My society has solar thermal energy filling in the role of coal in the 19th Century, as described in the accepted answer here: Could I have a 19th Century American Society Develop Solar Power from Blueprints?

I wanted to have 21st century solar panels, but... Not happening. However, Solar THERMAL energy is fully possible. One comment on the page, however, caught my eye:

You don't even need steam. You could use a lower boiling point fluid in your engine, as is often done with geothermal power plants

However, this setting being in the 19th Century of America, it has me wondering: What chemicals with a lower boiling point than water could be easily produced in the 19th Century, especially if it could be at least invented BEFORE the Industrial Revolution?

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    $\begingroup$ But why do you want lower boiling point with thermal solar? With enough mirrors, it is easy to go higher than the water boiling point. $\endgroup$
    – Alexander
    Dec 10 '20 at 18:04
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    $\begingroup$ Ethanol boils about 40°F below water. Ammonium Hydroxide (Ammonia + Water solution) boils at 100°F. Of course, they have their own usage issues. $\endgroup$
    – user535733
    Dec 10 '20 at 18:04
  • $\begingroup$ @Alexander: Because you need fewer mirrors to reach the lower boiling point, of course. $\endgroup$
    – jamesqf
    Dec 10 '20 at 19:17
  • $\begingroup$ @jamesqf so here is a trade-off between using mirrors or special liquid agent. Today, the economics dictate to use more mirrors and even go for an agent like molten salt. I don't think that in XIX century using a lower-boiling agent can be more practical than using more mirrors. $\endgroup$
    – Alexander
    Dec 10 '20 at 19:25
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    $\begingroup$ Please wait 24 hours before accepting an answer so folks around the world have a chance. $\endgroup$
    – StephenS
    Dec 10 '20 at 19:52


Produced in solution with water since the times of Babylon, it was already distilled in highest grades for distillates like whiskey, grappa, vodka and so on.

Solutions of water and alcohol start boiling at temperatures between 100 and 78 degrees Celsius, depending on the alcohol concentration.

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    $\begingroup$ Not bad! It could even spawn jokes: Industrial Revolution - Powered by Beer! $\endgroup$ Dec 10 '20 at 18:01
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    $\begingroup$ @TheDarkeLorde Beer? Nah its fuled by, WHISKEY! Or, if you like it dangerous: Quicksilver. $\endgroup$
    – Trish
    Dec 11 '20 at 9:03
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    $\begingroup$ Doesn't seem very safe. One leak goes BOOM! $\endgroup$
    – lvella
    Dec 11 '20 at 16:30

Besides alcohol, there were a number of other low-boiling liquids that were known prior to the 19th century. By 1860, it was possible to make sulfur dioxide and anhydrous ammonia industrially -- and both of these saw service as refrigerants beginning in the late 19th century, which means they're thermodynamically suitable for use in a Kelvin cycle or Brayton cycle (steam piston or steam turbine) engine.

Both are hazardous, but then so is high-proof alcohol.

The advantage of these fluids over water for a boiler/expander engine system isn't their lower boiling point, however (higher temperature gives better thermal efficiency, and this was known well before 1900): it's the much lower latent heat figures. Far less of the heat input to convert room temperature liquid sulfur dioxide to hot vapor is inaccessible as latent heat of vaporization than would be the case with water/steam. Having a freezing point lower than even common winter temperatures also means that the system won't freeze its pipes if it goes out of service for a few hours in mid-winter.

It's also worth noting that air engines, similar in operation to the Stirling cycle, were well known and commercially available before 1900; they were used as low power stationary engines (the same application as fixed steam engines, lighting gas or natural gas engines, etc.). All these need is heat input; the working fluid is the same air the operator breathes, and most of them don't care if the heat is supplied by burning wood, coal, gas, or alcohol, or pressurized hot water (from a solar field or geothermal well).

  • $\begingroup$ Re "both are hazardous", true, but then so is water. Look up e.g. boiler explosions. $\endgroup$
    – jamesqf
    Dec 10 '20 at 19:19
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    $\begingroup$ @jamesqf Oh, no question -- but a small leak from a steam boiler feed tank isn't dangerous; the same is not true of alcohol, ammonia, or sulfur dioxide. Two of the three are flammable, two are inhaled toxins. Ammonia and sulfur dioxide are far easier to detect in small quantities, however... $\endgroup$
    – Zeiss Ikon
    Dec 10 '20 at 19:44

I think it's worth a frame challenge here.

In the comments Alexander asks "why do you want lower boiling point with thermal solar?", to which jamesqf (not the OP) replies "because you need fewer mirrors to reach the lower boiling point, of course." I am guessing that this is also the OP's reasoning, and if so a frame challenge is definitely in order.

In the 18th and early 19th century, when steam engines were beginning to become viable, people worried a lot about this sort of thing. There were people working on "air engines" instead of steam engines, on the grounds that it seemed like steam engines were wasting energy boiling water when it could be used to expand air instead. I daresay there were others working on fluids with a lower boiling point - it seems an obvious thing to try.

This kind of thing eventually led to the development of thermodynamics, which was really a revolutionary science at the time. One of the outcomes of it was that you don't waste energy boiling water, because all of that energy goes into expanding the steam, and the boiling point of the working fluid doesn't directly affect the efficiency of the engine - what matters is the temperature difference between the boiler and the cooling fluid (a bigger difference is better).

Water, then, turned out to be a very good choice for the working fluid of a heat engine. It's non-toxic and readily available (hence a bit of leakage doesn't do too much harm, and there's no problem venting any excess or cooling it evaporatively in the open air). It's not corrosive if the machine components are prepared correctly. Its boiling point of 100°C is easy to reach and won't melt metal, but high enough that you can easily condense it even on a very hot day. It's also not flammable, which provides an enormous safety advantage over almost any other fluid, and evaporates fairly slowly in comparison to something like ethanol, which limits spoilage.

Even today, with the exception of internal combustion engines, virtually all heat engines run on steam. (They tend to take the form of turbines rather than piston engines these days, but the principle is exactly the same.)

There is no reason why all this wouldn't also apply to a 19th century solar thermal plant. It's true that with a lower boiling point you would need less mirrors to heat the boiler, but the engine would provide a correspondingly lower motive force, so in many ways this is a disadvantage - you're wasting energy by not using the sunlight to heat the boiler as hot as possible, which is easy to do. Sunlight has an effective temperature of about 6000°C, and until the target starts to get near that you can always make it hotter just by adding more mirrors.

Because of all this, it would make much more economic sense to just have more mirrors and use water/steam as the working fluid.

Of course, solar power also has the issue of intermittency - even in the sunniest places it doesn't work at night. This is why modern solar thermal plants generally heat a reservoir of molten salt, which can be heated to much higher temperatures than 100°C, which makes it more convenient for storing heat for long periods of time. But when you want to use that heat, as far as I know, you still just use it to boil water and run the engine on steam. I think this could all be done with 19th century technology, so I would expect it to be developed then if solar power was the main energy source.

  • $\begingroup$ Could you please explain what you mean with "Sunlight has an effective temperature of about 6000°C". My understanding is that it's a stream of photos with a certain frequency spectrum and intensity (photons/time) where the concept of temperature does not apply. Or are you talking about color temperature? Because with enough mirrors you could heat a substance to more than 6000 -- but any material would most probably evaporate or react violently with oxygen from the air or other nearby substances way before 6000 is reached. $\endgroup$
    – hochl
    Dec 12 '20 at 16:44
  • $\begingroup$ @hochl I'm talking about the temperature of the (approximate) black body radiation from the Sun. I can't find a good non-technical introduction, but thermal radiation behaves like it has the same temperature as the thing that emitted it, which in the case of the Sun's photosphere is about 6000K. The reason you can't heat something higher than 6000K is the Stefan-Boltzmann law. When the object gets to 6000K it will be glowing white hot and emitting its own radiation just as fast as you can pump heat into it with mirrors. If this wasn't true it would break the second law of thermodynamics. $\endgroup$
    – N. Virgo
    Dec 13 '20 at 3:19
  • $\begingroup$ (Colour temperature is related but not exactly the same thing. Black body radiation at 6000K does have a colour temperature of 6000K, sunlight included - but colour temperature just refers to the perceived colour and not to the thermodynamic properties, which is why you can have an LED lamp with a 6000K colour temperature that doesn't contain any components at anywhere near that temperature.) $\endgroup$
    – N. Virgo
    Dec 13 '20 at 3:38
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    $\begingroup$ Thanks, that was a nice explanation!!! :D $\endgroup$
    – hochl
    Dec 15 '20 at 11:40

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