Effects of sun radiation on building elements
The relevance of the effects of sun radiation on the thermal behavior of buildings can be demonstrated by an experimental study. In this study the short wave sun radiation is measured by a pyranometer. At the same time, the air temperature and the surface temperature of the building component have been measured as function of time. Knowing both temperatures, the overheating of the wall surface by radiation, the Surface-Overtemperature can be calculated.
Under adiabate conditions (no heat transfer to the interior) the Surface-Overtemperature would reach a maximum called Radiation-Overtemperature. The Surface/Radiation-Overtemperature ratio is a measure for the temperature amplitude attenuation that depends on the thermal storage capacity of the building element. Heavy buildings can store more heat than lightweight constructions; their temperature amplitude attenuation would be higher.
With knowledge of the Surface/Radiation-Overtemperatures, the long wave emission and the heat convection it is possible to calculate the heat load and the absorption coefficient of the building element. The study carried out in the frame of the Doctoral Thesis of Prof. Roloff, TU Dresden yielded absorption coefficients for a variety of surface colors and saturation degrees.
Experimental results
The first data plot below shows the influence of the surface color on the surface temperature of a heavy construction (concrete). The Surface-Overtemperature reaches from 7.5 K (white) to 30 K (black). The measurement was carried out at Jul 2-3, 1968 in Dresden, Germany.
The second data plot shows the influence of the heat storage capacity on the surface temperature. A difference of up to 9 K in Surface-Overtemperature between a heavy (concrete) and a lightweight construction (polystyrene) could be measured. A second effect could shown: the surface temperature falls below the air temperature in the night. This effect is called under-cooling by long wave emission in clear nights with little cloud covering. The measurement was carried out at Aug 14, 1968 in Dresden, Germany.
The third data plot shows again the influence of the heat storage capacity on the surface temperature. The under-cooling effect is relevant for lightweight constructions. All constructions on Polystyrene show under-cooling effects, the (blue marked) curve of the concrete construction stays clearly above the air temperature. The measurement was carried out at Aug 10-11, 1968 in Dresden, Germany.
Evaluation of results
As one result of the experimental study a relation between the Surface-Overtemperature and Absorption coefficients could be determined. This relation is basically linear but depends on the sun radiation intensity. The colors were assigned to the absorption coefficients, ranging from white to black.
Evaluation of the experimental results delivers absorption coefficients a function of the surface color, or better as function of the saturation degree of the surface color. The variation of the absorption coefficient within a color can be higher than within a specific saturation degree of different colors.
The table below is just an excerpt of the data gained from the experimental study and literature reviews.
