the cusps and the energy extracted by collectors, wouldn't the discharged Alpha's simply drop to the floor of the containment vessel under gravity. If its spherical, it would concentrate making extraction easier through a drain.
You probably mean Helium atoms. Once they passed the collector, the Alphas aren't ionized anymore (that's the whole point of the collector, right!) and will become simple 4He atoms.

'falling to the floor' under the influence of gravity is a bit of a longshot expression, but the atoms aren't whizzing by at stupendous energies anymore and will behave just like the background gas.

The vacuum will have to be sucked at some point in the reactor, but I doubt gravity will have a major influence. The pressure difference between the top of the reactor chamber and the bottom caused by gravity will probably be *very* minute.
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BenTC
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Post by BenTC »

Stoney3K wrote: Encasing the entire super-HV installation in a vacuum to prevent any arcing or air breakdowns would probably be a good idea as well. :)

Higher frequency power grids will reduce transformer footprints and will not pose health hazards from ELF radiation.
Vacuum would have a problem with heat dissipation. High pressure SF6 is another option since it has higher dielectric strength and higher thermal conductivity than a vacuum. (High Voltage Circuit Breakers: SF6 vs. Vacuum)

How does a high frequency power grid relate to a DC grid?

Stoney3K
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Post by Stoney3K »

BenTC wrote:How does a high frequency power grid relate to a DC grid?
Advantages of a HF power grid (with HF, I mean the 400-1000Hz range, like in aerospace): Less losses in transformers and smaller footprints (due to smaller cores), no health hazards from ELF radiation through transmission lines, possibility of three-phase transmission. Some people are sensitive for >100Hz pulsing of fluorescent and AC gas discharge lights running off 50Hz grids, this is also eliminated when using HF supply (most 'decent' fluorescent manufacturers also supply 400Hz, even dimmable, ballasts)

With the transition from incandescent to LED lighting the coming decade, the issue might even become more prominent as a lot of entry level LED drivers are synced to the line frequency and cause a very annoying flicker when operating, or suffer from the 'pinwheel' effect when in RGB drive mode.

DC has safety issues (arcs when switching) and is difficult to step down or up. A possible advantage is that constant power can be transmitted through one wire, where any AC transmission grid needs a polyphase system.

50 or 60Hz is an obsolete system which was useful in the early 20th Century. With modern-day electronics, a lot of equipment operates better with higher line frequencies, as the SMPS's in the units need to do less work or can be eliminated altogether (the reason we're using SMPS units in electronics is because the 50/60Hz is hard to transform, making a device ridiculously heavy.)
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BenTC
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Post by BenTC »

chrismb wrote:I did ask someone at Areva (one of the companies that install these systems) how they simultaneously switched all on at the same time without putting a huge pd across the last one to switch on (seeing as I've tried this myself and it ain't easy) but he only told me that it was their 'trade secret'.
In series IGBT chains, small switching differences on the order of 10 nanseconds are critical to the dynamic sharing of voltage across modules. One static method is a simple resistor voltage divider, with one resistor parallel with each IGBT module. See Fig 2.28 page 50 of Arrillaga, Flexible Power Transmission - The HVDC Options, Preview Here. The leakage current forces a static potential across each module regardless the switching order. Of course losses are increased, but apparently not too much.

btw, Fig 2.32 is also interesting showing future trends in silicon carbide device ratings.

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