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The project at Gemasolar uses molten salt to store heat. This heat is then used with water to drive steam turbines. Over time does this kind of repeated heat/cold cycle make the molten salt a less effective medium to store heat? How would the old medium/waste be disposed of?

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Gemasolar uses a mixture of sodium nitrate and potassium nitrate. There's an overview of this and other molten salt mixtures in this pdf.

The sodium/potassium nitrate is chosen because it has excellent stability, both chemical and thermal. The molten salt mixture will in principle last forever and wouldn't need to be replaced. Presumably the Gemasolar salt circulation system isn't perfect and will gradually build up impurities in the salt e.g. water from leaks or metal from wear in the pumps. How fast this will happen I don't know, and I suspect Gemasolar won't know either until they get experience at running the plant.

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In any case, the usual thing to do at that point is to re-purify the working fluid, not to replace it. Indeed many working loops have a purify-as-you-go cutout capability. –  dmckee Jul 14 '12 at 18:09

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