I'm using a 4'X8' heat exchanger from an old steam boiler in an insulated box with a thermopane window, on the roof of my shop, and a smaller heat exchanger in the 500 gallon tank. I plan on using a 50/50 mix of glycerol and water for the transfer liquid.
I also plan on installing a valve set so I can transfer the heated fluid to heat the shop when I'm not using the tank.
I tried a small test last summer with a hundred foot coil of black plastic pipe on the roof of my shop, and developed 185 deg temps with a 4 gal/min flow.
My friends call me Don.
"I always keep a supply of stimulants handy in case I see a snake, which I also keep handy" WC Fields
RIP Steve Price. A true friend and a tireless supporter of FTE