Once I read about an idea where the concrete floor of a big greenhouse was laid down with shallow channels molded into it, then covered with sturdy lids. Water was stored there, warmed during the day and giving off bottom heat through the night.
I forget whether they pumped it through a heat exchnager during the day. I think it was some different scheme where the North wall of the greenhouse had black 55 gallon drums stacked up where the sun would hit them and warm the water.
It seemed to me that the first idea would be easier to do, and maybe last longer, if you poured the concrete floor around PVC pipes. 1/2" or 3/4" if you relied on pumps to circulate water through the floor from a reservoir, or bigger pipes if the floor WAS the reservoir.. Since heat rises, I would think that ther best place to collect heat during the daytime would be near the ceiling.
