Large Thermal Storage Tank with Immersed Tank Heat Exchanger

This is a simple and elegant thermal storage tank and heat exchanger installation from Alan Rushforth of Rushforth Solar LLC.


Thanks very much to Alan for providing this material!

The tank provides thermal storage for a large solar domestic water heating system in a 22 unit apartment building.



The 2000 gallon storage tank is 7 ft high and 8 ft - 3 inches in diameter.  It is made from a roll of 0.058 inch thick sheet aluminum.  The tank is just sheet metal wrapped into a cylinder with the ends overlapped and fastened together.  This is an efficient design for a water tank, in that the water pressure loads are taken by simple tension in the cylinder wall -- there are no bending loads in the tank wall.


The tank is insulated with 2 layers of 1.5 inch rigid foam board insulation on the inside and bottom of the cylinder.  The insulation is polyisocyanurate with aluminum foil face sheets.   The tank is then lined with sheet EPDM.  The EPDM liner is a single piece that is just tucked and folded to fit the shape of the cylinder.


This is a drain back system, and the tank is vented to the atmosphere (not pressurized).   When there is enough sun, water is pumped from the tank and up to solar collectors mounted on the roof.  The heated water is returned to the storage tank.  Note the two pumps that are connected in series to pump the water from the basement all the way up to the roof.


The water to be heated for the people in the apartment flows through the two stainless steel tanks that are immersed in the large tank.  Heat is transferred from the storage tank water to the potable water by conduction through the stainless steel tank walls.  This preheats the water before it goes to the conventional water heater for the apartment.   The stainless tanks are about 50 gallons each.  During periods of low demand, the water in the immersed tank heats up to the full temperature of the storage tank water, and provides some storage of preheated water to satisfy high demands.  If the preheated water is not hot enough for final use, the conventional water heater will take it up to temperature.


Note that all plumbing connections are made above the waterline in the storage tank, so there are no penetrations of the liner below the waterline that might leak later.   The "U" shaped loop that goes down the side of the tank to the two pumps, and then back up to the output line to the collectors insures that the pumps retain their prime.


The immersed tanks appear to be doing a good job -- see the performance plot below.


So, this shows two nice ideas:

You probably don't need an installation this large for your house, but it can be scaled down appropriately to your water or space heating needs.


Another variation on this theme would be to use a large coil of PEX or even HDPE pipe in place of the immersed tank for the heat exchanger.  An immersed 300 ft coil of 1 inch pipe would hold 12 gallons of water, and even when the 12 gallons was exhausted, it would have enough surface area to act as a pretty good heat exchanger.


All of these immersed tanks or pipe coil schemes are single wall heat exchangers, so the fluid on the outside of the exchanger tank must be non-toxic, and you should check with your local code folks to make sure this arrangement is legal in your area.


From Alan:

"I am pretty pleased with the performance of the immersed tanks.  Two of them (100 gallons total) works pretty well.  Three would been a bit better. After that - diminishing returns.  I am still thinking about using galvanized tanks for that - maybe fed by pex/al/pex to minimize galvanic action."


The Alan made 2000 gallon thermal storage tank.




The two stainless steel heat exchanger tanks mounted inside the thermal storage tank.




Note that even with a 22 unit building, the "preheat out" temperature never falls far below the "Tank Top" temperature.


Alan Rushforth

Rushforth Solar LLC

Bryn Mawr, PA



Gary April 25, 2008