Solar pool heating systems are one of the most cost efficient uses of solar thermal technology. The cost of heating the average residential pool using a gas or electric heater is approximately $2,000 per year, according to the US Department of Energy. The cost of the average solar pool heating system is between $2,000 and $4,000. Although solar pool heating systems are exempt from federal and most state incentives and rebates, the payback period is usually between one to two years. After the payback period, owners of solar pool heating systems can enjoy their solar heated pool for free!

Solar pool heating systems work much the same way as drainback solar domestic hot water systems. In the collector loop, the pool water is heated in the solar collectors. The pool pump will be turned on and the flow of the pool water will go through the panels when the collector temperature is greater than the measured pool temperature. A pool heating system generally doesn’t include a heat exchanger. When the pool panels are no longer hotter than the pool (or the maximum desired temperature is reached) the flow is no longer directed through the panels. The panels (with the assistance of a vacuum breaker) would then drain the fluid back into the pool. If a certain temperature is required then a back-up heater would be installed after the pool panels prior to the heated water reentering the pool.
If your home already is already utilizing a closed loop solar domestic hot water system, you may be able to integrate a solar pool heating system. A valve would be installed on the glycol mixture’s return line. This valve would be able to divert the heated glycol mixture to a second heat exchanger to heat pool water. A sensor and controller would be required to prioritize the two loads. The domestic hot water system would receive first priority. The heated glycol mixture would be diverted to the pool heating system, the second priority, only after the required temperature of the domestic hot water system had been met.
The above integration of a solar pool heating system into an existing http://www.solarhotusa.com/index.html may not be employed in an open loop system.

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.”

I am a strong advocate of the work that the SRCC (solar rating and certification corporation) is doing in testing and certifying solar collectors but….  There are a number of factors that can have a significant impact on the certified performance that aren’t published in their certifications and can lead to significant differences in performance as well as longevity.  Below are a few things that have a significant impact on the performance of the solar collector that currently aren’t reported:

1.  Glass

1. Hardiness – as part of the certification process the solar collector glass has to undergo repeated thermal shocks to insure that it doesn’t fail.  It is commonly understood that in order to pass this test the solar collector glass must be tempered.  The tempering process is expensive for glass manufacturers and leads to significantly higher glass prices for the collector manufacturer.
2. Transparency – when the solar collector s are tested for certification they are tested as a whole and the transparency of the glass is not measured separately.  There are two steps that can be taken to increase the thermal transmission of the glass.  The first is going with low iron glass.  This is the most common grade of glass that is used in solar collectors.  The second step that can be taken is going with an anti-reflection coating on both sides of the glass.  This coating allows low angle light to pass through the glass rather than be reflected by it.  Unscrupulous manufacturers could substitute tempered, low-iron, anti-reflection glass for their certified collector (they are only required to produce 5 samples) and then use standard float glass to supply there everyday production.  This higher performing glass would give them a significantly higher performance number from the testing but then they could cut the cost of their glass by 75% by going with standard float glass for their production.

2. Absorber

1. Thickness – as a rule the thicker the absorber sheet the greater it’s ability to carry heat (less thermal resistance).  Unfortunately, a thicker sheet costs more to produce.  Currently, the published data does not specify at what absorber thickness the collector was tested so again an unscrupulous manufacturer could substitute a thinner material once they have received their solar collector certification with higher numbers.
2. Riser quantity – the shorter distance between the fluid channels (tubes) the less heat is lost.  The number and spacing of the tubes is not published.  It is possible (and has been reported to have happened) where a manufacturer increases the spacing between there tubes to save money on the solar collector and the contractor/homeowner are none the wiser since they are sold the solar collector based on the SRCC solar collector rating.
3. tube thickness – in order to pass the rigorous 160 psi standard required for open loop collectors manufacturers can reduce the wall thickness of their copper tube once the certification has been achieved.  This will save the manufacturer money but diminish the expected life of the collectors

Random variation

1. Every process has natural variation including the production of a solar collector.  Currently, the SRCC (or there certified testing labs) tests only 1 collector (from a batch of 5) to determine what the published data for that collector and manufacturer will be.  Even their testing procedures have variation in them (this has been much discussed in the industry).  With only taking a single sample you could infer that Michael Jordan either never missed a shot or never made one (both equally wrong).  Without several data points it is easy to see how seemingly identically constructed collectors have widely difference performance numbers.

So all I have shown is that unscrupulous companies can (and will) take advantage of the system but what can we do about it?

I would recommend that we convert the current system of 1 scheduled inspection every 10 years to one where the certified manufactures pay an annual fee for the testing that will be done that year.  The samples to be tested would then be pulled randomly from the manufacturers on the list based on surprise audits.  This would serve the purpose of leveling out the costs associated with the testing process (fees would be annual based on the number of models needing testing and therefore your chance of being selected in a given year), eliminating the complaints of back-logs in solar collector testing because you would have your certification by paying your fee until your solar collector is inspected,  and freeing the solar collector performance numbers from the influence of gaming that I discussed above.