I think the sand direction is deff an improvement over the 300 delta_C systems now with (NaNO3, KNO3, NaNO2) at the 900 delta_C in used in the pdf, but idk if it will win out in the long run with the need for direct contact vs some kind of sCO2 system with higher delta_C mixtures (for example a theoretical mixture CaCl2 + MgCl2 with 1500 delta_C, or even engineered metamaterials[0, i've only seen some specific examples of electromagnetic metameterials when i was in school about a decade ago, this post was about a year after i left] in a mixture to have even higher delta_c).
There's only so much one can do when one limits the materials used for heat capacity to naturally occurring ones (i imagine that compounds in the future could be engineered to be at least a order of magnatude higher in delta_c from natural mixtures).