Up until just a few years ago, solar swimming pool heating was far and away the largest use of "solar energy". The reason is simple and straightforward. It makes economic sense. It competes head on against very inexpensive natural gas for heating swimming pools. The unglazed (no glass over the collector) plastic collector is low cost per area exposed to the sun and when heating low temperature water like a swimming pool (or even pre-heating domestic hot water) the efficiency is high. In other words a large portion of the solar energy exposed to the collector is collected and delivered as energy to the load. This simple and obvious combination makes for the most cost effective and therefore most viable solar energy solution. Over the last 37 years this solar energy market has thrived but there have been ups and downs.
The first low cost unglazed solar collectors for swimming pool heating were introduced around 1969 under the trade name "Solaroll". The product was made of a synthetic rubber called EPDM. It was a flexible collector. It thrived. In 1972 "Fafco" introduced the first rigid collector. This collector, still in production today, was made of a type of polypropylene. The header pipes are permanently attached to the absorber sections in the factory resulting in fixed size collectors. 4x12 has become the most common size. The advantages to this design are that they are low cost to manufacture and low cost to install. Solaroll by comparison, required the installer, dealer, or homeowner to attach all the headers to the flexible fin-tubing after cutting the fin-tubing to length. This extra work traded off with the extra capability in terms of being able to fit the collector to the space. In addition these original flexible collectors could be used in areas where freezing was an issue. If water is trapped inside a rigid collector and it freezes the tubes break and the remedies require sealing off entire flow cores. The flexible Solaroll on the other hand could be mounted flat on a roof where water will not drain out. It could freeze solid with water in it. This was the state of the art in the industry 30 years ago? What happened since? Solaroll no longer exists and Fafco was mimicked over and over again and today 75% of all solar pool heating is done with rigid polypropylene boards floating around under straps. Most in the industry can tell you flexible EPDM synthetic rubber collectors like Solaroll had a fatal flaw. Chlorine in the swimming pools caused the flexible material to break down. One of the big chemical companies proposed a solution. It was a polypropylene based flexible extrusion called Santoprene. As the story goes, it broke down faster than EPDM and the result was an industry calamity. From this point forward the rigid polypropylene collector industry thrived.
But the rigid 4x12 panels were limiting. They couldn't be applied to flat roofs without fear of freeze damage. Available space could not be fully utilized and what could be less esthetically appealing than big black rectangles on a rooftop? Where there is demand there is supply. The epdm rubber tubing collectors came back, slowly and gradually. Its been 30 years since the major fallout. Hot Sun installed a 2000 sq ft system in North Carolina using EPDM and it took only 3 years in the field for the system to start churning black crud into the pool.
Luckily Hot Sun had decided this was risky and had already switched to a plastic based alternative. The experience in North Carolina was the final straw and we went 100% non EPDM after that point... because we could!
There is an undeniable appeal to using EPDM. Its tough. It usually lasts long enough that these issues occur well into the lifespan when most people don't even remember who they bought their systems from. Hot Sun's solution is not without its downsides too. There is no perfect answer. We have to use a mechanical or an adhesive connection to join the tubing and the headers. It takes more time to assemble. Theoretically flexible plastic is not as strong as flexible epdm so we have to be more careful with system pressure but what we've found are better and better thermoplastics such that today our flexible plastic is as strong as most competing epdm products with one major distinction. Powerstrip is guaranteed not to break down for 25 years. Some manufacturers of some EPDM products can't have water left in them over winter due to "freezing issues". That mindbender really means that if you leave water in the epdm tubing and it heats up the black crud will come off the solar panel and go into the pool. Freezing seems to trigger the release, we think just due to the physical stretching. We don't believe anyone else has even tried to resolve the issues with epdm in the solar business. My point is we can guarantee it won't happen for 25 years. EPDM solar manufacturers can't guarantee it won't happen in 10. The seal manufacturers have certainly acknowledged this problem. Here we're talking about chlorine in drinking water (much lower concentrations) and city water temperatures. The effect is thought to double with every 10 degree increase in temperature. The solutions are special compounds that are very expensive and even then the effect is just reduced.