Abstract
Transmission of solar radiation into greenhouses is an important factor in greenhouse design. Factors which influence the solar irradiation in a greenhouse can be divided into three groups. They are external, structual, and internal factors (see Fig. 1).This paper describes an analytical method for calculating the direct solar irradiation in an isolated gable roofed greenhouse. The effect of the first and second order internal reflection in a greenhouse is taken into consideration. The effect of incident angle on the reflectivity and the decrease of the solar radiation due to the shading effects of the structural elements have also been taken into account in the computational model. But the thickness of the structural elements, the decrease of transmissivity due to flirts on covering materials, and diffuse transmission (reflection) due to water droplets condensed on them are neglected. The calculation, therefore, may include some overestimations for an actual greenhouse.The computatianal procedure was prograammed for a computer, and was applied to four types of East-West oriented greenhouse: one of which is a usual glasshouse and the others are a glasshous with reflective mirror on the wall and/or roof. Calculations were made for the winter solstice at 35° latitude for use at Tokyo and smilar latitudes. The total direct solar radiation received by the floor of the house is assessed by summing the primary (or straight) and secondary (or reflected) direct solar radiation contributed by each wall and roof suface separately (see Fig. 2).The following onoclusions were obtained:1. The ratio of the daily integrated secondary solar irradiation on the floor to the direct irradiation outside is about 0.1 for a usual glasshouse placed in East-West orientation. But its distribution on the floor is not uniform.2. The average total direct solar irradiation on the floor in an East-West oriented glasshouse with reflective mirror on the north side wall is more than the direct solar irradiation outside.
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