$2K Solar Space + Water: Storage Tank Design

This section goes over the design of the tank used to store heat for the solar water and space heating system.

The tank is a very simple, inexpensive and easy to build design that provides large storage capacities at low prices. 

The design has a long, proven track record.

The tank can be built to accommodate odd shape space, and can accommodate spaces with limited access.


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The tank we use in this system is basically a plywood box that is framed with 2 by lumber.  The inside is insulated and then lined with EPDM rubber membrane.  The tank is vented to the atmosphere, so they only need be designed to resist the weight of the water, not house water pressure.  The tanks can be built for capacities from about 100 gallons on up to 800 gallons or more.

Plywood box, lumber frame, insulation inside

EPDM rubber lining with plastic wood edge.

This is my third system using this type of tank, and I have become a firm believer in the design -- which goes back to at least the 70's.

Pros and Cons of this Tank Design

It can provide large capacity inexpensively.  The larger capacity, well insulated  tank improves system performance.


EPDM lined tanks have proven to have a long life, and at the end of the liner life (15 years?), the EPDM liner can be replaced at a cost of about $50, and you are good to go for another 15 years.


The EPDM liner can handle temperatures up to 180F+, although life is reported to be longer if the tank is kept to 170F or below -- it should be really long at the 160F or less that this system normally operates at.


The tank can be made to fit in, and make efficient use of limited spaces (in my case a limited height crawl space). 
AND, it can be built in pieces that fit into areas with limited access (such as my crawl space). 


The tank is unpressurized and vented to atmosphere -- it can (and does) serve as both the hot water storage tank and the drainback tank for the collector.  This combined functionality eliminates one more expensive item from the system.


The design allows you to use as much insulation as you like to reduce heat loss.


The tank is large enough to allow the use of a large coil of plastic pipe as the heat exchanger.  This makes for an inexpensive and essentially 100% efficient heat exchanger.


The lack of any liner penetrations below the waterline eliminates the possibility of leaks at penetration points.


On the negative side, you need to be certain that the tank sits on level smooth ground, and that it does not sit in a puddle. 


Its not as nice to look at as a  cool new stainless steel tank -- but, you could always have the kids draw some artwork on it -- the couple thousand dollars you save will by a lot of crayons.

The Tank Construction section provides all the details on how to build the tank, and the precautions to take to insure that the tank is structurally strong enough.
The guidelines for choosing the right tank size are covered in the Collector Design and Sizing section.


If you decide the EPDM lined box tank described here is not for you, here are a few other alternatives:


- There is a scheme that uses multiple plastic barrels that is described on the JC-Solarhomes website.  One caution with this design is that maximum tank temperature is limited by the plastic material -- you will need a way to keep the system from heating the barrels over this limit.


- Some have used precast septic tanks for outdoor locations.  They must, of course, be well insulated.


- Our friend Tom at American Solartechnics makes several solar heated water storage tanks including a new kit version that is very reasonable in price.


- There are places that will make custom liners for any tank size you specify.  These liners will fit your tank without the folding needed for our single sheet EPDM liner.


- And there is this "hole in the ground" tank -- the ultimate in simplicity!

- There are a few more ideas for tanks on our tank page...






Gary February 16, 2011