The Thermochemical Separator has the same recipes as the Electric energy Injector once you've gotten all the research (which is trivial), but consumes MORE energy by default, and it uses heat.
The energy issue means you have to put efficiency modules in it for it to perform similarly to the alternative. This is just a building tax and should not be the case. There's no point in putting speed or gauss modules in it because it's already fast enough, and the only ingredient is water so gauss modules are useless.
The heat issue is worse, though! There's no way to detect how hot something is so you can't throttle your reactor to produce the correct amount of heat for a hydrogen setup using Thermochemical Separators. At best, if your Star Engines are in high enough demand, you can just leave a reactor running at all times to fuel the Separators and have Injectors as backup.
A special building that produces heat and then stops after reaching a high temperature would be much appreciated; too bad this mod isn't updated anymore.
As it stands, I'm just going to use a bunch of Electric energy Injectors. They take up like 10x more room and use around 20% more energy but that's fine.
