SOLAR TRANSMITTANCE OF GREENHOUSE COVERING MATERIALS

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.