Another issue in Finland is that the average soil temperature (like, a few meters down) is only five or six degrees. But that leave a lot of room for a heating

Another issue in Finland is that the average soil temperature (like, a few meters down) is only five or six degrees. But that leave a lot of room for a heating upgrade... IMHO if you can capture the massive surplus of sunlight in summer and use it in the winter, it would be a major win. Some way to set up a big underground heat reservoir ?