Читать книгу Solar Water Heating--Revised & Expanded Edition - Bob Ramlow - Страница 28

The single largest application of active solar heating systems in the world is in heating swimming pools. Special collectors have been developed for heating seasonal swimming pools: they are unglazed and made of a special copolymer plastic. Most are made of proprietary materials containing UV stabilizers to prolong their lifespan. The collectors don’t have to be glazed because they are used only when it is warm outside, and they are heating water to a relatively low temperature that is just slightly above ambient. These collectors cannot withstand freezing conditions.

Figure 3.7: Pool collector

Figure 3.8: Web-and-tube configuration

The absorber plate in copolymer collectors is an extruded mat that has many waterways, or risers, that are set close together. Many designs have been produced, but the design that has proven most successful is a web-and-tube configuration. With this design, the tubes are separated by a small web so they are not touching one another. Much like copper absorber plates, these collectors have a header running along the top and bottom of the absorber. The best designs, like web-and-tube absorbers, are made so that each riser is individually attached to the header, which allows the least back pressure within the collector and allows individual connections to be repaired, if needed. Because the risers are situated close together, this type of collector has a large wetted surface, adding to its efficiency. In fact, these collectors can outperform glazed collectors in seasonal pool heating applications.

Solar pool heating systems that use unglazed plastic collectors are direct systems in which the pool water is circulated through the collectors. This is the most efficient configuration as there are no heat exchanger losses. Most pool water contains additives such as chlorine, which is highly corrosive. This corrosive property of pool water makes copper absorbers or copper pipes noncompatible. This is one of the reasons the collectors are made of plastic, which is not affected by chlorine. The same holds true for the piping, which is typically PVC.

Plastic pool heating collectors are typically mounted flat on a roof. The collectors are held in place with a set of straps that go over the collectors but are not actually attached to them. The straps are often plastic-coated stainless steel and are threaded through special clips that are bolted to the roof. This method of holding down the collectors allows them to expand and contract on the roof without binding and makes them easy to install.