Hydrogen Iodide for more efficient heat engines

Another one of those 'crazy but works on paper' ideas

Supercritical Carbon Dioxide heat engines are the newfangled design to use in power plants. Since they’re both more efficient and don’t need water they apples-to-apples are clearly superior once you get them working. There are some subtle advantages to having a continuous transition between gas and liquid, but that’s dwarfed by the benefit of the working fluid having the density of, well, a fluid, which makes the whole engine vastly smaller, thus reducing its surface area and thermal losses (read: heat escaping off the surface). It also has less friction in its operation and requires less material to build, which are real benefits but much less of bump.

Carbon Dioxide is a great substance to use for many reasons. It has a good critical point of 31 celsius and 74 bar, is very inert, is fairly heavy with a combined atomic weight of 44, and is cheap. Its density isn’t much less than that of water. But in the interest of trying to improve on things I’d like to propose an alternative: Hydrogen Iodide. It has similar properties with a critical point of 151 celsius and 83 bar. I don’t know if it matters but the boiling point is also similar at -35 celsius versus -79 celsius (The triple point of Hydrogen Iodide isn’t readily available for some reason). The big advantage is that the combined atomic weight is 128, almost three times as much resulting in a density similar to concrete. The somewhat higher critical point means it can’t go down to room temperature but that’s unlikely to matter for the primary turbine in a power plant. (Power plants generally have more than one turbine with the second one being much larger and more impressively looking but only generating a small fraction of the power.) The additional cost of the material is only a tiny issue when used for a power plant.

Whenever there’s a plausible sounding idea like this the big question is: Why hasn’t it been done yet? There are several likely culprits in this case: The material in question is esoteric and unexpected: It’s the gaseous salt, sort of like how water is the liquid mineral. Given how nuts engineers get considering options I doubt that’s the whole story. Another potential downfall of it is that it may dissociate at high temperatures. It does dissociate and around 400 C at standard pressure and in the presence of water but these are very conditions the temperature of dissociation may be much higher, and it may be reasonable to keep it operating in a temperature band which stays below the breaking point.

But most likely the biggest issue so far has been that it’s corrosive. When people are considering supercritical materials Carbon Dioxide works so well and is so well behaved that people have been much more focused on getting it deployed than improving on it. If this really is the bottleneck and more detailed analysis indicates that there really is a theoretical advantage (both big ifs) then it may be clearly worth doing the necessary work to figure out what materials overcome the corrosion problems to take advantage of the improved efficiency.