[osolemio]

Very interesting!

I have a 1000 liter tank (~300 USG) which is made for this already, from the Austrian Okofen (US dealer is located at oekofen-usa dot com). The one I have has two heating circuits, a solar heat exchanger and hot water is flow-through in a stainless steel pipe, not "sump water" (always fresh water, both hot and cold).

The solar heat exchanger is at the bottom, but it has a "chimney", an internal vertical tube connected, with holes in it. The hot water in the buffer water will exit from this chimney at exactly the level where it does most good, as the density of the water changes slightly with temperature. If there is plenty solar heat, it goes to the top to give a good temperature for the hot water supply, leaving the bottom less heated for times where there is less heat from the solar panels.