Characterization of air velocities near greenhouse internal mobile screens using 3D sonic anemometry

In Dutch greenhouses, different screen types are used for different purposes (shading, energy saving, black-out, light emission, etc.). In order to quantify the energy and mass transfers through screens, characterization of air permeability through the screens is required. In the case of energy-saving screens, it is an essential parameter to estimate the energy saving of each screen. Air permeability can be measured under defined conditions in a laboratory. In order to select the appropriate equipment for air velocity measurements, the air velocity vector near screens in a practical situation in a greenhouse needs to be identified by measurements. Sonic anemometry techniques have been used extensively in different types of greenhouses: a) to study natural ventilation, with and without insect screens, and in different positions; b) to study airflow patterns in greenhouses with mechanical ventilation/pad and fan systems; c) to study airflow patterns induced by different types of heating systems, and d) for the estimation of crop evapotranspiration (i.e., eddy covariance). However, to the best of our knowledge, no research has been carried out to study the airflow near different types of screens in a greenhouse. Many Dutch growers are increasingly using various types of fans with different positions in the greenhouse for dehumidification and improved climate uniformity purposes. The effect of such fans on the air velocity near screens, and therefore the effect on energy and mass transfer, is unknown. For this purpose, air velocities near different types of screens in commercial greenhouses were measured using ultrasonic 3D anemometers. The results show that, in the absence of fans, air velocity near the screens is affected by vent opening. With vents closed, air velocities are hardly ever above 0.2 m s-1. Therefore, a simple air-suction device can be used to characterize permeability of screens at a very low Reynolds range.</p

An app to quantify radiative heat loss from greenhouse crops

Deploying a thermal screen in the night gives a significant reduction in radiative heat losses from the crop and heat losses of the greenhouse in general. The reduced radiative heat loss gives a smaller vertical temperature gradient in the crop. Deployment of a thermal screen results in increases in top-leaf temperatures of 1-2°C, which allows for a higher humidity set point without risk of wet leaves, even at higher humidity in the greenhouse. This increment in tolerance of humidity is the second contribution of thermal screens to energy saving. Both aspects of thermal screens have made increased screening one of the main pillars of “next-generation cultivation”, a term referring to growing strategies that reduce energy consumption while promoting crop production. In order to support knowledge on screens and to stimulate growers to apply the benefits of next-generation cultivation, an app was developed that quantifies the effect of screens on leaf temperature and transpiration. On top of that, the app computes the net radiation from the crop, a figure that has gained attention as more and more growers install net radiation sensors in their greenhouse. The effect of screens is, of course, dependent on the outside and inside climate conditions, the crop, the greenhouse covering material and the type of screens used. The app enables the user to select the screen and covering materials from a number of options and to select from a number of crops. Among the screens, a selection can be made from partly open shading screens to transparent energy screens and completely blocking blackout screens. Also, the effect of artificial light can be shown. The app solves the steady-state energy balance of the greenhouse to calculate the promptly presented output. With the output, a quick exploration of the effect of screens on radiative losses and crop vertical temperature profile can be made, to learn from this for practical use

Characterization of air velocities near greenhouse internal mobile screens using 3D sonic anemometry

In Dutch greenhouses, different screen types are used for different purposes (shading, energy saving, black-out, light emission, etc.). In order to quantify the energy and mass transfers through screens, characterization of air permeability through the screens is required. In the case of energy-saving screens, it is an essential parameter to estimate the energy saving of each screen. Air permeability can be measured under defined conditions in a laboratory. In order to select the appropriate equipment for air velocity measurements, the air velocity vector near screens in a practical situation in a greenhouse needs to be identified by measurements. Sonic anemometry techniques have been used extensively in different types of greenhouses: a) to study natural ventilation, with and without insect screens, and in different positions; b) to study airflow patterns in greenhouses with mechanical ventilation/pad and fan systems; c) to study airflow patterns induced by different types of heating systems, and d) for the estimation of crop evapotranspiration (i.e., eddy covariance). However, to the best of our knowledge, no research has been carried out to study the airflow near different types of screens in a greenhouse. Many Dutch growers are increasingly using various types of fans with different positions in the greenhouse for dehumidification and improved climate uniformity purposes. The effect of such fans on the air velocity near screens, and therefore the effect on energy and mass transfer, is unknown. For this purpose, air velocities near different types of screens in commercial greenhouses were measured using ultrasonic 3D anemometers. The results show that, in the absence of fans, air velocity near the screens is affected by vent opening. With vents closed, air velocities are hardly ever above 0.2 m s-1. Therefore, a simple air-suction device can be used to characterize permeability of screens at a very low Reynolds range.</p