What are Solar Thermal Storage Tanks?

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It doesn't matter whether you're installing a huge solar thermal system to heat the water for an entire community or a small residential setup to lower your power bills: you'll need a storage tank to keep hot water in. And since the sun has an annoying tendency to be weaker in winter and disappear at night, that tank needs to be insulated to keep your water hot as long as possible.

Most solar thermal installations use a traditional steel tank, coated on the inside with a baked-on layer of enamel (often called the "glass lining") which protects the metal against corrosion and gives the tank a much longer life. The tank is surrounded by foam insulation, on the outside of which a light-gauge steel jacket sits, to protect the insulation. Basically, it looks just like any standard water heater.

Higher-quality tanks usually come with an anode rod to prolong their life even further. This is a special metal rod, also called a "sacrificial" rod, which screws into the top of the tank and extends into the water. The anode's job is to rust before any other part of the tank, thus warning you of impending problems. Anode rods should be checked every five years or so; steel tanks usually last anything from 15 to 30 years, depending on tank quality, water quality and location.

The modern alternatives to a steel tank are plastic and fiberglass models. They haven't existed for very long because it's only recently that advancements in thermoplastics and fiberglass have allowed engineers to produce equivalent tanks of similar size, while keeping them competitive on cost.

Plastic and fiberglass tanks are both built much like steel tanks, except that the tank itself is made of the alternative material and the jacket is made of plastic. Their biggest advantage over the traditional steel tank is that they won't rust or corrode, assuming you choose a tank that is rated for constant temperature extremes. Bear in mind that "any old plastic" simply will not do the job.

The downsides of alternative tanks are twofold and are caused by the materials' relative weakness when compared to solid steel. The first is that the pipe connections must not be tightened too much. Most steel tank connections are cranked really tight when they're installed – it's more secure and, since they're made of steel, you can get away with it. But plastic and fiberglass can break, so use plenty of pipe dope, Teflon tape or sealant on the fittings and don't tighten them too much.

Secondly, you should make sure you buy and install a vacuum breaker. When you drain the tank for any reason, there is the possibility of creating a near-vacuum inside. Although plastic and fiberglass are both tough materials and can take a fair hammering, tanks like these are structurally weak when the pressure comes from inside. Steel can take it; alternatives can't. By ensuring there's no vacuum created during maintenance, you stop the tank from buckling.