Condensation and radiation transmittance of greenhouse cladding materials: Part 1, Laboratory measuring unit and performance

In order to assess the effects of condensation on the radiation transmittance of greenhouse cladding materials, controlled laboratory conditions were developed. The experimental unit consisted of two climate rooms (hot room/cold room) separated by a partition wall containing a sample of the cladding material under investigation. Climatic apparatus in both rooms were used to create condensation on the inner surface of the cladding. A HeNe-laser was used as a monochromatic radiation source in the photosynthetically active radiation (PAR) range, while a large integrating sphere with a photodetector measured the transmitted radiation flux. The measuring method included a calibration factor and a correction factor for the internal reflection fault. This arrangement enabled investigation of the relationship between the incidence angle and the transmittance of monochromatic radiation during a complete condensation cycle (dry phase; condensation without run-off; condensation with run-off; and evaporation phase) on different cladding materials. The performance of the climate and the optical apparatus of the laboratory unit for determining the transmittance of greenhouse cladding materials was assessed. To this end, climate factors inside the laboratory unit were measured and analysed during a complete condensation cycle. The reliability and the accuracy of the optical apparatus and the transmittance measuring method were ascertained by comparing the experimental angular transmittance results of a dry glass plate with simulation values, for perpendicular and parallel polarized radiation. The results demonstrated the reliable performance of the entire laboratory unit. (C) 1999 Silsoe Research Institute.