Today the plan for a Torus fusion reactor is to use the radiant energy to boil and superheat water for steam turbines. This represents a massive reduction in the potential thermal efficiency. At 100,000,000K the potential efficiency is close to 100% but the steam cycle at , say 500 DegC is only good for, say 32%. To realise the potential efficiency is, of course, an unknown technology today. My guess is we should use electromagnetic induction to directly provide electrical power. Does anyone know about attempts in this direction, or ideas?
cheers Martin Rose
One way is magnetohydrodynamic (MHD) generator,
http://a-smattering-of-subjects.blogspot.co.uk/2011/05/magnetohydrodynamic-drive.html
I think fusion people are proposing this with waste heat recovery via conventional steam.
The major issue is cooling of the anode and cathode to prevent melting, this reduces the effective efficiency. (Carbon is also sometimes used in non-oxidising environments)
Hope this helps
Paul
Dear Paul, thanks for the reply yes I had MHD in mind. But the idea of inserting electrodes sounds far fetched to me. The temperature of the plasma is so high anything would vapourise surely? I think there should be another way of using MHD. If the current in the plasma could be made to oscillate the resultant magnetic field would alternately grow and then shrink. If this change in magnetic field were to link with electical coils an induced EMF would result. Faraday learnt that the inductive effect only occurs as the field changes. But if we could have an unsteady field we dont need the electrodes at all! I am sure the physicists have sweated blood to stop oscillations, but maybe its just what is needed to extract power.
cheers Martin
Hi Martin,
the electrodes have to be cooled, and as long as their surface temperature is above the plasma condensing temperature, then it can work. Obviously this affects efficiency.
You have made me think of another idea though. I have worked with Thermo-acoustics for may years; heat to sound with no moving parts. (have a look at Greg Swift's book on the subject). It may be possible to cause an acoustic resonance in the plasma, which could then generate the oscillating magnetic field. One would still need a regenerator, which if a solid would limit operating temperature. I wonder if there is a way to get a thermal incline that can absorb and emit heat without there being a solid present?
Am happy to talk off line if you want to pursue this.
Best Regards
Paul
Dear Paul here's a link to my web site
http://www.sussex.ac.uk/profiles/356515
cheers M
It happened to me to work for some years both in the field of controlled nuclear fusion and in the field of thermo-acoustics. Trouble with MHD energy conversion is the need of electrodes which are both good electrical conductors (they carry no electric power otherwise) and thermal insulators (they are melt under heavy thermal load otherwise). These two requirements contradict each other: this is e.g. the reason no such a thing as the "Red October" submarine of Sean Connery's movie exists. As for thermo-acoustics, rather low values of conversion efficiency and power density (lower than in a BWR core) are no real problem. The real issue is the very high value of acoustic resonance, because of both the high plasma ion plasma temperature and the high magnetic field (Alfven slow magnetosonic frequencies are above 1 MHz, to be compared with the 100 Hz - 1 KHz frequency range of usual thermo-acoustic converters). Such high resonance frequency means small wavelength, i.e. poor coupling in devices of reasonable dimension (about 1 m; lower dimension is forbidden by exceedingly large heat loss fluxes at the wall).
A question that has troubled me also... So here I am leaving a link with information about direct energy conversion, but probably difficult to implement on a tokamak.
http://www.askmar.com/Fusion_files/Direct%20Energy%20Conversion%20in%20Fusion%20Reactors.pdf
Dear Daniel,
thanks for the link, I have read it and now realize that there is much activity to try and do what I was talking about and now I have a name for it too "Direct Energy Conversion". In a tokamak how does the fusion energy appear ? Your document appears to suggest that this question has various answers. If a configuration exists where the energy release leads to higher velocities of the charged particles then presumably we would have an increasing electrical current and a growing magnetic field. one of doughnut form. This may allow a Direct Energy Conversion via secondary coils to link with this field.
Dear Martin, it seems scarcely possibile to modify the carefully designed magnetic field lines of a tokamak just for Direct Energy Conversion (DEC) purposes: energy confinement depends critically on them, and even without DEC it is likely to be too poor to allow breakeven (not to mention ignition): after all, it is energy confinement which prevented mankind from achieving the goal of controlled magnetic nuclear fusion to date. DEC is more suitable to Field Reversed Configuration approach, if ever such approach will be successful.
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