4 ms·

Preface: I know nothing about this. How can it "produce more energy out than energy in"? I don't know if I am not aware of what you mean by that, but that does

• mark-r 9y ago
  That's the whole point of fusion: it produces energy. But you can't do anything practical with it until you have a reactor that produces more power than it takes to run.
  • gryfft 9y ago
    To clarify, it produces energy equal to the difference between the mass of the fuel and the mass of the final byproduct multiplied times the speed of light squared. Energy and mass are conserved, but some mass is converted into useful energy. Doing so in a way that produces sufficient energy to sustain the fusion reaction without creating an uncontrolled reaction ("boom") is the trick that always seems to be 30 years away.
    • orclev 9y ago
      > Doing so in a way that produces sufficient energy to sustain the fusion reaction without creating an uncontrolled reaction ("boom") is the trick that always seems to be 30 years away. Uh, no, not really, a run away fusion reaction has never really been a concern. In fact, that's one of the biggest advantages of a fusion reactor vs. a fission reactor. Fission is a self-sustaining reaction, once it starts you have to work to stop it (via injecting a mediator to interfere in the fission reaction), where as fusion requires constant energy input in order to maintain the reaction. The part that "always seems to be 30 years away", is achieving a fusion reaction that produces more energy than it takes to maintain (allowing some of the output energy to be siphoned off to maintain the reaction). There have been a number of techniques attempted to achieve this with the holy grail being so called "cold" fusion, where cold is defined in this context as something less than the surface temperature of the sun. It sounds like the ultimate solution to the problem though is simply better magnets, not cold fusion at all. Assuming this pans out, the real thing needed to make this viable as something other than a novelty is how much more efficient the reaction can be made. After all, if the output energy is just barely over the input energy you'd need to scale out to ridiculous extremes to produce enough usable energy, but if it's a significant amount higher then that makes more modest size plants viable.
  • spacehome 9y ago
    The parent is pointing out that mass-energy is conserved. There's nothing that can produce it.
• ivanhoe 9y ago
  Energy IN in this context is the energy needed to make the fusion possible and controllable, and energy OUT is what you get from the reactor. For this to work as a generator you want out to be bigger than in, obviously.
  • tudelo 9y ago
    I see. So the OP is just saying that it is above the breakeven point, as in we are not losing energy from running it. Seems like a pretty low bar for energy production :). FWIW, that is a pretty misleading statement when not contextualized and some people could become very confused.
• supermdguy 9y ago
  The energy produced by the fusion reaction is greater than the energy required to make the fusion reaction occur. The extra energy comes from inside the atoms being fused, it's not appearing out of nowhere.
• mykull 9y ago
  Previously its been possible to do contained fusion but the energy cost of containment was as high or higher than what could be gleaned from the reaction. Youre right to assume there can't be any new energy out of nowhere, it comes from the atoms smashing together and becoming a new element.
• tgb 9y ago
  It depends on what you count as "energy in". A solar panel produces more energy out than it takes in - if you don't count the sunlight as "energy in." A fusion reactor (hopefully) produces more energy out than energy in if you don't count the fuel used. Current designs require us to have another massive power source keeping the reaction going.
  • taeric 9y ago
    Aren't there some reactors that produce more fuel than they spend?
    • opwieurposiu 9y ago
      Deuterium-Tritium fuelled reactors can produce Tritium by wrapping the reactor walls in a "blanket" of lithium. The lithium captures neutrons and transmutes into Tritium. It should be possible to produce an more tritium then the reactor requires, but no one has actually done it yet for fusion reactors. Fission "fast" reactors can do something similar by turning uranium into plutonium. This has been demonstrated by several countries but so far has been uneconomic.
      • taeric 9y ago
        Thanks for the details. I don't think I even have to confess that my knowledge here is rudimentary. I just finished Atomic Awakening and I remember the phrase popping up a few times.
      • Nomentatus 9y ago
        Breeder reactors are possible because elements above the atomic weight of iron are storing a lot of energy that's released if you can bust them up (fusion.) Iron is the least (stored) energetic state, so below iron, fusion releases energy. It's a real nasty process to control, though since neutron radiation tears the shit out of container metals over time.
• isk517 9y ago
  Current fusion reactors require more energy put into them to start and maintain the fusion reaction than is harvested from the actual fusion reaction. Better superconductors means it will take much less energy to start and maintain fusion and thus the reactors should produce energy rather than consume it. It doesn't break the laws of thermodynamics since the energy produced is from fusing hydrogen into lower energy state helium, just currently we use more energy than the process produces to make it happen.
• luke0016 9y ago
  It's the opposite of fission. Fission produces energy by "destroying" matter (splitting it into smaller parts). Fusion produces energy by creating it (combining it into larger parts). The difference in mass between the inputs and outputs manifests as energy. In simple mathematical terms, thanks to Einstein, E=mc^2
  • slavik81 9y ago
    I don't quite understand what mass is lost. I thought fusion involved overcoming the electrostatic forces to get two protons close enough that they are pulled together by a nuclear force, thus releasing the nuclear potential energy. But, you start with two protons, and you end with two protons, so what's the mass that's converted to energy?
    • slavik81 9y ago
      Apparently I was mistaken. They don't just make a weird hydrogen isotope. They make plain-old hydrogen. 4 protons become 2 protons and 2 neutrons. The difference in mass between a proton and a neutron is 2.3×10^-30 kg. So, naively, you'd generate 4.1×10^-13 J per helium atom. Or, roughly 11 GJ (3 Mwh) per litre of helium at STP. Seems a little low, though I suppose 1L is only 1/5th of a gram of helium. I still might be missing something, too.
• Nomentatus 9y ago
  Campfires "produce more energy out than energy in" too - the kindling is energy in, but once you've established a chain reaction (no, not an out-of-control chain, but yes a self-sustaining chain) you get warm. Exothermic.