Collector type. Since UV radiation degrades plastic, appropriate UV inhibitors are important. Collectors that have been rated by the Florida Solar Energy Center (FSEC) have their absorber material listed with their performance rating. Standard materials include copolymer plastics, such as polypropylene and polyethylene. If collectors with metal absorbers are used, care must be taken to ensure that the fluid is not acidic, since this could corrode the absorbers.
Unglazed flat-plate collectors look like large plastic mats. This popular style of collector typically uses two large headers connected by several smaller risers that lie parallel to one another. This arrangement splits the flow between these risers and accommodates various pool pump flow rates, which may be as high as 75 gallons per minute (gpm).
Many unglazed flat-plate collectors are not self-supporting, and must be installed on a roof or a frame constructed specifically for the collector array. Since a strong wind could easily lift these lightweight collectors, they are strapped or clamped to the support structure.
Wind can also negatively impact unglazed pool collectors by stealing their heat. For example, a 10 mph wind can reduce a collector’s efficiency by 20%. Some pool collectors feature a plastic glazing to help reduce these losses.
Coil-type collectors consist of black, coiled tubing that is unglazed or includes an acrylic cover. Pool water is pumped through the tubing, which may be constructed from a variety of materials, including low-density polyethylene. Only two manufacturers—SolarTech International and Gull Industries—offer SRCC-certified coil-type SPH collectors, which feature warranties of up to 20 years. Other coil-type pool collectors can be purchased through retail outlets, but typically only offer warranties ranging from 90 days to one year (see “Collector Types” table).
Coil-type collectors provide greater pumping resistance than unglazed collectors, which split the flow through multiple pathways. This increased resistance—known as pressure drop or head loss—can be minimized if multiple collectors are installed in parallel, which splits the flow between collectors.
A system that utilizes five coil-type collectors in parallel and pulls water from the pool at a flow rate of 20 gpm will have a flow rate of roughly 4 gpm in each collector. If the system used only a single coil-type collector in the mechanical loop and the flow rate of the pool water was maintained at 20 gpm to ensure proper pool filtration, the flow through the single collector would be 20 gpm. With this configuration either the head loss in the single collector would require more pumping energy or the reduced flow rate caused by the increased resistance would reduce the efficiency of solar collection and impact water quality in the pool.
Some homeowners have installed a single, large coil-type collector. To maintain water quality and minimize additional pumping costs, a bypass can be installed. The bypass allows 4 to 5 gpm to flow through the collector, while the other 15 to 16 gpm returns to the pool.
Since the production of unglazed solar pool collectors is extremely low in cold weather, glazed flat-plate collectors are sometimes used in SPH systems for indoor pools. However, this brings an additional complication—almost every certified glazed flat-plate collector uses copper piping. If pool water is run through the collectors, it can cause deterioration of the copper and stain the pool. As a result, many indoor pool-heating systems isolate the pool water from the solar collectors with a stainless steel or titanium heat exchanger. The solar loop in a heat-exchanger-based pool heating system may be configured as a drainback or an antifreeze system to prevent freeze damage. (See “Drainback Solar Hot Water Systems” in HP138 and “Closed-Loop Solar Hot Water” in HP140.)
Warranty. The quality of a collector material and construction can be indicated by the warranty. Industry-standard pool collectors typically carry a warranty of 10 years or more. When considering the strength of a warranty, it is always beneficial to do some research into the viability of the manufacturer.