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"In addition to creating steam, the heat transfer fluid is used to heat molten salt in tanks adjacent to the steam boilers. The thermal energy storage system includes six pairs of hot and cold tanks with a capacity of 125,000 metric tons of salt, and the molten salt is kept at a minimum temperature of 530 degrees Fahrenheit."

(link at bottom of OP's article)

I believe salt is used for thermal storage, but oil is the heat transfer fluid used to generate steam.

solar -> oil -> steam || salt

salt -> oil -> steam

From what I gathered the one that has the largest storage capacity measured in "minutes at maximum power."