Masonry Magazine June 1981 Page. 18
bond patterns, projections and even sculptured brickwork to increase the asethetic value of the thermal storage media. Although at first the idea of leaving the brick exposed seems merely aesthetic, it does serve a function important to the thermal performance of the thermal storage media. Brick masonry does not require nor should it have any coverings: i.e., gypsum wall board, paints, wall papers, or carpeting which could decrease the emissivity of the surface. The addition of coatings and coverings not only may reduce the emissivity of the thermal storage element but will usually decrease the thermal conductivity thus decreasing the surface temperatures and the amount of surface radiant heat available.
If the value of emissivity, the surface temperature of the thermal storage element, and the surface temperature of the interior materials being radiated to are known, the amount of radiant thermal energy emitted may be approximated. The approximate amount of thermal radiation emitted per square foot of thermal storage surface area may be calculated using Equation 10:
q, = [0.174 × e × [T,^ - T^]]/10^
where: q, = Radiation, in Btu/hr/ft²
e = Emissivity.
T, = Temperature of the radiant surface measured from absolute zero, degrees Fahrenheit plus 459.6.
T. = Temperature of the receiving surface measured from absolute zero, degrees Fahrenheit plus 459.6. For most applications in passive solar energy systems, the value of T. is usually interior design temperature in °F plus 459.6, typically 72°F + 459.6 or 531.6.
Consider an average surface temperature of a radiating surface at 83°F and an interior design temperature of 72° F. Measuring these temperatures from absolute zero would result in T, = 83 + 459.6 or 542.6 and T. = 72 + 459.6 or 531.6. The radiation from a dark brown brick wall, with e = 0.93, may be determined using Equation 10 to be:
q, = [0.174 × 0.93 × [(542.6)^ - (531.6)^]]/10^
q, = [0.168 × (8.668 × 10¹º)]/10^
q, = (1.103 × 10¹º)/10^
q, = 11.03 Btu/hr/ft²
Absorptivity. The solar absorptivity of a material is mostly dependent on color. The solar absorptivity is the ratio between how much solar radiation is absorbed by a material to that absorbed by a standard black surface. Typically, passive solar thermal storage components (or any finish applied to such components) should be as dark a color as possible to provide sufficient energy absorption. However, trade-offs do exist between color, wall thickness and the amount of surface area exposed to sunlight. Trade-offs also exist between darkness of color and how much heat is desired and when the available heat is wanted. These trade-offs can only be adequately determined by rigorous analysis and are not recommended for use with rule-of-thumb approaches. Surfaces (such as frame walls) not being used for storage should be painted light colors in order to reflect as much energy as possible to the darker storage material. Although black is the most desirable storage material color from a thermal point of view, it has been determined that the darker natural brick colors (browns, blues and reds) will perform almost as effectively, without deterioration problems which may result when using paint or other coverings. Typical values for solar absorptivity of brick are given in Table 2. Brick with glossy glazed ceramic coatings should be avoided as they will reflect too great a percentage of the solar radiation striking them. Several brick manufacturers can supply brick with dull black ceramic glazed faces, which may increase the solar radiation absorbed.
Although it would seem at first glance that rough-textured brick, by providing more surface area for the collection of energy, would be more effective than smooth brick as an energy storage media, but it has been determined that this is not the case. It appears that brick texture does not have a major impact on the performance of passive solar installations and that any desired texture can be used without significant loss or gain in effectiveness.
TABLE 2
Absorptivity of Brick
| | Range | Average |
| :------------------ | :--------- | :------ |
| Dull Black Ceramic Glaze | 0.85 to 0.98 | 0.92 |
| Flashed (Blue) | 0.86 to 0.92 | 0.89 |
| Dark Brown | 0.79 to 0.85 | 0.82 |
| Red | 0.65 to 0.80 | 0.73 |
| Yellow or Buff | 0.50 to 0.70 | 0.60 |
| White or Cream | 0.30 to 0.50 | 0.40 |
GLAZING MATERIALS
General
Information regarding the transmittance, reflectance, absorptance, thermal performance and durability of glazing materials for passive solar energy systems should be obtained from the glazing manufacturer. Some general suggestions to assist in the design and selection of glazing materials for passive solar energy systems applications are discussed.