Polywell building difficulty compared to other power plants

[TallDave]

No, it's power. Net power, admittedly, but once the gain is comfortably over unity, net power is roughly proportional to power.

Heh, okay. "Net power" is probably more accurate than "gain," because gain is relative and net power is absolute.

I was only trying state the relatively obvious notion that you can only make money on the excess power produced over what you require to run the reactor.

There are perhaps 100 markets for a 25GW plant (metropolitan areas > 3.0 million).

There may not be any, unless the energy produced is very very cheap. This has been discussed a few times here; the problem is that utilities and their governing municipalities don't like big chunks of power, as that creates all sorts of transmission and reliability problems relative to having a bunch of little ones. IIRC, you don't really see much over a few GWs except from hydro (because hydro is both location-dependent and ridiculously cheap relative to other techs).

That's been a major criticism of the ITER/DEMO path -- even if they work as expected, and they solve the plant power density problems to get costs reasonable, they still may not have a market because of their size.

[Aero]

That's been a major criticism of the ITER/DEMO path -- even if they work as expected, and they solve the plant power density problems to get costs reasonable, they still may not have a market because of their size.

If you build it, they will come.

If the power is cheap enough and the location is reasonable, that is. But what power company would build one on spec? I guess, with enough subsidies, a lot of them would.

[D Tibbets]

93143's arguments seem reasonable with a couple of points added. Where is the cutoff between gain and net power dominance. In a Polywell burning P-B11 with a Q of ~ 3-5 , or perhaps a maximum of 20?

Q is sort of unimportant (as long as its above 1), a low Q just means more heat rejection equipment and a larger electrical generation system.

...

Well, sort of. The Pratical Q more accurately. A machine burning D-T or D-D with a thermal conversion steam plant requires a Q sufficiently high to compensate for conversion efficiency. If steam conversion efficiency is 25% overall, then a fusion Q of 4 is necessary to breakeven. With a P-B11 direct conversion of 80% you only need a fusion Q of ~ 1.3 to breakeven (energy in = useful energy out). That is why it is so important for the DPF approach. This is one reason I think P-B11 fusion with direct conversion (if it works) has significant advantages. Even with 1/3rd the fusion output, it will deliver the same amount of electricity as a thermal cycle reactor. In a since this is a power over fusion gain advantage. The P-B11 reactor might only have a fusion Q of 20, but an otherwise equal D-D fusion/ thermal cycle reactor would need a fusion Q of ~ 60 to match it.
Also, in a thermal cycle machine size where gain is the dominate factor, adding a direct conversion system to capture the portion of energy of the charged fusion products may make economic sense. While more difficult because of the greater energy spread, capturing the high energy protons, He3 and tritium ions from D-D fusion might result in an overall energy conversion efficiency of perhaps 50%. While this would be impossible(?) with a Tokamac machine, it would be possible with a cusp machine like the Polywell, where ignition is not an issue.


Dan Tibbets

[WizWom]

Steam system conversion is ~50-60% to Electrical. Second-stage systems can bring that up to 75-80%, using a second and/or third stage turbine with the lower pressure output from the first stage high pressure turbine.

Direct conversion is a nice dream; build it and the world will beat a path to your door. I could do it with mercury, because it takes a charge well.

[KitemanSA]

Steam system conversion is ~50-60% to Electrical.

In your dreams! Even with superheated steam, 45% is great.

[D Tibbets]

Well, a brief Google search revealed multistage steam turbines may reach 50% realistic gross efficiency. Natural gas fed gas turbines with secondary steam turbines may exceed this some. I'm assuming these are gross efficiencies with unusually high temperature steam- with associated high costs- can't use standard stainless steel pipes.
If gross efficiency, this does not account for cost, all the pumps, scrubbers, transformers, etc. Once they are accounted for the net efficiency is probably in the often quoted range of 25-30% for coal and perhaps 40% for natural gas combined cycle plants.

For that matter, I don't know if estimates of 80-85% efficiency for direct conversion is gross or net.

Dan Tibbets

[TallDave]

That's a good point -- does anyone have any data on the efficiency of direct conversion in other applications? I'm wondering if it's ever really been done outside a lab.

80% may end up being overly optimistic. As chrismb pointed out, there may be a considerable spread in alpha energies, which could be problematic. As best I can tell it's not even a given that today's direct conversion tech will necessarily exceed the efficiency of conventional thermal cycles in economic terms.

[ladajo]

I don;t think that would neccessarily be a problem if the direct converstion coolection grids were layered to capture the spread of energies.

Seems plausible (mythbusters analogy! )