Working Fluids for Supercritical Turbines

Some fun esoteric engineering

To this day most power plants work by making a lot of heat then converting the heat differential between that and the surrounding environment to make electricity. The most efficient heat engines are closed cycle supercritical turbines. They basically all use Carbon Dioxide as the working fluid. I’ve spent some time researching possible alternative working fluids and have come up with some interesting results.

The ideal working fluid would have all these properties: high temperature of decomposition, low corrosion, low critical point, high mass, high thermal conductivity, non-toxic, environmentally friendly, and cheap. That’s a lot of properties to get out of a single substance. Unsurprisingly Carbon Dioxide scores well on these, especially on low decomposition, low corrosion, non-toxic, and cheap. For the others it’s good but not unbeatable. It’s the nature of chemistry that you can always imagine unobtainium with magical properties but in practice you have to pull from a fairly short menu of things which actually exist. Large organic molecules can start to feel more engineered but that isn’t relative here because organic bonds nearly all decompose at the required temperatures.

There are all manner of fun things which in principle would work great but fail due to decomposition and corrosion. As much fun as it would be to have an excuse to make a literal ton of Tungsten Hexafluoride it’s unfortunately disqualified. The very short list of things which are viable are: Carbon Dioxide, noble gases above Helium (which unfortunately leeches into and destroys everything), and short chain Perfluorocarbons. That last one is fancy talk for gaseous Teflon. I have no idea why out of all organic bonds those ones are special and can handle very high temperatures. As they get longer they have an increasing tendency to decompose and given the different numbers from different sources I think we aren’t completely sure under what conditions perfluoropropane decomposes and anyone who is seriously considering it will have to run that experiment to find out.

With multiple dimensions of performance it isn’t obvious what should be optimized for when picking out a working fluid so I’m going to guess that you want something with about the density of Carbon Dioxide and within that limitation as low of a critical temperature and as high of a thermal conductivity as possible (yes that’s two things but the way it works out they’re highly correlated so which one you pick doesn’t matter so much.) The reasons for this are that first of all it would be nice to have something which could plausibly replace the working fluid in an existing turbine meant for Carbon Dioxide without a redesign and second it may be that it’s hard to make a turbine which can physically handle something much denser than Carbon Dioxide anyway and that may be part of why people haven’t been eager to use something heavier.

To that end I’ve put together this interactive (which should probably be a spreadsheet) which shows how different potential working fluids fare. It turns out that there’s a tradeoff between high thermal conductivity and high mass and using a mix of things which are good at either one does better than picking a single thing which is in the middle. The next to last column of this interactive shows a measure of the density of the gas when holding temperature and pressure constant and the final column gives the a measure of the thermal conductivity under those conditions. The units are a bit funny and I’m far from certain that the formulas used for the mixed values here are correct but the results seem promising.

The increased mass benefits of longer chain Perfluorocarbons go down after Perfluoroethane, mostly because at that point when mixing with Neon it’s mostly Neon anyway. (With only two things it isn’t really a ‘chain’ at that point either.) That gives a thermal conductivity value of 0.040 as opposed to Carbon Dioxide’s 0.017, which is a huge difference. That mix has some cost and environmental impact concerns but being within a closed cycle system they’re used for the life of the turbine so they’re part of capital costs and can be disposed of properly afterwards so aren’t a big deal.

The downside of that mix is that although it works great for the temperatures in nuclear plants and the secondary turbine of gas plants it might decompose at the much higher temperatures of the primary turbine of a gas plant. The decomposition problem is likely to be better with Carbon Tetrafluoride which knocks the value down to 0.037 but I’m not sure if even that’s stable enough and superheated elemental Fluorine is not something you want to have around. Going with pure noble gases will definitely completely eliminate decomposition and corrosion problems. Using a mix of Xenon and Neon has a value of 0.038 but probably isn’t worth it due to the ludicrous cost of Xenon. A mix of Krypton and Neon is still quite good with a value of 0.032 and beats Carbon Dioxide handily on all metrics except initial expense which still isn’t a big deal.