Solar photosynthetically active radiation transmission through greenhouse glazings

One critical factor for crop energy conversion for plant growth is photosynthetically active radiation (PAR) received by the plant. While it is important to know their total solar radiation transmission characteristics in the design of greenhouse for thermal environment management, it is also essential to understand their PAR transmission capability, especially over the winter period for high-latitude regions. This paper presents the results of PAR transmission of four different greenhouse glazings, measured at both the glazing and crop canopy levels. The glazings studied were single glass, double glass, twin-walled acrylic and air-inflated double polyethylene. The first three materials were tested at a commercial rose greenhouse range (gable type) in Connecticut and the double polyethylene greenhouse (bow type) was located at Cook College, Rutgers University. Also reported is the comparison between total solar radiation transmission and PAR transmission in the double polyethylene greenhouse. The glazing level PAR transmission showed mainly the effects of glazing materials, sky clearness and solar angle of incidence, whereas PAR received at the canopy level was strongly influenced by the greenhouse geometric configuration and internal structures. It was found that air-inflated double polyethylene transmitted a higher percentage when measured in the total solar radiation range than in the PAR range.