There are two goals for this project: Store excess solar/wind energy (i.e. when the grid cannot take the electricity aka curtailment) and buy energy when it's cheap for use when it's not. In a deregulated energy market, these are often the same thing: At high noon, the spot market price for electricity in a saturated PV market goes to zero so, since there's nowhere to put it, so you might as well use it to heat sand. Since homes in Finland can use low grade heat (for space and water heating) a large part of the year, there will be demand eventually if, as mentioned in the article, the system is well enough insulated to hold the heat for months ("seasonal storage"). The reason for using sand is that it's robust, doesn't wear out, and is almost literally as cheap as dirt. Because it can stand being heated to 500C, a given storage volume can store more thermal energy than would be possible with another medium (for example, water). Although it's not the exact same approach, see, for example, the Drake Landing Solar Community thermal energy storage project: www.dlsc.ca

The only time I ever heard Science Friday host Ira Flatow actually speechless (he's usually quite chatty) was when one of the NIST optical clock researchers told him that the Flatiron mountains on the southwest edge of Boulder significantly affect the accuracy of these clocks.
So, my question is: Where in the universe do you take them to decide the final calibration? Maybe Boulder CO USA is the center of the universe...