What is the type of energy we would harvest from nuclear fusion?

[Elite Engineer]

If we were theoretically able to carry out nuclear fusion, such as encasing a star in a reactor, how/what would be the type of energy we would be pulling from it 
to power the grid? I would it would be heat energy like in nuclear fission? I hesitate to believe this because isn't the energy from fusion way too high to use? 
Would it be akin to powering a 40 watt light bulb with 1,000 watt power source?

 

~ee
 

 

[Sensei]

It is kinetic energy of newly created particles and isotopes. Particle with high kinetic energy is causing ionization of matter through which it is flying, decelerating, and losing part of its energy. As a result medium particles are accelerated. Particle with high enough kinetic energy during collision can make pair of matter-antimatter e.g. electron-positron. They annihilate together producing gamma photons. Gamma photons passing through medium are scattering, ejecting electrons, ionizing atoms, etc. and losing their energies. From two gamma photons each with 0.511 MeV after a while there are created millions low energy photons.

45 minutes ago, Elite Engineer said:

Would it be akin to powering a 40 watt light bulb with 1,000 watt power source?

That depends on quantity of reactions per second. You need billions of them per second happening even to get 1 W. e.g. annihilation of electron-positron yields 1.022 MeV * 1.602*10^-19 J/eV * 10^6 = 1.637244e-13 J. 1 W / 1.637244e-13 = 6.108e+12 reactions needed per second !!

 

 

There is no "single fusion reaction". That all depends on fuel. Typical star fusion reactions are pretty inefficient (and good, otherwise stars could not work for billions of years). While talking about man-made fusion, typically we are talking about fusion of Tritium with Deuterium as it yields 17.6 MeV per reaction. Star-like fusion of H-1 + H-1 gives just 0.42 MeV (and half energy taken by neutrino!) + 1.022 MeV from annihilation.

Edited July 1, 2019 by Sensei

[Ken Fabian]

My (limited) understanding mostly they intend heating of some kind of fluid coolant, probably water, and using that directly or indirectly for steam power generation as the means of using the energy from fusion. Neutrons are captured by a surrounding jacket, heating it. Lithium has been proposed for such a jacket because the process should convert some into Tritium, that can pass back to the working plasma as more fuel. Direct energy capture has been proposed as another means of extracting the energy - a kind of magneto-hydrodynamic process, using the flow of fast charged particles in a magnetic field.

I'm not expecting any great results any time soon - and anything that is that hard to do at all is going to be difficult to optimise into a reliable and low cost form of energy generation. Worth trying I think - but so are other possible energy solutions that don't get nearly the levels of attention, support and funding that fusion does - some that look far more suitable to mass manufacture and worldwide everyday use (like a favourite of mine is optical rectennas that could work a bit like PV but should able to utilise IR, including from atmospheric back radiation and ground heat - ie would work at night. Or utilise low grade heat from waste heat or ground heat storage).

[Ken Fabian]

PS Just clarifying my answer above - the fluid coolant would collect heat from that (probably Lithium) 'jacket' that surrounds the containment chamber.

[swansont]

10 hours ago, Sensei said:

It is kinetic energy of newly created particles and isotopes. Particle with high kinetic energy is causing ionization of matter through which it is flying, decelerating, and losing part of its energy. As a result medium particles are accelerated. Particle with high enough kinetic energy during collision can make pair of matter-antimatter e.g. electron-positron. They annihilate together producing gamma photons. Gamma photons passing through medium are scattering, ejecting electrons, ionizing atoms, etc. and losing their energies. From two gamma photons each with 0.511 MeV after a while there are created millions low energy photons.

Converting kinetic energy of the particles is not the description one would use in physics. If you heated water and made steam to drive a turbine, you would describe it in terms of thermodynamics, and it would be heat transfer. 

If you captured photons and converted them to electricity you'd say it was photovoltaics.

11 hours ago, Elite Engineer said:

 I hesitate to believe this because isn't the energy from fusion way too high to use?  

As Sensei has said, it depends on the reaction rate. And there's plenty of demand for electricity, so e.g. a 1 GW reactor is only a drop in the bucket of what we use. Encasing a star is well beyond any technology we have.

The energy released per reaction of fusion is somewhat smaller (~10-100x) than that of fission. It can be higher on an energy per mass of reactants basis.

http://hyperphysics.phy-astr.gsu.edu/hbase/NucEne/fusion.html