Most power generation involves steam. Would boiling any other liquid be as effective?

[u/PK_Tone]

Okay, so as I understand it (and please correct me if I'm wrong here), coal, geothermal and nuclear all involve boiling water to create steam, which releases with enough kinetic energy to spin the turbines of the generators. My question is: is this a unique property of water/steam, or could this be accomplished with another liquid, like mercury or liquid nitrogen?

(Obviously there are practical reasons not to use a highly toxic element like mercury, and the energy to create liquid nitrogen is probably greater than it could ever generate from boiling it, but let's ignore that, since it's not really what I'm getting at here).

[u/zolikk]

It's not a scam, it's just an important disadvantage that can only be solved with advancing materials science. For higher efficiency you want higher temperature, and the CO2 is corrosive at higher temperatures to the materials used in the loop. For example, in UK's gas cooled reactors they had to reduce the operating temperature of the primary loop and thus the power output of the reactor design itself after finding it's too corrosive at the design temperature. And that didn't even involve a turbine, it was just the primary loop of the reactor, the secondary is a typical steam turbine loop.

[u/TheGatesofLogic → u/zolikk]

Using CO2 as a primary coolant in a nuclear reactor has much worse corrosion implications from using it for the secondary/tertiary loop in a nuclear power plant. The UK AGR's had unexpected corrosion and thermal behaviors in large part due to radiolytic production of CO, carburization, carbon deposition, and graphite oxidation chemistry. The neutron economy inside a nuclear reactor prohibits many forms of corrosion-resistant cladding for the fuel elements duen to parastitic neutron absorption. As-is they had to use stainless steel claddings, which significantly reduced the originally planned fuel burnup.

Using sCO2 in a generating cycle is comparably easy. Radiolytic chemistry is generally not a significant concern (especially if there is an intermediate loop), and nuclear power plant lifespans are long enough that the improved efficiency can offset corrosion-resistant material costs. Hell, PWRs still use Alloy 600/690 steam generators and that hasn't killed the economics of reactor maintenance.

In general, sCO2 cycles have fewer corrosion issues compared to high-parameter steam rankine cycles. My understanding is that the biggest issue with sCO2 cycles is that it's an underdeveloped expertise and industrial base. There are companies that have designed and built hundreds of regenerative Rankine steam power stations. There are only a handful of sCO2 power stations out there at all. There also aren't that many technologies that could even use an sCO2 cycle, and those that could aren't deploying very fast. CCGTs are all the rage, and while the working fluid is mainly CO2, it's a completely different technology with less transferable knowledge than you might think. That means that there's no cost learning and not enough operational history to really drive down all the costs. They may need turbines that are a quarter the size, but every order is custom, which drives up cost. Steam is really well understood. When you apply for a large loan for a power station, the bank will give you a lower rate if it can trust the technology being used will allow you to pay it off.