A flat black paint should be used to reduce possible reflection. Special coatings may reduce the amount of heat radiated from the plate to surroundings. These coatings absorb the same amount of radiant energy as regular black surfaces, but emit or radiate much less. Materials generally used for collector plates are copper, plywood, aluminum and steel. Copper is the most expensive but also has the highest thermal conductivity.
Steel is the least expensive but also has the lowest conductivity of the three.
For water heating collectors especially, the conductivity is important. For copper, the water tubes, which are bonded to the absorber plate, may be spaced farther apart than for a material with less conductivity. If the entire area of the absorber is swept by the transfer fluid, the conductivity of the plate is much less important. See Table 2 for more information. Radiant energy absorption and emittance for several surface materials.