Can a Piston Engine Harvest Fusion Energy?

[luetm]

Hi

I have a really weird idea that is robbing me of my sleep.

One of the many problems with fusion seems to be how to harvest the energy released by the reaction efficiently. So as I was slowly drifting into sleep I was wondering, how we usually harvest energy from reactions, and the piston engine came to mind... so:

If my thermodynamics don't fail me, we could inject a pre-heated DT gas-mixture into the cylinders, then the isentropic process would increase the temperature and pressure by about one order of magnitude. Ideally one would make this process symmetric (one piston from each side) so the impulse forces the nuclei into each other rather than into the cylinder bottom. The isochoric process would then increase temperature by the heat released from the fusion, as well as increase pressure from the higher heat capacity ratio of helium. This would result in quite some kinetic energy being created, more than just from the temperature increase itself.

I had some thoughts about having the piston be positively charged to increase the force on the nuclei towards the center and maybe even make them bounce back and forth a little, which would help increasing the reactivity by optimizing the angles. It might even speed up the transition from gas into plasma, but to be honest this is wild speculation.

I'm sure there are quite some reasons why this won't work (for instance I can think of the mixture cooling down in the piston) - but I thought it might be quite a cool thought experiment I might share.

But anyway. What do you think? And thank you for indulging me :)

luetm

 

[Drakkith]

At the temperature required for fusion the pistons would literally be melted from the inside-out, regardless of cooling.

 

[Orodruin]

Similar ideas have already developed, see for example https://en.m.wikipedia.org/wiki/General_Fusion

 

[mfb]

Harvesting the energy is easy, getting sufficient fusion is the hard part. Also, you certainly want to avoid any contact with material while the plasma is still 100 million degrees hot.

 

[Astronuc]

At the temperature required for fusion the pistons would literally be melted from the inside-out, regardless of cooling.

Except the density of the plasma is about 10¹⁴ cm^-3 as compared to metals which have atomic densities of ~10²² cm^-3, so there would be little heat input into the pistons, but the plasma would be quenched.

The average fission product energy is about 84 MeV, which is equivalent to 975 billion K. However, in the UO₂ ceramic, the fission rate is on the order of 10¹³ 1/cm³-s as compared to the ceramic density of about 10²² cm^-3, so the maximum fuel temperature is usually less than 1800 K in the center of the pellet and less than 700 K at the surface of the pellet.

Magnetic confinement could make use of the plasma pushing/expanding against the magnetic field, or use charge separation, or otherwise, use the thermal energy of neutrals and neutrons heating the first wall. The problem with the last scenario is the degradation of the first wall over time.

 

[mheslep]

At the temperature required for fusion the pistons would literally be melted from the inside-out, regardless of cooling.

At those densities, the first casualty would be the plasma, not the piston, with the plasma contaminated with high Z nuclei from the piston.

Except the density of the plasma is about 1014 cm-3 as compared to metals which have atomic densities of ~1022 cm-3, so there would be little heat input into the pistons, but the plasma would be quenched.

Yes, I'm scooped.

 

[mheslep]

Harvesting the energy is easy, getting sufficient fusion is the hard part. ...

Agree that getting sufficient fusion is proving very hard, but the heat transfer engineering in the case of fusion is also much harder than that for fission and I suspect is little talked of since, well, as you say fusion itself is so hard. A comparison of the two:

Fission:

Consider heat transfer in fission and fusion reactors. In today’s typical light-water reactor (LWR), there is generated by fission in fuel pins containing uranium. The heat is then transferred to the coolant at the surfaces of a relatively large number of small diameter pins. This arrangement provides a larger surface area to transfer heat than, say, a single large fuel cylinder. Indeed, by decreasing the diameter of the pins even further (but increasing their number to keep the amount of uranium unchanged), the total surface area available to transfer heat would be further increased. Thus, the actual heat-transfer rate through any given square inch of surface on a fuel rod is not critical. Sufficient heat can always be removed merely by increasing the total area.

Fusion:

This strategy does not work in a fusion reactor. The heat-transfer surface is limited to the inside of the wall surrounding the plasma, and the relatively small surface area of this wall cannot be increased without further increasing the size of the reactor. In fact, bigger reactors need larger heat-transfer rates. Thus, the actual heat-transfer rate per square inch must be extremely large and cannot simply be reduced by a design change.

 

[Drakkith]

At those densities, the first casualty would be the plasma, not the piston, with the plasma contaminated with high Z nuclei from the piston.

Well, that counts as melting the piston in my book.
(It's a small book full of scribbles and other nonsense)

But you are correct. The plasma would be quenched and contaminated with nuclei from the piston and cylinder. I wonder what the heat flux is for a difference in temperature of about a million+ degrees...

 

[mheslep]

... I wonder what the heat flux is for a difference in temperature of about a million+ degrees...

Astronuc has a good example above in post 5, that it is some product of delta T and density that define heat flux, with fission events at 975,000 million K which average out over the entire solid lattice from 1800K to 700K.

 

[mfb]

Without anything to contain the plasma, the atoms just hit the wall within microseconds (deuterium at ~10 keV => 1000 km/s), the atoms knocked out of the surface need a bit longer to hit another wall again, but it all cools down extremely fast.