Analysis of Cooling Ventilation Efficiency in a Naturally Ventilated Almeria-Type Greenhouse with Insect Screens

The main objective of this particular study was to analyse different methods of measuring the capacity of the greenhouse to cooling the crop zone by natural ventilation. The experimental measurements on an Almeria-type greenhouse presented here show that very low ventilation efficiency can sometimes occur. Air exchange rates deduced from direct determination of the heat flow throughout the greenhouse openings, using the eddy-correlation technique, are compared with local estimations of mean and turbulent flows, obtaining a mean coefficient of heat transport ventilation efficiency eta(Q)=0.41. The airflow at the side opening was influenced by the proximity of an obstacle to the greenhouse (an advertising sign close to the northeast side vent opening). The obstacle increases the turbulence intensity of the airflow and reduces its capacity to transport heat. The mean flow of sensible heat is estimated at between 86 and 97% of the total flux, and so the turbulent flow does not exceed 14%. The efficiency of a ventilation system can also relate to temperature in the greenhouse and at outlets by a thermal efficiency eta(T). It is observed that this ventilation efficiency does not change significantly with wind speed or direction, ranging between eta(T)=0.68 and 0.87 in the four cases analysed.