[martythemaniak]

This is either a bad joke, an incredible misunderstanding of physics. "perpetual motion machine" is not compatible with the laws of physics as we understand them. There's many working fusion reactors, we don't know which, if any, designs will be economical.

[oceanplexian]

A cup of water is enough to power a small city, if the atoms in it were perfectly fused. So for all practical purposes, clean fusion IS pretty close to what we imagine when we think of perpetual motion. The sun would burn out long before we ran out of usable fuel in our solar system.

[pfdietz]

No, I understand that. What I'm pointing out is that if something has no realistic chance of being economically viable, then from the end user's point of view that's just as bad as if it were physically impossible. The only difference is society is spending billions while pretending an economic miracle will happen, even if that miracle is as implausible as discovery of a violation of the law of conservation of energy.

[sam]

Fusion indeed has a realistic chance of being economically viable. Your comment assumes that current technologies remain entirely static which is the unrealistic position. Enabling technologies like high temperature superconductors, advanced computation/simulation capabilities, new high current/high voltage electronics, as well as recent advances in alternate (non tokamak) concepts bode well for the field.

[pfdietz]

Your comment is funny, since "assuming current technologies remain static" is what fusion proponents have done. They have to assume the competition doesn't get better. If solar and wind continue down their experience curves they will be delivering power at a fraction of a cent per kWh, when fusion will be lucky to deliver power at a cost 100x that.

[andrewmutz]

I would love to see a citation or source to back up your economically viable fusion energy is impossible

[pfdietz]

Just look at the multiple engineering miracles that would be needed. I'll mention a couple here.

Note that DT fusion is a thermal power technology. It makes heat that drives a thermal cycle. It turns out that all externally heated thermal cycles have become uncompetitive for power generation, just because of the cost of the non-reactor/non-boiler components. This includes fission, solar thermal, geothermal, and coal. So even if a DT fusion reactor is delivered FOR FREE from the fusion reactor fairy, it will not be competitive. Expecting capital equipment to have zero or negative cost is a good economic analogue for a perpetual motion machine.

So, what's the alternative? It's going to have to be something using advanced fuels and direct conversion. That means all the efforts with tokamaks are ruled out (they cannot work with advanced fuels and have no place for adding putative direct conversion.) And what advanced fuels are we talking about? D-3He would require space mining of the outer planets (the much-discussed lunar resource, even if it could be mined at 10 ppb in the regolith, would only last centuries before being exhausted.) And H-11B is extremely difficult, because the energy out is only about 10x the kinetic energy of the particles, so very little loss can be tolerated. In particular, systems in thermal equilibrium will have a very difficult time reaching breakeven, due to large photon losses.

Another showstopper is the absolutely terrible power density of fusion reactors. This follows from general principles (geometry and the square cube law). If you look at all existing fusion reactor proposals, ask what their thermal power/volume is. You will discover they are terrible compared to fission reactors (ITER 400x worse, ARC 40x worse). So ask yourself: why is this going to be cheaper than fission reactors? The fusion reactors are both larger, more complex, and made of much more sophisticated materials. And fission has already lost the economic race.

The larger more complex reactors are really bad news for reliability and maintainability. Estimates for the fraction of time a fusion reactor will be up, given current estimates of MTBF and MTTR, are just a few percent. This is inadequate even for an experimental reactor. There is not much budget space for operating costs from a fusion reactor before just that becomes prohibitive.

From a hard nosed engineering point of view, fusion is just ridiculous. It piles complexity upon complexity in a large machine that will be too radioactive for hands on maintenance. How could you think this would ever make sense?

[imtringued]

Well, it must be possible. If we can't build an artificial sun then we won't be able to leave the solar system.

[nl]

That doesn't follow at all.

I think you meant to say "I hope we develop fusion so we can leave the solar system".

[pfdietz]

Nonsense. It's quite possible to achieve interstellar travel without controlled fusion.