This paper describes an analytical method for calculating both the direct and the diffuse solar irradiation in an isolated gable roofed greenhouse. The effect of multiple reflection is neglected and the effect of incidence angle on the reflectivity and transmissivity is taken into consideration. The decrease of the solar irradiation due to the effect of the structural elements (frames etc.) has also been taken into account in the computational model.
The computational procedure was programmed for a computer, and was applied to a particular greenhouse with various orientations. Calculations were made for the winter solstice, the summer solstice, and the vernal equinox at Tokyo and similar latitudes.
The following conclusions were obtained from this analysis:
1. The variationof the space averaged transmissivity at the floor varies between about 55% and 70% depending upon the amount of structural elements shading the floor area. A higher transmissivity for direct solar radiation is obtained in an East-West orientation than with a North-South orientation in the winter wheras a lower transmissivity is obtained in an E-W orientation in summer and spring.
2. The ratio of the dialy integrated direct solar irradiation on the floor to the irradiation outside varies between about 0.52 and 0.80 in one case and between about 0.69 and 0.79 in another.
3. The structural elements have a more important effect on the distribution of the daily integrated direct solar irradiation than the shape or the orientation of the greenhouse.
4. The intensity of the diffuse solar irradiation is higher at the center of the floor than near the side walls. The difference of the transmissivity is about 10% for a usual greenhouse.