Numerical Experiments on Light Transmission in Greenhouses (1)

Abstract

Light transmission in the greenhouse is an important factor influencing the design and layout of commercial greenhouse enterprises, since the quantity of solar light is quite often the limiting factor for plant growth in the greenhouse during winter months in Japan.Recenty several workers have developed mathematical models which are capable of predicting the light transmission of greenhouses. While these models have been of great value, most of them have not considered the width and depth of the structural element, the shape of a frame, and the arrangement of structural elements. Several experiments, on the other hand, have shown that the structural frame can account for 60 to 70 percent of the total light losses in the greenhouse, and that a relationship exists between the arrangement of structural elements and materials of a greenhouse and the resulting distribution of transmitted solar light.This paper presents an analytical approach to this problem. A frame with a glass sheet is considered as a fundamental element of greenhouse walls for analyzing transmission mechanisms of direct solar light into a greenhouse. The approach is based on a general model so that it can be adapted to any single greenhouse for which the light transmission characteristics and dimensions of each frame have been defined. But no provisions are made for internal reflection of light and for light diffusing covering materials in the present model. Models considering these effects have already been described in the other paper. Applications of the present model to complete greenhouses will be described in a later paper.Transmissivities of solar light have been calculated of fundmental elements. The results obtained from this analysis show that the transmissivity for a fundamental element can vary widely with the dimensions of a frame and geometrical position of the sun (relative to the frame), and that the arrangement of structural elements has an important effect on the distribution of the daily integrated solar light in the greenhouse.Some of the results for the paticular fundamental element whose dimensions are given in Figs. 3 and 5 are as follows:1. The horizontal structural element running E-W direction constructing the south roof and wall causes an ununiform space distribution of the daily integrated light in the greenhouse.2. The depth of a frame can account for 20 to 30 percent of total light losses in the greenhouse.3. A higher transmissivity for a fundamental element is obtained in a greenhouse with square frames than with rectangular one.4. The transmissivity of a vertical fundamental element whose azimuth is south is 82% in winter and 42% in summer.