direct conversion voltage and converting to grid usable AC

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

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ohiovr
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direct conversion voltage and converting to grid usable AC

Postby ohiovr » Sun Dec 07, 2014 12:08 am

Hello,

How expensive would it be to convert the fusion products into grid commodity electricity (synced with the grid and everything)? Inverters cost more than $100 per kilowatt (grid synced). Is there a potentially cheaper way to get about 50 kilowatts of whatever voltage DC into something you could feed the grid with?

hanelyp
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Re: direct conversion voltage and converting to grid usable

Postby hanelyp » Sun Dec 07, 2014 2:11 am

$100/kW comes to just over 1cent/kW-hour if the inverter lasts one year at rated output. The cost of power conversion equipment also varies with scale. What scale is the >$100/kW figure? I also note that electric utilities have started using high voltage DC for very long range electric transmission, more efficient even after conversion than AC.
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Re: direct conversion voltage and converting to grid usable

Postby prestonbarrows » Sun Dec 07, 2014 6:26 am

HVDC grids are on the order of 100's of kilovolts. Fusion products have energies of the order of 10's of megavolts. Below is an image of what is needed to hold off 25 megavolts at an Oak Ridge accelerator. It is 100 feet tall, 33 feet in diameter, and under 6 atmospheres of pressurized SF6...

Image

The basic principles are the same on paper, but the practical engineering issues become vastly more difficult when dealing with those types of voltages.

ohiovr
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Re: direct conversion voltage and converting to grid usable

Postby ohiovr » Sun Dec 07, 2014 12:17 pm

Maybe steam conversion is better then

DeltaV
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Re: direct conversion voltage and converting to grid usable

Postby DeltaV » Sun Dec 07, 2014 4:40 pm


D Tibbets
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Re: direct conversion voltage and converting to grid usable

Postby D Tibbets » Mon Dec 08, 2014 3:59 pm

Direct conversion, I think, is not an issue of several MeV, but possibly several hundred KeV. That is the conversion is a multiple stage process. Each set of electrodes (as the radius increases) are biased several hundred KV each and there may be ~ 10-20 sets/ stages. Each draws off up to several hundred KeV, until finally the fusion charged particle is allowed to ground on ideally the final electrode/ shell with a small fraction of it's initial KE. This decreases the voltage load on each stage and allows for conversion of a range of particle KE within limits.

The KE of the alphas is probably ~ 4 MeV, 2.4 MeV and 2.4 Mev (there is some uncertainty - a Tri Alpha study gave different values). Added variation comes from the center of mass KE of the impacting fuel ions. If their net energy is 400 KeV, to the left, but the test alphas flies to the right, the net KE would be 400 KeV less and 400 KeV more if they flew off to the left. If flying up, down, or some other vector the KE of the alphas will vary accordingly. In the ideal situation (never happen) in a Pollywell, with perfect confluence, the vast majority of fusions would occur at the center with near zero net KE of the colliding fuel ions, as measured from the rest/ lab frame of reference. +200 KeV plus - 200 KeV = zero. In this case the alphas would have precise KE as listed above. Theoretically two grids, with the inner at - 2.399 MV and the outer at 1.98 MV would harvest almost all of the KE with only a few thousand KeV left in the alphas as they finally hit a surface. The process is, of course, much more messy and complex than this and direct converting ~ 80% to 90% of the energy with significantly more grids is probably an optimistic goal.

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Re: direct conversion voltage and converting to grid usable

Postby ladajo » Tue Dec 09, 2014 3:14 am

The output energies are dependent on the more than the primary reactions.
Each machine construct is going to produce a slightly or significantly different energy profile of resultants based on this.

I think that the polywell machine remains to be seen in a steady state reaction for what becomes the dominant reaction chains.

In any event, it will not require the behometh pictured above. There are several ways to skin the cat for collection. These will be primarily determined by the application IMO.
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Re: direct conversion voltage and converting to grid usable

Postby MSimon » Sat Jan 10, 2015 1:13 pm

There is also the possibility of slowing things down by magnetic conversion if the output of the Polywell is pulsed. This is not unlikely given the nature of the device. It may also be possible to operate it in a pulsed mode if necessary provided the pulse frequency is high enough - probably on the order of KHz.

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Re: direct conversion voltage and converting to grid usable

Postby Tyler Jordan » Fri Jun 19, 2015 6:32 am

Several things I'm confused about and several questions and thoughts in here ... so my apologies in advance for your efforts to answer.


With direct conversion, I've read (somewhere?) that direct conversion *ideally* would be at most 80% efficient.

The 20% that isn't recovered directly presumably ends up as heat.

Are these percentages familiar to anyone? Not sure where I took them from (sources?).

Clearly some percentage must be lost as x-rays and neutrons in the core. But of this percentage that's lost - how much of it is lost during the DC-AC conversion process and how much is this lost in the reactor itself.

One of the questions I'm trying to understand is how we dissipate and very possibly use this heat energy and in particular where is the heat coming from (obviously through-out the system as a whole) but percentage-wise how much from the reactor core and how much from the electrical conversion hardware.

I am guessing that the 80% ideal direct power means 20% remains uncaptured that becomes heat in the core.

meaning that there is going to be a percentage of loss in the direct conversion process too.

So 80% [gross ideally] becomes --> 60-75% [net ideally] and another 5-20% heat (depending on the efficiency of the conversion process - and presumably the cost of the components used in the converter).

All of this would mean that we get 60-75% actually electrical output from the 80% ideal unconverted 2+MV DC.

which means we would have 20% of heat in the core to deal with plus 5-20% heat from the converter to deal with. depending on the size of the power plant this will be a huge factor to be considered in the mechanical engineering.

... So

If we have such a large amount of heat to deal with in the first place, unless we have something useful to do with this heat as opposed to steam production, then it seems steam production is going to be a major component in any case. Given the potential costs of direct conversion (high), then despite the efficiency increases of direct conversion, might it make more economic sense just to go with a larger steam-turbine system? Or to design to plugin-into existing steam turbine systems?

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Re: direct conversion voltage and converting to grid usable

Postby MSimon » Fri Jun 19, 2015 10:29 am


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Re: direct conversion voltage and converting to grid usable

Postby MSimon » Fri Jun 19, 2015 10:41 am

DC to DC conversion is running 95% to 99% efficient these days. DC to AC conversion is in about the same range. Let us say we are at the low end. Overall conversion efficiency will be 90%. If we can get to the high end we are at 98% efficiency. Again you have a series of engineering/economic choices.

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Re: direct conversion voltage and converting to grid usable

Postby Tyler Jordan » Fri Jun 19, 2015 11:12 pm

Last edited by Tyler Jordan on Sat Jun 20, 2015 3:20 am, edited 1 time in total.
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Re: direct conversion voltage and converting to grid usable

Postby MSimon » Fri Jun 19, 2015 11:40 pm


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Re: direct conversion voltage and converting to grid usable

Postby MSimon » Fri Jun 19, 2015 11:51 pm

There are also "tricks" like counter flow heat exchangers to preheat the oil so the temperature delta through the coils is kept low enough to avoid thermal shocking the metal excessively.

All this is eased to some extent if you just throw away the waste heat. The delta Ts can be larger.

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Re: direct conversion voltage and converting to grid usable

Postby Tyler Jordan » Sat Jun 20, 2015 3:25 am

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