Gaskets and sealants must be carefully selected if a collector is to have a long life. Generally, the housing and the glazing have different rates of thermal expansion. Gaskets and sealants form a flexible interface between the two components and seal out moisture and other contaminants. If they fail, moisture fogs the glazing and may damage the absorber coating and the insulation. These problems can drastically reduce the thermal performance of the collector.
Two suitable sealing methods are shown in figure 15-3. The gaskets provide flexible support and the primary weather sealant ensures against ,oisture leakage. Desiccants are sometimes placed between the two glazings to absorb any moisture that may remain after cover installation.
When you are selecting collector gaskets and sealants, certain material requirements must be kept in mind. The gaskets and seals must do the following:
1. Withstand significant expansion and contraction without destruction
2. Adhere effectively to all surfaces
3. Resist ultraviolet degradation
4. Resist outdoor weathering
5. Not harden or become brittle
6. Withstand temperature cycling from -30F to 400F
Silicone sealants have been found adequate for use as gasket material. Silicone sealants have exceptional weathering resistance and have received widespread use for many years.
The choice of collector fluid is important because this is the lifeblood of the system. The cheapest, most readily obtainable, and thermally efficient fluid to use is ordinary water. However, water suffers from two serious drawbacks: freezing and corrosion. Therefore, the choice of collector fluid depends on the type of solar system the choice of components, future maintenance, and several other factors.
Implicit in this discussion is the use of fluid in the collector. As explained in table 15-1, an air solar system does not suffer from corrosion or freezing. The low density and heat capacity of an air solar system requires the use of fans, large clarge storage volumes, and it is not suitable for domestic water heating.
Figure 15-3. - Typical sealing methods for single or double glazing.Continue Reading