I'm no physicist but I assume that you'd need to put a lot more energy into the system before the water starts boiling, so you wouldn't have to worry about it?
If the energy would just be lost otherwise, you might as well go all the way to 100℃?
That's at 1 atm, perhaps the water is a bit pressurized as well?
A bit further down there's this confusing quote though:
> District heat is stored in two rock caverns in Mustikkamaa. The temperature of the water in the caverns varies between 50 and 90 °C.
That would mean that the rock baseline would be 10 degrees below freezing, which seems improbable. I think it would be close to the yearly average temperature for Helsinki (6.5°C) that close to the surface. Maybe I'm missing your point?
6.5 sounds right. Average ground temperatures down where they are stable are under ten degrees C. So this project will definitely warm up the surrounding rock, which (I guess) will increase its effective heat capacity.
> "perhaps the water is a bit pressurized as well?"
I suppose in such a large storage vessel, most of the water is naturally pressurised by its own weight. So most of it won't boil, even at 100°C, except right at the surface?
If the water is not being heated from the bottom then I'd expect all the hottest water at the top/and or at the point of entry to the system. Heat loss into the holding substrate will cause rather rapid convection and surface boiling.
