An 8-foot cube of water in the basement might store 15 kL of water, which is heated using a copper tube with fins in the collector. The performance of this can be further improved by putting the finned tube inside another layer of glazing at the back of the main collector, allowing the temperature to build up more than the surrounding air stream.
An 8-foot cube of water in the basement could hold 15 kL of water and is heated using a copper tube with fins in a collector.  Adding another layer of glazing behind the main collector can further improve its efficiency by raising the temperature above the surrounding air stream.