Solar Transmittance of Greenhouse Covering Materials H.-J. Tantau, J. Hinken and B. von Elsner Institute of Biological Production Systems, FG Biosystems- and Horticultural Engineering, Leibniz Universität Hannover, Hannover, Germany J. F. J. Max, A. Ulbrich and U. Schurr Institut für Bio- und Geowissenschaften, IBG-2: Pflanzenwissenschaften Forschungszentrum Jülich GmbH Jülich, Germany T. Hofmann Centrosolar Glas GmbH & Co KG, Fürth, Germany G. Reisinger Friedrich-Wilhelms-Universität Bonn Rheinbach, Germany Keywords: PAR transmittance, light transmittance, AR glass, PMMA, PE-film, ETFE-film, drop condensation, dirt Abstract Light is an important growth factor in greenhouses. The choice of a covering material strongly influences the light transmittance of a greenhouse. For low energy greenhouses double and even triple covering materials are available. Also covering materials with higher light transmittances than conventional float glass are available. When these materials are used for double glazing the light transmittance might be equal or even higher than that of single glazing units. The aim of this project was to measure the transmittance of different covering materials for solar radiation and PAR under greenhouse conditions. For this purpose 20 different covering materials were installed on the southern roof of an east-west oriented greenhouse. Underneath these specimens, PAR and solar radiation were continuously measured and compared to the outside conditions from 2006 to 2011. Transmittance was determined for different incident angles of solar irradiation, direct and diffuse radiation, as well as under the influence of condensation, dust and dirt accumulation, and the aging of the materials. Drop wise condensation on the inner surfaces of the covering materials reduced PAR transmittance. Dust had an impact on the transmittance as well but its influence varied over time since dust particles accumulated during dry periods were partially removed by rain. An influence of aging on the materials’ light transmission could, however, not yet be detected. INTRODUCTION Light is an important growth factor. Therefore the development of greenhouse structures aims at increasing the PAR transmittance of the greenhouse construction. In the last decades the overall PAR-Transmittance of newly constructed greenhouses has been increased from about 60 % to more than 70 % (v. Zabeltitz 1986; v. Zabeltitz 2011). This is due to a reduction of constructional elements, smaller glazing bars, larger panels and covering materials with a higher PAR transmittance. Different types of glass are available (Max et al. 2013). Float glass is the glass type most commonly used to cover greenhouses. For safety reasons in some countries it is required by law to use tempered float glass for roof glazing. Low iron glasses are characterized by a higher UV transmittance as well as a slightly higher PAR transmittance compared to customary float glass. Antireflection (AR) coatings can further increase the PAR transmittance. Glass with photo catalytic coatings is reported to have self-cleaning properties (Pilkington 2012). UV stabilized LDPE film is used world-wide as a cheap greenhouse covering material. ETFE-film has a higher UV and PAR transmittance but is more expensive than LDPE. LDPE and ETFE films are available with no drop treatments avoiding drop wise