[Physics] Why is steam a good working fluid for heat engines

Question

Steam (gas-phase H₂O) is very commonly used as the working fluid for external-combustion heat engines. It seems to me that it ought to be a poor choice, because:

• Unless I am seriously confused about something, you have no choice but to throw the working fluid's heat of vaporization away every time around a Rankine cycle. (You might be able to recover some of it with an economizer, but that's a bolt-on.) This drawback seems like it ought to apply to all heat engine designs involving a gas-liquid phase transition, but water should be particularly bad in this context because its heat of vaporization is so large.

• Relatedly, phase transitions are a source of inherent irreversibility and thus efficiency loss, aren't they?

• Water also has a very high specific heat capacity, both as liquid and as gas; I'm not sure about this part, but my best guess is that you want a working fluid with a low specific heat capacity, so that as much of its thermal energy as possible will be translation-kinetic energy of the molecules, and therefore directly convertible to motion.

• Water condenses to liquid at 100°C, which is well above the temperatures of readily-available cold sinks (ambient air usually from 10–30°C; cold water is often available at 5°C or so); shouldn't a working fluid that could be expanded all the way down to the cold-sink temperature allow greater efficiencies?

• "Wet" steam is reactive enough to cause a variety of engineering headaches, e.g. turbine blade erosion.

Instead, the abstractly correct choice would seem to be a permanent, inert gas; dry nitrogen and dry air are readily available for cheap and should be plenty inert enough for most applications (maybe at very high temperatures you go to helium or argon). But "hot air" reciprocating engines are rare, and nobody tries to build gas turbines that expand the gas all the way down to ambient—instead they use the exhaust as the hot reservoir for a steam engine!

There must be something I'm missing. Please explain.

Answer 1

Other points aside, I would start by stating that a fluid with low specific heat, as you speculated, is not necessarily a good choice; I suspect that would usually be bad indeed, though it depends on more factors.

How a heat engine works, in a single work cycle, can be roughly modelled by adiabatic expansion. I copied over the image:

The working fluid, typically gas, does (positive) work by moving bottom-right, increasing $V$ and decreasing $p$, also $T$. For a simple and rough estimate, let's assume the fluid is ideal gas; it usually is not very far from that in typical civilian engines. Then the isotherms have the form $$p \propto TV^{-1}$$ and the adiabatic curve $$p \propto V^{-\gamma}$$ where $\gamma = \frac{C_P}{C_V}$.

With these in mind, let's start interpreting this graph; first note the fact that

• the process starts at a point where $T$ is the maximum as heated by external heat source;
• the process stops at a point where $p$ is too low to keep a reasonable output force/torque/whatever.

Then let's think about its "slope": is it better to be steep, or flat?

I would say the flatter the better.

Being "steep" means it undergoes a greater change in $p$ and, somehow less (compared to a flatter one) of that in $V$; think about it. At the very best we can have $p$ stop at 1atm, so for a good output from a steep curve, we would need to start at a $p$ much higher than 1atm. That means we would need to design our engine so that many parts need to sustain a lot of pressure; we may need a super beefy pressure chamber for heating the working fluid. We would also need heavy moving parts so as to sustain, in case of failure e.g. the output shaft is stuck stationary, possibly the full pressure as it comes out of heating chamber. Such added weight would very likely contribute to the total efficiency negatively, as well as making the machine harder to build, and more dangerous.

So, assume you agree to that, we should prefer a flat curve, so that the pressure does not change dramatically, but doing the work gradually over a greater expansion. Observing that, in most cases, the isotherm curve shall decend (how could it be hotter after doing work?), meaning $\gamma > 1$, the best we can hope for is a $\gamma$ that is close to 1. That is achieved with a high specific heat.

And when it comes to high specific heat, water is probably the best choice that is cheap and safe. I do admit that the great amount of energy involved in phase trasition is usually lost, but you can recycle some of them, e.g. we can heat cold water with the exhaust vapour.