system
Given the following post, respond as if you're commenting on Reddit:
human
How close are we to be able to use nuclear fusion to produce our energy?

I was taught when I was in school that fusion, at that time, took more energy to achieve than was released. However I heard someone claiming that we are very close to achieving fusion with a net energy output (I think its called cold fusion?). How true is this?
gpt
Barring any unprecedented breakthrough, we're still very far (read: decades) from using nuclear fusion in a commercially viable manner.

We can already do fusion in experimental reactors. That's not new. We can also do fusion in a way that more energy is produced than is used in H-bombs. The problem is combining the two :)

A nuclear fusion reaction is rather hard to achieve and the essence of the problem is one of scale: The larger and more extreme the setup, the higher its efficiency. Current experimental reactors are too small to produce more energy than they consume. So the goal is to make a reactor that is bigger and operates at even higher temperature and pressure to go beyond the threshold of "Q = 1" (Q is the ratio between output and input energy, if Q > 1 then more energy is produced than consumed).

The current big project is ITER, this is planned to be the first fusion reactor to achieve Q > 1. The design goal is to reach Q = 10. But targets for continuous operation are only in the order of magnitude of hundreds of seconds (several minutes). Large parts of ITER and supporting infrastructure are currently being built in southern France. The project is scheduled to have its first plasma operations inside the reactor in 2020 and start fusion reactions in 2027. 

But these are just schedules and delays can't be excluded since there is a lot to figure out still. The conditions inside the fusion plasma will be unlike anything we've made so far and even though magnetic fields will be used to prevent the fusion plasma from touching the vessel walls, making a reactor vessel that can withstand such stresses and still allow for energy to be extracted is a challenge, to say the least.

And while ITER is designed to generate more energy than it consumes, it's still very much an experimental device, set up for scientific purpose and not commercial application. The followup to ITER, called DEMO is a prototype of a potential commercial plant built on the same principles as ITER, but slightly larger. Its very tentative schedule has first operation starting in 2033, with an upgrade/expansion already planned afterwards and the second, final phase of operation starting in 2040. But this very strongly depends on whatever we learn with ITER. And DEMO is still a demonstration model, not something that will be mass produced. 

Next to ITER and DEMO, which will use a technique called "magnetic confinement fusion" in a "tokamak reactor", there are several other avenues being explored. The donut-shaped tokamak reactor has an alternative in the so-called stellarator, which also uses magnetic fields to confine the plasma. In addition to magnetic confinement, fusion has been achieved using intertial confinement, which essentially involves a fuel pellet being forced to implode due to exposure to lasers, causing the outer layers being pushed inwards and confining the fusion reaction in the inner layers of the pallet.

All these techniques work, but none work at an efficiency that allows for commercial application. Currently tokamak reactors are in the lead when it comes to scientific attention, as evidenced by the ITER megaproject, but the jury is still out on what'll end up winning the race.

You hear cold fusion being mentioned from time to time as an easy and affordable alternative that does not involve effectively putting the sun in a small box and hooking it to the powergrid. In the late '80s there was some buzz about scientists allegedly finding fusion reactions taking place in conditions close to room temperature. 

However, these findings were quickly disproven. These days, there's still a fringe community that revolves around the notion that cold fusion is possible in the way that was described back then. They also use the term "Low Energy Nuclear Reaction" (LENR). From time to time, some news article appears claiming a new device delivering practically free energy using something related to cold fusion / LENR. 

Unfortunately, there has been no independent scientific proof of such a device actually delivering on its promises. When tests are being published, they're often of very poor quality, done by people affiliated by the creator or done without the researchers having full access to the device (At one point a test report described a cable being attached to the device without the researchers being allowed to see where the cable lead or to unplug it).

So for now, cold fusion / LENR is fringe activity that is riddled with scammers and people that are a bit too trusting. If cold fusion or something similar would be possible, it would be the holy grail to solve the energy puzzle. Because while "regular" nuclear fusion is a perfectly achievable process, actually getting useful power out of it is a large challenge it's unlikely that it will be the source of widely available, virtually unlimited clean energy this century.