Increase of light transmission of a Venlo-type greenhouse during winter by 10%: a design study

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During winter and at higher latitudes, natural light is the limiting factor for crop growth in greenhouses. The design of Venlo-type greenhouses in the Netherlands has not changed for many years, although recent developments of diffuse glass with anti-reflective coatings might necessitate alteration of the roof design for maximized light transmissivity during winter. Light transmissivity of a greenhouse, in general, is influenced by roof slope, gutter orientation, roof shape, dimensions, position and reflectivity of construction elements, screen installation, position and transmissivity of screen material, transmissivity and light-scattering pattern of the covering and the effect of condensation on the inner side of the covering material or screen. Using the ray-tracing model RAYPRO (Swinkels et al., 2001), the integral effect of all individual and combined measures on light transmission of the greenhouse at crop level was calculated. The results show that it is possible to increase light transmission by more than 10% with a novel greenhouse roof concept. Results of ray-tracing calculations for individual and combined measures are described in this paper. After identifying the roof concept with the theoretically highest increase in light transmissivity during winter in the Netherlands, economic feasibility and all constructional restrictions were taken into account in close cooperation with industrial partners. This has led to a novel roof design that was built as a demonstration greenhouse on a scale of 500 m2 in summer 2016 at Wageningen University & Research station in Bleiswijk. This “winterlight greenhouse” had resulted in a re-design of the traditional Venlo-type greenhouse roof, covering and screen.