Pool heating costs and energy use are significantly reduced by installing a solar pool heater. Their upfront cost is comparable with gas heaters, but they have very low annual operating costs. According to the U.S. Department of Energy, in some climates, solar pool heating is among the most cost-effective use of solar energy.

Beyond a pump and filter, most solar pool heating systems require only a solar collector to heat the water and a flow valve to direct water to the solar collector as needed. And in hotter climates, the system can be used to cool pool water if it is run at night.

Some systems include sensors and an automatic valve to bypass the collector if the sensor detects that the pool and collector temperature are similar.

Solar pool collectors are made out of different materials. The type needed depends on climate and seasonal use. If the pool will be used only when temperatures are above freezing, then an unglazed collector system is appropriate. Unglazed collectors don't have a glass covering. They are generally made of heavy-duty rubber or plastic treated with an ultraviolet (UV) light inhibitor to extend the life of the panels. Unglazed collectors are usually less expensive than glazed collectors. These unglazed systems can even work for indoor pools in cold climates if the system is designed to drain back to the pool when not in use.

Glazed collector systems are usually made of copper tubing on an aluminum plate with an irontempered glass covering, which increases their cost. In colder weather, glazed collector systems, with heat exchangers and transfer fluids, capture solar heat more efficiently than unglazed systems.

Glazed systems can be used yearround in many climates, and they can also can be used to heat everyday use water year-round.

Both glazed and unglazed collector systems must include freeze protection if they'll be used in colder conditions.

A solar pool heating system usually costs between $2,500 and $4,000 installed. Depending on local fuel costs and available solar resource, the return on investment is between 1 and 7 years. They also typically last longer than gas and heat pump pool heaters.

Before installing a solar pool heating system, it’s important to consider the following:

• Evaluate the site's solar resource.

• Determine the correct system size.

• Determine the correct orientation and tilt for the collector.

• Determine the system's efficiency.

• Compare system costs.

• Investigate local codes, covenants, and regulations.

Evaluating the Site's Solar Resource

The efficiency and design of a solar pool heater depends on how much of the sun's energy reaches the building site. Solar pool heating systems use both direct and diffuse solar radiation, so even sites that aren’t warm and sunny all of the time may still have an adequate solar resource. Basically, if the building site has unshaded areas and generally faces south, it's a good candidate for a solar pool heating system.

Sizing a Solar Swimming Pool Heater

Sizing a solar swimming pool heating system involves many factors:

• Pool size.

• Length of swimming season.

• Average regional temperatures.

• Desired pool temperature.

• Site's solar resource.

• Collector orientation and tilt.

• Collector efficiency.

• Use of a pool cover. Solar system contractors use worksheets and computer programs to help determine system requirements and collector sizing.

The surface area of the solar collector should equal 50% to 100% of the surface area of the pool. In cooler and cloudier areas, it may be necessary to increase the ratio between the collector area and the pool surface area. Adding collector square footage also lengthens the swimming season.

For example, a 15-by-30-foot outdoor swimming pool in Florida typically requires a collector that equals 100% of the pool's square footage to accommodate year-round use. This equals 450 square feet of collectors. In northern California, most people use outdoor pools 6–8 months per year, so they typically size their systems at 60% to 70% of the pool's surface area.

The use of a pool cover can usually decrease the required collector area.

A properly sized pool pump is also needed for a solar system. If a conventional pool heating system is being replaced with a solar system, it may be necessary to get a larger pump or a separate, smaller pump to move the pool's water to and through the collectors.

Placing a Solar Swimming Pool Heater's Collector

Collectors can be mounted anywhere near the swimming pool that provides the proper exposure, orientation, and tilt toward the sun. Both the orientation and tilt of the collector will affect the solar system's performance. This should be considered while evaluating the site's solar resource and sizing the system.

Collector Orientation

In general, the optimum orientation for a solar collector in the northern hemisphere is true south. However, recent studies have shown that, depending on the location and collector tilt, the collector can face up to 45° east or west of true south without significantly decreasing its performance.

It is also important to consider factors such as roof orientation (if the collector will be mounted on the roof), local landscape features that shade the collector daily or seasonally, and local weather conditions (foggy mornings or cloudy afternoons), as these factors may affect the collector's optimal orientation.

Collector Tilt

The angle at which a collector should be tilted varies based on the latitude and the length of the swimming season (summer or yearround).

Ideally, collectors for summer-only heating should be tilted at an angle equal to the latitude minus 10° to 15°. Collectors for year-round heating should be tilted at an angle equal to the latitude.

However, studies have shown that not having a collector tilted at the optimum angle will not significantly reduce system performance. Therefore, you can usually mount collectors flat on your roof, which might not be at the optimum angle but is more aesthetically pleasing.

It’s important to take the roof angle into account when sizing your system.

Determining the Efficiency of Solar Swimming Pool Heating System

The efficiency of a solar swimming pool heating system is based on the collector's thermal performance rating, which can be measured using a variety of units.

Thermal performance rating can be measured by Btu per square foot per day: Btu/(ft2day).

The rating can also be measured by kilowatt hours (kWh) per square meter per day: kWh/(m2day).

It can also be measured by Btu per day, which is simply the rating in Btu/ (ft2day) multiplied by the area in ft2.

Also used is kWh per day, which is the rating in kWh/(m2day) multiplied by the area in m2.

The higher the number, the greater the solar energy collection efficiency. However, because weather conditions, instrumentation accuracies, and other test condition constraints can vary, the thermal performance of any two collectors should be considered approximately the same if their ratings are within 25 Btu/(ft2day) of each other.

High efficiency solar collectors not only will reduce annual operating costs, but may also require fewer square feet of collector area to heat the pool.

Comparing Solar Swimming Pool Heating System Costs

Before purchasing a solar pool heating system, estimate and compare the costs of using different solar collector models. This will help determine the potential cost savings of a more efficient type of collector, which may require fewer panels for the collector area needed to heat the pool.

To estimate and compare costs, get the following information:

• A collector's thermal performance rating(Btu/day).

• Total number of collector panels or piping for the area needed to heat your pool.

• Total installed cost of system. It is then possible to calculate a collector's energy output per dollar invested using the following formula: (Btu/day X # of collector panels/ piping modules) ÷ total installed cost of system = Btu/$ per dollar spent For example: A collector’s thermal performance is rated 27,900 Btu/day, with 4 collector panels needed, at an installed cost of $3,000.

(27,900 X 4) Btu ÷ $3,000 = 37.20 Btu/day per dollar spent.

If one wishes to compare the efficiency of one solar panel to another, and the prices and thermal performance ratings (Btu/day) of the collectors are known, the following formula can be used to calculate the energy output for each dollar invested for different collectors: Btu/day ÷ collector price = Btu/day per dollar spent.

For example: A collector has a thermal performance rating of 21,000 Btu per day, and costs $387.

21,000 Btu ÷ $387 = 54.26 Btu/ day per dollar spent.

It isn’t recommended to choose a solar pool heating system or collector based solely on its estimated costs. Also consider all of the factors involved in the system's sizing and quality of the design and installation.