Numerical and experimental study of heat and mass transfers in an Almería-type greenhouse

The present work analyses the effect of natural ventilation in the energy balance of an Almería-type greenhouse by means of both experimental measurements and numerical modelling. We have observed a great influence of the ventilation heat flux is seen to have a great influence on the greenhouse energy balance, accounting for 30-50% of the inside solar radiation. Inside air temperature and humidity measurements were taken at 12 locations, while crop temperature, photosynthesis, evapotranspiration and stomata resistance were recorded with a portable photosynthesis system. Soil temperature was taken at 1, 10 and 20 cm depth, and soil heat flow at 20 cm depth; solar and photosynthetically active radiations were also measured. Ten 2D and two 3D sonic anemometers were used to measure the airflow through the 4 greenhouse openings. The results indicate that cool outside air enters through the two side openings while warmer air exits through the two roof vents. The low ventilation surface area (5%) allows air renewal rates of less than 20 h^-1 for winds speed of 4 m s^-1. These reduced ventilation rates produce inside-outside temperature gradients of up to 12°C at midday, when solar radiation is high (800 W m^-2). On cloudy days, with radiation around 500 W m^-2 and strong winds (7 m s^-1), the ventilation system allows air exchange rates of up to 35 h^-1, reducing the temperature gradient to appropriate values of 5-6°C. Plant temperatures were 2-10°C higher than the air temperature. The dynamic energy balance model predicts accurately the inside temperature using the outside climatic variables (solar radiation, air temperature and humidity, wind speed and direction) and greenhouse cover, ventilation system, crop and soil characteristic parameters as input.