Unusual Working Fluids

Updated: 14 June 2012
Aluminium bromide
Nitrogen dioxide
Nitrosyl chloride
Gallium iodide
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The Power section of the Museum of Retrotech contains many unusual engines- for some of them, their interest is in the choice of working fluid. Steam and air (the latter as in the internal combustion engine) reign supreme today, for very practical reasons. Both are chemically stable, non-poisonous, and cheap. However, in the past, several other fluids have been tried. Some of these have their own galleries in the Museum; this now includes mercury:

Liquids that vapourise easily, when used in a stand-alone cycle, are not more efficient than water as a working fluid. Quite the reverse. See: Carnot's Law on the thermodynamics page.

The use of working fluids other than water goes back to the early history of heat engines. Consider:

According to The History and Progress of the Steam Engine by Elijah Galloway and Luke Hebert, in 1830 Lieutenant-Colonel Torrens of Croydon was, like the Brunels with their 'gaz' idea, impressed by the high pressures that could be obtained when a liquefied gas was confined and vapourised by heat. A modest amount of heat could produce pressures of 40 atmospheres or more (approx 560 psi) which in the those days was considered extreme. This does not of course mean that a large amount of power is so produced, and of course power is required to liquefy the gases iin the first place. The good Colonel appears to have overlooked this. He planned to use hydrogen sulphide, carbon dioxide, nitrous oxide, ammonia, hydrogen chloride or ammonia.
The boiler and engine were intended to use oil at high pressure as a sealing medium, to prevent leaks of these corrosive and poisonous gases through stuffing boxes, in much the same way as Howard's alcohol engine.

It was reported in the French journal Nature for 21 July, 1888, that petrol, alcohol, ether, chloroform, and carbonic acid had all been tried as working fluids. As you can see from the pages listed above this is certainly true, though I have so far only been able to sniff out only the sketchiest details of what appears to be the only attempt to build a chloroform engine. Such an engine would have been interesting to work with; chloroform may be non-flammable, but it is poisonous as well as a (dangerous) anaesthetic.

When you dig a little deeper into the business of unconventional working fluids, you find that, far from being a forgotten dead end, they have been the subject of great interest over the last three decades. One application is the generation of power in space, where the heat rejection is at a high temperature, not withstanding the poor Carnot efficiency this implies, because it can only be rejected by radiation.
Another influence is the increasing drive for energy efficiency and renewable sources. Waste heat may be available in huge quantities but at a relatively low temperature, too low to make a steam Rankine cycle viable. Geothermal power frequently has to be generated from water or brine at modest temperatures, which is sometimes used to boil voltaile organic fluids rather than water.

A good example of recent research is a report on seven possible working fluids, produced by the Argonne National Laboratory in 1981. You can read the whole report here (external link)

We will now look at some of the more weird working fluids:

Now read on...

Galloway and Hebert's The History and Progress of the Steam Engine refers to an engine mentioned in the French journal Annales de Chemie which was claimed to have worked satisfactorily. It was powered by the pressure of the vapours boiled off bitumen in a furnace- presumably relatively heavy hydrocarbons. After use these vapours were used to fire the furnace.

However, Galloway and Hebert had reservations about it functioning properly, saying "... strong doubts may however be entertained, since nothing has been heard of it for the last three years."

A journal called The Repertory of Arts, Manufactures, and Agriculture claimed in 1823 that the Montgery engine could safely use very high pressures because "it is inclosed in a double case" and that the engine could therefore be forty or fifty times smaller than a steam engine of equal power. This was, of course, pure fantasy.

The Register of Arts, and Journal of Patent Inventions, Volume 3 (edited by Luke Herbert, no less) in 1829 was unimpressed, saying "...not having seen or heard anything of the matter for 3 or 4 years past, we are inclined to think that it was merely one of those evanescent meteor-like schemes. which blaze for a while in the brains of the inexperienced mechanic, and then vanish forever."

The boiling point of sulphur dioxide is -10 degC. It is definitely a minority choice when it comes to working fluids. It is poisonous, and in combination with water forms corrosive sulphurous acid, H2SO3. (not to be confused with sulphuric acid, H2SO4) So far I have found only one example of its use.

Henry E. Willsie was a pioneer of solar energy. A Willsie installation had large flat-plate solar collectors that heated water, which could be kept warm overnight in an insulated basin. Liquid sulphur dioxide was passed through pipes immersed in this basin, and it duly boiled to give a high-pressure vapour capable of driving an engine. The sulphur dioxide was then condensed for reuse; quite how, given that the boiling point of sulphur dioxide is -10 degC, is not currently known. Possibly the whole system was pressurised so that condensation could occur at ambient temperature.

In 1904 Willsie built two solar/SO2 power plants. One was a 6-horsepower installation in St. Louis, Missouri, and the other a 15-horsepower system in Needles, California.

Willsie believed that the ability to store energy so power could be generated at night made his system a commercial proposition. Unfortunately he was wrong; no buyers came forward. The long-term cost analysis might have looked good, but it appears potential purchasers were doubtful of the machine's durability; very possibly they thought corrosion from the sulphur dioxide would be a problem. They were also put off by the large amount of machinery required for a tiny power output, and the big initial investment it required. His solar energy company, like other pioneering efforts before it, disappeared.

An exotic working fluid that once had considerable hopes invested in it is diphenyl oxide, a very stable organic chemical with a boiling point of 258 degC at atmospheric pressure. It was explored as a way of getting heat into a thermodynamic cycle at a higher temperature, but nothing came of it. Oliver Lyle (author of The Efficient Use of Steam) wrote "Diphenyl oxide has been suggested as being more suitable than mercury." in 1958, which sounds as though it had not been tried in practice at that date.

Diphenyl oxide has the structure C6H5-O-C6H5. At room temperature it forms colorless crystals, with a smell of geraniums and phenol; in fact it is sometimes called "geranium crystals". When molten it is a colourless liquid. It is insoluble in water. It is relatively non-toxic as organic compounds go, but I wouldn't sprinkle it on my cornflakes.

Diphenyl oxide has lots of names: diphenyl ether, phenyl ether, 1,1'-oxybisbenzene, geranium crystals, oxydiphenyl, phenoxybenzene, and phenyl oxide are all the same chemical.

Diphenyl oxide is still used in specialised heat transfer applications at high temperatures, sometimes mixed with plain diphenyl; diphenyl is also called Biphenyl, Lemonene, Phenyl Benzene, Bibenzene, and Xenene, and has the structure C6H5-C6H5.

Nitrogen dioxide (NO2) is a reddish-brown toxic gas; see Wikipedia At low temperatures it changes into its natural dimer as dinitrogen tetroxide. (N2O4) A cycle that takes advantage of this compresses the N2O4 at low temperature; it is then heated. The higher temperature causes each N2O4 molecule to break into two NO2 molecules. This lowers the molecular weight of the working fluid, dramatically increasing the efficiency of the cycle. The NO2 is expanded through a turbine, and then cooled, causing it to recombine into N2O4. This is fed back by the compressor to continue the cycle.

According to P K Nag in Power Plant Engineering (3rd edition) aluminium bromide (AlBr3) is a possible high-temperature working fluid that might have the same applications as diphenyl oxide. At a pressure of 12 Bar its saturation temerature is 482.5 degC, well above 187 degC for water. (Mercury is 560 degC) The most common form of aluminium bromide is Al2Br6, which is a hygroscopic colorless solid at standard conditions. Information is scarce but it appears that aluminium bromide might be useful in a dissociation-recombination cycle like that described for nitrogen dioxide and nitrosyl chloride.
Judging by the scarcity of information on the WWW there is not much interest in this substance as a working fluid.

Nitrosyl_chloride (NOCL) is a very toxic yellow gas; see Wikipedia, and is also irritating to the lungs, eyes, and skin.
It is not very stable at high temperatures, but that is why it is interesting. A working fluid that dissociates and recombines appropriately can give major improvements in cycle efficiency. Kesavan and Osterle reported at the Intersociety Energy Conversion Engineering Conference, (16th, Atlanta, GA, August 9-14, 1981, proceedings. Volume 3. (A82-11701 02-44) New York, American Society of Mechanical Engineers, 1981, p. 2204-2209) that:

"A study of the Brayton cycle with dissociating nitrosyl chloride (NOCl) as the working medium is reported. With the turbine inlet conditions of the gas in a highly dissociated state (a mixture of NOCl, NO, and Cl2) and the compressor inlet at the combined state (NOCl), the dissociating NOCl cycle shows superior overall performance when compared with the Brayton cycle based on inert gases such as helium. The results of the analysis show considerable potential for reduction in power generation costs through higher cycle efficiencies and smaller component sizes."

Gallium iodide has the chemical formula Ga2I6. It is the most common iodide of gallium. The chemical vapor transport method for growing crystals of gallium arsenide uses iodine as the transport agent. Ga2I6 reversibly forms GaI3, and so is presumably of possible use in a dissociation-recombination cycle, but Google is silent on this point. In its GaI3 form it sublimes at 345 degC; the critical temperature is 951K.

Unfortunately, methanol is very toxic. As little as 10 ml swallowed can cause permanent blindness by destruction of the optic nerve. Its low boiling point indicates it would be even less efficient as a working fluid than ordinary alcohol (ethanol).
For more information on methanol see Wikipedia

A azeotropic mixture of 2-methyl pyridine and water. (An azeotropic fluid is one with a single boiling point , even though it is composed of two fluids)
For more information on 2-methyl pyridine see Wikipedia. Note that it is miscible with water.

"The chosen working fluid is 2-2-2 trifluoroethanol, CF3CH2OH, 85% mol-fraction with 15% mol-fraction of water - known as Fluorinol 85 or F 85. This azeotropic fluid is supplied by only one manufacturer, and its current demand is low. A significant demand increase could substantially reduce the manufacturing cost of the fluid . If this were accompanied by a substantial reduction of its price, systems based on Fluorinol 85 would become much more economically attractive. At present prices, the cost of a charge of working fluid for a Fluorinol 85 system could be as high as 15% of the initial system cost."
No Wikipedia entry, but there is some chemical info here.
This stuff has been used to make at least one experimental engine, built by the Thermo Electron Corp of Waltham, Mass, which we have come across before, and is described thus:
"The report describes the design of a 3 kw generator set driven by an organic Rankine cycle engine. Power is produced with a turbine-gearbox expander arrangement. The turbine speed of 70,200 rpm is geared down internally to the sealed system to 3,600 rpm so that a low speed shaft seal of proven performance can be used to transmit the turbine output to a standard military generator. The system is completely self-contained with controls designed to maintain working fluid cycle conditions and turbine speed constant and independent of load. The system uses an organic working fluid (Fluorinol-85) rather than water to avoid problems associated with freezing and high temperature lubrication. At full load the cycle operates with turbine inlet conditions of 530F and 480 psia and a condensing pressure of 29 psia (202F)."
Unfortunately the thermal stability of Fluorinol-85 is not perfect. At 600 degF, the annual degradation exceeds 17%. Worst of all, the most significant degradation product is hydrofluoric acid, which is both extremely corrosive and a contact poison, and just about the last substance in the world that you'd want washing around inside an engine.
Thermo Electron Corp now seem to be involved in making scientific instruments, which is no doubt a much safer line of work.

Unfortunately it's poisonous. For more information on toluene see Wikipedia

Regrettably, Freon R-11 has the highest ozone depletion potential of any refrigerant, by definition assigned the value 1.0. US production was ended in 1995. This is not promising for a potential working fluid.
For more information on Freon R-11 see Wikipedia

Also known as 1,1,2-trichlorotrifluoroethane. No Wikipedia entry.
"Fluorocarbon 113 also called FREON(R) 113 or refrigerant 113 is a colourless to water white, non-flammable liquid with a slight, ether like odor at high concentrations. It is noncombustible at ordinary temperature, but the gas will ignite and burn weakly at 1256 degF. Fluorocarbon 113 is chemically reactive with metals such as calcium, powdered aluminum, zinc, magnesium, and beryllium."

has been used as a working fluid in Stirling-cycle engines; it improves their efficiency but it is expensive, and one of the most limited resources on the globe. Hydrogen would be much cheaper and even more efficient but the very small molecules are hard to contain- hydrogen will diffuse through solid metal- and hydrogen embrittlement of the metal parts is also a problem. Hydrogen is also highly inflammable and explosive if (or rather when) it leaks out.

Some thoughts from Sadi Carnot, who knew a thing or two about thermodynamics:

"As to the other permanent gases, they should be absolutely rejected. They have all the inconveniences of atmospheric air, with none of its advantages. The same can be said of other vapours than that of water, as compared with the latter. "


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