Abstract
It is well known that the rough surfaces absorb more incident radiative energy than the smooth surfaces. In urban regions the surface of the earth is rough due to buildings built on it. Therefore it is expected that the surface of the earth in the urban regions has special absorption properties for solar radiation. To consider the absorption properties a simple model of the surface is used. The model is the surface on which rectangular parallelepipeds with square horizontal cross sections are laied out regularly at right angles. In the previous paper, reported by authors, the absorption properties of the model rough surfaces for sky radiation were discussed. Since sky radiation is diffusive, the absorption properties for sky radiation are determined only by the conditions of the rough surfaces. On the other hand direct solar radiation is directional and the absorption of it is affected by the direction of the radiation. Therefore absorption properties for direct solar radiation are more complex than for sky radiation. But the amount of energy contained in direct solar radiation is the greater part of solar energy on the earth. Thus it is essential for the consideration as to the absorption of solar radiation to clarify the absorptivities for direct solar radiation. In this paper the absorption properties of the model rough surfaces for direct solar radiation are calculated and discussed. It is shown that the absorption properties change considerably due to the incident angles of the radiation and the geometries of the surfaces. The changing patterns of the absorption properties due to the incident angles are classified into three basic types according to the geometric factors of the surfaces. The absorptivities of the rough surfaces for solar radiation change with time, because direction of solar radiation is a function of time. Therefore mean absorptivities over a whole day, considering the change of the location of the sun with time, are calculated. It is expected that the mean absorptivities change seasonaly, because direction of solar radiation changes seasonaly. Thus the mean absorptivities are calculated at the representative days in a year; the winter solstice, the vernal (autumnal) equinox day and the summer solstice. The absorptivities at the vernal equinox day lie between at the winter and summer solstices, namely they are average values in a year. It is noticeable that absorptivities for sky radiation, discussed in the previous paper, are nearly equal to those at the vernal equinox day. For the open rectangular two dimensional cavities, which are the special forms of the model rough surfaces, absorptivities for sky radiation are nearly equal to those at the vernal equinox day too. As a result average absorption properties of the model rough surfaces for direct solar radiation can be estimated by the diagrams for sky radiation shown in the previous paper.
