TRANSMISSION OF SOLAR-RADIATION BY A MULTISPAN VENLO-TYPE GLASSHOUSE - VALIDATION OF A MODEL

Transmission of solar radiation by a multispan Venlo-type glasshouse was measured at a height of 0.5 m below the gutters, using four tube solarimeters inside the glasshouse, mounted on a 3.8 m pipe which described half a circle around its middle in a horizontal plane every 8 min and rotated in the opposite direction during the following 8 min. Radiation measurements were averaged over 2 min intervals, thus representing average radiation level for a sector. Total global radiation and fraction diffuse were measured outside. The measurements showed almost constant values of about 62% for transmissivity on dull days (90-100% diffuse radiation), whereas average daily transmissivity for a sector ranged between 51% and 73% on those days. On clear days (when a large part of radiation is direct), transmissivity could increase from values below 40% in the early morning up to 70% around noon. Measurements were used to validate a glasshouse transmissivity model which predicts transmissivity for direct radiation based on solar position, roof angle, dimensions of the roof construction parts, transmissivity of the glass panes and the orientation of the glasshouse. For diffuse radiation, transmissivity was calcualted by integrating the weighted transmission of direct radiation over the sky hemisphere. Using measured fraction diffuse, glasshouse transmissivity for the actual radiation condition was calculated. Both predicted level and dynamics of transmissivity showed reasonable agreement with the measurements (clear and dull days in May and November).