"there are only a few days per year you get very cold temperatures coupled with nearly no wind at all"

This is a terrible handwave. How many days per year, in the middle of winter, in a cold country, are you OK with having no power?

The system in the article works alongside gas and wood chips heating, so there are other options in place if the sand battery cannot be "charged".

FTA:

> The project will cut fossil-based emissions in the Vääksy district heating network by around 60% each year, by reducing natural gas use bu 80% and also decreasing wood chip consumption.

Please read the hn guidelines and reconsider your participation.

This would be an argument for widespread backup power, actually. If every residence had enough backup power to get through 24 hours, it would be far easier to deal with these relatively rare doldrums.