Opened: 14 Nov 2008
Updated:
Powered by boiling potassium! |
Opened: 14 Nov 2008 |
There are some other working fluids which have their own galleries in the Museum:
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Potassium, a soft metal with a well-deserved reputation for exploding when dropped into water, is not the obvious choice for a working fluid. However, it is perfectly feasible to boil potassium and use the vapour to drive a turbine. Like the mercury turbine, the motivation is to have a high heat-rejection temperature. This is essential in a space power-plant because heat can only be rejected by radiation, and a body needs to be hot to radiate effectively. Because of its high boiling point potassium can also be used to make a topping cycle for a steam plant in the same way as mercury.
Potassium melts at 63.4 degC and boils at 759 degC; this is not ideal, as a working fluid that solidifies in the pipes when you switch the system off is somewhat less than convenient. One answer is the use of a mixture of sodium and potassium, (NaK) which has been used as a coolant for nuclear reactors. When sodium/potassium alloys have between about 40% and 90% potassium by weight they are liquid at room temperature, which is a very handy thing in 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.
Hot potassium is cerainly a hazardous substance, but it is not an insidious poison like mercury.
POTASSIUM ENGINES
Considerable experience in potassium power-plants was built up by USA agencies, the aim being the generation of large amounts of power in space- far more than could be obtained from solar panel arrays, especially during missions at a long distance from the sun. Here is a fascinating and up-to-date (2004) document from The Oak Ridge National Laboratory. Note that on page 23 (document page numbers, not PDF numbers) two systems using mercury boilers (SNAP-2 and SNAP-8) are mentioned.
 | Left: A two-stage potassium vapour turbine.
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