The Finnish system is just heat->heat. They generate heat when the wind is blowing, and inject heat back into the district system when it isn't. Super simple - resistive heating, and passive heat transfer.

This system produces electricity. Exciting, but much fancier.

A stirling engine can generate heat from electricity, such as those used in MicroCHP, you can burn wood, produce heat, and generate electricity from that heat. You could do the same with a pile of sand.

You just need to pipe liquid through the sand, and a supply of cooler water.

Yes, this involves higher temperature (the sand is stable up to ~1200 C) and transfer of heat from the sand to a working gas by means of Babcock & Wilcox's fluidized bed heat exchanger technology. This is a neat idea that intimately mixes the gas and sand for very rapid and compact heat transfer.

Using resistive heaters, round trip efficiency (back to electricity) is estimated to be around 52%.