eli5: To make fusion energy viable, we need to get out more energy than it takes to actually run the process. But doesn’t that contradict the laws of thermodynamics (getting more energy out then you put in)?

Question

eli5: To make fusion energy viable, we need to get out more energy than it takes to actually run the process. But doesn’t that contradict the laws of thermodynamics (getting more energy out then you put in)?

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Answer 1

You’re neglecting the fuel that’s consumed. In the case of fusion the fuel is hydrogen, and mass of the fuel is consumed. Energy input is needed to maintain pressure and to start it up in the first place, and that’s what is being talked about when talking about energy input.

Think of like a regular fire. You need to put energy in to start with a match, you need to recycle some of that energy to keep it warm (which is why blowing a fire puts it out, it blows away the residual heat which is needed to gassify the solid fuel and maintain the fire) and you get energy out.