Here is my plan for my storage tank,I can buy a galvanized stock tank for $200.It measures 2ft.x3ft.x8ft. 300 gallon cap.easy to fit in my cellar.I am planning to buy three of these tanks and stack them on top of each other.insulation will be fiberglass with ridgid foam top and bottom.For heat exchangers I am planning to use 3/4 type m copper and build a hx similar to Erics.the plan is to have a separate hx for each tank.1 inch header to connect all three.use counterflow to add or remove heat as needed.I still need to figure out statification problems in the hx design.Seems to me like the simplest and most cost effective way to do it.What do you think?