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Why do you not use sand? It would be much easier to treat?
Sand has a pretty low thermal conductivity. If there is a huge pile of sand it will be difficult to get the the heat transferred in/out.
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How big should the stones be?
Preliminary analysis would suggest that size of 1 meter or so would be optimal for both providing efficient heat transfer within the stones and enabling convection through the stones.
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Will the stones not fall a part eventually?
Yes, that is a risk that should be studied. However, it should be noted the we are not planning to trough water on the stones so the temperature gradient and hence thermal stresses within the stone hopefully will not degrade the stone.
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The method of using stones has been studied by smart people. What did they miss?
We believe the the circumstances in Finland with a lot of cheap wind energy, lot of empty space and plenty of suitable stones make it economical to build larger storage facilities than initially considered.
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What is the key challenge in the proposed facility?
A key challenge is how to design and control the flow of heat such that the stones are heated everywhere and can also release their heat in somewhat controlled manner. Could natural convection be used for our benefit. The design of the heat flow and heat exchangers is something that requires special knowhow.
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How would the facility be insulated?
It seems that mineral wool is suitable to insulate the top and the sides. However, the insulation for the bottom is more difficult since it will need to hold up the huge amount of stones. Possibly we need to be content with using sand for the bottom insulation. [/qanda]