SOLAR IRRADIATION INSIDE A SINGLE-SPAN GREENHOUSE WITH SHADING SCREENS

A model is offered which yields the solar irradiation at the ground of a single-span greenhouse fitted with screens, in terms of the outside solar irradiation, the position of the sun, the optical properties of the cladding and screening surfaces, the geometry of the screening su rface and orientation of the axis of the greenhouse. The model is math ematically exact. From the physical point of view it is founded on the assumption that both the cladding and the screening surfaces are full y diffusive and the radiation diffused through the atmosphere and by t he ground is fully isotropic. The importance of the geometry and of th e optical properties of the screen in blocking the admission of solar irradiation inside are indicated. It is found that the solar irradiati on inside a greenhouse, with any given cladding material, with any ori entation and fitted with shading screens, is mainly determined by the transmittance of the screening surface and to a lesser extent by its r eflectance. The collected radiation increases with the transmittance o f the screening surface and for a given transmittance, with the reflec tance of both the bottom side and topside of the screen. It is also fo und that a screening surface forming a closed cavity with the inside g round is always more efficient in blocking the admission of solar irra diation than any other shape.