Do we need breakeven to consider fusion power sources?

It may be difficult to separate "theory" from "application," but let''s see if this helps facilitate the discussion.
Calmarius
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Do we need breakeven to consider fusion power sources?

Post by Calmarius » Tue May 27, 2014 8:16 am

I did some back of the envelope calculations.

So the fusion gain factor is

G = Q_fusion / Q_in,

G: the factor
Q_fusion: fusion power released
Q_in: input power.

In a practical power plant the setup would probably turn everything into heat and feed into a heat engine. But probably better designs possible (eg. direct conversion with decelerators?)
Both the input power and the fusion power will be turned into heat. So the total heat generated is:

Q_fusion + Q_in = Q_in*(1 + G)

The efficiencies of real heat engines (Chabal-Novikov efficiency):

eta = 1 - sqrt(T_c / T_h)

This is much smaller efficiency than of the (impossible to build) Carnot engine.
And the efficiencies of the current power plants corresponds well to this value.

So in order to make it self sustaining, we need to feed back the electric power into the device:

Q_in = eta * Q_in * (1 + G)

If we solve for G, we have:

G = (1 - eta) / eta

Or for eta, we have:

eta = 1 / (1 + G).

I don't know what was the highest gain factor confirmed. So if you have exact numbers please let me know.

On breakeven the eta required is 0.5. Which means the hot side of the heat engine should be at 1200K. Look like it's doable.

If we consider that all parts of the heat engine is made of tungsten and calculate with 3000K on the hot side, G=0.5 would be enough to be sustainable.

On the forums I read that one would need a gain factor of 5-10 to make fusion power practical... So am I missing something?

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Chris Bradley
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Re: Do we need breakeven to consider fusion power sources?

Post by Chris Bradley » Tue May 27, 2014 9:03 am

'Sub-breakeven' devices have been discussed here and elsewhere at length, both for fusion devices, accelerators and fission piles.

In the case of deuterium fusion, the neutron emissions of a sub-breakeven reactor could be used to make fertile material fissionable and could be used to 'burn up' nuclear waste and long-lived actinides with a highly 'profitable' and practicable thermal output as well as helping eliminate radioactive fission waste.

Ultimately, though, you have to get within reasonable sight of breakeven at least. Let us know when you make one that can do better than 0.1% fusion energy output to energy input .....

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Re: Do we need breakeven to consider fusion power sources?

Post by Calmarius » Tue May 27, 2014 12:44 pm

JET claims they reached the gain factor of 0.6. Unless they are lying I would say they are almost there...

It seems there is a confusion about the breakeven, and I'm a bit confused too. When do we reach the breakeven? When fusion_power_released == power_used OR when electric_power_produced_by_the_reactor == electric_power_consumed?

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Chris Bradley
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Re: Do we need breakeven to consider fusion power sources?

Post by Chris Bradley » Tue May 27, 2014 1:25 pm

Well, they achieved 18MJ of fusion energy for ~10 seconds for 22MJ input into the plasma, but it also required 1GJ of magnetic energy put into the confinement fields first. They don't seem to trouble themselves with that little detail.

I'd suggest that to make what you are looking for worthwhile one would need to demonstrate a continuous fusion power output and one which approaches a reasonable fraction of breakeven whilst taking into account all energy inputs, would you not agree?

FWIW, ITER will require 41GJ of magnetic energy before it can begin to confine a fusing plasma. At a design output of 500MW, it has to run for at least 2 minutes before it has even generated the energy required to generate those magnetic fields.

Also FWIW, one tonne of TNT is around 4.1GJ of energy, so the magnetic field in ITER is equivalent to 10 tonnes of TNT. Now imagine that field collapsing all at once and giving up its magnetic energy to the confinement vessel. .... I wait with baited breath to see if ITER can survive its first plasma collapse!...
Calmarius wrote:It seems there is a confusion about the breakeven, and I'm a bit confused too. When do we reach the breakeven? When fusion_power_released == power_used OR when electric_power_produced_by_the_reactor == electric_power_consumed?
The reality is that practical fusion power is so far off it makes little difference! The convention is when the input energy into the plasma equals the fusion energy generated. But that is just a 'target' so far. Reality, as you allude to, is that it needs to be Q=10 (fusion energy created is 10 times the input energy), so breakeven at Q=1 is only a milestone and is no practical result per se. It's a bit like hitting 100mph in a car or celebrating the year 2000 - it's just a number with little practical bearing.

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Re: Do we need breakeven to consider fusion power sources?

Post by Calmarius » Tue May 27, 2014 9:27 pm

Well, they achieved 18MJ of fusion energy for ~10 seconds for 22MJ input into the plasma, but it also required 1GJ of magnetic energy put into the confinement fields first. They don't seem to trouble themselves with that little detail.
I thought they were thorough enough to measure the input power vs output power in watts. So they are basically lying.
Also FWIW, one tonne of TNT is around 4.1GJ of energy, so the magnetic field in ITER is equivalent to 10 tonnes of TNT. Now imagine that field collapsing all at once and giving up its magnetic energy to the confinement vessel. .... I wait with baited breath to see if ITER can survive its first plasma collapse!...
What would happen? Would it explode? It would be the most expensive fireworks in human history lol...

Valid point anyway. Seeing many fusors in youtube quietly glowing I forgot about the fact that tokamaks are still struggling with plasma instabilities.

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Re: Do we need breakeven to consider fusion power sources?

Post by Dave Xanatos » Tue May 27, 2014 11:21 pm

Isn't what we are really looking for called Ignition? Correct me if I'm wrong (and I know you will, and I probably am! :) ) - but as I understand it, ignition is the point at which the usable, probably electrical, energy harvested from the output is sufficient to power the reactor itself. In other words, self-sustaining as long as the fusion fuel is being continually fed, much like an ICE which, once going, will power itself as long as fuel is available.

Wow... what a day that'll be.
It would take decades of work, by thousands of scientists, in a particle accelerator powered by dump trucks of flaming grant money! - Professor Farnsworth/FUTURAMA

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Re: Do we need breakeven to consider fusion power sources?

Post by Chris Bradley » Wed May 28, 2014 12:40 am

Calmarius wrote:I thought they were thorough enough to measure the input power vs output power in watts. So they are basically lying.
Not sure who is 'lying', I've not seen any JET scientists claim anything not essentially true. Possibly disingenuous, but not false. What do you think you've seen that is a 'lie'?

Dave Xanatos wrote:Isn't what we are really looking for called Ignition?
'Ignition' is the point at which the other, thermal products from the fusion are sufficient to sustain the plasma temperature so that there is no need for an external power input into the plasma (not counting sustaining the magnetic field and ancillaries). This is like setting a dry piece of wood alight, it will continue to burn under its own exothermic output. For DT fusion, this is higher than the fusion power by a factor of 5, because only 20% of the fusion energy (alpha particles) is 'thermal' and the remaining 80% is the neutron flux that will heat an external thermal working blanket.

'Burning' is where external energy is pumped in and it results in more energy out than in, but it'd stop if you take the external energy source away. This is a bit like putting a piece of wet wood into a gas burner - it will add to the heat, but will fizzle out and not keep burning by itself.

Burning is a form of 'energy amplifying' in which the process requires an energy input, but overall is a net positive energy process. You might be interested in looking up Rubbia's 'energy amplifier', which is what the term has come to commonly reference, which uses neutron spallation with fertile fuel. It is being researched academically and commercially as a viable, sub-critical (therefore intrinsically 'safe') reactor.

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Re: Do we need breakeven to consider fusion power sources?

Post by prestonbarrows » Wed May 28, 2014 2:03 am

The hand-wavy detail you guys are missing is that JET and most other existing systems use resistive external coils to produce the vacuum field. This means that basically all of those gigajoules of energy input into the coils wind up as waste heat. In a legitimate power-producing tokamak, it's usually assumed you would have superconducting coils to drive the vacuum fields. These still require stupid amounts of energy to spool up, but basically none of it is lost as heat and the vacuum field just coasts without the need for further input power. The energy will stay stored in the magnetic field, like a taught spring, as long as they are kept cool. It could concieveably be recovered when the machine is powered down also. Think of a giant near-ideal inductor. This is why most numbers quoted in today's experimental reactors omit the power needed for the external coils.

This is similar to charging a large capacitor bank or flywheel energy storage or pumping water up into a hydroelectric storage reservoir. The input energy is not 'lost' just transferred into potential energy waiting to be used later. The only losses come from series resistance in the conductors or frictional losses as the case may be.

Most of the workhorse experimental tokamaks today are inductively driven. They essentially are set up as a single-turn stepdown transformer (with the plasma in the torus as the secondary) to drive the huge toroidal currents through the plasma required for confinement. Since you can't keep ramping up the current through the primary side of the transformer forever, there is an inherent run time limit on such devices. Again, a legitimate power-producing tokamak will likely need non-inductive current drive mechanisms such as neutral beam injectors to achieve anything more than a few seconds of run time.

Addressing these issues, among others, is the main goal for the upcoming ITER project if they can sort out their bureaucratic issues first...

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Re: Do we need breakeven to consider fusion power sources?

Post by Richard Hull » Wed May 28, 2014 4:08 am

Big, Big science like ITER, being multi-national, will suffer mightily as the science is at the mercy of governmental budgets, bureaucratic oversight and even the science becomes politically involved and entangled in a self-inflicted choke hold.

Will there even continue to be a stable world able to continue this long term work? Giant fusion budgets will not be the highest priority in a crumbling political and economic world infrastructure, should this become a reality in the near future. Thus, it isn't just the science and engineering of fusion involved here.

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Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Re: Do we need breakeven to consider fusion power sources?

Post by Chris Bradley » Wed May 28, 2014 5:32 am

prestonbarrows wrote:The hand-wavy detail you guys are missing is that
No handwavy or even detail missing. You seem to have missed the point that the OP was discussing an experiment as 'nearly there'. Yeah, sure, if they can achieve stability for more than a couple of minutes it might start making sense to argue that the magnetic field energy is recoverable.

So the only minor matters (as you appear to be suggesting they are all readily solvable) are that tokamaks have not yet been driven by a bootstrap current, which they would require, have not yet been driven by a superconducting magnet, which seems like it would be required, have not yet had their magnetic field energy recovered, which they might not require so long as they can demonstrate continuous stability, and that's all before the fact that no one has yet even shown more energy out than in for a tokamak, let alone Q=10.

I would not regard these matters as 'handwavy' in the context of someone initially believing that the experiment is 'almost there'.

So maybe we can have this conversation again when all the 'if's' and 'should be's' you've mentioned are ever turned into a plausible level of practicability, or maybe I'll be dead by then. It is no handwavy thing to mention that a thing is not 'almost there' if you are not likely to live long enough see it, within several decades.

On the contrary, the energy expense of the magnetic field energy is the thing that others handwavy at, just as you have done. Rather than be specific about how the magnetic energy would be recovered by the coils, you flap your hand a little that someone, somewhere, some time in the future will no doubt do it. Well, if it was that easy, why don't the experiments recover that energy now?

I find it ironic that the thread discussion is specific about how much energy is required for a JET plasma run, yet you suggest that discussion is handwavy when all you do is make vague suggestions towards how these things might be solved in the future with absolutely no guarantee that they are tractable problems.

These issues are ones that tokamak experiments have been trying to overcome for decades. Decades. In fact, scientists now responsible for these experiments are two generations younger than the ones that started it. As a point of discussion, how long do you run an experiment to try to see if something will work before deciding it won't work? Under what circumstances would 'tokamak' ever be deemed a failed experiment? I'm not saying they can't make it work, I'm just wondering if there are any circumstances anyone will decide it has 'failed' and stop funding it. Is it possible that anyone will ever regard it as 'failed', or will it just go on forever?

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