At what scale does this become efficient? I may have 1000 sqft to dedicate to this type of system on my lot. Feels like that’s at least an order of magnitude too small to maintain the energy through the seasons. Could we build one of these slightly larger systems for every square mile (~1000 homes), or does this only work at a 10,000 home scale? The article is showing a pile of dirt on the ground. Could this just be an area of the subsurface which is heated, or does ground water become too much of a problem?

There are some companies making heat batteries that you can install inside your house. They won't last very long but it's a great way to store excess energy from your rooftop solar and use it for heating and warm water. Also it allows you to timeshift when you draw power from the grid (i.e. when it's cheap rather than when you need hot water).

Larger scale batteries can store enough energy for seasonal storage. The larger the size, the better the insulation can keep the heat losses to a minimum. Basically you have a smaller surface area relative to the volume and mass. But even with a small unit, you can keep it hot for quite long.

Stuff like this is easier in areas that are already on some sort of district heating or have some kind of water based central heating. For those systems it's pretty much plug and play. You don't really need to modify the houses.

I think Helsinki has a few larger scale units already operational and a few more under planning / construction. I think the largest one will store 90ghw of heat. Which is quite a lot.

The beauty with thermal storage is that almost any kind of mass with enough heat capacity works. Water, rocks, sand, etc. All fine.

You can scale this down to be reasonable for homes, yes, but they're specifically trying to make it usable for large industrial processes: https://en.wikipedia.org/wiki/Seasonal_thermal_energy_storag...