The choice of mercury for a working fluid may seem bizarre- after all, it is both expensive and horribly poisonous. However, there were what appeared to be sound thermodynamic reasons for grappling with the challenges involved in its use. After these challenges proved too great, mercury had a renaissance in projects for power plants in space. Here is the story...

There are some other working fluids which have their own galleries in the Museum:

Ammonia Petrol and Alcohol Compressed Air Potassium (coming soon)

Carbonic Acid Ether & Chloroform Carbon Disulphide Other working fluids

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Mercury has got to be the ultimate dodgy working fluid mercury: as remarked elsewhere on this site, in the Steamwheel gallery, it is an insidious poison of a most unpleasant kind. It has a much higher boiling point than water, at 357 degC. Experimental power station installations were tested in the USA but ultimately nothing came of them. Thank heavens.

The man behind the mercury vapour turbines was William Le Roy Emmet of the General Electric Company. Emmet devoted a great deal of time and energy to their development and promotion, "as a more efficient power generation system than steam turbines." The difficulties and risks in using the new technology led to termination of the development after his retirement. He died in Erie, Pennsylvania, on September 26, 1941, at the age of 82, so he had presumably managed to avoid mercury poisoning.

Mercury has a much higher boiling point than water, so the Carnot efficiency of a thermodynamic cycle that uses it is higher, in theory at least. The plants were dual-cycle, (also known as binary cycle) the hot metallic exhaust from the mercury turbines being used to produce steam for a second stage of steam turbines.

1914 Plans are made to build an experimental mercury boiler and power plant. (see below)

1923 The Hartford Electric Light Company (Helco) installs an experimental General Electric mercury-steam generating unit at Hartford, Conn.

1928/9 The first commercial mercury cycle generating unit in the world is installed at Helco's South Meadow Station in Hartford. Power output is believed to have been 10 MW. According one of my sources (possibly not that reliable) the Hartford plant used 13 pounds of mercury per kilowatt generated; that would be 130,000 pounds or 58 tons, which seems like an awful lot. See Oliver Lyle's comments below.

1931 A 20 MW General Electric mercury plant is installed at Schenectady, New York State, for the New York Power and Light Company.

1947 The last mercury/steam plant is constructed at Portsmouth, NH.

Left: A mercury/steam power plant. This is a mercury topping cycle. The mercury is vapourised in the boiler, drives the mercury turbine, and is condensed by boiling water to steam. This steam is superheated in the mercury boiler and then drives a conventional steam turbine. The "bled steam heaters" refer to the practice of heating the feed water by bleeding steam from various points along the turbine; this increases cycle efficiency and had been a standard feature of steam power plants for many years. Trust me, it works. From "Electric Power Stations" Vol 1, by T H Carr, 4th edition, 1954

Left: A mercury/steam power plant. On close inspection this diagram shows exactly the same plumbing as the one above. I have however included it here, as diagrams of mercury/steam plants are, to put it mildly, rather rare. From "Electric Power Stations" Vol 1, by T H Carr, 4th edition, 1954. According to this book, the mercury circuit operated at 200psi and 538 degC. Electrically welded joints were used in the pipework because the poisonous nature of mercury was recognised, and leaks were not acceptable. Even so, it is hard to believe that a big installation could have been completely free from leaks, and I for one would not have cared to work there, or indeed within ten miles of it.

In one American plant (it is not clear which) the mercury circuit operated at 113 psi and 507 degC, driving a 15 MW turbine-alternator set at 720 rpm. The mercury condenser produced steam at 400 psi and 371 degC, at the rate of 200,000 lb/hour; this drove conventional steam turbines and alternators.

The extract below is taken from a journal that appears to have been called simply "The Tech" published by students of the Massachusetts Institute of Technology. This issue is dated Saturday, April 25, 1914.

Engineering and Scientific Notes: Mercury Turbines
"In a paper recently read before the American Institute of Electrical Engineers, there were described some interesting experiments to test the feasibility of employing mercury as the working agent in heat engines. Mercury boils under one atmosphere at a temperature of 677 degF (358 degC) and in a 28 inch vacuum it will condense at 455 degF (235 deg C). It is proposed to use the mercury vapor to drive a turbine and to utilize the heat from the exhaust in generating steam to drive a steam turbine. Despite the poisonous character of the vapor and the comparatively high cost of mercury, the liquid has several physical properties admirably suited for this purpose. Its high temperature is associated with very moderate pressure, it is clean in use and does not wet nor corrode the blades, while its high density makes possible a very moderate blade speed, an important factor. As to the question of cost, it has been estimated that the mercury would not cost more than a maximum of ten dollars per kilowatt output and, of course, the same mercury could be used repeatedly. It is claimed that a tandem mercury and steam plant would show a gain of 44 per cent on output per pound of fuel. A boiler of considerable size is now under construction in order to test such a plant on a large scale."

Left: A later article on mercury power. From "The Tech" for March 1, 1926.

FROM "THE EFFICIENT USE OF STEAM"
"The Efficient Use of Steam" by Oliver Lyle was the bible of steam usage in Britain during the second world war and after. It was aimed more towards process steam rather than power generation, but the sixth edition (1958) contained the following information on mercury/water binary cycles: "Mercury boils at 900F under the very modest pressure of 80psi. At 28.5 in vacuum mercury vapour has a temperature of 437 degF (225 degC). If therefore, mercury is boiled in a boiler, and the saturated mercury vapour passed through a mercury turbine the mercury vapour can be exhausted into a condenser which can be a water boiler, and can raise steam to 250psi.This steam can be given some superheat up to say 600 degF (316 deg C) from the flue gases of the mercury boiler. Owing to the low latent heat of mercury it is necessary to use about 10 pounds of mercury for every 1 pound of water."

and later in the text:

"The disadvantages are somewhat formidable. Mercury vapour is extremely poisonous. Mercury does not wet metal surfaces.* The plant is complicated and costly. But several large mercury-steam stations have been working for some years in the USA and have shown very high sustained overall thermal efficiencies. other fluids might give better results. Diphenyl oxide has been suggested as being more suitable than mercury."

* If the surfaces are not wetted by the working fluid, then heat transfer would be greatly impaired. But you will see below that mercury does wet tantalum.

Now that's an interesting point above about the amount of mercury required. And it's not cheap stuff.

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THE MERCURY POWER STATION AT PORTSMOUTH, NH

The Schiller power station at Portsmouth, NH appears to have been the last binary mercury/steam plant built. The site has two decomissioned GE Mercury Turbines and two B&W Mercury/Steam boilers. I t currently uses three 50 megawatt coal-fired steam boilers built in the 1950s. One of these is being replaced with a fluidized-bed boiler to burn whole-tree wood chips and other clean low-grade wood materials. Read about The Northern Wood Power Project (external link)

Left: Mercury turbine-alternator in the centre, with mercury/steam boilers either side. Looking at the unit from the Alternator end. The two units on the sides are the mercury/steam boilers; from these the steam traveled back to the top of the mercury boiler, was superheated and traveled out to a steam turbine that has since been sold to Mexico... Picture and info courtesy of David E Corey

The mercury turbines produced 15 MW and the steam turbine 26 MW.

Left: Mercury Vapor Lines . The top pipe in the foreground is a steam line returning to the boiler from one of the 2 mercury/steam boilers; the 3 pipes in the background are the three separate mercury lines that go to the turbine nozzles. The mercury lines each have their own manual throttles and isolation valves. Picture and info courtesy of David E Corey

Left: Mercury Drum at the mercury Boiler. Picture courtesy of David E Corey

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MERCURY POWER IN SPACE: 1969
I have unearthed a NASA report from 1969 that describes a nuclear-heated mercury power plant intended to generate large amounts of electricity in space. It is called :"DESIGN AND FABRICATION OF A COUNTERFLOW DOUBLE-CONTAINMENT TANTALUM-STAINLESS STEEL MERCURY BOILER" from the Lewis Research Center, Cleveland, Ohio. It was part of the SNAP-8 program to develop a Rankine cycle power system for space applications.

The reactor is cooled with a mixture of sodium and potassium, (NaK) which heats a mercury boiler. The mercury vapour drives a turbo-alternator and is then condensed and subcooled by a secondary NaK heat rejection loop which transfers the waste heat to a radiator for rejection to space. Why is a second NaK loop used? I have no idea, but this is NASA we're talking about so I bet there was a good reason.
The report concentrates on the design of the potassium/mercury boiler, which is an exotic device indeed. It is a spiral stucture designed to fit in a small toroidal space in a cylindrical spacecraft; the heat exchanger tubes are of tantalum, which is readily wetted by mercury, making for good heat transfer. The net power output was 37 kW.

Left: The NASA nuclear mercury turbine system. Note the extra cooling and lubricating loop using polyphenyl ether.

Left: The performance data. The extraordinary thing is that the overall efficiency seems horribly low at 6.9%. A conventional terrestrial steam power station would give some where around 40%. So why is it so bad? The Carnot efficiency E = (T1 - T2 ) / T1 where T1 is the heat input (top) temperature and T2 the heat rejection temperature. Looking at the diagram above, the mercury leaves the boiler at 670 degC. The effective rejection temperature in the condenser is a bit less clear but the NaK coolant leaves it at 335 degC. This gives a Carnot efficiency of 35%. That's obviously a theoretical maximum, but a long step from 6.9%; there must be some serious losses somewhere. The rejection temperature has to be high because in space there are no rivers flowing by to cool your condensers- the only way to lose heat is to radiate it away from a flat plate, which needs to be pretty hot to radiate effectively. Hence the radiators in the flow diagram; these are nothing like central-heating "radiators" which work mostly by convection. This is one reason- possibly the most important- for the choice of mercury as a working fluid; it permits a high heat rejection temperature so the radiators are efficient, and therefore relatively small and light.

This project was only one of several nuclear-powered sytems that were intended to provide large quantities of electricity for long-duration space missions. One of the reasons that they have not so far been used is that no-one is very comfortable with the notion of launching a nuclear reactor on top of a rocket.

You can read the full NASA report on the potassium/mercury boiler here (External link)

A word of explanation about the use of sodium and potassium alloys. When they are between about 40% and 90% potassium by weight they are liquid at room temperature, which is very handy for a cooling fluid. The mixture with the lowest melting point (the eutectic mix), consists of 78% potassium and 22% sodium, and is liquid over the enormous range of -12.6 to 785 °C.

Another exotic heat-transfer/lubricating fluid is used in this system- polyphenyl ether, which is interesting stuff in itself, consisting of rotatable benzene rings joined by ether (-O-) links. You can read more about it here (External link)

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