The material of a solar energy system piping may be copper, galvanized steel, stainless steel, or plastic. All pipes are suitable for normal solar system operation except plastic piping, which is used only for low temperature systems, such as swimming pool heating. Another problem related to plastic piping is its high coefficient of expansion, which is 3-10 times as high as that for copper pipes and causes deformation at high temperatures. Piping that carries potable water may be copper, galvanized steel, or stainless steel. Untreated steel pipes should not be used because they corrode rapidly.
System piping should be compatible with the collector piping material to avoid galvanic corrosion; for example, if the collector piping is copper the system piping should also be copper. If dissimilar metals must be joined, dielectric couplings must be used.
Pipes can be joined with a number of different methods, such as threaded, flared compression, hard soldered, and brazed. The method adopted also depends on the type of piping used; for example, a threaded connection is not suitable for copper piping but is the preferred method for steel pipes.
Pipes are usually installed on roofs; therefore, the piping layout should be designed in such a way as to allow expansion and contraction, have the minimum roof penetration, and keep the roof integrity and weatherability. A way to estimate the amount of expansion is indicated earlier in this chapter; the supports selected for the installation, however, have to allow for the free movement of the pipes to avoid deformation. An easy way to account for the expansion-contraction problem is to penetrate the roof at about the center of the solar array and allow for two equal lengths of loops on each side of the penetration point. If the pipes must be supported on the roof, this must be done in a way so as not to penetrate the weatherproof roof membrane. For this purpose, concrete pads can be constructed on which the pipe supports can be fitted.
Another important issue related to the installation of collector array piping is the pipe insulation. Insulation must be selected to have adequate R value to minimize heat losses. Other issues to be considered are insulation availability and workability, and because the insulation is exposed to the weather, it must have a high UV durability and low permeability by water. The last factors are usually obtained by installing a suitable protection of the insulation, such as aluminum waterproofing. Areas that require special attention in applying the waterproofing are joints between collectors and piping, pipe tees and elbows, and special places where valves and sensors protrude through the waterproofing. The types of insulation that can be used are glass fiber, rigid foam, and flexible foam.
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