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Solar Shed -- Thermal Storage Tank

This section shows the construction of the 500 gallon water tank that stores about one days worth of heat output from the 240 sqft of Solar Shed collectors. The tank is capable of storing about 75 KWH of thermal energy.

I considered various kinds of tanks to store the 500 gallons:

A Polyethylene cylindrical storage tank. These are available at the local AG stores in a wide variety of shapes, and for good prices. I decided against these because they are only rated for continuous exposure to 120F. Since the storage temperatures will be higher than this, I (reluctantly) decided against this option.
A Polypropylene storage tank. These are available with ratings up to 200F and more -- this would be plenty. I think they would do the job well, but they are kind of pricy.
A galvanized metal stock tank, possibly lined with EPDM. These are relatively cheap, and may be a good option, but the shapes available did not fit my space. They tend to be shallow tanks with large diameters.
A plywood and lumber framed tank with an EPDM liner.

Plywood/EPDM tank during construction

I decided on a plywood tank with an EPDM liner. These tanks have been successfully used for quite a while, and are reported to have a life of 10 to 20 years and more. They can be built to fit the space you have available. The tanks are easy to build, but the tanks must be structurally up to withstanding the fairly large loads from the water -- 500 gallons weighs 4150 lbs.

The tank is insulated to about R30 to reduce heat loss (1).

I am not a tank designer, and I will not guarantee that this design is the best, or even that it will hold together for you. But, I have indicated here why I designed it the way I did, and what (I think) is important from a structural point of view. I have heard a stories about these types of tanks failing when not built properly, so be careful -- 500 gallons could make a big mess!

Tank Details:

Note 1: The tank is insulated to R30 because it lives outside. With an ambient temperature of 30F, and a tank temperature of 110F, the tank loses 320 BTU/hr, which lowers the tank temperature by 0.077F per hour (about 1F overnight).

Gary 10/25/06