I referred to "a molten salt", which in practice is never sodium chloride; probably because its melting point is higher than some superior alternatives. Molten

I referred to "a molten salt", which in practice is never sodium chloride; probably because its melting point is higher than some superior alternatives. Molten salt is used for two reasons. It is the best real-world conductor of heat you will find. If you heat a miles-long pipe of molten salt on one end, that heat will make its way to a temperature rise on the other end in surprisingly rapid fashion. But this is only true when it is molten. Naturally, if molten salt encounters its solid salt, the solid melts pretty easily to expand the volume of molten salt. Molten salt stores quite a bit of energy in its phase change, and of course as a salt solidifies, it becomes more dense. This means the top salt (at the collector) will always be the slowest to solidify. Not sure where all this worry about "freezing to death" comes from. The salt's effectiveness comes from its conduction, not its flow.