Wood atmospheric solar storage tanks made of treated plywood are the most popular tanks for
do-it-yourself homeowners. Polyisocyanurate insulation is usually used, with an EPDM liner. Additional insulation outside the wooden tank is recommended. Cost will vary with each installation, with the polyisocyanurate insulation being the most expensive element of the design.

Fiberglass atmospheric solar storage tanks are extremely corrosion resistant and are maintenance free. They are constructed of a thick inner liner of fiberglass with an insulation layer, usually of polyurethane and a plastic outer layer. The insulation is generally installed on site although it can be purchased factory installed for some sizes. During installation, care must be taken not to over tighten fittings which can break if too much force is used. Fiberglass tanks will last from 20 to 30 years although they generally only come with a one year warranty. Although prices vary depending on the grade of the material these tanks can rival the lined steel tanks in economy.

Atmospheric solar storage tanks made of welded stainless steel (either 304 or 316) are readily available. Stainless tanks are fabricated based on the particular space and volume requirements of the system. It is also possible to use multiple stainless tanks manifolded together. This approach can give you the ability to use the combined storage capacity of two tanks that may not be able to be installed in a single unit. The disadvantage of stainless tanks are:
1) price – these are generally the most expensive of the atmospheric tank options although many times they are a fraction of the cost of pressurized large volume tanks
2) insulation – these tanks generally require insulating on site.

Polypropylene atmospheric solar storage tanks have the longest lifespan, at about 50 years. Polypropylene is a rigid plastic that is rotationally molded. Tanks are constructed of Polypropylene but would need to be insulated on site. Polypropylene tanks have a temperature rating of 200 degrees F, but are not recommended for use with temperatures below freezing. Caution: When looking for plastic tanks for your solar water storage needs you need to avoid the more common Polyethylene tanks. The polyethylene tanks can only handle temperatures of around 120 degrees and would not be suitable for solar applications. With polypropylene tanks you need to concern yourself about shipping costs since the large tanks generally only come from a few facilities in the country. These tanks are middle of the road price wise for atmospheric storage tanks.

Atmospheric solar tanks made of welded mild steel are common. Inside the tank, the steel is first line with polyiso foam insulation, and then lined with the EPDM material. EPDM stands for Ethylene Propylene Diene Monomer. EPDM is a synthetic rubber that can be made in a variety of grades. Although manufacturers of EPDM lined tanks indicate that they have an indefinite lifetime and come with a warranty against leakage, many solar experts are concerned about EPDM for solar storage, stating that the material will deteriorate under the high temperatures of solar hot water systems. These tanks are generally the least expensive tanks overall.

All atmospheric solar
storage tanks are constructed of an outer layer to withstand environmental conditions, an inner layer of some type of insulating material to reduce heat loss, and an inner liner to reduce corrosion and extend the life of the tank. However, atmospheric solar storage tanks are constructed of many different types of materials.

Since large capacity tanks are difficult to maneuver through doorways, assembly of some atmospheric tanks is done at the installation site. This may reduce the shipping costs as well.

next – stainless steel tanks

Solar water heating systems almost always require some kind of thermal storage for solar heated water. Tanks used in solar water heating systems are available in two basic types: pressurized and atmospheric.

Pressurized tanks, constructed of stainless steel or welded steel with a baked-on glass liner, are made to withstand city water pressure without rupturing. Polyurethane foam insulation is used in between the metal skin and interior tank to minimize heat loss. Pressure relief valves are included in the design of pressurized tanks. If the pressure builds up too high inside the tank, the relief valve opens and water is released. Pressurized tanks range from about $500 for an 80 gallon capacity to about $17,000 for a 1,000 gallon capacity. The life span of a pressurized tank around 13 years but can vary depending on local water conditions and tank maintenance (changing the anode rods regularly).

Atmospheric tanks are basically just non-pressurized containers to hold solar heated water. Since they are not completely sealed from the surrounding atmosphere, they should be well insulated. An atmospheric tank that is filled beyond its capacity will overflow. In addition, atmospheric tanks will lose water through evaporation. A variety of materials are used in the construction of atmospheric and include stainless steel, EPDM lined steel, EPDM lined wood, polypropylene and fiberglass. Tank costs and longevity vary with material selection.

A two tank solar water heating system consists of a solar storage tank (it can be atmospheric or pressurized) and a pressurized backup tank. A one tank solar water heating system uses a pressurized tank which serves both as the solar storage tank and the backup water heater.

Simply stated, when using an atmospheric solar storage tank in your design, heat from the collectors is delivered first to the atmospheric tank, then on to the pressurized tank. When no atmospheric tank is used in the
design, with only a pressurized tank, the solar heat can be sent directly to the one pressurized tank.

Should you incorporate an atmospheric solar storage tank in your solar thermal design? Your solar equipment manufacturer should be able to help you decide what is best for your individual needs. However, if your tank capacity needs to be over 240 gallons, it may make sense to include an atmospheric tank for economic reasons.

See upcoming blog article for more information regarding atmospheric tanks.

G. Paul Menyharth, director of the American Solar Institute, weighs in with the test data and collector performance concluding that “the selection should be based on how much energy could be collected per dollar spent for the desired application.”