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

Transmission of solar radiation by a multispan Venlo-type glasshouse w as measured at a height of 0.5 m below the gutters, using four tube so larimeters inside the glasshouse, mounted on a 3.8 m pipe which descri bed half a circle around its middle in a horizontal plane every 8 min and rotated in the opposite direction during the following 8 min. Radi ation measurements were averaged over 2 min intervals, thus representi ng average radiation level for a sector. Total global radiation and fr action diffuse were measured outside. The measurements showed almost c onstant values of about 62% for transmissivity on dull days (90-100% d iffuse radiation), whereas average daily transmissivity for a sector r anged between 51% and 73% on those days. On clear days (when a large p art of radiation is direct), transmissivity could increase from values below 40% in the early morning up to 70% around noon. Measurements we re used to validate a glasshouse transmissivity model which predicts t ransmissivity for direct radiation based on solar position, roof angle , dimensions of the roof construction parts, transmissivity of the gla ss panes and the orientation of the glasshouse. For diffuse radiation, transmissivity was calcualted by integrating the weighted transmissio n 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 s howed reasonable agreement with the measurements (clear and dull days in May and November).