Selective Surfaces

Collectors are black and gray in color and have a rough textured surface. The rough-surface absorbs solar rays better than a smooth surface. A smooth, shiny surface will reflect radiant energy away from the collector. Generally, surfaces are made of metal particles, rather than paint, because paint cracks and peels at high temperature.

Collector covers (glazes)

The transparent covers serve to admit solar radiation to the absorber while reducing convection and radiation heat losses from the collector. The covers also protect the absorber from dirt, rain, and other environmental contaminants.

The materials used as covers include glass or plastic sheets. Glass is most commonly used because of its superior optical properties and durability. Standard plate glass reflects about 8 percent and absorbs about 6 percent of normal incident solar radiation, resulting in a transmissivity of about 86 percent. Glass is subject to impact damage and is more expensive than plastic; however, it does not degrade in sunlight or at high collector temperatures and is more durable than plastic.

Although resistant to impact damage, plastic

generally degrades in sunlight and is limited as to the temperatures they can sustain without undergoing serious deformation. In general, acrylic is the most ultraviolet-resistant, and polycarbonate offers good impact and high-temperature properties.

Figure 15-2.-Types of solar heat collectors.

Collector Gaskets and Sealants

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.

Figure 15-3.-Typical sealing methods for single or double glazing.

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.

[ Back ] [ Home ] [ Up ] [ Next ]

Privacy Statement - Press Release - Copyright Information. - Contact Us - Support Integrated Publishing