Mercury as a Working Fluid.

MERCURY IN THE NINETEENTH CENTURY

This is part of a British Association Paper called "Some of the Developments of Mechanical Engineering during the Last Half Century" By Sir Frederick Bramwell. It appeared in The Scientific American Supplement No. 312, on 24th December, 1881. While the mercury is not being used here as the working fluid (steam is) it's an interesting account of a bizarre idea.

"There was another kind of marine engine that I think should not be passed over without notice; I allude to Howard's quicksilver engine. The experiments with this engine were persevered in for some considerable time, and it was actually used for practical purposes in propelling a passenger steam-vessel called the Vesta, and running between London and Ramsgate. In that engine the boiler had a double bottom, containing an amalgam of quicksilver and lead. This amalgam served as a reservoir of heat, which it took up from the fire below the double-bottom, and gave forth at intervals to the water above it. There was no water in the boiler, in the ordinary sense of the term, but when steam was wanted to start the engine, a small quantity of water was injected by means of a hand-pump, and after the engine was started, there was pumped by it into the boiler, at each half revolution, as much water as would make the steam needed. This water was flashed on the top surface of the reservoir in which the amalgam was confined, and was entirely turned into steam, the object of the engineers in charge being to send in so much water as would just generate the steam, but so as not to leave any water in the boiler. The engines of the Vesta were made by Mr Penn, for Mr Howard, of the King and Queen Ironworks, Rotherhithe. Mr Howard was, I fear, a considerable loser by his meritorious efforts to improve the steam-engine."

"There was used, with this engine, an almost unknown mode of obtaining fresh water for the boiler. Fresh water, it will be seen was a necessity in this mode of evaporation. The presence of salt, or of any other impurity, when the whole of the water was flashed into steam, must have caused a deposit on the top of the amalgam chamber at each operation. Fresh water, therefore, was needed; the problem arose how to get it; and that problem was solved, not by the use of surface condensation, but by the employment of reinjection, that is to say, the water delivered from the hot well was passed into pipes external to the vessel; after traversing them, it came back into the injection tank sufficiently cooled to be used again. The boilers were worked by coke fires, urged by a fan blast in their ashpits, but I am not aware that this mode of firing was a needful part of the system."

The idea of using an amalgam of mercury and lead as a reservoir of heat was not a good one, and one regrets that Mr Howard was a considerable loser. The liquid with the highest specific heat, and so greatest storage capacity, is not some exotic chemical. Remarkably, it is plain water. Much cheaper and much safer!

MERCURY IN THE TWENTIETH CENTURY

The man behind the use of mercury vapour turbines in electric power stations 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.

THE 1949 SOUTH MEADOW POWER STATION AT PORTSMOUTH, NH

Left: The mercury part of the plant at South Meadow in 1949. The mercury vapour leaves the mercury boiler drum, and drives the turbine at the top of the drawing; it then exhausts to the mercury condenser-boilers where water at 410 degF is converted to steam at 450 degF and 410 psi. This steam passes through a superheater in the top of the mercury boiler and then goes to the station steam header at 700 degF and 385 psi. The condensed mercury goes back to the mercury boiler drum. Note the use of air pre-heating; air was supplied to the burners at 523 degF. From Power Generation March 1950

THE SCHILLER MERCURY POWER STATION AT PORTSMOUTH, NH

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.